Text: Hayley Kruger
One of the biggest mysteries horse owners can face is why their horses may be lame. Lameness has a variety of causes, and it is often very difficult to pinpoint exactly what the cause of it is. With modern technology and extensive research, veterinary practices have advanced to making use of nerve and joint blocks to aid in deciphering where and why lameness is occurring in a lame horse. We explore the basics of what nerve blocking is, and what to expect out of conducting a nerve block.
What is nerve blocking?
Nerve and joint blocking is a method that has been developed by veterinarians to aid in the isolation and diagnosis of lameness in the horse. Nerve blocks generally occur in the lower limbs, particularly when trying to determine sources of lameness and reasons for the lameness. Nerve and joint blocks involve the injection of a local anaesthetic either close to a nerve or directly into a joint or into another synovial cavity, such as the navicular bursa or a tendon sheath. The local anaesthetic temporarily disrupts the function of any nerves or nerve endings. This results in isolated areas of desensitisation (numbing) where the nerve block was exercised.
When conducting nerve blocks, the vet will usually have the horse trotted up either in a straight line on a hard, level surface, and/or on the lunge in both directions, seeing that the degree of lameness is identified best in the trot gait. The nerve block is then applied, and the horse is trotted up again to see whether there has been any improvement in the degree of lameness. If the lameness has improved, it is likely that the site of pain causing the lameness has been desensitised by the block.
Degrees of lameness
Most veterinarians use a lameness grading system developed by the American Association of Equine Practitioners (AAEP). This system grades lameness on a score of 0 to 5, with the following descriptions:
- 0: Lameness not perceptible under any circumstances.
- 1: Lameness is difficult to observe and is not consistently apparent, regardless of circumstances (for example under saddle, circling, uphills, hard surface).
- 2: Lameness is difficult to observe at a walk or when trotting in a straight line, but consistently apparent under certain circumstances (for example weight-carrying, circling, uphills, hard surface).
- 3: Lameness is consistently observable at a trot under all circumstances.
- 4: Lameness is obvious at a walk.
- 5: Lameness produces minimal weight-bearing in motion and/or at rest, or a complete inability to move.
Nerve blocks and joint blocks
If local anaesthetic is placed around a nerve, such as the palmar digital nerve that runs down both sides of the back of the pastern before entering the foot, the areas connected to that nerve will become numb. If the horse has pain in this area, the nerve block will temporarily numb it and the lameness may disappear. This gives us an indication of where the lameness may be coming from, by process of elimination.
If local anaesthetic is placed directly into the coffin joint, it will numb all the nerve endings in the joint. If the horse has pain inside the joint, this will temporarily relieve that pain. Worth noting is that placing local anaesthetic into joints or other synovial cavities may affect nerves that touch or cross the lining of the cavity; thus, the joint block may desensitise other areas of the limb or foot in addition to the joint itself. This is why nerve blocking should not be analysed in isolation, but used as a tool to solve the lameness issue.
How does it work?
When the vet conducts a nerve block, sensory nerves are being blocked, and not motor nerves. Sensory nerves are nerves that carry sensory information to the central nervous system and can, in so many words, be associated with ‘feel’. Motor nerves carry signals to the muscles in order to get them to move, and as such can be associated with ‘do’. Thus, even when a sensory nerve block is placed, the leg will continue to be able to move, because the deeper motor nerves are unaffected by a nerve block. A nerve block generally lasts 45 minutes to an hour, depending on the location of the pain, and the amount of anaesthetic that the vet has administered.
Part 2 will be released tomorrow