Dahl's sign (also Thinker's sign or Target sign)Pavithran K. Thinker's sign. Indian J Dermatol Venereol Leprol 1992;58:266-7
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is a

clinical sign Signs and symptoms are diagnostic indications of an illness, injury, or condition. Signs are objective and externally observable; symptoms are a person's reported subjective experiences. A sign for example may be a higher or lower temperature ...

in which areas of darkened (

hyperpigmentation Hyperpigmentation, also known as the dark spots or circles on the skin, is the darkening of an area of Human skin, skin or nail (anatomy), nails caused by increased melanin. Causes Hyperpigmentation can be caused by sun damage, inflammation, or ...

) and thickened (

hyperkeratotic Hyperkeratosis is thickening of the stratum corneum (the outermost layer of the Epidermis (skin), epidermis, or skin), often associated with the presence of an abnormal quantity of keratin,Kumar, Vinay; Fausto, Nelso; Abbas, Abul (2004) ''Robbins ...

) skin are seen on the lower

thigh In anatomy, the thigh is the area between the hip (pelvis) and the knee. Anatomically, it is part of the lower limb. The single bone in the thigh is called the femur. This bone is very thick and strong (due to the high proportion of bone tissu ...

s and

elbow The elbow is the region between the upper arm and the forearm that surrounds the elbow joint. The elbow includes prominent landmarks such as the olecranon, the cubital fossa (also called the chelidon, or the elbow pit), and the lateral and t ...

s. It occurs in patients with longstanding severe

chronic obstructive pulmonary disease Chronic obstructive pulmonary disease (COPD) is a type of progressive lung disease characterized by chronic respiratory symptoms and airflow limitation. GOLD defines COPD as a heterogeneous lung condition characterized by chronic respiratory s ...

. The sign occurs because patients with COPD tend to sit forwards with their arms resting on their thighs, leading to chronic

erythema Erythema (, ) is redness of the skin or mucous membranes, caused by hyperemia (increased blood flow) in superficial capillaries. It occurs with any skin injury, infection, or inflammation. Examples of erythema not associated with pathology inc ...

of the skin at the points of contact. Over time, haemosiderin released from

red blood cell Red blood cells (RBCs), referred to as erythrocytes (, with -''cyte'' translated as 'cell' in modern usage) in academia and medical publishing, also known as red cells, erythroid cells, and rarely haematids, are the most common type of blood cel ...

s trapped in the skin is released causing a brown discolouration of the skin. Air trapping in the lungs of COPD patients causes the diaphragm to be pushed down and flattened, which reduces the effect of contraction of the diaphragm during inspiration. Sitting forwards pushes the abdominal contents upwards, increasing the curvature of the diaphragm and improving its effectiveness. Bilateral cubital tunnel syndrome may also be found in patients with Dahl's sign.Dale Berg, Katherine Worzala. ''Atlas of adult physical diagnosis'', page 365. Lippincott, Williams & Wilkins, 2006. . Dahl's sign was described by K. V. Dahl in 1970.

References

{{reflist Medical signs