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Superior Canal Dehiscence

Timothy C. Hain, MD   Page last modified: December 13, 2013

New: MRI scans can diagnose SCD.

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, perhaps because fluid is generally not in direct communication with an air-filled cavity, but a membrane is present which maintains a seal. Dehiscence is a rare condition compared to most other causes of dizziness.

SCD R
Figure 2a. Coronal thin cut CT scan showing superior canal dehiscence (SCD). This patient was reported in detail in (Ostrowski, Hain and Wiet, 1997) A much larger (and newer) temporal bone CT scan showing dehiscence of the R SCC, as pointed to by the green arrow. Image courtesy of Dr. Marcello Cherchi. This patient also had SCD on the other side as well as low threshold VEMP tests. The VEMPs were the trigger to get the CT-Temporal bone. Without them, this diagnosis would not have been made.

There are several types of dehiscence

Superior canal dehiscence:  Case 1: 

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). It is thought that there is a failure of postnatal bone development, resulting in thin roofs of the superior canals. The thin bone is worn down by age, and can be broken by minor trauma (Teixido et al, 2012). Occasionally, the bone over the superior canal is removed surgically, when there is an attempt to "blue line" the labyrinth. This surgical procedure can create a dehiscence.

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).  As people age, the frequency of X-ray findings of SCD increases (Nadgir et al, 2011). This suggests that SCD is at least part an acquired condition, and also that it should get worse with age.

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.

Mastoid dehiscence -- lateral canal dehiscence

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). It has also been reported to occur without any clear source (Zhang et al, 2011)

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. In Fenestration cases, nystagmus induced by straining is nearly always horizontal.

Fenestration Supplemental material on the site DVD: Movie of nystagmus elicited by Valsalva in person with fenestration

Posterior canal dehiscence

Ant and PC
Case with both anterior and posterior canal dehiscence (courtesy of Dr. Dario Yacovino). These are oblique cuts as described below.

Posterior canal 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 with axial cuts, or with cuts in the plane of the PC. The image above shows an example of a person with dehiscence in both the AC and PC, and cuts in the plane of the canals.

We have encountered a similar case in which there was both SCD and PCD on the same side, associated with exercise intolerance, conductive hyperacusis and an enlarged VEMP. In this case, the radiologist suggested that the PCD was due to an "aggressive arachnoid granulation". One would think instead that this highly unlikely jutaxposition of dehiscences in two different canals would be due to a congenital maldevelopment of the inner ear.

Cholesteatoma

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.

Other dehiscences

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).

A case of a carotid-cochlear dehiscence was reported by Lund and Palacios (2011). Of course, this type of dehiscence is associated with pulsatile tinnitus.

Symptoms of a Dehiscence syndrome

Dizziness:

Usually there is an unsteadiness which increases with activity and which is relieved by rest.

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 due to sampling bias (i.e. this isn't true).

Supplemental material on the site DVD: Movie of nystagmus elicited by Valsalva in person with Superior Canal Dehiscence (51 meg)

Pressure sensitivity:

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.

Supplemental material on the site DVD:

Sound sensitivity

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

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, the voice is "boomy", as if in a barrel. This is due to a longer resonant cavity in the middle ear.

Other auditory symptoms

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.

How does the doctor know if I have a 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). 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.

nystagmus elicited by Valsalva in person with L Superior Canal Dehiscence (51 meg)

R SCD nystagmus elicited by Valsalva in person with R Superior Canal Dehiscence -- figure 2b (2 meg)

Fenestration Supplemental material on the site DVD: 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).

Other office based tests:

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.

Vibration
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 insensitive.

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 sign is not unusual, but it usually 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.

SCD pulse synchronous nystagmus movie Supplemental material on the site DVD: ----Pulse synchronous nystagmus in SCD

SCD pulse synchronous nystagmus movieSupplemental material on the site DVD: ----Pulse synchronous nystagmus in R SCD -- figure 2b

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 !

 

scd audio
Figure 3b: Conductive hyperacusis in patient with bilateral SCD. This can easily be missed as audiologists just assume that one can't hear better than '0' dB, and don't always test the patient thoroughly. In other words, thresholds can be better than 0.

 

 

Laboratory based tests:

VEMPS (Vestibular evoked myogenic responses)

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.

VEMPs (and here we mean threshold VEMPs) 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 VEMP's are specific. We have never encountered a person with a positive threshold VEMP that did not have SCD.

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). 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.

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 "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.

Figure 4: Coronal CT scan of the temporal bone clearly showing missing bone at the top of the left anterior (superior) semicircular bone.

R SCD movie of X-ray of SCD (contributed by Dr. Dario Yacovino) (4 meg)

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.

Before beginning this discussion - -note that temporal bone CT scans are "high radiation" procedures because enough Xray energy must be used to "see" into a very hard bone (temporal bone). All Xrays increase cancer risk. Accordingly, CT scans should not be done to "screen" for SCD. Safe tests such as the VEMP should be used first. They also should not be done in a "successive approximation" mode -- i.e. you don't start with a poor scan, and then get a good one. If you are going to do it - - do it right the first time. Be extra cautious in scanning children, as radiation exposure increases cancer risk, and children have a long life expectancy (Tunkel et a, 2012).

CT should be done of the temporal bone with at 0.6 mm resolution or better (lower is better). It may be impossible to get a CT scan with a resolution < 0.6 mm. This is generally OK, but don't accept lower resolution (i.e. 1 or 1.25 mm is not good enough).

