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Diagnosing Incoordination (Ataxia) and Weakness in Horses
including ataxia, spasticity, and hypermetria
by Robert N. Oglesby DVM
If your horse is also showing changes in alertness or behavior along with the incoordination he have have a disorder of the brain, for which there is a separate article...for more information.
Labeling Symptoms AccuratelyThe nervous system can be grouped into functional regions that correspond with anatomical structures. The first step in diagnosing dieseases of the nervous system is defining the functional disorder which leads to an anatomical location of the disease. Once a anatomical location is identified a list of diseases that commonly effect that location can be generated and either further diagnostic work or therapy can be planned. The main functional disorders and definitions are:
Gait changes may also be seen as a result of unusual conformation or shoeing. Some horses can show excessive flexion, pronounced external rotation (winging out), circumduction, or decreased action at normal gaits. The owner, trainer, or handler may help provide valuable information to help distinguish between normal and abnormal behavior and gait for that individual horse. Once a behavioral or gait problem is recognized, regardless of its severity, it must be defined in its simplest form and a problem list must be generated. In addition, horses with musculoskeletal disease can show signs of weakness when painful limbs are passively lifted off the ground or spasticity when they use painful joints (reluctance to flex).
Many lesions affecting the nervous system can be localized to a single focus but when such localization is not possible, a diffuse or multifocal disease of the nervous system should be considered. Accurately labeling and listing of symptoms seen will help lead to accurate localization.
Neurological Examination for Ataxia and Weakness
Disease of the spinal cord is seen as three main symptoms with incoordination (ataxia) being the predominant one. Stiffness (spasticity) and paresis (weakness) may also be present. The location and severity of the symptoms of disease of the spinal cord is less effected by the cause of the disease and more by it's location. The spinal cord is ensheathed in the vertebral column which is then buried deep in the muscle masses of the neck and back direct examination is impossible. As a result it is often possible to know where the disease is but harder to know what is causing the disease.Without specific evaluation for spinal cord disease mild problems may be overlooked. The following procedures will help determine whether there is the presence of spinal cord disease:
The ??Slap Test??
This response is not consistently absent in horses with cervical vertebral malformation or other forms of cervical spinal cord disease (wobblers) and may inexplicably be absent in some apparently normal horses. Depression or absence of the reflex on the left side must be taken as strong evidence for the presence of recurrent laryngeal neuropathy or prior laryngeal surgery. Exercising the horse will be necessary to confirm any clinical problem arising from laryngeal paralysis. Bilateral absence of the palpable response in the absence of other signs of laryngeal or cervicomedullary disease must be interpreted cautiously particularly in excitable horses.
The second of these reflexes is twitching of all facial muscles including the cutaneous facei (facial subcuticular) muscle in response to the same cervical stimulus as for the first reflex. The same segmental cervical sensory input likely applies to this reflex. However, both the sensory and motor pathways may be more complex as there is an anastomotic connection from the C2 nerve (and subsequently the first six cervical nerves via the transverse cervical nerve) and a cervical branch of the facial (cranial nerve VII) nerve. The fact that this anastomosis could contain sensory or motor or both types of fibers makes accurate interpretation of this reflex enigmatic.
Finding depressed and particularly asymmetrical local cervical and cervicofacial reflexes can be useful in localizing a cervical spinal cord lesion. Because of the incomplete understanding of these reflexes interpretation may need to be as imprecise as "consistent with a caudal cervical lesion" or "consistent with a cranial cervical lesion."
Cutaneous Trunci Reflex and Cutaneous Sensation
Evaluating for hypalgesia over the trunk, as elsewhere, should be performed with a two-pinch test. This is performed by pinching the skin into a fold, inserting the fold into the jaws of a strong hemostat or needle holder and after the patient has settled to this, a brief, sharp squeeze is applied to elicit a behavioral response.
Gait and Posture
Extensor weakness in a limb is best evaluated by observing for muscle trembling, buckling on a limb when turning and the ease in which the patient can be pulled to the side by the tail, both while standing still and while moving. Flexor weakness may be more evident as dragging of a toe and a low foot flight, particularly while turning. Subtle degrees of weakness in the thoracic limbs may be accentuated by performing a hopping test wherein one forelimb is held up and the horse made to hop laterally away from the examiner on the other forelimb. A horse with extensor weakness often will buckle on an affected limb. Pelvic limb and/or thoracic limb weakness can be detected by attempting to pull on the halter and tail at the same time. This is particularly useful if there is asymmetry in the degree of weakness. Normal alert horses resist such pulling whereas a weak animal is easy to pull to the side.
