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Diagnosing Incoordination, Ataxia and Weakness
Diagnosing Incoordination (Ataxia) and Weakness in Horses
including ataxia, spasticity, and hypermetria
Introduction
Most folks understand what you mean when you say muscular weakness but incoordination can be a bit less clear and sometimes the two can look like each other. Just to get off on the right foot let's explain the difference. Muscular weakness is the inability for the muscles to properly support the forces it is working against. For example, it might be the inability of the muscles to support the horse standing or walking properly. Weakness can be either a muscle or neurological problem. Incoordination is the inabilty for the horse to coordinate the proper function of the muscles and always a neurological problem. Incoordination is often labelled "ataxia" by medical experts. Muscular weakness is often termed "paresis".
Coordination and strength requires the proper integration of the brain (cerebrum, cerebellum, and brainstem), spinal cord, peripheral nerves, and muscles. Failure at any point causes signs of ataxia and/or weakness. The nature and location of the symptoms helps localize the lesion in the neuromuscular system. Most diseases of the horse's neuromuscular system tend to localize to specific areas so by localizing the lesion you can generate a list of probable causes and focus further diagnostic efforts. This article describes the different forms that ataxia and weakness can take and how to localize the disease. From the localization a list of rule outs is provided with links to articles on specific diseases for further research.
Examination for Ataxia and Paresis
Prior to a specific examination for the type and cause of ataxia a complete neurological examination should be done to rule out other areas of the nervous system and even some non-neurological diseases...more.
For assessing ataxia and weakness the horse should be walked in a straight line in order to evaluate subtle asymmetries in stride length or signs of weakness. The horse should be turned tightly while walking. When stopped the horse should be observed for abnormal foot placement. Unusual limb and foot placement while moving or standing is defined as ataxia (sometimes termed "incoordination") while an inability of the horse to support itself should be labeled weakness.
Ataxia is a neurological symptom consisting of gross lack of coordination of muscle movements. Signs of ataxia and/or weakness include:
- stepping on himself
- excessive swinging outward or inward or the leg in a irregular inconsistent manner, usually best demonstrated as excessive outward swinging of the outside rear limb when turned in a tight circle.
- tilting, leaning, or falling over may be a sign of ataxia or weakness
- short gait may be a sign of ataxia or weakness
- when accompanied by stiffness this may be ataxia
- when accompanied by body dips or sway this may be weakness
- exaggerated limb flexion during movement
Vestibular Ataxia: head tilt, stiff gait, and loss of balance
The vestibular system maintains the appropriate spacial orientation of the body and head, and position of the eyes. In short it helps keep the eyes, head and body upright. The vestibular system includes the inner ear, eight cranial nerve, and the nucleus in the brain.
Losing the vestibular system on one side causes the head and in some cases the trunk to tilt towards the ground on the same side of the lesion. There are two areas where the vestibular system can be effected and symptoms depend somewhat on where the lesion is in the vestibular system. Signs include a head tilt and in some cases an eye tick (nystagmus), eye drop, and body tilt all on the side of the lesion. Horses with ataxia from vestibular disease adopt a wide based stance, exhibit short stiff strides, and a reluctance to move. When presented with a horse with a stiff gait tetanus should be strongly considered. In mild cases horses can compensate remarkably and the symptoms can be exaggerated by applying a blindfold but this must be done carefully as the horse may fall over.
Most often vestibular ataxia is due to damage to the inner ear or the eight cranial nerve so ataxia may be mild and quickly compensated for ...more on vestibular disease.
Cerebellar Ataxia: tremors and exaggerated limb action
The cerebellum regulates the quality of muscle activity by smoothing out action and preventing exuberant muscle action. Cerebellar lesions cause hypermetria with joints and limbs showing an increased range of movement. There is no weakness of abnormal limb placement while the horse is standing still. Also usually seen intention tremors of the head and reduced menace responses with intact vision. Cerebellar disease can be caused by any focal infection or lesion such as a tumor but is only seen commonly in foals...more.
Proprioceptive Ataxia and Weakness: Inconsisten mistakes of limb motion and positioning
Propioceptive ataxia is assessed by looking for inconsistent "mistakes" while moving and is almost always accompanied by weakness. Conscious proprioception is affected as a result of interrupted spinal pathways going to the brain but because the motor neurons travel in the same area of the spinal cord weakness usually accompanies propioceptive ataxia. Horses with this condition may step on themselves or show inappropriately abducted or adducted foot placement, crossing of limbs, or excessive circumduction of the outside pelvic limb. Signs of weakness would include a weak response to "tail pulling" as well as decreased joint flexion (scraping of hooves) while walking. There are many causes of spinal cord lesions and is the main focus of the rest of this article.
Spastic Ataxia: A Stiff Gait
Spasticity is a stiff, stilted gait, with a short stride and lack of joint flexion. Often it is compared to the walk of a tin soldier. This stiff gait helps seperate this from hypermetria (see below). Most often spasticity is seen in combination with other signs of brain disease, like cranial nerve involvement. When a horse displays primary signs of spasticity with dementia or cranial nerve signs usually the brain stem is involved...for more information.
- Hypermetric Ataxia: seen as increased or exaggerated joint flexion. When hypermetria presents as the primary sign frequently accompanied by tremors during voluntary movement (intention tremors) the lesion may localize to the cerebellum. Primary cerebellar disease is seen most often in foals...for more information.
- Propioceptive Ataxia: seen as increased swaying of the trunk, prolonged pelvic limb stride, waving of the limb in the air before placement, abduction of the limb during forward movement, crossing the limb under the body, and stepping on the opposite limb. The primary sign of spinal cord disease is ataxia (incoordination) and not weakness, though exceptions exist. The reason is the way the muscles and reflexes work. Even without the innervation of the upper motor neuron in the spinal cord muscles maintain tone. This is mainly do to the peripheral nerves (also known as the lower motor neuron) and their spinal reflexes. So if communication is lost futher up the spinal cord, the leg still has strength but loses its ability to coordinate with other limbs and be controlled by the brain, ie ataxia.
- Weakness or paresis: seen as knuckling, stumbling, dragging of the limb, and dipping of trunk during weight bearing. Weakness occurs when the peripheral nerve, neuromuscular junction, or muscle is dysfunctional. The exception is if the lesion in the cord happens to be where the spinal cord is effected where the peripheral nerve communicates with it. Careful consideration of the symptoms is sometimes needed to distinguish ataxia and weakness. For instance stumbling and swaying of the body occurs with both conditions: did it occur because the horse was unable to coordinate it's movements or because of an inability to move and support the limb because the muscle was not strong enough. Sometimes you must look at other signs to differentiate them and occasionally they happen together.
Neurological Examination for Ataxia and Weakness
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