Urticaria

Urticaria (Hives)
المرحلة الخامسة الاستاذ د. محمد الحطاب
It is a vascular reaction of the skin characterized by the appearance of wheals, white or red evanescent plaques, generally surrounded by a red halo or flare and associated with severe itching ,stinging or pricking sensations. These wheals are caused by localized oedema .Clearing of the central region may occur ,producing an annular pattern .Their size can vary from 0.5 to more than 10 cm .The eruption usually favors the covered areas such as the trunk ,buttocks, or chest.
Subcutaneous swellings (angioedema) especially of the eyelids or lips may accompany the wheals (50% of urticaria patients) or occur alone .Angioedema may target the gastrointestinal and respiratory tracts ,resulting in abdominal pain ,coryza, asthma and respiratory problems .Respiratory tract involvement can produce airway obstruction .Anaphylaxis and hypotension may also occur.
Pathophysiology of urticaria :
The main chemical substance responsible for urticarial wheal is
histamine which is usually released by mast cells.
When inject histamine i.d to the skin, this will lead to triple response of Leuis : there will be local erythema at the site of injection due to vasodilatation of blood vessels by effect of histamine ,then flare :erythema beyond local erythema due to axonal reflex ,then wheal: developed due to extravasations of fluid into extravascular spaces . Direct trauma to the skin results in degranulation of mast cells and release of histamine.