Timothy C. Hain, MD • Page last modified: August 22, 2020
New: we split up this very long web page into 4 as of 12/29/2019.
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.
|A temporal bone CT scan showing dehiscence of the R SCC, as pointed to by the green arrow. Image courtesy of Dr. Marcello Cherchi.|
There are several situations in which the inner ear membranes may be uncovered by bone. Superior canal dehiscence is the most common. 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. In our practice database, we have diagnosed SCD in 154 patients (as of 2018) out of a total of 27,000 patients. This compares to about 5000 patients with BPPV. Thus SCD is much less prevalent than BPPV (which affects 2% of the population). On the other hand, SCD is "forever" while BPPV comes/goes.
It is known that about 2% of the population has thinning of the bone that can lead to SCD. From our clinic numbers, 154/27,000 patients is about 0.5% of dizzy patients. Of course, this is an immensely rough estimate as it depends on many uncontrolled variables. SCD is more commonly diagnosed in women (1.2:1) and is also more common on the left side (Karimnejad et al, 2016).
SCD never gets better -- it only stays the same or gets worse (without surgery). Radiology studies show that the thickness of the bone over the SCC (Superior semicircular canal) decreases with advancing age as well (Davey et al, 2015).
|Figure 2a. Coronal thin cut CT scan showing superior canal dehiscence (SCD). This patient was reported in detail in (Ostrowski, Hain and Wiet, 1997)||Some patients have bilateral SCD. This pair of coronal images shows a wide open R SCC, and a partially open L SCC. Cervical VEMP on the left had a threshold of 55. Note also the thinning of the tegmen (see below).|
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.
The tegmen is the roof of the mastoid sinus. It lies between the brain and the open space of the mastoid cavity. It is very well established that patients with SCD on temporal bone CT, frequently have thin or no bone at all between the mastoid cavity and the dura of the brain. This makes them more susceptible to CSF leaks, and presumably the greater compliance of the cranial cavity may also predispose them to complications related to the brain. On some of the images above, it is easy to see that the tegmen is thin or perhaps nonexistent. It is not unusual to see thin tegmen in persons with no SCD, but patients with SCD seem to have it a lot more than the normal population. Whyte et al (2016) wrote that tegmen dehiscence was about 20 times more common in SCD patients (2016).
The consensus right now in the literature is that these defects are likely part of a developmental process that also created SCD (Rodrigo et al, 2016).
Nadaraja et al (2012) observed that "Seventy-six percent (29/38) of patients with unilateral or bilateral SSCD had a radiographic TD on at least 1 side compared with 22% (9/41) of the comparison group. "
Rizk et al (2016) stated "Patients with SSCD have a marked thinning of the lateral skull base, more so than patients with spontaneous CSF otorrhea and control groups with different BMIs. "
Lim et al (2012) recommended "In cases of SSCD requiring surgery, we support the view that elective surgical repair be recommended where asymptomatic ipsilateral encephaloceles are found, to reduce the risk of otitic meningitis. " We would say that this is rarely done.
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.
Supplemental material Movie of nystagmus elicited by Valsalva in person with fenestration
|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. Like SCD, PCD also exhibits low frequency conductive type hearing loss. (Bear et al, 2015)
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 mal development of the inner ear.
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).
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.
There is some theoretical overlap between SCD and enlarged vestibular aqueduct syndrome and jugular bulb dehiscence, as in both situations there is increased compliance of inner ear fluid pathways. The overlap here seems to be more theoretical than real. It "walks like a duck, and quacks like a duck", but doesn't have feathers. These patients seem closer to Meniere's.