Paradoxical pseudomyotonia is a muscular disorder characterised by exercise-induced generalised myotonic-like episodes of variable severity – muscles take longer to relax (remain stiff) after voluntary contraction. When having an episode dogs experience the sudden onset of muscle stiffness, causing them look like they are “running in a computer game”, get “stuck” while climbing or performing small jumps. Signs of the condition are first observed between three months and 2 years of age. Episodes vary in frequency but can occur up to 15 times per day in some cases. They usually last for a few minutes and resolve with rest. Severity and frequency tend to remain stable or decrease with age, and episodes can be decreased or eliminated by avoiding triggers (usually strenuous exercise).
Currently we do not know how prevalent the variant is in either breed, or if it is more prevalent in working or show lines.
Four videos of episodes are available with the original publication by Stee et el:
The single nucleotide stop-gain variant in the gene called SLC7A10 that causes Paradoxical pseudomyotonia in English Cocker Spaniel and English Springer Spaniel is autosomal recessive. This means that dogs that carry two copies of the mutation (homozygotes) will almost certainly develop Paradoxical pseudomyotonia during their lives. Dogs that carry a single copy of the mutation (also known as carriers or heterozygotes) will not develop Paradoxical pseudomyotonia as a result of the SLC7A10 mutation, but they will pass the mutation onto about half of any offspring they have. Breeding dogs that will not develop Paradoxical pseudomyotonia should be the breeder’s priority, with a reduction in mutation frequency within the whole breed being the secondary, longer-term target.
Carriers can be bred from safely, provided they are mated to a dog that has also been tested and is clear of the SLC7A10 mutation (i.e. carry no copies of the mutation). If a carrier is mated to a clear dog approximately half of the resulting puppies will also be carriers, so should be tested themselves prior to breeding. Breeding carriers to tested, clear dogs is safe, in terms of avoiding dogs affected with Paradoxical pseudomyotonia, and will help to maintain the genetic diversity of a breed. It is therefore encouraged, particularly in the first few generations following the availability of a new genetic test, so that other desirable characteristics and traits can be preserved before the frequency of the disease mutation within the breed is gradually reduced.