Laboratory Studies
No specific laboratory studies confirm the diagnosis of superior canal dehiscence syndrome (SCDS). Laboratory tests may be obtained to rule out other pathological causes of vertigo (ie, multiple sclerosis, syphilis).
Imaging Studies
Imaging studies are critical in the diagnosis of SCDS. A high-resolution computed tomography (HRCT) scan of the temporal bones without contrast is required to make a definitive diagnosis. Views that are oblique to the temporal bone must be obtained with a HRCT scan in order to properly see the superior semicircular canal. The image below displays the typical findings in a patient with SCDS.
Coronal high-resolution computed tomography scan (1-mm sections) that demonstrates the presence of the superior semicircular canal (figure A, black arrow). As the scan is followed posteriorly (figures B, C, D), the bony dehiscence over the superior canal (black arrow) becomes more apparent.
Magnetic resonance imaging (MRI) cannot be used to confirm the diagnosis of SCDS; however, it may be of benefit in ruling out a retrocochlear process.
Other Tests
Audiometric testing
Comprehensive audiometric evaluation is indicated in any patient with vestibular symptoms. In a patient with SCDS, key factors may be revealed. One key is the presence of normal symmetrical hearing, which helps to eliminate a retrocochlear process as a cause of vertigo.
Patients with SCDS may have conductive hearing loss. The air-bone gaps are typically greatest at frequencies below 1 kHz.
The conductive hearing loss is believed to be due to the third mobile window of the superior semicircular canal dehiscence, resulting in elevation of thresholds of air-conduction sounds and a reduction of thresholds for bone-conduction sounds.
Acoustic reflex testing is preserved in a patient with SCDS as a cause of conductive hearing loss, as opposed to a loss of the reflex with other conductive or mixed hearing loss conditions such as otosclerosis.
Electronystagmography testing
Routine electronystagmography (ENG) testing reveals no objective or pathognomonic signs of SCDS. All patients with suspicion of the disorder should undergo ENG to further eliminate other potential causes.
Video-oculography can be valuable in recording the vertical and torsional eye movements specific to SCDS.
Caloric test results are usually unaffected in patients with SCDS; however, when the dehiscence is large (>0.5 mm), reduced caloric test results may be demonstrated on the affected side.
Vestibular evoked myogenic potentials
Vestibular evoked myogenic potentials (VEMPs) have recently been suggested to help with the diagnosis of SCDS. The inferior vestibular nerve innervates the saccule, which has some sound sensitivity. The inferior vestibular nerve has its main input to the lateral vestibular nucleus (Deiter nucleus), where the 2 main postural tracts originate. The medial vestibulospinal tract is responsible for postural control of the neck, whereas the lateral vestibulospinal tract is dedicated to the lower trunk and limbs.
For the most part, sound-evoked VEMPs are considered completely unilateral. This test is performed by placing electrodes on the sternocleidomastoid neck muscle. Patients hold their head up unsupported, using only their anterior neck muscles. Patients are instructed to tense the muscle during acoustic stimulation and to relax after the stimulation stops. Loud clicks or tone bursts (95-100 dB nHL) are repeatedly presented to each ear. [6] If the neck muscles are activated at this level, a VEMP is produced.
In patients with SCDS, a response at very low thresholds (< 65 dB) can be noted to produce a VEMP on the affected side. This is thought to occur secondary to the hypercompliance of the vestibular system on the affected sided secondary to the third-window effect. A low-threshold VEMP raises the suspicion of SCDS.
A study by Verrecchia et al indicated that in ocular VEMP tests, an amplitude of greater than 33.8 μVolts has a sensitivity and specificity of 87% and 93%, respectively, for differentiating an ear with SCDS from a healthy one. [7]
Vibration-induced nystagmus
Most recently, patients with known SCDS underwent a series of cranial vibratory tests. A 100 Hz oscillator was placed against the cranium at different places for 10-15 seconds. Nystagmus was digitally recorded by infrared video oculography. All patients demonstrated distinct torsional vibration-induced nystagmus that was especially prominent with suboccipital vibration. Vibration of the suboccipital region and the demonstration of torsional nystagmus is an emerging technique for the diagnosis of SCDS. [8]
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Coronal high-resolution computed tomography scan (1-mm sections) that demonstrates the presence of the superior semicircular canal (figure A, black arrow). As the scan is followed posteriorly (figures B, C, D), the bony dehiscence over the superior canal (black arrow) becomes more apparent.
