Myotonia Congenita in Mastiffs: A Rare Diagnosis and Its Common Impostors

In true myotonia, muscle fibres fail to switch off. When an affected dog first stands, the hind limbs are rigid and the gait is stiff and hopping, sometimes with both back legs moving together. The dog may fall over if it tries to turn quickly.

The distinguishing feature is what happens next. As the dog keeps walking, the muscles begin releasing more normally and the gait improves substantially, sometimes to near-normal within a few minutes. This warm-up effect is unusual and is the single most useful clue in the whole condition.

Affected dogs are also often visibly over-muscled, particularly across the neck and shoulders, because muscle that is constantly contracting hypertrophies. Some show a stiff, open-mouthed appearance or difficulty swallowing. Signs are present from a young age rather than developing in middle life, which immediately separates it from the arthritic stiffness Mastiff owners are far more likely to be looking at.

Diagnosis rests on electrical testing of the muscle, which produces a characteristic pattern, alongside DNA testing where a variant is known for the breed in question. It is not diagnosed on appearance alone, and it is not something an owner can confirm at home.

Contrast this with arthritis. An arthritic Mastiff is also stiff on rising, but the stiffness eases gradually over a longer period, worsens after heavy exercise rather than improving, and gets worse in cold weather and across years.

These are the conditions worth working through with a vet when a Mastiff moves stiffly, all of which are more plausible in this breed than myotonia.

If you think your Mastiff has myotonia congenita, the plan is three steps: write down what you have seen and when it started, book a veterinary appointment rather than waiting for the next flare, and take video of the behaviour or symptom before you go — the thing you are worried about rarely happens in the consulting room. Screening in the parents covers hip dysplasia, elbow dysplasia, cancer.

What the vet visit should produce is a diagnosis and a written plan, not just reassurance: what is being ruled out, what the monitoring interval is, and which signs mean you come back sooner. Ask what the treatment costs across a year rather than per visit, because that is the number that decides whether you are managing this condition or reacting to it.

Between appointments, keep a short log — dates, what you saw, what changed. It is the single most useful thing an owner brings to a follow-up, and for myotonia congenita it is often what separates a clear pattern from a guess.