Vestibular Schwannoma

What are Vestibular Schwannomas (VS)?

Vestibular schwannomas (VS), also known as acoustic neuromas, are benign tumors affecting the balance and hearing nerves. While non-cancerous, their growth can disrupt critical neurological functions. This article provides a detailed exploration of symptoms, diagnostic approaches, and medical evaluations to empower patients with knowledge.

Simplified Anatomy of The Skull Base

To understand vestibular schwannomas and their symptoms, it helps to know the anatomy of the internal auditory canal (IAC) and the cerebellopontine angle (CPA). These areas are located in the skull base exactly where vestibular schwannomas start and grow. They are small but packed with vital nerves and blood vessels that control hearing, balance, facial movement, and sensation. Understanding this anatomy is key to understanding how symptoms develop and the implications of treating a VS (whether with surgery or radiotherapy).

The internal auditory canal (IAC) is a narrow bony tunnel in the temporal bone that connects the inner ear to the brain compartment. It is fluid filled, and five important structures travel through this canal:

1. Facial nerve (CN VII): Controls facial expressions and some taste.

2. Vestibulocochlear nerve (CN VIII): Divided into two branches:

   • The cochlear branch, responsible for hearing.

   • The vestibular branch, responsible for balance. The vestibular branch further divides into a superior vestibular nerve and an inferior vestibular nerve.

3. Labyrinthine artery: The only supply of blood to the inner ear. This artery is therefore critical to the hearing health.

As these nerves leave the IAC, they enter the cerebellopontine angle (CPA), which is a fluid-filled space in the brain compartment located between the cerebellum (controls coordination) and the pons (part of the brainstem). The CPA contains:

• The trigeminal nerve (CNV): A cranial nerve responsible for providing sensation to the face, and that controls the chewing muscles.  
  
  

• The origins of the vestibulocochlear nerve and facial nerve, which are critical for hearing, balance, and facial movement.

• The anterior inferior cerebellar artery (AICA), which supplies blood to nearby structures.

• The cerebellum: The balance and coordination part of the brain.

What Causes a Vestibular Schwannoma?

Vestibular schwannomas usually develop from an overgrowth of Schwann cells on the vestibular branch of CN VIII inside the IAC. Schwann cells normally insulate nerves like “electrical tape” around a wire. As vestibular schwannomas grow, they can press on nearby nerves and structures, leading to symptoms.

In most cases, the exact cause is unknown. Many of my patients wonder if it's due to cellphone use or electromagnetic radiation from wires. At this point we don't have a any evidence suggesting a link between such causes and VS. About 5% of patients have a genetic condition called neurofibromatosis type 2 (NF2), which leads to bilateral tumors. Researchers have linked the condition to a faulty gene on chromosome 22 that normally controls Schwann cell growth.

Symptoms: From Gradual Hearing Loss to Sudden Emergencies

No symptoms! 

Patients may be surprised to learn that a VS may present with no symptoms at all! In those patients, the VS is found incidentally. This means it was discovered by chance during an MRI conducted for unrelated reasons, such as an episode of loss of consciousness, hand tingling, or monitoring another area (these are examples). In my experience, this is a quite common presentation.

When there are symptoms, they often develop slowly but can escalate unpredictably: 

Core Symptoms

• Unilateral hearing loss (hearing loss in one hear):

  • The symptom that will most commonly alert the patient that something is wrong. That patient will end up consulting their doctor for their hearing loss, who will order an audiogram. The audiogram will confirm the asymmetrical hearing loss, and that patient will be sent for an MRI which will reveal a VS. 
    
    

  • Begins as difficulty hearing high-pitched sounds (e.g., phone conversations) and progresses to muffled speech perception.

  • In 4-5% of sudden sensorineural hearing loss (SSNHL) cases, vestibular schwannoma is identified.

  • Hearing loss is not necessarily related to tumor size, or tumor growth. There are many patients with very small tumors, but very bad hearing, and vice-versa. You can also have a stable tumor that has not grown in years, but hearing could nonetheless keep declining.
    
    
• Sudden sensorineural hearing loss: In patients that have a known VS, abrupt deafness (sudden hearing loss) occurs in up to 20% of cases (this statement is different from the one stated above, that if you have a sudden hearing loss, you have a 4-5% chance of having an underlying schwannoma). It is possible to recover hearing with steroids, even with confirmed tumors. To learn more about sudden sensorineural hearing loss, read this article. 
  
  

• Tinnitus: Persistent ringing or buzzing, often localized to one ear. Learn more about tinnitus here.

• Balance issues: Dizziness when turning quickly or walking in dark environments would be due to vestibular (balance) nerve compression by the tumor. This could happen even with small tumors, as the schwannoma arises from the balance nerve. Sometimes, the tumor can cause episodes of spinning vertigo. Learn more about vertigo here.

Signs of Advanced Tumors

• Facial numbness: Trigeminal nerve (also called cranial nerve V) compression by a large schwannoma

• Facial weakness: Facial nerve (also called cranial nerve VII) leads to asymmetry or twitching. This would be rare however, and would raise the possibility of a facial nerve schwannoma (instead of of a vestibular schwannoma).

• Hydrocephalus: Large tumors obstruct cerebrospinal fluid flow (the fluid that surrounds the brain), causing an accumulation of that fluid in the skull, called hydrocephalus. This will lead to headaches, nausea, and most dangerously, vision changes that could be permanent. This would be a rare but serious complication that requires urgent evaluation and treatment. 

Medical Evaluation: History and Physical Exam

A thorough clinical assessment guides further testing:

Patient History

• Key questions:

  • Onset and progression of hearing loss (gradual vs. sudden).
  • Episodes of vertigo or imbalance (e.g., triggered by head movements).
  • Tinnitus 
  • Facial numbness
  • History of neurofibromatosis type 2 (NF2) in the family.

