The Unnerving Reality of Perineural Nerve Blocks

Learn about the challenges associated with interpreting perineural nerve blocks used to help diagnose lameness.
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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

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Nancy S. Loving, DVM, owns Loving Equine Clinic in Boulder, Colorado, and has a special interest in managing the care of sport horses. Her book, All Horse Systems Go, is a comprehensive veterinary care and conditioning resource in full color that covers all facets of horse care. She has also authored the books Go the Distance as a resource for endurance horse owners, Conformation and Performance, and First Aid for Horse and Rider in addition to many veterinary articles for both horse owner and professional audiences.

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