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

To evaluate content validity, psychometric properties, and score interpretability of the Idiopathic Hypersomnia Severity Scale (IHSS).

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.

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 < |r| ≤ 0.90) correlations were found between IHSS and other assessments (eg, Epworth Sleepiness Scale, Clinical and Patient Global Impression of Change). A large effect size (Cohen's |d|=0.89) was found in change scores. Interpretation analyses demonstrated a 3-point minimum increase for detecting MWPC for IHSS total score.

The IHSS is a content-valid patient-reported outcome questionnaire that may produce valid, reliable, interpretable scores.

NCT03533114.