Antibodies to the acetylcholine receptor are found in 85 percent of people with generalised myasthenia gravis, and 50 percent of patients with ocular myasthenia gravis. These are detected by a blood test.
Electromyography (EMG) is performed by a specialist doctor and involves measuring the electrical response in the muscle with a very fine needle. An electrical stimulus is applied to a nerve and the response in the muscle is recorded. It is a very sensitive test, showing an abnormality in most cases, but is not available at all hospitals.
Tensilon® test, an injection of edrophonium is given, which results in a rapid but shortlived improvement in symptoms in many patients.
Chest scan should be done to check whether the thymus is abnormal as many patients with myasthenia gravis have an enlarged thymus, and some have a benign tumour.
What will the doctor do?
As myasthenia gravis is rare, the general practitioner (GP) usually refers the patient to a specialist neurologist for further assessment and tests, and for initiation of treatment. Once the diagnosis has been made, the GP has a very important role in prescribing and monitoring the medication.
What else could it be?
The key feature that differentiates myasthenia gravis from many other conditions is the fatigable character of muscle weakness and fluctuating nature of the symptoms. Conditions affecting the muscles themselves (e.g. mitochondrial cytopathy or muscular dystrophies) can cause several of the symptoms seen in myasthenia gravis, and the eye symptoms can present in-patients with thyroid disease. There are other even rarer conditions that can be confused with myasthenia gravis and they are described briefly below.
What other conditions can be mistaken for myasthenia gravis?
Myasthenia gravis is the most common of the neuromuscular junction conditions, but about three in 20 people presenting with symptoms of myasthenia gravis will not have antibodies to the acetylcholine receptor. Some of these will have antibodies to another muscle protein, called MuSK. MuSK antibody myasthenia is treated in much the same way as the usual form of myasthenia gravis, although thymectomy may not be needed.
Some will have the Lambert Eaton myasthenic syndrome. With this condition, people are also weak and fatigue easily, but eye symptoms are less common. About half are smokers and a particular type of lung cancer may be found.
Antibodies to another neuromuscular junction protein, the voltage-gated calcium channel, cause the condition. Treatment is similar to that for myasthenia gravis but thymectomy is not performed. There are also rare congenital myasthenic syndromes, caused by hereditary gene mutations in the acetylcholine receptor. Antibodies do not cause these and thymectomy and immunosuppressive drugs are not used.
Can women with myasthenia gravis have babies?
Many women with myasthenia gravis develop the condition as teenagers and it is well controlled before they want to have children. The symptoms sometimes get worse during pregnancy but equally often get better. Sometimes the baby is born with a transient form of myasthenia gravis, owing to the transfer of antibodies across the placenta, but these symptoms respond well to treatment and usually disappear within days or weeks and cause no permanent disability. On very rare occasions, where a mother is untreated, she may give birth to a baby with severe symptoms, (including joint deformities) requiring intensive care. This has never been reported in a mother who has been diagnosed and adequately treated for myasthenia gravis.