BMI, Comorbidity Burden, and Postoperative Delirium in Older Adults: A Retrospective Cohort Study.

BACKGROUND: Postoperative delirium (POD) is a serious complication in older surgical patients. Although both body mass index (BMI) and comorbidity burden influence perioperative outcomes, it is unclear whether an "obesity paradox" applies to POD and, crucially, whether comorbidity burden modifies that relationship. We therefore examined the independent association of BMI with POD and formally tested whether comorbidity burden, measured by the Charlson Comorbidity Index (CCI), modifies this association in older adults. METHODS: This retrospective cohort study included 4320 patients aged ≥65 years who underwent noncardiac, non-neurological surgery under general anesthesia (2015-2024). Patients with BMI <18.5 kg/m2 or missing key data were excluded. BMI was categorized as normal (18.5-24.9), overweight (25.0-29.9), class 1 obesity (30.0-34.9), and class ≥2 obesity (≥35.0). The CCI was dichotomized as low (0-2) or high (≥3). The primary outcome was POD within 7 postoperative days, assessed daily with the Korean-validated Confusion Assessment Method (CAM) or CAM for the intensive care unit (CAM-ICU). The prespecified primary analysis was a single multivariable logistic regression model including a BMI × CCI interaction term. Propensity score matching and a restricted cubic spline for BMI were predefined supportive analyses. RESULTS: POD occurred in 625 of 4320 patients (14.5%). Compared with normal weight, adjusted odds ratios (aORs) were 0.81 (95% confidence interval [CI], 0.68-0.96) for overweight, 0.76 (0.60-0.97) for class 1 obesity, and 1.02 (0.78-1.33) for class ≥2 obesity. The BMI × CCI interaction was significant (P = .040), and this was the central finding: in patients with low CCI, overweight and class 1 obesity were protective (aOR 0.69 [0.53-0.90] and 0.65 [0.46-0.92]), whereas in patients with high CCI no BMI category differed from normal weight. Propensity score-matched estimates were consistent (overweight OR 0.74 [0.60-0.90]; class 1 obesity OR 0.72 [0.53-0.98]). CONCLUSIONS: The relationship between BMI and POD is not uniform; it is conditional on comorbidity burden. Overweight and class 1 obesity are associated with reduced POD risk only in older adults with low comorbidity burden. BMI should therefore be interpreted together with comorbidity burden when assessing delirium risk in elderly surgical patients.