A 43-year-old man, previously healthy, had progressive muscle weakness and fatigability.

The patient is a 43-year-old man who presents with several months of fatigable and fluctuating limb muscle weakness, relatively worse in the later hours of the day. The patient has no cranial symptoms. Examination confirms the presence of proximal limb muscle weakness. The history suggests and the examination confirms the presence of fluctuating and fatigable muscle weakness.

Although MG presents with fluctuating or fatigable weakness, the majority of patients have some degree of cranial/bulbar involvement. Twenty-five percent of patients present with diplopia, 25% with eyelid ptosis, and by 1 month 80% have some degree of ocular involvement. Ten percent of myasthenics present with bulbar symptoms, 10% with lower extremity weakness, and 10% with generalized weakness, whereas respiratory failure is the presenting symptom in 1%.

LEMS is far less common than MG and is associated with proximal limb muscle weakness, particularly in the lower extremities, with a relative paucity of cranial weakness. The absence of dry mouth (and other antimuscarinic, anticholinergic symptoms), as well as the preservation of muscle stretch reflexes, would be against the diagnosis of LEMS in our patient. An acquired myopathy such as polymyositis, dermatomyositis, inclusion body myositis, or thyroid disease are also in the differential, but far less likely to cause the degree of fluctuation in strength described and shown by this patient. Metabolic or genetic disorders such as adult-onset acid maltase deficiency and Desmond myopathy are additional myopathies in the differential.