Development of an epilepsy-specific risk adjustment comorbidity index

Purpose: To develop an epilepsy-specific comorbidity risk adjustment index for mortality outcomes research. Methods: Data were extracted from five linked administrative databases in Calgary, Canada from April 1, 1996 to March 31, 2004. Epilepsy patients were defined using a validated ICD-9-CM- and ICD-10-CA-based case definition. An epilepsy-specific comorbidity index was developed using comorbidities from the Charlson and Elixhauser indexes and other relevant epilepsy comorbidities. In the final model, 14 comorbidities significantly associated with mortality remained and each was assigned a value of 1-6 based on the hazard ratio from the survival analysis. Total prognostic scores were calculated and compared for all subjects using the epilepsy- specific index and the Charlson index. Crude mortality and survival curves of both indices were compared. Key Findings: We identified 7,253 subjects who met our case definition for epilepsy. The mean age of participants was 38 years (range 0.03-96), and 52% were male. The mortality rate was 7.9%. High rates of chronic pulmonary disease (20.3%), hypertension (19.6%), cerebrovascular disease (13.7%), fracture (12.1%), depression (28.2%), and alcohol abuse (10.1%) were noted. Patients with lower total prognostic scores were more likely to survive than patients with higher scores, using both indices. However, increasing prognostic scores were more strongly associated with reduced survival using the epilepsy-specific index compared to the Charlson index. Significance: A new comorbidity index for epilepsy, designed to include clinically relevant conditions, provided better discrimination of crude mortality in a population- based group of epilepsy patients compared with the Charlson index.