Test Detail

Congenital myotonia is an inherited neuromuscular disease in which the muscles have difficulty relaxing after contraction (myotonia), due to an alteration of the chloride channels of the muscle membrane. It affects the skeletal muscular system: affected cats show stiffness that improves with continuous movement, a peculiar gait and difficulty getting up after rest. The disease does not usually markedly shorten life, but it limits welfare and physical function. There is no curative treatment, although the symptoms are usually managed with environmental management and antimyotonic drugs.

An uncommon disease described in the domestic cat, so the test is offered for all breeds. There are no reliable carrier frequencies by breed: limited data.

- Generalized muscle stiffness, more intense after rest\n- Improvement of stiffness with continuous exercise (phenomenon of improvement with movement)\n- Stiff gait, hopping or with rigid legs (rabbit-hop type)\n- Difficulty getting up after sleeping\n- Percussion myotonia: persistent dimple after percussing the muscle\n- Difficulty blinking or swallowing in marked cases

Congenital myotonia is classic in humans and in livestock (the bull disease) and natural cases were described in the cat in the late 20th century, with electromyographic characterization of the myotonic phenomenon. In the 2010s the molecular cause was identified in mutations of the CLCN1 gene, which encodes the muscle chloride channel, confirming autosomal recessive inheritance in the domestic cat. The DNA test was later incorporated into feline panels as a general test for all breeds, since the variant can appear outside specific breeds.

- Test breeding animals in any line with signs of muscle stiffness or a family history.\n- Do not cross two carriers with each other; a carrier can be mated to a clear animal without risk of affected offspring.\n- Test the offspring that remain in breeding and document the result.\n- Keep affected cats with gentle, regular exercise, avoiding intense cold and prolonged rest that accentuate stiffness.

Differential diagnosis with episodic hypokalemic myopathy (crises with cervical flexion, low potassium), muscular dystrophies, polyneuropathies and myositis. Electromyography with characteristic myotonic discharges is the reference functional test; muscle percussion with a slow dimple guides the examination. Antimyotonic drugs (membrane stabilizers such as mexiletine) can be used in marked cases under veterinary supervision, with gastrointestinal and cardiac monitoring.

1. Gandolfi B et al. (2014) A novel mutation in CLCN1 associated with feline myotonia congenita. PLoS One 9(10):e109926. PMID: 25356766
2. Woelfel C et al. (2022) A novel mutation of the CLCN1 gene in a cat with myotonia congenita: Diagnosis and treatment. J Vet Intern Med. PMID: 35815860
3. Corrêa S et al. (2023) Hereditary myotonia in cats associated with a new homozygous missense variant p.Ala331Pro in the muscle chloride channel ClC-1. J Vet Intern Med 37(6):2498-2503. PMID: 37668104
4. OMIA:000698-9685. Myotonia in Felis catus. https://omia.org/OMIA000698/9685/