# Vestibular Schwannomas and Skull Base Tumors
## Overview
Vestibular schwannomas (VS, also termed acoustic neuromas) are benign tumors arising from the Schwann cells of the vestibular portion of CN VIII. They account for approximately 8% of all intracranial tumors and 80-90% of cerebellopontine angle (CPA) tumors. Management options include observation, stereotactic radiosurgery (SRS), and microsurgical resection, with the primary goals being tumor control, hearing preservation, and facial nerve preservation.
## Epidemiology
Vestibular schwannomas have an annual incidence of 1-2 per 100,000, with peak incidence in the 5th to 6th decade and no sex predilection. Unilateral tumors account for over 95% of sporadic cases. Bilateral vestibular schwannomas are pathognomonic of NF2 (neurofibromatosis type 2), an autosomal dominant condition caused by mutations in the merlin/schwannomin gene on chromosome 22q12. NF2-associated tumors present at a younger age (teens to 20s) and tend to be more aggressive.
## Anatomy of the CPA and Internal Auditory Canal (IAC)
### Internal Auditory Canal
The IAC contains CN VII (facial nerve) and CN VIII (vestibulocochlear nerve). The transverse crest (crista falciformis) divides the IAC into superior and inferior compartments, while the vertical crest (Bill's bar) divides the superior compartment into anterior (facial nerve) and posterior (superior vestibular nerve) portions. Bill's bar is the key surgical landmark for identifying the facial nerve in the lateral IAC.
### CPA Cistern
The CPA cistern is bounded by the cerebellum, pons, and petrous bone. It contains CN V, VII, VIII, IX, X, XI, the anterior inferior cerebellar artery (AICA), and the flocculus. AICA loops into or near the IAC in 20-40% of cases and must be preserved during surgery. The labyrinthine artery, a branch of AICA, supplies the cochlea; injury to this vessel causes immediate hearing loss.
### Koos Grading (Tumor Size)
The Koos grading system classifies VS by size: Grade I is intracanalicular (confined to the IAC). Grade II describes a small CPA component under 2 cm that extends into the CPA but does not reach the brainstem. Grade III occupies the CPA cistern without brainstem compression (2-3 cm). Grade IV is a large tumor with brainstem compression and possible hydrocephalus (over 3 cm).
| Koos Grade | Size | Description | CN VII Outcome (HB I-II) |
|-----------|------|-------------|--------------------------|
| I | Intracanalicular | Confined to IAC | >95% |
| II | <2 cm CPA | Extends into CPA, no brainstem contact | >95% |
| III | 2-3 cm | Fills CPA cistern, no brainstem compression | 80-90% |
| IV | >3 cm | Brainstem compression ± hydrocephalus | 50-70% |
## Clinical Presentation
Unilateral sensorineural hearing loss is the most common presenting symptom, occurring in 95% of cases. It is typically progressive, asymmetric, and high-frequency. Unilateral tinnitus (often high-pitched) is common. Vestibular dysfunction manifests as disequilibrium rather than true vertigo, since the slow tumor growth allows gradual vestibular compensation. Large tumors may cause facial numbness from trigeminal nerve compression, though facial weakness is uncommon at presentation despite CN VII being stretched over the tumor. Hydrocephalus develops when large tumors obstruct the fourth ventricle. Sudden sensorineural hearing loss can occur from vascular compromise of the labyrinthine artery.
### Audiometric Evaluation
Pure-tone audiometry reveals asymmetric sensorineural hearing loss, and the speech discrimination score is critical for determining serviceable hearing. The AAO-HNS hearing classification defines Class A as PTA of 30 dB or less with SDS of 70% or greater (good serviceable hearing), Class B as PTA between 30-50 dB with SDS of 50% or greater (serviceable hearing), Class C as PTA above 50 dB with SDS of 50% or greater, and Class D as any PTA with SDS below 50% (non-serviceable). Auditory brainstem response (ABR) testing shows a prolonged wave I-V interval but has been largely supplanted by MRI for screening.
Coronal section through the internal auditory canal showing the relationship of CN VII (facial nerve, anterosuperior) and CN VIII (vestibulocochlear nerve, posteroinferior) separated by the transverse crest and vertical crest (Bill's bar). A vestibular schwannoma is shown arising from the superior vestibular nerve and expanding the IAC with extension into the CPA cistern. The AICA loop and labyrinthine artery are shown. The brainstem and cerebellum form the medial and posterior boundaries. Clean anatomical illustration with labeled structures.
## Management Options
### Observation (Watch-and-Wait)
Observation is appropriate for small tumors (under 1-1.5 cm), elderly patients, or those with poor surgical candidacy. Many vestibular schwannomas grow slowly at 1-2 mm per year or not at all, with approximately 50% showing no growth at 5 years. Serial MRI is performed every 6-12 months initially, then annually. Intervention is considered if significant growth exceeds 2-3 mm per year, hearing worsens, or new symptoms develop.
### Stereotactic Radiosurgery (SRS)
SRS is indicated for tumors under 3 cm (ideally under 2.5 cm), growing tumors, patients who are not surgical candidates, or as a matter of patient preference. Gamma Knife delivers a standard marginal dose of 12-13 Gy. Tumor control rates reach 93-97% at 10 years. Hearing preservation ranges from 50-75% at 5 years but declines over time. Facial nerve preservation exceeds 95-99%, and trigeminal neuropathy risk is 2-5%. Delayed complications include rare hydrocephalus and extremely rare (and debated) malignant transformation. SRS is not recommended for NF2-associated VS when hearing preservation is a priority due to higher failure rates.
