An Unusual Case of Chylous Ascites.

Chylous ascites occur when the lymphatic flow is blocked or disrupted, causing a leakage of fluid into the peritoneal space. It can be caused by a number of etiologies and identifying the exact cause can be challenging. We present the case of a 77-year-old man who presented with chylous ascites. An abdominal computed tomography scan revealed an abnormal spiculated mass and lymph nodes in the central mesentery encasing the pancreas, raising suspicion of pancreatic malignancy. A subsequent mesenteric biopsy indicated sclerosing mesenteritis characterized by dense fibrous tissue and positive Immunoglobulin G4 (IgG4) staining, leading to a final diagnosis of IgG4-related disease (IgG4-RD). The patient responded positively to a treatment regimen that included systemic steroids and rituximab. This case, with its atypical presentation of IgG4-RD, contributes to our understanding of this rare disease, providing valuable insights for future diagnosis and treatment.