Nager (4) in 1947 first described a bony dehiscence of the superior semicircular canal in the middle cranial fossa from an individual with no accompanying clinical history. He attributed the defect to “senile osteoporosis” of the temporal bone. A recent systematic survey of temporal bones by Carey et al. (5) found dehiscence of the bone overlying the superior semicircular canal in approximately 0.5% of temporal bones. Bilateral dehiscence was found in approximately half of the cases identified (5). Their findings also revealed that a thin covering of bone (0.1 ml or less) was present in 1.4% of all cases studied (5). The clinical histories of these patients did not reflect the typical symptoms associated with SSCD. It is likely that many if not most individuals who have bony dehiscence of the superior semicircular canal remain asymptomatic. This is further supported by incidental observation of superior canal dehiscence in temporal bone CT studies of nonvertiginous patients (H. Curtin, personal communication).

Factors that lead to the development of symptomatic bony dehiscence of the superior canal have yet to be determined. Our experience, and that of others, has been the occurrence of superior canal dehiscence in extremely well-pneumatized temporal bones with thin or defective tegmental plates, suggesting abnormal temporal bone development as one contributing factor. Some patients associate the onset of symptoms with minor head trauma, which has been suggested as a contributing factor in predisposed individuals. The third potential process leading to dehiscence is pressure-induced resorption of the tegmen tympani.

The mechanism of the evoked eye movements and vertigo can be explained by the presence of a third inner ear mobile window at the site of the superior semicircular canal dehiscence. Pressure changes transmitted through the third mobile window create motion of the endolymph and deflection of the superior canal cupula. Stimuli that produce inward displacement of the dehiscent membranous labyrinth result in ampullopetal deflection of the cupula. These inhibitory stimuli produce a decrease in the basal firing rate and include Valsalva maneuver, jugular venous compression, and negative middle ear pressure. Stimuli that cause outward deflection of the dehiscent membranous labyrinth produce ampullofugal deflection. These excitatory stimuli include increased middle ear pressure from Valsalva maneuver against closed nostrils, positive pressure on pneumotoscopy, and tragal compression.

Patients with SSCD syndrome may present with a variety of complaints, including hyperacusis, oscillopsia, auditory sensation with eye movement, and auditory- or pressure-evoked visual field shifts, vertigo, and disequilibrium. The auditory-and
pressure-evoked symptoms are most characteristic of SSCD, but chronic disequilibrium is also common and often the most debilitating symptom. A combined review of the clinical data from the Massachusetts Eye and Ear Infirmary and the one published series indicates 75% of patients complained of chronic disequilibrium; furthermore, this was the most common reason patients sought medical evaluation (6). Other common complaints included sound-evoked symptoms in 95%, pressure-evoked symptoms in 55%, and gaze-evoked symptoms in 20% of patients.

In most patients, careful examination reveals a vertical torsional nystagmus elicited in response to high level auditory stimuli, changes in middle ear pressure, or increased intracranial pressure. The torsional nystagmus is distinguishable from the horizontal nystagmus present in patients with a positive Hennebert’s sign or positive fistula test. The direction of the nystagmus corresponds to a change in the basal rate of firing from the superior canal cupula. Excitatory stimulation of the right superior canal will produce a verticaltorsional nystagmus with a slow phase that is upward and counterclockwise to the patient. The resulting nystagmus can be appreciated using Frenzel lenses or infrared video-oculography or by placement of specialized ocular coils that are capable of measuring torsional eye movements. It is important to note that standard vestibular testing with electronystagmography (ENG) or rotatory chair is unlikely to reveal an abnormality in these patients. Standard ENG ocular leads may be inadequate because they are incapable of sensing purely rotational eye movement. In the combined series of patients, nystagmus could be elicited by auditory stimuli in 84%, Valsalva maneuver in 53%, and tragal pressure in 47% of patients.

A small subset of patients with SSCD will experience vestibular symptoms without changes in eye movement in response to appropriate stimuli. When this occurs in the presence of a CT-confirmed unequivocal bony dehiscence, the diagnosis may be highly suspect but not absolute. Ultimately, it is the constellation of symptoms, the physical and radiographic findings, and the determination of an incapacitating disability that makes a particular individual a surgical candidate.

Patients with SSCD may also present with purely auditory complaints, including hearing loss, pulsatile tinnitus, and autophony. Conductive hearing loss may be present and in some cases may mimic the clinical features of otosclerosis. In recent years, at least four patients at the Massachusetts Eye and Ear Infirmary underwent surgical exploration and stapedectomy without improvement in hearing and ultimately were discovered to have SSCD as the cause of the conductive hearing loss. In all cases, the conductive loss was greatest in the low frequencies, and low frequency bone conduction scores were elevated above 0 dB. Most cases of SSCD are accompanied by a mild conductive loss 5 to 10 dB, and some cases have no conductive loss. At present the cause of the conductive loss is unknown and under investigation. Patients with conductive hearing loss from SSCD have intact stapedial reflexes and lower stimulus thresholds for vestibular evoked myogenic responses (VEMP), two distinguishing features that can be used to differentiate them from true conductive hearing loss. Because of the incumbent risks of middle fossa plugging of the superior canal, including sensorineural hearing loss, we have not repaired the SSCD for patients with purely auditory symptoms.

As is the case for all patients who present for vestibular evaluation, a complete neurotologic and neurologic history and examination provide the basis for establishing an accurate diagnosis. Because the symptoms associated with SSCD are often atypical and variable, the patient’s complaints may at first seem vague. It is unfortunately clear that many patients whom we have evaluated over the years for symptoms consistent with the diagnosis of SSCD have escaped accurate diagnosis. Details regarding the onset, frequency, and fluctuation of symptoms, as well as eliciting factors, such as auditory stimuli, tragal pressure, Valsalva maneuver, straining, and eye movements, must all be specifically probed. A complete review of auditory and vestibular symptoms, including hearing loss, tinnitus, aural fullness, oscillopsia, vertigo, and disequilibrium, must be conducted. Patients should be questioned about an individual and family history of migraine and other neurologic disorders.

Essential to the diagnosis of SSCD syndrome is the demonstration of a torsional vertical nystagmus in response to auditory stimuli or changes in middle ear or intracranial pressure. No patient should be considered a good surgical candidate unless vestibular symptoms can be provoked by auditory or pressure stimuli regardless of demonstrable changes in eye movement. Eye movements can be demonstrated in the office with the patient wearing Frenzel lenses. Nystagmus can often be evoked by one or more of the following: auditory stimuli from a Barany noisemaker placed directly in the suspected ear, positive or negative insufflation of the ear canal, and a Valsalva maneuver against a closed glottis or with plugged nose. If an immediate vertical torsional movement of the eyes cannot be readily demonstrated, efforts should be made to document the response with infrared video-oculography or with the use of specialized electromagnetic ocular coils capable of measuring rotatory nystagmus. As mentioned earlier, this is often not possible with conventional ENG leads and requires placement of a specialized coil directly on the surface of the cornea. If nystagmus is detected with conventional ENG leads, it may be horizontal and reflective of other inner ear pathology. This methodology, which is most often employed as a research tool, carries a low risk of minor corneal irritation and trauma, necessitating appropriate institutional approval and informed consent. A general neurologic examination, including cerebellar and cranial nerve evaluation, should be completed.