Progressive hemifacial atrophy (PHA), also known as Parry-Romberg syndrome, is characterized by slowly progressive deterioration of the skin and soft tissues on one side of the face. It sometimes occurs on both sides of the face and occasionally involves the arm, trunk, and/or leg. The condition may worsen for many years and then stabilize. The severity varies greatly. While the cause is not well understood, it may differ among affected people.