Clinical Reasoning: An unusual lung mass causing focal weakness

A 30-year-old right-handed man presented to the emergency room with right forearm numbness and elbow pain for 7 months. For the past 4 weeks he could not write or hold anything with his right hand. His primary care physician referred him to the hospital for further workup after noticing thenar atrophy in that hand. He also described decreased sensation over the ventromedial aspect of his right forearm. He denied any neck pain, changes in urinary or bowel habits, or other neurologic complaints. Initial inspection showed significant atrophy of the thenar muscles. Neurologic examination showed mild weakness in the right arm and wrist extension and in the abductor pollicis brevis, all interossei (including the first dorsal interosseus), lumbricals I–IV, and the abductor digiti minimi of the right hand. He had diminished pinprick and temperature sensation over the ventromedial aspect of his right forearm, from the elbow to the fourth and fifth digits, without splitting of the fourth digit. The right biceps reflex was normal, the brachioradialis was diminished, and the triceps reflex could not be elicited. The remainder of the neurologic examination, including mental status, cranial nerves, coordination, and gait, was normal. There was no Horner syndrome on the right. His general physical examination demonstrated a well-built man with no skin lesions, edema, or protrusions. The muscles involved and the sensory distribution implied involvement of multiple nerves, possibly by compression of the lower trunk of the brachial plexus. Subsequently, a chest X-ray was performed by the emergency room physician to screen for a mass lesion compressing the brachial plexus. The X-ray showed a large mass in the apex of the right lung (figure 1). Figure 1 Unusual finding on chest X-ray Chest x-ray shows a round circular mass (large arrow) in the apex of the right lung. ### Questions for consideration: 1. What are the possible etiologies of the mass? 2. What would …