Neuroma Akustik

01Pathophysiology

Acoustic neuroma is a misnomer. The tumor does not arise from the acoustic (cochlear) division of CN VIII but from the vestibular division. It is a schwannoma, meaning it originates from the Schwann cells that produce the myelin sheath of peripheral nerves. This makes it a benign, slow-growing, well-encapsulated tumor with no malignant potential in the vast majority of cases.

The tumor typically originates within the internal auditory canal (IAC) and, as it enlarges, extends medially into the cerebellopontine angle (CPA) cistern. This anatomical corridor is critical because it is a confined bony channel housing CN VII (facial nerve) and CN VIII together. Even a small mass in this space produces symptoms through direct compression of adjacent neural structures rather than through tissue invasion.

The earliest and most common symptom is unilateral sensorineural hearing loss, which develops because the expanding tumor compresses the cochlear nerve fibers or disrupts the vascular supply to the cochlea (the labyrinthine artery, a branch of AICA). The hearing loss is characteristically gradual and progressive, and patients often notice difficulty using the telephone on one side or trouble hearing in noisy environments. Speech discrimination is disproportionately reduced relative to the pure tone threshold, a hallmark finding that separates retrocochlear pathology from cochlear causes of SNHL.

Tinnitus, typically unilateral and high-pitched, accompanies the hearing loss in most patients and results from aberrant neural firing in the compressed cochlear nerve.

True rotational vertigo is uncommon despite the tumor's origin on the vestibular nerve. This is because the slow growth allows central vestibular compensation to occur gradually. Patients instead report a vague sense of imbalance or unsteadiness.

As the tumor grows beyond 2 to 2.5 cm, it begins to compress neighboring structures in the CPA:

• CN V (trigeminal nerve): produces ipsilateral facial numbness or a diminished corneal reflex, which is a frequently tested exam finding

• CN VII (facial nerve): despite being immediately adjacent, frank facial palsy is a late finding because the facial nerve is resistant to compressive injury; when present, it suggests a large tumor

• Brainstem and cerebellum: very large tumors can compress the pons or cerebellar peduncle, causing ataxia, and can obstruct the fourth ventricle, leading to obstructive hydrocephalus with headache, nausea, vomiting, and papilledema

The genetic basis is best understood in the context of Neurofibromatosis type 2 (NF2), an autosomal dominant condition caused by mutations in the merlin (schwannomin) gene on chromosome 22q12. Merlin is a tumor suppressor protein. Loss of merlin function leads to unregulated Schwann cell proliferation. The hallmark of NF2 is bilateral vestibular schwannomas, and this finding alone is considered diagnostic of NF2. Sporadic unilateral tumors also involve somatic mutations in the same gene but are not heritable.

02Classification and Clinical Manifestation

Koos Grading

Cranial Nerve Involvement

03Diagnostic Workup

Walkthrough of the diagnostic sequence:

The workup begins when a patient presents with asymmetric sensorineural hearing loss or unilateral tinnitus that cannot be explained by noise exposure, ototoxic medications, or age-related presbycusis. The best initial test is pure tone audiometry with speech discrimination testing. The classic audiometric pattern is a unilateral or markedly asymmetric high-frequency sensorineural loss. The critical finding is that the speech discrimination score is reduced out of proportion to the pure tone average. In a cochlear lesion like presbycusis, speech discrimination generally correlates with the degree of pure tone loss. In a retrocochlear lesion like acoustic neuroma, discrimination drops far more than the pure tone threshold would predict. This pattern is called "rollover" and is the audiometric red flag that points toward a retrocochlear pathology.

Once retrocochlear disease is suspected, the most accurate test is MRI of the internal auditory canals with gadolinium enhancement. This has a sensitivity approaching 100% for tumors as small as 1 to 2 mm. On MRI, the schwannoma appears as a well-defined mass arising from the IAC, often described as having an "ice cream cone" shape when it extends into the CPA cistern. It enhances avidly and homogeneously with gadolinium. MRI also delineates the tumor's relationship to the brainstem, cerebellum, and adjacent cranial nerves, which is essential for surgical planning.

Auditory brainstem response (ABR) was historically the primary screening test before MRI became widely available. ABR measures the electrical activity along the auditory pathway from the cochlea to the brainstem. In acoustic neuroma, the characteristic finding is a prolonged interpeak latency between waves I and V or complete absence of waveforms on the affected side. While ABR has reasonable sensitivity for tumors larger than 1 cm, it misses small intracanalicular tumors, which is why MRI has replaced it as the preferred confirmatory study. ABR remains useful when MRI is contraindicated (e.g., incompatible implants).

CT of the temporal bone may show a widened or eroded internal auditory canal (the "trumpeted IAC" sign) but is far inferior to MRI for detecting the tumor itself and is not the answer to a "best test" question.

