The Case of the Hairy Kidneys

Introduction: Erdheim-Chester disease (ECD) is a multi-organ infiltrative disease due to to non-Langerhans histiocytosis, causing mostly bone and skin lesions. Kidney involvement occurs in 30% of patients. Case Description: A 54-year-old male with hypertension experienced flank pain and was diagnosed with kidney stones 4 years prior. He had sclerotic bone lesions on imaging and biopsies demonstrated non-specific myelofibrosis. More recently, he had worsening flank pain and renal insufficiency. By computed tomography, extensive bilateral perinephric edema and fat stranding was seen, identified as "hairy kidney sign”, a finding pathognomonic for ECD. Kidney biopsy was performed to establish diagnosis and perform genetic sequencing and demonstrated macrophage-predominant histiocytic infiltrate. Tissue was BRAF negative by immunohistochemistry, but BRAF positive by molecular analysis. The patient was diagnosed with ECD based on his clinical, imaging, and biopsy findings and began MAPK/extracellular signal regulated kinase (MEK) inhibitor therapy. Discussion: Fewer than 1,000 cases of ECD are reported. Somatic mutations of BRAF or other MAPK signaling pathway components result in myeloid progenitor cell proliferation. Kidney involvement presents as reduced glomerular filtration rate, perinephric stranding, hydronephrosis, renovascular hypertension, or flank pain with infiltration and enlarged kidneys. Tissue biopsy (often skin or kidney) establishes diagnosis and identifies mutations for targeted therapy.CT scan with extensive bilateral perinephric stranding