IN-DEPTH: SPORT HORSE LAMENESS


sheath and axial to the accessory carpal bone.3 Images of the SDF and DDF muscles along with their musculotendinous junctions in the distal ante- brachium can be obtained with the probe placed at the back of the forelimb.4 The presence of muscle fibers interspersed with the superficial and deep tendons at the musculotendinous junctions makes image interpretation of these structures particularly difficult. It is helpful to routinely image both front limbs simultaneously to help make an accurate determination. Radiographs are often inconsistent at providing a diagnosis. Some bony lesions at the back of the radius, such as osteochondromas, are consistently identified radiographically. Osteochondromas are cartilage-capped outgrowths from the palmar radial cortex and consist of a mixture of cartilage and bone. They are typically located proximal to the physeal scar on the caudomedial radius. Osteochondromas can impinge on the DDFT within the carpal sheath, resulting in carpal sheath effusion and lameness.5 Other bony abnormalities such as physeal remnant spikes, located at the level of the physis on the caudal radius, or enthesiophytes on cortex of the proximal palmar metacarpal bone can be very diffi- cult to image because of their small size and super- imposition of other bony structures.


4. Diagnostic Approach


Acute lameness related to the proximal metacarpus and carpal sheath appears to be quite common in show jumpers and hunters. A typical history is the horse that exercises quite vigorously, either in the ring or on the lunge line, without apparent incident, and becomes acutely and often markedly unsound. Frequently, the lameness may not become apparent for more than 24 hours. In the author’s opinion, this is a classic “high suspensory” history. Fre- quently, the affected limb is unremarkable to phys- ical examination. Occasionally, there is mild venous congestion in the proximal, medial metacar- pus region, but generally the limbs are often surpris- ingly normal on palpation, given the magnitude of the lameness. It is important to rule out the lower limb as the source of lameness with systematic nerve blocks before anesthetizing the structures of the proximal metacarpus. As with palpation, im- ages of the affected proximal metacarpus are often unimpressive. In our practice, horses with this type of history and clinical symptoms are treated symptomatically with ice, nonsteroidal anti-inflam- matories, and rest, and they are rechecked in ap- proximately 10 days. Some horses improve dramatically in that time period. At the 10-day re-check, if the horse jogs soundly with no palpable abnormality and no demonstrable lesion on ultra- sound, we will typically start light exercise. This usually consists of walking the horse under tack for an additional 10 days followed by tack-walking with short, straight-line trotting intervals for another 10 days. If the horse remains normal on clinical and


242 2013  Vol. 59  AAEP PROCEEDINGS


ultrasound evaluation at that time (30 days after injury), it is gradually returned to normal exercise. For horses that show any lameness during this pro- gression, we return it to walking-only exercise for a 2-week period and try again. In horses with lesions of the proximal suspensory that can be identified on ultrasound examination, we begin the exercise pro- gression once the horse is sound and the lesion has resolved. In the show hunters and jumpers, it is important


to differentiate whether lameness localized to the proximal metacarpus is caused by proximal superfi- cial flexor tendonitis or injury to other soft-tissue structures. Proximal SDFT injury is often a pro- gressive, degenerative condition, which occurs more frequently in the older show hunters and jumpers.6 Affected individuals are often observed to have a subtle forelimb gait asymmetry when first ridden, which resolves quickly as the animal warms up. In the author’s opinion, these horses are often un- comfortable at the canter, and, in the early stages, the lameness may be transient or indiscernible at the trot. There may be a brief period of lameness after vigorous exercise, which resolves spontane- ously. Swelling of the proximal SDFT may be dif- ficult to identify because the affected tendon may be constricted within the carpal sheath, making effu- sion uncommon. However, the soft-tissue swelling may be present if the injury extends distally into the proximal metacarpus. Because proximal SDF ten- donitis is an important rule-out in the older show horse, diagnostic blocking patterns are an important consideration. Anesthesia of the lateral palmar nerve on the axial surface or at the base of the accessory carpal bone will frequently anesthetize both a proximal suspensory desmitis and a proximal SDF tendonitis. The lateral palmar nerve block is performed by injection of a small volume of anes- thetic solution into the longitudinal groove along the medial aspect of the accessory carpal bone in its distal one-third. The needle is advanced until it con- tacts the carpal bone, and the anesthetic solution is deposited.7 It is important to image all of the soft- tissue structures in the proximal metacarpus in horses whose lameness resolves with the lateral pal- mar nerve block, most particularly the SDFT and the origin of the suspensory ligament. Practitio- ners who diagnose proximal suspensory desmitis with local infiltration of the suspensory origin are at risk of missing a case of proximal SDF tendonitis if they do not also anesthetize the lateral palmar nerve, because local infiltration of the suspensory origin may not effectively anesthetize the proximal SDFT. Furthermore, if the SDF tendonitis extends proximally into the carpal canal, anesthesia of the lateral palmar nerve may result in improvement but not complete abolition of the lameness. It is partic- ularly important, then, to consider the horse’s age, history, and clinical presentation in making this diagnosis.


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