CM is Congenital Myotonia - it is a recessive disease
Myotonia slows the relaxation of muscles after voluntary contraction or electrical stimulation this shows up as difficulty rising to the horses feet, abnormal vocalization, a stiff gait, and rigin dlimbs, neck and trunk.
Myotonia has been found in the New Forest Pony.
There is no effective treatment.
Genetic Testing is available.