Page 410 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

Musculoskeletal system: 1.9 Muscle disorders of the horse 385

VetBooks.ir 1.756 1.757

Plasma muscle enzyme activity

6h 24h
Muscle injury Time after injury
CK AST
Fig. 1.756 A graph showing the change in muscle Fig. 1.757 Sequential urine samples from a horse
enzyme activity after muscle injury. Creatine kinase with rhabdomyolysis, showing urine discolouration
(CK) activity typically peaks within 6 hours and has due to the presence of myoglobin.
a half-life of 12 hours. Aspartate aminotransferase
(AST) activity increases and decreases more slowly,
typically peaking at 24 hours.

Urinalysis Electromyography
In cases with extensive muscle damage, urinalysis Electromyography (EMG) can be used to quantify
may reveal the presence of myoglobinuria. This gives electrical activity within muscle. The technique is
a dark brown colour to the urine (Fig. 1.757). Other usually performed in the standing sedated horse,
differentials for pigmenturia include haemolysis which can lead to movement artefact. Specialised
(lysed red blood cells [RBCs]) and haematuria (whole EMG equipment is generally restricted to referral
RBCs in the urine). Urinalysis reagent strips are hospitals. The technique can be used to aid differen-
unable to differentiate between these different causes tiation between neurological and primary myopathic
of pigmenturia. The association of pigmenturia with diseases.
high muscle enzymes usually indicates myoglobin-
uria. Centrifugation of urine will cause RBCs to settle Ultrasound
out, allowing crude differentiation from haematuria, Ultrasonography can be used to evaluate the gross
while haemolysis will often lead to discolouration of three-dimensional structure of muscle. Focal mus-
plasma if a blood sample is collected simultaneously cle injuries resulting in changes in gross archi-
and allowed to clot or be spun down. tecture, such as muscle tears and strains, may be
Myoglobin is a highly nephrotoxic compound. detected. Changes in overall muscle volume can
Attention should therefore be paid to evaluating also be evaluated. The overlying skin is usually
renal function in patients with myoglobinuria and clipped and cleaned. Comparison with the contra-
fluid therapy should be initiated. Urinary casts on lateral muscle group can be useful to confirm the
sediment analysis are indicative of renal tubular diagnosis (Fig. 1.758). It is important to ensure
damage, while increases in plasma creatinine and that the horse is equally weight bearing during this
urea suggest impaired renal function. Electrolyte comparison.
clearance ratios can be calculated, although they
are infrequently helpful. Abnormalities in fractional Scintigraphy
excretion of electrolytes may suggest altered renal Scintigraphic examination is rarely performed
function, but are of little use in determining the exclusively to diagnose muscle disease; however,
cause of muscle damage due to the large changes that an increased radiopharmaceutical uptake may
occur as a result of muscle injury. be detected in the muscle of horses undergoing