The Poor Doer...I’ve Tried Everything, So What’s Next?

Take Home Message—Underlying chronic disease can have profound systemic effects in horses, and can have insidious onset with signs that go unnoticed for some time unless objective measurements can be made.

I. INTRODUCTION

The ‘poor doer’ primarily requires definition. Is there concern over the health of the horse, or is there a real or merely perceived performance issue? Has the horse

experienced any loss of overall body condition, fat stores or muscle mass? Has there been a decrease in exercise tolerance or increased difficulty in performing tasks over time? If chronic underperformance is the issue, is the horse suited for the purpose for which it is being used? A retrospective study of 348 poor performing horses has been reported.¹ Of horses considered deficient during training, racing or showing, approximately three-quarters had a diagnosable condition thought responsible. Conditions included cardiac issues, dynamic airway obstruction, lameness and exertional rhabdomyolysis.

II. PERFORMANCE REDUCING SYNDROMES

A comprehensive approach to diagnosis is necessary as poor performance in any individual may be the result of multiple conditions and therefore require sophisticated diagnostic equipment.

Lameness and Neurological Conditions

Lameness and neurological deficits will be considered together, although lameness is the most common cause of diminished performance, as these conditions can and do occur concurrently. Signs may be subtle and orthopedic lameness can appear similar to neurological disease. Ruling out a musculoskeletal condition contributing to the gait deficit by regional analgesia may be useful.

Signalment and history of the horse must be considered as age, breed, conformation and usage can induce physical musculoskeletal lesions. Improvement with nonsteroidal anti-inflammatory drug (NSAID) usage suggests lameness, prompting further examination for lesion localization, as lameness appears consistent between strides of the horse; however, peripheral nerve lesions may be difficult to rule out. If neurological lesions are apparent in the equivocally lame horse, then a concurrent or precipitating neurological disease should be investigated. This should prompt a complete neurological evaluation and ancillary diagnostic testing (cervical radiography, serology, cerebrospinal fluid analysis) if indicated.

Cardiac Conditions (Structural and Functional)

Structural

Audible valvular regurgitation and ventricular septal defects (VSDs) are frequently found; however, importance is variable with performance and health affected in some cases but not others. In one study, 81% of racehorses had detectable murmurs that had no association with performance.² Although horses have considerable cardiovascular functional reserve, any compromise to maximal cardiac output will certainly have a limiting effect on strenuous exercise.

Physiological flow or ejection murmurs result from systolic passage of blood through the great vessels, and these may increase post exercise. They do not affect performance. Mitral regurgitation; however, is likely to cause clinical signs, with lesions of the valve leaflets causing increased left atrial pressure and pulmonary hypertension which diminishes performance, and structural cardiac changes. Echocardiography is necessary to formulate a prognosis for performance and longevity. Tricuspid regurgitation conversely is both very common and seldom performance limiting. In most cases this lesion is static or very slowly progressive.

Ventricular septal defects may occur in apparently normal horses; however, echocardiography is essential to establish prognosis (number and location of VSD, size and shunt characteristics, ventricular changes and valvular regurgitation). Aortic valve insufficiency develops with maturity, being slowly progressive and well tolerated by the horse. Arterial pulse character is a meaningful indication of severity of aortic valvular regurgitation. Pulmonic valve insufficiency is usually secondary to a primary mitral regurgitation with congestive heart failure, regrettably indicating a grave prognosis for life.

Echocardiography is essential to distinguish physiological from pathological murmurs once they have been detected. Pericarditis can also be detected and investigated as to degree and nature of any fluid or adhesions present.

Functional

Of the electrical disturbances, atrial fibrillation is the most common performance limiting arrhythmia. Uncoordinated electrical activity of the atria diminishes ventricular filling and disturbs ventricular rhythm. Maximal cardiac output during exercise is therefore limited. It produces a characteristic irregularly irregular rhythm which may be intermittent in nature. The main differential is second degree AV block which can also cause sustained pauses in heart rhythm.

