Equine Neurology - neurological disease doesn't always mean the end of a racing career | Trainer Magazine | European Trainer Article Index

Oct 7

Oct 7 Equine Neurology - neurological disease doesn't always mean the end of a racing career

NEUROLOGICAL problems in racehorses are not uncommon and when they do occur, they can be hugely significant. Horses suffering from neurological disease are often not 20 TRAINERMAGAZINE.com ISSUE 14 diagnosed that quickly because trainers have a tendency not to consider a neurological problem unless it becomes glaringly obvious. The complaint is often vague – an unusual lameness or gait, a history of clumsiness or falling, or a blow to the head. The neurological examination and localization of the problem The primary aim of the veterinary neurological examination is to establish whether a neurological problem is present. It is only after this that the vet can begin to analyze all the signs in an attempt to localize the problem, for example, to the brain or spinal cord. The physical examination begins with assessment of the horse’s general behavior and mental status before examining the nerve reflexes of the head, most of which can be easily tested with a mint. The horse hears the rustle of the mint wrapper, before smelling the mint, seeing it, taking it with its lips and tongue, using its facial muscle to chew it before finally swallowing. The next time readers feed a mint to a horse, they now know that they are actually testing many complex neurological pathways! The vet may then proceed to examining the horse’s body – starting with the neck before moving onto the fore limbs, the back, the hind limbs and finally the tail and perineal area. Horses are then assessed at walk and trot before being asked to perform some more demanding tests, for example the ‘tail-pull’ test, turning the horse in tight circles, or asking the horse to walk whilst blindfolded. Once the neurological examination is complete, the results are analyzed by the vet who may then be able to localize the problem. The presence or absence of different reflexes can tell the examiner a great deal about the exact location of the problem, whether it is in the brain or the spinal cord. For example, if a horse shows hind limb incoordination but its front limbs appear unaffected, then the problem lies in the spinal cord behind the withers but ahead of the tail. On the other hand, if the head reflexes are normal but the horse shows front and hind limb incoordination, then the problem must lie in the spinal cord between the head and the withers. Wobblers One of the most common and important neurological conditions of the racehorse is the ‘classical wobbler syndrome.’ The term ‘wobbler’ describes exactly what such horses look like – they are uncoordinated and wobbly. It is more accurately described as ‘cervical vertebral malformation’ as it is produced by bony abnormalities in the horse’s neck vertebrae putting pressure on the spinal cord and so causing the horse to be uncoordinated. There are two main types of bony abnormality – narrowing of the vertebral canal through which the spinal cord runs, which causes problems for the horse whichever way the neck is positioned, and instability of the neck vertebrae, which mainly causes problems for the horse when its neck is flexed upwards. However, determining the specific problem in a horse’s neck is often academic unless surgery is being considered and so the level of incoordination shown by the horse is the most important piece of information. The exact cause of why a horse should develop bony changes in its neck vertebrae and become a wobbler is unknown. There is some evidence that it might be genetic, however, successful stallions such as Anabaa, who was diagnosed as a wobbler in his youth, have gone some way to showing that genetics can only be part of the picture. It has also been suggested that the wobbler syndrome develops as a result of a developmental bony disorder similar to osteochondritis dissecans (OCD). This does fit in well with the fact that wobblers are most commonly seen in fast growing male Thoroughbreds and that nutrition of the growing horse also seems to play a part. Essentially, the spinal cord is tightly held in a small bony canal formed by the horse’s vertebrae and any tiny bony change to these vertebrae can put significant pressure on the spinal cord and hence cause incoordination. A fall or stumble almost always instigates the first veterinary examination and the appearance of a wobbler is quite characteristic. Flexion of the neck is often painful whereas back pain is unusual. Wobblers show equal incoordination on both sides and the hind legs are always affected while the front legs can be normal in some horses. Wobblers often show something called ‘hypermetria,’ which is also known as ‘goose stepping,’ where the horse over-flexes its leg, walking in an exaggerated way. Suspected wobblers are usually asked to perform some demanding tests, which are designed to highlight any incoordination. This