2/26/2010 - Nerve Blocks

My apologies for this late posting - problems connecting with Internet prevented uploading for the past 2 days!

Vet tip of the Day: Diagnosing Lameness with Nerve Blocks

Key Words: Peripheral nerve, intra-articular, neurovascular bundle, lameness, imaging, posterior digital nerve block, medial, lateral, palmar, plantar

I’ve been scratching my head over a horse with a challenging lameness for the past 2 weeks. He inspired Wednesday's blog concerning veterinary specialists, since I’ve sent his x-rays off to three different ACVS (American College of Veterinary Surgeons) Diplomates for their opinions. This gelding became acutely lame after an accident during training. While working in a round pen, the trainer observed the horse hit the inside of his left front cannon bone with the opposing front foot. The horse began limping immediately and was brought back to the barn. The area of impact became warm, swollen and painful to touch in under an hour. I was called and scheduled an appointment to see the horse that afternoon. Today’s vet tip will explain how we evaluate and diagnose lameness with an emphasis on understanding nerve blocks. Tomorrow we will return to the case and how it is progressing.

Lameness diagnosis and treatment composes a large part of practice with performance horses. There are four main parts to lameness diagnosis: physical examination, gait evaluation, localization with nerve blocks, and diagnostic imaging. These four tools form layers of information which when combined, should each complement the other, allowing your veterinarian to confirm the cause of your horse’s lameness. Often a client will call me and ask to have their horse "x-rayed".  However, because horses cannot tell us where they hurt, it is necessary to use the first three diagnostic tools before obtaining diagnostic images. It is a mistake to jump to diagnostic imaging, such as x-rays, before performing an exam and nerve blocks.

Here’s the problem: let’s say I see a horse limping on its right front foot. It is a 15 year old Quarter horse with narrow heels and upright pasterns. I assume it has navicular disease and x-ray the front feet. Sure enough, there are radiographic changes consistent with navicular disese. End of story, right? Wrong. Without evidence from physical examination and regional nerve blocks, I cannot prove that the abnormalities on the x-rays are responsible for the horse’s lameness. What if he also has a bone chip in his knee with associated arthritis and this in fact is the source of his lameness? Because horses cannot describe their pain to me, in order to localize lameness to a particular anatomical location, it is critical that I use peripheral and articular “blocks” to localize the pain. The theory is simple: if I have pain in the big toe on my left foot causing me to limp and somehow that big toe loses its sensation, my limp would be gone

A systematic approach is used to localize a lame horse’s pain. The lower limb is blocked in sequential regions, progressing from the foot upward, until the lameness resolves. There are two types of nerve block: the peripheral block, and the intra-articular block. Peripheral nerve blocks involve the subcutaneous instillation of a small amount of local anesthetic into the tissue immediately surrounding a particular nerve. These blocks are performed after a brief surgical prep, typically the veterinarian does not wear gloves, and the risk of complication is extremely low. Intra-articular blocks refer to the injection of sterile local anesthetic directly into a joint space. Intra-articular blocks must be performed very carefully under aseptic conditions, because of the risk of introducing infection into a synovial structure. A careful surgical prep is performed, the veterinarian always wears sterile gloves and follows strict aseptic technique.

The body is a complex highway of neurovascular bundles that supply blood and innervation to precisely mapped regions. These bundles consist of a vein, an artery, and a nerve. They are uniformly constructed, with the nerve lying along the back of the bundle, preceded by the artery and vein, respectively. In the leg, there are two long neurovascular bundles that run along the inside and outside of the limb from the knee to the ground , along the back of the cannon bone. The nerves in this bundle are called the medial and lateral palmar or plantar nerves . Medial: inside, lateral: outside, palmar: front leg, plantar:back leg. When “working up” a lameness of unknown origin, we begin by blocking the horse’s heel region using a posterior digital peripheral nerve block.

One milliliter of lidocaine or carbocaine is infiltrated just beneath the skin low in the pastern along the medial and lateral axial planes. It is very easy to palpate the neurovascular bundle in this location. Pick up your horse’s left front leg and hold the foot in your left hand, facing your horse’s head. Press your right thumb firmly against the back (palmar aspect) of the pastern just lateral to midline, then slide your thumb laterally (toward the outside of the pastern), continuing to press firmly. You will feel a rubbery cord slip beneath your thumb. This is the neurovascular bundle. It also is easy to palpate at the base of the fetlock, along the medial and lateral palmar borders.

After the anesthetic is instilled, and an amount of time passes appropriate for the block performed (this varies a bit from one anatomical location to another, but generally is between 5 and 10 minutes), the horse’s gait is re-evaluated. If the gait has improved 80% or more, the block is considered diagnostic. This means that the pain causing the lameness originates from a structure that lies below (distal to) the location of the nerve block. In the case of the heel block, or posterior digital block, the source of the pain must be in the back one third of the foot. This includes the navicular bone, navicular bursa, the ligaments which support the navicular bone (there are a bunch of these), the digital cushion (the pad-like structure that supports the heel region of the foot), and the sole in the heel region. Once localized to this area, further diagnostic tests such as x-rays, ultrasound, or in complex cases, MRI can be performed. The trick is then to correlate imaging abnormalities with the information derived from the nerve block response and physical examination.

In tomorrow’s Vet Tip of the Day we will return to the horse with the challenging lameness I’ve been looking at for the past 2 weeks and begin peeling back the layers of his diagnostic work up in an effort to determine the source of his problem. See you then.