Atypical myopathy (sycamore myopathy) is a severe and often fatal muscle disorder caused by ingesting sycamore seeds, leaves or seedlings.

It is fatal for around three quarters of affected horses. Some horses appear to be more susceptible than others, perhaps due to genetic differences.  Reports of cases began in the 1940s but there is evidence supporting a marked increase in recent years.

There are more than 25 species of Acer tree and and not all species have the HGA toxin. (Hypoglycin A).  HGA toxin concentrations vary between plant samples derived from different trees or seedlings from them. Understandably, horse owners and yard owners want to minimise the exposure to anything which is poisonous to horses. 

For your own copy of RVC's Owners' Fact File on Atypical Myopathy -   Download

Following research that was supported by the Horse Trust and RVC's Animal Care Trust, the RVC now offers

Vets : serum testing for the HGA toxin;

Owners: Sample test service for sycamore seed,seedlings & leaves - For your sample order & payment form -  Download here

Guidelines on how to take & submit your plant samples are included in the Owners Fact File on Atypical Myopathy  Download here

As part of RVC’s ongoing commitment to scientific innovation - our teams are involved in research (supported by the Horse Trust) concerning the influence of genes on susceptibility and the impact of different treatments on the toxic effects of hypoglycin A. They are also establishing the reasons why different trees vary in their toxin production and other risk factors


Atypical myopathy is caused by horses eating sycamore seeds that fall onto pasture in Autumn and Winter, and their germinating seedlings in Spring.

Seeds and seedlings contain the toxin hypoglycin A (HGA) which slows or stops energy production in muscle and heart 

Some apparently unaffected horses have high concentrations of HGA in their blood suggesting that some horses are more susceptible to the toxin than others.

Clinical Signs

  • General weakness : horses struggle to walk, stand and breathe
  • Many horses develop heart problems. 
  • Horses appear depressed with low hanging heads
  • Muscle trembling
  • Signs of severe colic - yet, unlike colic,  they still have an appetite 
  • Brown or dark red urine 
  • Severely affected horses become unable to stand


Vets consider the horse’s environment as well as symptoms reported and those observed during an examination.

Diagnosis requires a variety of tests available to referring vets from the RVC Comparative Neuromuscular Disesases Laboratory.

Find out more about vet sample submissions


As confirmation can take several days, vets often start treatment immediately and before official confirmation. Your horse may require admission to a specialist equine hospital for 24 hour advanced care. If horses survive the first few days of treatment they usually go on to recover completely, although this can take several months.


The following steps can be taken to reduce horses' risk to atypical myopathy

• Provide supplementary forage during Autumn

• Clear fallen sycamore leaves and seeds from grazing areas 

• Check neighbouring areas for high risk plants/seeds as some ‘helicopter’ seeds can travel up to 200 yards

• Test for the presence of HGA in your own horses’ pastures

The RVC offers horse owners & stable/yard owners a sycamore sample test for identifying plants that contain the HGA toxin known to cause atypical myopathy 

Factsheet on the condition & how to collect your sycamore leaves & seeds samples   Download

Download a sample submission form/payment details

Key points

Diagnosis of atypical myopathy, despite its name, has become more common in recent years.

Evidence suggests it is caused by horses eating fallen sycamore seeds and their germinating seedlings.

It is a rapid onset disease affecting muscles, including the heart.

It is fatal for around three quarters of affected horses.

Some horses appear to be more susceptible than others, perhaps due to genetic differences

RVC provides diagnostic tests for referring vets dealing with suspected atypical myopathy cases

RVC provides atypical myopathy tests for horse owners & yard owners to establish if they have plants on their property which contain the HGA toxin

For your RVC Owners Fact File to Equine Atypical Myopathy & sample guidelines  Download here

For your seeds & leavers sample order form & payment information  Download here

Frequently asked questions

Our Owners Fact File has a FAQ's section which should answer many of your queries

Download here

Otherwise - please email us at if the issue relates to your samples;

or for horse-related issues contact or call us on 01707 666297

Atypical Myopathy Tests for referring vets

For vets

Serum testing for the hypoglycin A toxin and its principal metabolite known to cause atypical myopathy is now available. This is a more rapid test than previously reported methods. We also offer combined profiles including hypoglycin A & acylcarnitine identification and hypoglycin A, acylcarnitine & organic acid identification at special prices. Please send either a whole blood tube, serum gel tube or separated serum using a next day delivery service and use the specific form below. For organic acids profiles please send a urine sample and for acylcarnitines please send a lithium heparin blood tube.

Vet Submission form for atypical myopathy samples

Whenever possible, samples received before 9am will be reported in 48 hours and samples received after 9am will be reported in 72 hours. Please note these turnaround times are for working days only (the lab is open Monday-Friday)

Prices (all excluding VAT) of the available atypical myopathy tests are as follows:

Serum hypoglycin A & MCPA- carnitine: £95

Serum hypoglycin A & MCPA- carnitine (6 or more samples): £85 per sample

Serum hypoglycin A, MCPA- carnitine & acylcarnitine profile: £179

Full atypical myopathy profile (HGA, MCPA-c, organic acids & acylcarnitine profiles): £275

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