Acoustic neuroma (AN), also known as vestibular schwannoma, arises from the neurilemman sheath of the superior vestibular nerve at the junction of the central and peripheral myelin. This point is called the Obersteiner-Redlich zone, located 8 - 12 mm distal to the brainstem close to the porus acusticus. AN arises as a result of inactivating mutations of the NF2 tumor suppressor gene in multiple or bilateral cases and in 60% of sporadic cases. There is also a loss of chromosome 22q.
Epidemiology
AN accounts for 8 - 10% of tumors in most series with an annual incidence of 0,78 - 1,15 cases per 100,000 population (2,280 new cases per year). It becomes symptomatic in patients of over 30, and is unilateral in 95% of cases. Peak age = 40 - 60 yrs (range = 30 - 70 yrs). Female:Male = 1:1. It is rare in children (unless NF2). Bilateral ANs are pathognomonic of NF2. Any patient < 40 years old with unilateral AN should be evaluated for neurofibromatosis Type 2 (NFT2). The ANs of NFT2 may infiltrate the nerve fibers.
AN represents most common cerebellopontine angle (CPA) - internal acoustic canal (IAC) mass (85 - 90%). 0,5% of lesions are associated with arachnoid cyst.
Pathology
AN is a round-ovoid, encapsulated mass that arises eccentrically from 8th cranial nerve, commonly in the Obersteiner-Redlich zone. Histologically there are differentiated neoplastic Schwann cells in a collagenous matrix. There are two cell types: Antoni A (compact and elongated cells) and B (with cluster of lipid-laden cells).
AN is a WHO grade I lesion. 75% of ANs are slow growing, while 10% grow rapidly (≥ 1 cm per year); 15% very slowly and can be left alone in older patients. Negative prognostic imaging findings for hearing preservation are: size > 2 cm and AN involving IAC fundus and/or cochlear aperture. The usually quoted range of growth rate is 1 - 10 mm/year. 6% of ANs decrease in size.
Imaging studies
CT findings: well-defined, enhancing mass of CPA-IAC cistern; no calcification; IAC may flare when larger; smaller intracanalicular lesions (< 6 mm) may be missed with CT.
MR findings: T1WI - intermediate signal; T2WI - high-resolution T2 MR: “filling defect” in high signal cerebrospinal fluid (CSF) of CPA-IAC cistern; T1 C - 100% enhance strongly.
Clinical presentation
Symptoms: hearing loss (98%); tinnitus (70%); dysequilibrium (67%); headache (32%); facial numbness (29%); facial weakness (10%); diplopia (10%); nausea and vomiting (9%); otalgia (9%); change of taste (6%). Obstruction of CSF circulation by a larger tumor (usually > 4 cm) may produce hydrocephalus with increased intracranial pressure. Symptoms are due to 8th nerve compression and 5th and 7th nerve involvement. Other symptoms are due to brainstem and other cranial nerve compression (in larger tumors).
Treatment options
Surgery
The translabyrinthine approach (TA) is considered for a tumor of any size and with Gardner-Robertson Class III or IV hearing. Patients with large tumors (>2,5 cm) and hearing in Class I or II should be recommended for TA due to the very low possibility of hearing conservation with such a large tumor.
The retrosigmoid approach is indicated for small and medium-size tumors and Class I or II hearing.
Epidemiology
AN accounts for 8 - 10% of tumors in most series with an annual incidence of 0,78 - 1,15 cases per 100,000 population (2,280 new cases per year). It becomes symptomatic in patients of over 30, and is unilateral in 95% of cases. Peak age = 40 - 60 yrs (range = 30 - 70 yrs). Female:Male = 1:1. It is rare in children (unless NF2). Bilateral ANs are pathognomonic of NF2. Any patient < 40 years old with unilateral AN should be evaluated for neurofibromatosis Type 2 (NFT2). The ANs of NFT2 may infiltrate the nerve fibers.
AN represents most common cerebellopontine angle (CPA) - internal acoustic canal (IAC) mass (85 - 90%). 0,5% of lesions are associated with arachnoid cyst.
Pathology
AN is a round-ovoid, encapsulated mass that arises eccentrically from 8th cranial nerve, commonly in the Obersteiner-Redlich zone. Histologically there are differentiated neoplastic Schwann cells in a collagenous matrix. There are two cell types: Antoni A (compact and elongated cells) and B (with cluster of lipid-laden cells).
AN is a WHO grade I lesion. 75% of ANs are slow growing, while 10% grow rapidly (≥ 1 cm per year); 15% very slowly and can be left alone in older patients. Negative prognostic imaging findings for hearing preservation are: size > 2 cm and AN involving IAC fundus and/or cochlear aperture. The usually quoted range of growth rate is 1 - 10 mm/year. 6% of ANs decrease in size.
Imaging studies
CT findings: well-defined, enhancing mass of CPA-IAC cistern; no calcification; IAC may flare when larger; smaller intracanalicular lesions (< 6 mm) may be missed with CT.
MR findings: T1WI - intermediate signal; T2WI - high-resolution T2 MR: “filling defect” in high signal cerebrospinal fluid (CSF) of CPA-IAC cistern; T1 C - 100% enhance strongly.
Clinical presentation
Symptoms: hearing loss (98%); tinnitus (70%); dysequilibrium (67%); headache (32%); facial numbness (29%); facial weakness (10%); diplopia (10%); nausea and vomiting (9%); otalgia (9%); change of taste (6%). Obstruction of CSF circulation by a larger tumor (usually > 4 cm) may produce hydrocephalus with increased intracranial pressure. Symptoms are due to 8th nerve compression and 5th and 7th nerve involvement. Other symptoms are due to brainstem and other cranial nerve compression (in larger tumors).
Treatment options
Surgery
The translabyrinthine approach (TA) is considered for a tumor of any size and with Gardner-Robertson Class III or IV hearing. Patients with large tumors (>2,5 cm) and hearing in Class I or II should be recommended for TA due to the very low possibility of hearing conservation with such a large tumor.
The retrosigmoid approach is indicated for small and medium-size tumors and Class I or II hearing.
References
- Handbook of Neurosurgery - fifth edition - Mark S. Greenberg - ed. Thieme.
- Ross JS, Brant-Zawadzki M, Moore KR, et al.. Diagnostic Imaging Spine. First edition - Amirsys - Elsevier Saunders - 2004.
