Myasthenia Gravis

Myasthenia Gravis (MG) is a rare muscle disease that is characterised by generalised muscle weakness. All the body's muscles are controlled by their own individual nerves. An electrical impulse travels down the nerve causing the release of a chemical called acetylcholine (ACT). This flows across the neuromuscular junction and fixes itself to a specific receptor on the muscle causing it to contract. In MG it is this transmission message between nerve and muscle that gets interrupted. If muscles are unable to contract, they become weak.

MG can be characterised by weakness of the musculoskeletal muscles leaving dogs unable to stand or exercise normally. Some dogs will demonstrate weakness of other muscles in the body such as the oesophagus, so their main symptoms could be problems swallowing or regurgitation of food.

There are 2 forms of MG. A congenital form that dogs are born with, or a more common acquired form that develops during the dog’s lifetime. Dogs with the congenital form are born with too few ACT receptors of their muscles, so when the ACT is released from the nerves it has nowhere to latch onto to trigger muscle contraction. Acquired MG is an autoimmune disease which means the body forms antibodies against the acetylcholine receptors. These antibodies treat the ACT receptors as if they were a foreign body and attach and destroy the receptors. It is not known why the immune system suddenly becomes faulty, but it is sometimes seen in dogs with cancer or other autoimmune diseases such as hypothyroidism. As the number of receptors reduces, the ACT can’t fix itself to the muscle to trigger contraction resulting in muscle weakness.

The most common symptom is severe weakness after only a short period of exercise. It can affect the hindlegs only or all 4 limbs, it is often preceded by a stiff gait and muscle tremors. The animal recovers after a short rest and can become active again, however the collapsing symptoms soon return once exercise is resumed. Other symptoms include excessive drooling, problems swallowing or regurgitation if the throat or oesophageal muscles are mostly affected.

If your vet suspects MG, it can be confirmed by a blood test which looks for the ACT receptor antibodies. Your vet would probably take routine blood tests first, to rule out other metabolic problems that could cause problems with muscle contraction. For congenital MG a diagnosis is made by a muscle biopsy.

Treatment is with immunosuppressive drugs to stop the destruction of ACT receptors in the acquired form, along with a drug to improve the transfer of the signal from nerve to muscle. Prognosis is good for a complete recovery as long as there are no other complicating factors such as inhalation pneumonia (from the regurgitation of food) or a cancer which has triggered the faulty immune system. The response to treatment is monitored by measuring the level of antibodies against the ACT receptors.