Review of Mistakes That Can Be Made When Interpreting the Results of Diagnostic Analgesia During a Lameness Examination

Results of diagnostic analgesia can be misinterpreted during lameness examinations. Many diagnostic tests have been known to produce both false positive and false negative results. Authors’ addresses: Department of Large Animal Clinical Sciences, College of Veterinary Medicine, University of Tennessee, Knoxville, TN 37996 (Jim Schumacher); Department of Clinical Sciences, College of Veterinary Medicine, Auburn University, AL 36849 (John Schumacher); Equine Clinic, National Veterinary School of Lyon, Marcy L’Etoile, France (Schramme); and Department of Clinical Sciences, College of Veterinary Medicine, Texas A&M University, College Station, TX 77845 (Moyer); email: schumjo@auburn.edu.

1. Introduction

Many diagnostic tests used during the clinical examination of a lame horse are known to produce both false positive and false negative results (e.g., application of the hoof tester, the distal interphalangeal extension test, flexion tests, and pressure applied to the proximal aspect of the suspensory ligament). Diagnostic intrasynovial and regional analgesia is, therefore, an important component of lameness examination if the site of pain causing lameness remains uncertain after the horse undergoes a thorough clinical examination. The results of diagnostic analgesia are usually subjectively evaluated, sometimes with difficulty, and errors in the subjective evaluation lead to errors in diagnosis and, therefore, in prognosis and treatment. To accurately evaluate the results of diagnostic analgesia, the clinician should have a thorough knowledge of the nuances of interpretation. Although interpreting the results of diagnostic analgesia is usually straightforward, the clinician should be aware of the many ways in which diagnostic analgesia can be misinterpreted.

The results of diagnostic analgesia can be misinterpreted if:

• The horse is inconsistently or insufficiently lame.
• Lameness improves or resolves as a result of exercise (i.e., warms out of lameness) rather than from diagnostic analgesia.
• The incorrect limb was chosen for evaluation. 
• The clinician is biased by the expected or desired result of an analgesic technique.
• The horse’s gait has been altered by the use of sedatives.
• Testing of regional desensitization after diagnostic analgesia is inaccurate.
• Administration of anesthetic solution is inaccurate.
• Local anesthetic solution has migrated proximally following perineural injection.
• Anesthetic solution leaks or diffuses from a synovial structure to desensitize an adjacent nerve.
• Time of gait assessment after an analgesic technique is inappropriate.
• The clinician does not understand what structures are desensitized by the diagnostic block.
• Disease of the subchondral bone contributes to joint pain.
• The horse has aberrant nerves.
• Long-term pain causes the horse to develop an abnormal protective gait that is unchanged by diagnostic analgesia.
• An abnormal gait is caused by mechanical restrictions rather than pain.
• Lameness is caused by severe pain that cannot be significantly ameliorated by diagnostic analgesic techniques.

2. Interpretation

For subjective evaluation of a horse’s gait, the horse should be consistently and sufficiently visibly lame before diagnostic analgesia is performed, so that any improvement in gait can be detected and attributed to the diagnostic analgesic injection. The clinician should be aware that lameness of some horses improves or resolves during exercise and, so, for these horses, a false positive response to diagnostic analgesia can occur if the horse has not been sufficiently exercised to attain a consistent state of lameness.

Often, the lameness may only be improved, rather than completely abolished, after administering diagnostic analgesia. Many clinicians consider a positive response to regional analgesia to be 70% or more improvement in gait¹ and a positive response to intrasynovial analgesia to be 50% or more (authors’ opinion). If the lameness is subtle, a 50% or even a 70% improvement in gait may be difficult to appreciate. Keegan et al.² demonstrated, in a study comparing objective and subjective evaluations of mildly lame horses, that agreement among experienced equine clinicians on subjectively determined lameness scores is poor.

Methods by which a subtle lameness can be made more apparent include exercising the horse before the examination (e.g., by riding or lunging), changing the ground underfoot (e.g., from soft to hard ground or vice versa) and evaluating the horse’s gait while the horse trots in a circle. Pain in the digit can usually be exacerbated by exercising the horse on a hard surface and pain more proximal in the limb can sometimes be exacerbated by exercising the horse on a soft surface.^3,4 In some cases, a lameness is subtle because the horse is bilaterally lame. When a nerve or joint block causes a temporary resolution of pain in one limb, lameness in the other limb may become obvious. When lameness is too subtle for consistent subjective evaluation, thereby prohibiting the use of diagnostic analgesia, the use of an automated lameness detection device, such as one that uses body-mounted, wireless, inertial sensors, may still enable measurement of mild but consistent gait asymmetry to allow the clinician to proceed with the use of diagnostic analgesia.^5-7

3. Evaluating the Incorrect Limb

The results of diagnostic analgesia can be misinterpreted if the incorrect limb is chosen for evaluation. Lameness of a single limb can cause alterations in gait symmetry that make the horse appear lame on other limbs, causing the wrong limb to be chosen for evaluation.⁸ Uneven loading in a fore limb can cause compensatory uneven loading in the hind limbs and vice versa. Keegan cited the “law of sides” to aid in deciding which limb is the truly lame limb.⁸ According to this “law of sides,” when a horse appears to be lame in the ipsilateral fore and hind limbs, the site of pain causing lameness is likely to be located in the hind limb. The ipsilateral fore limb appears lame because of a compensatory change in loading in the fore limbs. When a horse appears to be lame in a fore limb and the hind limb of the contralateral side, the site of pain causing lameness is most often in the fore limb. The hind limb appears lame because of a compensatory change in loading or push-off in the hind limbs. The second part of the “law of sides” is less frequently applicable because only marked fore limb lameness tends to cause a visibly noticeable compensatory decrease in impact loading in the ipsilateral hind limb and a compensatory increase in push-off in the contralateral (i.e., diagonal) hind limb. A mild lameness in a hind limb, however, may cause a change in gait that mimics a substantial lameness of a fore limb, whereas lameness of a fore limb is less likely to cause a visibly noticeable change in gait of a hind limb.⁸

4. Clinician Bias Towards the Expected or Desired Result of an Analgesic Technique

One clinical trial showed that clinicians are biased by knowing that a diagnostic block has been performed on a lame horse.⁹ This bias may reveal itself in a positive or a negative way and a clinician may be inclined to see an improvement in a horse’s lameness where there is none or fail to see the improvement. [...]