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 inten