Semicircular Canal Dehiscence (SCD) is a relatively rare vestibular disorder affecting the inner ear, the sensory organ responsible for both hearing and balance. This condition is often misunderstood or misdiagnosed due to the unusual nature of its symptoms. SCD involves an anatomical defect in the bone protecting the inner ear structures, leading to a unique set of auditory and balance problems.
Defining the Anatomical Defect and Effect
SCD is characterized by a thinning or complete absence, known as a “dehiscence,” of the bony layer covering one of the three inner ear semicircular canals. The superior semicircular canal is the most commonly affected structure, leading to Superior Semicircular Canal Dehiscence (SSCD). In a healthy ear, a thick layer of bone encases and protects the fluid-filled labyrinth from the adjacent cranial cavity, which is necessary for normal function.
The absence of this bone creates an abnormal opening into the inner ear fluid, a state termed the “Third Window Phenomenon.” The inner ear normally operates with two compliant openings: the oval window, where sound vibrations enter, and the round window, which acts as a pressure-release valve. The dehiscence provides a third, unintended mobile window, disrupting the normal mechanics of the inner ear fluid. This new pathway diverts acoustic and pressure energy away from the cochlea’s normal hearing pathway, shunting it instead to the vestibular system.
This diversion explains the unusual combination of auditory and balance symptoms experienced by patients with SCD. The abnormal opening acts as a low-impedance outlet, making the inner ear overly sensitive to both sound and pressure changes. Fluid movement within the dehiscent canal is easily triggered, activating the balance nerves even when the head is not moving. This mechanical hyper-responsiveness underlies the disorder’s unique clinical presentation.
Recognizing the Unique Symptoms
The abnormal communication created by the dehiscence results in a cluster of distinct symptoms affecting both hearing and balance. One specific manifestation is the Tullio phenomenon, the experience of vertigo or disequilibrium triggered by loud noises. Sound energy is transmitted through the dehiscence, inappropriately stimulating the superior semicircular canal and causing a sensation of motion. This sound-induced dizziness can be accompanied by oscillopsia, where the visual world appears to bounce or vibrate.
Another highly specific symptom is autophony, the abnormally loud hearing of internal bodily sounds. Patients often report hearing their own heartbeat, the sound of their eyeballs moving, the crunching of their joints, or their own voice reverberating loudly. This occurs because the third window lowers the resistance to bone-conducted sounds, allowing these subtle internal noises to be perceived with great intensity.
Pressure sensitivity is also a common complaint, where changes in middle ear or intracranial pressure can trigger dizziness. Actions that can induce brief episodes of vertigo or unsteadiness include:
- Straining
- Coughing
- Sneezing
- Nose-blowing
- Lifting heavy objects
Furthermore, many patients experience a conductive hearing loss, often more pronounced at lower frequencies, which is a direct consequence of the energy shunting away from the cochlea.
The Diagnostic Pathway
Diagnosing Semicircular Canal Dehiscence requires a combination of clinical evaluation, functional testing, and structural imaging to confirm both the symptoms and the anatomical defect. Initial assessment involves standard audiometry, which may reveal a low-frequency conductive hearing loss. This often includes bone conduction thresholds that are better than normal (known as an air-bone gap). This hearing profile is highly suggestive of a third window abnormality.
A primary functional test is the Vestibular Evoked Myogenic Potential (VEMP) test, which measures vestibular system responses to sound. In patients with SCD, the third window makes the inner ear structures hypersensitive, resulting in VEMP responses detected at much lower sound levels (a reduced threshold). Both the cervical VEMP (c-VEMP) and the ocular VEMP (o-VEMP) are utilized, often showing pathologically large responses.
The definitive structural confirmation relies on a High-Resolution Computed Tomography (HRCT) scan. This imaging technique visualizes the thin slice of bone covering the superior semicircular canal and measures the size of the dehiscence. Specialized imaging views, such as those reformatted in the plane of the canal, are required to accurately confirm the absence of the bone layer. A confident diagnosis requires the combination of characteristic symptoms, hypersensitive VEMPs, and radiological confirmation.
Management and Surgical Repair
For patients whose symptoms are mild or infrequent, the initial approach to managing SCD is conservative. This involves modifying lifestyle habits to avoid known triggers. Patients may be advised to manage symptoms by:
- Avoiding straining or heavy lifting
- Refraining from playing loud musical instruments
- Wearing hearing protection in noisy environments
- Avoiding activities involving significant pressure changes
This conservative strategy is often sufficient for individuals whose quality of life is not severely compromised.
For patients with debilitating or frequent symptoms, surgical intervention is the standard of care, aiming to eliminate the third window effect. The most common and successful surgical technique is canal plugging or resurfacing. This procedure involves accessing the superior semicircular canal and sealing the dehiscent area with bone cement or bone grafts to restore the bony barrier. Plugging the canal effectively isolates the dehiscent segment, restoring normal inner ear mechanics.
A less invasive option is round window reinforcement, though it is sometimes less effective for vestibular symptoms. This procedure involves placing material near the round window membrane to increase its impedance, rebalancing the pressure dynamics of the inner ear. Following successful surgical repair, patients typically report significant improvement or complete resolution of their vertigo, autophony, and pressure-induced symptoms, with success rates often exceeding 90 percent.