ABSTRACT
Etiologically, autism can be divided into idiopathic (without an identifiable risk factor) and secondary (with an identifiable risk factor). About 80-85% of cases of autism are idiopathic and 15-20% are secondary (1). In a meta-analysis of 23 surveys of autism published between 1966 and 1998, “a medical condition of potential causal significance” was found in only 6% of cases (2). Although many causes of autism have been proposed for the idiopathic group, few meet the essential criteria of causation. According to Hill, to be causal, an association should be strong, consistent, sufficient, and necessary, and should precede the condition. There should be a dose relationship between the cause and the condition, a plausible and coherent biological explanation, and an experimental model (3). No single factor meets all these criteria, suggesting that autism is not due to a single cause. Most likely it is a heterogeneous disorder caused by many factors that work either independently, or in tandem, or in concert to cause neurological dysfunction, that, in turn, manifests as the syndrome of autism (Fig. 1). The foremost among these factors is genetic susceptibility.
