Congenital Myasthenic Syndromes are a group of inherited neuromuscular disorders. Several genetic variants have been found to cause similar clinical signs in various species and breeds. One such variant in Russell Terriers has been identified in the CHRNE gene and is associated with postsynaptic CMS due to defective acetylcholine receptors in the neuromuscular junction. A CHRNE gene variant has also been identified in a litter of Heideterriers showing myasthenia gravis-like symptoms six days after birth. Affected puppies display skeletal muscle weakness progressing over the course of the day (fatigable weakness) with decreased front limb coordination and reflexes. Severely affected puppies may show collapse and recumbency but still attempt normal behaviors. The mentation and cranial nerve function remain normal in affected dogs. Unlike acquired myasthenia gravis, which tends to be diagnosed in early adulthood or after, CMS affected dogs are autoantibody negative.

There is no cure for this condition. Upon initial observation of clinical signs, affected dogs should be closely monitored to assess welfare and devise a supportive care plan. Genetic confirmation of the type of CMS can be important, as treatment response varies by type. No treatment for CMS identified in the Heideterrier has been described, but dogs affected by the CHRNE associated CMS in the Russell Terrier have responded to acetylcholinesterase inhibitors. Drug resistance does occur in Russells and humans with variants in this gene. Humans with CHRNE variants have also been treated with 3,4‐DAP, and delay of resistance has been demonstrated with use of a β2‐adrenergic receptor agonist. More research is needed to fully understand the prognosis of affected individuals with this form of CMS.

There are many responsibilities to consider when breeding dogs. Regardless of test results it is important that your dog is in good general health and that you are in a position to care for the puppies if new responsible owners are not found. For first time or novice breeders, advice can be found at most kennel club websites.

This disorder is autosomal recessive, meaning two copies of the variant are needed for a dog to be at an elevated risk for being diagnosed with the condition. A carrier dog with one copy of the Congenital Myasthenic Syndrome (Discovered in the Heideterrier) variant can be safely bred with a clear dog with no copies of the Congenital Myasthenic Syndrome (Discovered in the Heideterrier) variant. About half of the puppies will have one copy (carriers) and half will have no copies of the variant. Puppies in a litter which is expected to contain carriers should be tested prior to breeding. Carrier to carrier matings are not advised as the resulting litter may contain affected puppies. However, in order to reduce the prevalence of this variant in the breed population, use of dogs with one or two copies of the variant should be critically considered prior to matings. Please note: It is possible that disorder signs similar to the ones associated with this CMS variant could develop due to a different genetic or clinical cause.