Abstract Details

Title Psychometric Evaluation of the Idiopathic Hypersomnia Severity Scale

Topic Sleep

Presentation(s) P1 - Poster Session 1 (8:00 AM-9:00 AM)

Poster/Presentation
Number 1-001

Objective To evaluate the psychometric properties of the Idiopathic Hypersomnia Severity Scale (IHSS) and inform a meaningful within-patient change (MWPC) threshold for the IHSS total score.

Background The IHSS is a 14-item instrument that assesses the severity and impact of idiopathic hypersomnia (IH) symptoms. Change in IHSS total score was a key secondary endpoint in a phase 3, double-blind, placebo-controlled, randomized withdrawal study of lower-sodium oxybate in adult participants with IH.

Design/Methods Psychometric analyses were conducted using data from the phase 3 clinical study, blinded to treatment, to evaluate reliability and construct-related validity of the IHSS total score across assessment timepoints. Excessive daytime sleepiness (EDS) severity levels were defined based on the Epworth Sleepiness Scale: normal (score 0–10), mild/moderate (11–15), severe (16–24). Distribution and anchor-based analyses were conducted to inform MWPC interpretation thresholds related to IH symptoms.

Results In the psychometric analysis population (n=115; modified intent-to-treat population, including participants with ≥1 non-missing IHSS response), the IHSS demonstrated good internal consistency (Cronbach’s alpha ≥0.80, all timepoints) and acceptable test-retest reliability (intraclass correlation coefficients >0.8). Spearman correlations between IHSS total score and other efficacy measures were moderate to strong (>0.3 in expected directions), demonstrating acceptable convergent/discriminant validity. In known-group analyses, IHSS discriminated among EDS severity levels: normal (n=58; mean [SD] IHSS score: 16.3 [7.6]), mild/moderate (n=37; 28.0 [6.7]), severe (n=20; 32.4 [9.9]; P<0.001, Kruskal-Wallis test between groups). High sensitivity of the IHSS total score was confirmed by a large effect size (Cohen’s d=0.89). Distribution and anchor-based assessments converged on an MWPC threshold of ≥3 points.

Conclusions The IHSS total score is reliable, construct valid, and able to detect changes in the clinical condition over time. The suggested MWPC threshold is at least a 3-point change of the IHSS total score.

Authors/Disclosures
Benjamin Banderas, Other PRESENTER	Mr. Banderas has nothing to disclose.
	No disclosure on file
	No disclosure on file
	No disclosure on file
	No disclosure on file
Junji Lin, PhD (Jazz Pharmaceuticals)	Ms. Lin has received personal compensation for serving as an employee of Jazz Pharmaceuticals. Ms. Lin has stock in Jazz Pharmaceuticals.
Patricia Chandler	No disclosure on file
Yves Dauvilliers, MD, PhD (Hopital Gui De Chaulliac)	Yves Dauvilliers, MD, PhD has received personal compensation in the range of $500-$4,999 for serving on a Scientific Advisory or Data Safety Monitoring board for idorsia. Yves Dauvilliers, MD, PhD has received personal compensation in the range of $500-$4,999 for serving on a Scientific Advisory or Data Safety Monitoring board for JAZZ. Yves Dauvilliers, MD, PhD has received personal compensation in the range of $500-$4,999 for serving on a Scientific Advisory or Data Safety Monitoring board for Takeda. Yves Dauvilliers, MD, PhD has received personal compensation in the range of $500-$4,999 for serving on a Scientific Advisory or Data Safety Monitoring board for Avadel.

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