Superior Canal Dehiscence Syndrome (SCDS) is an inner ear disorder that often goes unrecognized or is misdiagnosed because its symptoms overlap with more common conditions. It stems from a structural defect in the bone of the skull surrounding the hearing and balance organs. This defect, which is a thinning or absence of the bone, results in a spectrum of unusual auditory and vestibular disturbances.
Defining Superior Canal Dehiscence Syndrome
Superior Canal Dehiscence Syndrome (SCDS) is defined by a defect in the bony covering of the superior semicircular canal, one of the three fluid-filled loops in the inner ear responsible for sensing rotational head movements. The term “dehiscence” refers to the thinning or complete absence of the dense temporal bone that typically encases this part of the inner ear. This structural gap creates an abnormal opening between the inner ear and the brain’s covering, the dura mater.
This bony defect is often referred to as a “third window” in the inner ear system. Normally, the inner ear has only two flexible openings—the oval and round windows—that allow sound energy to travel through the fluid-filled cochlea. The presence of a third window disrupts the normal mechanics, allowing fluid to move abnormally in response to sound and pressure. This shunting of energy away from the cochlea and toward the balance organ causes the characteristic hearing and balance symptoms.
The superior semicircular canal is part of the vestibular system, which provides the brain with balance information. When the bone covering this canal is absent, pressure waves from sound and changes in intracranial pressure are rerouted through the canal’s fluid. This abnormal stimulation of the balance organ is the direct anatomical cause of the unique vestibular and auditory symptoms experienced by patients.
Recognizing the Unique Symptoms
The abnormal mechanics of the inner ear lead to a distinct set of symptoms that distinguish SCDS from other ear disorders. Autophony is a specific complaint where patients hear their own body sounds with extreme intensity. Internal noises like one’s voice, heartbeat, footsteps, or even the movement of the eyeballs become abnormally loud in the affected ear. This occurs because the dehiscence makes the inner ear hypersensitive to sounds generated within the body.
Another hallmark symptom is the Tullio phenomenon, which is the occurrence of vertigo, dizziness, or nystagmus (involuntary eye movements) in response to loud sounds. Since the dehiscent bone allows sound waves to directly stimulate the fluid in the superior semicircular canal, a loud noise can trick the brain into sensing head movement. This sound-induced dizziness can be debilitating, turning common environmental sounds into sources of disequilibrium.
Patients may also experience pressure-induced vertigo, sometimes called the Hennebert sign. Activities that increase pressure, such as coughing, sneezing, straining, or heavy lifting, can trigger brief but intense episodes of dizziness. This happens because pressure changes are transmitted directly to the sensitive balance organs through the bony defect. SCDS can also cause a low-frequency conductive hearing loss because sound energy is diverted away from the cochlea. Some patients report hyperacusis, or an over-sensitivity to external sounds, often alongside a persistent feeling of aural fullness.
Confirming the Diagnosis
The diagnosis of SCDS requires correlating the patient’s specific symptoms with objective findings from specialized testing. High-resolution Computed Tomography (CT) scans of the temporal bone are the imaging standard for visualizing the bony defect. These scans demonstrate the absence of the bone over the superior semicircular canal, which is necessary for a definitive diagnosis.
The presence of a dehiscence on a CT scan alone is not sufficient, as some asymptomatic individuals also show this finding. Therefore, the diagnosis must be supported by specialized inner ear function tests, such as Vestibular Evoked Myogenic Potentials (VEMPs). VEMP testing measures the response of the inner ear’s balance organs to sound or vibration. SCDS patients often exhibit abnormally low thresholds for these responses, providing physiological evidence of the third window effect.
Comprehensive audiometry is also a standard part of the workup to assess hearing involvement. The characteristic audiometric finding is a low-frequency conductive hearing loss, meaning the patient hears bone-conducted sound better than air-conducted sound. This “air-bone gap,” when combined with a positive VEMP and clinical symptoms, helps differentiate SCDS from other disorders.
Treatment Options
The management of SCDS is tailored to the severity of the symptoms, ranging from conservative monitoring for mild cases to surgical intervention for debilitating complaints. For patients with minimal symptoms, a conservative approach is recommended. This involves avoiding known triggers, such as loud noises, activities that cause barometric pressure changes, or strenuous physical exertion.
When symptoms severely impact quality of life, surgical repair is the only method that resolves the underlying anatomical problem. The goal of surgery is to close the third window, restoring normal fluid dynamics within the inner ear. The two primary surgical techniques are canal plugging and resurfacing.
Canal Plugging
Plugging involves completely sealing the dehiscent superior semicircular canal with bone wax or cement, which effectively eliminates the abnormal opening.
Resurfacing
Resurfacing covers the bony defect with a graft, such as bone cement, without fully blocking the canal. This technique may reduce the risk of temporary post-operative balance issues. Both procedures aim to restore the integrity of the temporal bone and are reserved for patients whose symptoms are severe and disabling.