Text: Hayley Kruger
Continued from yesterday’s article…
Types of blocks
- The palmar digital nerve block is performed by injecting a small amount of local anaesthetic over the palmar digital nerves on both sides of the pastern, just above the bulbs of the heel and adjacent to the deep digital flexor tendon. Initially, vets believed that a positive response to this nerve block indicated pain in the back third of the foot (including the back of the coffin joint and the navicular bone). This block was considered one of the most reliable indicators of navicular disease. However, recent research has shown that the palmer digital nerve block numbs the entire coffin joint and most of the foot, except for the laminae and coronary band at the toe region. This type of nerve block is therefore useful to generally indicate foot pain, but not a specific area of foot pain.
- The abaxial sesamoid nerve block is conducted at the bottom of the sesamoid bones at the back of the fetlock. The entire foot, the pastern joint, the short pastern bone and their associated soft tissues are numbed. Depending on exactly where the block is performed, the back part of the fetlock joint may also be affected by this block.
- The coffin joint block is usually conducted by injecting local anaesthetic with a needle placed into the joint just above the coronet. This numbs the coffin joint, the navicular bursa, the navicular bone and the toe region of the sole. If a large volume of anaesthetic is injected, the heel region of the sole can also then be numbed. Although this block affects the navicular area, a negative response is not reliable to rule out navicular disease as the cause of lameness on its own. Also, lameness due to deep digital flexor tendonitis in the region of the navicular bone may or may not be affected by this block.
- The navicular bursa block is usually conducted by injecting anaesthetic between the bulbs of the heel just above the coronary band, and advancing it into the foot until it touches the navicular bone. A positive response to this block indicates pain in the navicular bursa, the navicular bone, the supporting ligaments of the navicular bone, the sole at the toe or the deep digital flexor tendon inside the foot.
- Other types of nerve blocks include injecting anaesthetic into the jaw and/or face to anaesthetise teeth for dental procedures, or on the eyelids to permit eye exams, as these are extremely sensitive areas.
Interpretation: what does this mean?
The interpretation of the results of nerve and joint blocks is often not straightforward. Whereas it used to be believed that the different blocks were very specific and limited to the areas of the foot they desensitise, it is clear that this is not the case. Most of the blocks will desensitise more than one specific area, and there is a lot of overlap that can occur between the different blocks. This leaves room for various reasons for lameness to remain in the equation. It is now recognised that many diseases of the foot involve damage to multiple structures within the lower limb.
Studies have shown that horses with navicular disease commonly have variable involvement of many soft tissue structures in addition to the navicular bone (including the navicular bursa, the deep digital flexor tendon, the impar ligament, the suspensory ligament of the navicular bone and the coffin joint). It is therefore not surprising that various horses diagnosed with navicular disease will respond differently to the various nerve and joint blocks, depending on which specific structures are damaged within the individual horse’s limb. Like most other parts of the lameness evaluation, nerve and joint blocks are only one part of the overall examination.
What are the risks?
The risks stemming from nerve blocking are minimal; however, some nerves run close to synovial structures, such as tendon sheaths or joints. When needles are used to inject anaesthetic into these areas, you run the risk of infection. It is therefore important for the vet to ensure the area being worked on is thoroughly disinfected and all equipment is sterilised. If there is any suspicion of infection, the vet should be consulted immediately.
In closing, if your veterinarian conducts a nerve block and the horse becomes sound, this does not mean we have a diagnosis. The nerve block only indicates the area that the lameness is coming from. Your vet will need to look at the bone and soft tissue structures within that region to figure out the exact source(s) of the lameness. However, nerve blocking does assist in narrowing down the potential sources of the problem. Thereafter, appropriate treatment can be given.