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Strabismus is a common paediatric eye disorder occurring in 1–3% of children. However, it can present at any age.[[1,2]] Strabismus can be defined as any misalignment of the visual axes and is often referred to as a squint. Strabismus may be congenital or acquired and results from any abnormality along the ocular and oculomotor neural pathways.

In children, risk factors for strabismus include a family history of strabismus, prematurity and low birth weight.[[3]] Comorbid ocular conditions, including any cause of vision deprivation, neuromuscular disorders such as cerebral palsy and any systemic disease affecting the extraocular muscles of the eye, are also associated with a higher incidence of strabismus.

The goals of treatment are to improve ocular alignment and to preserve binocular vision, including stereopsis. Operative and non-operative interventions are used in the treatment of strabismus, and management is dependent on the subtype and etiology.[[3]] Surgical intervention for the purposes of this study include resection, recession or transposition of the extraocular muscles, as well as the application of botulinum toxin to any of the extraocular muscles.

There is a large body of evidence that demonstrates differences in health outcomes between population groups in New Zealand, with a specific focus placed on Māori, Pacific Peoples and high socioeconomic deprivation. This study aims to support the mandate of the New Zealand Government to deliver equitable health outcomes for all New Zealanders by building an understanding of how ophthalmic surgical services are distributed in the New Zealand public health system.

Strabismus is an important condition to explore in the context of equity due to the potential functional and psychosocial impacts of strabismus on children. Children with strabismus experience visual field defects and impaired binocular vision, and they are at higher risk of vision loss, which has serious implications for their education and future opportunities.[[4]] A review of the psychosocial, quality-of-life and health impacts of strabismus found that children with strabismus suffer from headaches, eye strain and increased incidences of mental illness such as anxiety, social phobia and learning disorders. Buffenn et al also described a continuation of a decreased quality of life into adulthood and negative attitudes and preconceptions regarding intelligence and trustworthiness towards individuals with strabismus.[[4]]

Methods

A retrospective analysis of all surgical strabismus procedures within the New Zealand public health sector between 1 January 2005 and 31 December 2014 was performed. Surgical strabismus data for the time period were retrieved from the National Minimum Dataset (NMDS)[[5]].

The NMDS is a national collection of public and private hospital discharge information, including coded clinical data for inpatients and day patients for publicly funded events. Data are provided by public and the larger private hospitals in a standardised electronic file format. Publicly funded hospital events are required to be loaded onto the NMDS within 21 days after the month of discharge.[[5]] Data are coded according to the International Statistical Classification of Diseases and Related Health Problems (ICD-10) classification system. There is no single ICD-10 code for a primary diagnosis of strabismus or for a strabismus procedure. A number of codes have been identified to represent the variety of subtypes and operations performed (Appendix Figure 1, Appendix Figure 2).

All cases retrieved from the NMDS with a recorded strabismus-related procedure and an associated diagnosis of any subtype of strabismus between the ages of 0 and 19 years were included in the study. Cases retrieved with incomplete data for ethnicity or deprivation index were excluded from the study. Data entries with a primary diagnosis of diplopia were also excluded due to ambiguity of diagnosis.

Ethnicity data recorded in the NMDS is self-reported at the time of admission for the given procedure. The cases were grouped according to the New Zealand census groupings of European, Māori, Pacific Peoples, Asian and Middle Eastern/Latin American/African (MELAA).

The New Zealand Index of Deprivation (NZDep13) is an area-based measure of socioeconomic deprivation derived from census data: it combines census data relating to income, family structure, education, employment, housing, home ownership and access to transport and communications. Socioeconomic deprivation is estimated by geographical location according to the NZDep13[[6]] and recorded in the NMDS. NZDep13 groups deprivation by deciles giving a score from 1 to 10 for each meshblock in New Zealand. An NZDep13 value of 1 represents the least deprived 10% of areas, whereas a value of 10 represents most deprived 10% of areas in New Zealand. For the purposes of this study, socioeconomic deprivation deciles were further grouped 1–3, 4–7 and 8–10 to respectively represent low, medium and high deprivation.

Data retrieved from the NMDS were then compared to regional demographic profiles, constructed from the 2013 census data. In the analysis, district health boards (DHBs) were grouped according to census DHB groupings under four corresponding regions: Northern, Midlands, Central and South Island.

Results

Six thousand and ninety-five cases from the 10-year period were retrieved from the NMDS: 1,563 cases were 20 years of age or older, and a further 56 cases had incomplete ethnicity or socioeconomic data and therefore did not meet the inclusion criteria. The final analysis included 4,476 cases. The median age was 5 years and there was a 50:50 split of cases by sex.

The distribution of cases by key ethnic groups were 71.4% European, 15.6% Māori and 4.9% Pacific Peoples. Thirty-four percent of procedures were in cases from the highest socioeconomic deprivation group, 41% of cases were from medium deprivation and 25% from the lowest deprivation group.

Table 1: Cases by ethnicity and socioeconomic deprivation, n (%).

*MELAA: Middle Eastern/Latin American/African.

Figure 1: Incidence of strabismus surgery per 1000 aged 0–19 years.

Over the 10-year period, the national incidence of strabismus surgery was 3.38 per 1,000 aged 0–19 years. The incidence of strabismus surgery in the South Island was 4.85 per 1,000 aged 0–19 years, more than double that of the Midlands region at 2.24 surgeries per 1,000. The inter-regional variation in procedural incidence may represent inter-regional disparities in access to surgical ophthalmic services.

There was a higher incidence of surgeries performed in the European ethnic group: 4.1 surgeries per 1,000 aged 0–19 years. When compared to the incidence of priority ethnic groups of Māori and Pacific Peoples, Europeans were respectively 1.5 times and 2.6 times as likely to receive surgical intervention for strabismus.

Figure 2: Incidence of strabismus surgery per 1,000 population aged 0–19 years.

*MELAA: Middle Eastern/Latin American/African.

Figure 3: Incidence of strabismus surgery per 1,000 youth population aged 0–19 years.

The most socioeconomically deprived group had the highest incidence of strabismus procedures over the 10-year period at 3.87 per 1,000. There appears to be a relationship between increasing incidence of strabismus surgery and higher socioeconomic deprivation. However, when stratifying the distribution of strabismus procedures by NZDep13 deciles (Figure 2), there is a notable under-representation of the most deprived 10% of the population when compared to the cases from deciles 8 and 9, which also make up the highest deprivation group.  

Figure 4: Distribution of cases of strabismus surgery by NZDep13.

Re-operation rate for surgical failures

The re-operation rate was calculated on a population level by dividing the number of secondary or subsequent procedures by the total number of primary procedures for each ethnic and socioeconomic deprivation cohorts. The national re-operation rate over the 10-year period was 11.8%.

Figure 5. Reoperation rate by ethnicity.

*MELAA: Middle Eastern/Latin American/African.

The European ethnic group had the highest rate of re-operation, with 13.0% receiving subsequent procedures following a primary procedure. The re-operation rate was lowest in the MELAA cohort at 2.6%. In comparison to the European cohort, Māori, Pacific Peoples and Asian ethnic groups were also less likely to receive subsequent procedures following the primary procedure, with respective re-operation rates of 8.4%, 9.4% and 9.2%.

Figure 6: Re-operation rate by deprivation group.

When compared to the re-operation rate in the most deprived socioeconomic group (9.2%), the low and medium deprivation groups were 1.46 and 1.43 times as likely to receive subsequent procedures following a primary operation.

Early intervention

The distribution of strabismus procedures by ethnicity were stratified further by age group for cases under the age of 19 years and then compared to the ethnic distribution of the youth population in the 2013 census. There was a higher representation of the European ethnic group across all three age categories. The degree of over-representation of the European ethnic group was found to be most pronounced in the 0–4-years age group, representing 75% of all procedures despite only comprising 59% of the population 4 years or younger. Minority ethnic groups were found to be under-represented across almost all age-groupings when compared to their census population distribution, with the Pacific Peoples ethnic group being most greatly under-represented. These findings could suggest earlier access to surgical services for Europeans when compared to all minority ethnic groups.

