Lameness is an abnormal gait or stance of an animal that is the result of dysfunction of the locomotor system. In the horse, it is most commonly caused by pain, but can be due to neurologic or mechanical dysfunction. Lameness is a common veterinary problem in racehorses, sport horses, and pleasure horses. It is one of the most costly health problems for the equine industry, both monetarily for the cost of diagnosis and treatment, and for the cost of time off resulting in loss-of-use.
Causes of lameness Lameness is most commonly caused by pain, but may also be the result of neuromuscular disease or mechanical restriction. Lameness itself is a clinical sign, and not a diagnosis.
Pain Pain is the most common cause of lameness in the horse. It is usually the result of trauma or orthopedic disease, but other causes such as metabolic dysfunction, circulatory disease, and infection can also cause pain and subsequent lameness. Orthopedic causes of lameness are very common and may be the result of damage to the hoof, bone, joints, or soft tissue. Horses are predisposed to orthopedic lameness by conformational flaws, poor hoof balance, working on poor footing, repetitive movements, poor conditioning for a given activity, and competing at a very high athletic level. Metabolic causes of lameness include hyperkalemic periodic paralysis (HYPP) and polysaccharide storage myopathy, which directly affect muscular function. Circulatory causes of lameness occur when blood flow to an area is compromised. This may be due to abnormal blood clotting, as in the case of aortic-iliac thrombosis, or decreased blood flow (ischemia) to an area, such as is sometimes seen in laminitis. Infectious causes of lameness are the result of inflammation and damage to tissue. These include problems such as cellulitis, hoof abscesses, and septic arthritis.
Mechanical lameness Mechanical lameness is caused by a physical abnormality, such as scar tissue, that prevents normal motion of a limb. Mechanical lameness does not cause pain. Classic examples of mechanical lameness include upward fixation of the patella and fibrotic myopathy, but any type of adhesion (often secondary to scarring during healing post-injury) or fibrosis can cause mechanical lameness.
Upward fixation of the patella occurs when the normal locking mechanism of the stifle, which allows the horse to "lock" its hind legs into a fixed position so it may stand with minimal effort, engages at inappropriate times such as when the horse is walking. It results in an inability to flex the stifle, so the horse must walk with an extended hind leg. It is associated with straight hind limb conformation (post leg), poor muscling of the hind limb, stifle trauma, and genetics. Fibrotic myopathy is caused by damage to the hamstring muscles, usually from trauma or intramuscular injection. It results in the formation of scar tissue and produces a characteristic gait where the horse prematurely "slaps" the ground with its hind foot, shortening the stride length in the damaged leg.
Neurological lameness Neurologic lameness may be the result of infection, trauma, toxicities, or congenital disease. Neurological evaluation of a lame horse may be warranted if the cause of the lameness is not obvious. Signs more commonly associated with a neurologic cause include unilateral muscle atrophy, paresis, paralysis, or dysmetria. Neurologic causes of lameness include:
Cervical vertebral stenotic myopathy (Wobbler disease): compression of the spinal cord in the cervical (neck) region results in lameness, ataxia, and change in gait, especially in the hind legs, and neck stiffness or pain. Stringhalt: caused by damage to the long digital extensor muscle, or from eating the weed Hypochaeris radicata; it produces a gait characterized by rapid, spasmodic hyperflexion of the hind limbs. Shivers: a rare neuromuscular disorder characterized by muscle tremors, difficulty picking up the hind feet when asked to lift for farriery work, hyperflexion or hyperextension of the hind limbs, and abduction of the hind limbs. Normal athletic function is often maintained, at least initially. Cerebellar abiotrophy: a rare genetic disorder that occurs in Arabian horses, producing ataxia, a base-wide stance, proprioceptive deficits, and high-stepping gait. Damage to individual nerves will affect the muscles they innervate and subsequently alter gait. Radial nerve paralysis will cause a dropped elbow and make it difficult to extend the affected limb. Suprascapular nerve damage will lead to atrophy of the main muscles of the shoulder (sweeny). Femoral nerve paralysis causes hyperflexion of the stifles, hocks, and fetlocks and the horse usually walks on the toe of the hind foot. Infectious causes affecting the nervous system may be bacterial, viral, protozoal, or rickettsial. Those that commonly alter gait include tetanus, botulism, Lyme disease, equine protozoal myeloencephalitis (EPM), rabies, West Nile virus, equine encephalitis virus, and equine herpesvirus 1. Neoplasic (cancerous) changes in the brain or around the spinal cord may also cause alterations in gait.
Signs of lameness Manifestations of lameness include any alteration in gait from what is normal for the horse. In general, it is harder to detect hind limb lameness when compared to lameness in a front limb when using visual cues.
Identifying a front limb lameness The classic sign of lameness in a front leg is a prominent "head bob." Viewed from the side, the horse raises its head and neck when the lame leg hits the ground, which helps to unload the lame leg. This is sometimes remembered by the adage "down on sound." A head bob is usually easy to see when one leg is lame, but can be subtle in very mild unilateral lameness, or in the case of bilateral front limb lameness. A horse may also try to reduce impact on a lame front leg by tensing of the muscles of the shoulder. In this case, it will stiffen the limb just before it hits the ground, a sign that may be noticed by an astute observer.
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