Conventional CT scans of the brain are nearly always useless to diagnose SCD as their cut resolution is 8-10mm -- this is almost as big as the entire inner ear ! There is also a trade-off between radiation and resolution. One might argue that the tiny lesions that can be discovered with 0.1 mm cuts are not worth the radiation load. This issue is presently unclear. Although resolution is not quite as good as with direct coronal, the radiation load is half compared to the protocol that combines direct axials and coronals, and we think that this is a reasonable compromise.

 

SCD R
Figure 5. Temporal bone CT scan with images taken in plane of superior canal. There is a wide area of dehiscence seen at the top.

 

Reformatted sagittal views are essential for the situation where there is no direct coronal. Coronal reformats are a bare minimum. Better are oblique reformats parallel to and perpendicular to the plane of the superior semicircular canals.

Because the posterior canal on each side is perpendicular to the ipsilateral superior canal, oblique reformats allow the clinician to check for dehiscence in all canals.

In our opinion, this is what your CT-temporal bone prescription looking for dehiscence should say:

CT scan of the temporal bone, with high resolution (0.6 mm or less). Direct axial, with reformatted coronal and oblique views parallel to and perpendicular to the plane of the Superior Semicircular Canals.

DO NOT PROCEED If you cannot perform a CT scan of the temporal bone with a resolution of 0.6 mm or less. In this case, send the patient home so that an adequate scan can be done elsewhere.

MRI for SCD

MRI SCD

A properly done MRI can rule out SCD, as well as point strongly towards SCD (Browaeys et al, 2013). The method is to use a newer 3T scanner, and obtain T2-Coronal views. On T2, the fluid filled semicircular canals stand out from the bone, and one can usually see that the canal goes "right to the dura" -- in other words, there is no dark bone between the loop of the canal and the brain. The image above shows a man with SCD identified from his very large VEMP on the right side. His CT-temporal bone showed SCD.

An example of a coronal with no SCD is here (T2, 4 mm cut size): Note that there is space between the loop of the canal and the brain. Thus no SCD.

Coronal no SCD

The huge advantage of using MRI is that there is no radiation. Temporal bone CT scans need a lot of X-ray radiation to see through the hard bone of the temporal area. On the other hand, MRI is more expensive and slower. We think MRI is underutilized, as most patients get MRI's for other purposes. We suggest that the proper MRI should be a 3T (high field) MRI, with T2 direct coronal cuts, with resolution ideally of 3 mm. Browaeys et al (2013) used higher resolution Fiesta imaging in a lesser field strength scaner (1.5T). We are not sure which is better, but we think the 3T T2 images are very reasonable.

An MRI is also useful to exclude potentially confounding entities such as cholesteatoma or tumor. MRI is not as good a test for dehiscence as a temporal bone CT because it doesn't show the bone and the resolution is not as good as a temporal bone CT scan. However, MRI is the best way of showing other possibly confounding problems such as acoustic tumors, cholesteatoma, or multiple sclerosis plaques. Note that "basic" CT scans such as are done in emergency rooms are always useless for diagnosis of SCD.

Emerging tests:

Click-evoked VOR

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 response 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.

How are Dehiscence Syndromes Treated?

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.

Patients with dehiscence may wish to avoid:

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 therebye 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 (see video above). 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 of Superior Canal Dehiscence

Vlastaros et al reviewed studies of treatment (2009). The treatment of a dehiscence generally involves either closing the dehiscence (resurfacing or capping) 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 using a bone graft (roofing, resurfacing) 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. Roofing (resurfacing) can also lead to recurrent symptoms due to shifting or resorption of the bone. For these reasons, at this writing (2010), "plugging" or "capping" is favored (Vlastarkos et al, 2009). This is an evolving situation as of 2013, and the surgeon's experience with the technique must also be considered.

With respect to capping, the opening is closed with cement. This procedure is intermediate in effectiveness compared to plugging.(Vlastarkos et al, 2009). One would think that it would be equally risky. On the other hand, cement might be more durable than bone.  Plugging and capping would seem even more logical.

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, Chicago Dizziness and Hearing in Chicago Illinois, we have accumulated 30 SCD patients in our database over the last year. We diagnose a new patient based either on office exam (Valsalva) or VEMP results followed by a temporal-bone CT scan (see comments). However, only about 1/5 patients are operated. Patients have simply generally not opted for surgery because of the risk of hearing loss, and also anxiety about the craniotomy often recommended for the "roofing" or "capping" procedures. Patients more likely to opt for surgery are those with prominent auditory symptoms (e.g. hearing their voice in their head), or prominent dizziness (e.g. dizziness when doing something as simple as burping).

Patients who undergo surgery are unsteady for roughly 6 weeks after the repair (Janky et al, 2012).

New approaches (not recommended):

Round window plugging is a newly proposed surgery for SCD. The basic idea is that pressure changes in the inner ear can cause fluid shifts only if there there is a place for the fluid to go. There normally is a pressure relief part of the inner ear -- called the round window. When the stapes moves inward, the round window moves outward. When there is abnormal pressure presented to the opening in SCD, presumably the round window also moves back and forth and faciliates fluid movement. This is all quite logical. Several otolaryngologists have suggested that closing the round window, might be a successful treatment of SCD. The advantage of this method is that it is much less invasive than either the plugging or resurfacing approaches.

Recently, Dr. Silverstein and Van Ess reported a single case in which this approach was successful (Silverstein and Van Ess, 2009). The operative approach was do to a very thorough closure of the round window, using 3 layers. This is essentially what is done in perilymph fistula surgery, but using a more vigorous approach. It is too soon to know whether this method will be adopted. One wonders whether or not it might be reasonable to use an even more aggressive approach and "cement" the round window shut.

We don't recommend either of these procedures.

Research:

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.

 

References:

Other resources (off site link):

 

Copyright October 6, 2013 , Timothy C. Hain, M.D. All rights reserved. Last saved on October 6, 2013