Lameness is by far the most common cause of inadequate performance in the horse. The majority of horses currently in training have experienced lameness at one time or another. Accurate diagnosis of lameness requires a comprehensive understanding of equine anatomy and a methodical approach to examination.
Causes of lameness can be divided into two categories: primary causes and secondary causes.
- Primary causes represent abnormalities that did not occur as a result of another problem. Foot abscesses, acute fractures, soft tissue injury (e.g. from trauma) and some forms of arthritis are common primary causes of lameness.
- Secondary causes of lameness are present as a consequence of one or more other problems. Laminitis, stress fractures, and soft tissue inflammation (e.g. myositis or desmitis) are common examples of secondary causes of lameness. They occur as a result of the horse’s compensating for a primary cause of pain.
How do secondary lameness abnormalities occur?
This may best be illustrated by using examples.
- Scenario #1: A horse develops an acute fracture of the right third metacarpal (cannon) bone. The fracture is surgically repaired, although it remains a persistent source of pain in the right thoracic limb. As a consequence, the horse constantly bears weight on the left thoracic limb. Chronic overloading of the left thoracic foot results in the development of laminitis (inflammation in the foot; "founder"). Although the laminitis is a secondary cause of pain, it may become just as significant as the primary cause (the fracture).
- Scenario #2: A common primary cause of lameness in horses is distal tarsitis, or inflammation of the distal hock joints. Although this typically occurs bilaterally (on both sides), one pelvic limb is frequently more painful than the other. A horse with distal tarsitis (which is the primary cause of lameness) will tend to move asymmetrically, exhibit an abnormal pelvic limb gait, and overload the contralateral thoracic limb (in an attempt to reduce pelvic limb weightbearing). Secondary problems including back soreness, hip soreness, and thoracic suspensory ligament inflammation (desmitis) often develop consequently to distal tarsal pain. In fact, it is not uncommon for a secondary cause of lameness to become more clinically obvious than the primary cause. For example, many horses with primary left distal tarsitis will present with a complaint of right thoracic limb lameness occurring as a result of suspensory desmitis (which is a secondary problem).
- Scenario #3: A horse develops acute severe pelvic limb stiffness and has difficulty walking following a 50-mile endurance ride in hot weather. The horse is very dehydrated and bloodwork reveals an imbalance of electrolytes. Although a metabolic problem, the primary cause of lameness is dehydration coupled with electrolyte imbalance. Severe myositis, which is the cause of the discomfort and stiff gait, is secondary. To effectively treat this lameness, the veterinarian must first normalize the horse’s metabolic parameters.
Why should we differentiate between primary and secondary causes of lameness? Although treating secondary causes of lameness often improves the horse’s performance, they will recur and the lameness will persist as long as the primary cause(s) of lameness goes untreated. It therefore behooves both the horse and client to accurately diagnose the primary problem(s) as soon as possible. For example, treating a sore back via acupuncture, chiropractic adjustment, massage, magnetic therapy, muscle relaxants, etc. will certainly make a horse feel and perform better. However, if the sore back is secondary to pelvic limb lameness, it will continue to recur as long as the lameness persists. In this case, the best therapy for the back is to successfully treat the pelvic limb lameness. Once the lameness is eliminated, all secondary problems (including the sore back) should also disappear.
How can we differentiate between primary and secondary causes of lameness? A complete understanding of the horse’s anatomy, conformation, gait, and intended use(s) are essential in determining an accurate diagnosis. A proper lameness examination should include 1) a conformation evaluation, 2) a passive lameness evaluation, and 3) an active lameness evaluation. Performing a lameness examination is much like putting a jigsaw puzzle together. The exciting part is finding the one key piece of the puzzle (i.e. the primary cause) and then watching everything else fall into place.
- Conformation Evaluation: This portion of the lameness examination is designed to help identify predisposing factors to certain types and causes of lameness. For example, a horse that has extremely straight pelvic limbs will be more prone to the development of distal tarsitis and/or upward patellar fixation. Although it is usually impossible to treat conformation (at least in adult horses), understanding how an individual horse is built often provides invaluable insight into the cause(s) of lameness.
- Passive Lameness Evaluation: This, like the conformational evaluation, is usually performed with the horse at rest in the stall or barn aisle. The passive exam involves placing pressure over various structures along each limb, neck, and body and noting areas of sensitivity. This is a very effective way of identifying many secondary sources of pain, particularly those that are not readily apparent during the active examination. A careful passive examination will often make it possible to predict where and to what degree the horse will exhibit lameness prior to performing the active exam.
- Active Lameness Evaluation: Assessing the horse at a walk, trot, canter, up or down hills, before and after limb flexion, etc. are all part of the active examination. The location, type, characteristics, and severity of lameness depicted during the active examination provide the final clues to completing the lameness puzzle.
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