A Preventale winter Problem: The Subsolar Abscess

 A preventable mid-winter problem: The Subsolar Abscess

This time of year I commonly see horses with lameness caused by subsolar abscessation. It seems like part of the winter cycle - we discussed what you can do to avoid cold related problems during our December crazy cold spell, then we talked about the dangers of icy footing around Christmas time, and now as mid-winter arrives the preventable problem of the season is the foot abscess. 

Here is what happens: First, horses are likely to be confined to smaller areas and get less exercise (and therefore less grooming) during the winter months.  Second, weather conditions make manure collection and disposal challenging, so pens tend to be dirtier than normal.  Third, freeze/thaw cycles, urine spots and collected manure all contribute to the creation of deep, wet, bacteria rich footing in our horses pens.  Moisture and bacteria enter the sole through small defects that occur naturally.  But now the foot may not be getting picked out daily, the sole never dries, and bacteria migrate through the horny sole and begin to replicate in the space between the horny, insensitive sole tissue and the deeper soft, sensitive tissues of the hoof.  The body tries to fight this slow growing infection by sending white blood cells to the area.  The white blood cells break down to form a gooey, black semi-liquid pus.  This pus spreads between the superficial and deep sole layers, building up pressure until one day your horse is suddenly extremely lame. 

As with all abscesses, drainage is the key to cure.  Subsolar abscesses are resolved by aggressively removing the overlying dead sole and exposing the infected area and underlying sensitive tissue to air.  The area is packed and bandaged for 7 to 10 days to prevent re-abscessation and allow the exposed tissue to cornify and begin the transition from sensitive to insensitive sole.  Antibiotics are not part of my routine care of subsolar abscesses, but tetanus vaccination status must be confirmed, and pain control with phenylbutazone is usually warranted. So your horse gets better, but in the mean time he/she experiences severe lameness, and you are out there in the freezing cold when you get home from work every night in the dark struggling to change the darn bandage!

The good news is these winter weather condition related abscesses are preventable!  Your job is to work hard to keep you horse's pen as clean and dry as possible, not to neglect regular farrier visits every 6-8 weeks through the winter, and to pick out your horse's feet thoroughly every day through the winter months.  Some of these slow developing winter abscesses, such as the one pictured above, can be deep and extensive.  Please be a good partner to your horse and pay special attention to foot care during the off season, and you can spare your horse this painful problem.


I believe that education is the key to evolution. I believe that animals are the key to compassion. I believe the learning never stops.

4/2/2010 - The Prepurchase Exam

Vet tip of the Day: The Prepurchase Examination
Key Words: Intended Use, Physical examination, X-rays

 Today I am going to discuss some theoretical considerations regarding prepurchase examinations.  Tomorrow I will go over the nuts and bolts of the prepurchase exam including what you should expect your veterinarian to do during the exam, and how to determine if x-rays or blood work are indicated.  Right now let's begin with a look at the issue of suitability.

Today I completed a prepurchase examination on a 15 year old arabian endurance horse.  The horse was experienced, with 5,000 competition miles and no history of lameness problems.  The buyers were looking for a safe, experienced horse to do 25 to 50  mile endurance rides, but not at a highly competitive level.  The horse had not been competing for the past two years, but had been consistently ridden as an athletic trail horse.

I share this information with you, and chose this picture of the little cowgirl riding her barrel patterns on what appears to be a very wise old mare, for a reason.  The arabian gelding I saw today, and the old quarter horse poking around the barrel are two very different horses, but each is appropriate for their rider's needs.

A prepurchase exam is a veterinarian's evaluation of a horse's physical well being based on a single evaluation which takes from 45 minutes to a couple of hours to complete. While a tremendous amount of valuable information can be gathered based on a thorough physical examination, lameness evaluation, and diagnostic imaging, even the most rigorous prepurchase exam cannot replace a good history on a horse's past performance record and a buyer's solid understanding of their goals in purchasing this horse.  Obviously the little cowgirl's horse needn't demonstrate the same athletic ability as the 15 year old endurance horse.

