Page 1142 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

Nervous system 1117

VetBooks.ir epilepsy in horses. It is, therefore, assumed to be (e.g. diazepam, 0.1–0.2 mg/kg i/v in foals and 0.02–
There is no evidence of familial adult-onset
0.08 mg/kg i/v in adults) hyperpolarise neuronal cells
acquired and caused by an intracranial epileptogenic
focus or foci acquired during post-natal life. Foci by binding to GABA receptors, changing chloride
conductance and making cells resistance to depo-
may become active following recovery from a cere- larisation. The overall effect is to reduce the electri-
bral insult, although the interval from brain disease cal activity responsible for the seizure and increase
to the onset of seizure activity may be several years the seizure threshold. This is an effective first-line
and is referred to as epileptogenic ripening. Foci are treatment for seizures, but the half-life is short and
the nucleating sites for the spread of paroxysmal neu- so multiple doses may be required. Caution is war-
ronal changes that are evident clinically as seizures. ranted if prolonged treatment is administered to
foals, because drug accumulation may occur, causing
Clinical presentation respiratory depression. Intravenous phenobarbitone
Transient changes in the horse’s behaviour usually rapidly provides a high serum concentration; it is a
precede attacks. Seizures may be partial, if only a GABA agonist and inhibits calcium-gated excitatory
small focus of the cerebrum is affected. Partial sei- nerve terminals. A range of dose rates have been
zures have relatively localised signs such as unilateral published in recent years and readers are referred to
facial twitching, limb twitching, compulsive run- equine internal medicine texts for these.
ning in a circle or self-mutilation. A generalised sei-
zure involves the whole cerebral cortex, and results Preventing seizures
in widespread tonic–clonic muscle activity and a loss The goal of long-term therapy is to reduce the
of consciousness. Complex partial seizures are rela- severity and frequency of seizures without signifi-
tively common in foals and are referred to as ‘chew- cant adverse effects of the chosen treatment. Drug
ing-gum fits’ because the foals show persistent jaw choices for long-term anticonvulsant therapy in
chomping and lip-smacking. horses include phenobarbitone, bromide, phenytoin
In more detail, three distinct periods may be and primidone.
observed with seizures. First, immediately prior to A general recommendation is that the doses of
the seizure (pre-ictal phase/aura), the horse may show these anticonvulsant drugs can be reduced gradually
signs of anxiety or restlessness. Next, during the after 2 months without attacks, and the horse should
seizure (ictus/ictal phase), the horse may be recum- not be ridden until it has been seizure-free without
bent, and show tonic–clonic muscle contraction, medication for a minimum of 6 months. When mak-
eyeball movements, dilated pupils, ptyalism, trismus, ing this recommendation it is important to consider
opisthotonus, lordosis or kyphosis, uncontrolled the possible consequences of an unexpected seizure,
urination and defaecation, and excessive sweating. especially with a child’s pony.
After the seizure (post-ictal phase), horses may be
obtunded, disorientated and blind for a period of Prognosis
minutes, hours or, rarely, days. The prognosis is highly variable because of the idio-
pathic nature of the disease. A fair prognosis should
Management be given; however, some horses will not respond well
Treatment goals include controlling the seizure, to treatment. Horses with uncontrolled intermittent
addressing underlying diseases and, finally, prevent- seizure activity should be considered unsafe to ride,
ing further seizures. even if the seizures are very infrequent.

Controlling seizures NARCOLEPSY–CATAPLEXY
Prompt control of seizures is a priority to mini-
mise further neuronal damage, self-inflicted injury Definition/overview
or human injury, but should not be attempted if Narcolepsy is a sudden onset of excessive daytime
there is risk to human health. Benzodiazepines sleepiness. It is usually accompanied by cataplexy,