Acoustic neuroma

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Acoustic Neuroma (Vestibular Schwannoma)

High-Yield Revision Notes for NEET PG / NEXT

An acoustic neuroma is a benign, slow-growing, encapsulated tumor originating from the Schwann cells wrapping the vestibulocochlear nerve (CN VIII). Histologically and anatomically, it arises far more frequently from the inferior or superior vestibular divisions rather than the auditory division itself, characteristically localizing at the transition zone between central and peripheral myelin (the Obersteiner-Redlich zone) within the Internal Auditory Canal (IAC).

Axial T1-weighted contrast-enhanced MRI showing a large, intensely enhancing mass in the right cerebellopontine angle extending into the internal auditory canal, resembling an ice-cream cone, alongside an anatomical diagram.

Radiological Profile: Post-gadolinium axial MRI (left) showing a massive, heterogeneously enhancing cerebellopontine angle (CPA) tumor causing brainstem distortion and widening of the internal acoustic meatus, matching the structural diagram (right).

Genetics & Pathological Profile

  • Tumor Suppressor Gene Link: Involves a mutation or inactivation of the NF2 gene located on Chromosome 22q12, which encodes the structural protein **merlin** (schwannomin).
  • Bilateral Presentation (Board Target): Unilateral acoustic neuromas are sporadic and account for 95% of cases. Bilateral acoustic neuromas are pathognomonic for Neurofibromatosis Type 2 (NF2).
  • Histopathology: Demonstrates two distinct alternating cellular architecture configurations:• Antoni A: Highly cellular areas composed of spindle-shaped cells with elongated nuclei arranged in palisades surrounding acellular eosinophilic zones (Verocay bodies).

    • Antoni B: Hypocellular, loose, myxoid tissue matrices featuring microcystic spaces.

Anatomical Progression & Clinical Manifestations

Symptoms emerge in a predictable sequence as the expanding tumor fills the internal auditory canal and leaks out into the **cerebellopontine angle (CPA)**:

Phase / Target Structures Clinical Symptoms & Specialized Physical Signs
1. Otological Stage
(Early Canalicular)		 • Asymmetrical Sensorineural Hearing Loss (SNHL): The single most common presenting symptom (greater than 90%). It is typically slowly progressive and impairs high frequencies first, accompanied by poor speech discrimination.
• Unilateral high-pitched tinnitus and mild unsteadiness (true rotatory vertigo is rare due to slow compensation).
2. Trigeminal Involvement
(CPA Expansion)		 As the mass breaches the CPA, it compresses the adjacent Trigeminal Nerve (CN V).
• This results in the **loss of the ipsilateral corneal reflex** (earliest sign) alongside midface paresthesias or numbness.
3. Facial Nerve Compression The motor fibers of CN VII are highly resilient to slow stretching, so overt weakness is a late sign. However, sensory changes manifest early:
• Hitselberger’s Sign: Hypoesthesia or numbness localized to the posterior-superior wall of the external auditory canal due to compression of the sensory cutaneous fibers of CN VII.
4. Neurological Crisis
(Late Stage)		 • Compression of the ipsilateral cerebellar peduncle leads to ataxia, dysmetria, and nystagmus.
• Large tumors compromise the 4th ventricle, presenting with signs of increased intracranial pressure (headache, papilledema, vomiting) due to obstructive hydrocephalus.

Diagnostic Workup Protocol

  1. Gadolinium-Enhanced T1-Weighted MRI: The absolute gold standard confirmation diagnostic test. It displays an intensely and uniformly enhancing mass centered in the CPA. Small intra-canalicular lesions appear clearly, while larger variants showcase the classic pathognomonic **”ice-cream cone” appearance** (where the canalicular component mimics the cone and the CPA component represents the scoop of ice cream).
  2. Auditory Brainstem Response (ABR / BERA): An exceptional, highly sensitive non-invasive screening tool. Findings indicative of retrocochlear pathology include an **interaural latency difference of Wave V greater than 0.2 ms** or an absolute elongation of the Wave I-V interpeak interval.
  3. Pure Tone Audiometry (PTA): Confirms unilateral or asymmetric sensorineural hearing loss. Rollover index testing during speech audiometry displays a high diagnostic yield for retrocochlear lesions.

Management Paradigms

Management selection is highly customized and depends on tumor diameter, growth kinetics, baseline hearing thresholds, and patient age:

  • 1. Conservative Surveillance (“Wait and Scan”): Indicated for very small intracanalicular tumors (under 1 cm), elderly patients with significant medical comorbidities, or tumors demonstrating zero growth on serial scans. Requires annual gadolinium MRIs.
  • 2. Stereotactic Radiosurgery (Gamma Knife / CyberKnife): Delivers highly focused, single-fraction radiation dose matrices to arrest tumor replication. Ideal for small-to-medium-sized lesions (less than 3 cm in diameter) in elderly or medically unfit patients, or to treat residual fragments post-incomplete surgical resections.
  • 3. Microsurgical Resection: The definitive treatment option for expanding, large, or symptomatic tumors. Executed via three classic primary surgical corridors:• Translabyrinthine Approach: Excellent exposure of the entire IAC with no cerebellar retraction required. However, it **destroys all residual hearing completely**, making it suitable only when baseline hearing is non-serviceable.

    • Retrosigmoid / Suboccipital Approach: Versatile corridor for large tumors. It offers direct visualization of the brainstem and **allows for potential preservation of hearing**.

    • Middle Cranial Fossa Approach: Utilized exclusively for small, strictly intracanalicular tumors localized to the lateral aspect of the canal, with the explicit goal of preserving hearing.