Content Validity and Psychometric Evaluation of the Idiopathic Hypersomnia Severity Scale (IHSS) in Adults with Idiopathic Hypersomnia.

Purpose: To evaluate content validity, psychometric properties, and score interpretability of the Idiopathic Hypersomnia Severity Scale (IHSS). Patients and Methods: Adults with idiopathic hypersomnia recruited from US patient organizations participated in qualitative interviews; transcripts were coded and qualitatively analyzed. Using data from a phase 3 trial of low-sodium oxybate in adults with idiopathic hypersomnia (NCT03533114), IHSS scores were psychometrically evaluated at Baseline, Weeks 1 and 8, end of titration, end of Stable Dose Period, and end of Double-Blind Randomized Withdrawal Period. Reliability, validity, and meaningful within-patient change (MWPC) were assessed. Results: In qualitative interviews, 20 participants (mean [SD] age, 35.0 [10.2] years; 70.0% female; 90.0% White) reported 18 idiopathic hypersomnia-related symptoms. Most frequently reported symptom/impact domains were excessive daytime sleepiness (100.0%) and emotional impact (100.0%). All participants understood and interpreted IHSS instructions, items, and response options as intended. Using the psychometric sample (N=115 participants; mean [SD] age, 40.9 [13.9] years; 71.3% female; 80.9% White), IHSS scores were found to be sufficiently reliable with respect to internal consistency (α≥0.80, average inter-item correlations 0.15-0.50) and reproducibility (intraclass correlation coefficient range, 0.73-0.98). Moderate to strong (0.30 < | Conclusion: The IHSS is a content-valid patient-reported outcome questionnaire that may produce valid, reliable, interpretable scores. Clinical Trial Registration: NCT03533114.