Frontal fibrosing alopecia sparing a vascular naevus: the Renbök phenomenon.

The Renbök phenomenon (or Inverse Köbner phenomenon) describes how the emergence of one skin condition inhibits another skin condition. The term was first coined in 1991 by Happle et al. to describe retained hair growth localised to psoriatic plaques in a patient with coexisting psoriasis and alopecia areata (AA). Subsequently, cases of AA sparing congenital melanocytic naevi and naevus flammeus have also been described.