Management of acoustic neuromas. A review

Gamma surgery for vestibular schwannoma

OBJECT

The goal of this study was to assess the results of gamma surgery (GS) for vestibular schwannoma (VS) in 200 cases treated over the last 10 years and to review the role of this neurosurgical procedure in the management of VS.

METHODS

Follow-up reviews ranging from 1 to 10 years were available in 153 of these patients. Follow-up images in these cases were analyzed using computer software that we developed to obtain volume measurements for the tumors, and the clinical condition of the patients was assessed using questionnaires. Gamma surgery was the primary treatment modality in 96 cases and followed microsurgery in 57 cases. Tumors ranged in volume from 0.02 to 18.3 cm(3). In the group in which GS was the primary treatment, a decrease in volume was observed in 78 cases (81%), no change in 12 (12%), and an increase in volume in six cases (6%). The decrease was more than 75% in seven cases. In the group treated following microsurgery, a decrease in volume was observed in 37 cases (65%), no change in 14 (25%), and an increase in volume in six (11%). The decrease was more than 75% in eight cases. Five patients experienced trigeminal dysfunction; in three cases this was transient and in the other two it was persistent, although there has been improvement. Three patients had facial paresis (in one case this was transient, lasting 6 weeks; in one case there was 80% recovery at 18 months posttreatment; and in one case surgery was performed after the onset of facial paresis for presumed increase in tumor size). Over a 6-year period, hearing deteriorated in 60% of the patients. Three patients showed an improvement in hearing. No hearing deterioration was observed during the first 2 years of follow-up review.

CONCLUSIONS

Gamma surgery should be used to treat postoperative residual tumors as well as tumors in patients with medical conditions that preclude surgery. Microsurgery should be performed whenever a surgeon is confident of extirpating the tumor with a risk-benefit ratio superior to that presented in this study.

A study of the effects of cochlear loss on the auditory brainstem response (ABR) specificity and false positive rate in retrocochlear assessment

The auditory brainstem response (ABR) test is widely used as a screen for retrocochlear dysfunction in individuals suffering idiopathic auditory and vestibular disturbance. Although its sensitivity for lesions of the lower brainstem is well established, hearing loss is known to have significant effects upon the test's specificity and false positive rate. This study analysed the effects of aspects of cochlear hearing loss including level, slope and general audiogram shape on these properties of the ABR test in a large clinical group. The study comprised 306 patients (153 male, 153 female) referred for ABR screening to exclude retrocochlear dysfunction. Of 566 ears meeting the inclusion criteria, 85 (15 per cent) failed the test. On follow-up eight ears (3 per cent) were confirmed to have some form of retrocochlear abnormality, seven of which were pathological. All had abnormal ABR results. No cases of retrocochlear abnormality were found in the ABR pass group. In those patients diagnosed as free of retrocochlear problems, the importance of the I-V interval as a primary index of function was confirmed. It displayed a high specificity (>90 per cent) which was largely independent of the level or slope of high-frequency hearing loss. In contrast wave V specificity decreased with increasing loss and increasing slope. Its value as a functional index was limited with losses greater than 70 dB HL in whom specificity fell below 70 per cent. The study concluded that, despite reducing interval availability above 60 dB HL, wave V latency cannot be used as an alternative index because it displays weakening specificity over this range of loss. With the problems inherent in applying latency corrections, the author advocates the use of improved procedures for wave I identification in these cases.

Significance of hearing preservation in acoustic neuroma surgery

In the past 10 years, 43 patients with acoustic neuroma have been operated on by the middle cranial fossa approach. In all cases, meticulous care was taken to preserve the cochlear function regardless of the degree of preoperative hearing disorder. Thirty-nine of 43 patients had various degrees of residual hearing, hearing was preserved in 16 of these (41%). The best result was obtained in patients with a tumor located less than 1 cm from the porus. The hearing preservation rate was 64% (11/17). The patients recovered hearing acuity quite well, especially at low frequencies in 2 cases who had a pure tone hearing level of more than 50 dB and less than 50% of speech discrimination. These 2 cases had a history of sudden deafness that was intractable 5 months preoperatively. Thus it is not correct to set certain criteria for hearing preservation for patients whose preoperative hearing may not be serviceable.

Cystic vestibular schwannomas: neuroimaging and growth rate

In a series of 571 vestibular schwannomas (VS) operated on in the period 1976-1992, via the translabyrinthine approach, 23 tumours (4 per cent) from 23 patients were radiologically, peroperatively and histologically identified as cystic VS. Neuroimaging examinations (CT and MRI) revealed 15 tumours (2.6 per cent) with extracystic formations and in eight cases (1.4 per cent) intratumoural cysts. In 15 cases (65 per cent), tumours were located extracanalicularly with no involvement of the internal auditory canal (IAC). The extracanalicular tumour extension including the cystic elements ranged between 10 and 60 mm with a mean of 45 mm. In eight cases out of 23 (35 per cent) other diagnostic possibilities such as trigeminal schwannoma, epidermoid cyst and meningioma were proposed at the primary evaluation. Deformation, displacement and/or impression of the fourth ventricle was demonstrated in 17 cases (74 per cent). Three patients (13 per cent) had obstructive hydrocephalus which demanded shunt insertion. Residual tumour was detected in two patients (9 per cent). Growth rate in four cases was more than 10 times higher when compared to the annual growth rate of noncystic VS. The results of this study provide data on cystic VS attenuation demonstrated by CT or MRI which may be valuable in neuroradiological diagnosis of these cystic lesions and emphasizes that the wait-and-see policy should not be applied to the cystic variant of VS.

The burden of screening for acoustic neuroma: asymmetric otological symptoms in the ENT clinic

The rising numbers of legal cases relating to delay in the diagnosis of acoustic neuroma, combined with the increasing availability of magnetic resonance, is increasing pressure on otologists to make an early definitive diagnosis of cerebellopontine angle tumours. Unilateral or asymmetrical otologic symptoms not explained by external or middle ear disease are elicited in 16.6% of 500 consecutive attenders to an otolaryngology clinic. An agreed policy of risk stratification of patients with unexplained asymmetric otological symptoms is required if expense is to be limited and litigation minimized.

Intraoperative monitoring by transtympanic electrocochleography and brainstem electrical response audiometry in acoustic neuroma surgery

The preservation of hearing is a major aim of contemporary temporal bone surgery. Our present findings demonstrate that intraoperative monitoring is a key method for attaining serviceable postoperative hearing after the removal of an acoustic neuroma. Both electrocochleography (ECoG) and brainstem electrical response audiometry were performed in 96 patients operated on for acoustic neuromas. The specificity of the different monitoring methods was affected by surgical manipulations in addition to such non-specific influences as CSF drainage, core body temperature and anesthesia. In the present study ECoG was found to be more reliable in assessing the intra- and postoperative course with respect to the preservation of cochlear function.

Advantages of the enlarged middle cranial fossa approach in acoustic neurinoma surgery. A review

Utilizing an enlarged middle cranial fossa approach to the cerebello-pontine angle without destruction of the labyrinth or cochlea the authors have since 1981 operated on 263 unilateral acoustic neurinomas. Tumour sizes ranged between 3 mm intrameatal and 35 mm within the cerebello-pontine angle. Complete tumour removal was accomplished in 96%. There was one postoperative mortality, and only rarely neurological complications. Excellent function of the facial nerve was obtained in 78% (in small and medium sized neurinomas 90% House I and II) and severe paralysis persisted in only 6%. Preservation of hearing was possible in 70% of the small tumours, and in 50% of the total group. Against this background comparable data of the literature are reviewed, and the indications for the enlarged midfossa approach analyzed.