Where to go when nerve blocks don’t reveal the source of pain

When systematic nerve blocks fail to eliminate the lameness, this results in diagnostic dilemmas. Palpation is a crucial part of every examination. It is the systematic evaluation of the limb by touch (i.e., feel for structural changes and pain).¹ The examination needs to look at all portions of the limb but should be gauged so the most time is spent in examining the most common areas of injury. Manipulative tests or flexion tests are important adjunct to a lameness exam and can often give an important (lead to establish the cause(s) of lameness.^1-3 In many cases the pain caused by manipulation is so severe that the horse will retract the limb. In more subtle cases, the only satisfactory way of evaluating the effect of manipulation is to trot the horse immediately and note any exacerbation of a problem.

Thermography offers the veterinarian new diagnostic insight for their patients. Interpretation of thermal images must be based on good quality, properly positioned images. Thermal cameras are highly accurate instruments for measuring temperature and can provide invaluable information to determine a site of lameness, pain, or autonomic dysfunction. Scanning over an object will not magically show a “hot spot” at the site of the problem. A standard examination protocol for each body segment evaluated should be used.^5,6 Each point of focus on the body should include dorsal, palmar/plantar, medial, and lateral views when possible. Contralateral and dorsal views should be equidistant and fill the image screen. Whenever possible, it is recommended that the contralateral extremity should be captured in the same image. Imaging is contraindicated if bilaterally symmetrical images cannot be evaluated. Injured or diseased tissues will invariably have altered circulation.^4-6 One of the cardinal signs of inflammation is heat due to increased circulation. Thermographically, the “hot spot” associated with the localized inflammation will generally be seen in the skin directly overlying the injury. However, diseased tissues may, in fact, have a reduced blood supply either due to swelling, thrombosis of vessels, infarction of tissues or change in sympathetic tone. With such lesions the area of decreased heat is usually surrounded by increased thermal emissions, likely due to shunting of blood.

Multiple thermographic images of a suspect area should be made.⁵ The area in question should be evaluated from at least two directions approximately 90o apart, to determine if a “hot spot” or “cold spot” is consistently present. The horse’s extremities should be examined from 4 directions (circumferentially).^4-6 Significant areas of inflammation will appear over the same spot on each replicate thermogram.