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There has been an increasing awareness of the importance of differentiating NF1 and NF2 in recent years culminating in the mapping of the genes to different chromosomes"q in 1987. In the same year the NIH Consensus Conference statement defined diagnostic criteria for NF1 and NF2.5 The NF2 criteria include distinct ophthalmological findings (posterior subcapsular lenticular opacities), although the frequency of these had been assessed in relatively small numbers of patients at that time.6 The NIH criteria for NFl have been used in a large study and found to be satisfactory,7 but the NF2 criteria have not been similarly assessed.A large clinical and genetic study of NF2 has allowed us to address these and other issues. The study has highlighted the inadequacy of current counselling and screening of at risk relatives. In this paper we present the findings of the study with relevance to genetic counselling, namely, the adequacy of current diagnostic criteria, the natural history of NF2, and suggestion of a screening protocol for at risk subjects. Patients and methodsThe methods of patient ascertainment, clinical assessment, and diagnostic criteria are described in the preceding paper.8 In addition all cases who did not precisely fit the NIH NF2 criteria (table 1), but who were thought to have the disease on clinical grounds were studied to allow evaluation of the diagnostic criteria.

The use of a closed set neurofibromatosis type 2 questionnaire identified hearing problems and subsequent communication difficulties as the main problems faced by people with this condition.

Background-Some 4%-5% of those who develop vestibular schwannomas have neurofibromatosis type 2 (NF2). Although about 10% of these patients present initially with a unilateral vestibular schwannoma, the risk for a patient with a truly sporadic vestibular schwannoma developing contralateral disease is unknown. Methods-A United Kingdom survey of 296 patients with NF2 was reviewed for laterality of vestibular schwannoma at presentation and the presence of other NF2 related features. The time to presentation of bilateral disease was calculated for patients presenting with a unilateral tumour. Mutation analysis of the NF2 gene was carried out on all available cases presenting initially with unilateral disease. Results-Of 240 patients with NF2 with vestibular schwannomas, 45 (18%; 32 sporadic, 13 familial) had either a unilateral tumour or delay in detection between the first and contralateral tumours. Among those tested for NF2 mutations, eight of 27 and nine of 13 were identified among sporadic and familial cases respectively. Sporadic cases showed a high female to male ratio and 19 of 32 have not as yet developed a contralateral tumour (mean 4.1 years after diagnosis of the first). Thirteen of 32 sporadic patients developed a contralateral tumour (mean 6.5 years after the first tumour diagnosis, range 0-22 years) compared with 11 of 13 familial patients (mean delay 5 years, range 0-16 years). Seven of the 45 patients had neither a family history of NF2 nor evidence of related tumours at initial presentation (six before the age of 35 years). Conclusion-The risk of patients with sporadic unilateral vestibular schwannomata developing a contralateral tumour in the absence of family history or other features of NF2 is low, but those presenting with other neurogenic tumours in addition to vestibular schwannoma are at high risk of harbouring an NF2 mutation in at least a proportion of their somatic cells. (J Neurol Neurosurg Psychiatry 1999;66:764-767) Keywords: neurofibromatosis type 2; somatic mosaicism; mutation; vestibular schwannoma Type 2 neurofibromatosis (NF2) is an autosomal dominant inherited condition characterised by development of bilateral vestibular schwannomas, schwannomas of other cranial, spinal, and cutaneous nerves, and cranial and spinal meningiomas.1-3 The National Institutes of Health (NIH) defined diagnostic criteria for NF2 in 1987 4 and modified criteria to allow for sporadic cases have since been published. 2According to NIH criteria (table 1) a person with bilateral vestibular schwannomas is assumed to have NF2 and 50% of oVspring would be predicted to be aVected. As the isolation of the NF2 gene in 1993 5 6 mutation studies have included reports of germ line mutations.7-9 Detection rates using routine methodology have been disappointingly low even in classically aVected patients and cannot therefore be used as a means of excluding the condition.At presentation, 10%-20% of patients with NF2 have a unilateral vestibular schwannoma, although other features of the disorder may be identi...