Table 2: Distribution of strabismus surgery by age group compared with 2013 census distribution of youth population.

*MELAA: Middle Eastern/Latin American/African.

Discussion

This is the first population-based study to explore the association between the incidence of strabismus surgery, ethnicity and socioeconomic status in New Zealand. The over-arching aim of this study was to explore access to surgical intervention in the New Zealand public health system, with a specific focus on the surgical specialty of ophthalmology.

There is a body of evidence that demonstrates reduced access to care and poorer health outcomes for Māori, Pacific Peoples and those who experience greater socioeconomic deprivation. Few studies have directly compared access to surgical intervention with socioeconomic deprivation and ethnicity in New Zealand. Key strengths of the study were its large population-base of data analysed over a 10-year time period with over 97% of data complete for ethnicity and deprivation index.

Limitations of the study include inconsistencies in the NMDS data entries and collection; the use of NZDep13 to estimate socioeconomic status; self-reported ethnicity; and the unavailability of data for procedures performed in the private surgical healthcare setting.

A number of different ICD-10 codes had been used to represent various sub-types as the diagnosis for strabismus and operation type in the retrieved NMDS. On entering the data, clinicians were also required to enter a written diagnosis and/or operation. Few inconsistencies were found in the entries of written diagnoses or operation types requiring clarification between stated written entries and ICD-10 codes. However, ICD-10 codes were reliable overall, with no significant inconsistencies in the ICD-10 coding for diagnosis or operation type.

The NMDS includes discharge information from all public and larger private hospitals, including day stay facilities for publicly funded events. There are potentially missing entries for publicly funded procedures performed at smaller private and day-stay surgeries in New Zealand. There is a nominal three-hour threshold of admission for reporting events to NMDS. If a case doesn’t meet this threshold for reporting, it may not be reported by same-day eye facilities. Outpatient data for publicly funded surgical procedures may also be reported to the National Non-Admitted Patient Collection (NNPAC), but the reported data do not include diagnostic or procedural data, which were therefore not explored further for the purposes of this study. A large majority of publicly funded ophthalmic procedures are performed in public hospitals in New Zealand. There are three private day surgeries in New Zealand contracted by their DHBs to routinely provide publicly funded ophthalmic surgeries. Two of these do not provide general anaesthetic and therefore are unsuitable for most eye procedures in children; the third facility was contacted and consistently reports their day-stay data to the NMDS. The authors consider the data retrieved from the NMDS to be a reasonable representation of the publicly funded procedures for strabismus over the 10-year period.

NZDep13 is an area-based measure of socioeconomic deprivation, as estimated by a participant’s address, and therefore might not be a true reflection of a person’s socioeconomic status. Ethnicity recorded in the NMDS and the 2013 New Zealand census are both self-reported. However, in the census an individual may be able to be represented under a number of different ethnic groups, and in the NMDS only one primary ethnicity can be recorded.

The data retrieved from the NMDS represented publicly funded strabismus procedures from 1 January 2005 to the 31 December 2014, but population profiles were constructed from a static point in time at the 2013 census. There are small variations between regional age, ethnic and socioeconomic deprivation distributions between the 2006, 2012 and 2018 censuses. The overall trends in population distribution remain consistent between the three most recent censuses and thus the 2013 census is considered to be a reasonable estimate of the population profile over the study period.

A larger proportion of surgeries were performed in the highest deprivation cohort when compared with the lowest deprivation cohort. This demonstrates a greater distribution of public health services for New Zealanders who are more socioeconomically deprived. It is worth noting that procedures performed privately are not required to be reported to the NMDS. Therefore, it is possible that there was bias towards private surgical care for lower deprivation groups, who can more likely afford private healthcare or health insurance. A greater uptake of surgical procedures in a private setting for more affluent New Zealanders could be another possible explanation for a lower incidence of strabismus procedures in the least socioeconomically deprived group. It is also possible that children from less-deprived homes receive non-operative care earlier and thus have a lower need for surgical intervention.

It is possible that the lower incidence of surgeries performed in Māori and Pacific Peoples is due to a lower population prevalence of strabismus and therefore a reduced need for surgery. A retrospective analysis of the electronic records of 846,477 patients with strabismus in the United States found a higher prevalence of strabismus in non-Hispanic Whites: 2.9% compared with 2.4% for African American and 2.0% for Native Hawaiian.[[7]] Interestingly, the prevalence of strabismus was found to be greatest in bi-racial/multi-racial ethnicity at 3.3%.[[7]] However, this large US study included patients of all ages, with different risk factors and aetiology of strabismus in adults than in children. The Sydney Paediatric Eye Disease Study also demonstrated ethnic variation in the prevalence of strabismus in children 6–72 months, with 4% in South Asian, 3.5% in European Caucasian and 3.3% in other ethnicities.[[8]] Although global studies have showed some ethnic variation in the prevalence of strabismus, there is no current evidence to support a higher prevalence of strabismus in New Zealand European’s than in Māori or Pacific Peoples. The disparity in the incidence of strabismus surgeries between ethnic groups in this study is too large to account for a difference in ethnic prevalence alone and more likely reflects a component of differing access to health services. Further research is required to determine the specific ethnic variation of common ophthalmic conditions such as strabismus in a New Zealand context.

The national re-operation rate was estimated on a population level to be 11.8%. This was further explored by ethnicity and socioeconomic deprivation. Re-operation rates in this study were similar to those in a large population-based study in the United States with estimated re-operation rates for strabismus procedures between 6.7% and 11.5% across all age groups.[[9]] We found that Māori, Pacific Peoples and those in the highest deprivation grouping had lower rates of re-operation. Although the MELAA ethnic group was found to have an incidence of strabismus surgery similar to the national incidence, the rate of re-operation in the cohort was less than quarter of the national rate at 2.7%. This can be interpreted either as lower operative failure rates in these cohorts, or as a reduced access to subsequent procedures following an unsuccessful primary procedure. Strabismus procedures can be performed for both cosmetic and functional corrections of misalignment. Factors affecting the likelihood of re-operation for strabismus include the type of strabismus, case complexity, patient age, the experience of the surgeon, the surgical method and patient satisfaction. While appreciating the complexity of factors influencing reoperation rate, we still expect reasonable randomisation of these effects between groups given the large size of our sample. It is possible that the observed variation in re-operation rates may reflect intrinsic cohort characteristics, with European and less socioeconomically deprived groups being more likely to seek further medical attention if a primary operation was perceived as unsatisfactory.

The findings in this study suggest earlier access to surgical intervention for a European cohort when compared to all other ethnic groups. There appears to be a trend across all minority ethnic groups of an increasing incidence of procedures with increasing age. The greatest incidence of surgeries by ethnicity peak at 0–4 years for European, 5–9 years for Māori and 10–19 years for Pacific Peoples, Asian and MELAA ethnic groups.

For the purposes of this study, having received surgical intervention for strabismus was used as an estimate of a population’s access to healthcare. To further assess access to ophthalmic services in the New Zealand public health system, the number of referrals made to ophthalmology services and alternative treatment options offered by ethnicity and socioeconomic status could also be explored. By using an event of surgical intervention as an end point, this study did not directly compare health outcomes. Further research is required to explore long-term visual outcomes between these population groups and assess the clinical significance of these reported variations in surgical intervention for strabismus.

Conclusion

This research shows that disproportionately fewer strabismus surgeries are performed in Māori, Pacific Peoples and those in the lowest deprivation group.

Our findings also suggest that access to subsequent secondary procedures in minority ethnic and higher deprivation groups may also be reduced. Further research is needed to directly compare health outcomes between these higher-needs and lower-needs groups.

