Interesting fact: Patients who can "hear their eyes move" almost always have SCD.
Symptoms of an inner ear dehiscence syndrome
Dizziness: in SCD
Usually there is an unsteadiness which increases with activity and which is relieved by rest.
SCD is a type of inner ear fistula. Some people with fistulas find that their symptoms get worse with coughing, sneezing, or blowing their noses, as well as with exertion and activity. This sort of symptom goes under the general name of "Valsalva induced dizziness", and it can also be associated with other medical conditions in entirely different categories --for example, the Chiari malformation, and a heart condition called "IHSS". Oddly, a recent report suggests that the Chiari is far more common in SCD (Kuhn and Clenney, 2010) than the normal population. We think that this report is likely erroneous.
The changes in air pressure that occur in the middle ear (for example, when your ears "pop" in an airplane) normally do not affect your inner ear. When a fistula is present, such as in SCD, changes in middle ear pressure will directly affect the inner ear, stimulating the balance and/or hearing structures within and causing typical symptoms. Pressure sensitivity due to SCD generally causes much stronger nystagmus than pressure sensitivity in persons with round or oval window fistulae, presumably because the pressure stimulus is directly applied to a single semicircular canal in SCD rather than disturbing the inner ear in a less direct way. There are also a very few other conditions that can also cause pressure sensitivity such as Meniere's disease and vestibular fibrosis.
One would think that persons both on CPAP and SCD would have dizziness from the interaction between the two conditions. We do not know of any confirmation of this as yet.
In superior canal dehiscence or in persons with fenestrations, it is not unusual to notice that use of ones own voice or a musical instrument will cause dizziness (this is called the "Tullio's phenomenon").
There are also patients who can indicate that their voice sounds louder than normal to them. This is a form of "autophony". More commonly autophony is caused by a patulous eustachian tube, which is another subject entirely. In eustachian tube malfunction, such as the patulous ET, the voice is "boomy", as if in a barrel. This is due to a longer resonant cavity in the middle ear.
Persons with unilateral SCD may have a positive "hum" test. When they hum a pitch, it is louder on one side.
Rare patients with SCD can "hear their eyes move". Or hear themselves blink. (Bertholon et al, 2017) If the patients noticed this, without reading about it on the internet, it is very specific. Of course, people are suggestible and if they develop this after reading Dr. Google, it is much less reliable. Occasional people can "hear their eyes move" after acoustic neuroma surgery, presumably due to abberant regeneration.
Some people experience ringing or fullness in the ears, and many notice a hearing loss. According to Yuen et al (2009), 85% of persons with SCD have auditory symptoms including autophony (40%), hyperacusis to bodily sounds (65%), hearing loss (40%), aural pressure (45%), and tinnitus (35%). What is missing in this report is a comparison to a control group -- our experience with SCD does not bear out Yuen's observations. We think that the main presenting symptom of SCD is pressure or sound sensitivity. We don't find that these other symptoms or signs are generally troublesome.
Clearly there are some patients with hearing loss - -this is puzzling as the damage to the ear in SCD is nowhere near the cochlea. Perhaps the difficulty in SCD is that the pressure fluctuates too widely because the inner ear is directly connected to spinal fluid pressure through the opening. This might be a similar mechanism to the "enlarged vestibular aqueduct" syndrome.
Some patients with SCD experience pulsatile tinnitus. This appears to be a variant of autophony.
Dehiscence, being a bone defect, is nearly always diagnosed using a high resolution temporal bone CT scan. Of course, CT scans involve radiation, and radiation is a little damaging, and best avoided when feasible. Other tests, not involving X-rays, may provide a very good clue that an temporal bone CT is indicated. We think that generally either the valsalva test or a large amplitude of the oVEMP tests should be positive as an indication to do a temporal bone CT. We don't think that these CTs should be done in everybody.
Tests that may be helpful in the office (Valsalva is the best) are as follows:
Vestibular Laboratory tests that may be helpful or not helpful (VEMP is most useful) are the following:
Of the office based tests, the Valsalva is the most specific, though insensitive. Somewhat sensitive is asking patients if they can "hear" a tuning fork (128 hZ) applied to their wrist. The latter method is of course vulnerable to suggestibility. Patients who can "hear their eyes move" always have SCD.
Valsalva test: (follow link for more detail)
In SCD, positive pressure or Valsalva against pinched nostrils produces downbeating nystagmus, with a torsional fast phase consistent with stimulation of the affected ear (CCW for right ear, CW for left ear). See example below. Negative pressure or Valsalva against a closed glottis may produce upbeating nystagmus and nystagmus beating with the torsional fast phase in the opposite direction (CW for right ear, CCW for left ear). We ourselves prefer the Valsalva against a closed glottis.
Practically, we don't think that you can do this test without magnification -- i.e. a video-frenzel system with a good enough focus that you can see torsion.
Another method is to use an examining microscope focused on the sclera. We are less enthused about technique as it is very hard to keep the sclera in view while the patient is undergoing a maneuver. Also, the light can be uncomfortable.
For those familiar with posterior canal BPPV, the vector relationships between vertical and torsional components is reversed so that the upbeating nystagmus beats away from the "bad" ear, and downbeating, towards the "good" ear. More commonly, however, no nystagmus at all is produced by either maneuver. In persons with lateral canal fistulae (which are rare and usually confined to persons with cholesteatoma or after fenestration surgery), horizontal nystagmus can be produced (see example below). In persons with window fistulae, generally very little nystagmus is produced by Valsalva or for that matter, any maneuver.
Supplemental material Movie of nystagmus elicited by Valsalva in person with fenestration
Case example: In the man shown in figures 3 and 4, 10 seconds of straining produced a very powerful torsional nystagmus (and a lot of dizziness).
Our current feeling is that these tests are much lower yield than the Valsalva.
A fistula test , which entails making a sensitive recording of eye movements while pressurizing each ear canal with a rubber bulb, is occasionally helpful. A positive test is good grounds for a temporal-bone CT. Fistula tests are little used because they are difficult to do and insensitive. Fistula tests are often not available or even thought of. However, if a patient complains of dizziness during tympanometry, this is a clue that the patient has a positive pressure test.
A strong nystagmus (vertical and rotatory) may be produced by pressure in the external ear canal. However, we do not think that this is very sensitive. It is very specific
Asking patients if they can "hear" a 128 hz tuning fork on their wrist, is often positive, but of course this is vulnerable to suggestibility.
|Upbeating nystagmus provoked by vibration over the mastoid of person with left sided SCD. Image courtesy of Dr. Dario Yacovino.|
Vibration can occasionally produce nystagmus over the defective ear. An example of this is shown above. Again, our impression is that this is VERY insensitive. It is also nonspecific as there are far more patients without any SCD that will have vertical nystagmus.
Simple observation of the patient's eyes with appropriate equipment (such as video frenzel goggle) may also provide the diagnosis, as in some cases, there is a pulse-synchronous oscillation (Rambold, 2001; Hain et al, 2008), see videos below and case 2. This rare sign requires either use of an ophthalmoscope or video frensel goggles to see it. One also has to think of it (: this is usually the hard part) The main confounding possibility is oculopalatal myoclonus, which causes a similar but non-pulse synchronous oscillation. There are also many other rare sources of pendular nystagmus, but almost all of thse have visual or neurological symptoms.
Catellucci et al (2020) observed that persons with larger dehiscence on their CT scans, have more positive tests.
Figure 5 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.
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.
Figure 3: Conductive hyperacusis in patient with L SCD. 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 !
Figure 3b: Conductive hyperacusis in patient with bilateral SCD. In other words, thresholds can be better than 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 3), 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.
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.
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.