History central to diagnosing myasthenia gravis

Myasthenia gravis (MG) is caused by failure of chemical transmission at the neuromuscular junction. It is an autoimmune disorder in which antibodies interfere with neuromuscular transmission. It has a prevalence of around 20 per 100,000. The incidence is bimodal with a 2:1 female to male ratio in the younger population and a reversed sex ratio over the age of 60. Around 15% of cases are associated with a thymoma. MG presents with fatiguable painless muscle weakness. Diplopia and ptosis are the most common presenting features. Around 80% of patients presenting with ocular MG will subsequently develop more generalised weakness. Respiratory muscle weakness is the most serious manifestation of MG and can be fatal. A detailed history is the most valuable tool in the diagnosis of MG. This should elicit the pattern of weakness, severity and diurnal variation. Exacerbating factors including extremes of weather, emotional stress, menstruation and intercurrent illness should be enquired about. No one diagnostic test is 100% sensitive and patients who have negative antibodies and normal neurophysiology may still have MG. Treatment should be directed at ameliorating weakness with acetylcholinesterase blockers and modulating the immune system. Pyridostigmine is the most widely used acetylcholinesterase inhibitor. Most patients with generalised MG require immunomodulatory therapy and prednisolone is generally used as the first-line agent. Despite the availability of symptomatic and immunomodulatory treatment, up to 20% of patients will experience a myasthenic crisis requiring admission for ventilatory support at some stage.