Dahl's sign (also Thinker's sign or Target sign)
[Pavithran K. Thinker's sign. Indian J Dermatol Venereol Leprol 1992;58:266-7]
Full text
is a
clinical sign in which areas of darkened (
hyperpigmentation
Hyperpigmentation is the darkening of an area of skin or nails caused by increased melanin.
Causes
Hyperpigmentation can be caused by sun damage, inflammation, or other skin injuries, including those related to acne vulgaris.James, William; Ber ...
) and thickened (
hyperkeratotic
Hyperkeratosis is thickening of the stratum corneum (the outermost layer of the epidermis, or skin), often associated with the presence of an abnormal quantity of keratin,Kumar, Vinay; Fausto, Nelso; Abbas, Abul (2004) ''Robbins & Cotran Pathologi ...
) skin are seen on the lower
thigh
In human 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 ...
s and
elbow
The elbow is the region between the 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 the me ...
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 long-term respiratory symptoms and airflow limitation. The main symptoms include shortness of breath and a cough, which may or may not produce ...
.
The sign occurs because patients with COPD tend to sit forwards with their arms resting on their thighs, leading to chronic
erythema
Erythema (from the Greek , meaning red) 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 asso ...
of the skin at the points of contact. Over time,
haemosiderin
Hemosiderin image of a kidney viewed under a microscope. The brown areas represent hemosiderin
Hemosiderin or haemosiderin is an iron-storage complex that is composed of partially digested ferritin and lysosomes. The breakdown of heme gives rise ...
released from
red blood cell
Red blood cells (RBCs), also referred to as red cells, red blood corpuscles (in humans or other animals not having nucleus in red blood cells), haematids, erythroid cells or erythrocytes (from Greek ''erythros'' for "red" and ''kytos'' for "holl ...
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
Ulnar nerve entrapment is a condition where the ulnar nerve becomes physically trapped or pinched, resulting in pain, numbness, or weakness, primarily affecting the little finger and ring finger of the hand. Entrapment may occur at any point from ...
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
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Medical signs