Appendix

Appendix Table 1: ICD-10 strabismus sub-type codes. View Appendix Table 1.

Appendix Table 2: ICD-10 strabismus procedure codes. View Appendix Table 2.

Summary

Abstract

Aim

This study aimed to identify the relationship between the incidence of strabismus surgery, ethnicity and socioeconomic deprivation in the New Zealand public health system. Secondary outcomes explored the association between re-operation rate for surgical failures, ethnicity and socioeconomic deprivation.

Method

Cases receiving operative management for strabismus were retrieved from the National Minimum Dataset. The incidence of surgery was correlated to patient demographics by ethnicity and socioeconomic deprivation and compared to population profiles for 0–19-year-olds constructed from the 2013 census.

Results

There were 4,476 strabismus surgeries recorded over a 10 year period from 1 January 2005 to 31 December 2014 included in the study. There was a lower incidence of strabismus surgery performed in Māori, Pacific Peoples and the least socioeconomically deprived cohort. There were significant inter-regional variations in the incidence of strabismus surgery. The European ethnic group was 1.4 times as likely to receive subsequent procedures following a primary procedure than either Māori or Pacific Peoples.

Conclusion

Disproportionately fewer strabismus surgeries were performed in Māori, Pacific Peoples and New Zealanders from the lowest deprived group in the New Zealand Public Health System. Minority ethnic groups are less likely to receive secondary operations following a primary procedure when compared to a European cohort. Further research is needed to directly compare health outcomes between these high-needs and lower-needs groups.

Author Information

Cheefong Chong: Consultant Ophthalmologist Tauranga Eye Specialists; Paediatric Ophthalmologist, Waikato District Health Board; Consultant Ophthalmologist, Auckland District Health Board; Honorary Academic, Faculty of Medicine and Health Sciences, University of Auckland. Alexandra Lawrence: PGY1, Bay of Plenty DHB, Tauranga; Tauranga Eye Specialists, Waikato District Health Board, Faculty of Medicine and Health Sciences, University of Auckland; Waikato DHB and FMHS UOA. Dan Allbon: Ophthalmology Registrar, Southern DHB, Dunedin; Tauranga Eye Specialists, Waikato District Health Board, Faculty of Medicine and Health Sciences, University of Auckland; Waikato DHB and FMHS UOA.

Acknowledgements

Correspondence

Alexandra Lawrence, Park St Eye Clinic and Day Stay Theatre, 25 Park St, Tauranga, 3110, + 64 27 511 0355

Correspondence Email

alexandralawrence4@gmail.com

Competing Interests

Nil.

1) Torp-Pedersen T, Boyd HA, Skotte L, Haargaard B, Wohlfahrt J, Holmes JM, Melbye M. Strabismus Incidence in a Danish Population-Based Cohort of Children. JAMA Ophthalmol. 2017 Oct 1;135(10):1047-53. doi: 10.1001/jamaophthalmol.2017.3158. PMID: 28859196; PMCID: PMC5710488.

2) Hashemi H, Pakzad R, Heydarian S, Yekta A, Aghamirsalim M, Shokrollahzadeh F, Khoshhal F, Pakbin M, Ramin S, Khabazkhoob M. Global and regional prevalence of strabismus: a comprehensive systematic review and meta-analysis. Strabismus. 2019 Jun;27(2):54-65. doi: 10.1080/09273972.2019.1604773. Epub 2019 Apr 23. PMID: 31012389.

3) Williams C, Northstone K, Howard M, Harvey I, Harrad RA, Sparrow JM. Prevalence and risk factors for common vision problems in children: data from the ALSPAC study. Br J Ophthalmol. 2008 Jul;92(7):959-64. doi: 10.1136/bjo.2007.134700. Epub 2008 May 14. PMID: 18480306.Ono H, Mapp AP. A restatement and modification of Wells-Hering's laws of visual direction. Perception. 1995;24(2):237-52. doi: 10.1068/p240237. PMID: 7617427.

4) Angela N. Buffenn, The impact of strabismus on psychosocial heath and quality of life: a systematic review, Survey of Ophthalmology. Available from: https://doi.org/10.1016/j.survophthal.2021.03.005

5) Ministry of Health [Internet]. Data and Statistics National Minimum Dataset (Hospital Events) (NMDS). 2010. Available from: http://www.moh.govt.nz/moh.nsf/indexmh/dataandstatistics-collections-nmds

6) Atkinson J, Salmond C, Crampton P. 2014. NZDep2013 Index of Deprivation. Dunedin: University of Otago.

7) Repka M, Lum F, Burugapolli B. Strabismus and strabismus surgery in the United States: analysis from the IRIS registry. Journal of American Association for Pediatric Ophthalmology and Strabismus. 2018;22(4):e6.

8) S. Sharbini. Prevalence Of Strabismus & Associated Risk Factors: The Sydney Childhood Eye Studies. The University of Sydney. 2015 Oct.

9) Repka MX, Lum F, Burugapalli B. Strabismus, Strabismus Surgery, and Reoperation Rate in the United States: Analysis from the IRIS Registry. Ophthalmology. 2018 Oct;125(10):1646-53. doi: 10.1016/j.ophtha.2018.04.024. Epub 2018 May 18. PMID: 29779683.

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Strabismus is a common paediatric eye disorder occurring in 1–3% of children. However, it can present at any age.[[1,2]] Strabismus can be defined as any misalignment of the visual axes and is often referred to as a squint. Strabismus may be congenital or acquired and results from any abnormality along the ocular and oculomotor neural pathways.

In children, risk factors for strabismus include a family history of strabismus, prematurity and low birth weight.[[3]] Comorbid ocular conditions, including any cause of vision deprivation, neuromuscular disorders such as cerebral palsy and any systemic disease affecting the extraocular muscles of the eye, are also associated with a higher incidence of strabismus.

The goals of treatment are to improve ocular alignment and to preserve binocular vision, including stereopsis. Operative and non-operative interventions are used in the treatment of strabismus, and management is dependent on the subtype and etiology.[[3]] Surgical intervention for the purposes of this study include resection, recession or transposition of the extraocular muscles, as well as the application of botulinum toxin to any of the extraocular muscles.

There is a large body of evidence that demonstrates differences in health outcomes between population groups in New Zealand, with a specific focus placed on Māori, Pacific Peoples and high socioeconomic deprivation. This study aims to support the mandate of the New Zealand Government to deliver equitable health outcomes for all New Zealanders by building an understanding of how ophthalmic surgical services are distributed in the New Zealand public health system.

Strabismus is an important condition to explore in the context of equity due to the potential functional and psychosocial impacts of strabismus on children. Children with strabismus experience visual field defects and impaired binocular vision, and they are at higher risk of vision loss, which has serious implications for their education and future opportunities.[[4]] A review of the psychosocial, quality-of-life and health impacts of strabismus found that children with strabismus suffer from headaches, eye strain and increased incidences of mental illness such as anxiety, social phobia and learning disorders. Buffenn et al also described a continuation of a decreased quality of life into adulthood and negative attitudes and preconceptions regarding intelligence and trustworthiness towards individuals with strabismus.[[4]]

Methods

A retrospective analysis of all surgical strabismus procedures within the New Zealand public health sector between 1 January 2005 and 31 December 2014 was performed. Surgical strabismus data for the time period were retrieved from the National Minimum Dataset (NMDS)[[5]].

The NMDS is a national collection of public and private hospital discharge information, including coded clinical data for inpatients and day patients for publicly funded events. Data are provided by public and the larger private hospitals in a standardised electronic file format. Publicly funded hospital events are required to be loaded onto the NMDS within 21 days after the month of discharge.[[5]] Data are coded according to the International Statistical Classification of Diseases and Related Health Problems (ICD-10) classification system. There is no single ICD-10 code for a primary diagnosis of strabismus or for a strabismus procedure. A number of codes have been identified to represent the variety of subtypes and operations performed (Appendix Figure 1, Appendix Figure 2).

