[Frequency of hereditary neurologic diseases. A clinical study]

The nervous system is affected in 30% of hereditary monogenic disorders and as many as 500 single-gene disorders display major neurologic symptoms. We have studied the frequency of hereditary neurological diseases to assess their importance in daily hospital activity. Only single-gene hereditary diseases with central or peripheral nervous system involvement were considered; thus chromosomal diseases and diseases with multifactorial etiology were excluded.

We surveyed admission to in- and out-patient departments of Neurology, Pediatrics, and Dermatology of the Aosta Regional Hospital for the calendar years 1982-1991, collecting 229 cases, 95 women and 134 men. Out-patient departments held 126 patients, the others came from in-patient departments. Admission to the neurological in-patient department were 1.8% of total neurological admissions in the same period. Each diagnosis was assigned to the code number of the International Classification of Diseases (ICD-IX Revision, 1975).

We found 33 different phenotypes. Most frequent diagnoses were: essential tremor (89 patients), hereditary sensory-motor neuropathy (HSMN) type I (28), Huntington's chorea (13), progressive muscular dystrophy limb-girdle type (8), neurofibromatosis type I (9), HSMN type II (9), spinocerebellar ataxia (9), hereditary spastic paraplegia (7), spinal muscular atrophy type IV (5), myotonic dystrophy (5), cerebellar ataxia (4), HSMN type III (4), spinal muscular atrophy type II and III (3), tuberous sclerosis (3). Essential tremor mostly affected persons in the over-50 age groups. On the contrary, the other neurologic monogenic diseases were diagnosed in all ages with the following age-group breakdown: 0-9, 11%; 10-19, 16%; 20-29, 15%; 30-39, 8%; 40-49, 11%; 50-59, 19%; 60-69, 14%, 70+, 7%. Consistently with the general rule, autosomic recessive diseases have the earliest onset and autosomic dominant ones the latest; HSMN, spinal muscular atrophy and Huntington's chorea were the disorders diagnosed in older age group.

Although the frequency of the single neurologic monogenic disease is low (with the exception of essential tremor), their overall prevalence is higher than the prevalence of multiple sclerosis or the peripheral neuropathies. All age-groups are involved. We separated three groups of diseases: 1) two relatively benign diseases, essential tremor and the HSMN, affecting half of our patients; 2) five severe and more common diseases (Huntington's chorea, progressive muscular dystrophy limb-girdle type, neurofibromatosis type I, spinocerebellar ataxia, hereditary spastic paraplegia), affecting 1/4 of patients; and 3) a group of rare, mostly severe diseases, affecting the remaining 1/4 of patients.