Linear and whorled nevoid hypermelanosis (LWNH) is a rare sporadic disorder in which multiple linear and swirled streaks of hyperpigmentation appear along lines of Blaschko without a preceding inflammatory stage. The hyperpigmentation typically becomes apparent soon after birth and remains stable, though it may progress for 1-2 years before stabilizing. Although originally described in 1988 in patients with only cutaneous findings, there have been several subsequent cases of LWNH with various systemic anomalies in the neurologic, cardiac, and musculoskeletal systems. The largest case series of patients with LWNH found a 30% incidence of extracutaneous abnormal features, primarily neurologic in nature.

Both the pigmentary and systemic findings are thought due to genetic mosaicism, with mutations in various genes leading to the variety of phenotypes observed. Mosaicism for c-KIT ligand (KITLG) in the hyperpigmented affected skin has been detected in one well-studied case.