Superior Canal Dehiscence Syndrome

What is Superior Canal Dehiscence Syndrome (SCDS)?

SCDS is a condition where there's an abnormal opening in the bone covering the superior semicircular canal in the inner ear. This canal is part of the vestibular system, which helps us balance and maintain our spatial orientation. The condition was first identified by Dr. Lloyd Minor in 1998.

How Common is SCDS?

While the anatomical defect is found in about 1% of the population, not everyone with this defect experiences symptoms. The actual number of people with SCDS symptoms is much lower. It affects both men and women equally and can occur at any age, though symptoms often appear in the fifth and sixth decades of life.

Anatomy of the Semicircular Canals

The inner ear contains three semicircular canals: superior, posterior, and lateral. These canals are filled with fluid and are crucial for maintaining balance, as they sense angular (rotation) acceleration. They are part of the vestibular system, which works with the cochlea to help us hear and maintain our balance.

How They Work:

• The superior semicircular canal is located vertically and helps detect movements in this plane. It's positioned to sense rotational movements that occur in a vertical axis.
  
  
• The posterior semicircular canal is also vertical but oriented differently to detect other types of movements. It helps with balance by sensing movements in a different vertical plane.
  
  
• The lateral semicircular canal is positioned at a slight angle to the horizontal plane. It senses rotational movements in the horizontal plane, which is important for maintaining balance while walking or running.

How Does SCSD Affect the Ear? (Pathophysiology)

In SCDS, the abnormal opening in the superior semicircular canal acts as a "third window" in the inner ear. Normally, there are only two windows: the oval window for sound entry and the round window for sound exit. This extra window allows sound and pressure to enter and exit the inner ear abnormally, leading to the symptoms of SCDS.

How Symptoms Occur:

• Abnormal Sound Transmission: Sound vibrations can travel through the dehiscence, stimulating the vestibular system abnormally. This is why patients might experience vertigo or oscillopsia (blurry vision) when they hear loud noises. By the same token, sounds reaching the eardrum and would normally entirely be transmitted to the inner ear will in part escape through that 3rd window (the dehiscence) leading to hearing loss.
  
  
• Pressure Changes: Changes in pressure, like those from sneezing or straining, can also affect the vestibular system through the dehiscence, causing dizziness or blurry vision.

What Causes The Bony Defect To Begin With? (Etiology of SCDS)

The exact cause of the bony defect in SCDS is not fully understood, but it's thought to be a mix of congenital (present at birth) and acquired factors. Some possible causes include:

• Congenital Factors: The bone over the superior semicircular canal might not develop properly. This could be due to genetic factors or developmental issues during fetal growth.
  
  
• Acquired Factors: Head trauma or increased intracranial pressure might contribute to the development of SCDS. Elevated intracranial pressure is more common in overweight individuals. Over time, the bone covering the canal might thin out, leading to a dehiscence. I often explain to my patients that it's similar to how ocean waves gradually wear down rocks on a beach. Positioned just above the superior canal is the brain, which pulses with each heartbeat. Due to the substantial blood volume it holds, these pulsations are quite pronounced, just like big waves. Over time, they will wear away the already thin bone covering the superior canal, much like how waves smooth and erode rocks.

What Are the Symptoms of SCDS?

People with SCDS often experience a range of unusual symptoms that can significantly impact their quality of life. These symptoms are due to the abnormal transmission of sound and pressure through the dehiscence in the superior semicircular canal. Each individual will have a different combination of symptoms and severity.

• Hearing Loss: SCDS can cause a conductive hearing loss, particularly at lower frequencies. This occurs because the dehiscence acts as a "third window" in the inner ear, allowing acoustic energy to escape away from the cochlea. Read more about the types of hearing loss here.
  
  
• Hyperacusis to Bone-Conducted Sound (and Autophony): This means to hear certain sounds much louder than normal. Bone-conducted sounds are noises that travel through the bones in your head, like your own footsteps or the sound of chewing. Autophony is when you hear your own voice, breathing, or even eye movements unusually loud or distorted. These symptoms happen because SCDS creates an extra opening in the inner ear, allowing sounds to travel in ways they shouldn't. As a result, normal body sounds become amplified and can be very bothersome. You can read more about autophony in this article.
  
  
• Sound-Induced Vertigo and/or Oscillopsia: Feeling dizzy or having blurry vision when you hear loud noises. This can happen with sounds like a car horn, a loud voice, or even music. The vertigo can be severe enough to cause nausea or loss of balance, while oscillopsia can make it difficult to focus on objects. It is important to note that the vertigo and the oscillopsia will be very brief in duration (a few seconds).
  
  
• Pressure-Induced Vertigo and/or Oscillopsia: Feeling dizzy or having blurry vision when there are changes in pressure, like when you sneeze, strain, or even change altitude. This can be very disorienting and affect daily activities. For instance, a person might feel dizzy when they blow their nose or lift heavy objects. The sensation is also very brief in duration.
  
