Poor Performance Requires Accurate Diagnosis

Daniel J. Burba, DVM
Associate Professor of Veterinary Surgery
Diplomate, American College of Veterinary Surgery

George S. Martin, DVM, MS, MBA
Professor of Veterinary Surgery
Diplomate, American College of Veterinary Surgery

Rustin M. Moore, DVM, PhD
Associate Professor of Veterinary Surgery
Diplomate, American College of Veterinary Surgery
Assistant Director, Equine Veterinary Research Program

Poor performance is a common problem in horses of all breeds and athletic uses. Poor perfomance can result from exercise intolerance associated with abnormalities of the cardiovascular or respiratory systems, endocrine (hormonal) alterations, inadequate thermoregulation, neurologic abnormalities, or pain associated with the musculoskeletal system resulting in lameness. The treatment of poor performance is dependent upon determining an accurate diagnosis or cause of the problem. Regardless of the cause, the diagnostic approach involves taking an accurate and complete history and conducting a thorough physical examination. After the initial steps are completed, ancillary diagnostic procedures are performed according to the most likely cause of poor performance in each individual horse. A description of the procedures and their clinical indications and uses are discussed in the remainder of this article.

The history is focused on prior medical abnormalities. A series of questions are asked by the clinician to help first develop a diagnostic plan. What did the owner or trainer first notice (respiratory noise, cough, lameness, slow speed, refusing to take a barrel, etc.) and did the problem have a gradual or rapid onset? Has the condition (clinical signs) improved, worsened, or remained static? Has the horse been treated for the problem, for how long, with what, and did the problem improve with treatment? Has there been any change in the exercise regimen (longer, more frequent, different surface, different use, etc.)? Does the problem occur seasonally, after being laid off (stalled or turned out), or is it constant in its presence and severity? Have there been any changes in management including diet or stabling practices? Does the horse cough, have discharge (blood, mucus, watery fluid, or mucopurulent) from the nostrils or make respiratory noise? Has the horse been noticed to be lame, and how severe is the observed lameness? Does the horse show any signs of incoordination (stumbling, etc.) or appear to have trouble or resist taking leads or turning around barrels? Does the problem worsen with someone on the horseÕs back, or is it noticeable when lunged or ponied? Does the horse warm out of the problem?

Cardiovascular diseases that manifest as poor performance include arrhythmias (irregular heart beat), the most common of which is atrial fibrillation. These abnormalities can be diagnosed based upon auscultation (listening with a stethoscope) and performing an electrocardiogram (ECG). Some cardiac arrhythmias are intermittent and require the use of a Holter monitor to record the ECG during exercise or over an extended period of time. Other causes include congenital or acquired conditions in the heart including cardiomyopathy (abnormal function of cardiac muscle), valvular problems or abnormal openings in the wall between heart chambers. These abnormalities can be suspected based upon a physical examination and close auscultation of the heart for murmurs. They can be confirmed with echo-cardiography (cardiac ultrasound).

Diseases of the upper and lower respiratory tract can be another cause of poor performance. Upper respiratory tract abnormalities involve either static or dynamic obstruction of the airway by masses (tumors, abscesses, granulomas, etc.), or functional abnormalities. The most common upper respiratory tract abnormalities include left laryngeal hemiplegia (roarer), dorsal displacement of the soft palate, or entrapment of the epiglottis by the aryepiglottic membrane. These upper respiratory tract conditions are usually diagnosed with endoscopy and/or radiography. Sometimes, dynamic endoscopy is performed immediately after exercise or while the horse is exercising on a high-speed treadmill.

Lower respiratory tract diseases that can lead to poor performance include reactive (allergic) airway disease, pneumonia, pleuropneumonia, and exercise-induced pulmonary hemorrhage (i.e., bleeder). Physical examination with careful auscultation of the lung fields often will identify a problem related to the lower respiratory tract; however, ancillary diagnostic techniques are useful to help confirm the diagnosis. Radiographs are important to identify abnormalites within the lungs, such as pneumonia, pulmonary abscess, obstructive airway disease, or hemorrhage. Thoracic ultrasonography is especially useful to identify fluid (purulent material associated with pleuropneumonia) within the thoracic cavity between the lung and body wall. Endoscopic examination of the trachea and bronchi allows one to examine for evidence of infection or bleeding. Samples can be collected from the lower respiratory tract via broncoalveolar lavage, transtracheal wash, or endoscopic-guided aspiration for cytologic examination and culture.

