Equine Lameness: Clinical Judgment Meets Diagnostic Imaging
Equine veterinarians have tools at their disposal--from traditional lameness exams to advanced diagnostic imaging--to help them make educated clinical judgments about lamenesses, their structural causes, and prognoses.
Photo: Stephanie L. Church, Editor-in-Chief
The athletic horse is subject to a variety of injuries and expresses the pain they cause in various ways, from behavioral quirks to obvious limping. Subtle lameness problems can be especially challenging to identify and manage. Fortunately, equine veterinarians have tools at their disposal to help them make educated clinical judgments about lamenesses, their structural causes, and prognoses.
Each year at the American Association of Equine Practitioners (AAEP) Convention organizers select a renowned practitioner to present the Frank J. Milne State-of-the-Art lecture on their topic of expertise. Sue Dyson, MA, Vet MB, PhD, DEO, FRCVS of the Animal Health Trust in Great Britain, who has devoted her professional life to the art and science of lameness diagnosis, was the distinguished lecturer at the 2013 convention, held Dec. 7-11 in Nashville, Tenn. She pulled from her experiences as veterinarian and accomplished rider to tackle this topic.
She prefaced her presentation with a quote from Sophocles: “Look and you will find. What is unsought will go undetected.” While this is true of life in general, she said, it is extremely appropriate to the process of working up a lameness in a horse.
Dyson prefers to apply a different lameness scale than the AAEP's five-grade system. Using a scale of 0-8, she grades the horse at each gait, on different surfaces, and both in hand and under saddle. She recommended embellishing on each numeric grade with verbal descriptions.
During a lameness exam Dyson views the horse from the front, behind, and side. Next, she listens to the footfalls for gait irregularity, preferring to do so while longeing the horse rather than trotting him in hand.
“On the longe line, it is easier to assess how the horse is holding its body as an adaptation to pain,” Dyson says. “It is possible to compare the horse’s natural balance on left and right reins, the head and neck position and movement, the excursion of the inside hind limb, and the swing of the back. We can look at quality of the canter, whether there is a delay to departure into canter, and the head and hindquarter movement and/or presence, absence, and symmetry of toe drag.”
She reminded the audience to scrutinize downward transitions from canter to trot, as well: “Alterations in body posture and balance during transitions are telling.” She also recommended exercising the horse on a fine gravel surface with some traction rather than concrete so the horse isn’t tentative and instead strides freely.
“In most cases, foot lameness worsens in circles as compared to straight lines,” she remarked, adding that about 20% of the time, the horse’s lameness worsens when the lame foot is on the outside of the circle.
She cautioned that a horse with bilateral (in both limbs of a pair) lameness can be difficult to grade. Putting a horse under saddle with a rider often exacerbates an unsoundness issue—in fact, ridden exercise is the only circumstance in which lameness is evident in some scenarios--so it can be another useful exam tool. Dyson said she likes to compare how the horse moves when the rider sits on both the correct and incorrect diagonals, as well as the difference in gait when the horse is ridden on contact versus moving on a long rein. “But," she cautioned, remember that “a good rider can mask lameness while a bad rider can induce it.”
As practitioners evaluate the quality of horses’ paces, rhythm, and swing through the back, Dyson recommended they take note of the steadiness of head carriage and any overbending and “curling up” in front. Do any aberrations change between right and left reins? She commented that a horse that ducks his head behind the vertical is likely displaying discomfort.
While most veterinarians focus their lameness evaluations on the horse at trot, Dyson also finds the canter useful. Canter locomotion places different biomechanical loads on the limbs, she said. She urged the examiner to look for behavioral resistance (bucking, kicking out, tail-wringing), loss of three-time canter, or difficulty with lead changes. “Failure to extend the hind fetlocks (at the canterduring the stance phase) is another important hint of a musculoskeletal problem,” she said.
Asking the horse to perform lateral work can bring out the worst in a sore limb. The most information comes with a skilled rider on board who can ask for correct lateral exercises. An experienced rider is also able to communicate what he or she is feeling under saddle, which is particularly helpful in subtle lameness situations. “What a rider feels may be different from what we see,” Dyson said.