As a veterinarian, one of the most challenging situations when performing a prepurchase examination is trying to evaluate a horse with an unknown history which is not performing the job for which it is intended.  For example, last fall I was asked to evaluate a 9 year old thoroughbred mare being purchased as a children's show jumper.  The mare had done some showing as a 5 and 6 year old, and then for an unknown reason had become a brood mare.  She had 2 foals then didn't conceive last year and now was for sale. 

Evaluating a horse like this is really difficult.  She has not been in any kind of exercise program for 3 years and there is no explanation for her change from performance horse to brood mare.  She may have a perfect prepurchase examination on the day I look at her.  She may be sound, have good conformation and pass all my flexion tests and limb palpations with flying colors.  She may have normal x-rays of her hocks, and front feet.  From my perspective, she is a sound horse.  However, I always caution buyers of such animals that the horse is not performing at the level of intended use and has an incomplete history therefore it is impossible for me to have great confidence in assessing the horse's long term prospects for continued soundness.   In fact, when this mare was put back in regular work and began jumping she developed a hind limb lameness that was attributed to a suspensory injury, which very possibly had been the reason she became a brood mare in the first place.

The moral of the story is this:  when purchasing a horse, always look for an animal that is in work, and performing at least close to the level at which you intend to use it.  It is then much more likely that your veterinarian will be able to accurately identify issues of concern and discuss them with you.  Make every effort to obtain a history of the horse's past performance and ask the seller why the horse is for sale.

A prepurchase examination is a very important part of making a wise investment.  Often I identify problems during a prepurchase exam about which the seller was unaware - that is why we subject the horse to unusually rigorous conditions during the examination such as trotting small circles on very hard and very soft surfaces.  But at the end of the day, the ethical horse seller with a complete history on their horse can be equally as valuable as your veterinarian's assessment.  Tomorrow I will discuss in detail what you should expect from your veterinarian during a prepurchase examination.

The wind is howling - Mother Nature has been very restless throughout March and appears to be continuing her mood right into April.  Keep your head down and your spirits high.

3/14/2010 - Spring is Coming

Vet tip of the Day: Thinking about spring and your horse's feet

Wow, the sun came out and I wasn't wearing 4 layers today.  It was such a wonderful feeling not to feel that chill in my bones.  With spring come those freeze thaw cycles when the ground is frozen in sharp, uneven layers at night only to thaw and turn into a slippery slimey mess during the day.  For many of us our horse's turn out areas are a mess this time of year. Particularly around feeders and water troughs, footing tends to be wet during the day and frozen at night.

Because our soil is usually so dry, our horse's feet are adapted to a dry environment.  After a long wet winter as we've had this year, the manure and urine soaked into high traffic areas where horses stand a lot, combined with the freeze/thaw cycles, create a bad formula for our horses feet.  This time of year I see LOTS of severe subsolar abscesses, particularly in older horses.  Here's what happens: first, the horse living in a dry desert climate typically has a hard, somewhat brittle sole.  Over the winter, if the horse's environment is not kept clean and dry, then wet, manure packed material accumulates in the sole and along the sulci of the frog.  This material is full of bacteria.  It softens the frog which becomes recessed, thereby leaving the sole in direct contact with the ground.  The horse steps on a frozen jagged mud edge, creating a tiny defect in the sole which provides access for bacteria to infiltrate into the foot.  The bacteria become trapped beneath the hard sole where there is no air supply.  They begin to proliferate, spreading between the natural layers of the sole.  In response to the bacterial proliferation, the body mounts an inflammatory response, resulting in the accumulation of pus.  Eventually this pus produces enough pressure within the hoof that the horse becomes acutely severely lame.

