Preserved, Deteriorated, and Premorbidly Impaired Patterns of Intellectual Ability in Schizophrenia
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Objective: Although impaired general intellectual ability is a prevalent feature in schizophrenia, patterns suggesting preserved, deteriorated, and premorbidly impaired intellect have also been identified. The main purpose of this investigation was to examine the clinical, cognitive, and neuroanatomical characteristics of these intellectual subtypes, and to establish the value and validity of this approach for reducing the heterogeneity of schizophrenia. Methods: A total of 71 patients with a diagnosis of schizophrenia or schizoaffective disorder and 66 healthy controls were assessed. A preserved performance pattern (n=29) was defined by average-range estimated premorbid and current IQ with no evidence of decline (premorbid- current IQ difference <10 points). A deteriorated pattern (n=14) was defined by a difference between estimated premorbid and current IQ estimates of 10 points or more. A premorbidly impaired pattern (n=14) was defined by below average estimated premorbid and current IQ and no evidence of decline greater than 10 points. The groups were compared on demographic, neurocognitive, clinical, and neuroanatomical variables. Results: Patients with the preserved pattern outperformed those meeting criteria for deteriorated and premorbidly impaired intellectual ability on a composite measure of neurocognitive ability, as well as on indicators of processing speed, attention, working memory, verbal and visual memory, and social cognition. However, preserved patients scored lower than control participants on tests of processing speed, verbal memory, and reasoning/problem solving. Patients demonstrating the deteriorated and premorbidly impaired patterns were indistinguishable across all cognitive measures. The patient groups were clinically indistinguishable from each other and showed a similar pattern of widespread cortical thinning compared to controls. Conclusions: Cognitive impairment is a core feature of schizophrenia present to some degree in all patients, regardless of their intellectual status. Therefore, IQ fails to capture the true breadth of cognitive impairment in schizophrenia. Although the preserved subtype has partial validation, comprehensive neurocognitive data provides little support for the distinctiveness of deteriorative relative to premorbid intellectual compromise. Cognitive ability and symptom severity represent independent disease processes in schizophrenia, and cortical thinning across the brain appears to reflect a shared disease process with no association to intellectual or cognitive status.