Horse & Hound

Ataxia explained Why horses become weak, wobbly and unable to stand

Why might a horse become weak, wobbly or unable to stand? Dr Philip Ivens MRCVS discusses potential causes

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ATAXIA refers to incoordina­tion, which can affect one or more of the limbs and also the neck and body. While this complex condition can result from problems with the vestibular apparatus in the inner ear, or in a part of the brain called the cerebellum, ataxia often arises due to an issue in the spinal cord.

Spinal ataxia occurs when there is a lack of informatio­n coming up the horse’s spinal cord to tell his brain where his body parts are in space, and the state of muscle contractio­n at any one time. Ataxia is therefore not painful for the horse; rather, he has no sense of the position and movement of affected areas.

The hallmark of ataxia is inconsiste­ncy. Most orthopaedi­c lamenesses are consistent­ly repeatable in any one gait, or on a particular surface, but with ataxia the gait changes all the time. A leg might swing out to the side or under the body; a joint might overflex, or a foot scuff or drag.

If he trips, an ataxic horse may be slow to correct himself and potentiall­y fall. His trunk, neck or both might sway. He may tread on himself, or struggle to turn – or instead of crossing his legs underneath as he does turn, he stays rooted to the spot, pivoting while his legs eventually catch up.

A grading system can document the severity of the problem, starting with a zero for normal strength and coordinati­on. Grade one applies to a horse who walks normally in a straight line, but shows a slight deficit when walking on tight circles or with his head extended. He may also sway when pulled by the tail.

A mild increase in muscle tone or weakness in all four limbs, with ataxia at all times – especially during the manipulati­ons mentioned – is graded two, while grade three denotes a more marked increase in stiffness and obvious ataxia, with a tendency to buckle and fall while being circled vigorously, backed up or swayed. A grade four case will spontaneou­sly trip, stumble and fall; at grade five, the horse is recumbent and unable to stand.


CERTAIN developmen­tal conditions present at birth can cause ataxia. These include abnormal underdevel­opment of the cerebellum, called cerebellar abiotrophy, and abnormal formation of the first and second cervical vertebrae, termed atlantoaxi­al malformati­on.

The degenerati­ve condition cervical vertebral malformati­on (CVM), or wobblers syndrome, leads to compressio­n of the spinal cord and inhibits its ability to carry messages to the brain. And synovial cysts, vertebral fracture and different types of tumour – melanoma, plasma cell myeloma and lymphoma – can press on the nerves and spinal cord.

Infectious disease can result in ataxia: viruses, such as the equine herpes virus-1 (EHV-1, see box), which is endemic in the UK, and the exotic disease West Nile virus, which is becoming establishe­d in central Europe and likely to come to the UK at some point. Bacterial meningitis, diseases caused by protozoa – seen mainly in north America – and parasitic causes have also been documented.

Ataxia can be caused by a long list of toxins, the most common of which in the UK is severe nettle rash. Heavy metals such as lead and arsenic may be to blame, or certain therapeuti­c drugs if used inappropri­ately – ionophores, for example, such as monensin, found in cattle and poultry feed.

Reaching a diagnosis begins with considerat­ion of the horse’s age and breed and a thorough examinatio­n of his history. Has he fallen, for example, or are there stinging nettles in his field?

A routine examinatio­n will assess factors such as heart rate, respirator­y rate and temperatur­e, to identify any other physical abnormalit­ies that may contribute to or be part of the neurologic­al problem. For example, it is possible for a horse with a foot abscess to look ataxic or for primary muscle disease to cause weakness and similar signs. Perhaps other body systems are involved, such as the skin. If fever is present, an infectious cause becomes more likely.

A neurologic­al examinatio­n then follows, comprising tests to identify the part of the nervous system that is affected. This process, neurolocal­isation, is a key part of pinpointin­g the cause and is used to determine whether the spinal cord alone is involved, or if the brain and the peripheral nerves are also implicated. The disease may be focal – centred in one location – or multifocal, affecting multiple places.

Based on these findings, auxiliary diagnostic tests may include a blood cell count, with serum biochemist­ry, to look for signs of inflammati­on and other body system involvemen­t. Diagnostic imaging is becoming increasing­ly helpful; the quality of X-rays of the neck has improved, while advanced techniques such as computed tomography (CT) and magnetic resonance imaging (MRI) are feasible for at least part of the top of the neck.

Sampling the cerebral spinal fluid, either at the atlantoocc­ipital joint ( just behind the ears) or in the lumbosacra­l area (behind the saddle), is possible and in some cases helpful.


THE prognosis for ataxia is variable, depending on the diagnosis. Stinging nettle-induced ataxia can resolve quickly and fully once the horse is removed from the nettles, sedated and provided with appropriat­e antiinflam­matories. Sadly, many conditions affecting the spinal cord have a guarded to poor prognosis, due in part to a horse’s size and the safety concerns regarding him and his handlers.

Treatments, including surgery, are improving all the time. When compared with humans and small animals such as dogs, however, the number of affected horses that can be sufficient­ly improved to be considered safe is still relatively small.

Repeated neurologic­al examinatio­n and diagnostic tests can aid and guide decisionma­king, ensuring that equine welfare is always put first.

“Ataxia is not painful; rather, the horse has no sense of movement”

 ??  ?? When assessing a horse for ataxia, a vet will look for any inconsiste­ncy in the normal pattern of movement, which may be more obvious when the horse is turned in a tight circle
When assessing a horse for ataxia, a vet will look for any inconsiste­ncy in the normal pattern of movement, which may be more obvious when the horse is turned in a tight circle
 ??  ??
 ??  ?? A horse affected with ataxia may sway when his tail is pulled
A horse affected with ataxia may sway when his tail is pulled
 ??  ?? The yellowing of fluid is seen in cases of EHM (see box, above)
The yellowing of fluid is seen in cases of EHM (see box, above)
 ??  ?? Cerebral spinal fluid is collected under general anaesthesi­a
Cerebral spinal fluid is collected under general anaesthesi­a
 ??  ?? Radio-opaque dye detects narrowing of the spinal cord
Radio-opaque dye detects narrowing of the spinal cord

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