Comorbidity and the increased mortality after hospitalization for stroke: a population-based cohort study

Background: Comorbidity is prevalent among stroke patients, affecting post-stroke survival. It remains unknown whether comorbidity impacts post-stroke mortality beyond the combined individual effects of stroke and comorbidity. Methods: Using nationwide Danish databases, we performed a cohort study of 201 691 patients >= 18 years old with incident ischemic stroke, intracerebral or subarachnoid hemorrhage, or unspecified stroke during 1995-2012, and 992 942 adults from the general population, matched to stroke patients by birth year, sex and individual comorbidities in the Charlson Comorbidity Index. During up to 5 years of follow-up, we computed standardized mortality rates (SMRs) to assess interaction contrasts as a measure of excess mortality not explained by the additive effects of stroke and comorbidity acting alone. Results: Five-year post-stroke mortality was 48%, corresponding to an SMR of 187 deaths per 1000 person-years. During the 30-day peak post-stroke mortality (SMR, 180 per 1000 person-months), interaction with comorbidity represented 23%, 34% and 51% of post-stroke mortality rates among patients with low (score = 1), moderate (score = 2-3) and high (score = 4+) comorbidity based on Charlson Comorbidity Index scores. The interaction accounted for 5% to 32% of subsequent 31-365-day post-stroke mortality rates, depending on comorbidity level. The interaction contrasts were most notable among comorbid patients with cancer, particularly with hematological or metastatic disease, followed by patients with moderate-to-severe liver or renal disease. Conclusion: Comorbidity, notably cancer and advanced renal or liver disease, increased 1-year mortality after stroke beyond the combined effects expected from either disease acting alone.