A bidirectional association between cognitive ability in young adulthood and epilepsy: a population-based cohort study.

ABSTRACT

Aim: To investigate the bidirectional association between cognitive ability in young adulthood and epilepsy. Methods: This cohort study included 1 159 076 men enrolled in the mandatory conscription board examination from the Danish Conscription Database (DCD; 658 465 men examined 1957-84), the Danish Defence Personnel Organization Database (DPOD; 216 509 men examined 1987-2005) and the Danish Conscription Registry (DCR; 284 102 men examined 2006-15). A supplementary analysis included 14 814 female volunteers. Cognitive ability was measured at conscription, and epilepsy was ascertained by physician diagnoses in the Danish National Patient Registries 1977-2016 [using International Classification of Diseases (ICD) codes ICD-8-345; ICD-10-G40-G41]. Differences in cognitive ability in relation to epilepsy status at the time of conscription (age 19) were calculated using linear regression. The risk of epilepsy associated with cognitive ability was estimated using Cox regression models, split at age at follow-up (40 and 60 years) and adjusted for year of birth, cerebrovascular disease, traumatic brain injury and education. Results: In all, 5097 (1.0%) men from the DPOD and DCR were diagnosed with epilepsy before conscription, and they had about 0.25 standard deviation (SD) lower cognitive scores than men without epilepsy. The largest difference in cognition was seen for those with the largest number of hospital contacts. A total of 22 364 (1.9%) men developed epilepsy, and cognitive ability was inversely associated with the risk of epilepsy. With the end of follow-up at age 40 years, the adjusted hazard ratio (HR)per SD increase was 0.75 (95% confidence interval = 0.73-0.77). The association attenuated with increasing age at diagnosis. The findings were replicated in female conscripts. Conclusions: The cognitive impairment seen in adults with epilepsy seems to reflect combined effects of epileptic processes and lower premorbid cognitive ability.