Introduction to superior semicircular canal dehiscence syndrome (SCDS)
Superior semicircular canal dehiscence syndrome (SCDS) is characterised by an opening in the bony covering of the upper(superior) semicircular canal in the inner ear. Merely detecting a dehiscence on a computed tomography (CT) does not automatically mean a diagnosis, as the syndrome refers to the specific clinical manifestations associated with this condition. The first description was by Lloyd Minor in 1998 (1).
While a dehiscence is seen in approximately 1% of the population, not all individuals with superior semicircular canal dehiscence experience symptoms. About 2% of the population has very thin bone covering the superior semicircular canal. In some cases, superior semicircular canal dehiscence can affect both ears, with symptoms typically more pronounced on one side. Diagnosis of superior semicircular canal dehiscence syndrome (SCDS) occurs around the age of 45, equally in males and females (2).
The exact cause of the dehiscence is unknown. Some individuals experience arrest of the bony development covering the superior semicircular canal in the first year of life. It is not linked to osteoporosis and is rare in children, suggesting that the condition may develop later in life as the thin bony plate covering the canal erodes over time due to a yet unknown cause (2).
Does loud sound make you dizzy? You may suffer from a superior semicircular canal dehiscence (SCD)
The concept of the third “mobile” window
The inner ear is a complex structure responsible for processing sound waves and transmitting them to the brain for interpretation. Within this delicate system, there are two main functioning windows known as the oval window and the round window.
The oval window serves as the primary entrance for sound energy into the inner ear. When sound waves enter the ear canal, they eventually reach the oval window and are transferred into the inner air. The sound energy then travels as a traveling wave around the cochlea, where it is converted into electrical signals that can be interpreted by the brain.
On the other hand, the round window acts as an exit point for sound energy that has been processed by the inner ear. As the traveling wave circulates around the cochlea, it causes the round window to bulge outward, allowing the energy to be released back into the external environment. This process ensures that the inner ear remains sensitive to new incoming sound waves.
In some cases, a third mobile window may be present in the inner ear, typically located in the superior semicircular canal (3). This additional opening can lead to sound energy escaping to the outside, creating an alternate route. Bodily sounds and pressure from the skull can also to enter the inner ear. When this occurs, patients may experience symptoms characteristic of superior semicircular canal dehiscence syndrome (SCDS).
Symptoms of superior semicircular canal dehiscence syndrome(SCDS)
Symptoms of superior semicircular canal dehiscence syndrome (SCDS) 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, heartbeat, breathing, eye movements, joint movements 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)
- Loud sounds and vibrations cause dizziness. (Tulio phenomenon) (4)
How is superior semicircular canal dehiscence syndrome (SCDS) diagnosed?
In order to diagnose superior semicircular canal dehiscence, the criteria set by the Classification Committee of the Barany Society must be met (5). This includes the presence of at least one of the following symptoms indicative of a ‘third mobile window’ in the inner ear:
- Bone conduction hyperacusis
- Sound-induced vertigo and/or oscillopsia that is time-locked to the stimulus
- Pressure-induced vertigo and/or oscillopsia that is time-locked to the stimulus
- Pulsatile tinnitus
Additionally, there must be at least one of the following signs or diagnostic tests indicating the presence of a ‘third mobile window’ in the inner ear:
- Nystagmus characteristic of excitation or inhibition of the affected superior semicircular canal triggered by sound, changes in middle ear pressure, or intracranial pressure
- Low-frequency negative bone conduction thresholds on pure tone audiometry
- Enhanced VEMP responses, such as low cervical VEMP thresholds or high ocular VEMP amplitudes (6)
Furthermore, high resolution temporal bone CT imaging with multiplanar reconstruction must show evidence of dehiscence of the superior semicircular canal, and the symptoms cannot be better explained by another vestibular disease or disorder.
Computed tomography for superior canal dehiscence syndrome (SCDS)
A standard CT scan (axial and coronal views) may not be sensitive enough to diagnose a superior semicircular canal dehiscence. The typical slice thickness of a conventional CT scan is greater than the required resolution for identifying such a small anatomical abnormality. The CT slices should be less than 0.6mm in thickness to visualise a superior semicircular canal dehiscence.
By using specialised techniques such as reconstructing the superior semicircular canal in the Poschl and Stenver view, the sensitivity of the CT scan can be significantly increased to more than 90%. This allows for a more accurate and reliable diagnosis of a superior semicircular canal dehiscence, essential for guiding treatment decisions and improving patient outcomes (7).
Cone beam CT scan causes far less radiation than conventional multi-slice CT (8).