At this point the sole must be removed to allow drainage of the dead tissue and expose the area to air.  In the worst case scenario, it is possible for the bacteria to migrate far enough into the hoof to cause infection of the coffin bone, although this is NOT the typical outcome.  Once the dead sole is removed exposing the area where the bacteria have infiltrated, with appropriate treatment the sole will harden and repair itself without long term complications.  However, subsolar abscesses are extremely painful, and can be frustrating and time consuming to treat, especially in horses living in outdoor settings.

Prevention is worth a pound of cure, and subsolar abscesses that occur secondary to poor footing conditions during the winter are preventable. First, be sure your horse has regular farrier care.  Second, PICK YOUR HORSES FEET OUT THOROUGHLY EVERY DAY!  Third, try to keep the area around feeders and water troughs as dry as possible, and remove manure from these areas.  If this is not possible,  using a tooth brush rub strong iodine (7%) into your horse's soles and frog several times a week after you clean out the feet.  Following these three simple rules may save you and your horse a major aggravation.  Again, this problem is especially prevalent in older, less active horses, so don't forget about the feet of the retirees, even though they are no longer performance horses.

3/9/2010 - Osteochondrosis - What is it?

Vet Tip of the Day: Osteochondrosis - Understanding the disease Process
Key Words: Bone, Cartilage, Mineralization, endochondral ossication

Yesterday I discussed the lameness workup on Classy, a 5 years old thoroughbred mare with OCD of the stifle.  So what is OCD anyway?
Today Iwill describe the disease process, and some of the factors we believe predispose juvenile horses to this problem.  Tomorrow I will go over some of the more common location for OCD lesions.

OCD stands for Osteochondrosis dissecans.  Just to give you a hint at how complex this disease is, there is still debate over whether this even is an accurate name for the problem.  To understand this disease at all, you must first understand how bones grow.  A joint is a moving part, consisting of bones that slide along each other, separated and lubricated by joint fluid.  The surface of the bone is covered by a layer of cartilage, which is softer and more compliant than bone, and therefore stands up better than more rigid bone to the forces exerted on joints during athletic activity. 

Think about it - how do your foal's bones get longer and thicker as the foal grows?  What happens is a process called endochondral ossification.  The bones grow from the surface cartilage toward the underlying bone.  The cartilage cells, called chondrocytes, divide and increase in number.  As they mature, they become mineralized and eventually transform into bone.  This is a rapidly ongoing process in the growing foal.  If the transition from young chondrocyte to mineralized chondrocyte to bone doesn't occur correctly, there is a defect in bone maturation.  Thus, osteochondrosis is a defect in endochondral ossification. 

Try this image to help you imagine what happens.  You are painting the jumps in your arena.  You are getting tired, so instead of putting on several thin coats of paint, and allowing each to dry in between, you start globbing on thick layers of paint, not letting each layer dry.  When you lay the paint on correctly, each layer adheres to the one beneath and you end up with a shiny surface of smooth paint that doesn't crack or peel.  If you glob the paint incorrectly, you end up with bubbles underneath the surface and cracks and flakes on the surface soon after the paint dries.  Exactly the same thing happens in foals with OCD.  The cartilage to bone development is abnormal, and the resulting defects include cysts (just like the bubbles under your paint surface) and cartilage flaps that detach from the underlying bone just like your flaky paint, because the attachment to the underlying layers is not healthy. 

Cysts lying just below a thin layer of unhealthy cartilage, flaky, cracked cartilage, and actual flaps of calcified cartilage that separate from the underlying bone are all manifestations of OCD.  In all three cases, the smooth, gliding surface that is critical to pain free, athletic joint function, is lost.  The result is swelling and pain during athletic activity.  Many factors predispose horses to develop OCD.  The particular combination of events in a given foal that result in OCD are complex and inter-related.  Some of the major forces at play are: genetics, rapid growth and large body size, excess feeding of carbohydrate, abnormal stress and trauma, and mineral imbalance, specifically copper deficiency.

