Early versus adult onset of schizophrenia: an examination of premorbid and current IQ.

Background: Cognitive deficits are core findings in schizophrenia, but whether the severity of impairments is related to the age of onset remains unclear. We hypothesized that early onset schizophrenia (EOS; onset before age 19) is associated with lower IQ compared to adult-onset schizophrenia (AOS; onset from age 19). Methods: We included 99 adult patients with EOS (age of onset: 15.3 ± 2.8 years), 282 adult patients with AOS (age of onset: 26.5 ± 7.4 years), and 863 adult healthy controls (HC). We assessed current IQ with Wechsler Abbreviated Scale of Intelligence (WASI) and estimated premorbid IQ with National Adult Reading Test (NART). Results: Both patient groups had lower current IQ than HC (p < 0.001). Full-scale (p = 0.004), performance (p = 0.003) and verbal (p = 0.011) current IQ were significantly lower in EOS than in AOS, with 5 IQ units difference for all three measures. EOS and AOS did not differ in premorbid IQ, but EOS showed a steeper IQ decline from premorbid levels than AOS (11.4 vs. 8 IQ units, respectively, p = 0.013). Conclusion: EOS had lower current IQ than AOS, but did not differ in premorbid IQ, suggesting a larger decline from premorbid IQ levels. This could imply different neurodevelopmental processes underlying cognitive dysfunction related to age of onset in schizophrenia, underscoring the necessity for further inquiry into the mechanisms driving this decline and strategies for its prevention.