Clinical Reasoning: A case of bilateral foot drop in a 74-year-old man

A 74-year-old man with no relevant medical history presented with 5 years of slowly progressive bilateral foot drop. He had used ankle foot orthoses for 3 years prior to presentation. There was no report of upper extremity weakness, numbness, paresthesia, myalgias, muscle cramps, or stiffness. He had attained age-appropriate developmental milestones as a child and was athletic, keeping up with his peers. His mother had bilateral foot drop, ankle contractures, and difficulty with ambulation. His brother, 2 daughters, and a son lacked similar symptoms. Neurologic examination showed bilateral distal lower extremity weakness (Medical Research Council grade 0/5 ankle dorsiflexion and eversion and grade 4/5 ankle plantar flexion and inversion), symmetric distal leg and intrinsic foot muscle wasting, absent patellar and Achilles tendon reflexes, and a steppage gait. Tone was reduced at bilateral ankles. Sensory examination including light touch, pinprick, vibration, and proprioception was intact. Mild bilateral pes cavus and hammertoes were noted. The remainder of the neurologic and physical examination was normal.