Cognitive decline in psychotic disorders: A longitudinal matched case-control study

Abstract

Background Patients with psychotic disorders often exhibit premorbid cognitive deficits and future cognitive changes following the onset of illness. This study examined cognitive trajectories in schizophrenia (SCZ), schizoaffective disorder (SZA) and bipolar disorder (BD) compared with healthy matched controls. Method Cases included individuals with SCZ (n = 24), SZA (n = 15), or BD (n = 11), and 50 controls matched for age, sex, and socioeconomic status (SES). Participants underwent IQ testing at age 17 as part of mandatory military screening (T1); the same tests were administered after illness onset (T2). Results Participants were assessed approximately 10 years after T1 (SD = 2.6). Cases with SCZ had a non-significant 4-point lower IQ than controls at T1. At T2, SCZ cases showed a 4-point IQ decline, whereas controls demonstrated a 5-point increase, resulting in a significant time × group interaction (F(1, 46) = 12.30, p = .006). At T2 the IQ of cases with SZ was 13 points lower than that of controls (96.97 ± 16.57 vs. 110.19 ± 11.87, respectively; Cohen's d = 0.91; p = .009). A similar trend was observed among cases with SZA and BD, although the magnitude was smaller and the interaction was non-significant. In SCZ, premorbid IQ was positively correlated with years of education (r = 0.670, p < .01), while IQ decline was associated with more psychiatric hospitalizations (r = −0.547, p < .01). Conclusion SCZ showed cognitive decline relative to controls over time, whereas SZA and BD demonstrated less pronounced cognitive changes. These findings are consistent with both neurodevelopmental and neurodegenerative models of psychotic illness.