Congenital onychodysplasia of the index finger, also known as Iso-Kikuchi syndrome, is a disorder characterized by a variety of nail deformities of the index fingers and bone abnormalities. Nail abnormalities include micronychia, "rolled" micronychia, polyonychia, hemionychogryphosis, malalignment, anonychia, and onychoheterotopia. The most common bone abnormality is a Y-shaped bifurcation of the distal phalanx, which may occur under both normal and abnormal nails. The defects occur most commonly on the radial side of the index fingers.