Case Report: Neurolymphomatosis of the sciatic nerve as a presentation of relapsed diffuse large B-cell lymphoma

Neurolymphomatosis (NL) is an uncommon manifestation of lymphoma caused by direct infiltration of the peripheral nervous system by lymphoma cells. Early recognition is particularly difficult when deep nerves such as the sciatic nerve are involved. Combining contrast-enhanced MRI and PET/CT to assess anatomic extent and metabolic activity, together with nerve ultrasound (US) to identify characteristic signs and guide biopsy, may improve diagnostic accuracy.

A 71-year-old man with systemic diffuse large B-cell lymphoma (DLBCL), histologically confirmed by gastric biopsy, developed progressive left-leg neuropathic pain, numbness, and foot drop during rituximab maintenance after achieving complete remission (CR). Common peroneal nerve decompression yielded only transient pain relief. Nerve US subsequently showed marked thickening of the sciatic nerve and its branches, with a surrounding hypoechoic rim forming the characteristic "fried egg sign," together with increased perineural echogenicity. MRI showed abnormalities along the sciatic nerve course, and whole-body ^18F-FDG PET/CT demonstrated metabolically active disease along the sciatic nerve course and defined the overall extent of involvement. Ultrasound-guided core-needle biopsy (US-CNB) confirmed NL, and immunohistochemistry demonstrated CD20 positivity, supporting B-cell lymphoma involvement. Given the patient's age and risk of neurotoxicity, treatment with cyclophosphamide preconditioning followed by glofitamab plus obinutuzumab achieved rapid clinical improvement and partial remission (PR) on imaging. At 3 months, MRI showed interval reduction of the lesion, and follow-up US demonstrated resolution of the hypoechoic rim with decreased diameters of the sciatic, tibial, and common peroneal nerves.

NL should be considered in patients with otherwise unexplained, progressive lower-limb neuropathy and a current or prior history of lymphoma. In this case, nerve US played a central role by demonstrating the characteristic "fried egg sign," localizing the lesion, and enabling US-CNB for tissue confirmation, while contrast-enhanced MRI and whole-body ^18F-FDG PET/CT provided complementary information on neural involvement and overall disease extent. An ultrasound-centered multimodal approach may help shorten time to diagnosis and facilitate timely treatment and neurological recovery.