Specificity of Digital Flexor Tendon Sheath Nerve Blocks

Diagnosing lameness in horses has often been termed an art and a science, in part because the use and interpretation of nerve blocks to isolate sources of pain is more art than exact science. In recent years, several blocks have been found to numb more structures than scientists previously thought based on research performed at Auburn University; this, in turn, impacts the interpretation of lameness that is attenuated by those blocks. To help sort out one particular block, Justin Harper, DVM, a resident in equine surgery at Auburn, presented the results of a study on analgesia of the digital flexor tendon sheath (DFTS) at the 2007 American Association of Equine Practitioners Convention, held Dec. 1-5 in Orlando, Fla.

He said the study was designed to answer one question: Is analgesia of the DFTS specific, only desensitizing the digital flexor tendon sheath and its contents, or is it nonspecific, blocking more structures distal to (lower than) or outside the confines of the sheath itself?

Six horses with no pre-existing forelimb lameness were studied in three trials (using temporary lameness induction methods); the first used set screws to induce solar pressure at the heel or toe, the second used endotoxin to create joint inflammation in the coffin joint, and the third used endotoxin to create inflammation in the navicular bursa. Each horse was videotaped walking and trotting toward and away from the camera before and after lameness induction, then the horses were blocked at the DFTS and videotaped again at 10 and 20 minutes after the block was applied.

Four investigators blinded to the video timing evaluated the videos on a 10-point lameness scale. Median lameness scores after trial 1 (sole pain) were not significantly improved at 10 minutes post-block, but they were significantly improved after 20 minutes. In trials 2 and 3 (coffin joint pain and navicular bursa pain), there were no significant improvements in lameness at 10 or 20 minutes after the blocks were applied.

"Analgesia of the DFTS using the palmar axial sesamoidean approach does not interfere with pain originating from the sole, coffin joint, or navicular bursa if evaluated within 20 minutes," Harper concluded. "So, if pain is attenuated with the DFTS block, chances are the pain is in the deep digital flexor tendon."

About the Author

Christy M. West

Christy West has a BS in Equine Science from the University of Kentucky, and an MS in Agricultural Journalism from the University of Wisconsin-Madison.

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