Immune-mediated inflammatory myopathy associated with a missense variant in MYH1 (myosin), with stiffness, pain, loss of performance and possible muscle atrophy. Dominant inheritance with incomplete penetrance. The test is complementary to the clinical examination and muscle biopsy.

Tiro belga, Cuarto de Milla and Welsh Pony. Frequencies: limited data.

- Muscle stiffness and pain
- Muscle swelling and reluctance to move
- Poor performance and exercise intolerance
- Sweating and inflammatory responses
- Possible muscle atrophy in chronic cases

The dossier records OMIA:002141 and the variant MYH1 NC_009154.3:g.53345548T>C (c.959A>G; p.Glu320Gly; rs3435577028). No author or year is stated.

- As it is dominant with incomplete penetrance, not all carriers express the disease
- Assess the genotype before breeding
- Avoid concentrating the allele in lines with clinical cases
- Record episodes of stiffness or rhabdomyolysis

Differential diagnosis with PSSM1/PSSM2, HYPP, myotonia, exertional rhabdomyolysis syndrome and Streptococcus equi myositis. Complementary: CK/AST, muscle biopsy and electromyography.

1. OMIA:002141 Miositis inmunomediada (MYH1) del caballo