Diagnosis

The diagnosis of myasthenia gravis is made after the sudden or gradual onset of specific symptoms and after diagnostic testing. During the physical examination, the doctor obtains a complete medical history, and may also ask if there is a family history of any medical problems.​



Patients who experience undue weakness which improves with rest have the hallmark signs of myasthenia gravis (MG) - it is a key sign. A GP (general practitioner, primary care physician) may also suspect MG if the patient has impaired eye movements or muscle weakness which is not accompanied by the loss of the ability to feel things. Nevertheless, as MG shares signs and symptoms with some other conditions, the diagnosis will need to be confirmed.



MG patients whose muscles are weak respond well to the application of ice to the affected area. Some doctors may try this initially as they gather data to help them make a diagnosis.



If the patient sees a GP who suspects possible MG, he/she will most likely be referred to a neurologist (a doctor specialized in the human nervous system) to confirm diagnosis. The following diagnostic tests will probably be ordered:



Edrophonium test 

This test is usually only ordered when other tests have not yet yielded a conclusive diagnosis. Edrophonium chloride (Tensilon, Reversol) or neostigmine (Prostigmin) is injected into a vein - the drug clocks the breakdown of acetylcholine by cholinesterase (cholinesterase inhibitors) and temporarily increases the levels of acetylcholine at the neuromuscular junction - put simply, edrophonium bocks an enzyme that breaks down acetylcholine, the chemical that transmits signals from the nerve ending to the muscle receptor sites. Some patients may experience a brief period in which muscle weakness is relieved, especially those with weakness in the eye muscles.



Blood tests

The aim here is to identify certain antibodies:
One test is for antibodies against the acetylcholine receptor. This test has a reasonable sensitivity of 80% to 90%. However, it may be as low as 50% in ocular myasthenia patients (those with just eye muscle weakness).
Some MG patients without antibodies against the acetylcholine receptor have antibodies against the MuSK protein. A blood test may detect those antibodies.
In some rare cases, when the doctor may want to rule out Lambert-Eaton syndrome, the blood test will attempt to detect antibodies against a voltage-gated calcium channel.

Repetitive nerve stimulation

Electrodes are attached to the skin over the affected muscles. Small electrical pulses are sent through the electrodes to measure how well the nerves send a signal to the muscle. The electrical pulses will be applied several times to determine whether signals get worse when the muscle is tired.



Single-fiber electromyography (EMG) 

This measures the electrical activity that flows between the brain and the muscle. A very fine wire electrode is inserted through the skin, into the muscle. Some patients may find this test uncomfortable. In a more precise version of this test, called single-fiber EMG, a single muscle fiber is tested.



Imaging scans
A chest X-ray is commonly performed, as it may point towards alternative diagnoses, such as Lambert-Eaton due to a lung tumor, and comorbidity (the presence of one or more disorders/diseases in addition to a primary disease/disorder, or the effect of such additional disorders or diseases).
A CT (computed tomography) or MRI (magnetic resonance imaging) scan may be performed to identify a thymoma (tumor in the thymus gland). CT or MRI scans are better at detecting thymomas than X-rays.

Pulmonary function test (spirometry) 

The aim here is to determine whether the patient is breathing adequately. The forced vital capacity (the maximum amount of air a person can expel from the lungs after a maximum inspiration) may be periodically measured so as not to miss a gradual worsening of muscular weakness in the lungs. MG patients with severe symptoms are at risk of respiratory failure due to exhaustion of the respiratory muscles. Respiratory failure is when there is inadequate gas exchange by the respiratory system, with the result that arterial oxygen and/or carbon dioxide levels cannot be maintained within their normal ranges.



Muscle biopsy

This is only done if the diagnosis is in doubt and a muscular condition is suspected. A needle or small incision is used to remove a small sample of muscle. The patient will receive a local anesthetic.