This is a very basic overview of the disease process called OCD.  Tomorrow we will look at some of the most common sites in the body where OCD occurs, and the clinical significance of some of these sites.

3/9/2010 OCD

Vet Tip of the Day: Osteochondrosis Dissecans - More Lameness!
Key Words: OCD, stifle, developmental orthopedic disease

I'm going to continue discussing lameness today.  I'm also going to confuse you by presenting another case in which I did not follow the cardinal rules of lameness diagnosis: 1) physical examination 2) gait evaluation 3) diagnostic nerve blocks 4) diagnostic imaging.  In the horse I will discuss today, I skipped step 3, and will explain why.

In today's discussion I am simply going to present this case, and tomorrow will discuss the disease process in more detail.  Look at the x-ray here - this is the horse's stifle, the joint which corresponds to our knee.  The patella is the triangular shaped bone at top right of the picture.  It rests against the bottom of the femur (your thigh bone) and slides up and down on two ridges called the trochlear ridges.  These ridges end in the condyles, which on the x-ray are the two rounded knobs at the end of the femur.  Below the femur is the tibia (your shin bone). 

Look on the trochlear ridge just below the tip of the patella and you will see a small piece of bone separated from the trochlear ridge by a black shadow.  This is a classic lateral trochlear ridge OCD lesion.  Now here is the case presentation.

Classy is a 5 year old thoroughbred mare.  She never raced, but did train for the track as a 2 year old.  She was pulled from training due to "splint bone issues" and was turned out to pasture for a few years.  Recently she was sent out for 60 days basic flat work training which she completed successfully without complaint of lameness from the trainer.  Upon return to her owner's barn, both the owner and trainer at the barn noted that Classy would buck when asked to canter and just "didn't seem right behind".

On physical examination Classy had no remarkable abnormalities execept for moderate fluid filling in her femoropatellar joints on both hind legs.  The filling was soft and non-painful.  On gait evaluation Classy was grade 1 lame (see previous post on lameness grading) on her right hind.  She had a mild positive response to right hind hock/stifle flexion.  She was sound and negative to flexion of the left hind.  When observed on a longe line on a soft surface, Classy did not show overt lameness, but her hind limb gait was characterized by a shortened step and a rapid downward placement of her feet at the trot.  When asked to canter, she had obvious difficulty executing a canter departure.  When travelling to the right, she would only canter disunited, with the forelimbs following a right lead foot cadence and the hindlimbs following a left lead foot cadence.  Tracking to the left she would occasionally canter in a united left lead canter, but also frequently cantered disunited.  Her hind limb gait at all times during the canter was characterized by poor independent motion of her two hind legs - they tended to move together, suggesting a "bunny hop" quality.

The combination of physical examination findings and gait evaluation were suggestive of a stifle problem.  Radiographs were performed immediately for three reasons.  First, intra-articular anesthesia (joint block) of the femoro-patellar joint to identify subtle lameness such as Classy exhibited is not routinely reliable - horses will not always "block out" even though the stifle is the source of the lameness. In general, lameness should be at least a grade 2 before intra-articular anesthesia is relied upon as a diagnostic tool.  Secondly, from a strictly practical point of view, we were under time constraints, the intra-articular anesthesia would incur cost without a high likliehood of definitive results, and Classy was a somewhat fractious 5 year old likely to be uncooperative during placement of needles in her stifle without significant sedation.  Thirdly, OCD was the most likely presumptive diagnosis for Classy, and radiographic confirmation of this diagnosis in combination with Classy's signs is considered definitive without intra-articular anesthesia.

Classy was sedated and 3 views of each stifle were taken.  The radiographs confirmed OCD in both stifles, the right significantly worse than the left.  The x-rays were reviewed by a board certified surgeon and surgery is scheduled for next week!  Cost $2500 - prognosis for soundness and full athletic function excellent - layup time 6 weeks.

