Physical urticaria is a subcategory of chronic urticaria induced by exogenous physical stimuli. In all types of physical urticarias, areas of the skin subjected to the stimulus react, forming wheals and/or angioedema. Typically, reactions are limited to the areas of exposure; however, some patients may develop a more generalized response. Pathogenesis is based on mast cell degranulation. The exact method of triggering of this pathophysiologic response varies between subtype.
The types of physical urticarial include the following (refer to individual subtypes for more detailed information and images):
Dermographism (also known as dermatographism or urticaria factitia) – Occurs at sites of rubbing, scratching, stroking, or any form of firm friction to the skin. Wheals form in the shape or pattern of the inciting trigger. This phenomenon can be elicited by stroking the skin with a clean, firm object, such as a tongue depressor or a retracted pen tip. Wheals develop within seconds to minutes after the stimulus.
Pressure urticaria – Occurs after exposure to sustained pressure to the skin, such as tight clothing, walking (sustained pressure to the plantar surfaces of the feet), and sitting down (sustained pressure on the buttocks). Symptoms include burning, pain, wheals, and/or angioedema. Can confirm with testing by suspension of a weight over the shoulder or thigh. Subtypes include immediate pressure urticaria, which develops in minutes of the stimulus, and delayed pressure urticaria, which develops in 4-8 hours of the stimulus. Some patients experience systemic symptoms, such as malaise and arthralgias.
Vibratory urticaria – Occurs after exposure to vibratory stimulus, such as operating tools, jogging, or riding a motorcycle or lawnmower. Typically, wheals are not seen but rather swelling, erythema, and pruritus. Can be confirmed by observing response after exposure to laboratory vortex mixer / shaker.
Heat urticaria – Rare; wheals occur after localized exposure to heat. Can confirm by using a test tube or other cylindrical object filled with 45°C-50°C water as a stimulus to the skin for 5 minutes.
Cold urticaria – Occurs after rewarming post-exposure to cold air, liquid, or objects. Can lead to generalized systemic involvement and even anaphylaxis. Subtypes include primary cold contact urticaria, secondary cold contact urticaria, reflex cold urticaria, and familial cold urticaria. For primary cold contact urticaria, diagnosis can be confirmed with an ice cube test, where an ice cube is placed in a plastic bag and applied to the patient's forearm.
Solar urticaria – Occurs after direct sunlight exposure. Typically, lesions are wheals, but generalized systemic symptoms and anaphylaxis can occur. Photo testing can be used to diagnose.
Aquagenic urticaria – Induced by exposure to water, usually on the upper body. Use water challenge test with a wet compress saturated in room-temperature (35°C) water (to avoid confusion with heat or cold urticaria).