Neurocognition in schizophrenia: a 20-year multi-follow-up of the course of processing speed and stored knowledge

Abstract

Individuals with schizophrenia have relative deficits in cognition, although little is known regarding the course of such deficits across the life span and at various stages of the illness. Furthermore, the relationship between psychosis and cognition has not been adequately explored to this point. Prospective, longitudinal, multi-assessment studies of the same patients across time are rare in the field and provide a unique opportunity to examine long-term changes in cognition among individuals with schizophrenia. As part of The Chicago Follow-up Study, we prospectively assessed 244 psychiatric inpatients, including individuals with schizophrenia, other psychotic disorders, and nonpsychotic depression. Assessments were conducted 7 times (once at index hospitalization and then 6 times subsequently for the next 20 years) to provide longitudinal data about cognition and symptoms, with a focus on 2 aspects of cognition: processing speed and the ability to access general knowledge. The Digit Symbol-Coding and Information subtests from the Wechsler Adult Intelligence scale were used to measure the 2 cognitive domains at each assessment. At all 7 assessments, individuals with schizophrenia performed more poorly than the other diagnostic groups on the 2 cognitive measures. However, after the acute phase (index hospitalization), individuals with schizophrenia demonstrated significant improvements in cognition and did not show evidence of cognitive decline over the remaining 6 assessments spanning 20 years. Our data support the presence of relative cognitive impairment in schizophrenia, as well as a pattern of stability in some cognitive areas after the acute phase. In addition, we find evidence for an association between relative cognitive impairment and psychosis.

Figure 1

Processing speed at Index Hospitalization and six follow-up evaluations in each diagnostic group

Figure 2

Performance on measure of accessing general knowledge at Index Hospitalization and six follow-up evaluations in each diagnostic group

Figure 3

Percentage of patients from each diagnostic group showing improvement in processing speed from Index Hospitalization to 2-year follow-up.