Superior Canal Dehiscence Syndrome (SCDS) is a rare inner ear disorder affecting balance and hearing. This condition arises from an abnormal thinning or complete absence of the bone covering a specific inner ear structure. The resulting defect creates an abnormal opening that disrupts the normal mechanical function of the inner ear, leading to a set of symptoms.
The Anatomy and Mechanism of SCDS
SCDS involves a defect, or “dehiscence,” in the bony layer covering the superior semicircular canal, one of the three fluid-filled tubes regulating balance within the temporal bone. While the exact cause is unknown, it is believed to result from a failure of the bone to thicken properly during development. Symptoms may become apparent following trauma or pressure changes later in life.
This bony defect creates an abnormal third opening into the inner ear, in addition to the oval and round windows. This “third window effect” allows pressure and sound energy to dissipate abnormally into the cranial cavity instead of being directed through the cochlea. This disruption alters the transmission of sound and pressure, causing the inner ear to become hypersensitive to internal and external stimuli.
Distinctive Auditory and Vestibular Symptoms
The symptoms of SCDS are divided into auditory and vestibular manifestations. A defining auditory symptom is autophony, where a person hears their own internal body sounds with unusual loudness. This includes hearing one’s voice, heartbeat, footsteps, chewing, or eye movements.
Another common auditory symptom is hyperacusis, a hypersensitivity to external sounds, often accompanied by a conductive hearing loss at low frequencies. The most dramatic auditory manifestation is the Tullio phenomenon, which involves sound-induced vertigo or dizziness. Loud noises cause an abnormal fluid movement in the superior canal, leading to the sensation of spinning and involuntary eye movements.
The vestibular, or balance, symptoms stem from the same abnormal fluid dynamics. Patients frequently experience chronic imbalance or disequilibrium, making walking challenging. Vertigo can be triggered by loud sounds or by pressure changes, such as coughing, sneezing, straining, or blowing the nose. This pressure-induced dizziness occurs because internal pressure is transmitted directly through the dehiscence, stimulating the balance organ. The sensation of oscillopsia, where stationary objects appear to be moving, may also occur during these events.
Diagnostic Procedures
Confirming a diagnosis of SCDS requires a combination of functional and structural tests. The gold standard for visualizing the bony defect is a High-Resolution Computed Tomography (HRCT) scan of the temporal bone. This imaging uses specific oblique views to demonstrate the absence of bone over the superior semicircular canal. However, a dehiscence visible on a CT scan does not automatically confirm the syndrome, as many people have the anatomical defect without symptoms.
To confirm the physiological effect of the dehiscence, a Vestibular Evoked Myogenic Potential (VEMP) test is used. This test measures the ear’s hypersensitivity to sound by assessing muscle reflexes in the neck (cervical VEMP) or eyes (ocular VEMP) in response to auditory stimuli. In SCDS patients, the dehiscence causes the inner ear to respond to much quieter sounds than normal. The combination of characteristic symptoms, a low-threshold VEMP response, and the bony defect on HRCT provides the definitive diagnosis.
Management and Treatment Options
Treatment for SCDS is tailored to the severity of symptoms, starting with conservative management for milder cases. This approach focuses on lifestyle adjustments, such as avoiding known triggers like loud noises or activities that cause sudden pressure changes. Vestibular rehabilitation therapy can also help the brain adapt to imbalance.
For patients whose symptoms severely impact their quality of life, surgical intervention is required. The goal of surgery is to close the abnormal third window to restore normal inner ear mechanics. There are two primary surgical approaches: canal resurfacing and canal plugging. Canal resurfacing involves covering the bony defect with material like bone cement or fascia to cap the area, sealing the opening without disabling the canal’s function. Canal plugging, or occlusion, involves completely filling the superior semicircular canal with bone wax or cement, obliterating its function. While plugging is considered more definitive for long-term relief, the approach is chosen based on the dehiscence location and the surgeon’s expertise.