A 76 year old male with recently diagnosed AL amyloidosis treated with CyBorD (cyclophosphamide, bortezomib, dexamethasone) presented with subacute progressive lower extremity proximal weakness resulting in recurrent falls and distal, painful sensory loss. EMG showed findings most consistent with a length-dependent axonal sensorimotor peripheral neuropathy and no evidence of myopathy. Contrast-enhanced lumbar plexus MRI showed diffuse fascicular T2 hyperintensity and enlargement of the lumbosacral plexus without significant enhancement. Laboratory evaluation (CRP, ANCA, ENA, HbA1c, SPEP) and contrast-enhanced lumbar spine MRI were unremarkable.

Sural nerve biopsy revealed multifocal and severely decreased myelinated fiber density, increased axonal degeneration with secondary demyelination, perivascular epineurial inflammatory collections with transmural inflammation and hemosiderin deposition, perineurial thickening, and neovascularization. Vastus lateralis biopsy showed denervation atrophy. Congophilic deposits were absent in both biopsies.

His proximal leg weakness was attributed to a motor-predominant lumbosacral polyradiculoneuropathy or plexopathy due to nerve microvasculitis from the bortezomib, which was discontinued. He was started on weekly IV methylprednisolone with significant improvement at 4-month follow-up.