Page 430 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

Musculoskeletal system: 1.9 Muscle disorders of the horse 405

VetBooks.ir Aetiology/pathophysiology Differential diagnosis
Hypocalcaemia, exertional rhabdomyolysis and
Endurance exercise leads to the generation of consid-
erable intramuscular heat, which must be shed to the
external environment. This is achieved by shifts of various causes of colic should be considered.
heat from the muscle and core to the skin via periph- Diagnosis
eral vasodilation and increased skin temperature. Diagnosis is based on the horse’s history, clinical
The skin is then cooled by convection, radiation and signs and laboratory evidence. On blood analysis,
evaporation of sweat. Increased ambient temperature the serum activity of muscle enzymes is often within
and humidity make this evaporative gradient less normal limits or only slightly increased. The haema-
efficient. Hence, endurance exercise, particularly in tocrit and protein concentration are increased and
hot, humid conditions, causes considerable sweat- an ionogram reveals hyponatraemia, hypokalaemia,
ing, muscle fasciculations and ‘cramps’, due to heavy and mild hypocalcaemia (ionised fraction). There is
losses of fluid and ions, including sodium, potassium, usually metabolic alkalosis and a partial respiratory
chloride and calcium. Fluid losses cause hypovolae- alkalosis due to hyperventilation, which only com-
mia and haemoconcentration. Poor oxygenation of pounds the metabolic alkalosis. Dehydration and
the muscles promotes an anaerobic metabolism, with renal disease may be apparent from elevated blood
subsequent lactate accumulation and consumption urea nitrogen and creatinine. Urinalysis may reveal
of the stored glycogen. highly concentrated urine (with dehydration and
Dehydration and electrolyte depletion are the normal renal function) or poorly concentrated urine
major overriding physiological factors causing (indicative of intrinsic renal disease).
the signs of exhaustion. Dehydration compounds
the problem of heat retention, because the horse has Management
less extracellular fluid available to circulate heat to The horse should cease exercise immediately and be
the skin surface and to make sweat in order to lose placed in a cool shaded area, preferably with fans to
heat by evaporation. This dehydration can be severe cool the immediate environment. Cooling by con-
enough to lead to hypovolaemic and circulatory tinued application of large volumes of cold water
shock, resulting in physiological events that may be over the entire body has been shown to be particu-
irreversible despite therapy. larly effective to reduce hyperthermia.
Oral fluid therapy may be considered if the patient
Clinical presentation is mildly affected. However, because the exhausted
The disturbances provoke a myositis-like syndrome horse is usually unwilling to drink sufficiently, iso-
with a similar presentation to hypocalcaemia or exer- tonic mixed electrolyte solutions can be adminis-
tional rhabdomyolysis, although the serum calcium tered by nasogastric tube, as long as the horse has
concentration is often normal and no myoglobinuria normal gut sounds and no evidence of colic or gas-
is noted. These signs include elevated temperature, tric reflux.
pulse rate and respiratory rate. The temperature If the horse is severely affected or fails to respond
often fails to return to normal on cessation of exer- to oral fluid therapy, intravenous fluid therapy should
cise. Other signs include depression, anorexia (includ- be instigated immediately. Large volumes are neces-
ing for water), a stiff, stilted gait, localised or more sary to reverse the effects of exhaustion. Balanced,
diffuse muscle cramping, hardening and pain on pal- polyionic solutions are warranted and are the treat-
pation. Signs of dehydration include poor skin tur- ment of choice to re-establish normal circulation.
gor, poor capillary refill time, dry and tacky mucous A more specific correction of electrolyte imbalance
membranes, minimally concentrated urine, dry fae- may be undertaken with care, including potassium
ces and sunken eyes. Less common signs include (10 mEq/l). If present, hypocalcaemia is treated as
synchronous diaphragmatic flutter, atrial fibrillation, described above. Glucose should be added to the
diarrhoea, colic and laminitis. fluids (10 g/l).