Many horse owners have had the unfortunate experience of heading to the stable to ride, only to discover their horse limping as it approaches the gate or stall door.  In some situations, the lameness has developed suddenly and unexpectedly.  At other times, the horse’s performance has been decreasing during the previous weeks without the horse showing any obvious signs of lameness. 

In these situations, a decision is typically made to call the veterinarian and find the source of the problem.  Lameness examinations are one of the many important services provided by equine practitioners.   A thorough clinical examination, including palpation of limbs for heat, swelling, and pain, hoof testing and flexion of joints, is usually involved when trying to determine the source of the lameness.   A horse owner must understand the clinical examination is important, but often does not identify the sourceof the lameness.  For this reason, practitioners will utilize diagnostic anesthesia procedures (“nerve blocks” or “joint blocks”) to identify or verify the source of the pain leading to the lameness.

In order to understand the benefits of utilizing “blocks” in a lameness examination, it is important to know the steps involved with performing diagnostic anesthesia.  Nerve blocks and joint blocks are two different forms of diagnostic anesthesia.  The term “diagnostic anesthesia” adequately describes the procedure: “diagnostic” because the block assists with determining a diagnosis (the disease process present) and “anesthesia” because the block inhibits sensation in the nerves. 

To perform an effective block requires strong knowledge of a horse’s anatomy, appropriate preparation of the injection site, lidocaine or cabocaine, and adequate patient restraint to avoid injuries.  Preparation for nerve blocks involves scrubbing the nerve block site to remove debris and prevent infection.  In some situations, it may be necessary to clip the hair at the site to allow adequate cleansing and identification of the nerves.  Additional scrubbing is necessary and hair clipping is more common when joint blocks are performed. 

Depending on the disposition of the patient and the block procedure being performed, the restraint procedures required will vary.  Because sedating can modify the gait during the lameness examination, avoiding the use of sedation is preferred.  Adequate restraint is important for several reasons.  It allows placement of the needle in the appropriate location to achieve a successful block and it can help avoid broken needles.  Although not a common occurrence, needles breaking in a joint during a joint block are possible in an inappropriately restrained patient.  The safety of both the veterinarian and the patient are also important reasons to take precautions when blocking nerves and joints.  Performing the blocks in a quiet environment with minimal activity and appropriate restraint decreases the potential for injuries occurring during the procedure.

In order to properly localize the lameness, a solution is injected under the skin over a nerve or into the joint.  The nerve then becomes “numb” and sensation to the limb is eliminated below the region of the block or the joint where the solution was injected.  The block is usually performed beginning in the lower limb and working up toward the shoulder or hip, for lameness in the forelimb and hind limb, respectively.  By following this pattern, veterinarians can determine which region of the limb is the source of the pain.  Localizing the source of pain allows veterinarians to evaluate the region by utilizing diagnostic equipment such as radiography and ultrasonography.  If the source of pain is not localized before performing further diagnostics, the injury may be missed and the financial investment may produce limited results.

Although blocks are very useful diagnostic tools, it is also important for owners to be aware of the limitations.  Many horses will have the expected distribution of nerves, but some patients will have variation.  Variation in the distribution of nerves can lead to an incomplete block of the nerve and cause difficulty during interpretation of the results.  Two of the disadvantages of joint blocks are the additional time required for the block to take effect and the invasiveness of the procedure. Remember, the only block that is significant is the one that works. In other words, if a block doesn’t produce a sound horse, it doesn’t mean the anesthetized region is not involved somehow. It is not unusual to repeat diagnostic blocking on another day to confirm or deny results.
         

Although there are limitations, performing blocks greatly assists veterinarians when determining the source of lameness, which leads to diagnosis of the injury.  As a result, you will hopefully receive the greatest benefit of all-appropriate therapy to help your horse get back on its feet again…or should I say, back on its “hooves” again.