Low BMI (< 10th percentile) increases complications and readmissions after posterior spinal fusion in adolescent idiopathic scoliosis

Purpose: While obesity has been shown to predict negative outcomes following PSF in AIS patients, less is known about the effects of low BMI. We sought to elucidate the impact of low BMI on 30-day outcomes in this population. Methods: Adolescent idiopathic scoliosis patients undergoing PSF were identified using the 2015–17 ACS-NSQIP-Pediatric database. Patients were placed in underweight (UW, < 10th percentile) and normal weight (NW 10–90th percentile) cohorts based off CDC BMI-to-age growth charts. Demographics, comorbidities, intra-, and postoperative factors were compared via univariate analysis with Benjamini–Hochberg adjustment. Multivariable logistic regression models were generated to assess UW status as a predictor of complications. Results: Two thousand seven hundred and ninety-nine AIS patients undergoing PSF (NW = 2517, UW = 282) were identified. UW patients were older (15.6 vs. 14.7 years), less female dominant (62.4% vs. 79.5%), and had more pulmonary (2.5% vs. 0.4%) and minor cardiac comorbidities (6% vs. 1.7%) compared to NW patients (p < 0.001). UW patients had a greater percentage blood loss (6.7% vs. 5.3% of total blood volume, p < 0.001) and higher complication (3.9% vs. 1.4%, p = 0.008), pneumonia (1.4% vs. 0.1%, p = 0.006), and readmission (3.5% vs. 1.2%, p = 0.001) rates compared to NW patients. UW status was a predictor of ≥ 15% blood volume loss (adjusted OR = 2.65; 95% CI = 1.76–3.97; p < 0.001), pneumonia (aOR = 6.57; 95% CI = 1.80–24.00; p = 0.004), and hospital readmission (aOR = 2.47; 95% CI = 1.02–6.01; p = 0.046). Conclusion: There is a higher occurrence of complications in UW AIS patients undergoing PSF. Low BMI is an independent predictor of ≥ 15% blood loss, pneumonia, and readmissions. Like their overweight counterparts, underweight AIS patients have an increased postoperative risk for negative complications.