An Owner’s Guide to Exercise-Induced Muscle Stiffness in Spaniels
Paradoxical myotonia (also known as paradoxical pseudomyotonia) is a rare inherited muscle disorder seen primarily in English Springer Spaniels and English Cocker Spaniels. Although the episodes can look alarming, the condition is usually non-painful, non-progressive, and most affected dogs continue to enjoy an excellent quality of life.
What is paradoxical myotonia?
Paradoxical myotonia is a condition in which the muscles have difficulty relaxing after exercise. Unlike normal muscles, which contract and then immediately relax, the muscles of affected dogs temporarily remain stiff.
The term “paradoxical” refers to the fact that the stiffness gets worse with repeated movement or exercise, rather than improving as the muscles warm up.
Most episodes last only a few seconds before resolving completely with rest.
What causes it?
We now know that paradoxical myotonia is an inherited genetic condition caused by a mutation in the SLC7A10 gene. This gene is involved in transporting the amino acid glycine, an important inhibitory neurotransmitter within the nervous system. When this transporter does not function normally, muscle relaxation after exercise becomes impaired.
The disease is inherited in an autosomal recessive manner, meaning an affected dog inherits an abnormal copy of the gene from both parents.
What are the signs?
Most affected dogs develop their first episodes before two years of age, often when they are puppies or young adults.
Although all affected dogs have the same underlying condition, the way it presents can vary considerably between individuals. In medicine, this is known as having different phenotypes, which simply means different outward appearances of the same disease.
Watch Video 1 to see how dogs appear when they collapse.
Examples of the different ways paradoxical myotonia can present include:
- A dog suddenly collapsing while walking.
- A dog becoming stiff and collapsing after running.
- A dog becoming “stuck” while trying to climb stairs.
- A dog becoming rigid after jumping.
- A dog running with a stiff, awkward gait that one owner memorably described as looking like “a dog running in a computer game.”
Although these episodes can look very different, they are all caused by the same underlying disorder.
Importantly, affected dogs remain fully conscious and aware throughout the episode. Most episodes last only a few seconds, and the muscles relax rapidly once the dog stops exercising and rests.
What triggers an episode?
Episodes are almost always triggered by vigorous exercise, particularly activities involving sudden bursts of speed such as chasing a ball or playing with other dogs.
Some dogs are also more likely to have episodes when they are:
- excited
- stressed
- exercising in very hot or very cold weather
- swimming.
Fortunately, many owners quickly learn which situations are likely to trigger an episode and can adapt exercise accordingly. Avoiding known triggers is often all that is needed to minimise episodes.
How is it diagnosed?
Diagnosis is based on:
- the characteristic history
- videos of the episodes
- a normal neurological examination between episodes
- exclusion of other muscle and neurological disorders.
Blood tests, MRI scans, electromyography (EMG) and other investigations are usually normal, helping to rule out other conditions. Genetic testing for the SLC7A10 mutation is now available and can confirm the diagnosis in many English Springer Spaniels and English Cocker Spaniels.
Can it be treated?
There is currently no cure, but most dogs do not require medication.
Simply avoiding situations that trigger episodes is enough to allow most dogs to lead completely normal lives.
In the small number of dogs with severe or frequent episodes, medication such as clonazepam may help reduce the frequency or severity of the episodes, although side effects such as drowsiness can limit its use.
What is the outlook?
The prognosis is generally excellent.
Unlike many inherited neurological diseases, paradoxical myotonia is not progressive. In many dogs, the episodes become less frequent as they mature, or owners simply learn how to avoid the situations that trigger them.
With sensible exercise management, most affected dogs continue to enjoy a happy, active life and have an excellent quality of life.
Key points
- Paradoxical myotonia is a rare inherited condition affecting mainly English Springer Spaniels and English Cocker Spaniels.
- It is caused by a mutation in the SLC7A10 gene.
- The condition causes brief episodes of muscle stiffness during or immediately after exercise.
- Different dogs can show different patterns of episodes, including collapse while walking or running, difficulty climbing stairs, stiffness after jumping, or a characteristic stiff “computer game” running gait.
- Dogs remain fully conscious throughout the episodes.
- Most episodes resolve within seconds once the dog rests.
- Most affected dogs require little or no treatment and enjoy an excellent long-term quality of life.