Objective: Premorbid intellectual impairment has been described in schizophrenia, but little is known about premorbid intellectual functioning in bipolar disorder or other psychoses. In this study, premorbid intellectual ability was investigated in individuals with bipolar disorder, schizophrenia, or other psychoses.
Method: Results on verbal, arithmetic, and visuospatial reasoning tests were obtained for 195,019 apparently healthy male subjects conscripted into the Finnish Defense Forces during 1982-1987 (mean age, 19.9 years). Linkage with the Finnish Hospital Discharge Register (mean follow-up time, 7.1 years) identified conscripts later diagnosed with bipolar disorder (N=100), schizophrenia (N=621), or other psychoses (N=527).
Results: Poor performance on the visuospatial reasoning test at age 20 was associated with higher risks for all three disorders. The odds ratios indicating the difference in risk of illness between the lowest and highest of nine performance categories were 34.65 (95% confidence interval [CI], 4.05-296.44) for bipolar disorder, 13.76 (95% CI=5.49-34.47) for schizophrenia, and 4.28 (95% CI=2.09-8.77) for other psychoses. In contrast, the higher the score for arithmetic reasoning, the greater the risk of bipolar disorder; a high score was associated with a more than 12-fold greater risk. Verbal test performance was not associated with higher risk for psychiatric disorder.
Conclusions: These results indicate that premorbid visuospatial reasoning is impaired in bipolar disorder and schizophrenia and, to a smaller extent, in other psychoses. This suggests that a subtle neurodevelopmental aberration is involved in the etiology of bipolar disorder and schizophrenia. High arithmetic test performance may be associated with greater risk for bipolar disorder.