There is general agreement that autistic disorder has an organic basis, but there is less agreement on the frequency with which it is associated with known medical conditions. After a review of the existing literature, Rutter, Bailey, Bolton, and Le Couteur (1994) concluded that the rate of known medical conditions in autism is probably about 10%, and that the rate is higher in autistic disorder associated with profound mental retardation and in cases of atypical autism. Fombonne, du Mazaubrun, Cans, and Grandjean (1997a) concluded from a French epidemiological survey that known medical disorders (excluding epilepsy and sensory impairments) accounted for fewer than 10% of the cases of autism. Gillberg and Coleman (1996) reported a higher rate of 24.4% with known medical conditions, but with a similar trend for higher rates of medical disorders among individuals with both autism and severe mental retardation. Barton and Volkmar (1998) retrospectively reviewed medical records of 211 subjects with autism and found the prevalence of associated medical conditions varied between 10% and 15%, and with less strict definition of medical condition, between 25% and 37%. However, these medical conditions or disorders are not considered to be the causes of autism.

Neurological abnormalities have been reported in 30% to 50% of several series of autistic patients (reviewed by Tsai, 1999). These abnormalities include hypotonia or hypertonia, disturbance of body schema, clumsiness, choreiform movements, pathological reflexes, myoclonic jerking, drooling, abnormal posture and gait, dystonic posturing of hands and fingers, tremor, ankle clonus, emotional facial paralysis, and strabismus. These are all signs of dysfunction in the basal ganglia, particularly in the neostriatum, and closely related structures of the medial aspects of the frontal lobe or limbic system. . .

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