First, because of the importance of the questions addressed; is neuropsychological impairment in schizophrenia a reliable, trait-related characteristic of the illness, or does it fluctuate with clinical symptoms; is schizophrenia associated with progressive cognitive decline and, if so, how often does this occur and what are the baseline characteristics of the patients who are affected in this way? A second reason I think it is cited frequently is that the study methods improve on those of previous studies of this topic: This was a study of large groups of patients with schizophrenia (n=142) and normal comparison subjects (n=206) who were followed longitudinally for up to 10 years (mean=3) with standardized clinical and comprehensive neuropsychological testing. In addition to comparing changes in group means, norms for neuropsychological change were developed with the control subjects and applied to individual subjects with schizophrenia, in an effort to identify and characterize any subgroup of patients who experience cognitive decline.

I think using neuropsychological norms for change in this type of research has important advantages, especially when one is trying to identify a phenomenon—cognitive change, in this case, deterioration—that does not occur in all or even in most people with the disease in question.

Early observers considered schizophrenia a dementing illness, i.e. dementia praecox, yet most modern, cross-sectional studies of patients with schizophrenia have not found that cognitive "impairment", corrected for normal aging, is greater in patients who are older or more chronic; i.e. most cross sectional data do not suggest a progressive course of cognitive decline in schizophrenia.

In our sample of ambulatory patients with schizophrenia, cognitive functioning over time was highly reliable and stable. Their cognitive status was not altered by fluctuations of psychotic symptoms, and we were unable to identify even a small subgroup of patients whose cognition deteriorated over the follow-up period. A limitation of this study is that we did not include chronically institutionalized patients, so our results cannot be generalized to this relatively small but important subgroup.

Robert K. Heaton, Ph.D.,
Professor of Psychiatry, 
University of California at San Diego.