The prevalences of epilepsy and mental retardation in the general population are both close to ! per cent. Epilepsy is more common and more difficult to diagnose and to treat in people with mental retardation than in those with normal intellect. Population-based studies have revealed that mental retardation occurs in at least 30 to 40 per cent of individuals with epilepsy, (2!> while the prevalence of epilepsy in the mentally retarded population is about 20 to 25 per cent. (22) It is higher in the more severely retarded (IQ < 50) than in the mildly retarded (IQ < 50-70)—30 to 50 per cent and !5 to 20 per cent respectively. Brain damage tends to be more extensive when epilepsy, mental retardation, or cerebral palsy are complicated by each other or by other conditions of brain origin. It is unlikely that specific causal factors of epilepsy, mental retardation, or cerebral palsy will ever be positively identified, for these are non-specific clinical features of brain disorder. Epileptic fits themselves, especially if they are persistent, may produce brain damage and play a part in producing a progressive decline in the intellectual functioning of patients. Further apparent deterioration of intellectual functioning may be the result of excessively high doses of anticonvulsant drugs.
Epilepsy occurs in all the main aetiological categories of mental retardation. In a series of !000 mentally retarded patients, epilepsy was less frequent in the prenatal category than in the rest of the series (182/515 or 35.3 per cent versus 260/485 or 53.6 per cent). Of the main types of epilepsy, partial epilepsy is more frequent in the prenatal and postnatal aetiological categories and in the category of infections and intoxications. (23>
In people with Down syndrome the frequency of epilepsy is 5 to !0 per cent. There is an age-related bimodal distribution with about 40 per cent of seizures starting before the age of ! year and another 40 per cent starting after the third decade. (24> Roughly 25 per cent of individuals with fragile X syndrome have epileptic seizures which are usually infrequent, mild, easily controlled, and typically disappear in adolescence, as in benign rolandic epilepsy. In Angelman syndrome epilepsy is present in more than 90 per cent of the affected individuals.(25) In Rett syndrome epilepsy affects up to 90 per cent of patients. (16> Seizures are usually benign during the early years of life. In patients with aspartylglucosaminuria, epilepsy is found in 28 per cent of adults and in 2 per cent of children. (26) Epilepsy is common (up to 100 per cent) in patients with the various forms of neuronal ceroid lipofuscinoses, especially during the last years of life, (27> and also in other inborn errors of metabolism leading to mental retardation such as sialidosis type !, Tay-Sachs disease, type 3 Gaucher disease, mitochondrial encephalopathy with lactic acidosis and strokes, and myoclonic epilepsy with ragged red fibres.
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