Nerve Blocks 

Lameness can be a frustrating condition for any owner and is the most common reason for enforced time out of work. Of the many conditions causing lameness foot pain – especially in the front feet, is the most frequently encountered problem. 

Quite often a horse will present as lame in a specific leg but in the absence of specific clinical signs other than the lameness it can be difficult to localise the site of pain. Nerve blocks and joint blocks are very useful in such situations to rule in or rule out a specific part of the lame limb as the origin of the site of pain.  

Although the results can be extremely useful in localising the site of pain, recent research suggests that nerve and joint blocks may not be as useful or accurate as onced believed.  

What are nerve and joint blocks? 

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 it contacts. This results in areas of desensitisation. The desensitised area depends on which nerves or nerve endings are affected.  

The palmar digital nerves run down both sides of the pastern before entering the foot. When these two nerves are blocked the areas of the foot supplied by these nerves become desensitised. If the pain causing the lameness arises from these areas then the pain causing the lameness should be reduced or abolished meaning that there will be an improvement in the grade of lameness or in some cases a complete abolishment of the lameness. 

If local anaesthetic is placed directly into a joint, it will desensitise all the nerve endings in the joint. If the horse has pain inside the joint, this will temporarily relieve that pain and again should result in an improvement in the grade of the lameness.  

Unfortunately once placed inside a joint cavity local anaesthetic can diffuse into the surrounding structures that touch or cross the lining of the joint. So a joint block may desensitise other areas of the foot in addition to the joint itself. 

Nerve and joint blocks are used as one part of an examination of the lame horse. In a typical lameness work up a horse is trotted up (in a straight line on a hard, level surface and/or on the lunge in both directions), and the degree of lameness is and graded. Once the initial lameness has been graded a nerve or joint block can then be performed and the horse 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 it is situated in the area of the foot that has been desensitised by the block.  

What nerve and joint blocks are performed in the lower limb, and what do they mean?  

There are many different nerve and joint blocks routinely performed in the horse's limb. The most common blocks are as follows:  

Palmar Digital 

The nerve block is performed by injecting a small volume of local anaesthetic drug (usually 1.5 - 2ml) just adjacent to palmar digital nerves on both sides of the pastern, just above the bulbs of the heel. 

Recent research has shown that this block desensitises the entire coffin joint and most of the foot apart from the dorsal laminae and coronary band at the toe region. The block is therefore relatively non-specific, and really only indicates foot pain.  

Abaxial Sesamoid

The abaxial sesamoid nerve block is performed at the bottom of the sesamoid bones at the back of the fetlock. This desensitises the entire foot, the pastern joint, the short pastern bone and their associated soft tissues. Depending on exactly where the block is performed, the back part of the fetlock joint may also be affected by this block.  

Low 4 / 6 point Block (Front / hind limb)

The Low 4 or 6 point nerve block is performed above the fetlock. The injection sites are just above the fetlock tendon sheath, the distal end of the splint bones and in the hind limb around the extensor tendon above the fetlock. This block desensitises the fetlock, pastern and foot.

Coffin (DIP) Joint 

This joint block is performed by injecting local anaesthetic through a needle placed into the joint capsule just above the coronet at the toe. This will not only desensitise the coffin joint, but also the navicular bursa, the navicular bone and the toe region of the sole. With a greater volume of local anaesthetic injection the heel region can also be desensitised.  

Although this block affects the navicular area, a negative response cannot be relied upon to rule out navicular disease as the cause of lameness. A deep digital flexor tendonitis in the region of the navicular bone also  may or may not be affected by this block.  

Navicualar Bursa

This block is usually performed by placing a 3.5in spinal needle between the bulbs of the heel just above the coronary band, and advancing it into the foot until it touches the navicular bone. A small volume (2-3 ml) of local anaesthetic solution is injected after correct placement of the needle is confirmed with a radiograph. 

A positive response to this block indicates pain in the navicular bursa, the navicular bone, the supporting ligaments of the navicular bone, the sole of the toe or the deep digital flexor tendon inside the foot.  

Tarsometatarsal (TMT) Joint Block

This blocks out the most distal of the four joints found in the hock. The joint has a variable communication with the distal intertarsal joint above it (5-30% of horses have communication). A needle is placed just above the plantar aspect of the lateral splint bone head and towards the back of the limb adjacent to the flexor tendons. This is about level with the chesnut but on the opposite side of the limb.

This joint is the most common joint for osteoarthritis of the hock to occur in and a positive response to this joint block is often indicative of bone spavin in the hock.

Interpretation of the results of nerve and joint blocks 

The interpretation of the results of nerve and joint blocks is often not straightforward. It used to be believed that the different blocks were very specific and limited to the areas of the foot they desensitise, it is now recognised that this is not the case. Most of the blocks will desensitise more than one specific area, and there is much crossover between the different blocks. 

In addition, it is now recognised that many diseases of the foot involve damage to multiple structures. For example, recent MRI 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 with navicular disease will respond differently to the various nerve and joint blocks, depending on what specific structures are damaged.  

Like most other parts of the lameness evaluation, nerve and joint blocks are only one part of the overall examination. 

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