Validation and Regression-Based Normative Study of the Idiopathic Hypersomnia Severity Scale for Assessing Hypersomnolence Symptoms in Community-Dwelling Adults

The Idiopathic Hypersomnia Severity Scale (IHSS) has been developed to assess hypersomnolence symptoms and their functional consequences associated with idiopathic hypersomnia. Hypersomnolence symptoms covered by the IHSS are common in the general population, and are linked to female gender, younger age, and lower education. We evaluated the psychometric properties and the structural invariance of the IHSS in a population-based sample of adults. We also provided norms adjusted to demographics.

A total of 612 adults underwent a face-to-face clinical interview and completed several questionnaires, including the IHSS. Exploratory factor and confirmatory analyses were conducted. Reliability, convergent, and divergent validities were tested. Regression-based norms were produced.

Exploratory and confirmatory factor analyses showed a three-factor structure with adequate internal consistency (ω = 0.85). Good convergent and divergent validities were documented. Younger age (p < 0.001) and female gender (p = .02) were associated with higher level of hypersomnolence symptoms. No significant effect was found for education (p = 0.70). Regression equations to calculate Z-score are presented. Almost a quarter of the participants scored at or above the threshold for idiopathic hypersomnia screening (i.e., ≥ 22).

The IHSS has good psychometric properties and a structural invariance from the community-dwelling adults, with a three-factor solution in accordance with the clinical expression of hypersomnolence symptoms. We also provide normative data taking into account demographics. In our sample, the currently recommended threshold score seems to overestimate the risk of positive screening for idiopathic hypersomnia in the general population, in particular for younger women.