Many times it can help to reassess the horse after he exercises and then stands for at least an hour. An injury that might have loosened up could get tight again and this will become evident in gait alterations as he begins another round of exercise.
“Behavioral signs are other indicators of lameness,” Dyson said. A horse that spooks, evades, tilts his head, opens his mouth, is stiff through the neck, resists turning, rushes, runs away or is reluctant to go forward, displays tension, and/or bucks or rears is a horse that is communicating some level of discomfort. It is prudent to watch the horse at his specific athletic pursuit–for example, a jumping horse should jump or a dressage horse should be asked to perform exercises at his skill level.
Another observable feature Dyson described as useful in a lameness diagnosis is saddle slip. While this might be a manifestation of an ill-fitting saddle or a crooked rider, it is often related to back asymmetry and/or hind-limb lameness. In Dyson's recent study of 128 horses with saddle slip, she determined that 4% were affected with forelimb lameness while 54% were lame in one or both rear legs. In 86% of the lame horses, the saddle tended to move to the side of the lame limb.
“Hind limb lameness is the most frequent cause of saddle slip and is not necessarily a result of the rider," she concluded. "Diagnostic analgesia (nerve blocks) of hind limb lameness eliminated saddle slip 97% of the time.”
Nerve blocks are a useful tool that can help pinpoint an area of discomfort. Dyson said that for best results a systematic approach is necessary—the veterinarian should numb each area of the lame limb in a methodical manner, starting from the ground and working up. While diagnostic anesthesia is crucial to lameness investigation, she said, “It has its limitations.” For instance, some pain is difficult to abolish altogether. And, she added, “There may be more than one source of pain causing lameness. Less than 70% improvement after diagnostic anesthesia often indicates an additional source of pain.”
Over the years practitioners have discovered that results of diagnostic anesthesia can be confusing. What they see following blocking areas of the limb with anesthesia depends on numerous factors: How accurate is the injection? How much anesthetic is injected? How far does anesthetic diffuse upward?
Dyson reported a confounding study with results that departed from the expected: Five percent of horses with primary foot pain required anesthesia above the fetlock to improve and, therefore, diagnose the lameness. Usually, foot lameness is localized by diagnostic nerve blocks below the fetlock.
She said nerve block responses don’t seem to differ based on injection approach, whether it’s in the weight-bearing or flexed limb or if the horse walks or stands for a few minutes after injection. Inadvertent injection into the wrong structure can result in no improvement in lameness, or at the very least there might be delayed numbing of that region or unintended anesthesia of other structures.
Dyson suggested practitioners interpret regional anesthetic nerve blocks carefully and corroborate results with all other exam findings.
When it comes to evaluating the various soft tissue structures in the equine foot, Dyson has found that advanced imaging modalities like MRI lend a hand. She has been using MRI for as long as it has been available in veterinary medicine, so she has many years of insight to impart on imaging a variety of injuries.
When it comes to evaluating the various soft tissue structures in the equine foot, Dyson has found that advanced imaging modalities like MRI lend a hand.
Photo: Stephanie L. Church
Collateral ligament injuries First she reviewed the difficulty of diagnosing injuries to the collateral ligaments (CL) of the coffin joint. These ligaments are at risk of damage when the foot is placed asymmetrically, particularly when the quarters of the hooves are different heights--lateral or medial rotation and sliding of the coffin bone relative to the short pastern bone above it stresses the ligaments and/or the coffin joint. The results of using diagnostic nerve blocks to isolate CL injuries can be disappointing, and she noted that ultrasound exam only provides a limited view of the foot structures; 27% of ultrasound exams miss the pertinent injury. Also, the longer the length of the hoof capsule, the more difficult it is to obtain effective views with ultrasound. Dyson reported that in comparing MRI findings to histopathology (examination of ligament tissue slices with a microscope) from euthanized horses, she observed a good correlation to actual damage within the collateral ligament, thereby confirming the value of MRI for diagnosing this injury. In general, MRI reveals that chronic CL lesions are degenerative in nature.
