Superior canal dehiscence (SCD)
In superior canal dehiscence (SCD) the uppermost semicircular canal of the inner ear, which is part of the organ for balance, has an opening, creating a direct connection between the intracranial space with the brain and the inner ear. The exact reason for this is not known but it may be due to trauma. The opening is also sometimes referred to as a third window. Patients may complain of hearing loss, balance problems, dizziness, tinnitus and other ear symptoms. Sometimes patients may complain that loud noise makes them dizzy (known as the Tullio phenomenon). Patients often complain that they can hear their own heartbeat, breathing, footsteps and even eye movements in their ear. SCD is diagnosed with hearing tests, vestibular (balance organ) testing and computed tomography (CT) scanning. This condition may mimic otosclerosis, which is another cause for hearing loss. SCD can be managed in different ways, but surgery is usually required.
Does loud sound make you dizzy? You may suffer from a superior canal dehiscence (SCD)
Superior semicircular canal dehiscence (SCD).
Superior canal dehiscence (SCD)
Superior canal dehiscence or superior canal dehiscence syndrome(SCD) is characterised by the absence of the bony covering of the uppermost (superior) semicircular canal of the inner ear resulting in an opening or hole (dehiscence) in the canal. The inner ear is then in continuation with the inside of the skull. It is rare for one of the other canals to be dehiscent.
This condition is present in up to 1% of the population but not all patients have symptoms. Of the population up to 2% have very thin bone overlying the superior canal. It occurs equally in the right and left ears and may involve both sides. In patients with involvement in both ears the symptoms are usually greater on one side. If a patient has a SCD on one side the bone over the other side may be very thin. The mean age of diagnosis is 45 years. It occurs equally in males and females.
The reason for the dehiscence is not known. The reason why some people with a SCD do not have symptoms is a mystery. A genetic cause is not established. It is extremely rare in children and suggests that something happens later in life over time that erodes the thin bony plate and cause the canal to open. A sudden increase of pressure in the head and head trauma are triggers that may open an already thin bone plate over the canal. It is not due to osteoporosis.
Symptoms of superior semicircular canal dehiscence(SCD) may include:
- Dizziness, vertigo and balance problems.
- Hearing loss.
- Hypersensitivity to sound(hyperacusis).
- Hearing of the own voice in the ear(autophony/autophonia)or other sounds like footsteps, heart beat, breathing, eye movements, joint movemnets and chewing.
- Fullness in the affected ear.
- Pulsating noise in the ear (tinnitus).
- Brain fog and fatigue.
- Inability to maintain a stable horizon when walking (oscillopsia).
- Dizziness caused by loud sounds and vibrations.(Tulio phenomenon)
How is SCD diagnosed?
In a patient with symptoms suggestive of SCD a high resolution computed tomography (CT) scan of the temporal bone will show the dehiscence. The CT scan needs to be performed in a specific way and reconstruction of the images into a 3D image is possible. Audiometry, acoustic reflexes, vestibular evoked myogenic potentials (VEMP) and nystagmography are commonly utilised. The Tullio phenomenon may be demonstrated in some cases.
Computed tomography (CT) scan showing a SCD (white arrow).
3D reconstruction of a CT scan showing a SCD (blue arrow).
A patient undergoing a VEMP test.
A VEMP graph. The VEMP may be elicited at a low threshold in a patient with a SCD.
Posterior semicircular canal dehiscence and lateral canal dehiscence
A canal dehiscence of any of the semicircular canals may evoke various auditory or vestibular symptoms. The bony dehiscence result in the formation of a third window into the inner ear. Posterior canal dehiscence and lateral canal dehiscence are rarer entities. Less than 20% of all radiological dehiscences on CT scan are due to these. The symptoms may be indistinguishable from SCD but the direction of nystagmus may give an indication of which canal is affected. PCD is often associated with a high jugular bulb, fibrous dysplasia (a type of bone disorder) or surgical injury. Lateral canal dehiscence is often associated with chronic otitis media with cholesteatoma or surgical trauma.
Posterior canal dehiscence (PCD)
Posterior canal dehiscence in contact with jugular bulb (yellow arrow).
How can SCD be managed?
Management of superior canal dehiscence(SCD) includes observation, noise protection, avoidance of triggering factors such as loud noise and vibration and a surgical operation.
Medication is of little help but may reduce symptoms temporarily.
Symptoms may stabilise or worsen over time. In a lot of cases it can lead to psychological problems and depression in patients. Patients may also be at risk of becoming dizzy, losing their balance, falling and injuring themselves and others when symptomatic.
Surgery can permanently solve the problem.
The different operations
The surgical operation can either be performed through the mastoid (back of the ear) or through the middle cranial fossa (above the ear). A neurotologist and neurosurgeon work together to offer the patient the best outcome.
The three types of operations in order of most to least preferred are:
Middle cranial fossa(MCF) approach
Reinforcement of the round window
The dehiscence in the canal can be plugged, resurfaced or capped. A combination of these can also be performed. The superior canal can also be blocked off (amputated).
Before surgery it is advised that the SCD questionnaire is completed.
Closing options for a semicircular canal dehiscence.
A superior canal dehiscence (PCD).
Reinforcement of the round window
The inner ear has two functioning windows, openings, that emit energy and eventually normal movement of fluid in the inner ear. A superior canal dehiscence is a “third window” causing all the problems. The theory behind partially closing, plugging or reinforcement of the round window with soft tissue and cartilage is that avoiding major surgery it closes off, one of the three windows, re-establishing a two window system. Unfortunately there is still not enough evidence either supporting or rejecting this procedure. It does, undoubtedly, have the potential of less side-effects than middle cranial fossa or transmastoid surgery. In my own experience of over 30 cases it is a definite option to consider in symptomatic patients.
Dr Hofmeyr’s experience
Dr Hofmeyr has managed more than 50 patients with superior canal dehiscence (SCD). In 2005 he diagnosed and operated the first patient in South Africa with SCD in collaboration with Dr Herman Hamersma. Dr Hofmeyr has performed over 200 middle cranial fossa approaches for different reasons and over 30 specifically for SCD. Recently Dr Hofmeyr has started performing the round window reinforcement procedure in some selected cases.
Evaluation and Management of Superior Canal Dehiscence Syndrome