Abnormal renal morphology Hairy kidney sign 2025-09-01T10:45:00Z Erdheim-Chester disease is a non-Langerhans cell histiocytosis characterized by the infiltration of foamy histiocytes with fibrosis that can affect many organs, most commonly long bones. Renal involvement commonly manifests in this disease as a histiocytic infiltration into the perirenal fat. A radiological sign related to infiltration of histiocytes into the kidneys and perirenal fat. This infiltration, along with thickening of fibrous bands within the perirenal fat, creates a hairy appearance on cross-sectional imaging studies. Perinephric infiltration