Acoustic schwannoma (neurnoma, neurolemmoma) is the mass formation of the bridge of the cerebellopontine angle, which grows into the auditory canal, expanding it into the cranial cavity, affecting mainly adults; bilateral schwannomas are associated with neurofibromatosis type II.
Neuroma of the anterior-cochlear nerve (synonyms: schwannoma, neurolemmoma) is a benign volumetric formation, with fairly distinct and relatively even contours, located along the course of the VIII pair, with a tendency to slow expansive growth.
By localization, acoustic neurinomas can be:
- in-channel (located in the internal auditory canal);
- extrachannel (located in the bridge of the bridge);
- mixed (growing in the canal and in the bridge of the bridge).
The structure of the tumor can be a cystic, solid and mixed structure.
Intrachannel neurinomas are completely solid, and intracranial, as well as mixed, can be cystic, solid and cystic-solid.
During growth, the tumor increases the volume and causes atrophy of the canal walls from pressure, expanding the internal auditory meatus.
When the tumor reaches a diameter of up to 1.5-2 cm, it can exert a mass effect on adjacent structures, subjecting them to compression.
At the MRI, a solid component of the tumor ↓ by T1, → or ↑ by T2, the cysts have a liquid MR signal. On CT, neurinoma → and ↓ depending on the structure (solid or cystic).
Acoustic neuromas (schwannoma) account for approximately 80 per cent of extra-axial lesions in the region of the cerebellopontine angle and 4–10 per cent of intracranial tumours overall (Mahaley et al. 1990; Grant et al. 1996).
Other tumours in the region of the cerebellopontine angle include:
- meningiomas (10 %),
- primary cholesteatoma (5–10 %),
- glomus jugulare tumours (1 %),
- facial or trigeminal neuroma (1–2 %),
- metastasis (1–5 %).
The incidence of acoustic neuroma is approximately 1/100 000/yr.
The acoustic nerve is the most common site for a neuroma (85 %), although they can also arise from:
- trigeminal nerve (1–8 %),
- facial nerve (0.5–1 %),
- and spinal roots (10–15 %).
Schwannomas arise from the junction between the peripheral Schwann cell nerve sheath and the central glial nerve sheath. Ninety five per cent of acoustic neuromas are sporadic and 5 per cent are dominantly inherited as part of neurofibromatosis type 1 (NF1) or type 2 (NF2).
Karyotype analysis in sporadic schwannomas may be normal. The most common abnormality is monosomy of chromosome 22 and there may be deletions in the long arm of this chromosome (22q). The gene for NF2 has been isolated to a 6 Mb region of the q12 band of the long arm of chromosome 22, and it is highly likely that the relevant area on this chromosome involves a tumour suppressor gene. Mutations to the NF2 gene are also likely to be an important step in the pathogenesis of sporadic unilateral acoustic neuroma and have been found in 40–70 per cent of cases.
The majority of acoustic neuromas arise from the vestibular branch of the nerve (85–90 per cent). Acoustic neuromas present with unilateral slowly progressive hearing loss (95 per cent), sometimes associated with non-specific unsteadiness (77 per cent), tinnitus (71 per cent), or vertigo. Commonly, the sensorineural deafness (90 per cent) is associated with facial sensory loss (50 per cent) or facial weakness (10 per cent). Hydrocephalus due to obstruction of CSF pathways can lead to raised intracranial pressure. MRI is the scanning procedure of choice.
The tumours are very slow growing. Almost half of the tumours do not grow perceptibly over a 5-year follow-up. Of those that do enlarge, in 75–80 per cent of cases the growth rate is only 1–2 mm/year (Bederson et al. 1991). There may therefore be a case for conservative management with careful follow-up rather than intervention, especially in the elderly or those in poor general health, or where the tumour is small or there is contralateral deafness and retained hearing in the affected ear. Growth rate apparently doesn't correlate with the age of the patient, the size of the tumour, or the duration of symptoms (Bederson et al. 1991).
Surgical management has a high morbidity, particularly in patients with retained hearing. A suboccipital approach has the advantage of possibly retaining any existing hearing, but may require cerebellar traction and can cause postoperative headaches and cerebellar symptoms. The translabyrinthine approach has the advantage of requiring little in the way of cerebellar retraction and, because the surgery is largely extradural, complications of meningitis and headache are less; however, hearing is always lost postoperatively. The middle fossa approach is useful for small tumours and may spare hearing; however, there are increased complication rates from facial nerve paresis and possibly temporal lobe sequelae (e.g. seizures, dysphasia, etc.) from temporal lobe traction. Monitoring brainstem auditory evoked potentials (BAEPs) intraoperatively can significantly decrease the morbidity of surgery, especially when trying to preserve hearing. Prolongation of the latency of wave V of the BAEPs is usually an early sign that the acoustic nerve is being compromised. Acoustic neuromas may require a joint surgical approach by both ENT and neurosurgeons. Surgery should aim to remove the tumour completely and to preserve facial nerve function and, where possible, preserve hearing. Neurosurgery for acoustic neuroma has a postoperative mortality of approximately 5 per cent. Mortality is related to the size of the tumour and age of patient (Hardy et al. 1989).
