Clinical reasoning: a 56-year-old man with progressive spasticity.

A 56-year-old man presented with progressive neurologic symptoms that developed more than 14 years ago. His symptoms started with numbness in the toes, followed by slowly progressive lower extremity weakness, increase in tone, and imbalance. He also developed bladder and bowel urgency, constipation, and erectile dysfunction. He did not have any symptoms in his arms.