Myotonia Congenita (MC) is a hereditary neuromuscular disorder characterized by persistent contraction (or delayed relaxation of muscles), particularly during the muscle movement. The word myotonia derives from the Greek word "myo" for muscle and Latin word "tonus" for tension. This disease is caused by an autosomal recessive point mutation and it is not breed-specific. Other than cats, the disease has been described in humans, dogs, horses, goats, mice and water buffalos too.
Congenital myasthenic syndrome (CMS) is a genetic neuromuscular disorder caused by defects at the neuromuscular junction. In cats, this condition is associated with a deficiency of the acetylcholinesterase (AChE). The deficiency of the enzyme is caused by a truncation of the collagen-like tail subunit of acetylcholinesterase (COLQ). The deficiency of the signal transduction termination leads to prolonged muscle contraction and muscle stiffness (spasticity) which interferes with normal movement. The disease seems to be associated with Devon Rex and Sphynx cats.