Polysomnography and 24-hour observation in the evaluation of equine sleep disorders: narcolepsy or REM-sleep deficiency?

Introduction

Narcolepsy is a chronic neurological disorder characterized by excessive daytime sleepiness, cataplexy, sleep paralysis, and hallucinations. Although the pathogenesis is not completely understood, it has been shown that a lack of the hypothalamic hormone hypocretin is crucial in humans and dogs. In the horse a similar aetiology is suspected, but there are no studies to support this hypothesis and some cases are described as examples of sporadic idiopathic hypersomnia instead.

Aim of the study

To investigate the pathogenesis of atonic collapses through ethological, clinical and polysomnographic examinations.

Material and methods

A call to owners was made, followed by a survey which was completed by 177 owners of narcoleptic horses. 39 horses were studied via clinical examinations, polysomnographic measurements, and 24-hour observation. Furthermore the individual management and stabling conditions were investigated.

Results and conclusions

37/39 horses showed up to 199 collapses (MW 64.0 ± 40,5) per day. In 20/39 horses a correlation between the onset of collaps and an event such as change of stabling or illness was observed. In 36 % (14/39) the stable size was too small. 34 horses refused to lie down and showed an altered, restless sleep profile. Collapses mainly occurred during nighttime (p < 0.001) and in 86.7 % during rapid eye movement (REM) sleep while standing. Naturally REM-sleep occurs every night and only in a recumbent position, which can be explained by the low muscle tone during this sleep stage (1). Presumably „narcolepsy“ of adult horses is not a neurological disorder but rather a REM-sleep deficiency due to recumbent sleep deprivation caused by illness, husbandry shortcomings or ethological deficits. As a result the horses fall into REM-sleep while in a standing position, leading to collaps. A positive correlation between the amount of REMsleep and the number of collapses was observed (p = 0.001).

References

1. Kalus M. Schlafverhalten und Physiologie des Schlafes beim Pferd auf der Basis polysomnographischer Untersuchungen: Ludwigs-Maximilians-Universität München; 2014.