Myasthenia gravis is a condition where muscles become easily tired and weak. It is due to a problem with how the nerves stimulate the muscles to contract. The muscles around the eyes are commonlyaffected first. This causes drooping of the eyelid and double vision. Treatment is usually effective.
Myasthenia gravis literally means “grave muscle weakness“. However, myasthenia gravis is not as “grave“ as its namedescribes as most people with this condition lead normal lives as there is effective treatment available.
1) Understanding muscle: Each muscle is supplied by a nerve which splits into smaller nerves that spread along the muscle fibres. There is a tiny gapbetween the ends of the nerves and the surface of the muscle. This gapis called the neuromuscular junction (see diagram).
The brain sends messages down the nerves to the muscles it wants tocontract. The nerve endings release a chemical called aneurotransmitter into the neuromuscular junction. Thisneurotransmitter is called acetylcholine. The acetylcholine quicklyattaches to receptors on the muscles. This in turn triggers the muscleto contract. There are many acetylcholine receptors on each musclefibre.
2) What happens in myasthenia gravis and what causes it: People with myasthenia gravis have a fault in the way nerve messages are passed from the nerves to the muscles. Themuscles are not stimulated properly, so do not contract well and become easily tired and weak.The fault is due to a problemwith the immune system. Myasthenia gravis is an ‘autoimmune disease’. This means that the immune system (which normallyprotects the body from infections) mistakenly attacks itself.
3) What causes the abnormal antibodies to be made: The reason why the body’s immune system starts to make abnormal antibodies against muscle receptors is not known.Abnormal antibodies are made in various parts of the immune system which includes: the bone marrow; blood; lymph glands;thymus gland. However, the thymus gland is thought to be a main source of the abnormal antibodies (see below).
4) Who gets myasthenia gravis: Myasthenia gravis can develop at any age but most commonly affects women under 40 years and men over 60 years. It affectsapproximately 1 in 20,000 people. Women are three times more likely than men to have myasthenia gravis. It is not aninherited condition, although it can occur in more than one member of the same family.
5) Relation of thymus gland with myasthenia gravis: The thymus gland is a small gland at the upper chest just behind the sternum (breastbone). It is part of the immune system.The thymus is abnormal in many people with myasthenia gravis, particularly in those who develop myasthenia gravis before theage of 40. The exact role of the cells in the thymus is not clear but they may have something to do with ‘programming’ ormaking antibodies against acetylcholine receptors. For some people, removal of the thymus gland by an operation cures theirmyasthenia gravis.A small number of people with myasthenia gravis develop a tumour (growth) of the thymus gland called a thymoma. If it occursit is usually benign (non-cancerous) but it is malignant (cancerous) in a very small number of cases.
6) What are the symptoms of myasthenia gravis: The typical main symptom is weakness of muscles that gets worse with activity and improves with rest. In effect, affectedmuscles tire or become fatigued very easily. This means that symptoms are usually worse at the end of the day and afterexercising. Symptoms may come and go at first when using the muscles. The symptoms vary enormously between people withmyasthenia gravis.
• The muscles around the eyes are most commonly affected first as these are constantly used and can quickly tire. Thiscauses drooping of the eyelid (ptosis) and double vision. In some people the muscles around the eyes are the onlyones affected (when the level of abnormal antibody is low). If symptoms only affect the muscles around the eyes forlonger than two years then the condition is unlikely to progress to other muscles. This is known as ocular myastheniaand affects 1 in 6 people with myasthenia gravis.
Muscles around the face and throat are also often affected. Difficulty in swallowing and slurred speech may be thefirst signs of myasthenia gravis.
• Weakness in the arms, hands, fingers, legs and neck may develop.
• Weakness in the chest muscles sometimes occurs. If this is severe a ‘myasthenic crisis’ may result (see below).The severity of symptoms (how easily the muscles tire) can vary from mild to severe. Infection or stress can make symptomsworse.
7) How is myasthenia gravis diagnosed:
• Muscle and nerve tests (NCV, RNS, EMG) is needed in most cases.
• A blood test can detect the abnormal antibody and confirm the diagnosis in most people.
• A scan of the upper chest may be performed to calculate the size and shape of the thymus gland.
• Breathing tests are performed in those people who have weakness of the chest muscles.
8) What is the treatment for myasthenia gravis?Anticholinesterase medicines
These medicines delay the breakdown of acetylcholine when it is released from the nerve endings. More acetylcholine is thenavailable to compete against the abnormal antibodies for the muscle receptors which then improves the strength of themuscles. These medicines work best when the disease is mild and the level of antibody is low. The most commonly prescribedanticholinesterase medicine is called pyridostigmine.
Thymectomy (removal of the thymus)
This is an option in some cases. A thymectomy can improve symptoms in more than 7 in 10 people with myasthenia gravis andmay even cure some.
Steroid medication such as prednisolone tablets is often used in the treatment of myasthenia gravis. Once improved, the doseis commonly reduced gradually to find the lowest dose needed to prevent symptoms. In some people the dose of steroidneeded to control the disease may be quite high and lead to side-effects.
An immunosuppressant medicine such as azothiaprine may be advised in addition to steroid medication. These medicines workby suppressing the immune system.A steroid plus an immunosuppressant tends to work better than either alone. Also, thedose of steroid needed is often less if an immunosuppressant is added which reduces the risk of side effects with steroids.
9) Myasthenic crisis: A myasthenic crisis is rare. This occurs when the muscles that control breathing weaken to the point that breathing becomesvery difficult. Admission to hospital is usually needed and sometimes assisted breathing by a ventilator machine is performedfor a short while. Plasma exchange or IvIg (Intravenous immunoglobin) may be required. This quickly gets rid of the abnormalantibody for a short while. This then gives time for other treatments described above to be started or modified.
10) What is the course of the disease and prognosis (outlook): Treatment usually works well. For most people with myasthenia gravis, treatment significantly improves their muscleweakness. Myasthenia gravis often goes into remission temporarily meaning the medication can be stopped for periods of time.
Compiled in year 2008.This article is for information only and should not be used for the diagnosis or treatmentof medical conditions. We have used all reasonable care in compiling the information but make no warranty as to itsaccuracy. Consult a doctor or other health care professional for diagnosis and treatment of medical conditions.
Dr. Arun K. Dhanuka
MBBS, MD, DM (Neurology)
1799/1, Maharaj Nagar, Near PAU Gate No. 3,Ludhiana (Punjab).Ph: 0161-4662345, 98550-30365, 98147-20265.