All cases retrieved from the NMDS with a recorded strabismus-related procedure and an associated diagnosis of any subtype of strabismus between the ages of 0 and 19 years were included in the study. Cases retrieved with incomplete data for ethnicity or deprivation index were excluded from the study. Data entries with a primary diagnosis of diplopia were also excluded due to ambiguity of diagnosis.

Ethnicity data recorded in the NMDS is self-reported at the time of admission for the given procedure. The cases were grouped according to the New Zealand census groupings of European, Māori, Pacific Peoples, Asian and Middle Eastern/Latin American/African (MELAA).

The New Zealand Index of Deprivation (NZDep13) is an area-based measure of socioeconomic deprivation derived from census data: it combines census data relating to income, family structure, education, employment, housing, home ownership and access to transport and communications. Socioeconomic deprivation is estimated by geographical location according to the NZDep13[[6]] and recorded in the NMDS. NZDep13 groups deprivation by deciles giving a score from 1 to 10 for each meshblock in New Zealand. An NZDep13 value of 1 represents the least deprived 10% of areas, whereas a value of 10 represents most deprived 10% of areas in New Zealand. For the purposes of this study, socioeconomic deprivation deciles were further grouped 1–3, 4–7 and 8–10 to respectively represent low, medium and high deprivation.

Data retrieved from the NMDS were then compared to regional demographic profiles, constructed from the 2013 census data. In the analysis, district health boards (DHBs) were grouped according to census DHB groupings under four corresponding regions: Northern, Midlands, Central and South Island.

Results

Six thousand and ninety-five cases from the 10-year period were retrieved from the NMDS: 1,563 cases were 20 years of age or older, and a further 56 cases had incomplete ethnicity or socioeconomic data and therefore did not meet the inclusion criteria. The final analysis included 4,476 cases. The median age was 5 years and there was a 50:50 split of cases by sex.

The distribution of cases by key ethnic groups were 71.4% European, 15.6% Māori and 4.9% Pacific Peoples. Thirty-four percent of procedures were in cases from the highest socioeconomic deprivation group, 41% of cases were from medium deprivation and 25% from the lowest deprivation group.

Table 1: Cases by ethnicity and socioeconomic deprivation, n (%).

*MELAA: Middle Eastern/Latin American/African.

Figure 1: Incidence of strabismus surgery per 1000 aged 0–19 years.

Over the 10-year period, the national incidence of strabismus surgery was 3.38 per 1,000 aged 0–19 years. The incidence of strabismus surgery in the South Island was 4.85 per 1,000 aged 0–19 years, more than double that of the Midlands region at 2.24 surgeries per 1,000. The inter-regional variation in procedural incidence may represent inter-regional disparities in access to surgical ophthalmic services.

There was a higher incidence of surgeries performed in the European ethnic group: 4.1 surgeries per 1,000 aged 0–19 years. When compared to the incidence of priority ethnic groups of Māori and Pacific Peoples, Europeans were respectively 1.5 times and 2.6 times as likely to receive surgical intervention for strabismus.

Figure 2: Incidence of strabismus surgery per 1,000 population aged 0–19 years.

*MELAA: Middle Eastern/Latin American/African.

Figure 3: Incidence of strabismus surgery per 1,000 youth population aged 0–19 years.

The most socioeconomically deprived group had the highest incidence of strabismus procedures over the 10-year period at 3.87 per 1,000. There appears to be a relationship between increasing incidence of strabismus surgery and higher socioeconomic deprivation. However, when stratifying the distribution of strabismus procedures by NZDep13 deciles (Figure 2), there is a notable under-representation of the most deprived 10% of the population when compared to the cases from deciles 8 and 9, which also make up the highest deprivation group.  

Figure 4: Distribution of cases of strabismus surgery by NZDep13.

Re-operation rate for surgical failures

The re-operation rate was calculated on a population level by dividing the number of secondary or subsequent procedures by the total number of primary procedures for each ethnic and socioeconomic deprivation cohorts. The national re-operation rate over the 10-year period was 11.8%.

Figure 5. Reoperation rate by ethnicity.

*MELAA: Middle Eastern/Latin American/African.

The European ethnic group had the highest rate of re-operation, with 13.0% receiving subsequent procedures following a primary procedure. The re-operation rate was lowest in the MELAA cohort at 2.6%. In comparison to the European cohort, Māori, Pacific Peoples and Asian ethnic groups were also less likely to receive subsequent procedures following the primary procedure, with respective re-operation rates of 8.4%, 9.4% and 9.2%.

Figure 6: Re-operation rate by deprivation group.

When compared to the re-operation rate in the most deprived socioeconomic group (9.2%), the low and medium deprivation groups were 1.46 and 1.43 times as likely to receive subsequent procedures following a primary operation.

Early intervention

The distribution of strabismus procedures by ethnicity were stratified further by age group for cases under the age of 19 years and then compared to the ethnic distribution of the youth population in the 2013 census. There was a higher representation of the European ethnic group across all three age categories. The degree of over-representation of the European ethnic group was found to be most pronounced in the 0–4-years age group, representing 75% of all procedures despite only comprising 59% of the population 4 years or younger. Minority ethnic groups were found to be under-represented across almost all age-groupings when compared to their census population distribution, with the Pacific Peoples ethnic group being most greatly under-represented. These findings could suggest earlier access to surgical services for Europeans when compared to all minority ethnic groups.

Table 2: Distribution of strabismus surgery by age group compared with 2013 census distribution of youth population.

*MELAA: Middle Eastern/Latin American/African.

Discussion

This is the first population-based study to explore the association between the incidence of strabismus surgery, ethnicity and socioeconomic status in New Zealand. The over-arching aim of this study was to explore access to surgical intervention in the New Zealand public health system, with a specific focus on the surgical specialty of ophthalmology.

There is a body of evidence that demonstrates reduced access to care and poorer health outcomes for Māori, Pacific Peoples and those who experience greater socioeconomic deprivation. Few studies have directly compared access to surgical intervention with socioeconomic deprivation and ethnicity in New Zealand. Key strengths of the study were its large population-base of data analysed over a 10-year time period with over 97% of data complete for ethnicity and deprivation index.

Limitations of the study include inconsistencies in the NMDS data entries and collection; the use of NZDep13 to estimate socioeconomic status; self-reported ethnicity; and the unavailability of data for procedures performed in the private surgical healthcare setting.

A number of different ICD-10 codes had been used to represent various sub-types as the diagnosis for strabismus and operation type in the retrieved NMDS. On entering the data, clinicians were also required to enter a written diagnosis and/or operation. Few inconsistencies were found in the entries of written diagnoses or operation types requiring clarification between stated written entries and ICD-10 codes. However, ICD-10 codes were reliable overall, with no significant inconsistencies in the ICD-10 coding for diagnosis or operation type.

The NMDS includes discharge information from all public and larger private hospitals, including day stay facilities for publicly funded events. There are potentially missing entries for publicly funded procedures performed at smaller private and day-stay surgeries in New Zealand. There is a nominal three-hour threshold of admission for reporting events to NMDS. If a case doesn’t meet this threshold for reporting, it may not be reported by same-day eye facilities. Outpatient data for publicly funded surgical procedures may also be reported to the National Non-Admitted Patient Collection (NNPAC), but the reported data do not include diagnostic or procedural data, which were therefore not explored further for the purposes of this study. A large majority of publicly funded ophthalmic procedures are performed in public hospitals in New Zealand. There are three private day surgeries in New Zealand contracted by their DHBs to routinely provide publicly funded ophthalmic surgeries. Two of these do not provide general anaesthetic and therefore are unsuitable for most eye procedures in children; the third facility was contacted and consistently reports their day-stay data to the NMDS. The authors consider the data retrieved from the NMDS to be a reasonable representation of the publicly funded procedures for strabismus over the 10-year period.

