Narcolepsy is a neurological sleep disorder characterized by recurrent episodes of uncontrollable drowsiness and sudden loss of muscle tone (cataplexy) that precipitate collapse into sleep. In horses, narcolepsy is rare and can be idiopathic or secondary to underlying conditions such as liver disease, equine protozoal myeloencephalitis, or hypersomnia associated with sleep deprivation from an inability to achieve recumbent rest. Affected horses exhibit characteristic behavior: they become progressively drowsy, the head drops, the knees buckle, and the horse may collapse partially or fully before partially awakening. Skin abrasions on the knees, fetlocks, and muzzle from repeated partial falls are a common physical finding. Diagnosis requires ruling out other causes of episodic weakness or collapse, including paralysis, hypoglycemia, and cardiac arrhythmia. The orexin (hypocretin) system implicated in human and canine narcolepsy is also relevant in horses; imipramine has been used diagnostically and therapeutically. Environmental assessment is important because horses that cannot lie down comfortably in their stabling may develop secondary sleep deprivation that mimics narcolepsy. See also motor pathway disruption as the primary differential when distinguishing true paralytic episodes from narcoleptic collapse, and fetlock anatomy for understanding the abrasion pattern that accumulates from repeated partial falls.
Further Reading
- narcolepsy (Wikipedia)
- For differential diagnosis of sleep-related and neurologic behavior issues: behavior problems in horses — MSD Veterinary Manual