Endoscopy

The endoscope has become the most useful diagnostic tool in evaluating the upper respiratory tract of performance horses. With the development of fiberoptics and video microchip technology, veterinarians are able to view the inside of the horse's nasal cavity, throat, and windpipe. The video endoscopic camera is built into a long snake-like tube (approximately 2 meters long and 12 mm diameter). The endoscope is passed up a nostril of the horse and the upper respiratory tract is examined by viewing a video monitor. Endoscopy is performed while the horse is standing and with the horse exercising on the high-speed treadmill. Scoping a horse while exercising on the treadmill allows the function of the upper respiratory tract, particularly the throat, to be evaluated, allowing a problem to be detected that otherwise would not have been seen at rest. This is called dynamic endoscopy. The most common problems detected in the upper respiratory tract of horses include left laryngeal hemiplegia (paralyzed flapper), dorsal displacement of the soft palate, epiglottic entrapment, arytenoid chondritis, and subepiglottic cysts.

Lameness Examinations

Lameness examinations are commonly performed on athletic horses because they frequently sustain injuries. The basic purpose of the exam is to isolate the source of the lameness to one or more anatomic locations. The stages of the exam are:
1) examination of the horse at a walk and trot to establish the baseline appearance of the lameness;
2) isolation of the painful location with local anesthesia; and
3) examination of the painful area with radiographs.

The initial stages of the exam focus on what the horse's lameness looks like prior to any physical manipulations, such as flexion tests. Throughout the exam, the horse's way of travel is compared after local nerve or joint blocks to its baseline lameness. The trot is the most useful gait at which to observe a horse's movement patterns and to assess the lameness degree.

The American Association of Equine Practitioners has created a five level grading scale that is useful for categorizing lamenesses.

The lameness grades are:
¥ Grade 1 - Difficult to observe; not consistently apparent regardless of circumstances (i.e., weight carrying, circling, inclines, hard surface, etc.)
¥ Grade 2 - Difficult to observe at a walk or trotting straight line; consistently apparent under certain circumstances (i.e., weight carrying, circling, inclines, hard surface, etc.)
¥ Grade 3 - Consistently observable at a trot under all circumstances
¥ Grade 4 - Obvious lameness, marked nodding, hitching or shortened stride
¥ Grade 5 - Minimal weight bearing in motion and/or at rest; inability to move

Usually the next step is to assess the patient's response to one or more flexion tests. If the lameness involves the forelimbs, then a lower limb flexion test (fetlock flexion) will be used first. If the lameness involves a hind limb, then a hind limb flexion test (hock flexion, spavin test) will be used first. The purpose is to make a subtle lameness more visible and to give clinicians a general clue about the location of the lameness. After flexion tests, hoof testers are used to look for areas of pain within the hoof capsule. When a horse flinches reflexively and consistently to pressure applied to some areas of the foot, it is suggestive of a soreness in the foot.

To know the location of the lameness, local anesthesia is used to block out the lameness, using a process which starts low on the horse's leg and works upward until the lameness has been eliminated or improved. When the lameness is eliminated, it is evident that its source was located in the area just blocked. Clinicians then know what part of the horse's body to radiograph or ultrasound. When the lameness comes from an area high on the horse's body, such as the hip or humerus, it can be impossible to block out. In these situations, examination by nuclear scintigraphy (bone scan) can be very helpful.

Radiography

Radiographic images of the horse's bony structure are used to determine the type of problem and the degree of severity of the problem when a skeletal structure is considered the cause of a lameness. Typically, the area is imaged by taking five views of the joint or structure. This is done because radiographs are two-dimensional images of three-dimensional objects. By taking views at various angles, veterinarians can interpret the three-dimensional object appropriately.

In horses, the radiographs contain information about bone only. Bone will react in specific patterns when injured, and by knowing the pattern of reaction in specific areas, veterinarians can predict how the patient will respond to therapies, such as rest, immobilization, surgery, steroid injection, or hyaluronate injection. Generally, bone reacts to injury by either producing more bone or by removing bone. Thus, the radiographs of a joint may show spur formation (new bone) at the margins of a joint or decreased bone mineral density (removal) along the underlying joint surface. Both of these changes are radiographic evidence of arthritis.

