Mononeuritis multiplex with tumefactive cellular infiltration in a patient with reactive lymphoid hyperplasia with increased immunoglobulin G4-positive cells.

We describe a 54-year-old man with mononeuritis multiplex and reactive lymphoid hyperplasia with increased immunoglobulin G4 (IgG4)-positive cells. Asymmetrical numbness and weakness had advanced stepwise for 6 years. Serum immunoglobulin G, IgG4, and immunoglobulin E levels were elevated, whereas M protein was not detected. Chest and abdominal computed tomography showed generalized lymphadenopathy. Inguinal lymph node biopsy revealed expansion of the interfollicular area with infiltration of IgG4-positive cells, of which the absolute number was greater than 100 per high-power field, and the percentage of IgG4+/immunoglobulin G+ plasma cells was 33%. Sural nerve biopsy disclosed axonal neuropathy with tumefactive lymphoid infiltrate in epineurium, but IgG4-positve plasma cells and fibrosis were not detected. Symptoms and laboratory data were improved with oral glucocorticoid therapy at a dose of 0.6 mg/kg per day. Although the causal mechanisms of neuropathy should be determined in future studies, peripheral nerve involvement may occur in patients with reactive lymphoid hyperplasia with increased IgG4-positive cells.