Understanding Third Window Syndrome (TWS)

As healthcare providers, we occasionally encounter patients with dizziness that does not fit the usual patterns of vestibular disorders. One such condition is Third Window Syndrome (TWS), which most commonly presents as Superior Semicircular Canal Dehiscence (SSCD). There are other presentations of TWS aside from SSCD and TWS itself was first described by Minor in 1998. 

In this blog, we will cover what TWS is, its causes, symptoms, triggers, diagnosis, and treatment options.

What is Third Window Syndrome?

Third Window Syndrome (TWS) encompasses a group of inner ear disorders characterized by an abnormal opening or thinning in the bony structures of the inner ear. The most well-known form is Superior Semicircular Canal Dehiscence (SSCD) which has a prevalence of 2.1%–10.7% on temporal bone CT and 0.5%– 0.6% at postmortem studies (Ho 2017). 

Other variants include: perilabyrinthine fistula, enlarged vestibular aqueduct, dehiscence of the scala vestibuli side of the cochlea, X-linked stapes gusher, and bone dyscrasias (Ho 2017).  These abnormalities disrupt the normal pressure and fluid dynamics within the inner ear, leading to symptoms that not only include dizziness and imbalance, but other aural symptoms such as hearing loss, sensitivity to loud sounds, tinnitus, and ear fullness.

The Mechanisms Behind Third Window Syndrome

The inner ear is a fluid filled, closed or sealed unit composed of the cochlea, the three semicircular canals, and two otolithic organs. The cochlea contains hair cells that detect pressure changes in the fluid of the inner ear that is created by the stapes of the middle ear vibrating against the oval window (let's call this window #1). The pressure changes in the inner ear is buffered through the deflection of the round window (let's call this window #2).

Head motion can also cause pressure changes with the fluid of the inner ear, stimulating the hair cells in the semicircular canals and otolithic organs.

Now, TWS occurs when an abnormal opening or thinning of tissue causes a "third window," and disrupts the pressure balance in the inner ear. The third window now shunts the pressure changes to the vestibular portion of the inner ear.  The third window also creates an increase in bone conduction sensitivity. These changes result in symptoms like dizziness and other aural symptoms outlined below. 

Symptoms of Third Window Syndrome

Patients with TWS can experience a variety of symptoms, which may include:

• Dizziness and Vertigo: Sensations of spinning or movement, often triggered by changes in head position, increased pressure, and/or loud noises.
• Imbalance: Difficulty maintaining balance, especially during rapid movements.
• Difficulty Focusing: Visual disturbances (impaired gaze stabilization with head motion, loud sounds, and pressure changes).
• Hearing Loss: Typically low-frequency conductive hearing loss.
• Hyperacusis: Reduced tolerance to sound (i.e. other people chewing food, car engine, hitting keys on key board).
• Autophony: Hearing one's own voice, heartbeat, stomach sounds, eye movement, or other internal sounds abnormally loudly.
• Tinnitus: Ringing or buzzing in the affected ear.
• Aural Fullness: A sensation of fullness or pressure in the affected ear.
• Oscillopsia: A visual disturbance where objects appear to bounce or move.

Triggers of Symptoms

Several factors can exacerbate symptoms of TWS:

• Loud Noises: Sounds like musical instruments, traffic, or loudspeakers can provoke dizziness or vertigo.
• Pressure Changes: Activities that cause rapid changes in pressure, such as weightlifting, heavy lifting, nose blowing, bearing down while having a bowel movement, sneezing, coughing, and/or altitude changes can trigger symptoms.
• Head Movements: Sudden or rapid changes in head position can induce dizziness and vertigo.

Causes of Third Window Syndrome

In our review to determine the cause, most sources indicate that it occurs during fetal development where the uppermost semicircular canal fails to close or thicken normally. These sources are also only referring to SSCD. VEDA on the other hand states that TWS could be due to: anatomical variations, genetics, trauma, surgery, barotrauma, and other disorders (i.e. growths, tumours, infections, or elevated intracranial pressure). The reason we believe that VEDA's list is longer is that they consider all of the causes of a third window and not just SSCD.

Diagnosis

Diagnosing TWS can be done by ruling out other conditions and matching the reported symptoms with the clinical examination, which can include:

• Hearing Test: An audiologist assesses for hearing loss, bone conduction, and heightened sensitivity.
• Cervical & Ocular Vestibular-Evoked Myogenic Potentials (cVEMP and oVEMP): Measures muscle reflexes and eye muscle responses to sound respectively. cVEMPS have a 85-91% sensitivity and 90-96% sensitivity with detecting SSSD. 
• Vestibular Testing: Includes Tullio, Valsalva, and Skull Vibration Induced Nystagmus tests to evaluate balance function and nystagmus.
• CT Imaging: High-resolution CT scans can identify defects in the inner ear bones. Note that 2% of the adult population normally has a dehiscence and that CT diagnosis of dehiscence alone overestimates the diagnosis of SSCD by 6-20 fold (Carey 2000).

Treatment Options

Conservative Management

1. Observation and Lifestyle Modifications: Patients may opt for conservative management that includes avoiding the following:
   
   • Avoid loud noises (i.e. musical instruments, industrial noise, and other loud environments).
   • Avoid pressure fluctuations (i.e. weightlifting, strenuous activities, nose blowing, air travel, scuba diving).
2. Ventilation Tube: Can help reduce some of the symptoms by minimizing middle ear pressure changes.

Surgical Treatment

If conservative measures are insufficient, surgical options are considered to repair the third window and alleviate symptoms. Dr. T. Hains website provides a good review of the surgical options, with images. On his website he indicates that only about 1 in 5 patients opt for surgery, often due to the risks of hearing loss and concerns about craniotomy (refer to below). Patients more likely to opt for surgery were those with prominent auditory symptoms and/or significant dizziness from minor activities like burping. Ward (2017) on the other hand indicated that 50% of their patients go on to have surgery. 

If surgery is performed patients can experience unsteadiness and adjustment issues post-surgery, typically improving over several weeks. Complications can include CSF leaks, pneumolabyrinth, plugging material affecting the posterior canal, BPPV, hearing loss, and the potential need for revision surgery.

Conclusion

The choice between conservative and surgical treatment for TWS depends on symptom severity, the size and location of the defect, and patient preference. Conservative management focuses on lifestyle changes to avoid symptom triggers, while surgical options aim to repair the defect. 

As physical therapists, our role in managing TWS includes assessing patients clinically, referring them to otolaryngology specialists if they present with a history and finding consistent with TWS, as well as educating them about treatment options and prognosis. 

Understanding and awareness among healthcare providers is vital for timely and accurate diagnosis, ensuring better outcomes for patients with TWS.