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Kissing Spines in Horses: More Than Back Pain

By Jackie Hill

If you’ve ever managed a horse with back pain, then you’ve probably heard the term kissing spines floated around. But what exactly are kissing spines, and what does it mean if your horse is diagnosed with them?

Kissing spines, or overriding spinous processes, are reported to be one of the most common causes of back pain in horses. Though back pain was originally described in the 1960s, it wasn’t until recent years that veterinarians and researchers devoted more attention to it as a primary problem in horses as opposed to secondary to hind-limb lameness. As technology has progressed to produce higher-quality X rays and our knowledge of equine back anatomy’s complexity has expanded, we have a greater appreciation for and understanding of kissing spines.

A horse’s spine is composed of individual vertebrae connected by ligaments and surrounded by muscles. Each vertebra has a bony prominence that sticks up—the spinous process. In a normal horse, the spinous processes are spaced evenly, allowing a horse to both flex and extend his back. With kissing spines, the spinous processes are too close to each other or even touching. The lack of space between processes reduces back mobility and causes pain during movement as the spinous processes interfere with each other.

In this back X ray, note the lack of space between the spinous processes and evidence of bone bridging between some of the severely affected processes.

The underlying cause of kissing spines is still largely unknown. The condition can occur in any horse, but some breeds, such as Thoroughbreds and Warmbloods, seem to develop it more than others. Horses are most commonly diagnosed around 5 to 10 years of age, but younger and older horses can develop it, too. Kissing spines typically occur in the last few thoracic vertebrae—right where a saddle and rider would sit along the horse’s back.

Horses with kissing spines can demonstrate a variety of clinical signs. These can be subtle, such as poor performance or decreased range of motion when asked to flex or extend the back, all the way up to more noticeable behaviors, such as a painful reaction to back palpation, reluctance to be saddled or ridden, cross-­cantering, and bucking under saddle.

Veterinarians typically diagnose kissing spines using a combination of clinical signs and X rays of the horse’s back. X rays are the best way to assess the distance between spinous processes and to look for evidence of problems in the bones, such as increased density or ­cysticlike lesions. The tricky aspect of diagnosing kissing spines is that researchers have shown that 39% of horses have changes on their back X rays consistent with kissing spines, yet they show no signs of back pain. Similarly, the angle from which the veterinarian takes the X rays can influence the apparent spacing between processes and might result in overdiagnosis.

Other diagnostic modalities your veterinarian might use include nuclear scintigraphy (bone scan), ­ultrasonography, and the injection of local anesthetic around the suspected painful spinous processes. Unfortunately, each of these modalities has its limitations. For example, injecting local anesthetic into the area does not produce reliable diagnostic results, meaning veterinarians cannot predict which horses will respond to treatment. Therefore, the most reliable way to diagnose kissing spines continues to be looking at changes on back X rays in conjunction with clinical signs of back pain.

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