Idiopathic hypersomnia affects approximately one person in two thousand in the general population, with a female predominance. It manifests as excessive daytime sleepiness, prolonged sleep duration over twenty-four hours, and/or reduced sleep onset latency. Despite considerable social, occupational, and personal consequences, this disorder remains underdiagnosed and is often inadequately treated. Its pathophysiology is still poorly understood, but recent work points to possible abnormalities of the circadian system and of phototransduction. The absence of specific biomarkers further complicates the differential diagnosis, as idiopathic hypersomnia shares symptoms with certain mood or attention disorders.
Light exerts powerful effects on physiology and behavior, primarily through intrinsically photosensitive retinal ganglion cells, which contain melanopsin. These cells contribute to the pupillary reflex and send projections to various brain regions, thereby participating in the regulation of sleep and alertness. To search for a disease marker, the authors investigated the post-illumination pupil response, a parameter that specifically reflects the melanopsin-mediated pupillary response. This measurement was obtained using a dedicated pupillometry protocol, comparing patients with idiopathic hypersomnia and prolonged sleep duration (greater than 660 minutes) with healthy subjects.
The study included twenty-eight patients (86% women, mean age 25.4 ± 4.9 years) and twenty-nine controls (52% women, 27.1 ± 3.9 years). After correction for baseline pupil diameter, the relative post-illumination pupil response was found to be significantly lower in patients than in controls (32.6 ± 9.9% versus 38.5 ± 10.2%, p = 0.037), suggesting a reduced melanopsin response. Notably, this response was correlated neither with age, chronotype, total sleep time, nor depressive symptoms. This lack of correlation indicates that the observed decrease probably reflects neither the severity of the disorder nor the presence of a depressive comorbidity, but rather constitutes a feature specific to the disease.
By demonstrating a reduced melanopsin-mediated pupillary response in patients, this work identifies this measurement as a potentially innovative marker of idiopathic hypersomnia. Pupillometry thus emerges as a promising tool for better characterizing hypersomnias.