Superior Semicircular Canal Dehiscence

History

• 1.

  History of present illness

  • a.

    Does the patient experience dizziness?

    Two types of elicited dizziness are commonly associated with SCD syndrome: sound- and pressure-induced dizziness. Sound-induced dizziness occurs in response to exposure of the affected ear to loud noise, such as that from loud machinery, music, or crowd noise. Pressure-induced dizziness in SCD syndrome occurs in response to the transient elevation of intracranial pressure, as occurs with heavy lifting or bearing down. The dizziness that occurs in response to these inciting events is usually transient. Dizziness may be elicited by positive pressure in the external meatus, as with finger manipulation. Some patients with SCD syndrome have nonspecific chronic dizziness with or without elicited dizziness.

  • b.

    Does the patient have autophony?

    Autophony is the hearing of internal body sounds. Examples of common sounds heard by patients with SCD syndrome include hearing one’s eyes move, the sound of one’s heels hitting the floor, hearing one’s heartbeat, the sound of chewing, or hearing one’s voice reverberate within one’s head. Note that autophony of breathing is not expected in SCD; if this is present, alternative diagnoses such as patulous Eustachian tube should be considered.

  • c.

    Does the patient have hearing loss?

    Hearing loss is common in SCD syndrome. The dehiscence of the superior canal results in a third mobile window (the oval and round windows being the first and second) into the inner ear. This third mobile window allows for the shunting of air-conducted sound energy away from the cochlea and leads to hearing loss.

  • d.

    Is the patient asymptomatic?

    Superior canal dehiscence can present as an incidental finding discovered on imaging performed for another reason. Patients with asymptomatic SCD do not require surgery and should be observed.

• 2.

  Past medical history

  • a.

    Prior treatment: Has the patient had prior treatment for SCD syndrome or for dizziness thought to be from another etiology?

  • b.

    Medical illness

    • 1)

      What is the hearing status in the contralateral ear?

    • 2)

      What is the vestibular function of the contralateral ear?

    • 3)

      Is there a history of migraine? Migraine may interfere with or prolong compensation following a vestibular insult, such as that which occurs following plugging of the semicircular canal.

    • 4)

      Is the patient’s overall medical health sufficient to tolerate a craniotomy?

  • c.

    Surgery

  • d.

    Family history

    • 1)

      Some cases of familial SCD syndrome have been described. Most cases, however, appear to be sporadic.

  • e.

    Medications

    • 1)

      Is the patient currently on anticoagulation therapy that would interfere with the performance of a craniotomy?

    • 2)

      Is the patient currently taking vestibular suppressant medication? Vestibular suppressants can interfere with vestibular compensation following surgery.

Physical Examination

• 1.

  Otoscopy

• a.

  Otoscopic examination in SCD syndrome is normal.

• 2.

  Tuning fork examination

  • a.

    Weber lateralizes to the affected ear and Rinne can be negative (bone conduction perceived louder than air conduction). (Note: Not all patients with SCD have demonstrable hearing loss or an air-bone gap. Therefore the findings of tuning fork examination are variable in SCD and are often in line with audiometric findings.)

  • b.

    When the vibrating tuning fork is placed on the lateral malleolus of the ankle, it can often be perceived in the affected ear.

• 3.

  Tullio phenomenon

  • a.

    Presentation of a loud sound (as with a Barany noise box or delivered with an audiometer) to the affected ear results in dizziness.

• 4.

  Hennebert sign

  • a.

    Pneumatic otoscopy or tragal pressure to the affected ear results in a vertical-torsional nystagmus with associated dizziness. Note that this nystagmus is likely to be difficult to perceive without the assistance of Frenzel lenses.

Audiometric Findings and Other Testing

• 1.

  Audiogram

  • a.

    The characteristic audiometric findings of SCD include supranormal bone conduction thresholds and a predominantly low-frequency air-bone gap ( Fig. 147.1 ).

    

• 2.

  Acoustic reflex testing

  • a.

    The audiometric findings in SCD may mimic those seen in otosclerosis. Acoustic reflex testing can be used to distinguish between these diagnoses. In SCD, acoustic reflexes are present. In contrast, in otosclerosis, acoustic reflexes are absent because the stapes is immobilized.

• 3.

  Vestibular evoked myogenic potential (VEMP)

  • a.

    Presentation of a loud tone (or click) to the ear and evaluating for an inhibitory response in the ipsilateral sternocleidomastoid muscle is the cervical VEMP test. Reduced thresholds or increased amplitude on the test is consistent with SCD.

  • b.

    The ocular VEMP test (excitatory response measured in the contralateral inferior rectus muscle) has been suggested to be a more sensitive and specific test for SCD than the cervical VEMP test.

Imaging

Computed tomography (CT) of the temporal bone with fine cuts should be obtained when SCD syndrome is suspected. Multiple planes should be evaluated to confirm the diagnosis ( Fig. 147.2 ). An image guidance study should be performed if the use of image guidance is planned for surgery.

Indications

• 1.

  SCD syndrome—Patients should make the final decision as to when they want to intervene based on their perceived severity and disability.

Contraindications

• 1.

  Anatomic SCD without symptomatology

• 2.

  An active coplanar semicircular canal problem (e.g., contralateral posterior semicircular canal benign paroxysmal positional vertigo [BPPV]) should be treated prior to proceeding with surgery.

• 3.

  SCD syndrome in an only one hearing ear: Very strong consideration should be given to alternatives to surgery owing to the risk of hearing loss in the operative ear.

• 4.

  Medical conditions that would preclude a craniotomy