8.3.3 Neurological causes of collapse
There are a number of different neurological conditions which result in collapse. Those which produce flaccid paralysis should be distinguished from those which produce recumbency as a result of ataxia or due to seizure activity. Spinal injusuch as cervical malformation and fractures resulting in spinal cord trauma, may also cause flaccid collapse. Ataxia is not usually associated with cardiocauses of collapse except when the animal tires during arduous exercise. Cardiovascular disease seldom results in seizures.
Seizures are characterised by involuntary motion, including paddling of the limbs. Urination and defecation may occur. Behavioural abnormalities may be noted in the form of pre-ictal aura and post-ictal bewilderment. Neurological causes of seizures include epilepsy, toxins, viral conditions and trauma.
An important differential diagnosis in cases of flaccid collapse is narcolepsy. This is a sleep disorder which results in cataplectic collapse and may appear very similar to vaso-vagal syncope. An inciting cause may or may not be identified. Inciting causes have been described which are similar to those listed for vaso-vagal syncope. Narcoleptic attacks may also occur during light exercise. The condition has been recorded in a variety of breeds including related Shetland ponies and Suffolk horses. Any age or sex of animal can be affected. Animals may fall, or may catch themselves in the process of falling. Narcoleptic, areflexic collapse may result in trauma to the carpi and head.
Diagnosis in cases of narcolepsy can be made in some cases on the basis of a physostigmine stimulation test. The test is best performed on a deep straw bed or in an induction/recovery box. Around 0.08 mg/kg is injected slowly intravecollapse may follow in 5-20 minutes. Administration of physostigmine can result in colic. The drug can be difficult to obtain and is highly toxic. Failure to respond to administration of physostigmine does not exclude a diagnosis of narcolepsy.
Prevention of narcolepsy has been recorded following administration of 20~60 mg atropine intravenously or imipramine at a dose of 250 mg intraveor 750 mg orally. However, collapse is seldom sufficiently frequent for this to be used diagnostically, and it is not practical to continue treatment in the long-term. Some animals recover from the condition for unknown reasons.
Psychological/behavioural causes of collapse
It has been suggested that some horses may collapse for behavioural reasons. This may be because the animal does not want to be groomed, saddled or ridden. Back pain might be responsible for these behavioural abnormalities in some horses, but back problems are notoriously difficult to diagnose with certainty. Ponies have been known to collapse by rearing and falling backwards when attended in these ways. It is difficult to prove that behavioural abnormalities are responsible. Vaso-vagal syncope occurs in similar situations and may be related. It may be difficult to distinguish some of these episodes from narcolepsy. It is possible that collapse can become a patterned behaviour and it is worthwhile changing the management of the animal so that the inciting associations are broken.