Once the veterinarian makes a definitive diagnosis, he or she can suggest a prognosis and implement a treatment plan. “The outcome for sport horses with collateral ligament injury is better than for the general purpose horse, possibly because there is earlier recognition in horses in steady training and competition,” she said.
Sidebone Another condition that practitioners have historically overlooked is collateral cartilage ossification (conversion to bone), referred to as sidebone. Dyson’s comments shed light that there is often an association between sidebone and CL injury: “Extensive sidebone increases the risk of injury to the collateral ligaments and their coffin bone attachments as well as the ossified cartilage being susceptible to fracture from trauma.”
DDFT injuries The deep digital flexor tendon (DDFT) courses behind the pastern to insert on the bottom of the coffin bone; its anatomical arrangement serves to stabilize the coffin joint. Dyson said MRI is useful for assessing the DDFT; in nearly 83% of horses with foot-related lameness, she identified DDFT lesions around the navicular bone using this modality.
Dyson noted that elite show jumpers are at particular risk for DDFT injury within the hoof, with increased strain on the tendons as fences get higher. Also, 10- to 15-year-old horses are most at risk of these injuries, as opposed to horses younger than six. Dyson suggests that this is probably due to cumulative loading or age-related degeneration in the distal (lower) part of the tendon. An affected horse shows more lameness when turned on a circle or exercised on firm footing, and lameness improves with rest. Horses with DDFT injuries often stand pointing the lame foot, much like horses with navicular disease.
Dyson also remarked that there is no definite relationship between foot conformation, including coffin bone angle, and DDFT injury. That said, she added, “Of horses with DDFT lesions, about 15% have low heels and/or broken-back hoof-pastern axis.” Veterinarians can miss these lesions on ultrasound but see them clearly using MRI.
Dyson cited one study in which 32% of horses with distal DDFT injuries returned to full athletic function after 6-12 months of stall rest and controlled walking. These injuries are often chronic by the time they are identified, and injury duration affects treatment outcome.
Navicular problems Navicular disease has been a challenge for practitioners to diagnose and manage, in part due to the extensive interconnection of soft tissues around the navicular bone and coffin joint. “A normal radiographic (on X ray) appearance of the navicular region does not preclude the presence of significant abnormal pathology (damage or disease),” Dyson stressed. She offered information about a “new form of navicular disease: Abnormalities are restricted to the marrow fat and the thin struts of bone (trabeculae) within the navicular bone.” This type of injury is not visible with radiography and can only be seen using MRI.
Additional pathology the MRI reveals in the navicular structures also has relevance. “Distal border fragments are now considered significant to lameness,” Dyson said. In nearly half of radiographic studies, veterinarians miss these and only identify them with MRI. In addition, the distal sesamoidean impar ligament, attached to the navicular bone, is at risk of injury; distal border fragments are found within the ligament, which may have adjacent tears (basically, from where the ligament has pulled away from the bone and taken fragments with it). The ligament is also highly innervated with many sensory nerve endings so such damage is likely to be a source of pain.
In summary, Dyson reported that we have learned quite a bit from MRI over the years. Some findings might seem like common sense, but we can now confirm these conclusions with definitive information:
- She emphasized, “There are an infinite number of structures that can be injured in many different ways.”
- Veterinarians have a lot more to learn about how lesions develop and what causes pain. For example, it is not uncommon to see bilaterally similar lesions in a unilaterally lame horse, begging the question, “Why isn’t the horse bilaterally lame?” Usually, with both limbs affected, one would expect to see the horse lame in both legs. Yet, this isn’t always the case.
- Signal intensity changes of MRI imaging might not resolve over time; veterinarians need to determine what chronic pathology is significant and what isn’t. It can be a bit confusing to correlate MRI abnormalities with active and painful lesions.
- Not all horses need an MRI!
- Veterinarians’ current ability to diagnose far exceeds their ability to treat successfully.
As Dyson wrapped up her talk, she took one final opportunity to impart a bit more wisdom, quoting Nobel Prize-winning physiologist Albert Szent-Gyorgi: “Discovery consists of looking at the same thing as everyone else and thinking something different.” Approaching equine lameness diagnosis with just such an inquisitive and creative mind is what will generate advancements in this field, she noted.
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