Palpating nerves to prepare for a nerve block.
Ever feel lost when your veterinarian starts talking about nerve blocks? Nerve blocks are a basic diagnostic tool that veterinarians can use to localize lameness to a certain area of the body- usually this is the “distal extremity” also known by most of us as the “lower leg.” Here are the answers to some common questions that stump horse owners from time to time.
What exactly is the vet doing when he or she places a nerve block?
The horse has sensory nerves going down the legs that lie just under the surface of the skin. These nerves send information about stimuli up to the brain. If the veterinarian anesthetizes (“numbs”) these nerve at a certain location, everything on the leg below that location will cease to feel stimuli- including pain. A nerve block consists of the veterinarian placing a small amount of numbing agent (usually lidocaine or carbocaine) strategically near nerves to selectively anesthetize an area of the leg. This is done by wiping the skin with some rubbing alcohol and then using a very small needle to inject the agent under the skin.
Once the area is numb, how does the vet determine if it is the location of the lameness?
Prior to placing a nerve block, the veterinarian will observe the horse moving- usually at the trot but sometimes even at the walk. Once the vet sees which leg is lame and the degree of lameness, he or she can use this as a comparison point for after the nerve block is place. So, for instance, if a horse is lame at the trot prior to a nerve block being placed, but then trots sound following the nerve block, the vet knows that the source of the lameness is coming from the location that was just anesthetized.
Speaking of degrees of lameness, how does the vet quantify them?
Most veterinarians use a lameness grading system developed by the American Association of Equine Practitioners (AAEP.) It 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 (e.g. under saddle, circling, inclines,
hard surface, etc.)
2: Lameness is difficult to observe at a walk or when trotting in a
straight line but consistently apparent under certain circumstances
(e.g. weight-carrying, circling, inclines, hard surface, etc.)
3: Lameness is consistently observable at a trot under all
circumstances (e.g. trotting in a straight line.)
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.
If my horse’s leg is numb, how come he can still move it?
When the vet places a nerve block, he or she is blocking sensory nerves. But sensory nerves are different than motor nerves, which innervate muscles in order to get them to move. 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.
How long do nerve blocks last?
About 45 minutes to 1 hour.
Are there any risks?
Risks are minimal however some nerves run close to synovial structures such as tendon sheaths or joints. For that reason, infection is possible as a result of the introduction of the needle into that area. Therefore, for certain nerve blocks, the vet will take a few extra minutes to wipe betadine and rubbing alcohol on the leg.
How many nerve blocks are there?
There are about a dozen areas of the body that can be nerve blocked. Most nerve blocks are performed on the legs, but they can also be performed on the jaw and face to anesthetize teeth for dental procedures or on the eyelids to permit eye exams. The most common nerve blocks are as follows:
· Palmar Digital “PD”- anesthetizes the heel and bottom of the foot.
· Abaxial- anesthetizes the foot and pastern.
· Low 4-point- anesthetizes the foot from the fetlock down.
· High 4 or 2 point – anesthetizes the leg from just below the carpus or hock
· Radial/ Ulnar (front) or Peroneal/ Tibial (hind)- anesthetizes the leg from the
carpus down on the front or the hock down on the hind.
If my veterinarian places a nerve block and my horse goes sound, does that mean we have a diagnosis?
No. The nerve block just tells you the area that the lameness is coming from. Next your veterinarian will need to look at the bone and soft-tissue structures within that region to figure out the exact source(s) of the lameness.
Now when the vet starts talking about nerve blocks - you will be in the know!