Urticaria

Definition:

Urticaria is characterised by the occurence of pruritic, raised lesions with surrounding erythema that are transient and resolve without scarring.

Angioedema is characterised by swelling involving the lower dermis and subutis. It is also tranisent but generally longer lasting than urticaria. Angioedema occurs in 40 to 50 % of patients with urticaria.

Classification:

It is important to differentiate physical from spontaneous urticaria.

Physical urticaris is provoked by external physical stimuli, usually soon after contact ( except in the case of delayed pressure urticaria ).

• Dermographism - induced by shearing forces on the skin
• Cholinergic urticaria - triggered by changes in core temperatures such as exercise, hot baths and stress.
• Delayed pressure - occurs four to eight hours after exposure to pressure. It typically involves the palm, soles and buttocks.
• Cold urticaria - stimulated by a sudden drop in skin temperature.
• Heat urticaria
• Solar urticaria - due to serum factors acting as IgE dependent photoallergens.

There are usually no serious underlying conditions associated with the physical urticarias, except cold urticaria.

Causes:
For the purpose of classifying aetiology, urticaria is subdivided into acute ( less than 6 weeks ) and chronic.

Acute:
Most acute urticaria has identifiable triggers, but it can be spontaneous which can be:

• IgE mediated

Food allergy
Hymenoptera venom - bee, wasp, ant
Medications

• Complement mediated

Post viral infections

• Direct mast cell degranulation

Radiocontrast media
Opioids
Vancomycin

• Imbalance of arachidonic acid metabolism

NSAIDs
Aspirin

Chronic:
Unlike acute urticaria, chronic urticaria is unlikely to be IgE mediated. An external cause for the conditon cannot be found in at least 80 - 90 % of patients with chronic urticaria.

Conditions associated with chronic urticaria are:
autoimmune thyroid disease
hepatitis B, A

Management:

History taking is the most important tool for identifying the cause for urticaria.

Acute urticaria is self limiting and does not require laboratory investigation.

• Laboratory tests should be tailored individually and requested based on clues in the history.
• Many experts recommend routine FBC, ESR and CRP. Screening for thyroid hormones, thyroid autoantibodies and antinuclear antibodies can be requested if the history is suggestive in chronic urticaria.

Treatment:
Step 1: H1 receptor antagonist
Step 2: If there is no improvement, add an H2 receptor antagonist.
Step 3: If there is no improvement and/or sleep disturbance, consider adding doxepin to the regimen.
Step 4: Referral to immunologist, if there is no response to above or suspected of urticarial vasculitis or chronic angioedema without urticaria.