Myotonia congenita is an inherited muscle disorder affecting dogs. The muscle cells of an affected dog are over-excitable, which causes muscles to remain contracted rather than relaxing after voluntary activity. Signs of the disorder usually present when puppies begin to walk. Their gait may appear stiff and uncoordinated and they may fall frequently. Affected dogs frequently have a “bunny hop” type gait. Episodes do not appear to be painful and the muscle stiffness may improve with increased exercise. Episodes can worsen with cold and excitement. Other features include enlargement of the muscles, abnormal posture, an upper jaw that is much longer than the lower jaw, and dental abnormalities. Dogs may also exhibit excessive panting and salivation, have an abnormal bark and loud raspy breathing, and have difficulty swallowing. Dogs with this disorder typically have a normal lifespan.