04Management and Treatment

Observation is the initial approach for patients with small, incidentally discovered tumors or minimal symptoms. Because these tumors are benign and slow-growing (average growth rate approximately 1 to 2 mm per year), many will remain stable for years. The protocol involves serial gadolinium-enhanced MRI at 6-month intervals initially, then annually if the tumor is stable. If sequential imaging shows no growth over 2 to 3 years, the interval can be extended. Observation is also favored in elderly patients or those with comorbidities that increase surgical risk.

Microsurgical resection is definitive treatment when the tumor is large, actively growing, or producing progressive neurological deficits. The choice of surgical approach depends on the tumor size, the patient's residual hearing, and the surgeon's expertise:

• The translabyrinthine approach is used when hearing is already non-serviceable. It sacrifices the labyrinth and all residual hearing but provides excellent exposure of the tumor and facial nerve with low risk of cerebellar injury.

• The retrosigmoid (suboccipital) approach allows hearing preservation and is used for medium to large tumors. It provides wide CPA exposure but carries a higher risk of postoperative headache.

• The middle fossa approach is reserved for small intracanalicular tumors (under 1.5 cm) with serviceable hearing. It accesses the IAC from above through a temporal craniotomy.

The primary intraoperative concern is preservation of the facial nerve (CN VII), which is monitored with continuous electromyography during surgery. Hearing preservation is attempted when preoperative hearing is still serviceable (defined by the AAO-HNS classification: pure tone average under 50 dB and speech discrimination above 50%).

Stereotactic radiosurgery (Gamma Knife, CyberKnife, or fractionated stereotactic radiotherapy) is an alternative for tumors under 3 cm. It delivers a focused dose of radiation (typically 12 to 13 Gy to the tumor margin in a single fraction for Gamma Knife). The goal is growth control, not tumor eradication: the tumor stops growing but rarely disappears. Tumor control rates exceed 90% at 10 years. Risks include delayed sensorineural hearing loss (occurring in 50 to 70% of patients over 5 to 10 years) and facial neuropathy (under 5% with modern dosing). Radiosurgery is favored for patients who are elderly, medically unfit for surgery, or who prefer a less invasive option.

For large tumors causing brainstem compression or obstructive hydrocephalus, surgical intervention is urgent. Hydrocephalus may require CSF diversion (external ventricular drain or ventriculoperitoneal shunt) before or at the time of tumor resection. In some cases, intentional subtotal resection is performed to decompress the brainstem while preserving the facial nerve, with residual tumor monitored or treated with adjuvant radiosurgery.

There are no standard chemotherapy agents for vestibular schwannoma. However, in NF2-associated tumors, the VEGF inhibitor bevacizumab has shown benefit in reducing tumor size and improving hearing in select cases and may appear in vignettes about NF2 management.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The single most common trap with acoustic neuroma questions is confusing the presentation with Meniere disease. Both involve unilateral hearing loss and tinnitus, and students often anchor on the vestibular symptoms. The discriminator is the pattern of hearing loss and the nature of the vertigo. Meniere produces episodic, dramatic spinning vertigo with low-frequency hearing loss that fluctuates between attacks. Acoustic neuroma produces steady, progressive high-frequency hearing loss with mild imbalance, not episodic vertigo. If the vignette describes a patient with slowly worsening hearing on one side, persistent tinnitus, and unsteadiness rather than spinning, the answer is acoustic neuroma.

A second common error is selecting the wrong diagnostic test. Students sometimes choose CT scan or ABR when the question asks for the best next step. The answer is almost always MRI with gadolinium once retrocochlear pathology is suspected. CT is useful only for bony anatomy and surgical planning; it is not the confirmatory test. ABR is a historical screening tool with lower sensitivity than MRI and is not the best answer unless MRI is explicitly contraindicated.

The third trap involves NF2 recognition. When a vignette describes a young patient (under 30) with bilateral hearing loss, bilateral CPA masses, and possibly a family history of brain tumors, the diagnosis is NF2, not "bilateral acoustic neuromas" as an isolated entity. The question may then pivot to ask about the genetic basis (chromosome 22q12, merlin gene) or associated tumors (meningiomas, spinal ependymomas, posterior subcapsular cataracts). Do not miss the NF2 connection when the tumors are bilateral.

Finally, watch for the "decreased corneal reflex" clue. This finding, which reflects CN V compression in the CPA, is a favorite test-writer insertion that points directly to a CPA mass rather than a peripheral ear problem. Any time a vignette pairs unilateral hearing loss with an absent or diminished corneal reflex, the answer is a CPA lesion until proven otherwise.

The core competency being tested is the ability to recognize a retrocochlear pattern of hearing loss (progressive, unilateral, high-frequency SNHL with disproportionately poor speech discrimination), select the correct confirmatory imaging (MRI with gadolinium), and distinguish this diagnosis from its closest mimics based on the temporal pattern of symptoms and imaging characteristics.