Page 414 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

Musculoskeletal system: 1.9 Muscle disorders of the horse 389

VetBooks.ir Table 1.5 Breeds in which PSSM1 has been signs of poor performance, other common causes of
this presenting complaint, such as orthopaedic con-
identified to date
ditions, upper and lower respiratory tract disease and
• Quarter horses • Suffolk Punch cardiac abnormalities, should all be considered.
• Appaloosa • Hanoverian
• Paint • Warmblood Diagnosis
• Belgian draught horse • Morgan • Blood muscle enzyme activity may be
• Percheron • Cob persistently elevated, even at rest, in many cases
• Haflinger • Connemara cross but some draught breeds may only reveal mild
• Shire • Arab cross elevations following exercise.
• Polo ponies • A genetic test is now available for PSSM1. This
can be performed on DNA extracted from blood
collected into EDTA or hair roots.
A high prevalence has been described in continental • PSSM1 can also be diagnosed from a muscle
draught breeds. The disease is associated with the biopsy.
accumulation of glycogen and abnormal polysac- • PSSM2 can only be diagnosed from a muscle
charide inclusions within muscle fibres and with a biopsy.
reduction in cellular energy availability. In 2008,
an autosomal dominant mutation in the glycogen Management
synthase 1 (GYS1) gene was identified as a cause of Cases presenting with acute rhabdomyolysis are
PSSM in approximately two-thirds of horses with treated as described under RER. In other presenta-
the disease. This has led to the recent reclassification tions, long-term management changes are essential
of the disease as PSSM1 for horses with the GYS1 in resolving the clinical signs.
mutation and PSSM2 for horses that have abnor-
mal polysaccharide within their skeletal muscle but • A regular, graduated daily exercise programme
lack this gene mutation. Recent work has suggested plus daily pasture turnout is advised.
that the abnormal accumulation of polysaccharide in • A diet low in starch and sugar (<10% digestible
horses with PSSM2 results from disruption of the energy [DE] as non-structural carbohydrates
myofibrillar proteins rather than a primary glycogen [NSC]) and relatively high in fat (13–20% DE)
storage disease. should be provided.
• Horses should continue to receive 1–2% of their
Clinical presentation bodyweight as good-quality forage, ideally with
The clinical presentation in PSSM can vary from a low (<12%) NSC content.
vague signs of poor performance or back pain to loss • Decrease weight in overweight animals.
of muscle and hindlimb weakness to acute rhabdomy- • Grazing may need to be restricted at certain
olysis. Horses with PSSM1 are more likely to present times of the year when the NSC content of grass
with rhabdomyolysis than horses with PSSM2, and is particularly high.
the latter are more likely to present with subtle signs • There are specifically formulated commercial
of gait abnormality such as an undiagnosed lameness diets available (such as Dodson and Horrell ERS
or weakness. Pellets or Saracen ReLeve ). Alternatively,
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a low starch diet may be supplemented with
Differential diagnosis vegetable oil, up to a maximum of 1 ml/kg
In the horse with exertional rhabdomyolysis, other bodyweight, to provide sufficient calories.
causes listed in Table 1.4 should be considered. To In some cases, diets with a slightly lower fat
date, PSSM1 has not been identified in a purebred content may be more palatable yet still be
Thoroughbred horse. In horses with more subtle sufficient to control the condition.