Congenital myotonia of the Miniature Schnauzer is a hereditary neuromuscular disease affecting the chloride channels of skeletal muscle. It is characterised by difficulty in relaxing the muscles after a contraction, producing stiffness, weakness and a stiff gait.\n\nAffected dogs show muscle stiffness during the first steps after a period of rest, which improves with continued exercise ('warm-up' phenomenon).

- Miniature Schnauzer: in the original screening study, 20.4% were carriers and 1.1% affected (Bhalerao et al. 2002); later frequencies may be lower due to selection.\n- The p.Thr268Met variant appears to be specific to the Miniature Schnauzer; other breeds (Australian Cattle Dog, Labrador, American Bulldog, French Bulldog) have distinct variants of CLCN1.

- Muscle stiffness during the first steps after rest\n- Stiff gait that improves with exercise ('warm-up' phenomenon)\n- Difficulty in getting up after lying down\n- Clenched fist (flexion of the limbs)\n- Hypertrophic myopathy (increased muscle size in chronic cases)\n- Difficulty in swallowing (dysphagia, in severe cases)\n- Stiffness worsens with cold

Canine congenital myotonia was described clinically in the Miniature Schnauzer (Vite et al., 1999). In 1999, Rhodes and colleagues identified the causal mutation in the CLCN1 gene (skeletal muscle chloride channel, chromosome 16): a missense substitution that reduces the probability of channel opening and produces muscle hyperexcitability and difficulty in relaxing. In 2002 a specific molecular test was developed (Bhalerao et al.).

- Test Miniature Schnauzer breeding animals before breeding (molecular test available since 2002).\n- Do not mate two carriers: 25% risk of affected offspring.\n- A carrier can be mated to a clear animal; test the offspring intended for breeding.\n- Do not use affected animals for breeding.\n- The p.Thr268Met variant is breed-specific; a common carrier stud dog explained the spread of the allele.

The differential diagnosis includes mitochondrial myopathies, myasthenia gravis and other neuromuscular diseases; genetic testing is definitive. Symptomatic management relies on drugs that reduce muscle stiffness (e.g. procainamide or mexiletine), according to veterinary judgement and clinical response. With appropriate treatment, many affected dogs maintain a relatively normal life.

1. Rhodes TH, Vite CH, Giger U, Patterson DF, Fahlke C, George AL Jr. A missense mutation in canine C1C-1 causes recessive myotonia congenita in the dog. FEBS Lett 1999. PMID:10452529.
2. Vite CH, Melniczek J, Patterson D, Giger U. Congenital myotonic myopathy in the miniature schnauzer: an autosomal recessive trait. J Hered 1999. PMID:10544501.
3. Bhalerao DP, Rajpurohit Y, Vite CH, Giger U. Detection of a genetic mutation for myotonia congenita among Miniature Schnauzers and identification of a common carrier ancestor. Am J Vet Res 2002. PMID:12371774.
4. Finnigan DF, Hanna WJ, Poma R, Bendall AJ. A novel mutation of the CLCN1 gene associated with myotonia hereditaria in an Australian cattle dog. J Vet Intern Med 2007. PMID:17552451.