Superior Canal Dehiscence (SCD): Laboratory diagnosis.

Timothy C. Hain, MD   • Page last modified: July 24, 2021

See also: SCD (overview) SCD bedside diagnosisSCD lab diagnosisSCD radiology diagnosisSCD conservative treatment sSCD surgical treatmentSCD references

By laboratory diagnosis, we mean diagnosis using tests outside of the radiology department, as there is a separate page on that.

As an overview, the oVEMP test is the most sensitive test for SCD, and the CT scan of the temporal bone (covered under radiology) is the most specific test for SCD. Catellucci et al (2020) observed that persons with larger dehiscence on their CT scans, have more positive tests, so these are not "yes you have it" or "no you don't have it" situation, but rather you can infer (roughly) how big the dehiscence is, and also (roughly) how soon someone will deteriorate.

Vestibular Laboratory tests that may be helpful or not helpful (VEMP is most useful) are the following:

Of course, we often do not know that a patient has SCD, and the purpose of these lab tests that are insensitive to SCD is to detect other causes of dizziness.

Below we are excluding Xrays and MRI scans, that are sometimes or usually positive in SCD. Or in other words, we are just discussing audiometry for SCD and VEMPs for SCD.

 

Audiometry for diagnosis of SCD -- detected by enlarged air-bone gaps and conductive hyperacusis.

Audiograms are not nearly as good as oVEMPs for sensitivity, but still one can sometimes detect SCD with this common procedure, given that it is done properly (i.e. bone tested even when the individual is not a hearing aid candidate).

The bottom line is that one should always do a careful audiogram when one is looking for SCD.
Conductive Hyperacusis
Conductive hyperacusis in patient with L SCD. The ']' is better than perfect (which is 0). VEMP testing was much stronger on the left side, which is the one with the air-bone gap. From this, the audiologist concluded that the patient had SCD, and she was right !

 

scd audio
Air-bone gaps in patient with bilateral SCD (the little ['s and ']'s, which are bone thresholds, are higher than the circles and x's). In audiology speak, this means that the bone thresholds are "lower" than the air thresholds. In addition, the '[]' are right on 0. This is not conductive hyperacusis because the brackets never go above 0. "

 

Audiometry is generally done as a preliminary test in evaluation of patients with potential SCD, and an alert audiologist who knows about SCD may make the diagnosis on the spot. In patients with SCD (see figure abiove), audiometry may show bone conduction scores better than air (conductive hyperacusis). This is not universal -- but occurs in roughly 40% (Yuen et al, 2009). If there is a simultaneous sensorineural hearing loss in SCD, the overall picture may mimic the conductive hearing loss pattern of otosclerosis (Mikulec et al, 2004). However, as VEMP's are present in SCD, but absent in conductive hearing loss, it is easy to tell these two apart. The main pitfall here is that audiologists may not check for conductive hyperacusis, as this is not very relevant to hearing aid dispensing. This is the reason that it is usually best to "redo" audiograms done in practices that are not experienced with SCD (such as. most hearing aid dispensors).

Tympanometry may induce dizziness, which may lead to the diagnosis. This generally requires a very large dehiscence, but it is worth being attentive.

 

VEMPS (Vestibular evoked myogenic responses) -- including oVEMP and cVEMP for diagnosis of SCD

The bottom line for this section is that VEMPs should be done if one suspects SCD. A more thorough description of VEMP tests is found here:

Left: cVEMP obtained in an individual shown in figure 3, who has left sided superior canal dehiscence, using a Bio-Logic Navigator Pro. The left side is much larger than the right.

Right: Threshold VEMP in same person, showing lower threshold on the left side.

VEMPs are very useful in dehiscence syndromes because they quantify sound sensitivity. There are two general flavors -- cVEMPs (cervical) and oVEMPs (ocular). Both are useful for diagnosis. These sound evoked vestibulocollic evoked potentials have been described as useful in diagnosing Tullio's phenomenon (sound induced dizziness) from superior canal dehiscence (Brantberg et al, 1999; Watson et al, 2000). The side with the larger cVEMP (figure 5 left) or lower threshold (figure 5 right) is the abnormal side. A threshold at or lower than 65 is very suggestive of SCD.

Amazingly, some health insurance companies fail to cover VEMP testing (i.e. Aetna), claiming that it is not proven to be useful. We think this policy is designed to save Aetna money, and is not an optimal one for the health of their participants.

cVEMPs (and here we mean threshold cVEMPs) are not always positive. In other words, it is very clear that one may have SCD on X-ray, and a normal VEMP. The lack of sensitivity probably is due to a mixture of "autoroofing" of SCD by the dura, and the usual decline in VEMPs with age or other ear disorders.

On the other hand, threshold VEMPs are fairly specific. We have rarely encountered a person with a positive threshold VEMP that did not have SCD. The exceptions are generally young women, who tend to have very large VEMPs.

oVEMPs are a more recent development. oVEMPs can be far more obviously positive than cVEMPs, because the potential on the symptomatic side can be 10 times larger -- this is not possible with a cVEMP.

We currently think the oVEMP is the most sensitive laboratory test for SCD. An amplitude > 20 is very suggestive. There is both a false positive and false negative problem however.

ENG testing often shows a minor reduction in responses on the dehiscence side. Also a downbeating nystagmus may be seen on positional testing, which resembles that of anterior canal BPPV. Most of the time though, ENG testing is not diagnostic.

An "ECochG", or electrocochleography may be of help also, although only in rare instances. The main role of ECochG is to diagnose Meniere's disease, which is a common alternative source of pressure sensitivity. ECochG is technically challenging and it may be difficult to locate a laboratory that does it well. We would not do this test at all if the VEMP is abnormal -- we would go right to the CT.

Rare tests for SCD:

Click-evoked VOR -- never became popular

This test was described by Halmagyi and others (2003). Event triggered averaging is used to detect electro-oculographic responses to loud clicks -- intensities ranging from 80 to 110 Db. 128 clicks were delivered at a rate of 5/s from 60 to 110 db, in 10 db steps. Normal subjects have no response or a very low amplitude response of < 0.25 deg at 110. The latency was 8 msec. This test is not generally available, and since there have been no adopters since 2003, this is probably not going to be pursued.

Summary:

There are several laboratory tests that are very sensitive to SCD (especially oVEMP). These are both cheaper and don't have any radiation (like a CT scan). However, once you have a positive test like this, often you will be recommended to have a radiology test to confirm or perhaps weaken the diagnosis. These are discussed next on this page. SCD radiology diagnosis

See also: SCD (overview) SCD bedside diagnosisSCD lab diagnosisSCD radiology diagnosisSCD conservative treatmentsSCD surgical treatmentSCD references