Locks, Keys and Muscular Weakness
In this article, Neurology Expert Laurent Garosi explains an important cause of muscular weakness and collapse in dogs…Myasthenia Gravis.
What is myasthenia gravis?
Myasthenia Gravis (pronounced my-as-theen-ia grar-viss) is a common cause of generalised weakness in dogs and occasionally cats. It also occurs in humans.
What happens in myasthenia gravis?
Nerves work like small electrical cables. They pass an electrical current into the muscles to make them contract or shorten. The nerve doesn’t directly stimulate the muscles to contract; instead it releases a chemical into a small gap that exists between the end of the nerve and the outer lining of the muscle. This gap is called the neuromuscular junction. The chemical that is released is called a neurotransmitter and in most of our muscles this neurotransmitter is a chemical called acetylcholine. It is packaged into tiny pockets at the end of the nerve and it is released when a nerve impulse comes along.
In a fraction of a second the acetylcholine spurts out from the end of the nerve and sticks to the outer membrane of the muscle. This allows a small electrical current to flow into the muscle and the muscle contracts. To create this effect the acetylcholine must attach to a specific receptor on the muscle surface. This can be likened to a lock and a key opening a door. The lock is the receptor and the key is the acetylcholine. The two fit together perfectly and a microscopic door in the muscle wall then opens to allow the electrical current into the muscle cell. It has to be an exact fit so that the muscle only contracts when the nerve ‘tells’ it to.
In myasthenia gravis this chemical transmission across the neuromuscular junction becomes faulty and if the muscles are unable to contract properly they then become weak. Muscle weakness can affect the limbs so that animals are unable to stand or exercise normally but can also affect other muscles in the body. The muscles of the oesophagus (the pipe carrying food from the mouth to the stomach) are often weak in dogs with myasthenia gravis and this means that affected animals may have problems swallowing and often bring back food after eating.
What causes myasthenia gravis in dogs?
Some animals are born with too few acetylcholine receptors on the surface of their muscles i.e. it is a congenital disease. More commonly however, myasthenia gravis is an acquired condition that comes on later in life and here it is caused by a fault in the animal’s immune-system.
Antibodies usually attack foreign material such as bacteria and viruses. In acquired myasthenia gravis, the immune system produces antibodies which attack and destroy the animal’s own acetylcholine receptors. No-one really knows why the immune system should suddenly decide to attack these receptors in some dogs. In rare cases, it can be triggered by cancer.
Whatever the reason, when the number of receptors is reduced, acetylcholine cannot fix itself to the muscle to produce normal muscle contractions and muscle weakness results.
How do I know if my pet has myasthenia gravis?
The typical picture is severe weakness after only a few minutes of exercise. This weakness might affect all four legs or only affect the back legs. It is frequently preceded by a short stride, stiff gait with muscle tremors. The patient eventually has to lie down. After a short rest they regain their strength and can be active again for a brief period before the exercise-induced weakness returns. Other signs of myasthenia are related to effects on the muscles in the throat and include regurgitation of food and water, excessive drooling, difficulty swallowing, laboured breathing and voice change. In the most severe form, the animal can be totally floppy and unable to support its weight or hold its head up.
Which dog and cat breeds are most commonly affected?
The acquired form is seen most commonly in Akitas, terrier breeds, German Shorthaired Pointers, German Shepherd Dogs and Golden Retrievers. In cats it is Abyssinians and a close relative, Somalis. The rare congenital form has been described in Jack Russell Terriers, Springer Spaniels and Smooth-haired Fox Terriers.
What conditions can it be confused with?
Diseases of the muscles (a myopathy) or nerve disease (a neuropathy) can mimic signs of myasthenia gravis and should be considered in the diagnosis.
How is myasthenia gravis diagnosed?
Your vet will often be very suspicious based on the clinical signs and medical history alone. A blood test can be performed to detect antibodies directed toward the acetylcholine receptor. Another test that is sometimes performed is known as the ‘Tensilon test’. Here the patient is given a short-acting antidote that boosts the effects of acetylcholine on the muscle. It is injected into a vein and in patients that have myasthenia gravis there will be a dramatic increase in muscle strength immediately after injection and collapsed animals may get up and run about. However the effects wear off after a few minutes.
In specialist centres an electromyogram (EMG) may be performed. An EMG machine can be used to deliver a small electrical stimulation to an individual nerve or muscle in an anaesthetised animal. Using an EMG machine we can measure how well the muscles respond to stimulation.
The rare congenital form of myasthenia is usually diagnosed by special analysis of a muscle biopsy.
Chest X-Rays will also be performed to look for cancer in the chest cavity which can trigger myasthenia gravis. The chest x rays will also help to evaluate possible involvement of the oesophagus and to detect pneumonia secondary to inhalation of food.
Can myasthenia gravis be treated?
Specific treatment of myasthenia gravis is based on giving a long-acting form of a drug that works like the Tensilon in the test described above. These chemicals boost the effects of acetylcholine on the muscle cells and improve the transfer of the electrical signal from the nerves to the muscles. It may also be necessary to give drugs that will suppress the immune system to stop it attacking the acetylcholine receptors. This will make the patient more susceptible to infections and so they would require careful monitoring whilst on immunosuppressive drugs, particularly with the increased risks of pneumonia is myasthenia patients.
What is the outlook for dogs and cats with myasthenia gravis?
The outlook is generally good unless the patient develops a secondary severe pneumonia due to the inhalation of food material. Treatment usually lasts many months and your vet will need to re-examine your pet on a regular basis to check that they are improving. Repeated blood tests to measure the levels of the antibodies against the acetylcholine receptors will also be required.
Myasthenia gravis is a serious and very debilitating disease. However, with an early diagnosis and a high level of care the symptoms may be controlled so that your pet has an active life. Some patients may make a full recovery.
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