Page 409 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

384 CHAPTER 1

VetBooks.ir 1.9 Muscle disorders of the horse

INTRODUCTION the stable. Does the horse readily move around the
box? Is there any evidence of sweating, muscle fas-
There are many diseases that affect the skeletal ciculations or pain? Is the horse well-muscled? Are
muscle of the horse. In some cases, the presentation there general or focal areas of muscle atrophy? A full
is pathognomonic, whereas in others the clinical physical examination should include obtaining vital
signs are vague and the differential diagnosis more parameters (e.g. heart and respiratory rate and rec-
obscure (i.e. skeletal muscle disease should always be tal temperature), assessment of hydration status and
considered in the differential of poor performance). cardiovascular auscultation. Palpation of the major
Diseases of skeletal muscle can be broadly divided muscle groups should allow the firmness of muscles
into those that present following exercise, which is to be determined. Percussion of the muscles is useful
the most common scenario, and those that are not to determine the presence of sustained contraction
associated with exertion, such as atypical myopathy. as is often seen in myotonic diseases. If the horse is
Muscle enzyme activity is the most useful tool for able to walk, seeing the horse move can allow gait
confirming underlying muscle damage, although abnormalities to be determined. Horses with rhab-
skeletal muscle biopsy is required to determine the domyolysis may appear stiff whereas horses with
underlying aetiology. Other imaging modalities may other myopathies may present with weakness or
also be useful in certain situations. In general, the apparent lameness. In many myopathies, specific gait
prognosis for horses with skeletal muscle disease is abnormalities may not be detected.
good, when appropriate management recommenda-
tions are followed. DIAGNOSTIC TECHNIQUES

CLINICAL HISTORY AND PRESENTATION Muscle enzymes
Serum muscle enzyme activity is often measured
When obtaining the clinical history for the horse to screen for muscle disease. This usually includes
with suspected muscle disease, it is important to creatine kinase (CK) and aspartate aminotransferase
establish the following facts: (AST). CK rises quickly following injury, peaking
at 4–6 hours, and returns to baseline rapidly with
• The signalment of the animal. a half-life of approximately 12 hours (Fig. 1.756).
• Has the horse had episodes of muscle disease AST rises more slowly, peaking at around 24 hours
previously? following muscle injury and declines over days to
• Is there any history of known muscle disease in weeks. CK is considered to be specific for muscle
related animals? damage, although it is also found in the muscle of
• How long has the horse been in the owner’s/ the heart, and in the brain. AST is not specific for
trainer’s possession? muscle damage as it is also released from the liver
• What discipline and level of work the horse is in particular. Evaluation of other liver enzymes can
currently doing? Has this changed? How has the therefore be useful to rule out liver disease as a cause
horse tolerated this? of increased AST activity.
• How is the horse managed (i.e. diet, turnout, Measuring muscle enzyme activity following
training, recent competition, etc.)? 10–15 minutes of submaximal exercise can increase
• Has the body condition of the horse changed? the sensitivity of detecting an exercise-related myop-
athy. A baseline blood sample should be collected
A thorough physical examination should prior to exercise with a second sample between 4 and
begin with observing the horse unrestrained in 6 hours later.