Veterinary Tool: Bone Scan

Perplexing lamenesses seem to be more the norm than the rarity for veterinarians. The broad scope of medications and treatments available today virtually eliminate many of the simpler soundness problems that commonly put a horse into the retirement field 25 years ago. If your veterinarian mentions taking your horse for a bone scan, don’t cringe. The cost isn’t that bad, relatively anyway, and the information it provides may surprise you.

The Candidate
A 10-year-old open jumper is at a university clinic with complaints including poor impulsion from behind, difficulty with tight combinations, tendency to jump long and flat, and trouble with flying changes. He’s also interfering sometimes, something he never did before.

The rider can feel a problem behind, but no one can see much. Lameness workups included nerve/joint blocks, flexion tests and X-rays. Although there have been some positive findings, things just don’t add up right. The hocks do have some arthritic changes, but the nerve blocks don’t change the horse much.

The Procedure
A diagnostic test is suggested: a bone scan. In this study, the horse is given an intravenous injection of a radioactive compound that has a high attraction to bone. After waiting for the radioactivity to clear from the blood and the soft tissues, the horse is scanned with a gamma camera that picks up the level of radioactivity in the bones. Areas that are injured or otherwise metabolically active will light up more on the scanner. A computer program then integrates the gamma-radiation counts and produces an image of the area.

The amount of radiation needed to do a whole-body bone scan is small, similar to that from a set of chest X-rays. As a safety precaution, the horse must stay at the hospital or clinic until the radioactivity has been eliminated, usually 24 hours.

The Impression
Our horse did show some increased radioactive uptake in the hocks, in the same areas as identified on the X-rays, but was also found to have a nondisplaced fracture in the pelvis. Since the blocks of the hocks hadn’t changed the horse’s way of going much, the pelvic fracture was diagnosed as the cause.

A bone scan is also particularly useful for situations where routine X-rays have trouble penetrating heavy muscle masses, such as on the upper limb. It can also be helpful for horses that are known to have multiple problem areas but it’s not clear which one(s) might be active at the time and causing him problems, or for cases where nerve block results were unconvincing or gave only partial relief of symptoms.

The Drawbacks
The only real drawback to nuclear scanning is its extreme sensitivity. Areas may light up for reasons that are unrelated to the lameness. For example, a cannon bone rapped on a jump rail may show increased activity on the scan. Unexplained hot spots may also appear.

Therefore, bone-scan results taken alone may be misleading but when combined with the history, physical examination findings, X-rays and nerve-block results it is a useful tool in lameness diagnosis.

The cost of a bone scan varies among clinics but usually runs about $500. Pharmacy costs, clinic and professional fees are extra, as are the costs of any additional X-rays, ultrasounds, etc. of areas highlighted by the bone scan.

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