Bilateral superior semicircular canal dehiscence: bilateral conductive hearing loss with subtle vestibular symptoms

Abstract

Superior semicircular canal dehiscence is caused by a bone defect on the roof of the superior semicircular canal. The estimated prevalence when unilateral varies between 0.4% and 0.7% and is still unknown when bilateral. Patients may present with audiologic and vestibular symptoms that may vary from asymptomatic to disabling. We report a case of a 72-year-old Caucasian woman presented to otolaryngology department reporting imbalance, bilateral pulsatile tinnitus, hypoacusis while being very sensitive to certain sounds. Physical examination was unremarkable, except for the Rinne test that was negative in both sides. The patient underwent an audiometry revealing a mild bilateral conductive hearing loss. A temporal bone CT scan was performed which evidenced bilateral superior semicircular canal dehiscence. Cervical vestibular evoked myogenic potentials and electrocochleography confirmed diagnosis. Although rare, superior semicircular canal dehiscence shall be considered in conductive hearing loss with vestibular symptoms.

Background

Superior semicircular canal dehiscence (SSCD) is caused by a bone defect of the roof of the superior semicircular canal that creates a ‘third window’.¹ All the semicircular canals can be dehiscent, but the superior is the most common.²

This condition was first described in 1998 by Minor et al³ and was defined as a defect on the bony roof of the superior semicircular canal exposing the membranous vestibular labyrinth on the medial cranial fossa.

It is known for a long time that fenestration of the labirinth may cause symptoms. Early experiments in pidgeons in 1880s by Ewald proved that pressure applied to surgically fenestrated canals could cause nystagmus on the same plane of the fenestrated canal (Hennebert sign).⁴ In 1929, Tullio, in his experiments in dogs with surgically fenestrated superior canals, showed that loud sounds could induced nystagmus on the plane of the fenestrated canal (Tullio phenomenon).⁵

The incidence of SSCD is not known. Studies in cadaveric temporal bones have found dehiscence of the bone overlying the superior canal in 0.4%–0.7% of temporal bone specimens.² On CT scans studies, the overall incidence vary between 2% and 13%.²

Although pathogenesis is uncertain, prior studies have implicated possible genetic, embryologic, metabolic and traumatic etiologies,⁶ and suggest a higher incidence in older patients.^{2 7}

Patients with SSCD may present vestibular and auditory symptoms. The symptoms are caused by the third mobile window of the inner ear. It is theorised that lowering the cochlear impedance experienced by the stapes footplate causes the shunting of fluid flow mostly on the dehiscent canal, and also reduces the pressure gradient across the cochlear partition.¹

The vestibular symptoms usually are provoked by intense sounds or pressure changes. Vertigo or oscillopsia caused by high-intensity sounds are present in 90% of patients.⁸ In patients with the Tullio phenomenon, the Hennebert sign is also often present.⁹

The auditory symptoms are also common. Fifty-two per cent of patients report hyperacusis to their own heart beating, eye movements or their own footsteps. This is caused by a supranormal bone conduction (BC). At the same time, it is frequent that the patientsreport hearing problems because some of the acoustic energy is lost for the third window.⁸ Autophony is present in 60% and aural fullness and tinnitus are also frequent.¹

Diagnosis is based on clinical manifestations, but at the same time, some complementary diagnostic examinations should be realised. High-resolution computerised tomography (HRCT) of the temporal bone is required, preferentially performed with <0.5 mm slices proving dehiscence of the superior semicircular canal. The audiometric findings may include air-bone gap, a negative BC threshold on pure tone audiometry.¹⁰

Consistent findings on vestibular evoked myogenic potentials (VEMPs) may help to confirm the diagnosis with low Cervical Vestibular-Evoked Myogenic Potential (cVEMPs) threshold. More recently, abnormal electrocochleographic findings have been documented on SSCD.¹⁰

Case presentation

A 72-year-old woman presented to our otolaryngology department referred by her family doctor, reporting imbalance, bilateral pulsatile tinnitus, hypoacusis while being very sensitive to certain sounds. She noticed when walking on firm ground, she could hear her own footsteps. These symptoms had been reported for years, but recently got worse than before. The patient did not report vertigo, but referred some difficulties on stabilising the gaze with blurred images when moving her head or when driving.