often begins with asking the horse to turn in tight circles, where it may find it hard to place its feet correctly and often ends up standing on the other foot, stumbling and stopping in an abnormal position. Having assessed the horse turning, many will then ask the horse to reverse, which also shows off incoordination as moving backwards requires more coordination than moving forwards. The list of manipulative neurological tests is nearly endless but many of these are very useful when assessing a potential wobbler. For example, the ‘tail-pull test’ reveals hindlimb weakness, the ‘hop test’ reveals any front leg incoordination, and asking a wobbler to walk when holding its head up high can cause increased incoordination in horses with neck vertebrae instability. The incoordination of a wobbler can be graded using a number of different classifications, most of which vary from zero to five, with zero being normal and five being a severely uncoordinated horse that cannot stand let alone walk. This scale is useful for monitoring progress and for insurance purposes. Some insurance  policies do pay out if a horse is diagnosed as a wobbler of a certain grade, but as with most types of insurance, wobbler clauses are not always straightforward. Wobblers are usually diagnosed using a combination of clinical signs and x-rays. The neck is x-rayed and abnormalities are searched for that might explain the incoordination. It is usually possible to find small bony changes associated with some of the neck vertebrae and if so, this will be the most likely site of the spinal cord compression that is causing the incoordination. It is also possible to inject a dye into the spinal column that shows up brightly on x-ray – something that is known as a myelogram. When performed, this shows very clearly where any compression may be. However, this is not commonly carried out because it is a dangerous procedure for a number of reasons – the dye is an irritant, and any injection here is dangerous, with the uncoordinated horse then expected to get up smoothly after the anesthetic without injuring itself further. Unfortunately, little can be done to aid the recovery of a wobbler. Wobblers are often treated with long courses of antiinflammatories and rested for up to six months in the hope that the problem may settle. However, this is not straightforward as the horses often become very fresh. Another issue with wobblers is that even if they recover, how safe are they to ride? If their incoordination has been blamed on changes seen on neck x-rays, then the problem could recur. Neck surgery is sometimes attempted but success is far from guaranteed. All of these concerns and thoughts come together to make the diagnosis of a wobbler a damning one in so many cases, which makes the fairytale stories of Anabaa and Shamardal even more extraordinary. Equine Protozoal Myelitis (EPM) Some say that EPM is now the most common neurological disease affecting horses in the USA. The disease causes very similar signs to those shown by a wobbler but cases can have a considerably better outcome. The initial signs are often vague findings such as lethargy and horses knocking themselves at exercise but these may progress to obvious incoordination and weakness and sometimes severely affected horses cannot even stand. Although more common in young horses, the disease can affect horses of any age. In 1991 researchers found that EPM was caused by a small protozoal organism called Sarcocystis neurona, which is common in North and South America. The Sarcocystis lifecycle must involve its definitive host or carrier, the Virginia Possum, and for a horse to become infected it must ingest the Sarcocystis from possum feces when, for example, grazing in a field. The horse does not pass on Sarcocystis and so becomes infected with the organism, which targets its brain and spinal cord resulting in an uncoordinated horse. Several new studies, however, have shown that the exact cause of EPM is not as clearcut as it seemed. Uninfected horses were fed up to 60 million Sarcocystis neurona sporocysts from the feces of infected possums but the signs of EPM were not always seen. In essence, EPM continues to puzzle vets. Currently, it seems that there are two possibilities. Firstly, it is another species of Sarcocystis that causes EPM and not Sarcocystis neurona. Secondly, another factor or agent predisposes a horse to develop the disease. Nevertheless, it does appear to be fairly concrete that possums play a pivotal role in horses becoming infected with the disease and so horse owners are well advised to try and keep the two species separate. The task facing the vet is often to decide whether a particular horse has EPM or classical wobbler syndrome. The most common clinical differences are that horses with EPM often show forelimb incoordination and usually display more advanced signs on one side, whereas wobblers often show only hind limb incoordination and are usually equally affected on both sides. Confirmation of the diagnosis