A 57-year-old man developed numbness and tingling in his toes, which progressed over 3 to 4 months to involve his legs, hands, and lower torso. He then developed trouble walking owing to imbalance and had increasingly frequent falls. His hands felt clumsy, and he began dropping objects. He reported several instances of urinary incontinence in the weeks before presentation, but no bowel symptoms. He denied focal weakness or changes in speech, swallowing, or breathing. His sisters brought him to the emergency room after a fall in which he hit his head on a kitchen stool, and he was admitted to the neurology service for further evaluation.