Is autism one or multiple disorders?

From the earliest description of autism in 1943 to the present day, there has been a widely held view that the behavioural anomalies associated with the disorder occur more often together than would be expected by chance, and therefore there will be a single causal pathway that explains the non-random co-occurrence of these symptoms.1 The phenotypic variability of autism has proved to be a major stumbling block for aetiological research. The heterogeneity spans the entire range of intelligence quotients (IQs) and language abilities, as well as other behavioural, communicative and social functions. While any psychiatric condition is likely to incorporate a degree of heterogeneity, the variability in the nature and severity of behaviours observed in autism is thought to exceed that of other disorders.

An instructive example here is cerebral palsy. In the mid 19th century, cerebral palsy was thought to be a unitary disorder caused by anoxia secondary to trauma occurring during labour and delivery. However, the variability in the nature of impairment between individuals with cerebral palsy, spanning varying degrees of motor, intellectual and sensory difficulties, led researchers to hypothesise that there may be many causal pathways, with only a minor proportion of cases being a direct result of perinatal hypoxia. Other identified causes include a range of genetic syndromes, neuronal migration disorders, complications of preterm birth, infections and inflammation in utero, and postneonatal causes such as bacterial meningitis.5 Contemporary international agreements for diagnosis therefore emphasise that cerebral palsy is an umbrella term covering a wide range of syndromes that arise secondary to a variety of brain lesions or anomalies occurring early in development.6

Current evidence suggests that autism may also best be conceptualised as an umbrella term for a collection of behavioural disorders resulting from a range of causal pathways.

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