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Page last modified: January 5, 2008
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Figure 1. Anatomy of the normal inner and middle ear. In superior canal dehiscence (SCD) bone is missing over the top (superior) semicircular canal, uncovering a membrane. This dehiscence makes the ear more sensitive to pressure and noise. |
There are several situations in which the inner ear membranes may be uncovered by bone. These conditions are generally recognized because pressure in the ear, changes in intrathoracic pressure, or loud noises can cause strong vertigo and jumping of the eyes (nystagmus). While similar to a perilymph fistula, these conditions are generally less bothersome than than the oval or round window fistulae, as fluid is generally not in direct communication with an air-filled cavity, but a membrane is present which maintains a seal. Dehiscence is a very rare condition compared to most other causes of dizziness and hearing loss.
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| Figure 2. Coronal thin cut CT scan showing superior canal dehiscence (SCD). This patient was reported in detail in (Ostrowski, Hain and Wiet, 1997) |
In the "superior canal dehiscence" syndrome (Minor et al, 1998; Minor 2000) , the roof of the superior semicircular canal is missing. A conductive hearing loss similar to that of otosclerosis can be found in some individuals with SCD (Minor et al, 2003), see case example. SCD can also cause improvement in bone conduction. SCD can be distinguished from otosclerosis by a temporal bone CT scan or an intact VEMP test, as VEMP's are generally absent in otosclerosis.
Roughly 2% of persons at autopsy are found to have thinning of bone, which is thought to predispose them to this syndrome (Carey et al, 2000; Hirvonen et al. 2003). Eye movements in this syndrome align with the superior canal (Ostrowski, Hain and Wiet, 1997; Cremer et al, 2000). A similar dehiscence can be caused by other processes that wear away bone, such as vascular malformations (Brantberg et al, 2004).
Case example: a commercial airline pilot complained that on landing, the world would tilt 15 degrees to the side. A CT scan of the temporal bone showed a dehiscence. He indicated that he always had his co-pilot land the plane.
Fistulae can also occur as a late complication of mastoid surgery using the canal wall down technique. In this instance, the fistula is usually caused by repeated infections in the opened mastoid. (Hakuba et al, 2002).
Dehiscence is also the result of a surgery is done called "fenestration" (previously done for otosclerosis, this procedure is no longer used). In this operation, there is an opening created between the lateral semicircular canal and an artificially created cavity in the mastoid sinus area. In animals, fenestrations create pressure sensitivity (Hirvonen et al. 2001), and this is nearly always the case in people who have had this obsolete surgery.
Case example: An 80 year old woman came in to have her mastoid bowl cleaned out. When asked, she said that ever since her fenestration surgery in the 1940's, she had gotten dizzy from loud noses. Comment: as this surgery is no longer done, most persons with it are in their 80's.
Posterior dehiscence (PCD) is rarely reported compared to superior canal dehiscence, but nevertheless a few cases have been identified (Di Lella et al, 2007). PCD has been reported in association with a high jugular bulb (Mickulec and Poe, 2006). PCD is identified using high-resolution CT scans, either axial cuts, or with cuts in the plane of the PC.
In years gone by, a condition called "cholesteatoma" was a common cause of this problem also, but this condition is now encountered only rarely because of improved antibiotic treatments.
It is possible that there are occasionally small cracks in the bone between the middle and inner ear. They are sometimes called "fistulae", although it seems unlikely that in this situation there is a break between the inner ear membranes and the middle ear. Their significance is controversial.
A case of a fistula (air in labyrinth) was reported after cochlear implantation of the HiFocus II implant In this case, vertigo occurred after the patient blew his nose. It was suggested that the connective tissue seal between the electrode and positioner extends into the tympanic cavity and predisposes this type of implant to this type of fistula (Hempel et al, 2004). Fistulae have also been reported in other cochlear implant cases (Kusuma et al, 2005)
Dizziness:
Usually there is an unsteadiness which increases with activity and which is relieved by rest. Some people experience ringing or fullness in the ears, and many notice a hearing loss.
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".
Supplemental material on the site DVD: Movie of nystagmus elicited by Valsalva in person with Superior Canal Dehiscence (51 meg)
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, changes in middle ear pressure will directly affect the inner ear, stimulating the balance and/or hearing structures within and causing typical symptoms. There are a number of other conditions that can also cause pressure sensitivity such as Meniere's disease and vestibular fibrosis.
Supplemental material on the site DVD: Movie of nystagmus elicited by pressure
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").
Supplemental material on the site DVD: Movie of
nystagmus elicited by sound. For other examples see the page on Tullio's
(note that we did not indicate that hearing loss is a symptom of dehiscence).
Dehiscence, being a bone defect, is nearly always diagnosed using a high resolution temporal bone CT scan. Other tests, not involving X-rays, may provide a clue that an temporal bone CT is indicated.
Tests that may be helpful in the office (Valsalva is the best) are as follows:
Laboratory tests that may be helpful (VEMP is most useful) are the following:
Of the office based tests, the Valsalva is the best.
Valsalva test:
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). 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). 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.
For those familiar with 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), horizontal nystagmus can be produced. In persons with window fistulae, generally very little nystagmus is produced by Valsalva.
nystagmus elicited by Valsalva in person with L Superior Canal Dehiscence (51 meg)
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. Simple observation of the patient's eyes with appropriate equipment (such as VNG) may also provide the diagnosis, as in some cases, there is a pulse-synchronous oscillation (Rambold, 2001).