In Classy's case, the owner's astute attention to a relatively mild gait abnormality resulted in a timely diagnosis of a problem that left untreated would likely result in degenerative changes in the joint with resultant chronic lameness.  Over the next few days we will look at OCD in more depth, and then return to Classy's case and discuss why her particular manifestation of this developmental orthopedic disease carries with it an excellent prognosis following surgical repair.

3/5/2010 Winding up with Flash

.Vet tip of the Day:  Challenging lameness
Key words: Carpus, metacarpal bones

Finally I'm back after a few down days - my apologies to those of you following daily.

Let's finish up with Flash, the gelding with the challenging lameness.  Quick review: the take home messages you should remember from following this case are: the importance of client communication, the need to treat each lame horse individually, and the importance of patience and care to do know harm when approaching a challenging lameness.

Two weeks after my initial evaluation of Flash I returned to repeat x-rays and check on his progress.  His lameness was slightly improved, but he continued to be very uncomfortable when stepping down from his stall or turning.  Repeat x-rays of his splint bone in the area of the trauma still showed no fracture.  There still was no swelling in the joints above or below the site of trauma, but there continued to be some pitting edema (soft swelling) along the length of the medial splint bone (metacarpal II) and Flash continued to show a painful response when palpated along the proximal splint and at the back of the knee where the splint bone articulates with the second carpal bone (see red arrow on diagram).

Although Flash remained very lame, I decided to performa an abaxial sesamoid nerve block, which would remove sensation to structures below the fetlock.  There was no indication that the lameness originated in the area, but because I so far had failed to identify a definitive cause for the lameness, I wanted to be sure that I wasn't making a mistake concentrating on the splint trauma.  The foot block did not change Flash's gait.  I then did a local block, injecting local anesthetic directly into the region around the focal swelling which had orignially been so painful when the trauma occurred.  Again, no change in the lameness. 
At this point I decided to examine the leg more proximally - above the site of the injury.  We took several x-rays of the knee and found that there was evidence of trauma to the second carpal bone and extensive remodelling of the proximal second metacarpal bone (the medial splint).  Again, look at the red arrow on the diagram - the second carpal bone is the bone immediately above the arrow, sitting right on top of the head of the medial splint bone.  The radiographic changes appeared chronic - in other words the character of the abnormalities suggested they had been present for more than two weeks.  I e-mailed these x-rays to my consulting surgeons, who agreed that it appeared that some trauma involving the structures of the carpus in this region probably occurred at some time in the past and this recent injury led to a flare up of an old problem. 

It is important to remember that the x-ray only shows bones.  There is a very complex system of ligaments and tendon sheaths that overlies the medial aspect of the carpus exactly where the second carpal bone meets the medial splint.  Here is a somewhat out of focus (sorry, my limited tech skills showing through) diagram of these structures.  The boney remodelling present on the x-ray very likely is accompanied by additional damage to the associated soft tissue structures.  Ultrasound or MRI would be useful in further specifying such damage.

With these changes identified radiographically, and with the recommendation of the consulting surgeons, I went ahead and did an intra-articular block (use google bar at top of blog and search for "intra-articular block" and you'll find blog entry describing this) to anesthetize the carpo-metacarpal joint and Flash's gait improved about 75%.  With an articular injury, and likely additional pain arising from soft tissue structures outside the joint, this degree of improvement is considered diagnostic.

I felt comfotable that this unusual lameness story was finally coming to a conclusion.  The treatment would be rest and slow rehabilitation.  I left instructions for Flash to remain on stall rest for 60 days, at which time radiographs and full lameness exam would be repeated.  And in fact Flash's gait improved dramatically by 21 days post injury. I'll give you an update on his progress in May.

Please feel free to comment on these blog entries and let me know which are your favorites.  My goal is to provide continuing education that is both interesting and useful to you.  Give me some feedback to help direct my future efforts!

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.