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Atypical Myopathy

A myopathy is a condition affecting the muscles where damage to the muscle cells releases enzymes into the blood, causes significant pain and limits the horse’s ability to move. Typical myopathies are related to exercise such as ‘tying up’ or rhabdomyolysis as it is technically known and conditions such as PSSM. Atypical myopathy refers to a myopathy which is not related to exercise and appears to come from nowhere. It is caused by ingestion of sycamore leaves, seeds or seedlings which contain a toxin called hypoglycin A. It is therefore also known as sycamore poisoning. There appears to be variability in the susceptibility of individual horses and also in how badly affected horses are in different years and seasons. The disease is seen in autumn and winter when the sycamore seeds are on the ground and again in spring when the seedlings are growing, and young horses are most at risk although it can affect any age.

Symptoms

Horses may demonstrate one or more of the following symptoms:

• Weakness or walking oddly
• Low head carriage
• Muscle tremors
• Sweating
• Laboured breathing
• Looking like they have colic but they usually still have an appetite
• Dark coloured urine – brown or red
• Down and unable to stand

Diagnosis

Clinical examination will raise suspicion including recognising the above symptoms as well as the horse having an elevated heart rate but normal temperature and gut sounds usually so it can be a rather confusing clinical picture. A blood test will confirm a significant myopathy by elevations in blood enzymes called CK and AST. In severely affected horses the kidneys will be damaged too which will also show on the blood test. A confirmation of the disease can be achieved via a muscle biopsy being sent to a laboratory which can also be used to prove cause of death via a postmortem examination.

Treatment

1. Stopping any further ingestion of toxin by removing the horse from the pasture. Stomach tubing with activated charcoal may be used to reduce further absorption of toxins from sycamore in the horse’s digestive system.
2. Aggressive fluid therapy via a drip to support the kidneys, maintain hydration and flush the enzymes and toxins through the system.
3. Pain relief.
4. Vitamin E for muscle support and antioxidant properties.
5. Nursing – meeting the horse’s care needs such as support in feeding or comfort if recumbent.

Due to the intensity of care required in these cases referral to an equine hospital may be required.

Prognosis

Unfortunately, atypical myopathy carries a very poor prognosis, and it leads to death in 75% of affected horses. Poor prognostic indicators include kidney failure showing on the bloods, horses which are down and unable to stand and horses which are not clinically improving within the first few days of treatment. Due to the grave prognosis it is reasonable to consider early euthanasia in severely affected horses or  if financial and practical situations preclude an attempt at the intensive treatment required.