Vestibular symptoms to sound should arise suspicion for semicircular canal dehiscence (SCD) syndrome, especially when Tullio phenomenon or Hennebert’s sign are present. But diverse clinical presentation makes this diagnosis not straightforward. Repeating episodes of positional vertigo in a patient with posterior semicircular canal dehiscence is herein presented case. Inconsistent history of patients with SCD makes this diagnosis challenging. BPPV has been described in various inner ear disorders but not yet in the SCD syndrome.
Patients with SCD have a diverse history. The symptoms mostly suggestive for dehiscence are vestibular symptoms to sound or pressure changes, but chronic disequilibrium and audiological symptoms of autophony, aural fullness, hyperacusis, hearing loss and tinnitus are very frequently encountered also [1-3]. Some patients can hear their own eye movements and such hyperacusis to sounds transmitted through bone are called conductive hyperacusis . It’s still unclear why in some SCD patients no vestibular symptoms are present, but only auditory . Characteristic audiogram for the SCD is conductive hearing loss, mostly pronounced in low tones with decreased bone conduction threshold [2,4]. But other forms of audiograms has been described also . Air-bone gap with normal tympanograms and acoustic reflex present should raise suspicion for the third window lesion . VEMPs are very sensitive detector of the third window lesion and show decreased threshold on the side of the lesion . CT scan studies showed that the dehiscence of the posterior canal is almost as frequent as the superior one .
The symptoms of posterior canal dehiscence seem to be almost the same as superior canal dehiscence . Etiology of the superior canal dehiscence is still under debate, but it’s believed that developmental failure of outer bone layers over superior canal, makes it a predilection place for dehiscence . A posterior canal dehiscence mainly results from jugular bulb erosion, while a lateral canal dehiscence is usually from cholesteatoma erosion . SCD represents a third window on a bony labyrinth. It makes sound pressure from the oval window to dissipate and leak through dehiscence resulting in hearing loss. At the same time third window increases pressure difference between the two sides of cochlear partition, making cochlea more sensitive to bone conduction . In the normal ear only strong sound stimuli can stimulate sensory epithelium of the maculae which are the bases of VEMP testing. But in the patient with the dehiscence of the semicircular canal, acoustic flow to the place of small impedance (third window) deflects also vestibular hair cells in the affected canal and makes cupula sensitive to sound stimulus . Recorded eye movements are in a plane of dehiscent canal .
A 49 year old man presented with a history of positional vertigo. Short vertigo spells were mostly pronounced in the morning when getting up and were repeating with bending and other provocative positions characteristic for BPPV. Usually these episodes of positional vertigo were lasting for a week and repeated two times per year. It has started few years ago and the last one was a month ago. Also, he has tinnitus (continuous high pitched) in the right ear for 5-6 years and during that period he noticed slowly progressive diminishing of hearing in the right ear. He denied other audiological symptoms like aural fullness, autophony and pressure or sound induced vertigo.
Neurotological exam was normal and Dix-Hallpike test was negative. VHIT demonstrated good VOR gain in all six canals. Audiometry on the right ear showed mild to moderate down sloping curve with mixed, predominantly conductive type of
hearing loss with bone conduction thresholds in low tones
below normal (Figure 1). Since otoscopy and tympanometry
didn’t show any middle ear pathology (Figure 2), cVEMPs were
performed and showed normal amplitudes and latencies at
both sides (Figure 3). It was a sign to ask for a CT scan which
confirmed third window lesion - the posterior canal dehiscence
due to high jugular bulb (Figure 4).
The history of the presented patient corresponds to the
recurrent BPPV. Additionally his audiological symptoms of
continuous high pitched tinnitus with mild to moderate hearing
loss didn’t arouse suspicion for a SCD. But conductive hearing loss
without obvious middle ear pathology especially with decreased
bone conduction thresholds in low tones was indication to check
for the VEMPs. Presence of vestibulo-cervical reflex pointed that
the air-bone gap is of inner ear origin. Finally, CT scan confirmed
dehiscence of the posterior semicircular due to the high jugular
bulb. Discovered dehiscence explains the right sided hearing
loss and tinnitus, but recurrent episodes of positional vertigo
haven’t been described in SCD syndrome yet. As there is no proof
for canalolithiasis of the posterior canal it can be only assumed
according to the characteristic history that he had episodes of
BPPV. The explanation might be in the pressure of the jugular bulb threw dehiscent posterior canal and influence on fluid
dynamics in it.
Another would be that acoustic flow through vestibular
part makes more frequent detachment of otoconia. The second
explanation is less probable in this patient as he hasn’t have the
vestibular symptoms to sounds or pressure stimuli. VHIT showed normal VOR gains in
all six canals including the dehiscente one. The reduced VOR
gain could be expected in dehiscence bigger than 5mm .
Dehiscence can be successfully cured surgically, but it’s usually
not recommended in cases with mild symptoms as is the case of
the presented patient . The progression of the high jugular
bulb is unpredictable and it’s advised to monitor this patient.
The characteristic signs of SCD as Tullio phenomenon
and Henneberth’s sign are not seen in every patient with SCD
syndrome. Also, inconsistent history of the patients with
SCD, makes this diagnosis challenging and probably often
undiscovered . Whenever conductive hearing loss is without
obvious signs of middle ear pathology it should be checked if the
air-bone gap is due to inner ear pathology as is a case of a SCD.
VEMPs are very sensitive for the presence of the third window
lesion . It has been suggested that oVEMPs are even more
sensitive for SCD than cVEMPs [11,12]. Dehiscence is mostly
prevalent on the superior canal but it’s not so infrequent finding
in the posterior one also . Usually it’s due to high jugular bulb
[13,14]. The unusual cases of multi dehiscent bony labyrinth has
been described also [15,16]. Recurrent BPPV has been seen in
various inner ear disorders but hasn’t been described in patients
with SCD syndrome yet, except the unusual case of a patient who
developed posterior canal BPPV after canal plugging [16,17].