of EPM is not straightforward as a blood test only shows that a horse has come into contact with the disease (not necessarily become infected with it) and although a spinal fluid sample can confirm the disease, it is still possible to get false positive and false negative results. Recent extensive blood testing has revealed that over half the horses in areas with high rates of EPM, for example, the Midwest and Northeast, seem to have been exposed to Sarcocystis but only a small percentage develop neurological signs. Nevertheless, perhaps the biggest difference between EPM and the classical wobbler syndrome is that EPM is treatable in a large proportion of cases if therapy is initiated quickly. Several different types of anti-parasitic and anti-inflammatory drug have to be given to the horse for a long period of time, but a significant percentage of horses can recover and race successfully and only a small percentage have to be euthanized. There is also the possibility of a vaccine becoming available in the future. Stringhalt ‘Stringhalt’ is an involuntary, repetitive, exaggerated flexion reflex of the hock. As the horse walks, it flexes its hock in an exaggerated manner with every stride. Some horses have a mild form of the condition in which it is only noticeable at walk and has no effect on the horse’s athletic ability. However, horses with a severe form of the condition can actually touch their belly with their hoof on every stride and the stringhalt can limit their athletic ability. Treatment is often ineffective but when the condition first becomes apparent, rest and anti-inflammatories are usually initiated to try and resolve the problem or at least limit its severity. There is a surgery in which a specific tendon is cut, which can abolish the condition in some horses but this is not often attempted unless the condition is preventing the horse from being an athlete. The stringhalt described above is often referred to as ‘classical stringhalt’ in order to differentiate it from ‘Australian stringhalt,’ which is seen in the southern hemisphere. Australian stringhalt causes affected horses to walk with the same strange gait described above but it often affects both hind legs. It is caused by the ingestion of plant toxins and so is usually found in several horses grazing on a specific pasture. Acute Head or Neck Trauma Horses bang their heads with alarming regularity; however, if a horse receives a blow of sufficient strength in a specific place, it can result in severe neurological consequences. When the neurological signs do not disappear immediately, the attending vet will perform a thorough neurological examination which attempts to localize the problem to a specific part of the brain, or at the very least to establish whether the problem is in the brain or the spinal cord. Head and neck x-rays are carried out and sometimes the cause becomes clear so the required treatment is obvious. However, more commonly, the x-rays show no significant abnormalities and then deciding upon a treatment plan difficult. For many years, both doctors and vets administered the strongest type of antiinflammatory at their disposal – the corticosteroids. However, new extensive research carried out in human patients has surprisingly revealed that not only do patients not seem to improve for the administration of corticosteroids but that these drugs actually seem to be potentially harmful. While some vets still give corticosteroids to horses who suffer from acute head or neck trauma, this human research has encouraged many of them not to use this type of drug. As a result, treatment now comprises of good management of the situation, for example, maintaining breathing, reducing swelling using other drugs, preventing secondary injury and good nursing. The prognosis for horses following head trauma and neurological signs is variable and highly unpredictable. Other neurological conditions There is a long list of equine neurological conditions but there are a few more that are worthy of mention particularly with regard to racehorses. Equine Herpesvirus (EHV-1) is best known for causing respiratory problems in horses or abortion in pregnant mares but it can also cause neurological disease. Treatment is not easy and hence many vaccinate against the disease. There are also a number of conditions that result from damage to specific nerves with perhaps the best example being ‘Sweeney.’ This condition results from damage to the suprascapular nerve that is usually caused by trauma, for example, a collision with another horse or a solid object such as a fence. The nerve damage then slowly leads to marked muscle loss of the shoulder, and a few months after the incident the amount of muscle loss can be quite alarming. Fortunately, the condition usually resolves with rest but the muscles often require up to eighteen months to regenerate.

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James Tate BVMS MRCVS (14 October 2009 - Issue 14)

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