Head and neck examination and otoscopy examination were unremarkable. Tullio phenomenon and Hennebert could not be elicited. On neurotologic examination no nystagmus was found and all the vestibular signs were normal. Rinne test was negative bilaterally and Weber test was centred.

Investigations

Pure tone audiogram revealed bilateral mild to moderate conductive hearing loss, worse at low frequencies with ascending curves (figure 1).

Figure 1.

Pure tone audiogram: bilateral mild to moderate conductive hearing loss, worse at low frequencies with ascending curves.

Word-recognition score showed a speech-reception threshold of 35 dB on the right side and 30 dB on the left side (figure 2).

Figure 2.

Word-recognition score showed a speech-reception threshold of 35 dB on the right side and 30 dB on the left side.

Tympanogram showed type A curves with positive acoustic reflexes bilaterally (figure 3).

Figure 3.

Impedanciometry: type A tympanogram with positive acoustic reflexes bilaterally.

HRCT scan was obtained and revealed bilateral superior semicircular canal dehiscence (figures 4 and 5).

Figure 4.

High-resolution temporal bone CT scan (right side): superior semicircular canal dehiscence.

Figure 5.

High-resolution temporal bone CT scan (left side): superior semicircular canal dehiscence.

The patient also underwent cVEMPS which confirmed high amplitudes with lower threshold on the left side. The ratio between both ears were 0.30 (figure 6).

Figure 6.

cVEMPS: high amplitudes with lower threshold on the left side.

Electrocochleography showed abnormal summating potential/action potential (SP/AP) ratios in both ears (figure 7).

Figure 7.

Electrocochleography: abnormally high SP/AP ratios bilaterally. SP/AP, summating potential/action potential.

The diagnosis of a bilateral SSCD was defined.

Differential diagnosis

As a result of a conduction hearing loss and normal otoscopy, some patients with primarily auditory symptoms have historically been misdiagnosed as having otosclerosis.⁹ There are some differences that can be readily identified. The hyperacusis does not occur in otosclerosis, and the stapedial reflexes are normal on SSCD, and absent in otosclerosis.

There are some other conditions that may cause conductive hearing loss, such as ossicular discontinuity, tympanosclerosis, malleus head fixation, Paget’s disease and osteogenesis imperfecta. Others may cause pseudoconductive hearing loss, such as enlarged vestibular aqueduct syndrome.¹¹ All these conditions were ruled out after HRCT of the temporal bone, audiometry and tympanometry.

Treatment

To assess the vestibular impact of the disease, the patient underwent Dizziness Handicap Inventory (DHI) with a result of 10 points (out of 100). Regarding the auditory symptoms, the patient explained that were not disabling, and she was able to control them well.

We recommended her to consider the use of hearing aids, to avoid noisy environments and to avoid driving. After discussing a possible neurosurgical correction with the patient, both parts considered not to be currently beneficial, due to the mild symptoms and high risk involved.

Outcome and follow-up

The patient was followed up during 6 months and reported to be well adapted to the symptoms. She refused to use hearing aids because aesthetical and financial reasons. She also stopped driving after the diagnosis.

Discussion

Although the estimated prevalence of unilateral SSCD is between 0.5% and 0.7% in cadaveric studies, only an unknown number of patients are symptomatic. It is rarest when bilateral, but the prevalence is still unknown.

Cremer and colleagues stated that superior semicircular canal function is only impaired by dehiscence superior to 5 mm when compression of the membranous canal by dura impedes endolymphatic flow.¹²

Most of the symptomatic patients with SSCD search for medical treatment because of vestibular symptoms, such as vertigo, unsteadiness or oscillopsia.⁸ In some cases, these symptoms occur after sudden pressure changes on the middle ear or intracranially. Loud sounds may also induce the same symptoms.