Mild degrees of ataxia can be detected by performing additional postural maneuvers. Considerable time usually is spent in performing serpentine maneuvers, circling wide and tight, elevating the head while walking the patient on a flat and on a sloped surface, turning tightly upon stopping abruptly from a trot and backing. These maneuvers alter visual, gravitational, vestibular and proprioceptive input to the nervous system such that any subtle sensory or motor deficit can become more clearly expressed. The overall severity of any gait abnormality in each of the four limbs can be graded one through four, as subtle, mild, moderate or severe.
Rather than manually placing limbs in abnormal positions to evaluate conscious proprioception, it appears more reliable to maneuver the horse rapidly (say in a circle) and stop the maneuver abruptly. This often results in an initial awkward placement of a limb and then the examiner can determine how long the horse leaves the limb in such an abnormal posture to determine the presence or not of a conscious proprioceptive deficit.
To determine the presence of weakness in the limbs of a horse suffering from spinal cord disease the three most useful tests are the tail pull, the tail and halter pull and thoracic limb hopping (Fig. 10). Pulling the tail while the patient remains static initiates, an extensor (patellar or quadriceps) reflex. This reflex is poor with lower motor neuron disease at the level of L3 - 4 and the patient will demonstrate weakness while standing still (hypotonia) as well as voluntary extensor weakness while moving. In contrast, a horse with an upper motor neuron lesion (wobbler) will have good resting muscle tone and be difficult to pull to the side in a singular movement while standing still. However, such a patient will be easily pulled to the side while walking. This demonstrates voluntary extensor weakness but the presence of intact or even hyperactive extensor reflexes in the pelvic limb.
Pulling on a lead rope and the tail simultaneously while circling the horse around the examiner is a postural reaction that also evaluates voluntary extensor strength and in addition can exaggerate a patient?s tendency to pivot on a hind limb and to maneuver limbs in an ataxic fashion. Flexor weakness leads to the patient not flexing the affected limb well and thus dragging the toe on the ground. A worn toe will result. Some neurologically normal horses will "toe drag"; many of these will have orthopedic disease.
A horse that has extensor weakness in a thoracic limb often will tend to tremble on the limb while the opposite thoracic limb is held up on initiation of the hopping test. It also will have difficulty in hopping to the side when pushed with the examiner's shoulder.
Evaluation of horses while being walked across curbs has not proven to be a useful test of proprioceptive dysfunction. Normal horses, particularly if distracted, often will stumble and those that are quite weak and ataxic but moving cautiously often can maneuver such obstacles. In the author?s experience blindfolding a horse suspected of suffering from spinal cord disease usually has not added anything substantial to the neurological evaluation. Normal horses react in different ways, from extremes of excitement to calmness and the subsequent movements they make depends on this behavioral response.
Signs of dysmetric ataxia and loss of balance will be markedly exacerbated when a blindfold is applied to a horse suffering from vestibular or occasionally from spinocerebellar disease.
Equine practitioners do find cases for which there is some indication of spinal cord involvement but no definitive proof. These cases usually are suspected of suffering from a painful musculoskeletal disorder, a peripheral neuromuscular spastic disorder, a behavioral problem (belligerency), laziness or back disease. Such patients may show one or more of the signs listed in Table 4. Other forms of frantic behavior have been associated with a strong suspicion of exposure to nettles or poison ants, but in these situations the signs usually abate with time. With horses that demonstrate a mild or unusual gait or postural abnormality, emphasis often will be on detecting evidence of spinal cord, peripheral nerve and muscle disease and distinguishing such evidence from signs resulting from primary orthopedic disorders.
Syndromes Wherein an Organic Spinal Cord or Vertebral Column Lesion May Be Suspected but Usually Not Proved
Localizing Lesions of the Spinal CordAfter the precise type of gait abnormality present has been determined, the most likely site of an acute spinal cord lesion and the pathways involved frequently can be accurately defined, with some exceptions. With peracute lesions, particularly those of an inflammatory nature and those with soft tissues compressing the spinal cord (such as with caudal cervical synovial cyst formation), resulting signs can wax and wane quite dramatically over periods of hours to days. Such signs usually stabilize with subacute to chronic lesions.
On the other hand, a horse with chronic spinal cord disease may show quite different neurological signs. For example a horse that has suffered a single insult of cervical spinal cord compression a year before examination may have an unusual, perhaps hypermetric, mild ataxia in the pelvic limbs with no evidence of weakness. There may be no signs in the thoracic limbs save for a questionable response to hopping.
