AL amyloidosis presenting with isolated lumbosacral radiculoplexus neuropathy

A 45-year-old man presented with an isolated sciatic mononeuropathy, which then evolved into a lumbosacral radiculoplexus neuropathy. His initial symptoms included lower limb pain, sensory disturbance and later weakness, without autonomic dysfunction. Neurophysiology suggested a postganglionic neuropathy. MR and ultrasound scans of the thighs showed right sciatic nerve thickening, and CSF analysis showed albuminocytological dissociation. Fluorodeoxyglucose positron emission tomography (FDG PET) was unremarkable. He then developed orthostatic symptoms and urinary disturbance, and was found to have an IgM paraprotein. Fat aspirate, cardiac and whole-body imaging found no amyloid deposition, and genetic testing for transthyretin amyloidosis was negative. A bone marrow biopsy was unremarkable. However, neuropathology review of a proximal, fascicular nerve biopsy identified a lambda chain-restricted plasma cell population with positive Congo red staining, leading to a diagnosis of peripheral nerve restricted amyloid light amyloidosis. We discuss the diagnostic approach to this case from the perspectives of neurology, neurophysiology, radiology and neuropathology.