What is Myasthenia Gravis?
Myasthenia gravis (MG) is an autoimmune disorder in which the affected individual’s immune system produces antibodies which attack the neuromuscular junction. The neuromuscular junction is the space between nerve and muscle where chemical signals released from the nerve ending activate muscle contraction. People who have myasthenia gravis experience fluctuating muscle weakness.
MG is relatively uncommon. There are between 1.7 to 10.4 new cases of MG per million in the general population.
What are the causes?
MG is caused by self-produced antibodies directed against various components of the muscle endplate which is in the neuromuscular junction. In most cases, the antibodies attack the acetylcholine receptor (AChR) on the muscle endplate. A less common target for autoimmune attack is the muscle-specific receptor tyrosine kinase (MUSK) on the muscle endplate. About 10-15% of MG sufferers have a thymoma, a tumour of the thymus gland.
What are the symptoms?
The key feature of MG is fatiguable weakness, that is, weakness that worsens with activity and improves with rest. The weakness often fluctuates from day to day or even hour to hour. The muscles affected are often the muscles controlling the eyes and eyelids, the facial muscles, the muscles controlling speech and swallowing, the limb muscles, the axial muscles (which keep us in an erect posture), and the muscles controlling breathing. As such, symptoms include droopy eyelids, double vision, nasal speech, swallowing difficulties such as coughing or throat clearing with food, hoarseness, jaw or tongue fatigue, weakness of eyelid closure or facial movements, limb weakness, head drop, and shortness of breath on exertion or when lying down.
How is it diagnosed?
MG is often suspected on the basis of the patient’s reported symptoms and a consistent physical examination. Investigations done to confirm the clinical diagnosis include blood tests measuring levels of antibodies to AChR or MUSK, electrophysiological tests such as repetitive nerve stimulation or single fibre electromyography, and sometimes bedside tests such as the edrophonium test or the ice pack test.
What are the treatment options?
MG treatment can be divided into treatment of symptoms and treatment to reduce the underlying autoimmune attack. Treatment of symptoms is often with a drug called pyridostigmine. It does not affect disease progression but can improve symptoms.
Treatment of the underlying autoimmune condition can be classified under short term and long term therapies. Short term therapies are used in severe cases to rapidly decrease the concentration of circulating antibodies and bring about quick clinical improvement. Short term treatments are plasma exchange or intravenous immunoglobulin. Long term therapies are oral medications which suppress the immune system. These include steroids, azathioprine, mycophenolate mofetil, cyclosporin and others. Patients with thymoma are usually treated with thymectomy (surgical removal of the thymus).
What is the prognosis?
Most individuals with MG improve significantly with treatment and can lead normal or nearly normal lives. 10-20% of MG sufferers may go into remission—either temporarily or permanently – so that medications can be discontinued. In a few cases, the severe weakness of MG may cause respiratory failure, which requires immediate emergency medical care.