Functional Sections of the Equire Spinal Cord
Prominent Gait and Postural Abnormalities Present with Neurologic Lesions at Different Locations
UMN, upper motor neuron; LMN, lower motor neuron; 0, not usually expected; +, mild if present; ++, usually present; +++, quite characteristically present.
With the neurological examination completed the examiner may be able to decide if and where any possible lesions exist. If this is not clear then it is often worthwhile returning to the patient and performing an even more critical evaluation. With a very fractious or a very excited horse suspected of having a neurological abnormality involving the limbs, exercise such as lunging or running on very soft going for 20 minutes can be undertaken and then a reevaluation made.
When lameness is present or is suspected, possibly interfering with interpretation of a horse’s gait and posture, then appropriate regional analgesia or short acting systemic analgesia (e.g. using synthetic opioids), may be used. In more chronic cases, nonsteroidal anti-inflammatory drugs may be given at a relatively high dose for several days (or weeks) and then the horse’s gait can be reevaluated, when lameness often will be reduced.
Neurological Examination of Foals
In addition to the evaluation of gait and reflexes in non recumbent adults, other postural reactions can be performed in foals and are of most benefit in detecting subtle proprioceptive and motor pathway lesions when the gait is normal. These include wheelbarrowing the patient to make it walk on just the thoracic limbs, hopping it laterally while supporting weight on just the left then the right thoracic limb in turn and hemistanding and hemiwalking the patient by making it stand and then move laterally on both left, then both right limbs. Spinal cord lesions cause postural reaction deficits on the same side as the lesions. Lesions involving the proprioceptive and motor pathways to the limbs result in an extremely slow or absent hopping response in that limb.
Objective Neurophysiologic Testing
Diseases of the Spinal CordOther conditions such as injection-site abscesses can occur, but less frequently. A history of previous cervical injection can be helpful in making the diagnosis.
Cervical vertebral malformations
Equine Degenerative Myelopathy (EDM)
Spinal cord trauma
Paresis and Weakness, without pronounced ataxiaWeakness, limb hyporeflexia, muscle hypotonicity, rapid muscle atrophy, and decreased pain sensation in any or all four limbs can be anatomically localized to the peripheral nerve, neuromuscular junction, or muscle itself. Weakness or paresis can be observed as muscle trembling, knuckling, stumbling, dragging of the limb, and dipping of trunk during weight bearing. When the horse does not stumble, foot flight and placement are usually normal. Weakness can be exacerbated by observing for buckling on a limb when turning and the ease in which the patient can be pulled to the side by the tail, both while standing still and while moving. Muscle atrophy is common with these diseases. Diseases of the peripheral nerves can be divided into several categories, depending on their anatomic location along the motor unit ( peripheral nerve, neuromuscular junction, and muscle):
Multifocal Diseases: Ataxia and Dementia TogetherDementia, cranial nerve signs, ataxia, and weakness in one or several limbs in an unequal distribution are problems that cannot be localized to a single focus and thus are considered diffuse or multifocal. Subtle differences in the clinical problems exist between these conditions, which may help in their differentiation:
ConclusionVariations in number and shape of vertebrae as well as prominent performance related and ageing changes in vertebrae need to be understood to interpret disease states more accurately. More objective measurement of vertebral canal and myelographic diameters in different sized horses will definitely improve identification of spinal cord compression. The roll of cranial cervical afferent input to the vestibular system and the clinical syndromes seen with lesions to cranial cervical nerves need to be better understood. Finally, critical documentation of localizing signs such as hypalgesia, hyporeflexia and sweating over the neck and trunk is often difficult but can be extremely useful in localizing spinal cord disease.
Scientific ReportsVet Q 2001 Jan;23(1):49-50
Neurological signs in a horse due to metastases of an intestinal adenocarcinoma.
Spoormakers TJ, IJzer J, Sloet van Oldruitenborgh-Oosterbaan MM
Department of Equine Sciences, Faculty of Veterinary Medicine, Utrecht University, The Netherlands.
A 22-year-old Dutch Warmblood mare was referred to Utrecht University with progressive left hind limb paresis and hyporeflexia. The preliminary clinical diagnosis was the neurological form of equine herpes virus (EHV-1) infection. Within 1 day of admission, the mare became recumbent and deteriorated rapidly. Postmortem examination revealed an adenocarcinoma of the caecum, with metastases in all regional lymph nodes and extending from the lumbar nodes into the vertebral canal, causing spinal cord compression and destruction of the left 4th and 5th lumbar nerves.