Unfortunately, radiographs give very little information about the soft tissue of the horse's legs. Therefore, it is not uncommon to have a very obvious lameness without any abnormal changes on the radiographs. Ultrasonography is the current diagnostic modality used to image soft tissue in equine practice. Eventually, Magnetic Resonance Imaging (MRI) technology will be available, but that is probably several years in the future.

Ultrasonography

Ultrasonography is a method of imaging that uses ultrasonic sound waves to create an image of the soft tissue. Tendons are the most frequently imaged structure, but ligaments, muscles, and joint capsules can also be seen. The ultrasonic probe is both a source of ultrasonic sound and a receiver of the echo that returns from the tissue. The pattern of the echo is created by the tissue, and abnormal changes in soft tissue can be detected by the changes in the echo pattern. Regarding tendon tissue, the changes in echo pattern are more sensitive than other methods of examination, such as palpation with hands. Thus, ultrasonic exams can sometimes detect early lesions before they become catastrophic bowed tendons.

After injury, ultrasonic exam should be used to gauge the severity of the injury. The damaged areas of the tendon will show as areas of decreased echoes (hypoechoic areas), which is caused by swelling or torn tendon fibers in the damaged tendon. More severely damaged areas will appear as completely dark areas (anechoic areas), in which there has been hemorrhage resulting in a blood clot within the tendon. The presence of a large anechoic area may be reason to consider surgical intervention (tendon splitting) to create a pathway for removal of the clot and for ingrowth of blood vessels, which brings with it cells that help heal the tendon.

Ultrasonic examination should also be used to stage the healing of the injured tendon, so that the horse is not returned to athletic activity too soon. A new tendon treatment drug, Bapten¨ (beta-aminoproprionitrile), has recently been approved for use in horses. The protocol for treatment involves monitoring the tendon with ultrasound after injection to stage the healing of the tendon. This new treatment represents an important improvement in the treatment of bowed tendons.

Treadmill Evaluation

Use of a high-speed treadmill is an important part of performance evaluation of a horse. It is the basis on which other diagnostic modalities are performed. Treadmill evaluation involves the use of a specially designed stationary treadmill on which the horse is exercised. The treadmill consists of a rubber belted revolving surface. When a horse is walked onto the treadmill, the surface is started in motion. The horse is maintained in the center of the working surface of the treadmill by guard rails and with the aid of assistants holding a lead line attached to each side of the halter. The horse is acclimated to exercising on the treadmill at a walk, trot, and canter. This is usually done for 15-20 minutes daily for two days. Racehorses can be worked at racing speed. Once the horse appears comfortable with the routine, then a particular diagnostic test, such as an upper respiratory tract endoscopy, is performed during exercise on the treadmill or immediately thereafter. The treadmill evaluation is designed to simulate exercise, thus a prerequisite is that the horse be in good physical condition before this type of evaluation is considered.

Bone Scan (Nuclear Scintigraphy)

Nuclear scintigraphy is used as an aid in the diagnosis of skeletal injuries. Commonly called bone scanning, it is a rapid and effective means of identifying and locating bone damage, particularly in the limbs and pelvis of horses. Bone scans are primarily performed in situations where the bone injury is minimal and undetectable using conventional methods such as x-rays. This diagnostic modality is most indicated in horses with a relatively acute (short duration), moderate to severe lameness that cannot be localized or diagnosed with a thorough lameness exam and radiographs.

A bone scan is performed by injecting a radioactive isotope into the bloodstream and then scanning a particular area of the horse's body with a gamma camera. Technetium 99m is the radioisotope (radioactive compound) of choice for bone scanning. Technetium 99m is bound to a phosphate compound (99mTc-MDP; commonly called the "bone seeking agent") which the body incorporates into bone undergoing rapid turnover (fracture, stress fracture, infection, etc.). The Technetium 99m emits gamma rays which are detected by the gamma camera. This results in an image of the bone(s) produced by the gamma camera. Uptake of 99mTc-MDP is greater in regions of increased bone activity, such as with fractures. The more active an area of bone is, the more uptake of 99mTc-MDP will occur, giving what is called a "hot spot" on the bone scan image. Nuclear scintigraphy is indicated in horses with obscure, unlocalized lameness or when radiographs fail to demonstrate a bone lesion.

The Equine Surgery and Medicine section of the Veterinary Teaching Hospital and Clinics has recently obtained a gamma camera with state-of-the-art computer technology, which will provide an additional diagnostic aid for determining the cause of difficult lamenesses in equine patients.