Underweight patients are the highest risk body mass index group for perioperative adverse events following stand-alone anterior lumbar interbody fusion.

Background Context: Prior studies investigating the association between Body Mass Index (BMI) and patient outcomes following spine surgery have had inconsistent conclusions, likely owing to insufficient power, confounding variables, and varying definitions and cutoffs for BMI categories (eg, underweight, overweight, obese, etc.). Further, few studies have considered outcomes among low BMI cohorts.

Purpose: The current study analyzes how anterior lumbar interbody fusion (ALIF) perioperative outcomes vary along the BMI spectrum, using World Health Organization (WHO) categories of BMI.

Study Design/setting: A retrospective cohort study.

Patient Sample: Patients undergoing stand-alone one or two-level anterior lumbar interbody fusion (ALIF) found in the 2005-2018 American College of Surgeons National Surgical Quality Improvement Program (NSQIP) databases.

Outcome Measures: Thirty-day adverse events, hospital readmissions, post-operative infections, and mortality.

Methods: Stand-alone one or two-level ALIF surgical cases were identified and extracted from the 2005-2018 National Surgical Quality Improvement Program (NSQIP) database. Posterior cases and those primary diagnoses of trauma, tumor, infection, or emergency presentation were excluded. Patients were then binned into WHO guidelines of BMI. The incidence of adverse outcomes within 30-day post-operation was defined. Odds ratios of adverse outcomes, normalized to the average risk of normal-weight subjects (BMI 18.5-24.9 kg/m), were calculated. Multivariate analysis was then performed controlling for patient factors.

Results: In total, 13,710 ALIF patients were included in the study. Incidence of adverse events was elevated in both the underweight (BMI<18.5 kg/m) and super morbidly obese (>50 kg/m), however, multivariate risks for adverse events and postoperative infection were elevated for underweight patients beyond those found in any other BMI category. No effect was noted in these identical variables between normal, overweight, obese class 1, or even obese class 2 patients. Multivariate analysis also found overweight patients to show a slightly protective trend against mortality while the super morbidly obese had elevated odds.

Conclusions: Underweight patients are at greater odds of experiencing postoperative adverse events than normal, overweight, obese class 1, or even obese class 2 patients. The present study identifies underweight patients as an at-risk population that should be given additional consideration by health systems and physicians, as is already done for those on the other side of the BMI spectrum.