### Microsurgical Approaches
#### Translabyrinthine Approach
This approach sacrifices hearing completely by removing the labyrinth. It is the best approach when hearing is non-serviceable, for any tumor size, and when direct early identification of the facial nerve at the lateral IAC is desired. Its advantages include early CN VII identification at Bill's bar, minimal cerebellar retraction, and a direct path to the tumor. Disadvantages include complete hearing loss, CSF leak risk requiring abdominal fat graft closure, and the need for otology team involvement.
#### Retrosigmoid (Suboccipital) Approach
This approach preserves the possibility of hearing. It is appropriate for CPA tumors with limited IAC extension, medium to large tumors where hearing preservation is desired, and posterior fossa tumors other than VS. Advantages include wide CPA exposure, hearing preservation potential, and familiarity for neurosurgeons. Disadvantages include required cerebellar retraction, limited lateral IAC access (cannot drill past the posterior semicircular canal), higher postoperative headache rate, and the fact that the facial nerve is often on the anterior surface of the tumor and thus hidden from the surgeon's initial view.
| Feature | Translabyrinthine | Retrosigmoid | Middle Fossa |
|---------|-------------------|--------------|--------------|
| Hearing preservation | No (sacrificed) | Possible | Best rates |
| Tumor size | Any | Medium-large | <1.5 cm |
| CN VII identification | Early (Bill's bar) | Late (anterior tumor surface) | Within IAC |
| Cerebellar retraction | Minimal | Required | None |
| Ideal candidate | Non-serviceable hearing | CPA tumor, hearing preservation desired | Small intracanalicular, serviceable hearing |
| Key disadvantage | Complete hearing loss | Limited lateral IAC access, headache | Temporal lobe retraction, limited for large tumors |
#### Middle Fossa Approach
This approach offers the best hearing preservation rates and is strictly indicated for small intracanalicular or small CPA tumors under 1.5 cm with serviceable hearing. Its advantages include excellent hearing preservation for small tumors and direct IAC exposure. Disadvantages include temporal lobe retraction, limited exposure for larger tumors, technical difficulty, and the risk of CN VII injury within the narrow confines of the IAC.
### Facial Nerve Management
The facial nerve is displaced and thinned by the tumor, with its position varying but most commonly found anterior and superior to the tumor. The nerve is often paper-thin and adherent to the tumor capsule. Intraoperative facial nerve monitoring with continuous EMG is mandatory, and direct facial nerve stimulation identifies the nerve throughout dissection. Facial nerve outcomes correlate strongly with tumor size: Koos I-II tumors achieve greater than 95% House-Brackmann grade I-II outcomes, while Koos IV tumors achieve 50-70%. Facial reanimation procedures such as CN VII-XII anastomosis or cross-face nerve grafting are options for patients with permanent facial paralysis.
## Other CPA Tumors
### Epidermoid Cyst
The epidermoid cyst is the third most common CPA lesion. It appears as a pearly white, flaky tumor that insinuates around neurovascular structures. On MRI it is bright on DWI (the key distinguishing feature from arachnoid cyst), T1 hypointense, and T2 hyperintense. Complete removal is curative but recurrence occurs if capsule fragments remain.
### CPA Meningioma
This is the second most common CPA tumor. It has a broad dural base on the petrous bone and does not extend into the IAC (unlike VS). It shows homogeneous enhancement with a dural tail.
### Glomus Jugulare (Paraganglioma)
This tumor arises from glomus bodies in the jugular foramen and shows characteristic "salt and pepper" enhancement on MRI due to flow voids. Preoperative embolization is often required. Lower cranial nerve deficits (CN IX, X, XI) are typical.
Comparison of the three microsurgical approaches for vestibular schwannoma resection. The translabyrinthine approach shows the lateral-to-medial dissection through the mastoid with early facial nerve identification at Bill's bar. The retrosigmoid approach shows posterior fossa exposure behind the sigmoid sinus with cerebellar retraction. The middle fossa approach shows temporal lobe elevation with superior IAC exposure. Each approach is shown as a cross-sectional diagram with the tumor, facial nerve position, and key anatomical landmarks labeled. Indications for each approach are listed below. Surgical anatomical illustration.
## Clinical Pearls
Asymmetric sensorineural hearing loss warrants MRI with gadolinium to rule out VS, as audiometry alone is insufficient for screening. Bill's bar (the vertical crest) is the most reliable intraoperative landmark for locating the facial nerve in the lateral IAC. Facial nerve function is the primary outcome measure of VS surgery, and nerve preservation should be prioritized over total tumor removal; leaving a thin rind of tumor on a functioning facial nerve is acceptable. AICA and the labyrinthine artery must be identified and preserved during surgery, since injury causes cerebellar infarction or immediate deafness, respectively. Bilateral VS mandates genetic testing for NF2; management differs, with hearing preservation being paramount, bevacizumab potentially useful, and cochlear implants considered. Postoperative CSF leak is the most common surgical complication (approximately 10%), and the translabyrinthine approach requires meticulous closure with fat graft, dural sealant, and bone wax. House-Brackmann grade should be documented preoperatively, postoperatively, and at every follow-up to track facial nerve recovery.
## References
- Carlson ML, et al. "Management of Vestibular Schwannoma -- What Is the Best Strategy?" *NEJM*. 2021;384(13):1230-1242.
- Stangerup SE, et al. "Natural History of Vestibular Schwannoma." *Otol Neurotol*. 2006;27(4):547-552.
- Samii M, Matthies C. "Management of 1000 Vestibular Schwannomas: Surgical Management and Results." *Neurosurgery*. 1997;40(1):11-23.
- Koos WT, et al. *Microsurgery of Cerebellopontine Angle Tumors*. Thieme, 2002.
- Rhoton AL Jr. "The Cerebellopontine Angle and Posterior Fossa Cranial Nerves by the Retrosigmoid Approach." *Neurosurgery*. 2000;47(Suppl 3):S93-S129.