  
• Pulsatile Tinnitus: Hearing a ringing in your ears that sounds like a heartbeat. This can be very distressing and disrupt sleep. The sound should be synchronized with the heartbeat. This is different from non-pulsatile tinnitus (ringing or buzzing) with would not be caused by SCDS. I mention this because it is important for patients to realize that their ringing will usually not be cured by surgery even if they have a SCDS.
  
  
• Ear fullness: Due to the nature of the opening that puts the inner ear in contact with the brain compartment, patients may report an ongoing fullness in the ear.
  
  
• In some patients, we see a range of neuro-cognitive symptoms such as memory problems, "brain fog", headaches, non-specific dizziness and imbalance. It is generally accepted that SCDS is not the cause of these symptoms. These patients often have another concomitant disorders such as migraines, concussions, anxiety, PPPD (persistent postural-perceptual dizziness), or another medical diagnosis. Providing appropriate guidance to these patients and setting realistic expectations is crucial, as surgery is very unlikely to resolve these symptoms. See the section on treatment below for more information.

Physical Exam Findings

• Tuning Fork Tests: These tests can show unusual hearing patterns. For example, patients might hear sounds better through bone conduction than air conduction in certain frequencies. In that test, called the "Rinne" test, the tuning fork is placed on the bone behind the ear for a few seconds than placed in front of the ear. The patient will report hearing better when the tuning fork was placed behind the ear. This is known as a "bone conduction over air conduction" phenomenon, which is abnormal and can indicate SCDS.
  
  
• Tullio Phenomenon: Vertigo or nystagmus (uncontrolled eye movements) triggered by sound. This is a key indicator of SCDS. During the exam, the patient might be exposed to loud noises to observe if they experience vertigo or nystagmus.
  
  
• Fistula Test: Nystagmus triggered by pressure changes. This test involves changing ear pressure to see if it causes eye movements. The patient might be asked to perform a Valsalva maneuver (like blowing against a pinched nose) to change the pressure in the ears, or the doctor can manually generate pressure by pressing on the tragus (the cartilage in front of the ear).
  
  
• Nystagmus: Abnormal eye movements in response to sound or pressure. This can be observed during the physical exam. The presence of nystagmus in response to sound or pressure changes is a strong indicator of vestibular dysfunction.

How Do We Diagnose SCDS?

Diagnosing SCDS involves several steps:

1. Patient History and Physical Exam: Described in the sections above. This is of course the most important step.
   
   
2. Audiogram: Shows how well you can hear different sounds. In SCDS, the audiogram might show low-frequency conductive hearing loss or unusually "good" bone conduction thresholds (better than the average ear, which is a consequence of the conductive hyperacusis decried in the section above). This is because the dehiscence affects how sound is transmitted through the inner ear. Read more about audiograms here.
   
   
3. VEMP (Vestibular Evoked Myogenic Potentials): Measures the response of muscles to sound and vibration, which can be abnormal in SCDS. In SCDS, the VEMPs typically show usually low stimulation threshold and high amplitudes of the recording, both indicating a 3rd window effect. Read more about VEMPs here.
   
   
4. vHIT (Video Head Impulse Test) and VNG (Videonystagmography): Evaluate how well your vestibular system is working. These tests assess eye movements and vestibular reflexes to determine if there's any dysfunction. The vHIT and VNG are typically normal in SCDS. They are often done to rule out other anomalies, and also as a pre-operative baseline evaluation of the vestibular system.
   
   
5. High-Resolution CT Scan of the temporal bones: Essential for confirming the dehiscence with detailed images. The scan is done with special reconstruction cuts (called Pöschl's and Stenver's views) to clearly visualize the superior semicircular canal. This imaging is crucial for diagnosing SCDS and planning surgical intervention. Be careful, as a standard scan without reconstruction might lead to an overdiagnosis of SCDS. In some cases, a dehiscence appears on a regular scan, but upon reconstruction using the two specified views, it becomes clear that it is not present.
   
   
6. MRI: It is usually not required for the diagnosis of SCDS.

Differential Diagnosis (What else could cause similar symptoms)?

SCDS symptoms can be confused with other conditions that affect the ear and balance. As we discussed in previous section, accurate diagnosis requires a combination of clinical findings and diagnostic tests.

Common Similar Conditions:

• Otosclerosis: A condition affecting the middle ear that can cause hearing loss and rarely balance issues. However, otosclerosis typically involves a different set of symptoms and diagnostic findings, such as a fixed stapes bone. Read more about otosclerosis here.
  
  
• Patulous Eustachian Tubes: A condition where the Eustachian tube remains open, causing hearing symptoms. This condition often involves autophony just like SCDS but is not typically associated with vertigo.
  
  
• Ménière's Disease: A disorder of the inner ear that affects balance and hearing. Ménière's disease involves episodes of vertigo, hearing loss, tinnitus, and ear fullness, but it does not typically cause sound-induced vertigo or the specific audiometric findings seen in SCDS.
  