Some patients do not have vestibular symptoms and visit an otolaryngologist because of auditory symptoms ranging from hyperacusis to conductive hearing loss or even to autophony or tinnitus.^{13 14}

The diagnostic is rarely obvious on the first consultation. The anamnesis and the physical examination are of superlative importance, but some exams are also key to define the diagnosis such as HRCT, audiometry, stapes reflexes or VEMPs.

In some patients with only conductive hearing loss, otosclerosis may be wrongly diagnosed, submitting the patient to unnecessary surgery. On SSCD the conductive hearing loss is characterised by a low frequency air-bone gap sometimes with a negative BC. The stapes reflexes are also present on SCDD and absent on otosclerosis.

For the diagnosis of SSCD to be considered, HRCT must show the absence of bone over the superior canal.⁹ If the superior canal appears surrounded with bone on HRCT, the diagnosis of SSCD is excluded; however the appearance of a dehiscence does not rule out thin bone layer covering the superior canal.⁹ In a study of Belden et al, thin sections on CT scans helped to reduce the false positive. The positive predictive value of an apparent dehiscence improved from 50% with the 1.0 mm collimated CT to 93% with 0.5 mm collimated CT with reformation in the plane of the superior semicircular canal.¹⁵ ‘The HRCT specificity and positive predictive value are improved with narrowing the collimation width and with reformatting the images into the plane of the uperior semicircular canal (SSC). In some cases, the bone is so thin that a determination of whether or not the roof of the SSC is intact will not be possible even with the 0.5 mm collimation.’¹⁵ cVEMPS are an indirect measure of otolith function, measured through sound and vibration responses recorded in the ipsilateral sternocleidomastoid muscle. Usually, on the SSCD, there is a reduced threshold and increased amplitude of vestibular evoked myogenic potentials. In our case, only the left side had diagnostic criteria for SCDD, with high amplitudes potentials with lower threshold. Niesten et al in 2012 described that in patients with bilateral SSCD, the cVEMPS threshold in the less symptomatic SSCD ear showed similar values to the unaffected ear in patients with unilateral SSCD.¹

Park and colleagues in 2015¹⁰ evaluated the electrocochleographic findings of patients with SSCD and concluded that mean SP/AP ratio of SSCD ears was significantly higher than that of unaffected ears and SP were significantly elevated in affected ears, whereas AP were similar for affected and unaffected ears. SP/AP ratio showed a sensitivity of 92,3% and a specificity of 94%.¹⁰ These findings support our diagnosis of bilateral SSCD.

The decision about the treatment to adopt on SSCD is often as difficult as the diagnosis. The treatment should be tailored for each patient and depends on the severity of the symptoms. Patients with mild symptoms are advised to lifestyle changes and to avoid situations that exacerbate their symptoms, such as loud noises.⁹ Other patients are disabled by their symptoms and surgery is the only viable option for them to have an acceptable quality of life. There is also a subset of patients where the decision of surgery versus conservative treatment is less clear. It may be helpful to assess the vestibular symptom severity through a validated scale such as the DHI, which assess the severity of vestibular symptoms on a scale from 0 to 100. However, patients with very low DHI values may be candidates to surgery as well if the auditory symptoms are severe. Symptoms as severe autophony or abnormal sound of own voice may be disabling and surgery may be the only relief. Conductive hearing loss is a common symptom, and sometimes may be the only symptom.¹⁶ In these cases, patients should be first encouraged to consider non-surgical options, such as hearing aid.⁹

Learning points.

• Bilateral superior semicircular canal dehiscence is a very rare disease caused by a bone defect of the roof of both superior semicircular canal.

• Patients may present with audiologic and vestibular symptoms that may vary from asymptomatic to disabling.

• For the diagnosis, anamnesis and physical examination must be supplemented with high-resolution computerised tomography, audiometry, stapes reflexes and cVEMPS.

• Treatment should be adapted to the severity of patient’s symptoms ranging from plugging the superior semicircular canal to only lifestyle changes.