About 15% to 20% of the general population experience urticaria at least once in their lifetime. Urticaria is characterized by transient localized skin edema (wheal) with surrounding erythema (flare) and pruritus. The edema in urticaria affects the superficial dermis and individual wheal typically lasts less than 24 hours. On the other hand, angioedema is characterized by ill-defined skin edema with a normal or paler overlying skin as the condition affects the deeper dermis and subcutaneous tissue. Instead of pruritus, pain may be the predominant symptom. In addition, angioedema usually lasts longer than urticaria. However, both angioedema and urticaria share a common pathomechanism of histamine released from mast cells, subsequent increase in vascular permeability of the skin, and stimulation of C-fibers responsible for pruritus. It is not uncommon for both urticaria and angioedema to co-exist in the same patient.
Urticaria and angioedema are broadly classified by the duration of illness. Six weeks is arbitrarily taken as the cut-off for acute versus chronic lesions. This classification is helpful in considering the etiologies of these conditions. Common causes of acute urticaria/angioedema include acute viral illness, drug allergy, food allergy, and hymenoptera stings. Chronic urticaria is characterized by evanescent wheals occurring at least three times a week for more than 6 weeks. Urticaria which occurs less frequently may be better considered as “episodic” urticaria.
Chronic urticaria/angioedema may be spontaneous or triggered by physical factors. Physical urticaria includes dermographism, delayed pressure urticaria, cholinergic or exercise-induced urticaria, cold contact urticaria, solar urticaria, vibratory angioedema, and aquagenic urticaria. Dermographism, delayed pressure urticaria, cholinergic urticaria, and exercise-induced urticaria are among the more common types of physical urticaria. Avoidance of the known physical triggers and the use of H1-antihistamines are needed to bring about control of these conditions.
Chronic spontaneous urticaria, which is previously known as chronic idiopathic urticaria, is the most common subtype of chronic urticaria. As the term implies, there is no observable external stimuli for this form of urticaria. Up to 50% of chronic spontaneous urticaria may be autoreactive in nature. It can be a frustrating disease to both affected patients and their treating physicians due to its apparent lack of causative factor, challenges in control of the disease, its chronicity, and lack of clear prognostic factors. However, studies have shown associations with autoimmune thyroid disorders in about 20% of patients with chronic autoreactive urticaria. Infections have also been reported to have links with chronic spontaneous urticaria, particularly Helicobacter pylori infections, but these links have proven to be inconsistent. There are also studies looking at the role of pseudoallergens in chronic spontaneous urticaria, but a pseudoallergen-free diet may only help in at most one-third of the patients. About 30% of patients with chronic spontaneous urticaria have intolerance to NSAIDs. Conversely, about one-third of patients with NSAID intolerance may develop chronic urticaria.
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