Long-term systemic inflammation and cognitive impairment in a population-based cohort

Evidence suggests inflammation is associated with cognitive impairment, but previous epidemiological studies have reported conflicting results.

Prospective population-based cohort.

Epidemiology of Hearing Loss Study participants.

Individuals without cognitive impairment in 1998-2000 (N = 2,422; 1,947 with necessary data).

Cognitive impairment (Mini-Mental State Examination score <24 or diagnosis of dementia) was ascertained in 1998-2000, 2003-2005, and 2009-2010. Serum C-reactive protein (CRP) and interleukin-6 (IL-6) were measured in 1988-1990, 1998-2000, and 2009-2010; tumor necrosis factor-alpha was measured from 1998-2000.

Participants with high CRP in 1988-1990 and 1998-2000 had lower risk of cognitive impairment than those with low CRP at both time points (hazard ratio (HR) = 0.46, 95% confidence interval (CI) = 0.26-0.80). Risk did not differ according to 10-year IL-6 profile or baseline inflammation category in the whole cohort. In sensitivity analyses restricted to statin nonusers, those with high IL-6 at both times had greater risk of cognitive impairment than those with low IL-6 at both times (HR = 3.35, 95% CI = 1.09-10.30). In secondary analyses, each doubling of IL-6 change over 20 years was associated with greater odds of cognitive impairment in 2009-2010 in the whole cohort (odds ratio (OR) = 1.40, 95% CI = 1.04-1.89), whereas a doubling of CRP change over 20 years was associated with cognitive impairment only in statin nonusers (OR = 1.32, 95% CI = 1.06-1.65).

With data collected over 20 years, this study demonstrated greater likelihood of cognitive impairment in individuals with repeated high or increasing IL-6. The inconsistent CRP findings may reflect effects of statin medications, survival effects, or adverse effects associated with chronically low CRP. Further studies of long-term inflammation and cognitive impairment are needed.