OBJECTIVES: Hypoglossal nerve stimulation (HGNS) is a treatment option for patients with CPAP-intolerant (CPAPi) obstructive sleep apnea (OSA). The modified sleep apnea severity index (mSASI) combines patient anatomy, weight, sleep study metrics, and symptoms into a composite index ranging from 1 (least severe) to 3 (most severe). Prior studies have associated mSASI with quality of life, CPAP adherence, and hypertension, but its utility in CPAPi patients is unknown. We evaluate the relationship between mSASI, HGNS efficacy, and adherence. METHODS: Retrospective cohort study of consecutive CPAPi OSA patients who underwent HGNS from 2014 to 2023. Patients were included if data were available to calculate preoperative mSASI and postoperative HGNS adherence/efficacy. Kruskal-Wallis rank-sum, Fisher's exact, and Chi-squared tests were performed. RESULTS: 264 patients were included (mean age = 61.3 years, 95% White, 66% Male). Preoperatively, 168 (64%) patients had mSASI of 1, 81 (31%) mSASI = 2, and 15 (5.7%) mSASI = 3. At 3 months post-op, patients with baseline mSASI of 1, 2, and 3 showed 6.72, 6.39, and 5.88 hours/night of device usage (p = 0.4). This pattern persisted, although showing no significance, at 6 months, 12 months, and most recent follow-up. There were similar reductions in Epworth Sleepiness Scale (ESS) across cohorts, with the mSASI3 group having the highest ESS postoperatively (p <
0.01). Sher15 response was similar between cohorts (mSASI1 = 52%, mSASI2 = 40%, mSASI3 = 25%, p = 0.2). CONCLUSION: Preoperative mSASI was not significantly correlated with HGNS adherence. Higher baseline mSASI was associated with greater postoperative daytime sleepiness. Further study is needed to evaluate mSASI as a tool within this population. LEVEL OF EVIDENCE: 3 Laryngoscope, 135:1212-1217, 2025.