At Table Topic sessions during the annual American Association of Equine Practitioners (AAEP) Convention, roomfuls of horse doctors discuss important elements of equine practice. One of those conversations, which took place at the 2014 edition, held Dec. 6-10 in Salt Lake City, Utah, covered the challenges associated with interpreting perineural nerve blocks that veterinarians use to help diagnose the source of lameness in horses.
Michael Schramme, DrMedVet, CertEO, PhD, Dipl. ECVS, ACVS, professor at the Ecole Nationale Vétérinaire in France, and Katherine Garrett, DVM, Dipl. ACVS, of Rood & Riddle Equine Hospital, in Lexington, Kentucky, led the discussion.
Veterinarians administer nerve blocks in a diligent, sequential fashion starting as far down on the limb as possible and with as little anesthetic (even as little as 1 mL of mepivicaine) as possible. Their goal is to numb certain areas of a lame limb to see if the horse trots sound, narrowing the cause of the lameness to the numbed area. Mepivicaine lasts longer than lidocaine, the veterinarians noted, but lidocaine produces quicker pain relief. Researchers have shown that lidocaine can irritate tissue, however, and is more toxic to cartilage than mepivicaine, making the latter the perineural nerve blocking agent of choice.
Attendees discussed how MRI studies have demonstrated that the palmar digital nerve (PDN) block, often used to block out the rear portion of the foot, actually can block lesions higher in the leg because the anesthetic migrates as high as the fetlock. At a 2012 AAEP Convention presentation, Natasha Werpy, DVM, Dipl. ACVR, clinical associate professor at the University of Florida's College of Veterinary Medicine, showed that as many as 10% of horses block out to the palmar digital nerve block because their lameness originates in the fetlock, not the foot.