Urticaria, commonly known as hives, is a frequent skin reaction characterized by the sudden appearance of itchy, raised welts on the skin. While the classic presentation involves distinct, swollen patches called wheals, hives can also be flat. The appearance of urticaria can vary widely, sometimes manifesting as flatter, less elevated patches, particularly during the early or resolving stages. This reaction is caused by the release of chemical mediators that cause temporary swelling and redness.
Defining the Appearance of Flat Urticaria
The classic hive, or wheal, is a raised, often pale swelling of the superficial skin layer (dermis), typically surrounded by a red ring called a flare. This elevation is caused by the localized leakage of fluid from small blood vessels, a process called edema, which is triggered by the release of histamine. These raised lesions are transient, appearing and disappearing within 24 hours without leaving marks or bruising.
A flatter presentation, sometimes described as a macule or patch, occurs when fluid leakage is less pronounced. This results in a lesion that is only slightly raised or appears entirely flat against the skin. These flatter areas are often pink or red and retain the characteristic itchiness associated with urticaria.
A useful identifying feature of both raised and flat urticaria is blanching, meaning the skin temporarily turns white when pressure is applied. The appearance of hives can also be influenced by conditions like urticarial vasculitis, where lesions persist for longer than 24 hours, often sting or burn, and may leave residual bruising.
Common Triggers and Etiology
Urticaria develops when mast cells in the skin are activated, releasing chemical messengers, most notably histamine. This release causes local blood vessels to dilate and become more permeable, allowing fluid to leak into the surrounding tissue and cause swelling. Triggers for this reaction are numerous and categorized into allergic and non-allergic causes.
Allergic Triggers
Common allergic triggers include specific foods (peanuts, eggs, shellfish), certain medications (antibiotics, NSAIDs), and insect stings. In these instances, the immune system mistakenly identifies a harmless substance as a threat, initiating the mast cell response.
Non-Allergic Triggers
Non-allergic causes are also frequent, with acute infections, particularly viral infections, being a major contributor, especially in children. Physical stimuli can also trigger a response, leading to chronic inducible urticaria. Examples include:
- Exposure to cold temperatures.
- Pressure from tight clothing.
- Increased body heat from exercise.
- Friction or scratching of the skin.
In many cases, particularly chronic urticaria lasting longer than six weeks, a specific cause is never identified. This situation is referred to as idiopathic or spontaneous urticaria.
Management and When to Seek Professional Help
The primary goal of managing mild urticaria is to alleviate intense itching and reduce inflammation. First-line treatment involves using over-the-counter, non-sedating H1 antihistamines, such as cetirizine or loratadine. These second-generation medications block the effects of released histamine and are favored due to their effectiveness and lower risk of drowsiness compared to older antihistamines.
Applying cool compresses or taking a cool shower can also help soothe the irritated skin and reduce itching. If a trigger is known, avoiding that specific food, medication, or physical stimulus is the most effective way to prevent future episodes. For mild, acute urticaria, symptoms often resolve on their own within a few hours to days, even without specific treatment.
You should seek professional medical help if the hives are persistent, occurring nearly daily for more than six weeks, classifying the condition as chronic. Immediate emergency care is necessary if the hives are accompanied by signs of a severe systemic reaction, such as angioedema. Angioedema is a deeper swelling that commonly affects the lips, eyelids, tongue, or throat and may feel painful or tight rather than itchy.
Signs of a life-threatening allergic reaction, or anaphylaxis, include:
- Difficulty breathing.
- Wheezing.
- Dizziness.
- A sudden drop in blood pressure.
If any of these symptoms occur alongside the hives, it indicates a medical emergency requiring prompt treatment, often with an epinephrine auto-injector.