Diagnostic anesthesia (nerve blocks) comprise an essential part of lameness examinations for many practitioners. Their function is to anesthetize specific structures in the lower leg and thus allow identification of the location of pain. However, results are not always straightforward. Kent Carter, DVM, MS, Dipl. ACVIM, professor of medicine, section chief of internal medicine, and a lameness specialist at Texas A&M University, discussed potential areas of confusion with diagnostic anesthesia at the 2005 American Association of Equine Practitioners Convention, held Dec. 3-7 in Seattle, Wash.

“I’m one of the world’s biggest fans of this procedure; I’d be a cripple without it,” he said. “But sometimes results are unexpected. Limitations of diagnostic anesthesia that warrant consideration are mechanical lameness that does not respond to anesthesia, difficulty in accurately determining the extent of anesthesia (e.g., the skin can be desensitized without deeper structures being desensitized), and partial responses to anesthesia. Some lame horses simply do not respond to appropriately applied diagnostic anesthesia procedures, and this can add confusion to the lameness examination.

“Despite some limitations in performing and evaluating diagnostic anesthesia, it is my opinion that it is the most definitive method of determining or confirming the source of lameness in the majority of horses,” he continued.

Carter explained that important considerations in administering diagnostic analgesia include the location and direction of needle placement, volume of local anesthetic needed, diffusion patterns to different structures, and time constraints