Differential effects of delirium on fluid and crystallized cognitive abilities

Patients with delirium (acute confusional state) show extensive cognitive deficits. These deficits have typically been measured using tests of fluid cognition, which involve the active processing of mental representations. However, the effects of delirium on stored, crystallized dimensions of cognition, such as well-learnt word pronunciation knowledge, are not known. In this study 37 patients (aged 60-85 years) without delirium were recruited before undergoing cardiac surgery. Cognitive assessments were performed 0-8 days before surgery and again 2-9 days after surgery in order to determine the effects of post-operative delirium (POD) on fluid and crystallized aspects of cognition. Crystallized cognition was tested with the National Adult Reading Test (NART). Fluid cognition was tested with digit span, verbal fluency and Stroop tests. Nine patients (24%) developed delirium post-operatively. Patients with delirium showed significant post-operative deficits on most tests of fluid cognition, but no change in the NART measure of crystallized cognition (p=0.95). These results parallel recent findings in Alzheimer's dementia and suggest that, despite showing extensive deficits of fluid cognitive processing, crystallized cognition is preserved in delirium. The results also suggest that the NART may be a useful tool for assessing premorbid ability in patients with delirium.