IMPORTANCE: As US society ages and the climate changes, extreme outdoor heat may exacerbate the health burden of Alzheimer disease and related dementias (ADRD), but where, when, and among whom extreme heat may increase hospitalizations with ADRD remains understudied. OBJECTIVE: To investigate the association between extreme heat and the risk of hospitalization with ADRD, and to explore how associations differ across climates and population subgroups. DESIGN, SETTING, AND PARTICIPANTS: Population-based cohort study, using a time-stratified case-crossover design, of Medicare fee-for-service (Part A) claims from 2000 to 2018 among beneficiaries aged 65 years or older in the contiguous US
time-stratified case-crossover design implemented with distributed lag nonlinear models using conditional logistic regression. Data were analyzed from October to November 2024. EXPOSURES: Daily maximum heat index converted to percentiles of climate-specific warm season heat index distributions. MAIN OUTCOMES AND MEASURES: The main outcome was each beneficiary's first hospitalization with an ADRD diagnosis code, and other measures were county-level climates (arid, continental, temperate, or tropical). RESULTS: The sample included 3 329 977 beneficiaries (2 126 290 [63.9%] female, 33 887 [1.0%] Asian, 354 771 [10.7%] Black, 61 515 [1.8%] Hispanic, 2 831 391 [85.0%] White, and 891 815 [26.8%] dual eligible for Medicaid). The odds ratio (OR) of hospitalization with ADRD comparing days in the 99th vs 50th percentile of the heat index distribution was 1.02 (95% CI, 1.01-1.02), corresponding to 0.8 (95% CI, 0.5-1.1) additional hospitalizations with ADRD per 1000 beneficiaries. Results suggest extreme heat associations persist for 3 days beyond the initial day. The cumulative OR of hospitalization with ADRD after 4 days of continuous exposure to heat indexes at the 99th vs 50th percentile was 1.04 (95% CI, 1.03-1.04), or 1.7 (95% CI, 1.3-2.0) additional hospitalizations with ADRD per 1000 beneficiaries. Extrapolating these estimates to the 6.7 million adults currently living with ADRD suggests that each day of extreme heat could contribute to at least 5360 added hospitalizations with ADRD nationwide. Effects estimates were similar in temperate and continental climates. Arid and tropical climate estimates were somewhat similar but more uncertain. OR point estimates for hospitalization from 4 days of continuous extreme heat exposure for beneficiaries identifying as Asian (OR, 1.09
95% CI, 1.02-1.17), Black (OR, 1.07
95% CI, 1.05-1.10), and Hispanic (OR, 1.08
95% CI, 1.03-1.13), were 2.6 to 3.2 times larger than for White beneficiaries (OR, 1.03
95% CI, 1.02-1.04). CONCLUSIONS AND RELEVANCE: This study found that extreme heat may pose a growing threat to older adults living with ADRD. This threat may be larger among Asian, Black, and Hispanic racial and ethnic groups. Clinicians should consider counseling patients living with ADRD on extreme heat risks, and policymakers should devise risk mitigation programs.