Page 171 - The Veterinary Care of the Horse

Following the injection, the site is lightly massaged and the horse may be gently walked

around to assist spread of the local anaesthetic around the nerve or within the synovial
VetBooks.ir structure. The block may take between 10–30 minutes to be fully effective. With some nerve
blocks, skin sensation can be tested by assessing the horse’s response to deep pressure after

5–10 minutes to check whether it is working.

ASSESSING THE RESULT

If the horse becomes sound or is significantly improved, then the cause of the lameness is

very likely to be within the last region of the limb to be blocked. However, interpretation of
the results is not always that simple for the following reasons.

• Many horses have multiple sites of pain contributing to their overall lameness. Taking the
pain away from the most obvious site may cause the degree of lameness in that limb to

improve but not be abolished. Alternatively, the horse may now show lameness in a
different leg.

• Very severe pain may not be eliminated. For example, horses with pedal bone fractures,
unrelieved foot abscesses or laminitis do not always respond to local anaesthesia of the

foot.

• Joint pain can arise from many sources. These include the synovial membrane, the
fibrous joint capsule and associated ligaments, the periosteum and subchondral bone.

These structures are not reliably blocked by intra-articular anaesthesia and so the horse
may remain lame after local anaesthetic has been injected into the affected joint.
Subchondral bone pain, in particular, is difficult to eliminate.

• Local anaesthetic may diffuse from the site of injection to numb other nerves or nearby

structures, giving a misleading result.
• Occasionally the pain is localized to a particular region, but it is not possible to identify

the source, despite follow up with diagnostic imaging techniques. One example is the
equine foot. The nerve blocks are not specific enough to pinpoint individual structures.

The use of magnetic resonance imaging has improved our diagnosis and understanding of
foot pain.

Possible complications

These are rare but include:
• local inflammation over the injection site, causing transient pain and lameness

• inflammation of the synovial membrane following a nerve block; this is known as ‘joint

flare’ and usually settles down after a couple of days