SUPERStudy: Acoustic Neuroma (Vestibular Schwannoma)

Introduction: Vestibular schwannomas (VSs) (acoustic neuromas) are nerve sheath tumors of the superior and inferior vestibular nerves (cranial nerve VIII). They arise in the medial internal auditory canal (IAC) or lateral cerebellopontine angle (CPA) and cause clinical symptoms by displacing, distorting, or compressing adjacent structures in the IAC and the CPA.

Epidemiology: Vestibular schwannomas are by far the most common tumors involving the cerebellopontine angle. Vestibular schwannomas make up 80% of cerebellopontine angle tumors and 8% of all intracranial tumors.There is no gender bias and the age of presentation is between 40 and 60 years of age.

Etiology:

1.Sporadic (Most Common): Occurs without a known cause in the majority of cases. Typically unilateral.

2.Neurofibromatosis Type 2 (NF2): A genetic condition associated with bilateral acoustic neuromas. Caused by mutations in the NF2 gene encoding merlin, a tumor suppressor protein.

Pathogenesis: VSs originate in the Schwann cells of the superior or inferior vestibular nerves at the transition zone (Obersteiner-Redlich zone) of the peripheral and central myelin. This transition zone occurs in the lateral cerebellopontine angle or medial internal auditory canal. Therefore, VSs most often arise in the internal auditory canal and occasionally in the cerebellopontine angle.

Clinical Features:

1.Hearing Loss: Unilateral, gradual, sensorineural hearing loss is the most common symptom (present in 95% of patients). Sudden hearing loss can also occur in some cases.

2.Tinnitus: Persistent ringing or buzzing in the affected ear.Tinnitus is present in 65% of patients. The tinnitus is most often constant with a high buzzing pitch. 

3.Vertigo: The majority of patients have self-limiting episodes of vertigo, because the gradual vestibular deficit is compensated centrally as it develops.

4.Balance Problems: Unsteadiness, dizziness, or vertigo due to vestibular nerve involvement.

5. Trigeminal and facial nerve dysfunction: midface numbness, absent corneal reflex, numbness of the posterior external auditory canal (Hitselberger sign), and facial weakness or spasm

6.Large Tumors: May compress the brainstem, leading to hydrocephalus, headache, altered mental status, dysphagia, hoarseness, or more severe neurological deficits.

Diagnosis:

1.Clinical Examination: Assess for asymmetric hearing loss, cranial nerve deficits, and balance abnormalities.

2.Audiometry: Confirms unilateral sensorineural hearing loss.

3.Vestibular testing: The most common test ordered to evaluate vestibular complaints includes an electronystagmogram (ENG). An ENG in a patient with VS will show a reduced caloric response in the involved ear.

4.Imaging: 

MRI with gadolinium: Gold standard for detecting acoustic neuromas. Shows a well-defined, enhancing mass in the cerebellopontine angle.

CT scan: When MRI scans cannot be used or are not accessible, a computed tomography (CT) scan with iodine contrast or an auditory brainstem response (ABR) offers a reasonable alternate screening modality.

Management:

1.Observation: For small, asymptomatic tumors or patients with significant comorbidities.Regular MRI and audiometric monitoring.

2.Surgical Resection: Indicated for symptomatic, large, or growing tumors.

Techniques:

• Translabyrinthine approach: Sacrifices hearing but provides good tumor access.
• Retrosigmoid approach: Preserves hearing in select cases.

• Middle cranial fossa approach: Used for smaller tumors with hearing preservation potential.

3.Stereotactic Radiosurgery (Gamma Knife): Stereotactic radiation therapy (SRT) is indicated when tumors are less than 3 cm in size, in patients who are not surgical candidates due to advanced age or other factors, or due to patient preference over surgery.

4.Rehabilitation and Support:

• Hearing aids or cochlear implants for hearing loss.
• Vestibular rehabilitation for balance issues.

Complications:

• Facial Nerve Damage: Can occur during surgery or due to tumor compression.
• Hearing Loss: Permanent sensorineural hearing loss is common after treatment.
• Hydrocephalus: Caused by brainstem compression from large tumors.

Prognosis: Acoustic neuromas are benign and slow-growing, with a generally good prognosis if treated appropriately. Early detection and management help prevent complications and preserve function.

Key Points:

1.Acoustic neuroma is a benign tumor of the vestibulocochlear nerve, commonly presenting with unilateral sensorineural hearing loss, tinnitus, and balance problems.

2.MRI with gadolinium is the gold standard for diagnosis, revealing a mass in the cerebellopontine angle.

3.Treatment options include observation, stereotactic radiosurgery, or surgical resection, with the choice depending on tumor size, symptoms, and patient preference.

4.Regular follow-ups are essential to monitor tumor growth and preserve function.

References: 

-Chapter 22: Dizziness and Vertigo 

Mark F. Walker; Robert B. Daroff; Harrison’s Principles of Internal Medicine, 21e

-Chapter 63: Vestibular Schwannomas 

Maja Svrakic; Anil K. Lalwani; Current Diagnosis & Treatment Otolaryngology—Head and Neck Surgery, 4e