NZDep13 is an area-based measure of socioeconomic deprivation, as estimated by a participant’s address, and therefore might not be a true reflection of a person’s socioeconomic status. Ethnicity recorded in the NMDS and the 2013 New Zealand census are both self-reported. However, in the census an individual may be able to be represented under a number of different ethnic groups, and in the NMDS only one primary ethnicity can be recorded.

The data retrieved from the NMDS represented publicly funded strabismus procedures from 1 January 2005 to the 31 December 2014, but population profiles were constructed from a static point in time at the 2013 census. There are small variations between regional age, ethnic and socioeconomic deprivation distributions between the 2006, 2012 and 2018 censuses. The overall trends in population distribution remain consistent between the three most recent censuses and thus the 2013 census is considered to be a reasonable estimate of the population profile over the study period.

A larger proportion of surgeries were performed in the highest deprivation cohort when compared with the lowest deprivation cohort. This demonstrates a greater distribution of public health services for New Zealanders who are more socioeconomically deprived. It is worth noting that procedures performed privately are not required to be reported to the NMDS. Therefore, it is possible that there was bias towards private surgical care for lower deprivation groups, who can more likely afford private healthcare or health insurance. A greater uptake of surgical procedures in a private setting for more affluent New Zealanders could be another possible explanation for a lower incidence of strabismus procedures in the least socioeconomically deprived group. It is also possible that children from less-deprived homes receive non-operative care earlier and thus have a lower need for surgical intervention.

It is possible that the lower incidence of surgeries performed in Māori and Pacific Peoples is due to a lower population prevalence of strabismus and therefore a reduced need for surgery. A retrospective analysis of the electronic records of 846,477 patients with strabismus in the United States found a higher prevalence of strabismus in non-Hispanic Whites: 2.9% compared with 2.4% for African American and 2.0% for Native Hawaiian.[[7]] Interestingly, the prevalence of strabismus was found to be greatest in bi-racial/multi-racial ethnicity at 3.3%.[[7]] However, this large US study included patients of all ages, with different risk factors and aetiology of strabismus in adults than in children. The Sydney Paediatric Eye Disease Study also demonstrated ethnic variation in the prevalence of strabismus in children 6–72 months, with 4% in South Asian, 3.5% in European Caucasian and 3.3% in other ethnicities.[[8]] Although global studies have showed some ethnic variation in the prevalence of strabismus, there is no current evidence to support a higher prevalence of strabismus in New Zealand European’s than in Māori or Pacific Peoples. The disparity in the incidence of strabismus surgeries between ethnic groups in this study is too large to account for a difference in ethnic prevalence alone and more likely reflects a component of differing access to health services. Further research is required to determine the specific ethnic variation of common ophthalmic conditions such as strabismus in a New Zealand context.

The national re-operation rate was estimated on a population level to be 11.8%. This was further explored by ethnicity and socioeconomic deprivation. Re-operation rates in this study were similar to those in a large population-based study in the United States with estimated re-operation rates for strabismus procedures between 6.7% and 11.5% across all age groups.[[9]] We found that Māori, Pacific Peoples and those in the highest deprivation grouping had lower rates of re-operation. Although the MELAA ethnic group was found to have an incidence of strabismus surgery similar to the national incidence, the rate of re-operation in the cohort was less than quarter of the national rate at 2.7%. This can be interpreted either as lower operative failure rates in these cohorts, or as a reduced access to subsequent procedures following an unsuccessful primary procedure. Strabismus procedures can be performed for both cosmetic and functional corrections of misalignment. Factors affecting the likelihood of re-operation for strabismus include the type of strabismus, case complexity, patient age, the experience of the surgeon, the surgical method and patient satisfaction. While appreciating the complexity of factors influencing reoperation rate, we still expect reasonable randomisation of these effects between groups given the large size of our sample. It is possible that the observed variation in re-operation rates may reflect intrinsic cohort characteristics, with European and less socioeconomically deprived groups being more likely to seek further medical attention if a primary operation was perceived as unsatisfactory.

The findings in this study suggest earlier access to surgical intervention for a European cohort when compared to all other ethnic groups. There appears to be a trend across all minority ethnic groups of an increasing incidence of procedures with increasing age. The greatest incidence of surgeries by ethnicity peak at 0–4 years for European, 5–9 years for Māori and 10–19 years for Pacific Peoples, Asian and MELAA ethnic groups.

For the purposes of this study, having received surgical intervention for strabismus was used as an estimate of a population’s access to healthcare. To further assess access to ophthalmic services in the New Zealand public health system, the number of referrals made to ophthalmology services and alternative treatment options offered by ethnicity and socioeconomic status could also be explored. By using an event of surgical intervention as an end point, this study did not directly compare health outcomes. Further research is required to explore long-term visual outcomes between these population groups and assess the clinical significance of these reported variations in surgical intervention for strabismus.

Conclusion

This research shows that disproportionately fewer strabismus surgeries are performed in Māori, Pacific Peoples and those in the lowest deprivation group.

Our findings also suggest that access to subsequent secondary procedures in minority ethnic and higher deprivation groups may also be reduced. Further research is needed to directly compare health outcomes between these higher-needs and lower-needs groups.

Appendix

Appendix Table 1: ICD-10 strabismus sub-type codes. View Appendix Table 1.

Appendix Table 2: ICD-10 strabismus procedure codes. View Appendix Table 2.

Summary

Abstract

Aim

This study aimed to identify the relationship between the incidence of strabismus surgery, ethnicity and socioeconomic deprivation in the New Zealand public health system. Secondary outcomes explored the association between re-operation rate for surgical failures, ethnicity and socioeconomic deprivation.

Method

Cases receiving operative management for strabismus were retrieved from the National Minimum Dataset. The incidence of surgery was correlated to patient demographics by ethnicity and socioeconomic deprivation and compared to population profiles for 0–19-year-olds constructed from the 2013 census.

Results

There were 4,476 strabismus surgeries recorded over a 10 year period from 1 January 2005 to 31 December 2014 included in the study. There was a lower incidence of strabismus surgery performed in Māori, Pacific Peoples and the least socioeconomically deprived cohort. There were significant inter-regional variations in the incidence of strabismus surgery. The European ethnic group was 1.4 times as likely to receive subsequent procedures following a primary procedure than either Māori or Pacific Peoples.

Conclusion

Disproportionately fewer strabismus surgeries were performed in Māori, Pacific Peoples and New Zealanders from the lowest deprived group in the New Zealand Public Health System. Minority ethnic groups are less likely to receive secondary operations following a primary procedure when compared to a European cohort. Further research is needed to directly compare health outcomes between these high-needs and lower-needs groups.

Author Information

Cheefong Chong: Consultant Ophthalmologist Tauranga Eye Specialists; Paediatric Ophthalmologist, Waikato District Health Board; Consultant Ophthalmologist, Auckland District Health Board; Honorary Academic, Faculty of Medicine and Health Sciences, University of Auckland. Alexandra Lawrence: PGY1, Bay of Plenty DHB, Tauranga; Tauranga Eye Specialists, Waikato District Health Board, Faculty of Medicine and Health Sciences, University of Auckland; Waikato DHB and FMHS UOA. Dan Allbon: Ophthalmology Registrar, Southern DHB, Dunedin; Tauranga Eye Specialists, Waikato District Health Board, Faculty of Medicine and Health Sciences, University of Auckland; Waikato DHB and FMHS UOA.

Acknowledgements

Correspondence

Alexandra Lawrence, Park St Eye Clinic and Day Stay Theatre, 25 Park St, Tauranga, 3110, + 64 27 511 0355

Correspondence Email

alexandralawrence4@gmail.com

Competing Interests

Nil.