What happens when you have some symptoms but do not meet the criteria of superior semicircular canal dehiscence syndrome (SCDS)?
The criteria for diagnosing superior semicircular canal dehiscence syndrome (SCDS) is not always straightforward and can sometimes require a multidisciplinary approach. Due to the varied nature of symptoms and overlapping features with other conditions, a “committee diagnosis” may be necessary, where specialists from different fields come together to review a patient’s case and collectively determine the best course of action (9).
It is important to note that even if a patient does not meet all the criteria for a formal diagnosis of superior semicircular canal dehiscence syndrome (SCDS), it does not mean that nothing can be done to help them. Dr. Hofmeyr’s experience in managing patients with hypersensitivity to sound (hyperacusis) using round window reinforcement surgery is a testament to the fact that individualized treatment options may still be available based on a patient’s unique presentation and needs. This personalised approach underscores the importance of considering all factors and exploring alternative strategies beyond strict diagnostic criteria in the management of complex conditions like superior canal dehiscence syndrome (SCDS).
Posterior semicircular canal dehiscence and lateral canal dehiscence syndrome
A dehiscence of any semicircular canal may evoke various auditory or vestibular symptoms (10). Posterior semicircular canal dehiscence and lateral semicircular canal dehiscence are rarer entities, occurring in less than 20% of all radiological dehiscences on CT. The symptoms may be indistinguishable from superior semicircular canal dehiscence syndrome (SCDS) but the direction of nystagmus may indicate which canal is affected. Posterior semicircular canal dehiscence syndrome is often associated with a paraganglioma tumour, high jugular bulb, fibrous dysplasia (a type of bone disorder) or surgical injury. Lateral semicircular canal dehiscence is often associated with chronic otitis media with cholesteatoma or surgical trauma.
Otic capsule dehiscence syndrome
Otic capsule dehiscence syndrome is characterised by hearing loss, vertigo, and tinnitus that suggest a third mobile window but occurs without radiological evidence of a defect in the otic capsule. The otic capsule is a structure in the inner ear that surrounds the cochlea and semicircular canals. In this syndrome, patients experience symptoms typically associated with a defect or abnormality in the otic capsule despite no visible abnormalities on imaging studies such as CT scans. The exact cause of otic capsule dehiscence syndrome is not well understood (11).
Management of superior canal dehiscence syndrome (SCDS)
The management of superior semicircular canal dehiscence syndrome (SCDS) includes observation, noise protection, avoidance of triggering factors such as loud noise and vibration and a surgical correction of the defect (12).
The 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.
Medication is of little help but may reduce symptoms temporarily.
Surgery can permanently solve the problem.
Surgery for superior semicircular canal dehiscence syndrome (SCDS)
The surgical procedures include:
- middle fossa approach (capping, resurfacing, occlusion or a combination)
- transmastoid approach (capping, resurfacing, occlusion or a combination)
- transcanal approach (reinforcement of the round window)
Surgical options for superior semicircular canal dehiscence syndrome (SCDS) have evolved over the years as our understanding of the condition has improved. One common surgical approach involves directly addressing the dehiscent semicircular canal. This can be done through a middle fossa approach, where the skull is opened above the affected ear, or a transmastoid approach, where the surgery is performed through the mastoid bone behind the ear. These surgeries aim to either cover the dehiscence with a piece of material, resurface the dehiscence or occlude the canal to prevent abnormal sound energy transmission through the canal.
Another surgical concept for treating superior semicircular canal dehiscence syndrome (SCDS) involves reinforcing the round window to restore a two-window system in the inner ear. This is done by approaching the round window through the ear canal and placing cartilage on it to help block sound transmission through the dehiscent superior semicircular canal. While this method is less effective than directly addressing the dehiscence, it can still relieve some patients (13).
Superior semicircular canal dehiscence syndrome (SCDS) questionnaire
To assess subjective symptoms and outcomes, I use a modified questionnaire based on Silverstein and colleagues’ work in my pre and postoperative evaluations (14). This psychometric scale helps patients define their symptoms and how much they are affected by them, aiding in setting postoperative goals. While the scale is unvalidated, it is an internal measurement tool for tracking my surgical results and improving patient care. By using this questionnaire, I can better understand and address patients’ needs, ultimately leading to more successful outcomes.
Dr Hofmeyr’s experience
Dr Hofmeyr has performed over 200 middle cranial fossa approaches for different reasons and over 50 specifically for superior semicircular canal dehiscence syndrome (SCDS).
He has also performed over 50 transmastoid approaches with excellant results and utilise this approach now as his preferred option.
Recently, Dr Hofmeyr abandoned the round window reinforcement procedure as it seems to be less effective.