The aims of the present study were to compare NP performance between schizophrenia, schizoaffective disorder, psychotic major depressive disorder, and psychotic bipolar disorder, estimate the prevalence of NP impairment using different criteria, and examine the possibility of NP normality in these disorders. Despite extensive NP investigations of these disorders, particularly schizophrenia, this is the first epidemiological investigation of these questions simultaneously. Using a well-characterized, first-admission sample, and on the basis of a comprehensive NP examination, it can be concluded that the 4 diagnostic groups vary only minimally in their NP performance profiles, characterized by common relative deficits in memory, attention and processing speed, and executive functions. This supports previous observations in clinical samples of first-admission and chronic patients that persons with schizophrenia and mood disorder with psychotic symptoms have similar relative NP strengths and weaknesses but that persons with schizophrenia manifest more severe impairments.12,18,20,21
This may further suggest similar pathophysiology, possibly involving frontal lobe circuits, underlying the NP deficits in different psychotic disorders.12
In keeping with prior studies,3,4,6
a substantial proportion of persons with schizophrenia or schizoaffective disorder were classified as NP impaired. NP impairment was also common in psychotic affective disorders. Nevertheless, only 2 pairs of classification methods converged well across diagnostic groups: GDS and CSCI. A third method (IPR) seemed less sensitive to impairments, perhaps due to applying a more stringent criterion. The GDS and CSCI may, however, have a low threshold for classifying impairment. For example, performance of 1 SD or more below mean on 2 ability areas is sufficient for a patient to be classified as “neuropsychologically impaired” using the CSCI criterion.3
Because rates of NP impairment vary depending on the criteria applied, and convergence across criteria is only moderate, a careful selection of criteria for NP impairment should be applied to both everyday clinical NP assessment of psychotic persons, as well as for the design and selection of participants in pharmacological studies targeting cognition in schizophrenia and other psychotic disorders.
Despite NP abnormalities, NP normality was detectable, even among persons with schizophrenia. In this epidemiological sample, 15% of the schizophrenia group was classified as neuropsychologically normal across all criteria. First, this calls into question the concept of NP impairment as a core feature of schizophrenia. One potential criterion for a “core” deficit is that it should be present in all patients.38
Applying this criterion, the present study argues against NP impairment as a core deficit. Further supporting this are the results that NP deficits are qualitatively similar across all psychotic disorders. Second, the existence of such a group of psychotic persons may have important implications for the efforts to understand the underlying brain mechanisms of the disorder. While studying the physiological mechanisms of persons with schizophrenia who have impaired cognitive functions has the potential of elucidating mechanisms associated with poor performance, studying persons with schizophrenia with normal cognitive functioning has the potential of identifying unaffected, or protective, neural functions,39
justifying further research in this area.
This study has several strengths. First, the patient groups were drawn from an epidemiological sample and are, therefore, representative of the patient population. Second, the sample has been particularly well characterized and consensus diagnoses were based on semistructured interviews. Nevertheless, limitations of the study should be acknowledged and results should be interpreted in light of these limitations. The sample size for the schizoaffective group was small. Second, standard scores were calculated based on published norms rather than on NP assessment of a control group of healthy subjects. However, the individual tests in the NP assessment battery were selected on the basis of both relevance to schizophrenia and the availability of comprehensive norms. Furthermore, the use of norms in the present study is probably a more ecologically valid method: Standard NP assessment practice uses identical methods and relies on available norms, not control subject groups. Nevertheless, education-adjusted norms were not available for the majority of tests. Given that education and NP performance are highly correlated, estimates of rates of impairment might be biased. However, diagnostic groups did not differ in level of education, suggesting that bias, if exists, should not affect group comparisons. Fourth, it has been suggested that deterioration from premorbid levels of intellectual performance may be important in establishing rates of NP impairment.4,8
Thus, even among the group of patients who perform within normal limits on the present battery of tests, there could be undetected impairments or discrepancies between premorbid IQ and current functions. Although we did not have a reliable measure of premorbid intellectual functioning, previous studies4,8
used only estimates of premorbid intellectual functioning which may introduce bias. More importantly, Kremen et al37
have demonstrated that differences in current NP function in schizophrenia are attributable primarily to current IQ, rather than IQ trajectory over time. Fifth, the methods for determining individual impairment status assume that tests that tap the same cognitive function are grouped together. Although we did not demonstrate this empirically (ie, using factor analysis), classification of individual NP measures to ability areas followed common, well accepted, classifications.32
Sixth, although there were correlations between NP impairment and symptoms severity across the different diagnostic groups, the majority of those were small, indicating that symptoms account for only a small proportion of the variance in NP impairment and cannot explain group differences in rates of impairments. Finally, this is a study of first-admission psychotic cases. Previous studies of individual classification of NP impairment used predominantly chronic patients.3,4,6
Furthermore, cases were tested only at one point in time. Thus, although it is not possible to draw definitive conclusions about rates of impairments across the life course of psychotic illness, rates of impairment in the present study are within the range previously reported supporting the validity of individual classification methods across duration of illness and effects of long-term treatment with antipsychotic medications.
In conclusion, in a longitudinal, epidemiological study of first-admission psychotic patients, we found strong evidence supporting the notion that differences in NP performance between schizophrenia and psychotic affective disorders are largely quantitative. We demonstrated that NP impairment is common in psychotic affective disorders but that a significant minority of individuals with psychotic disorders (including schizophrenia) perform within the normal range.