1) Torp-Pedersen T, Boyd HA, Skotte L, Haargaard B, Wohlfahrt J, Holmes JM, Melbye M. Strabismus Incidence in a Danish Population-Based Cohort of Children. JAMA Ophthalmol. 2017 Oct 1;135(10):1047-53. doi: 10.1001/jamaophthalmol.2017.3158. PMID: 28859196; PMCID: PMC5710488.

2) Hashemi H, Pakzad R, Heydarian S, Yekta A, Aghamirsalim M, Shokrollahzadeh F, Khoshhal F, Pakbin M, Ramin S, Khabazkhoob M. Global and regional prevalence of strabismus: a comprehensive systematic review and meta-analysis. Strabismus. 2019 Jun;27(2):54-65. doi: 10.1080/09273972.2019.1604773. Epub 2019 Apr 23. PMID: 31012389.

3) Williams C, Northstone K, Howard M, Harvey I, Harrad RA, Sparrow JM. Prevalence and risk factors for common vision problems in children: data from the ALSPAC study. Br J Ophthalmol. 2008 Jul;92(7):959-64. doi: 10.1136/bjo.2007.134700. Epub 2008 May 14. PMID: 18480306.Ono H, Mapp AP. A restatement and modification of Wells-Hering's laws of visual direction. Perception. 1995;24(2):237-52. doi: 10.1068/p240237. PMID: 7617427.

4) Angela N. Buffenn, The impact of strabismus on psychosocial heath and quality of life: a systematic review, Survey of Ophthalmology. Available from: https://doi.org/10.1016/j.survophthal.2021.03.005

5) Ministry of Health [Internet]. Data and Statistics National Minimum Dataset (Hospital Events) (NMDS). 2010. Available from: http://www.moh.govt.nz/moh.nsf/indexmh/dataandstatistics-collections-nmds

6) Atkinson J, Salmond C, Crampton P. 2014. NZDep2013 Index of Deprivation. Dunedin: University of Otago.

7) Repka M, Lum F, Burugapolli B. Strabismus and strabismus surgery in the United States: analysis from the IRIS registry. Journal of American Association for Pediatric Ophthalmology and Strabismus. 2018;22(4):e6.

8) S. Sharbini. Prevalence Of Strabismus & Associated Risk Factors: The Sydney Childhood Eye Studies. The University of Sydney. 2015 Oct.

9) Repka MX, Lum F, Burugapalli B. Strabismus, Strabismus Surgery, and Reoperation Rate in the United States: Analysis from the IRIS Registry. Ophthalmology. 2018 Oct;125(10):1646-53. doi: 10.1016/j.ophtha.2018.04.024. Epub 2018 May 18. PMID: 29779683.

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Strabismus is a common paediatric eye disorder occurring in 1–3% of children. However, it can present at any age.[[1,2]] Strabismus can be defined as any misalignment of the visual axes and is often referred to as a squint. Strabismus may be congenital or acquired and results from any abnormality along the ocular and oculomotor neural pathways.

In children, risk factors for strabismus include a family history of strabismus, prematurity and low birth weight.[[3]] Comorbid ocular conditions, including any cause of vision deprivation, neuromuscular disorders such as cerebral palsy and any systemic disease affecting the extraocular muscles of the eye, are also associated with a higher incidence of strabismus.

The goals of treatment are to improve ocular alignment and to preserve binocular vision, including stereopsis. Operative and non-operative interventions are used in the treatment of strabismus, and management is dependent on the subtype and etiology.[[3]] Surgical intervention for the purposes of this study include resection, recession or transposition of the extraocular muscles, as well as the application of botulinum toxin to any of the extraocular muscles.

There is a large body of evidence that demonstrates differences in health outcomes between population groups in New Zealand, with a specific focus placed on Māori, Pacific Peoples and high socioeconomic deprivation. This study aims to support the mandate of the New Zealand Government to deliver equitable health outcomes for all New Zealanders by building an understanding of how ophthalmic surgical services are distributed in the New Zealand public health system.

Strabismus is an important condition to explore in the context of equity due to the potential functional and psychosocial impacts of strabismus on children. Children with strabismus experience visual field defects and impaired binocular vision, and they are at higher risk of vision loss, which has serious implications for their education and future opportunities.[[4]] A review of the psychosocial, quality-of-life and health impacts of strabismus found that children with strabismus suffer from headaches, eye strain and increased incidences of mental illness such as anxiety, social phobia and learning disorders. Buffenn et al also described a continuation of a decreased quality of life into adulthood and negative attitudes and preconceptions regarding intelligence and trustworthiness towards individuals with strabismus.[[4]]

Methods

A retrospective analysis of all surgical strabismus procedures within the New Zealand public health sector between 1 January 2005 and 31 December 2014 was performed. Surgical strabismus data for the time period were retrieved from the National Minimum Dataset (NMDS)[[5]].

The NMDS is a national collection of public and private hospital discharge information, including coded clinical data for inpatients and day patients for publicly funded events. Data are provided by public and the larger private hospitals in a standardised electronic file format. Publicly funded hospital events are required to be loaded onto the NMDS within 21 days after the month of discharge.[[5]] Data are coded according to the International Statistical Classification of Diseases and Related Health Problems (ICD-10) classification system. There is no single ICD-10 code for a primary diagnosis of strabismus or for a strabismus procedure. A number of codes have been identified to represent the variety of subtypes and operations performed (Appendix Figure 1, Appendix Figure 2).

All cases retrieved from the NMDS with a recorded strabismus-related procedure and an associated diagnosis of any subtype of strabismus between the ages of 0 and 19 years were included in the study. Cases retrieved with incomplete data for ethnicity or deprivation index were excluded from the study. Data entries with a primary diagnosis of diplopia were also excluded due to ambiguity of diagnosis.

Ethnicity data recorded in the NMDS is self-reported at the time of admission for the given procedure. The cases were grouped according to the New Zealand census groupings of European, Māori, Pacific Peoples, Asian and Middle Eastern/Latin American/African (MELAA).

The New Zealand Index of Deprivation (NZDep13) is an area-based measure of socioeconomic deprivation derived from census data: it combines census data relating to income, family structure, education, employment, housing, home ownership and access to transport and communications. Socioeconomic deprivation is estimated by geographical location according to the NZDep13[[6]] and recorded in the NMDS. NZDep13 groups deprivation by deciles giving a score from 1 to 10 for each meshblock in New Zealand. An NZDep13 value of 1 represents the least deprived 10% of areas, whereas a value of 10 represents most deprived 10% of areas in New Zealand. For the purposes of this study, socioeconomic deprivation deciles were further grouped 1–3, 4–7 and 8–10 to respectively represent low, medium and high deprivation.

Data retrieved from the NMDS were then compared to regional demographic profiles, constructed from the 2013 census data. In the analysis, district health boards (DHBs) were grouped according to census DHB groupings under four corresponding regions: Northern, Midlands, Central and South Island.

Results

Six thousand and ninety-five cases from the 10-year period were retrieved from the NMDS: 1,563 cases were 20 years of age or older, and a further 56 cases had incomplete ethnicity or socioeconomic data and therefore did not meet the inclusion criteria. The final analysis included 4,476 cases. The median age was 5 years and there was a 50:50 split of cases by sex.

The distribution of cases by key ethnic groups were 71.4% European, 15.6% Māori and 4.9% Pacific Peoples. Thirty-four percent of procedures were in cases from the highest socioeconomic deprivation group, 41% of cases were from medium deprivation and 25% from the lowest deprivation group.

Table 1: Cases by ethnicity and socioeconomic deprivation, n (%).

*MELAA: Middle Eastern/Latin American/African.

Figure 1: Incidence of strabismus surgery per 1000 aged 0–19 years.

Over the 10-year period, the national incidence of strabismus surgery was 3.38 per 1,000 aged 0–19 years. The incidence of strabismus surgery in the South Island was 4.85 per 1,000 aged 0–19 years, more than double that of the Midlands region at 2.24 surgeries per 1,000. The inter-regional variation in procedural incidence may represent inter-regional disparities in access to surgical ophthalmic services.

