BASICS
REM sleep disorders result in excessive daytime sleepiness and paroxysmal sleep attacks with REMs (narcolepsy) or complete loss of muscle tone and reflexes (cataplexy).
In laboratory species and dogs, it has been shown that the neuropeptide hypocretin (orexin) is central to the control of sleep and arousal. Hypocretin neurons project to areas involved in these processes, including the ascending reticular activating system; hypocretin levels fluctuate across the sleep–wake cycle and increase with sleep deprivation. Hypocretin neurons activate brainstem “REM-off” neurons and reduce the activity of “REM-on” neurons, acting as a gate to entry into REM sleep. Recent work suggests that an excess of a class of neurons containing histamine may be the cause of the loss of hypocretin cells in narcoleptics, and inhibiting histamine signaling could be a new way to treat narcolepsy. In humans, narcolepsy has been associated with reduced hypocretin levels, while in dogs narcoleptic lines that lack hypocretin receptors have been bred for narcolepsy research projects. Neither reduced hypocretin levels nor the lack of receptors has been demonstrated in horses, and animals with “sleep attacks” may simply be REM sleep deficient (i.e. not lying down enough).
Narcolepsy-like episodes can be seen transiently in foals, particularly in miniature horses, Fell Ponies, Shetland and Welsh Ponies, and Appaloosas. Usually, the episodes resolve with time. More commonly, it is seen in aged horses.
- An attack may progress from buckling at the knees without falling to sudden and total collapse and areflexia, usually with maintenance of some eye and facial responses and normal cardiorespiratory function
- Each recumbent episode may last up to hours if the foal is totally undisturbed, but the patient usually can be aroused from this state with varying degrees of difficulty
- Most commonly, this includes horses resting at the back of a field that can be seen to “buckle” at the knees, rarely resulting in collapse
- Excessive somnolence is easily noted by owners with the patient often resting the head or hindquarters on objects
- Most times the animal awakes with the (impending) fall to resume the somnolent state or revert to wakefulness
- Some horses have had relentless persistence of the syndrome to the point of severe knee and face trauma
- In some patients, episodes may be triggered by specific stimuli such as saddling, hosing down, or feeding, with no permanent consequences. Only occasionally do episodes occur with the excitement of being ridden
- No specific cause has been identified, but rigorous assessment of hypocretin levels has not been undertaken
- In some older horses, the cause may well be due to sleep deprivation, perhaps because the horse is unwilling to lie down as a result of joint pain or fear of enclosed spaces
- Risk factors have not been identified
DIAGNOSIS
TREATMENT
MEDICATIONS
FOLLOW-UP
MISCELLANEOUS