  
• Perilymphatic Fistula: A condition where there's an abnormal connection between the inner and middle ear, leading to balance and hearing symptoms induced by pressure or sounds just like SCDS. There is however no autophony and no conductive hearing loss. It is often associated with a history of trauma or surgery.
  
  
• Vestibular migraines: Vestibular migraines and SCDS often present with overlapping symptoms, making diagnosis challenging. Both conditions can cause vertigo, dizziness, and sensitivity to sound. The vertigo in SCDS is very brief however, and very specifically triggered by sounds and pressure, which is a feature that can help distinguish it from migraines. Patients with either condition may experience nausea, unsteadiness, and a feeling of fullness in the ears. In fact, it's not uncommon for individuals to have both vestibular migraines and a superior canal dehiscence on CT scan. However, as we mentioned earlier, it's not because there is a dehiscence on a scan that there is necessarily the clinical syndrome of superior canal dehiscence. The similarity in symptoms can lead to misdiagnosis or delayed diagnosis, as healthcare providers may initially focus on one condition while overlooking the other. This overlap highlights the importance of comprehensive evaluation and consideration of multiple conditions when patients present with complex vestibular symptoms. It is very important to try a medical treatment for migraines in these individuals before planning a surgical repair of the SCDS.

Importance of Accurate Diagnosis:

Accurate diagnosis is crucial because each of these conditions requires different treatments. For example, SCDS often requires surgical intervention, while Ménière's disease might be managed with medication or lifestyle changes.

Treatment Options for SCDS

Treatment for SCDS often involves surgery to repair the dehiscence. The goal is to close the abnormal "third window" and alleviate symptoms.

Surgical Approaches:

• Middle Fossa Repair: This is the most common method, involving a craniotomy (a window opening in the skull) to access the superior semicircular canal. The bony defect is usually plugged and the area along the skull base repaired. This approach is the most effective but is more invasive because we need to open the skull. This is the method I use.
  
  
• Transmastoid Approach: Less common, this involves accessing the canal through the mastoid part of the temporal bone. It's less invasive but may have higher recurrence rates because you cannot directly correct the area of the bony defect.

What Are The Outcomes (and Risks) of Surgery?

• Overall Success: Surgery is generally highly successful, with around 90% of patients seeing improvement or resolution of their main symptoms.
• Symptom Relief: Both auditory (hearing-related) and vestibular (balance-related) symptoms improve significantly for the majority of patients. 
• Hearing: Hearing usually improves and the autophony resolves. 
• Risks: There are risks, including hearing loss, facial paralysis and CSF leaks, but these are relatively rare, and recurrence rates are low.
  

Some people report neuro-cognitive symptoms such as memory problems, "brain fog", headaches, non-specific dizziness and imbalance. It is generally accepted that SCDS is not the cause of these symptoms, and they are usually the least responsive to surgery. These patients often have another concomitant disorders such as migraines, concussions, anxiety, PPPD (persistent postural-perceptual dizziness), or another medical diagnosis. Providing appropriate guidance to these patients and setting realistic expectations is crucial, as surgery is very unlikely to resolve these symptoms. Unfortunately, it is quite common in my practice to see patients who, after receiving the diagnosis of SCDS on a CT scan, have had all their neuro-cognitive symptoms attributed to SCDS by other healthcare providers (such as neurologists, family physicians or physiotherapists). These practitioners often refer patients to otologists like myself, with the expectation that surgery will resolve all issues. As a result, patients often come to our consultations with unrealistically high expectations and may leave feeling disappointed. In these patients, I always advocate for a trial of a medical treatment for their concurrent diagnosis (example a trial of migraine treatment) before considering surgery. 

Living with SCDS - Tips and Tricks If Surgery Is Not For You

For individuals with (SCDS) who prefer to avoid surgery, there are several strategies to manage symptoms and protect themselves:

1. Avoid triggers: Stay away from loud noises and situations that change pressure in your head, such as straining, heavy lifting, or sudden movements.
2. Use hearing protection: Wear earplugs or noise-canceling headphones in noisy environments to reduce sound-induced symptoms.
3. Seek vestibular physical therapy: A certified vestibular physical therapist can teach exercises to improve balance and reduce fall risk.
4. Lifestyle modifications: Adjust daily activities to minimize symptom triggers. This may include avoiding certain exercises or movements that worsen symptoms.
5. Manage mental health: Since SCDS can impact mental well-being, consider counseling or support groups to cope with anxiety or depression related to the condition.
6. Educate others: Inform family, friends, and colleagues about your condition to help them understand your needs and limitations.
7. Consider workplace accommodations: If necessary, request reasonable accommodations at work to manage your symptoms effectively.

While these strategies can help manage symptoms, it's important to note that SCDS doesn't go away without surgery. If symptoms become unmanageable or significantly impact quality of life, it may be worth reconsidering surgical options in consultation with a healthcare provider.

Conclusion

Surgical repair can significantly improve symptoms and quality of life for patients with SCDS. However, it should be considered carefully, weighing the potential benefits against the risks. This is a discussion that every surgeon must have with his/her patient. After all, this surgery is entirely elective and only done to improve quality of life.