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| 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 ! |
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Figure 5 left: VEMP 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. |
VEMP's are very useful in dehiscence syndromes because they quantify sound sensitivity. 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 VEMP (figure 5 left) or lower threshold (figure 5 right) is the abnormal side.
Audiometry is generally done as a preliminary test, 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). 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.
Also, tympanometry may induce dizziness, which may lead to the diagnosis.
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 "ECOG", or electrocochleography may be of help also, although only in rare instances. The main role of ECOG is to diagnose Meniere's disease, which is a common alternative source of pressure sensitivity. ECOG 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.
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| Figure 4: Coronal CT scan of the temporal bone clearly showing missing bone at the top of the left anterior (superior) semicircular bone. |
A CT scan of the temporal bone should generally be obtained in persons with sound or pressure sensitivity. CT of the temporal bone is supposedly very accurate in identifying canal fistulae (Fuse et al, 1996), although as there is really no other good way to identify canal fistulae, it is hard to be sure that it is picking them all up. As SCD is a type of canal fistula and it is moderately common, the main reason for this procedure is to check for SCD. CT should be done of the temporal bone with at .6 mm resolution or better (lower is better).
Direct coronal scans are the best protocol for SCD -- in our opinion, reformatted images are not nearly as good. At this writing (2006) all University hospitals should be able to do a direct coronal CT. It is very important to insist on this -- radiology technicians have been known to do reformatting, figuring that it doesn't matter. It does matter ! Usually you only get one shot at this, so make sure it is done right the first time.
In our opinion, this is what your test prescription should say:
CT scan of the temporal bone, with high resolution (0.6 mm or less). Please do direct coronals, using a bone dedicated window (width 1600 HU and center at 400 HU).
Axial cuts are not necessary to diagnose SCD. Nevertheless, at our instititution, the radiologists like to do them anyway. One could argue that without axial cuts, the very rare PCD syndrome might go undiagnosed. On the other hand, one could also argue that the axial cuts are generally not necessary, and add cost and radiation to the patient experience. At this writing, this issue is simply not resolved.
An MRI is also a good idea to exclude potentially confounding entities such as cholesteatoma or tumor. MRI is not a test for dehiscence because it doesn't show the bone and resolution is not as good as CT scan. However, MRI is the best way of showing other possibly confounding problems such as acoustic tumors, cholesteatoma, or multiple sclerosis plaques.
This test was described very recently 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 or a very low amplitude response of < 0.25 deg at 110. The latency was 8 msec. This test is not generally available, but appears promising.
Basically, one can either do nothing and avoid things that make you dizzy, or you can get an operation.
Conservative approach: Dehiscence is not likely to resolve on it's own, so the real choice is between adjusting ones life, or having surgery done.
These are the things you might wish to do to mitigate symptoms:
1. Avoid loud nose -- for example, if you play a musical instrument, better find another hobby or get used to being dizzy.
2. Avoid pressure fluctuations between your ear and the rest of your body -- this isn't easy, as nearly any type of exertion has the potential for producing pressure fluctuation. It is not that there is a danger from this activity -- it just may give make you dizzy. Weight lifting, straining to do things, strenuous sexual activity - -these are all things that might cause trouble.
3. Avoid pressure fluctuations between your middle ear and external ear -- in other words, avoid situations where your ear might pop. Again, there is no real danger (other than that of falling or driving off the road), but you might get pretty dizzy.
On airplanes, our patients have indicated that ear plugs are often helpful in this situation also. The "ear plane" ear plugs are designed to reduce pressure fluctuation, and may be useful. If these are not possible or available, we suggest using a nasal decongestant at least one half hour prior to landing.
4. A ventilation tube may help. This positive effect is due to reduced movement of the tympanic membrane, ossicular chain, and stapes footplate, and there bye reduction of pressure on the middle ear.
5. Medications are not terribly useful, but those in the benzodiazepine family are sometimes helpful.
Case example: An otherwise healthy man developed positional vertigo. On examination he was noted to have both a positional nystagmus (downbeating) as well as pulse-synchronous nystagmus. A VEMP was abnormal and this was followed by a temporal-bone CT scan which documented clear superior canal dehiscence. As his symptoms were minor, he opted to do nothing.
Surgical treatment :
The treatment of a dehiscence generally involves either closing the dehiscence or plugging of the canal. This is appropriate, for example, in superior canal dehiscence. Results are claimed to be good (Mikulec et al, 2005), although in a condition like this without clear cut objective endpoints, it might be difficult to be sure.
The closing of the dehiscence (roofing) appears to be the riskier procedure -- sometimes dura is stuck to the membranes of the inner ear and an attempted repair results in deafness instead. At this writing (2006), a combined procedure -- "roofing", and "plugging" is favored.
Some authors report large series of operated patients. For example, Hillman and associates (2006) reported 30 patients, in whom 14 were operated. In our own practice in Chicago Illinois, only about 1/5 patients are operated. Patients have simply generally not opted for surgery because of the risk of hearing loss.
While the diagnosis of SCD has become much easier in recent years, treatment has lagged behind. More work is needed to work out the best approach for treatment.
Acknowledgement: Figure 1 is courtesy of Northwestern University.
| © Copyright May 22, 2008 , Timothy C. Hain, M.D. All rights reserved. Last saved on May 22, 2008 |
