Clinical reasoning: a 55-year-old man with weight loss, ataxia, and foot drop.

A 55-year-old man with prior alcohol abuse and an 80 pack-year smoking history was referred for evaluation of a 3-month history of subacute-onset, progressively worsening imbalance without back pain. He began using a cane to ambulate after multiple falls. He also described recent right foot weakness, numbness in his feet and fingertips, and unintentional 25-pound weight loss over the past year. His medical history was significant for hypertension, gastroesophageal reflux disease, diverticulitis, and pelvic abscesses. A paternal grandfather had lung cancer. He reported a remote history of IV drug use. General examination revealed cachexia. Neurologic examination findings were complex. Gait examination revealed severe ataxia, a high steppage gait on the right, and a positive Romberg sign. The total ataxia score using the Scale for Assessment and Rating of Ataxia (higher scores indicate increased severity) was 14/40, including gait, 5/8; stance, 4/6; sitting, 1/4; speech disturbance, 0/4; finger chase, 0/4; nose-finger test, 0/4; fast alternating hand movements, 2/4; and heel-shin slide, 2/4. Nystagmus was not present. Strength testing revealed hip and knee flexion weakness bilaterally (grade 4/5) and severe (grade 2/5) weakness of right ankle dorsiflexion and eversion but preserved inversion strength. Reflexes were brisk in the upper extremities and normal in the lower extremities and plantar responses were flexor. Sensory testing revealed absent lower extremity vibration, absent joint position at the toes, and reduced pinprick in the feet without a sensory level. Initial laboratory testing revealed a hemoglobin of 9.3 g/dL (normal range 13.5–17.5).