Etiology

Superior Semicircular Canal Dehiscence (SSCD) is an exceedinglyrare vestibular condition where the roof of the superior canal  has become damaged by head trauma to the temporal bone or is thin from birth (Minor, 2005). This loss of bone density leads to interesting auditory and vestibular symptoms. These symptoms can include a mild to moderate conductive hearing loss, vertigo with loud sound or pressure changes, abnormal increased sensitivity to bone conducted sound, auto-phony, and general disequilibrium (Minor 2005; Minor, Cremer, Carey, Della, Streubel, Weg, 2001). These symptoms are attributed to the change in inner ear fluid dynamics as a result of the dehiscence that causes sound and pressure energy to be siphoned directly to the
vestibular system through a third opening within the inner ear (Minor, 2005). This third opening causes the patient`s vestibular systems to become much more sensitive to sound and pressure changes than they would be with the two normal principal energy reception sites, the oval and round windows of the cochlea. This spread of sound and pressure energy away from the cochlea to the vestibular system activates the superior semicircular canal with the dehiscence leading to the unpleasant perception of spinning or rolling (Minor, 2005; Minor 2001).

Frequency of Disease

Carey, Minor, and Nager, (2000) found that between 0.5 and 1.5% of cadaver temporal bone specimens had complete loss or thinning (< 0.1mm) of the bone overlaying the superior canal. Prevalence or incidence rates other than from this study are currently lacking in otologic research.

Diagnosis

The primary means of diagnosis is through case history and CT imaging studies. In Minor, (2005) the most common complaints for those with confirmed SSCD included vertigo caused by loud sounds or vertigo caused by external pressure changes. The symptoms of auto-phony or the ability to hear one’s internal sounds, such as eyes moving, was a common complaint at around 60% (Minor, 2005). Some patients had a mild to moderate conductive hearing loss which was inconsistent with normal tympanometry and acoustic reflexes. This audiometric profile often gets confused with true middle ear pathology or perilymph fistula leading to unnecessary surgical investigation or repair (Minor, 2005). The presence of any of these symptoms as well as
confirmed thinning or absence of the temporal bone above the superior canal in a fine grained CT scan makes a diagnosis of SSCD possible (Minor et al, 2001).

The most sensitive audiological test for SSCD is the vestibular evoked myogenic potentials (VEMP) assessment. Patients with this disorder have much higher VEMP amplitudes with lower P13-N23VEMP thresholds. The average VEMP threshold for SSCD patients is around 80dB nHL while control normal patients have thresholds that are much higher at 100 dB nHL (Minor, 2005). More energy from sound is directed to the ear with SSCD because the dehiscence acts as an energy exit point of the vestibular system leading to higher activation at lower level sound inputs (Minor et al, 2001).

Treatment

Surgical management is available but carries tremendous risk to a patient`s hearing and may not correct the sound or pressure induced vertigo that patients find the most difficult to live with. Minor, (2005) investigated eleven patients who went for
temporal bone resurfacing or superior canal plugging procedures. The temporal bone resurfacing requires the surgeon to graft bone with fascia from the cranial fossa over the dehiscence. The canal plugging procedure is the actual blockade of the superior canal so that it no longer reacts to sound, external pressure, or vestibular changes. Overall, eight had long term cessation of SSCD symptoms which was considered a great result. However, four patients had temporary resolution of the SSCD symptoms that lasted one to four months followed by the same vertigo and disequilibrium. Two patients suffered a moderate to severe hearing loss in the surgically repaired ear one to two months post-operation. The recommendation to surgically repair SSCD should be made only in cases where the patient is suffering from debilitating vertigo or other SCCD symptoms.

Another alternative for those who suffer primarilyfrom pressure induced vertigo is to use a pressure equalization tube to reduce
the inner ear`s response to common internal and external pressure changes such as coughing, sneezing, and during physical exercise. This is a safe and quick surgical procedure that should be considered prior to more invasive surgical management (Minor, 2005).

Prognosis

Very few patients withSSCD should get to the point in the disease process where surgical procedures are the only viable option. Successful surgical management as mentioned above can be effective at alleviating the symptoms of SSCD for most patients. However, the risk of hearing loss and symptom exacerbation because of bone graft or canal plug migration is high. A person diagnosed with SSCD should be counselled on adapting their lives to avoid the events that cause the negative aspects of the  disorder. They should be monitored annually by a collaborative team of professionals including an audiologist, otolaryngologist, occupational therapists and physical therapist. If the symptoms are continuously or progressively debilitating surgical referral should be made. Post-op recovery and rehabilitation would again require a team approach and close monitoring to ensure positive outcomes from the procedure (Minor, 2005).

References

Carey, Minor, and Nager. (2000) Dehisence or thinning of bone overlying the superior semicircular canal in a temporal bone survey. Arch. Otolaryngol. Head Neck Surg. 126,137 – 147

Minor, LB., Cremer, PD., Carey, JP., Della, CC., Streubel, SO., Weg, N. (2001). Symptoms and signs in superior canal dehiscence syndrome. Ann N Y Acad Sci USA;942:259–73.

Minor LB. (2005). Clinical manifestations of superior semicircular canal dehiscence.Laryngoscope;115:1717–27.