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
Cervical or Ocular VEMP Testing
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.
Patients with SCDS will often have LOW threshold cVEMP responses (Less than 70 dB thresholds to tone-burst testing at 250 or 500 Hz). Click-evoked cVEMP responses are slightly higher - 75 to 80 DB or less in SSCD patients.
However, there are some patients with normal thresholds on cVEMPs and so if the clinical presentation is still suspicious imaging should be obtained. In these cases, ocular VEMPs (oVEMPs) are a new way to test the vestibular, or balance pathways and may be a more specific measure of a change in the function of the superior semicircular canal when a dehiscence is present. oVEMPs may be a better way to determine whether the dehiscence is anatomic (where the CT scans shows SCDS but that the dehiscence doesn't affect function of the inner ear) or physiologic (when the SCDS is affecting the function of the inner ear and may be the cause of a patient's symptoms) when the cVEMPs do not show a change in thresholds or amplitudes in an ear that is symptomatic for SCDS and the CT scans show a dehiscence. oVEMPs may be helpful in patients who have bilateral SCDS and bilateral symptoms to determine the ear that has been affected the most by the dehiscence.
Cervical Vestibular Evoked Myogenic Potentials (cVEMP) testing in the office. VEMP testing involves introducing sound to one ear, while monitoring activity of the sternocleidomastoid muscle (SCM), a major muscle of the neck.
The test is painless, non-invasive and is relatively easy to perform for most patients in the hands of an experienced audiologist. The threshold, or intensity, of the sound required to generate a muscle response is recorded-several frequencies in the low to mid-range are tested.
Patients with SCDS will have decreased thresholds (increased) sensitivity) to sound as measured by a response of the SCM. In addition the amplitude of the cVEMP response in a SCDS ear will be much larger than the uninvolved ear (unless both ears have SCDS). In contrast, a patient with Meniere's disease or otosclerosis may have increased thresholds, or absent responses. With an experienced audiologist trained to perform evoked response potentials, cVEMP testing is reliable and provides a good measurement of vestibular sensitivity, with respect to the functioning of the saccule - inferior vestibular nerve.
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.
Will a standard head CT or low resolution temporal bone CT will show SCD?
No. A typical SCD is 1 to 4 millimeters-the size of a pinhead or only slightly larger! Therefore, standard head CTs, low resolution temporal bone CTs and MRIs will not be able to detect SSCD reliably.
The BEST imaging for SCDS is a high resolution 0.5 mm collimation or less CT of the temporal bones with images that are REFORMATTED to include cuts in the plane of (Poschel) and perpendicular (Stenver) to the superior canals.
If surgery is considered as an option, we would recommend that the best possible imaging be obtained prior to undergoing a craniotomy procedure as negative explorations where no dehiscence was found have been reported in the clinical literature.
RADIOLOGIC FINDINGS IN SCDS
As stated above, conventional temporal bone CT scans may not identify small dehiscences of the superior canal. Although a recent study by the University of Pittsburgh demonstrated that an experienced neuroradiologist could identify SCD reliably in standard 0.5 collimation coronal temporal bone cuts (and these were compared with images reformatted in the planes parallel to (Poschel) and orthogonal (Stenver), negative surgical explorations have been reported and so we believe that reformatted images to examine superior canals should be standard of care ESPECIALLY if surgery is being considered.
Temporal bone CT #1:
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.
The patient had presented with conductive hearing loss following a failed stapedotomy procedure on the right side-the prosthesis is shown in the scan. Stapedial, or acoustic reflexes were present prior to the stapes surgery. This patient is being observed as surgery is NOT performed for hearing loss alone.
Temporal bone CT #2:
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.
Vestibular-evoked myogenic potential testing revealed decreased thresholds bilaterally. His temporal bone CTs revealed bilateral superior semicircular canal dehiscences. The patient was managed with observation, and fitted with hearing aids. Surgical repair of the SCD in this case was not offered.