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Atypical Myopathy

Updated: Feb 22, 2023

Author: Katherine Murray BVMedSci BVM BVS CertAVP MRCVS

What is atypical myopathy?

‘Atypical’ means irregular or abnormal and ‘myopathy’ is a general term used to describe a disease or disorder affecting skeletal muscles. So ‘atypical myopathy’ is an abnormal disorder affecting the muscles, it is thought to be fatal in approximately 75% of cases.

What causes it?

It is caused by ingestion of: - Sycamore seeds or leaves that fall onto

pasture in Autumn and Winter - Sycamore seedlings that germinate in Spring

- Specifically, it is caused by ingestion of the

Hypoglycin A toxin within the plant.

Hypoglycin A (HGA) toxin

- This toxin is found in some but not all species of sycamore trees, the European Sycamore (Acer Pseudoplanatus) is the one currently linked to UK cases

- The concentration of the toxin varies between plants

- Research shows some horses are more susceptible to the toxin than others

- HGA slows or stops energy production in skeletal and cardiac (heart) muscles.


What are the clinical signs?

- Weakness of the skeletal muscles including:

  • Difficulty walking

  • Muscle tremors

  • Difficulty breathing

  • Recumbency (unable to stand)

- Heart issues (due to cardiac muscle weakness)

- Depressed with a low head carriage

- Colic signs but often appetite remains

-Dark red or brown urine

Normal Abnormal

The dark urine is due to muscle damage. A product called myoglobin is released from damaged muscle cells and is removed from the body in the urine, giving it a dark red/brown colour (container on right).

If you are suspicious that your horse may be showing any of the clinical signs listed above call your vet immediately.


How is it diagnosed?

Diagnosis is based on - History eg. Time of year - Environment eg. Are sycamore trees

within or close to grazing areas?

- Clinical signs (listed above)

- Clinical examination (heart and respiratory rate, temperature etc.)

- Blood samples submitted for laboratory tests

eg. Testing for high muscle enzymes which would indicate muscle damage


Can it be treated?

Up to 75% of cases can be fatal HOWEVER early diagnosis and treatment will increase chances of survival. Clinical signs often get worse before they improve so even if signs are mild, transportation to a hospital to start intensive treatment is recommended to give your horse the best chance of survival.

Treatment includes:

- Intravenous fluid therapy (IVFT) – very large volumes are needed to:

  • Protect the kidneys from long-term damage

  • Prevent ongoing dehydration

- Pain relief – ‘multimodal analgesia’

  • A combination of various types of intravenous pain relief may be required

  • Often strong pain relief such as opioids are required which cannot be administered outside of a hospital setting

- Supplementation of vitamins

  • Carnitine, vitamin B1 and vitamin B2 may support muscle cell function

  • Vitamin C and Vitamin E are anti-oxidants and may be a useful adjunctive treatment

The recovery period is long, often several months. However, studies show that horses which survive to day 5 after the onset of clinical signs are likely to recover fully with no long-term side effects.


Prevention – How to reduce the risk

Given that the disease is caused by ingestion of sycamore seeds and leaves avoid this by:

- Reducing the chance of your horse scavenging for alternative food sources if grazing is tight in Autumn and Spring:

  • Provide supplementary forage

  • Reduce stocking density

- Remove fallen sycamore leaves and seeds from pasture or fence off affected areas

- Test your pasture for the presence of Hypoglycin A toxin

- The ‘helicopter’ seed shape means that the seeds can travel around 200 yards so check neighbouring areas also for high-risk plants

If you have any concerns about your horse, pony or donkey please do not hesitate to contact the practice on 01635 39039.


Olga Witkowska-Pilaszewicz, et al.(2019) ‘Equine atypical myopathy – a review’, Animal Science Papers and Reports, 37(3), pp: 233-242.


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