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Summary Table

Category	Details
Risk Factors	Neurofibromatosis type 2 (bilateral tumors are pathognomonic); sporadic cases in adults aged 40 to 60; prior radiation exposure to the head and neck
Etiology	Benign neoplasm of Schwann cells arising from the vestibular branch of cranial nerve VIII, almost always at the cerebellopontine angle (CPA)
Patient Presentation	Gradual, progressive unilateral sensorineural hearing loss (SNHL); persistent unilateral tinnitus; imbalance or unsteadiness rather than true spinning vertigo
Classic Physical Exam	Weber test lateralizes to the unaffected ear; Rinne test shows air conduction greater than bone conduction bilaterally (consistent with sensorineural pattern); decreased corneal reflex (CN V involvement with larger tumors); ipsilateral facial weakness or numbness in advanced cases
Key Diagnostic Results	MRI with gadolinium contrast shows an enhancing mass at the CPA/internal auditory canal; audiometry reveals asymmetric high-frequency SNHL with disproportionately poor speech discrimination scores
Management	Observation with serial MRI for small, asymptomatic tumors; microsurgical resection for large or symptomatic tumors; stereotactic radiosurgery (e.g., Gamma Knife) for tumors under 3 cm in select patients
Keywords	"Unilateral hearing loss," "tinnitus," "cerebellopontine angle mass," "poor speech discrimination out of proportion to pure tone loss," "bilateral acoustic neuromas = NF2," "decreased corneal reflex"

1. Pathophysiology

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.

2. Classification and Clinical Manifestation

Koos Grading

Grade	Size / Extent	Typical Clinical Features
Grade I	Small, intracanalicular (confined to the IAC)	Unilateral SNHL, tinnitus; may be incidental
Grade II	Extends into CPA but does not contact the brainstem	Progressive hearing loss, mild unsteadiness, tinnitus
Grade III	Fills the CPA cistern and contacts the brainstem without displacing it	Above findings plus facial numbness, decreased corneal reflex (CN V compression), possible facial twitching
Grade IV	Large tumor compressing and displacing the brainstem	Brainstem compression signs: ataxia, cranial neuropathies (CN V, VII), obstructive hydrocephalus, papilledema

Cranial Nerve Involvement

Structure Affected	Manifestation	Exam Significance
CN VIII, cochlear division	Unilateral SNHL, poor speech discrimination, tinnitus	Earliest and most common symptom; tested in nearly every vignette
CN VIII, vestibular division	Mild imbalance, unsteadiness (NOT classic spinning vertigo)	Distinguishes from Meniere disease and BPPV
CN V (trigeminal)	Ipsilateral facial numbness, absent corneal reflex	A classic "next step" physical exam finding in vignettes
CN VII (facial)	Facial weakness (late finding)	If present early, consider malignancy rather than schwannoma
Brainstem / cerebellum	Ipsilateral ataxia, gait instability, long tract signs	Indicates large (Koos IV) tumor
Fourth ventricle obstruction	Headache, nausea, vomiting, papilledema (obstructive hydrocephalus)	Late and ominous; demands urgent intervention

3. Diagnostic Workup

Test	Role	Key Findings
Pure tone audiometry + speech discrimination	Best initial test for any patient with asymmetric hearing loss	Unilateral or asymmetric high-frequency SNHL; speech discrimination score disproportionately poor relative to pure tone loss
MRI of the IAC and CPA with gadolinium	Most accurate (confirmatory) test	Well-circumscribed, homogeneously enhancing mass centered on the IAC, often extending into the CPA
Auditory brainstem response (ABR)	Screening tool; less sensitive than MRI	Prolonged wave I to V interpeak latency or absent waveforms on the affected side; used when MRI is unavailable or contraindicated
CT of the temporal bone	Adjunct for surgical planning	Widened IAC on the affected side; less sensitive than MRI for soft tissue detail

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.

4. Management and Treatment

Clinical Scenario	Management	Details
Small tumor (under 1.5 to 2 cm), minimal symptoms, elderly or high surgical risk	Observation ("watch and wait")	Serial MRI every 6 to 12 months; audiometry monitoring; appropriate for tumors with no growth on interval imaging
Growing tumor, symptomatic hearing loss, CN compression, or younger patient with long life expectancy	Microsurgical resection	Approaches: translabyrinthine (sacrifices residual hearing), retrosigmoid/suboccipital (hearing preservation possible), middle fossa (small intracanalicular tumors with serviceable hearing)
Tumor under 3 cm, patient declines or is unfit for surgery	Stereotactic radiosurgery (Gamma Knife)	Single-fraction focused radiation; tumor control rates above 90%; goal is growth arrest, not tumor elimination; risk of delayed facial neuropathy and hearing loss
Large tumor with brainstem compression or hydrocephalus	Urgent surgical decompression	Priority is brainstem decompression; CSF diversion (ventriculostomy or shunt) if hydrocephalus is present; subtotal resection may be acceptable to preserve CN VII

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.

5. Differential Diagnosis and Distractors

Differential	Why It Is Similar	Key Discriminator
Meniere disease	Both cause unilateral hearing loss, tinnitus, and vestibular symptoms	Meniere causes episodic attacks of spinning vertigo lasting 20 minutes to hours, with fluctuating low-frequency SNHL and a sensation of aural fullness; acoustic neuroma causes gradual, progressive high-frequency SNHL without episodic vertigo
Sudden sensorineural hearing loss (idiopathic)	Both present with unilateral SNHL	Sudden SNHL has an acute onset (within 72 hours), often with a viral prodrome; acoustic neuroma is insidious and progressive over months to years; MRI should be obtained in all sudden SNHL cases to exclude a schwannoma
Meningioma of the CPA	Both are enhancing CPA masses on MRI	Meningioma has a broad dural base ("dural tail" sign), is eccentric to the IAC rather than centered within it, and is more likely to cause hyperostosis of adjacent bone on CT
Epidermoid cyst of the CPA	Both are CPA masses causing cranial neuropathy	Epidermoid does not enhance with gadolinium; it appears bright on DWI (diffusion restriction) and follows CSF signal on T1/T2 but does not suppress on FLAIR
Cholesterol granuloma of the petrous apex	Can cause hearing loss and cranial nerve symptoms	Appears bright on both T1 and T2 (due to cholesterol and methemoglobin content), unlike schwannoma which is T1-dark and T2-bright before contrast
Otosclerosis	Causes progressive hearing loss	Otosclerosis produces conductive hearing loss (Rinne: bone conduction > air conduction; Weber lateralizes to the affected ear), not sensorineural loss; normal MRI
Presbycusis	Bilateral high-frequency SNHL in older adults	Presbycusis is symmetric and bilateral; speech discrimination is proportional to the degree of hearing loss; no CPA mass on imaging
NF2 with bilateral schwannomas	Both involve CN VIII schwannomas	NF2 is bilateral by definition; vignettes will mention a young patient (teens to 20s) with bilateral hearing loss, family history, and other CNS tumors (meningiomas, ependymomas)

6. Traps 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.

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