Clinical Reasoning: A 61-Year-Old Man With Progressive Right Leg Numbness and Weakness.

Approaching patients with lumbosacral plexopathies or radiculoplexus neuropathies can be challenging and a well-defined strategy considering specific etiologies is necessary to arrive at the correct diagnosis. In this case, a 61-year-old man presented with a 10-year history of slowly progressive right lower extremity numbness and weakness. His examination was marked by right lower extremity weakness, decreased temperature/pinprick and proprioceptive/vibratory sensations, hypotonia, muscle atrophy, and absent right patellar, and ankle deep tendon reflexes. His workup was notable for electrodiagnostic findings of chronic lumbosacral radiculoplexus neuropathy and neuroimaging revealing marked enlargement, T2 signal abnormality, and faint contrast enhancement in multiple nerves. A targeted fascicular nerve biopsy yielded the final diagnosis of a rare disease. This case highlights the differential diagnoses of lumbosacral plexopathies or radiculoplexus neuropathies, including a guided workup. We also discuss the typical features of a rare diagnosis and its therapeutic options.