How Can You Test For SCDS?
A full audiogram should be performed for both ears. Specifically, BONE conduction should be performed at -5 and -10 dB to exclude supranormal thresholds that are common in SCDS patients. Additional tests that should be performed include a tympanogram, acoustic reflexes, and otoacoustic emissions are crucial to exclude otosclerosis or ossicular fixation, middle ear fluid, Eustachian tube dysfunction. Often, SCDS patients will have supranormal bone conduction or conductive hyperacusis with an air-bone gap and PRESENT acoustic reflexes and normal tympanograms (unless previous surgery or concurrent ear infection is present).
Caloric testing may be important to exclude unilateral vestibular hypofunction caused by Meniere's or vestibular neuronitis/labyrinthitis
There are two types of VEMP testing - cervical or ocular VEMPs. cVEMPs have become a standard test in the workup of a patient with suspected SCDS. VEMP stands for vestibular evoked myogenic potential testing and tests for a reflex that begins as sound presented to the ear and ends as an inhibitory response of the ipsilateral (same side) neck muscle called the sternocleidomastoid. The pathway is sound-tympanic membrane-ossicles-saccule-inferior vestibular nerve-vestibulospinal tracts-sternocleidomastoid muscle.
A Temporal Bone CT scan with true coronal cuts and reconstructions that specifically examine the superior canals should be obtained. Unfortunately, this cannot be performed in the majority of neuroradiology centers (yet). MRI scans with contrast are useful in the workup of dizziness and should be obtained to exclude a tumor but do not show SCD.
RADIOLOGIC FINDINGS IN SCDS
This fine cut temporal bone CT, 0.2 mm cuts parallel to the plane of the superior (anterior) semicircular canal, reveals a right-sided dehiscence of the superior canal, about 4mm in length. A thin, but intact covering of bone over the membraneous left-sided superior canal is also shown.
This fine cut temporal bone CT, 0.2 mm cuts (Stenvers view), orthogonal to the plane of the superior semicircular canal, reveals a dehiscence of bone overlying the arcuate eminence. This patient was seen by an outside specialist for possible otosclerosis, and presented with bilateral hearing loss but denied dizziness. The audiograms revealed bilateral conductive hearing loss and present, bilateral stapedial reflexes.