Diagnosis
Due to weakness being a common symptom of many other disorders, the diagnosis of myasthenia gravis is often missed or delayed in people who experience mild weakness or in those individuals whose weakness is restricted to only a few muscles. Generally an individual with potential symptoms of MG would be referred to a neurologist for diagnosis, but sometimes an ophthalmologist or an immunologist will also make the diagnosis.
The doctor investigating the individual should be aware as much as possible about all the symptoms that the patient is experiencing- what symptoms are occurring, the time of day, the circumstances, what brings it on and what relieves the individuals symptoms. The doctor should also be aware of which specific muscles are more affected. If the doctor then suspects myasthenia gravis then further tests are carried out.
A neurological exam can then be performed on the individual who is suspected of myasthenia gravis however the doctor may also look for findings that may indicate an alternative diagnosis. An examination focused on MG will test various general areas: the eyes, speech and swallowing, the neck, and the arms and legs.
- When examining the eyes the Dr may check the eyelids for drooping, limitation of eye movements or double vision.
- Examining the throat the Dr will look at the ability of the individual to talk for a sustained period of time and their capability to swallow and drink safely.
- In the arms and legs the Dr will check for the strength of the reflexes of the muscles especially those around the shoulder and the hip.
The Tensilon test is often used to diagnose myasthenia gravis. Edrophonium chloride (Tensilon) is a drug that temporaily blocks the action of acetylcholinesterase. By blocking the action of acetylcholinesterase, Tensilon prolongs muscle stimulation and temporarily improves strength. Tensilon is administered intravenously and the muscle response is then evaulated. In cases of MG, muscle weakness improves within 1 minute. The Tensilon test is more effective in diagnosing myasthenia gravis when easily observed weakness is present. However the side effects of the test include temporary abnormal heart rhythms.
Another diagnostic test is a blood test for serum antibodies to the AChR, this is not a routine blood te
st. The standard myasthenia gravis blood test is for the detection of serum antibodies to the acetylcholine receptor. AChR antibodies are only detectable in about 50% of patients with myasthenia gravis confined to the eyes, and about 80% of patients with generalised myasthenia gravis- where the disease nots not only affects the eyes but also other muscles of the body. Therefore some patients with myasthenia gravis are missed by the blood test, and as a result it cannot be relied upon to detect all cases of myasthenia gravis. The concentration of AChR antibodies varies amoung sufferers of the disease and the level of the antibodies does not predict the severity of the condition. The levels of AChR antibodies increases as the condition progresses. 20% of generalised myasthenia gravis sufferers have no AChR antibodies detectable in their blood. However about half of these individuals have recently shown to have antibodies to MuSK (muscle specific tyrosine kinase) another protein present at the neuromuscular junction.
Single fibre electromyography (EMG) is used by physicians to detect impaired nerve to muscle transmission. EMG measures the electrical potential of muscle cells when single muscle fibers are stimulated by electrical impulses. Muscle fibers in myasthenia gravis, as well as other neuromuscular disorders, do not respond as well to repeated electrical stimulation compared to muscles from non sufferers. However this diagnostic test is much more difficult for the doctor to perform and often requires a study of up to an hour long. Therefore generally the other tests are performed first, and single fibre electromyography is then used if the other tests reveal a normal result but myasthenia gravis is still suspected.
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