HealthDay News — Autism spectrum disorders (ASDs), schizophrenia, and bipolar disorder seem to share etiologic factors — having a first-degree relative who has schizophrenia or bipolar disorder increases the risk for having an ASD.

The autism risk was 2.9-fold higher with schizophrenia in parents and 2.6- to 12.1-fold elevated with schizophrenia in a sibling across various cohorts studied by Patrick F. Sullivan, MD, of the University of North Carolina at Chapel Hill, and colleagues.

They conducted a case-control evaluation of histories in first-degree relatives of probands in three samples (population registers from Sweden, Stockholm County in Sweden, and Israel). Probands met the criteria for ASD, and the affection status of their first-degree relatives was established. Study findings were published in the Archives of General Psychiatry.

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The national inpatient and outpatient registries in Sweden included 25,432 ASD cases, each matched to 10 controls for sex, birth year, and sex of relatives. The Stockholm County cohort included 4,982 ASD cases and 49,844 controls from 1984 through 2007. The Israeli cohort included 386 ASD cases born in the 1980s and 436,311 controls.

The likelihood of autism spectrum disorder was 2.9-fold elevated with a parental history of schizophrenia in both the Swedish national cohort (95% confidence interval 2.5 to 3.4) and the Stockholm County cohort (95% CI 2.0 to 4.1).

Schizophrenia in a sibling raised the risk 2.6-fold in the Swedish national cohort (95% CI: 2.0-3.2) and 12.1-fold in the Israeli cohort (95% CI: 4.5-32.0).

Similar patterns of association were seen for bipolar disorder, with lesser magnitude. Bipolar disorder in a parent was associated with a 90% elevated odds of an ASD in the Swedish national cohort (95% CI 1.7 to 2.1) and a 60% elevated odds of an ASD in the Stockholm cohort (95% CI 1.1 to 2.1).

“In conclusion, our findings suggest that ASD, schizophrenia, and bipolar disorder share common etiologic factors,” the researchers wrote.

The findings suggest that schizophrenia, bipolar disorder, and autism are different manifestations of the same root causes, they noted.

Sullivan PF et al. Arch Gen Psychiatry 2012; doi: 10.1001/archgenpsychiatry.2012.730.