- intense sound stimuli or
- changes in intracranial or middle ear pressure.
This pressure in the middle ear can be changed by a dehiscence of the bony layer that covers the superior semicircular canal, one of the three semicircular canals responsible for maintaining balance. However, although it is not so common, some individuals diagnosed with this syndrome only have hearing loss, and no imbalance symptoms.
Prevalence of the condition
The prevalence of complete dehiscence of the superior semicircular canal is approximately about 0.7% in the general population. However, not all patients with this syndrome have all the symptoms, so experts still don’t know the exact percentage of symptomatic patients among them. There is still no report of children being affected by the syndrome.
Mechanism of the condition
The cochleovestibular system of structures in the middle ear responsible for normal hearing and balance has two functional windows.
The oval window: Located at the footplate of one of the smallest bones in our body-the stapes, this window’s function is to allow sound to enter the inner ear (vestibule). This opening allows the mechanical wave to be transduced into neural activity, because it is the only way a sound can be perceived.
The round window: The function of this opening is a bit more controversial; it is considered to have several roles. Its first role is thought to involve the release of sound and mechanical energy from the tympani part of the ear. Another proposed role is its participation in the secretion and absorption of substances in the inner ear.
These two windows of the inner ear work together to regulate hearing and balance. It is important to understand that all this can work only as previously described. In case of a dehiscence in the superior semicircular canal, a third-window effect is thought to take place. When that happens, endolymph within the labyrinthine system continues to move in relation to sound or pressure, which causes an activation of the vestibular system.
Experts proposed several different theories regarding the cause of superior canal dehiscence syndrome. An embryological etiology of this syndrome has been proposed, and is generally considered the most probable. According to this theory, the condition is caused by a postnatal failure of bone formation over the superior semicircular canal. Researchers used a computer to make a simulation model, and determined that the cause of bony dehiscence of the superior semicircular canal was due to a mal-positioned primitive otocyst-cell that later forms the bone.
What are the symptoms of the dehiscence syndrome?
As previously mentioned, not all patients with this syndrome have the symptoms, but when they do, these are the most common:
Dizziness: The great majority of patients reports unsteadiness. This imbalance increases with activity and is relieved by rest. That’s why people often feel fine in the morning, but as the day passes, their balance gets progressively worse. Besides these balance problems, some patients also report a ringing in the ears. Other patients experience problems only when coughing, sneezing, or blowing their noses. This form of disease is tentatively named "valsalva-induced dizziness".
Pressure sensitivity: It is quite normal for a person to feel altered pressure sensations when moving from a high-pressure area to a low-pressure one. Such sensations are usually felt in airplanes or after diving. The changes in air pressure that occur in the middle ear normally do not affect the inner ear. However, in the case of dehiscence syndrome, changes in the middle ear pressure will directly affect the inner ear, stimulating the balance and causing typical symptoms.
Sound sensitivity: Most patients also report a special kind of sound sensitivity. It is not unusual for patients with this syndrome to notice that the use of one’s own voice or a musical instrument will cause dizziness.
Diagnosis of dehiscence syndrome
Ideally, dehiscence syndrome diagnosis is done using a high resolution temporal bone CT scan.
Other tests available are:
- Valsalva test
- Tullio test (done in office, not very sensitive)
- Fistula test (best done in office, but can be done in a lab)
Laboratory tests that may be helpful include:
- MRI scan
- Temporal bone CT scan, high resolution
- VEMP (vestibular evoked myogenic potentials)
Treatment of dehiscence syndrome
In most cases, the treatment is not necessary but surgical care is sometimes needed; it is reserved for patients with severe disabling symptoms. There are several forms of operation but these two are the most common: Middle fossa craniotomy and repair of fistula, and the transmastoid superior canal occlusion.
Middle fossa craniotomy and repair of fistula
The patients undergo a middle cranial fossa craniotomy on the affected side after which the temporal lobe is gently retracted. A dehiscence of the superior semicircular canal is typically covered with bone wax, bone cement, or fascia.
Transmastoid superior canal occlusion
This procedure is also an effective surgical method of treatment. First, mastoidectomy is being done, meaning a removal of the bone located behind our ears. After this, the superior semicircular canal is identified near the ossicular heads. The superior semicircular canal is then ablated with a combination of tissue and fascia, like in the first form of operation described above.
The success rate in the treatment of superior canal dehiscence is quite high, about 95%. A control study with 20 patients showed remarkable results. Nine patients had canal plugging and eleven had resurfacing procedures. Complete resolution of all vestibular symptoms and signs was achieved in 8 of the 9 patients after the canal was plugged. Seven of the 11 with resurfacing procedures had resolution of their vestibular complaints.
- Avoid loud nose - no playing instruments or loud singing.
- Avoid pressure fluctuations between the ear and the rest of the body. This includes weight lifting, straining to do things, or even strenuous sexual activity.
- Pressure fluctuations between the middle ear and external ear should also be avoided. Patients should avoid situations where the ear might pop; for example, wear ear plugs or a nasal decongestant when traveling by plane.
- Medications that can relieve symptoms are not very helpful in most cases, but sometimes benzodiazepines can help.