There was a higher incidence of surgeries performed in the European ethnic group: 4.1 surgeries per 1,000 aged 0–19 years. When compared to the incidence of priority ethnic groups of Māori and Pacific Peoples, Europeans were respectively 1.5 times and 2.6 times as likely to receive surgical intervention for strabismus.

Figure 2: Incidence of strabismus surgery per 1,000 population aged 0–19 years.

*MELAA: Middle Eastern/Latin American/African.

Figure 3: Incidence of strabismus surgery per 1,000 youth population aged 0–19 years.

The most socioeconomically deprived group had the highest incidence of strabismus procedures over the 10-year period at 3.87 per 1,000. There appears to be a relationship between increasing incidence of strabismus surgery and higher socioeconomic deprivation. However, when stratifying the distribution of strabismus procedures by NZDep13 deciles (Figure 2), there is a notable under-representation of the most deprived 10% of the population when compared to the cases from deciles 8 and 9, which also make up the highest deprivation group.  

Figure 4: Distribution of cases of strabismus surgery by NZDep13.

Re-operation rate for surgical failures

The re-operation rate was calculated on a population level by dividing the number of secondary or subsequent procedures by the total number of primary procedures for each ethnic and socioeconomic deprivation cohorts. The national re-operation rate over the 10-year period was 11.8%.

Figure 5. Reoperation rate by ethnicity.

*MELAA: Middle Eastern/Latin American/African.

The European ethnic group had the highest rate of re-operation, with 13.0% receiving subsequent procedures following a primary procedure. The re-operation rate was lowest in the MELAA cohort at 2.6%. In comparison to the European cohort, Māori, Pacific Peoples and Asian ethnic groups were also less likely to receive subsequent procedures following the primary procedure, with respective re-operation rates of 8.4%, 9.4% and 9.2%.

Figure 6: Re-operation rate by deprivation group.

When compared to the re-operation rate in the most deprived socioeconomic group (9.2%), the low and medium deprivation groups were 1.46 and 1.43 times as likely to receive subsequent procedures following a primary operation.

Early intervention

The distribution of strabismus procedures by ethnicity were stratified further by age group for cases under the age of 19 years and then compared to the ethnic distribution of the youth population in the 2013 census. There was a higher representation of the European ethnic group across all three age categories. The degree of over-representation of the European ethnic group was found to be most pronounced in the 0–4-years age group, representing 75% of all procedures despite only comprising 59% of the population 4 years or younger. Minority ethnic groups were found to be under-represented across almost all age-groupings when compared to their census population distribution, with the Pacific Peoples ethnic group being most greatly under-represented. These findings could suggest earlier access to surgical services for Europeans when compared to all minority ethnic groups.

Table 2: Distribution of strabismus surgery by age group compared with 2013 census distribution of youth population.

*MELAA: Middle Eastern/Latin American/African.

Discussion

This is the first population-based study to explore the association between the incidence of strabismus surgery, ethnicity and socioeconomic status in New Zealand. The over-arching aim of this study was to explore access to surgical intervention in the New Zealand public health system, with a specific focus on the surgical specialty of ophthalmology.

There is a body of evidence that demonstrates reduced access to care and poorer health outcomes for Māori, Pacific Peoples and those who experience greater socioeconomic deprivation. Few studies have directly compared access to surgical intervention with socioeconomic deprivation and ethnicity in New Zealand. Key strengths of the study were its large population-base of data analysed over a 10-year time period with over 97% of data complete for ethnicity and deprivation index.

Limitations of the study include inconsistencies in the NMDS data entries and collection; the use of NZDep13 to estimate socioeconomic status; self-reported ethnicity; and the unavailability of data for procedures performed in the private surgical healthcare setting.

A number of different ICD-10 codes had been used to represent various sub-types as the diagnosis for strabismus and operation type in the retrieved NMDS. On entering the data, clinicians were also required to enter a written diagnosis and/or operation. Few inconsistencies were found in the entries of written diagnoses or operation types requiring clarification between stated written entries and ICD-10 codes. However, ICD-10 codes were reliable overall, with no significant inconsistencies in the ICD-10 coding for diagnosis or operation type.

The NMDS includes discharge information from all public and larger private hospitals, including day stay facilities for publicly funded events. There are potentially missing entries for publicly funded procedures performed at smaller private and day-stay surgeries in New Zealand. There is a nominal three-hour threshold of admission for reporting events to NMDS. If a case doesn’t meet this threshold for reporting, it may not be reported by same-day eye facilities. Outpatient data for publicly funded surgical procedures may also be reported to the National Non-Admitted Patient Collection (NNPAC), but the reported data do not include diagnostic or procedural data, which were therefore not explored further for the purposes of this study. A large majority of publicly funded ophthalmic procedures are performed in public hospitals in New Zealand. There are three private day surgeries in New Zealand contracted by their DHBs to routinely provide publicly funded ophthalmic surgeries. Two of these do not provide general anaesthetic and therefore are unsuitable for most eye procedures in children; the third facility was contacted and consistently reports their day-stay data to the NMDS. The authors consider the data retrieved from the NMDS to be a reasonable representation of the publicly funded procedures for strabismus over the 10-year period.

NZDep13 is an area-based measure of socioeconomic deprivation, as estimated by a participant’s address, and therefore might not be a true reflection of a person’s socioeconomic status. Ethnicity recorded in the NMDS and the 2013 New Zealand census are both self-reported. However, in the census an individual may be able to be represented under a number of different ethnic groups, and in the NMDS only one primary ethnicity can be recorded.

The data retrieved from the NMDS represented publicly funded strabismus procedures from 1 January 2005 to the 31 December 2014, but population profiles were constructed from a static point in time at the 2013 census. There are small variations between regional age, ethnic and socioeconomic deprivation distributions between the 2006, 2012 and 2018 censuses. The overall trends in population distribution remain consistent between the three most recent censuses and thus the 2013 census is considered to be a reasonable estimate of the population profile over the study period.

A larger proportion of surgeries were performed in the highest deprivation cohort when compared with the lowest deprivation cohort. This demonstrates a greater distribution of public health services for New Zealanders who are more socioeconomically deprived. It is worth noting that procedures performed privately are not required to be reported to the NMDS. Therefore, it is possible that there was bias towards private surgical care for lower deprivation groups, who can more likely afford private healthcare or health insurance. A greater uptake of surgical procedures in a private setting for more affluent New Zealanders could be another possible explanation for a lower incidence of strabismus procedures in the least socioeconomically deprived group. It is also possible that children from less-deprived homes receive non-operative care earlier and thus have a lower need for surgical intervention.

It is possible that the lower incidence of surgeries performed in Māori and Pacific Peoples is due to a lower population prevalence of strabismus and therefore a reduced need for surgery. A retrospective analysis of the electronic records of 846,477 patients with strabismus in the United States found a higher prevalence of strabismus in non-Hispanic Whites: 2.9% compared with 2.4% for African American and 2.0% for Native Hawaiian.[[7]] Interestingly, the prevalence of strabismus was found to be greatest in bi-racial/multi-racial ethnicity at 3.3%.[[7]] However, this large US study included patients of all ages, with different risk factors and aetiology of strabismus in adults than in children. The Sydney Paediatric Eye Disease Study also demonstrated ethnic variation in the prevalence of strabismus in children 6–72 months, with 4% in South Asian, 3.5% in European Caucasian and 3.3% in other ethnicities.[[8]] Although global studies have showed some ethnic variation in the prevalence of strabismus, there is no current evidence to support a higher prevalence of strabismus in New Zealand European’s than in Māori or Pacific Peoples. The disparity in the incidence of strabismus surgeries between ethnic groups in this study is too large to account for a difference in ethnic prevalence alone and more likely reflects a component of differing access to health services. Further research is required to determine the specific ethnic variation of common ophthalmic conditions such as strabismus in a New Zealand context.

