High-grade B-cell lymphoma masquerading as brachial plexopathy: a diagnostic challenge.

Neurolymphomatosis, resulting from direct infiltration of the peripheral nervous system by lymphoma, is a rare presentation of aggressive B-cell lymphoma and an uncommon cause of brachial plexopathy. We report a 32-year-old man who presented with a painful swelling around the right shoulder followed by rapidly progressive weakness, complete sensory loss, and marked muscle wasting confined to the right upper limb, associated with a 10-kg unintentional weight loss over five months. Neurological examination localized the lesion to the right brachial plexus. Radiographs and computed tomography demonstrated an infiltrative soft-tissue lesion with cortical destruction of the proximal humerus. Magnetic resonance imaging of the shoulder and brachial plexus revealed diffuse thickening and T2/STIR hyperintensity of the distal divisions, cords, and branches of the brachial plexus, with contiguous infiltration of the humeral marrow, surrounding musculature, and adjacent soft tissues, strongly suggesting lymphomatous infiltration. Subsequent bone marrow biopsy confirmed Stage IV high-grade B-cell lymphoma with a high proliferative index. The patient was treated with R-CHOP chemoimmunotherapy and demonstrated significant clinical and neurological improvement on follow-up, with marked reduction in shoulder swelling and substantial recovery of motor and sensory function. This case highlights the importance of considering neurolymphomatosis in patients presenting with rapidly progressive painful brachial plexopathy, particularly when associated with shoulder swelling, constitutional symptoms, and infiltrative imaging findings. Early clinicoradiological recognition and prompt tissue diagnosis are essential for timely initiation of lymphoma-directed therapy and favorable neurological outcomes.