A comprehensive cardiac work-up requires careful auscultation and knowledge of the appropriate electrical and echocardiographic activity of the heart. To this end, timing of the murmur in the cardiac cycle (systolic, diastolic), location (with respect to the pulmonic, aortic, mitral or tricuspid valve), degree of radiation (area of the chest wall over which murmur sounds radiate), severity of murmur (lower grade less likely associated with performance limitation), associated pulse quality (bounding, weak), and any concurrent rhythm disturbances provide the basis of the initial cardiac examination. Further investigation involves defining the electrical activity of the heart (electrocardiogram) and imaging during the cardiac cycle via echocardiography, this benefitting greatly from the referral opinion of a specialist.

Cardiorespiratory Problems (Coexistent Cardiac and Respiratory Conditions)

Hypoxia during exercise caused by airway compromise may predispose the horse to a clinically relevant arrhythmia. In one study, horses with cardiac anomalies had arrhythmias alone, dynamic airway collapse with concurrent arrhythmias, and diminished post exercise contractility. Cardiac evaluation may therefore be indicated in cases of upper airway collapse confirmed by resting or dynamic upper airway endoscopy.

Respiratory Conditions (Structural, Functional, Infectious and Inflammatory)

Upper airway issues are common performance-limiting conditions in all disciplines and breeds.³ As the upper airway is responsible for the majority of airway resistance, any narrowing has profound effects on overall airflow. Tissue vibration and turbulent airflow are responsible for noise generation, although not all horses that generate noise have performance limitations. Noise present only on inspiration implies airway collapse, whereas during both phases of respiration a fixed obstruction should be suspected.

Laryngeal hemiplegia is a frequent cause of exercise intolerance and prevalence increases with age. Breed differences in occurrence exist (in descending order of frequency): draft horses, Tennessee Walking Horses, Saddlebreds, Thoroughbreds, Standardbreds, and Quarter Horses. Dorsal displacement of the soft palate predominates in Standardbreds, the condition during exercise differing from that occurring at rest (the latter condition typically of neurological origin and potentially leading to aspiration). Hypoplasia of the epiglottis may occur in these horses. Epiglottic entrapment is common in racing Standardbreds and Thoroughbreds but uncommon in other breeds. It may be present at rest and an incidental finding with no effects, or may create noise during exercise. Arytenoid chondritis varies from mild thickening to ulceration and deformation, and prognosis for performance is poor if the contralateral cartilage is abnormal. This inflammatory condition is of unknown origin. Obstruction of the nasal cavity by ethmoid hematomas may be clinically silent during development except for occasional epistaxis.

Lower airway disease may affect upper airway function (precipitating collapse or diminishing laryngeal function) or diminish exercise tolerance in its own right.^4,5 Inflammatory airway disease (IAD) and exercise induced pulmonary hemorrhage (EIPH) are the most common performance limiting lower respiratory tract diseases, although in the older horse developing heaves may be present. Changes may be precipitated by both infectious and non-infectious agents, including bacteria, viruses, allergens and particulate irritants. In Thoroughbreds and Standardbreds IAD is widespread, with diminished performance and cough commonplace. Nasal discharge is variably present. The majority of racehorses and other breeds/disciplines that require short bursts of maximal exercise experience EIPH. This may be related to the occurrence of IAD. Alternatively, repeated EIPH episodes initiate a self-perpetuating cycle of inflammation and hemorrhage. In both conditions, bronchoalveolar lavage is a useful diagnostic technique. Heaves may also be confirmed by this technique. Lower respiratory tract infection is usually associated with systemic signs of illness; however, stress associated with exercise and inflammatory conditions as detailed above may predispose to infection. Impairment of pulmonary defenses, irritant insults, and EIPH create a favorable environment for subclinical infection to progress to pneumonia and pleuritis.

The anhidrotic horse can appear similar to one affected by performance limiting pulmonary disease, with elevated respiratory rate, poor exercise tolerance and hyperthermia during forced exercise.

Muscle (Myopathy, Exertional Rhabdomyolysis and Storage Diseases)

Injury, overexertion and myopathies lead to performance limitation. Myopathies may be inherited, due to metabolic abnormalities, toxic in origin, or the result of an inflammatory stimulus.⁶ [...]