Page 1143 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

1118 CHAPTER 10

VetBooks.ir a sudden and profound loss of muscle tone. Both • Sleep-deprivation narcolepsy has a classic
history. It occurs when horses cannot (due to
have been recognised in several breeds and are
classified as two different syndromes. The first
herd hierarchy) lie down to sleep. REM sleep
is a fairly common transient condition affecting orthopaedic pain) or will not (due to anxiety/
primarily foals of light breeds and is frequently is not achievable when horses doze standing
induced by restraint. The second is a rare but per- up. If they will not lie down to sleep, they
sistent form that appears to be familial in miniature become sleep deficient and prone to sleepiness
horses and has also been seen in ponies and Suffolk and falling down while standing or being
Punch horses. tacked up.
• Vasovagal reflex also has a characteristic history,
Aetiology/pathophysiology where the horse collapses and falls backwards
The aetiology is unclear. A familial predisposition is (usually conscious throughout) when bridled
suspected in certain breeds. Often a specific stimu- or when a dental gag is opened while attached
lus is associated with initiation of an episode. In rare to the head. The exact nature of the reflex is
cases, signs have been reported in association with not fully understood but direct arterial blood
EPM. The pathogenesis of the condition is not fully pressure monitoring in these cases reveals a drop
understood but is suspected to involve abnormalities in blood pressure as the mouth opens, leading to
of the neuropeptides (hypocretins and orexins) that hypotensive collapse (syncope).
are linked to the regulation of sleep. • Cardiac syncope: horses suffering from
cardiogenic hypotension/hypoxic episodes
Clinical presentation do not usually lose consciousness, are more
The intermittent episodes are characterised by likely to suffer an episode during exercise
lowering of the head and buckling of the fetlocks, and have cardiac abnormalities detected on
with occasional collapse and rapid eye movement electrocardiography and/or echocardiography.
(REM) sleep. Between episodes, animals are clini-
cally normal. Management
If an inciting cause can be identified, it should be
Differential diagnosis avoided. The signs of adult-onset narcolepsy usually
Differential diagnoses include other causes of col- persist for life. Treatment with the tricyclic antide-
lapse, such as syncope or seizures, and any disorder pressant imipramine (0.5–2.0 mg/kg i/m, i/v or p/o
that may prevent a horse from lying down, lead- q6–12 h) may improve clinical signs in some animals.
ing to excessive sleepiness (e.g. a musculoskeletal Oral absorption is poor, and the oral route of admin-
problem). istration may not provide an acceptable response.

Diagnosis Prognosis
Diagnosis is based on history, clinical signs and Narcolepsy–cataplexy is not a life-threatening con-
exclusion of other problems. Affected horses are nor- dition; however, affected horses are not safe to ride,
mal between episodes and routine clinicopathologi- even when being treated. Some foals may outgrow
cal evaluation is normal. Intravenous administration the condition and would be safe for use provided a
of physostigmine salicylate (0.1 mg/kg i/v) may elicit long period (at least 6 months) has passed since the
signs of narcolepsy within minutes in some individ- last episode.
uals, but this response is not consistently found in
all animals with narcolepsy. Sudden death has been EQUINE DEGENERATIVE
reported with the intravenous injection of physo- MYELOENCEPHALOPATHY
stigmine in horses.
Ruling out other differential diagnoses for col- Definition/overview
lapse is important when trying to make a diagnosis Equine degenerative myeloencephalopathy (EDM) is
of true narcolepsy: a diffuse, non-compressive, symmetrical degenerative