# Lateral Skull Base: Acoustic Neuroma (Vestibular Schwannoma)

## Introduction

**Vestibular schwannomas** (commonly termed acoustic neuromas) are benign tumors arising from the Schwann cells of the vestibular portion of cranial nerve VIII. They represent the most common tumor of the **cerebellopontine angle (CPA)**, accounting for approximately 80% of CPA lesions. Management requires understanding of lateral skull base anatomy, audiologic assessment, and the nuances of surgical versus conservative approaches.

## Epidemiology and Pathophysiology

Incidence: approximately 1-2 per 100,000 per year. Peak presentation: 4th-6th decades of life. Most arise from the **inferior vestibular nerve** at the Obersteiner-Redlich zone (glial-Schwann cell junction). **Bilateral vestibular schwannomas** are pathognomonic for **Neurofibromatosis Type 2 (NF2)** — autosomal dominant, chromosome 22q12 mutation. Slow-growing (1-2 mm/year on average), but growth rate is variable.

## Clinical Presentation

**Asymmetric sensorineural hearing loss (SNHL)**: most common presenting symptom (>90%). **Tinnitus**: unilateral, often high-pitched. **Imbalance** and unsteadiness (true vertigo is less common). **Facial numbness** (CN V compression) with larger tumors. **Facial weakness** (CN VII) is rare at presentation but indicates large tumor. Brainstem compression: hydrocephalus, ataxia, headache (late findings).

## Diagnostic Workup

### Audiometry
**Asymmetric SNHL** with poor speech discrimination disproportionate to pure tone thresholds. **ABR (auditory brainstem response)**: prolonged wave I-V interpeak latency; sensitivity ~95% for tumors >1 cm.

### Imaging
**MRI with gadolinium**: gold standard; enhancing mass in the IAC/CPA. Evaluate tumor size, relationship to brainstem and cranial nerves, presence of hydrocephalus. **CT of temporal bone**: assess bony anatomy of the IAC for surgical planning.

![MRI with gadolinium demonstrating a vestibular schwannoma in the right cerebellopontine angle with IAC extension](/images/vestibular-schwannoma-mri.jpg)

## Classification

### Koos Grading System

| Grade | Description | Size |
|-------|-------------|------|
| I | Intracanalicular tumor | Confined to IAC |
| II | Extends into CPA | <2 cm |
| III | Occupies CPA cistern, no brainstem contact | 2-3 cm |
| IV | Brainstem compression | >3 cm |

**Grade I**: intracanalicular tumor. **Grade II**: extends into CPA, <2 cm. **Grade III**: occupies CPA cistern, 2-3 cm, no brainstem contact. **Grade IV**: >3 cm, brainstem compression.

## Management Options

### Observation (Watch and Wait)
Appropriate for small tumors, elderly patients, only-hearing ears. Serial MRI at 6 months, then annually, Up to 50% of tumors show no growth over 5 years.

### Stereotactic Radiosurgery (Gamma Knife / CyberKnife)
Typically for tumors <3 cm without significant brainstem compression. Tumor control rates: 90-95% at 10 years. Hearing preservation: 50-70% at 5 years. Risk of delayed facial neuropathy: 1-5%.

### Microsurgical Resection

| Approach | Hearing Preservation | Best Indication | Key Advantage |
|----------|---------------------|-----------------|---------------|
| Translabyrinthine | No (sacrificed) | Non-serviceable hearing | Excellent facial nerve exposure |
| Retrosigmoid (suboccipital) | Possible (30-50%) | Medial tumors with serviceable hearing | Good access to CPA |
| Middle cranial fossa | Best (50-70%) | Small intracanalicular tumors | Superior hearing preservation |

**Translabyrinthine approach**: sacrifices hearing; excellent facial nerve exposure; ideal for non-serviceable hearing. **Retrosigmoid (suboccipital) approach**: hearing preservation possible; good for medial tumors. **Middle cranial fossa approach**: best hearing preservation for small intracanalicular tumors with serviceable hearing.

![Illustration comparing the three surgical approaches to vestibular schwannoma: translabyrinthine, retrosigmoid, and middle fossa](/images/vs-surgical-approaches.jpg)

## Surgical Considerations

**Intraoperative facial nerve monitoring** (EMG) is mandatory. **ABR monitoring** for hearing preservation approaches. Facial nerve is typically displaced anteriorly and superiorly by the tumor. Goal is **gross total resection** while preserving facial nerve function. A thin rind of tumor may be left on the facial nerve to preserve function.

## Outcomes and Complications

**Facial nerve outcomes** (House-Brackmann scale): HB I-II in 80-95% for experienced surgeons. **Hearing preservation**: 50-70% for middle fossa, 30-50% for retrosigmoid. **CSF leak**: 5-15%, managed with lumbar drain or wound revision. **Headache**: common after retrosigmoid approach. **Meningitis**: 1-2%.

![Intraoperative photograph showing facial nerve preservation during translabyrinthine vestibular schwannoma removal](/images/vs-facial-nerve-intraop.jpg)

## Key Clinical Pearls

Any patient with **asymmetric SNHL** or unilateral tinnitus should have MRI to rule out vestibular schwannoma. **NF2 should be suspected** in any patient with bilateral vestibular schwannomas or a young patient with unilateral VS. The translabyrinthine approach provides the **best facial nerve visualization** but sacrifices all residual hearing. Small, stable tumors in elderly patients are often best managed with observation. Serviceable hearing is defined as **AAO-HNS Class A or B** (PTA <=50 dB, SDS >=50%).

## References

1. Carlson ML, Link MJ, Wanna GB, Driscoll CLW. Management of sporadic vestibular schwannoma. *Otolaryngol Clin North Am*. 2015;48(3):407-422.
2. Kanzaki J, Tos M, Sanna M, et al. New and modified reporting systems from the Consensus Meeting on Systems for Reporting Results in Vestibular Schwannoma. *Otol Neurotol*. 2003;24(4):642-649.
3. Goldbrunner R, Weller M, Regis J, et al. EANO guideline on the diagnosis and treatment of vestibular schwannoma. *Neuro Oncol*. 2020;22(1):31-45.
4. Brackmann DE, Cullen RD, Fisher LM. Facial nerve function after translabyrinthine vestibular schwannoma surgery. *Otolaryngol Head Neck Surg*. 2007;136(5):773-777.