Dahl’s sign also known as Thinker’s sign or Target sign.
A clinical sign in which areas of hyperpigmentation are seen on the skin of the lower thighs and elbows.
It occurs in patients with longstanding severe chronic obstructive pulmonary disease.
Due to the fact patients with COPD tend to sit forwards with their arms resting on their thighs, leading to chronic erythema of the skin at the points of contact.
Over time, hemosiderin released from red blood cells trapped in the skin is released causing a brown discoloration of the skin.