A significant number of pain-related gait abnormalities in horses are evident only when the horse is ridden, and are not apparent when the horse is hand-walked or longed. Even when these horses are ridden, the lameness might not be overt.

While there have been many recent technical advancements in the objective assessment of gait, these are sometimes of limited value for detection of bilaterally symmetrical alterations in gait that result in reduced performance such as generalized stiffness, lack of willingness to work, alteration in quality of movements such as lack of hind-limb engagement and impulsion, and alteration in the rider’s feel of the contact via the reins and bit to the horse’s mouth.

A rider often assumes that these problems are attributable to thoracolumbar region pain, because the problems are only manifest when the horse is ridden. When observed on the longe, such horses might lean into the circle—often more on one rein than the other—and show exaggerated contractions of the epaxial muscles. However, studies have shown that experimentally induced forelimb or hind-limb lameness could reduce range of motion of the thoracolumbosacral vertebral column. Radiographic examination can reveal impinging spinous processes, and this finding often results in an erroneous conclusion implicating thoracolumbar pain as the primary problem. Researchers at the Animal Health Trust, in Newmarket, U.K., have demonstrated that by using diagnostic analgesia to abolish overt or subclinical lameness, the rider often appreciates an increased range of motion of the horse’s back.

To investigate these clinical observations, the researchers studied normal horses subjectively free from lameness in hand, after flexion test, on the longe on both soft and firm surfaces and when ridden. The team objectively measured body lean on the longe and range of movement of the thoracolumbar region using inertial measurement units placed at predefined locations on the thoracolumbar and pelvic regions. These studies established normal ranges of motion for the thoracolumbosacral spine and demonstrated that sound horses have a small degree of bilaterally symmetrical body lean on the longe.

The researchers also measured body lean on the longe in lame horses and demonstrated that there is frequently asymmetry between left and right reins, with greater lean compared with normal on at least one rein. Substantial improvement in lameness by performing diagnostic analgesia resulted in reduced body lean on both reins and reestablishment of symmetry between left and right reins. Likewise, when lameness was improved by diagnostic analgesia, range of motion of the thoracolumbosacral regions increased, especially in the caudal thoracic and lumbar regions.

Further, the researchers have observed that the tendency of a saddle to slip persistently to one side is most frequently associated with hind-limb lameness. Abolition of lameness by diagnostic analgesia results in resolution of the saddle slip. The saddle most commonly slips to the side of the lame or more lame hind limb, but less frequently slips toward the less lame limb. Presumably saddle slip is induced by altered range of motion of the thoracolumbosacral region, which can vary among horses. Saddle slip can actually be an indicator of the likely presence of hind-limb lameness.

Overt lameness might not be apparent when a horse is trotting, but musculoskeletal pain can manifest at a canter, evidenced by the horse’s tendency to become disunited or repeatedly change leading limbs behind or in front, crookedness, loss of a normal three-time rhythm, placing the hind limbs either abnormally close together spatially and temporally, or placing the limbs remarkably far apart.

These observations can be apparent either on the longe or when the horse is ridden. Abolition of baseline lameness seen in hand can paradoxically make the canter appear worse if sacroiliac pain is contributing to pain and poor performance.

These observations highlight the importance of evaluating horses with performance problems in hand, on the longe, and ridden, preferably by the normal rider. Horses should be assessed in walk, trot, and canter, bearing in mind that while one aspect of the gait might improve with diagnostic analgesia, another could deteriorate. Horses should also be assessed performing the movements which they find most difficult, because in some horses this could be the only condition when the problem is manifest.

CONTACT: Sue Dyson, MA, VetMB, PhD, DEO, FRCVS—sue.dyson@aht.org.uk—+44 (0)1638 7519098— Animal Health Trust Centre for Equine Studies, Newmarket, Suffolk, United Kingdom

This is an excerpt from Equine Disease Quarterly, funded by underwriters at Lloyd’s, London.