The national re-operation rate was estimated on a population level to be 11.8%. This was further explored by ethnicity and socioeconomic deprivation. Re-operation rates in this study were similar to those in a large population-based study in the United States with estimated re-operation rates for strabismus procedures between 6.7% and 11.5% across all age groups.[[9]] We found that Māori, Pacific Peoples and those in the highest deprivation grouping had lower rates of re-operation. Although the MELAA ethnic group was found to have an incidence of strabismus surgery similar to the national incidence, the rate of re-operation in the cohort was less than quarter of the national rate at 2.7%. This can be interpreted either as lower operative failure rates in these cohorts, or as a reduced access to subsequent procedures following an unsuccessful primary procedure. Strabismus procedures can be performed for both cosmetic and functional corrections of misalignment. Factors affecting the likelihood of re-operation for strabismus include the type of strabismus, case complexity, patient age, the experience of the surgeon, the surgical method and patient satisfaction. While appreciating the complexity of factors influencing reoperation rate, we still expect reasonable randomisation of these effects between groups given the large size of our sample. It is possible that the observed variation in re-operation rates may reflect intrinsic cohort characteristics, with European and less socioeconomically deprived groups being more likely to seek further medical attention if a primary operation was perceived as unsatisfactory.

The findings in this study suggest earlier access to surgical intervention for a European cohort when compared to all other ethnic groups. There appears to be a trend across all minority ethnic groups of an increasing incidence of procedures with increasing age. The greatest incidence of surgeries by ethnicity peak at 0–4 years for European, 5–9 years for Māori and 10–19 years for Pacific Peoples, Asian and MELAA ethnic groups.

For the purposes of this study, having received surgical intervention for strabismus was used as an estimate of a population’s access to healthcare. To further assess access to ophthalmic services in the New Zealand public health system, the number of referrals made to ophthalmology services and alternative treatment options offered by ethnicity and socioeconomic status could also be explored. By using an event of surgical intervention as an end point, this study did not directly compare health outcomes. Further research is required to explore long-term visual outcomes between these population groups and assess the clinical significance of these reported variations in surgical intervention for strabismus.

Conclusion

This research shows that disproportionately fewer strabismus surgeries are performed in Māori, Pacific Peoples and those in the lowest deprivation group.

Our findings also suggest that access to subsequent secondary procedures in minority ethnic and higher deprivation groups may also be reduced. Further research is needed to directly compare health outcomes between these higher-needs and lower-needs groups.

Appendix

Appendix Table 1: ICD-10 strabismus sub-type codes. View Appendix Table 1.

Appendix Table 2: ICD-10 strabismus procedure codes. View Appendix Table 2.

Summary

Abstract

Aim

This study aimed to identify the relationship between the incidence of strabismus surgery, ethnicity and socioeconomic deprivation in the New Zealand public health system. Secondary outcomes explored the association between re-operation rate for surgical failures, ethnicity and socioeconomic deprivation.

Method

Cases receiving operative management for strabismus were retrieved from the National Minimum Dataset. The incidence of surgery was correlated to patient demographics by ethnicity and socioeconomic deprivation and compared to population profiles for 0–19-year-olds constructed from the 2013 census.

Results

There were 4,476 strabismus surgeries recorded over a 10 year period from 1 January 2005 to 31 December 2014 included in the study. There was a lower incidence of strabismus surgery performed in Māori, Pacific Peoples and the least socioeconomically deprived cohort. There were significant inter-regional variations in the incidence of strabismus surgery. The European ethnic group was 1.4 times as likely to receive subsequent procedures following a primary procedure than either Māori or Pacific Peoples.

Conclusion

Disproportionately fewer strabismus surgeries were performed in Māori, Pacific Peoples and New Zealanders from the lowest deprived group in the New Zealand Public Health System. Minority ethnic groups are less likely to receive secondary operations following a primary procedure when compared to a European cohort. Further research is needed to directly compare health outcomes between these high-needs and lower-needs groups.

Author Information

Cheefong Chong: Consultant Ophthalmologist Tauranga Eye Specialists; Paediatric Ophthalmologist, Waikato District Health Board; Consultant Ophthalmologist, Auckland District Health Board; Honorary Academic, Faculty of Medicine and Health Sciences, University of Auckland. Alexandra Lawrence: PGY1, Bay of Plenty DHB, Tauranga; Tauranga Eye Specialists, Waikato District Health Board, Faculty of Medicine and Health Sciences, University of Auckland; Waikato DHB and FMHS UOA. Dan Allbon: Ophthalmology Registrar, Southern DHB, Dunedin; Tauranga Eye Specialists, Waikato District Health Board, Faculty of Medicine and Health Sciences, University of Auckland; Waikato DHB and FMHS UOA.

Acknowledgements

Correspondence

Alexandra Lawrence, Park St Eye Clinic and Day Stay Theatre, 25 Park St, Tauranga, 3110, + 64 27 511 0355

Correspondence Email

alexandralawrence4@gmail.com

Competing Interests

Nil.

1) Torp-Pedersen T, Boyd HA, Skotte L, Haargaard B, Wohlfahrt J, Holmes JM, Melbye M. Strabismus Incidence in a Danish Population-Based Cohort of Children. JAMA Ophthalmol. 2017 Oct 1;135(10):1047-53. doi: 10.1001/jamaophthalmol.2017.3158. PMID: 28859196; PMCID: PMC5710488.

2) Hashemi H, Pakzad R, Heydarian S, Yekta A, Aghamirsalim M, Shokrollahzadeh F, Khoshhal F, Pakbin M, Ramin S, Khabazkhoob M. Global and regional prevalence of strabismus: a comprehensive systematic review and meta-analysis. Strabismus. 2019 Jun;27(2):54-65. doi: 10.1080/09273972.2019.1604773. Epub 2019 Apr 23. PMID: 31012389.

3) Williams C, Northstone K, Howard M, Harvey I, Harrad RA, Sparrow JM. Prevalence and risk factors for common vision problems in children: data from the ALSPAC study. Br J Ophthalmol. 2008 Jul;92(7):959-64. doi: 10.1136/bjo.2007.134700. Epub 2008 May 14. PMID: 18480306.Ono H, Mapp AP. A restatement and modification of Wells-Hering's laws of visual direction. Perception. 1995;24(2):237-52. doi: 10.1068/p240237. PMID: 7617427.

4) Angela N. Buffenn, The impact of strabismus on psychosocial heath and quality of life: a systematic review, Survey of Ophthalmology. Available from: https://doi.org/10.1016/j.survophthal.2021.03.005

5) Ministry of Health [Internet]. Data and Statistics National Minimum Dataset (Hospital Events) (NMDS). 2010. Available from: http://www.moh.govt.nz/moh.nsf/indexmh/dataandstatistics-collections-nmds

6) Atkinson J, Salmond C, Crampton P. 2014. NZDep2013 Index of Deprivation. Dunedin: University of Otago.

7) Repka M, Lum F, Burugapolli B. Strabismus and strabismus surgery in the United States: analysis from the IRIS registry. Journal of American Association for Pediatric Ophthalmology and Strabismus. 2018;22(4):e6.

8) S. Sharbini. Prevalence Of Strabismus & Associated Risk Factors: The Sydney Childhood Eye Studies. The University of Sydney. 2015 Oct.

9) Repka MX, Lum F, Burugapalli B. Strabismus, Strabismus Surgery, and Reoperation Rate in the United States: Analysis from the IRIS Registry. Ophthalmology. 2018 Oct;125(10):1646-53. doi: 10.1016/j.ophtha.2018.04.024. Epub 2018 May 18. PMID: 29779683.

Contact diana@nzma.org.nz
for the PDF of this article

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