I'm including this section on the extremely rare condition feline congenital myotonia (i.e., myotonia congenita) because one of our cats suffers from this disorder, it was a challenge to find information about it, and I thought it might save others time tracking down the literature if I provided a brief summary and some citations.
The first case studies of feline congenital myotonia did not appear in the literature until 1998, and to date there have only been 5 publications focusing on this condition exclusively (i.e., that do not also address canine myotonia, feline dystrophy, myotonia more generally):
Toll, Cooper, & Altschul: "Congenital Myotonia in 2 Domestic Cats" Journal of Internal Veterinary Medicine, 1998, vol. 12, pages 116-119.
Hickford, Jones, Gething, Pack, & Alley: "Congenital Myotonia in Related Kittens" Journal of Small Animal Practice, June, 1998, vol. 39, #6, pages 281-285.
Toll & Cooper: "Feline Congenital Myotonia" in Journal of Small Animal Practice, October, 1998, vol. 39, #10, p. 499.
Jones, Hickford, Gething, Pack, & Alley: "Congenital Myotonia in Related Cats" in Feline Practice (supplement), 1999, p. 33.
Hickford & Jones: "Congenital Myotonia in the Cat" in Bonagura (editor), Kirk's Current Veterinary Therapy XIII, Small Animal Medicine, 1999, pages 987-989.
A more recent article that discusses the condition is Vite's "Myotonia and Disorders of Altered Muscle Cell Membrane Excitability" in Veterinary Clinics of North America: Small Animal Practice, 2002, vol. 31, #1, pages 169-187. (A section on page 177 discusses the condition as it occurs in "Domestic Longhair and Shorthair Cats.")
Information can also be found in Shelton's "Neuromuscular Disorders Affecting Young Dogs and Cats" which is available on the web.
According to the available literature, feline congenital myotonia is an inherited disease, though how it is inhereted is unknown. There is a substantial literature on the occurrence and treatment of congenital myotonia in dogs (and various other species) but there has yet to be a published report of treatment for the condition in cats. There are risks in trying to treat cats with medications effective for dogs, especially when so little is known about the disease itself.
The cat we adopted showed many characteristics associated with congenital myotonia: she walks in a stiff and awkward manner; she moves more easily when the room is warm rather than cold; she is more stiff immediately after getting up and becomes more limber as she continues to move; when startled, she becomes temporarily paralyzed, falling over on her side with all 4 legs extended, and becomes gradually able to move again after about 10 seconds; she has difficulty eating; she is heavily muscled with *very* little body fat. Her muscles have difficulty relaxing: they continue to contract after they are stimulated, which makes it difficult for her to move or chew.
The shelter from which we adopted her thought she might have a seizure disorder. They had substantial time to observe her: she lived at the shelter from shortly after her birth (she was found out behind a store) until she was about 1 year and 10 months old. Although her sibblings had all been quickly adopted, they had been healthy; she had striking symptoms that made it difficult for the shelter to find a home for her. We had thought she might have cerebellar hypoplasia (because of her difficulty moving; 2 of out other cats have that condition) or perhaps hypertrophic cardiomyopathy (because of her fallingover; one of our other cats has that condition). These 3 possibilities--along with others such as muscular dystrophy, liver shunt, and thyroid disorders--are part of the rule-out process of diagnosis for this condition.
If feline congenital myotonia is suspected, it makes sense to have a specialist who is familiar with the condition and the literature on the condition, to conduct or consult on the diagnostic process and the treatment phase. Harmony--the cat whom we adopted from the shelter last month and who has congenital myotonia--will undergo a carefully administered clinical trial, one which places her at no risk for harm, to see if there is a way to help her with her condition.