© 2019 Massachusetts Medical Society. A 22-year-old woman with a history of sickle cell disease and migraine headaches presented to the emergency department with weakness that had caused her to fall that day. She reported a gradual progression of weakness over a period of 6 months, which had led in the previous few weeks to difficulty standing, climbing steps, and lifting her child. Her weakness was symmetric in both her arms and her legs, and she had hand stiffness and dull, aching pain in both thighs. She reported no cognitive impairment, paresthesia, numbness, or urinary incontinence.