Abstract
Intellectual functioning (IQ) is lower in schizophrenia patients compared to healthy controls, with bipolar patients intermediate between the two. Declines in IQ mark the onset of schizophrenia, while stability is generally found post-onset. There are to date few studies on long-term IQ development in bipolar disorder. This study presents 10-year follow-up data on IQ, including premorbid IQ estimates, to track the developmental course from pre-onset levels to long-term outcomes in both patient groups compared to healthy controls. We included 139 participants with schizophrenia, 76 with bipolar disorder and 125 healthy controls. Mixed model analyses were used to estimate developmental slopes for IQ scores from estimated premorbid level (NART IQ) through baseline (WASI IQ) measured within 12 months post-onset, to 10-year follow-up (WASI IQ), with pairwise group comparisons. The best fit was found using a model with a breakpoint at baseline assessment. Only the schizophrenia group had significant declines from estimated premorbid to baseline IQ levels compared to controls. When comparing patient groups, schizophrenia patients had steeper declines than the bipolar group. Increases in IQ were found in all groups over the follow-up period. Trajectories of IQ from premorbid level to 10-year follow-up indicated declines from estimated premorbid level to illness onset in both patient groups, followed by increases during the follow-up period. Schizophrenia patients had a steeper decline than bipolar patients. During follow-up, increases indicate developmental improvement for both patient groups, but with a maintained lag compared to healthy controls due to lower premorbid levels.
Highlights
Intellectual functioning (IQ) encompasses the ability to reason and adaptively solve problems in everyday life (Wechsler, 1944)
We found no difference in the change over time depending on baseline IQ and, despite a baseline difference between completers and noncompleters in the schizophrenia group, proceeded with using the entire sample in mixed model analyses
We found negative correlations between IQ and both antipsychotic (WASI IQ: r = −.34, p = 0.003, NART IQ: − 0.31, p = 0.010) and antiepileptic (WASI IQ: r = −0.28, p = 0.02) dosage at baseline, but no significant association with lithium
Summary
Intellectual functioning (IQ) encompasses the ability to reason and adaptively solve problems in everyday life (Wechsler, 1944). IQ is associated with symptom severity and clinical outcome (Leeson et al, 2011), and may indicate rehabilitation potential (Amoretti et al, 2018) Together these observations underline the importance of increasing our knowledge about IQ development in schizophrenia and bipolar disorder as it could provide insight into the mechanisms of these disorders. The present study seeks to model IQ development from premorbid levels to long-term follow-up in schizophrenia and bipolar disorder, compared to healthy controls. This study presents 10-year follow-up data on IQ, including premorbid IQ estimates, to track the developmental course from pre-onset levels to long-term outcomes in both patient groups compared to healthy controls. During follow-up, increases indicate developmental improvement for both patient groups, but with a maintained lag compared to healthy controls due to lower premorbid levels
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