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7
EQUINE MATTERS
MUSCL E D I SEASES
Veterinary Surgeon
Laura Sharpe
XLEquine Practice
Chapelfield Veterinary
Partnership Ltd
Muscle
Diseases
and Disorders
Laura Sharpe BVSc Cert AVP MRCVS,
Chapelfield Veterinary Partnership
Many articles in the equestrian literature cover lameness topics involving bones,
joints, tendons and ligaments. There is no doubt that injuries and diseases affecting
these tissues form the majority of the causes of lameness. However, the topic
of muscle disease is infrequently discussed, despite there being a number of
important and relatively common conditions that affect muscles. Broadly speaking,
muscle diseases are either present from birth
(genetic disorders)
or develop
spontaneously during adulthood
(acquired disorders)
.
The medical term for the most common
muscle disorder (myopathy) is exertional
rhabdomyolysis (ER). This results in a s
yndrome of moderate to severe muscle
cramping that occurs during or immediately
following exercise. ER is more commonly
known as tying up, set-fast and Monday
morning disease. Horses that experience ER
either have an underlying myopathy or have
physically overexerted themselves.
This is a disease characterised by
repeated episodes of ER that may be
induced with very minimal amounts of
exercise, typically within the first 30
minutes. Affected animals are stiff, painful,
sweaty, and may be reluctant to move.
Horses can also present with progressive
poor performance, muscle wasting,
weakness or back pain. Examination of
a muscle biopsy is necessary to confirm
a diagnosis of PSSM.
A gene mutation is responsible for some
cases of PSSM and this can be confirmed
on a blood test, but whilst a positive test
is helpful, some horses not carrying the
gene may still have PSSM or other
causes of ER.
In many animals this disease can be
well managed. Keeping the horse fit,
lean and in regular exercise will help,
and many can be used for their original
purpose. Regular and prolonged periods
of grazing for as long as possible also
reduces the frequency of episodes. The
diet should be changed from any
carbohydrates to one high in fibre
with additional oil if needed for energy,
along with a complete vitamin and
mineral supplement.
Genetic Disorders
1.
Polysaccharide Storage
Myopathy (PSSM)
This inherited disorder results in problems
with the pump that moves the electrolyte
potassium in and out of muscle cells. It
occurs in QHs, Paint Horses, Appaloosas
and others that have bloodlines traced
back to the sire Impressive. These horses
tend to be well muscled, and may do
well showing. Between episodes they
are normal. Clinical signs during an
episode include facial muscle spasms,
drooling, sweating, noise whilst
breathing, shoulder, neck and flank
muscle trembling, an inability to lift the
head and neck and some patients are
unable to stand. These episodes typically
last for 30-60 minutes. Sudden death
during an episode can occur. Usually
an episode has been noticed before the
horse reaches three years old and
triggers including cold, periods of fasting,
heavy sedation, anaesthesia and rest
after exercise.
The best way to confirm a diagnosis of
HYPP is to take a hair or blood sample
and have it genetically tested. Horses can
be managed by feeding a diet low in
potassium, feeding a grain-based feed
twice daily and keeping them in light
regular exercise.
2.
Hyper-kalaemic Periodic
Paralysis (HYPP)