These percentages are heavily affected by selection of patients, experience of the surgeon, and possibly surgical approach. Mortality using the translabyrinthine approach is probably not significantly different from the suboccipital or middle fossa approaches when patient characteristics are taken into account. There is a view that small tumours are better approached by a translabyrinthine approach whereas large tumours are best approached suboccipitally or by middle fossa approach. Translabyrinthine surgery results in complete hearing loss but this is not a problem if hearing is already lost preoperatively, and there may be slightly more chance of preserving facial nerve function. Anatomical preservation of the facial nerve can be achieved in about 80–90% of cases, but anatomical preservation is not always associated with good facial nerve function. It is exceptionally rare for postoperative hearing to be better than preoperative hearing, using either the suboccipital or middle fossa approaches. Where hearing preservation is the main aim, for instance if there is already contralateral deafness and the affected ear has maintained hearing, a suboccipital approach has advantages.
In selected centres it has been shown that with experienced neurosurgeons, using the retromastoid route and evoked potential monitoring, complete resection of the tumour can be accomplished with preservation of hearing in 50 per cent of patients with tumours smaller than 2 cm and more than 80 per cent of patients who have a tumour of less than 1 cm diameter (Post et al. 1995). Indeed, if preoperative hearing loss is slight, then the retromastiod approach is preferred for small tumours. However, in a recent review of the literature it is clear that only a few patients have truly normal hearing after surgery (Sanna et al. 1995). Delayed deterioration in hearing, years after successful operation, is well recognized in up to 50 per cent of patients, although the cause remains uncertain (Shelton et al. 1990; Ogunrinde et al. 1994). Attempts at maintaining hearing by minimizing resection can be complicated by recurrence of the tumour.
In some cases, where hearing is preserved and the tumour is small, stereotactic radiosurgery or streotactic radiotherapy using a linear accelerator can be effective. Stereotactic radiosurgery uses a single fraction of high dose but small volume radiation to the tumour. Conformal-beam stereotactic radiotherapy, using a linear accelerator and fractionating the treatment over several days and reducing the dose of each fraction, has potential advantages in that radiation-induced neural side-effects are reduced by reducing the fraction size. Radiation therapy is not usually advised for tumours larger than 3 cm because of the increased risks of central nervous system side-effects. The aim of radiation therapy is to prevent growth of acoustic neuroma. Tumours smaller than 3 cm in diameter commonly show shrinkage (35 per cent) or ‘stabilization’ (60 per cent) at 2 years after stereotactic radiosurgery (Ogunrinde et al. 1994).
This apparent success has to be compared with the natural history of acoustic neuroma. One study of conservative management demonstrated that 71 per cent of acoustic neuromas do not enlarge over 3.4 years (Deen et al. 1996). Short-term complications from stereotactic radiosurgery or stereotactic radiotherapy using a linear accelerator are infrequent; however, it will be some years before one can fully ascertain the effect of radiation on the acoustic nerve and surrounding structures, particularly in patients with normal preradiation hearing. Hearing is very likely to become impaired with time: only 50 per cent of patients with preserved hearing following radiation therapy will maintain this at 6 months, and only 45 per cent at 1–2 years (Ogunrinde et al. 1994).
Two years after stereotactic radiosurgery, preserved facial nerve function was achieved in 90 per cent and trigeminal nerve function in 75 per cent of cases who had no deficit immediately post-radiotherapy. The results of conformal-beam stereotactic radiotherapy are as good as stereotactic radiosurgery, but long-term side-effects appear to be less, probably reflecting the fractionation schedule and reduction in fraction size. Where surgery is contraindicated because of poor health or poor risk–benefit ratio, radiosurgery or conformal stereotactic radiotherapy should be considered the treatment of choice, and there is adequate evidence that these treatments have a therapeutic role. The role of radiotherapy in the treatment of small acoustic neuromas remains controversial.
The coauthor of the article: radiologist, Ph.D. Vlasov Evgeniy Alexandrovich
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