Diagnosing myasthenia gravis

Myasthenia gravis is an autoimmune disorder, with antibodies to the postsynaptic acetylcholine receptor (Ach-R) or (less commonly) the muscle-specific tyrosine kinase (MuSK). Using tests available at the time of writing, antibodies are not detected in some patients. Myasthenia gravis presents most often in females aged 10 to 30 years and males aged 50 to 70 years. To confirm a diagnosis of myasthenia gravis, muscle fatigability must be shown, typically in the ocular, facial, bulbar or limb muscles. Ptosis may improve after applying ice to the eyelid. Myasthenia gravis does not cause sensory loss, sphincter disturbance or loss of reflexes—if these are present, consider an alternative diagnosis and refer for expert advice.

Many patients with ocular symptoms only (ocular myasthenia) are at risk of developing generalised disease within 2 years of onset. This is more likely in patients who have anti–Ach-R antibodies. Observational studies suggest that corticosteroids may reduce progression to generalised myasthenia gravis, or else mask the symptoms of generalised disease—the potential benefit needs to be weighed against the harms of immunosuppression.

When myasthenia gravis is suspected, investigations that can help confirm the diagnosis include:

blood tests for Ach-R antibodies—these are positive in about 85% of patients. If the result is negative, consider testing for MuSK antibodies, especially if the patient has bulbar symptoms
neurophysiological tests—appropriate findings are a decremental response on repetitive stimulation or positive single-fibre electromyography.

Perform thyroid function tests and measure vitamin B₁₂ concentration in all patients, because patients with myasthenia gravis are at higher risk of other autoimmune disorders. Also perform a computed tomography scan of the chest, to check for a thymoma.

If the patient has ocular signs only and serology is negative, consider magnetic resonance imaging of the brain to exclude a structural abnormality.

When the investigations above are equivocal or negative but myasthenia gravis is still suspected, refer for expert advice and consideration of an edrophonium (Tensilon) test—this test should be performed under expert care in a hospital setting with resuscitation facilities.

Many drugs can make myasthenia gravis worse (see advice).