Clinical Reasoning: A 35-Year-Old Man With Recurrent Asymmetric Lumbosacral and Cervical Plexopathy

A previously well 35-year-old right-handed man developed abrupt severe lower back pain radiating to the anteromedial and anterolateral right thigh. The pain began as constant with superimposed lancinating exacerbations and was uniquely provoked by minimal stretch maneuvers, such as straight leg raise beyond 20° or hip extension, leading to functional immobility. Within 2 weeks, he developed prominent proximal right lower limb weakness. Over the following 3 months, his course was characterized by repeated relapses with episodic worsening of pain and new contralateral involvement despite corticosteroids and intravenous immunoglobulin. Examination disclosed marked proximal right lower limb weakness with relative distal sparing, intact sensation, brisk knee jerks with intermittent ankle clonus, and normal paraspinal muscle examination. Electrophysiology showed reduced compound muscle action potential amplitudes in the right femoral distribution with preserved sensory nerve action potentials, abundant fibrillation potentials and reduced recruitment in proximal muscles, and no conduction block or temporal dispersion. Magnetic resonance neurography demonstrated asymmetric fascicular enhancement of lumbosacral plexus components with corresponding neurogenic muscle oedema. The clinical, electrodiagnostic, and imaging features point to a focal, relapsing inflammatory process selectively affecting motor fibers at the plexus level. The report describes the diagnostic reasoning and investigations that distinguished this entity from other painful plexopathies.