Hairy kidney sign (Erdheim-Chester disease)

The hairy kidney sign refers to the soft tissue rind of perirenal infiltration seen on cross-sectional imaging studies in Erdheim-Chester disease, and is considered to be pathognomonic of this disease. The ‘‘hairy’’ description refers to the associated thickening of the bridging perirenal septa (Kunin septa) which are bands of fibrous tissue that extend between the renal capsule and the perirenal fascia .