Mistakes in Interpreting Diagnostic Analgesia

Veterinarians, avoid these common mistakes to make the most out of your nerve blocks during lameness exams.
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Diagnostic nerve blocks are a mainstay of lameness diagnosis, but they’re fraught with problems of reliability—some tests produce false positive results while others produce false negatives.

“Subjective evaluation of diagnostic analgesia can lead to errors in diagnosis and, therefore, prognosis and treatment,” explained Michael Schramme, DrMedVet, CertEO, PhD, Dipl. ECVS, ACVS, professor at the Ecole Nationale Vétérinaire in Lyon, France.

During a presentation at the 2014 American Association of Equine Practitioners Convention, held Dec. 6-10 in Salt Lake City, Utah, Schramme identified possible reasons for faulty diagnostic nerve block interpretation. He first listed all 16 reasons, which include:

  • A subtle or inconsistent lameness;
  • Lameness that improves or resolves with warm-up exercise rather than due to the nerve block;
  • Inaccurate administration of diagnostic analgesia;
  • Evaluation of the incorrect limb;
  • Clinician bias toward an expected or desired result;
  • Gait alterations from sedative use;
  • Inaccurate testing of desensitization after placing a nerve block (Following a nerve block, the practitioner will press on the numbed tissues with a tool—such as a hoof pick—to see if the horse reacts. If he doesn’t, it generally means the area is numb and the horse can’t feel the pressure. However, some horses won’t respond even if they feel the pressure, which can make the veterinarian misinterpret the area as being blocked.);
  • Upward migration of anesthetic solution that blocks out more than the intended area;
  • Leakage or diffusion of anesthetic from a synovial (joint) structure to an adjacent nerve;
  • Inappropriate timing of gait assessment after nerve blocking;
  • Lack of understanding by the clinician as to which structures are blocked;
  • Subchondral (deeper in the bone beneath the cartilage) bone disease contributing to joint pain that could lead to false negative block results, since the nerve block won’t numb this area of bone;
  • The presence of aberrant (unusually placed and/or extra) nerves that aren’t anesthetized by conventional approaches;
  • Anesthetic’s inability to affect the protective gait that a horse has developed due to chronic pain;
  • Mechanical restrictions rather than pain causing an abnormal gait; and
  • Pain severe enough (fractures, laminitis, sepsis) that it cannot be blocked out.

Of these, Schramme described four interpretation quandaries in more detail. He said one of the biggest mistakes in interpretation is related to inaccurate administration of diagnostic analgesia. If the practitioner places anesthetic around the nerve (perineural fascia) rather than directly over the nerve, the numbing effect will be delayed. If the practitioner inadvertently instills the solution into a synovial structure, which can happen up to 50% of the time in blocks above the fetlock, this will obscure results because this numbs the joint rather than the nerves

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Written by:

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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