Urticaria is common in children: 15 to 20% have at least one episode by adolescence. It is a result of transient extravasation of plasma into the dermis, which causes changes in the skin. Involvement in the upper dermis results in wheal formation, and localised vasodilatation produces redness.
- Allergy testing with skin prick testing or radioallergosorbent testing (RAST) is helpful only if there is strong suspicion of immediate urticaria.
- Urticaria can have varying appearances, including wheals and erythema with circles or polycyclic borders on the trunk, limbs or face.
- Persistent purple discolouration or purpuric centres may be a vasculitis.
- The physical urticarias (triggered by changes with pressure, heat, water, cold, sunlight or exercise) are non-IgE mediated.
- The commonest form of childhood urticaria is acute, self-limiting and non-IgE mediated and is usually due to viral infections with or without drug intake.
- Urticaria associated with atopic dermatitis and angio-oedema of the lips may be due to foods.