Dermographism

The term dermographism (or dermatographism) literally means writing on the skin. Dermographism is a common form of chronic hives called physical urticaria, which gets its name because the hives are caused by a certain stimulus on the body. Firm stroking of the skin produces:

• an initial red line (capillary dilatation),
• an axon-reflex flare with broadening erythema (arteriolar dilatation)
• formation of a linear wheal (transudation of fluid /edema); 

=> these events are collectively termed the triple response of Lewis(due to release of histamine) .

An exaggerated response to this constitutional whealing tendency is seen in approximately 2-5% of the population and is referred to as dermographism. In a minority of people, it is accompanied by itching (symptomatic dermographism).
   
Diagnosis
Dermographism is diagnosed clinically based on the typical appearance of the rash. To make the diagnosis, the doctor may take a tongue blade and draw it over the skin of the arm or back to cause a
 hive.