Duration of illness and structure of symptoms in schizophrenia

Background. Previous research has mainly focused on the cross-sectional structure of symptoms in schizophrenia. This meta-analysis examined the association of duration of illness with the structure of symptoms. Methods. Using explicit criteria, 22 studies reporting on the correlations of symptoms in 2665 schizophrenic patients were selected. From each study, symptom-pair correlations for negative symptoms as rated by Scale for the Assessment of Negative Symptoms (SANS) and positive symptoms as rated by the Scale for the Assessment of Positive Symptoms (SAPS) were extracted. Variability among symptom-pair correlations across studies was assessed using tests of homogeneity. For symptom-pair correlations which were not found to be homogeneous, the association of average duration of illness with the symptom-pair correlations were examined. Results. There was considerable variability in symptom-pair correlations across studies. Part of this variability was explainable by variations in average duration of illness. Longer duration of illness was associated with lower negative-negative symptom-pair correlations and higher negative-positive symptom-pair correlations. Conclusions. The findings suggest that the structure of symptoms in schizophrenia evolves over time, following a consistent pattern. In the early stages of illness, negative and positive symptoms form cohesive dimensions. With time, these dimensions become less cohesive and the boundaries between them, less clear.