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| Journal of the New Zealand Medical Association,
27-August-2010, Vol 123 No 1321
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Bilateral superior canal dehiscence
syndrome
Jeremy Hornibrook, David O’Neill-Kerr, Latham Berry,
Grant Carroll
Superior canal dehiscence (SCD) syndrome is a newly
recognised condition1–4 where dehiscence
of bone over the superior semicircular canal can lead to unusual auditory and
vestibular symptoms and signs.
In the 1880s, Ewald demonstrated that pressure applied to
surgically fenestrated canals in pigeons could cause a nystagmus in the plane of
the stimulated canal. This has long been recognised as a possible complication
of cholesteotoma eroding bone over the horizontal canal accounting for
horizontal nystagmus induced by applying alternating pressure to the external
ear canal, called Hennebert’s sign.
In 1929, Tullio5 showed
that in dogs with surgically fenestrated superior canals loud sounds could
effect a nystagmus in the plane of the canal. Also, in rabbits and pigeons with
intact labyrinths loud sounds could induce vestibular responses with diconjugate
rotation of the eyeballs, tilting of the head and leg flexion. These vestibular
responses are now called the Tullio phenomenon.
More recent electrophysiological studies in humans showed
that a myogenic response to a loud click stimulus occurs in the sternomastoid
muscle.6 The response—named the
vestibular-evoked myogenic potential (VEMP)—is generated by the saccule
and is non-hearing dependent. Ears affected with the Tullio phenomenon have an
abnormally low VEMP threshold.7
Case reportA 58-year-old female presented complaining of intermittent
dizziness, oscillopsia and aural symptoms. For 2 years there had been pulsatile
tinnitus in both ears, particularly the right ear. More recently stooping,
nose-blowing or a sneeze would elicit brief vertigo and oscillopsia. Also she
had noticed that when walking on firm ground she could hear her footsteps:
“my footsteps just echo inside my body”.
On examination ear drums, hearing and acoustic reflexes were
normal. There was no spontaneous nystagmus. Vestibulo-ocular reflexes were
normal. When the patient performed a valsalva manoeuvre a brief down beat and
slightly right-torsional nystgmus occurred. This was reproduced and recorded in
infrared light by a right eye camera (Figure 1 – mpeg video-clip
accessible at http://www.nzma.org.nz/journal/123-1321/4307/video.mpg).
Further provocative tests for Tullio phenomena were done.
However, straining against a closed glottis, alternating pressure in the
external ear canal, or a 0.5kHz tone at 90dB and a 1kHz tone at 120dB did not
induce symptoms or nystagmus.
CT scans with 0.625m slices in the coronal plane showed a
bony dehiscence of both superior semicircular canals (Figure 2). MRI scans imply
an abnormal absence of bone between the membranous portion of the superior
canals and the overlying dura (dark space), in comparison with a subject with a
normal bony covering (Figure 3).
Cervical VEMPS were measured with surface electrodes over
the sternum and each flexed sternomastoid muscle, using 0.1 msec clicks from a
headphone in 5 dB descending steps. On the right the threshold for a clear
N13/P23 wave-form was 73dB, and 83 dB on the left, both significantly below the
normal (96±5 dB)8 (Figure 4).
Figure 4. Cervical VEMPS
 In this patient there is both CT scanning and VEMPS evidence
of bilateral SCD syndrome.
DiscussionBony dehiscence over the superior canal can result in
vestibular or auditory symptoms and signs, or both. The reasons for the
differences are not known, but the dehiscence creates a so-called “mobile
third window” effect. The vestibular symptoms are vertigo and oscillopsia.
A valsalva manoeuvre with closed nostrils, positive pressure
in the ear canal cause an ampullofugal deflection (excitatory) of the superior
canal cupula. In contrast—a valsalva manoeuvre against a closed
glottis—bilateral jugular venous compression and a negative pressure in
the ear canal can cause an ampullopetal (inhibitory) cupula deflection. The
auditory symptoms can be autophony (hearing one’s own voice loudly in the
ear), hypersensitivity for bone-conducted sounds, a blocked ear or pulsatile
tinnitus.
In a microscopic study of 1000 temporal bones sectioned
vertically in the plane of the superior canal, 0.5% had a complete dehiscence
and 1.4% had bone so thin it could appear dehiscent even on high resolution CT
scanning.9 Conventional CT scans are displayed
in the axial and coronal planes. False positive dehisences can be reduced with
0.5-mm-collimated helical scans with reformation of the images in the plane of
the superior canal.10,11
The onset of SCD symptoms is typically in adulthood when,
presumably, abnormally thin bone over the canal is disrupted by trauma or eroded
by overlying structures. In most patients (as in this report) the symptoms are
mild, and are prevented by avoiding the stimuli that cause them. In rare cases
with disabling symptoms (sound-induced vertigo, pulsatile oscillopsia) surgical
treatment is justified, either by closure of the defect with fascia and bone
cement12 or by superior canal
plugging.13
In summary, SCD syndrome can cause unusual auditory and
vestibular symptoms. Elicitation of nystagmus by a valsalva manoeuvre, sound
stimulation or external canal pressure and a CT scan implying dehiscence are
strongly suggestive, but a reduced VEMP threshold is required for certain
diagnosis.
Author information: Jeremy Hornibrook,
Otolaryngologist–Head and Neck Surgeon, Christchurch Hospital; David
O’Neill-Kerr and Latham Berry, Radiologists, Christchurch Hospital and
Christchurch Radiology Group; Grant Carroll, Clinical Neurophysiologist,
Department of Neurology, Christchurch Hospital, Christchurch
Correspondence: Jeremy Hornibrook,
Department of Otolaryngology-Head and Neck Surgery and Audiology, 2 Riccarton
Avenue, Christchurch 8011, New Zealand. Fax: +64 (0)3 3642073; email: jeremy@jhornibrook.com
References:
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