Physical Examination

• Otoscopy: Checking the eardrums and ear canals, should be normal in VS. 
  
  

• Cranial nerve testing:

  • Facial nerve (VII): Check for asymmetry or weakness during eyebrow raises or eye closures.

  • Trigeminal nerve (V): Assess facial sensation with light touch.

  • Vestibulocochlear nerve (VIII): Use tuning forks (Rinne/Weber tests) to differentiate conductive vs. sensorineural hearing loss.

• Balance tests: Romberg test (standing with eyes closed) to detect instability. Head-impulse test and head-shake test can help determine a weakness in vestibular system of the affected ear. Sometimes, there is associated benign positional vertigo (learn more about BPPV here), so a Dix-Hallpike maneuver is necessary. 

Medical Work-Up: What Tests are Needed?

1. MRI of the internal auditory canal (MRI IAC): The Gold Standard

This test is the most effective method for identifying almost all cases of vestibular schwannomas.

• Protocol:

  • T1-weighted with Contrast: This imaging technique is used to make tumors more visible by enhancing (lighting up) their appearance on the scan by use of an intravenous dye (called contrast). The dye is called gadolinium.
  • High-resolution T2 (CISS/FIESTA): This method provides detailed images of nerve structures and helps in identifying small tumors, and also helps tract the trajectory of the nerves along the tumor.
  • Diffusion-weighted Imaging (DWI): This scan helps differentiate between tumors and other conditions like cysts or epidermoid tumors by analyzing the movement of water molecules in tissues.

• Findings: Tumors appear as masses that are contrast-enhancing (lighting up). The usually involve the internal auditory canal (a bony canal in the skull base) and the cerebellopontine angle (a fluid-filled space close to the brain).

2. Audiogram: Measuring Hearing Loss

Since VS develop from the nerves responsible for hearing and balance, it is essential to evaluate a patient's hearing levels. This evaluation is crucial because it guides doctors like myself in recommending the most suitable treatment options, which will change based on the extent and quality of the patient's hearing. You can learn more about audiograms here.

• Key metrics:

  • Pure-tone average (PTA): Thresholds >50 dB suggest significant loss.

  • Speech discrimination score (SDS): Scores <50% indicate poor word recognition, even with mild PTA changes. A %SDS less than 50% indicates what we consider a "non-functional" ear, and will affect the treatment option (see below).

• Screening criteria: MRI is recommended for patients with:

  • Asymmetrical hearing loss: 15 dB hearing difference (between the good and bad ear) at two adjacent frequencies (e.g., 2000 + 3000 Hz).

  • More than 12-16% asymmetry in the speech discrimination score.

3. Vestibular/Balance Testing (Optional)

• Videonystagmography (VNG): Detects asymmetric caloric responses (weakness in the vestibular part of the affected ear). Read more about VNG here.

• Vestibular evoked myogenic potentials (VEMP): Assesses utricle, saccule and superior and inferior vestibular nerve functions, often abnormal in schwannomas. Read more about VEMPs here.

Navigating Vestibular Schwannoma Treatment: Options and Decision-Making

Choosing the right treatment for a vestibular schwannoma (acoustic neuroma) depends on tumor size, hearing status, symptoms, and patient medical status and preferences. Here’s a breakdown of the three main approaches—observation, radiotherapy, and surgery—and how specialists decide between them.

1. Observation (“Wait-and-Scan”)

• This is the most commonly recommended trajectory for patients at the initial consultation when only 1 MRI has been done (unless there are clear indications for surgery, see below), because we do not have proof of tumor growth at that time. Since 20% of tumors may not grow, there is a chance we may never need to treat it.
  
  

• Best for: Small tumors (<1.5 cm) with mild or no symptoms (e.g., stable hearing, no balance issues).

• Process: Regular MRI scans (every 9–12 months) monitor tumor growth.

• Pros: Avoids risks of surgery/radiation.

• Cons: Hearing may decline over time, and sudden growth could require urgent treatment.

2. Surgery: Three Key Approaches

Surgery is typically recommended for growing tumors (regardless of size) or for large tumors (>3 cm, regardless of growth) compressing the brainstem or causing severe symptoms, or for patients that are looking for a cure regardless of growth or size.

• Pros : Achieves compete cure in over 95% of cases, possibility to preserve hearing.
• Cons : Invasive, higher risk of complcations including facial nerve injury.
  
  
• Hearing Outcomes: Depends un tumor size, but studies show that for tumors less than 1.5 cm and good pre-operative hearing, close to 70% of patients their hearing after surgery. Balance often improves post-surgery as the brain adapts.

There are 3 main surgical approaches (retrosigmoid, middle fossa and translabyrinthine). The approach chosen by your specialist depends on tumor size, location, and hearing goals:

3. Radiotherapy: Gamma Knife vs. CyberKnife

Radiation halts tumor growth (does not remove and cure the tumor) over a period of 20-25 years in ~90% of cases. It is ideal for small-to-medium tumors (1–2.5 cm). It is not possible for tumors larger than 3 cm.

Pros: Non-invasive, shorter recovery.
Cons: Hearing decline in over 85% of patients over 5–10 years; rare facial numbness, risks of tumor transformation into cancer (very rare)

Untreated Vestibular Schwannomas: What Should You Expect? 

Final Thoughts

Evaluating vestibular schwannomas requires a careful and comprehensive approach. Thanks to advancements in MRI technology and hearing tests, doctors can now diagnose these tumors earlier. Additionally, balance tests can help determine if nerves are affected. If you notice any changes in your hearing, whether they develop slowly or happen suddenly, it is crucial to seek medical evaluation promptly. Early assessment can help you explore treatment options and prevent permanent damage.