2/26/2010 Friday - More on Flash's lameness

Vet tip of the Day: Understanding the lameness exam

Key Words: splint bone, cannon bone, fracture, sclerosis, lysis

Today I’m going to continue following the trail of Flash’s lameness which remains unsolved. A quick review of the initial findings: an acutely grade 4 lame 11 year old gelding with history of trauma to the medial  left front mid-metacarpus . There was hot painful swelling mid-way down the splint bone, no other pertinent findings on physical examination.

The American Association of Equine Practitioners has established a uniform grading system to describe equine lameness. If you Google - AAEP lameness - you can read more on how veterinarians evaluate lameness. Here is the scoring system:

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. 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.

While this grading system is useful, it does not specifically describe the characteristics of an individual horse’s lame gait. When watching Flash move, what concerned me was not only the degree of lameness – clearly visible at the walk, but the inconsistency of the lameness. When turning, or stepping onto uneven ground Flash would occasionally take steps that were almost non-weight bearing. The veterinary vernacular for this type of gait is “fracture lame”. When walking carefully on a flat surface Flash’s gait would improve to a fully weight bearing walk.

When evaluating a lame horse it is important not to develop tunnel vision. While Flash had obvious evidence of trauma to the splint bone area of the forelimb, the character of the lameness was not characteristic of that produced by a splint bone fracture. I was immediately concerned that there might be a non-displaced fracture (a crack, more or less) of the cannon bone or first pastern bone, although these injuries are uncommon. Also the occasional “bad steps” suggested pain in the hoof. Flash wore shoes with full pads, making it impossible to apply hoof testers to his foot reliably to check for a signs of pain originating from the foot.

A second concern when examining a horse with a grade 4 lameness is making a bad situation worse. If a diagnostic nerve block is performed which eliminates the pain, the horse may no longer protect the injured area and in bearing full weight, cause a non-displaced fracture to become unstable, or comminuted, with disasterous consequences. So, on my first examination of Flash I broke the rules of lameness evaluation that you learned in yesterday’s blog. I skipped the diagnostic nerve blocks and only evaluated the gait at a walk. I completed a careful examination of the limb and went straight to x-rays to rule out a fracture.

Radiographs of the medial splint bone and cannon bone ( MCII and MCIII – see blog archive from Feb 7th for review) did not reveal a fracture, although there was obvious enlargement of the splint bone at the swelling site. So what to do? Small, fresh, non-displaced fractures are not always immediately apparent on radiographs, even with excellent quality digital x-rays. Keeping the veterinary code of “above all else, do no harm” foremost, I decided to treat Flash conservatively and repeat the x-rays in two weeks. There were no open wounds or swollen joints, so the chance of infection requiring immediate treatment was minimal, and if there was a non-displaced fracture present that I had failed to detect, it was best treated with stall rest and observation at this point.

I prepared an anti-inflammatory “sweat” containing furacin, glycerin and DMSO and instructed the trainer to apply it daily to the swelling under a bandage. I prescribed a low dose of phenylbutazone to decrease pain and inflammation. The dosage was very conservative because I did not want Flash to feel too comfortable and not protect his leg. I instructed the trainer to call me immediately if the lameness worsened or did not improve somewhat over the next 2-4 days, and scheduled a recheck in two weeks barring complications.

Hopefully the past two blogs have helped illustrate some of thought processes we employ when approaching lame horses. The take home message: for chronic, low grade lamenesses, we use a systematic approach:

Step 1: physical examination

Step 2: gait evaluation

Step 3: diagnostic nerve blocks

Step 4: Diagnostic imaging

When evaluating acute, severe lameness, additional concerns come into play which may alter this plan. Two important things to keep in mind: 1: Do No Harm, 2: Don’t get tunnel vision.

More on Flash on Monday – what do you think the answer is?

Enjoy your weekend – the 4th chapter of Veterinary Tales will be ready next week, introducing one of my favorite clients, getting Sticky’s histopathology results, and treating a horse with a mysterious swelling!

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.