Page 412 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

Musculoskeletal system: 1.9 Muscle disorders of the horse 387

VetBooks.ir removal at 10–14 days. The muscle should be trans- These involve biopsy needles or conchotome for-
Following biopsy, horses are rested until staple
Less invasive biopsy techniques are available.
ported by same day or next day courier service to an
experienced laboratory. ceps. These yield smaller muscle samples but mini-
mise the need for rest following the procedure.

MUSCLE DISORDERS

EXERCISE-INDUCED MYOPATHIES RecuRRent exeRtional Rhabdomyolysis

EXERTIONAL RHABDOMYOLYSIS Aetiology/pathophysiology
RER is a disease that affects Thoroughbred and
Exertional rhabdomyolysis (otherwise known as Standardbred horses, mainly when they are in
azoturia or ‘Monday morning disease’) is a syn- training regimes. It may be more common in ner-
drome of muscle pain that occurs during or follow- vous animals and possibly fillies. RER is associated
ing exercise. It can be caused by several different with abnormal release of calcium within muscle
conditions that each affect skeletal muscle function cells, resulting in abnormal muscle contracture.
in different ways and result in a common clinical The pattern of inheritance in several breeding
presentation. lines suggests that RER is a genetic condition,
Some horses experience isolated bouts of tying- transmitted as an autosomal dominant trait, with
up, thought to be trigged by acquired environmental the recurrent episodes triggered by factors such as
factors such as overexertion (Table 1.4). Other horses stress, variations in exercise intensity or duration,
experience recurrent bouts of rhabdomyolysis due and diet.
to an underlying susceptibility caused by specific
diseases, and possibly triggered by environmen- Clinical presentation
tal factors. Some of these diseases have a proven or Horses with RER present with signs of muscle
likely genetic basis to their aetiology. Two genetic pain and stiffness, during or immediately following
conditions have been identified to date: recurrent exercise. Depending on the severity of the episode,
exertional rhabdomyolysis (RER) and type 1 poly- horses may have mild gait abnormalities, be reluc-
saccharide storage myopathy (PSSM). tant to move or even be recumbent. Some cases can
be particularly distressed, and in the most severe
there is dehydration, congested mucous mem-
Table 1.4 Causes of exertional rhabdomyolysis in branes, tachycardia/tachypnoea, colic-like signs,
the horse myoglobinuria and hyperthermia. Moving the
horse may worsen the clinical signs. Muscles are
ACQUIRED GENETIC often firm on palpation, with sweating and muscle
Overexertion, inadequate training Recurrent exertional fasciculations. More subtle signs may include poor
rhabdomyolysis performance and lack of power and collection. The
Electrolyte abnormalities Type 1 polysaccharide hindlimb, pelvic and epaxial muscles are most com-
storage myopathy monly affected.
Antioxidant deficiencies Type 2 polysaccharide
(e.g. selenium and vitamin E) storage myopathy
Hormones Differential diagnosis
Other acquired or genetic causes of exertional rhab-
Infectious agents, especially post domyolysis (see Table 1.4); other causes of sudden-
viral infections onset lameness including laminitis; colic; severe
Dietary imbalances such as excess neurological disorders.
energy (grain)