Most people recognize hives—those sudden instances of swelling of the skin that usually itch, burn, or sting. Less familiar, however, is the condition’s close cousin, angioedema, a similar kind of swelling of the deeper tissues beneath the skin.

Hives and angioedema are related because they are similar reactions to the same basic cause, a chemical called histamine. In most cases, histamine is a byproduct of the body’s specialized mast cells as they go about their job of destroying allergens, which are any substances that cause allergic reactions. Fighting allergens is not the only way that histamine is produced, however. Irritation caused by sunlight, some medications, and unknown sources can make mast cells release histamine, too.

Whatever the reason for its release, histamine produces hives and angioedema by dilating the small blood vessels in the skin and causing fluid to leak from them. This in turn generates swelling. It also can stimulate gastric acid secretion and cause certain smooth muscles to contract.

Hives—or more technically, urticaria—are pale red welts that can appear anywhere on the body in splotches as small as a pencil eraser and as large as a dinner plate. These splotches sometimes join together to form larger areas known as plaques. Whether large or small, hives and plaques often fade within a few hours, but can last three days or longer.

In general, angioedema lasts longer than urticaria, but the swelling usually goes away within 24 hours. Symptoms include deep swelling around the eyes and lips, and sometimes swelling of the genitals, hands, and feet. Occasionally, severe and prolonged swelling of the tissue can be disfiguring. In rare instances, angioedema of the throat, tongue, or lungs can block the airways, making breathing difficult and endangering the life of the victim.

Types of urticaria and angioedema
Doctors usually classify hives and angioedema according to the following categories:

• Acute urticaria is a case of hives that lasts less than six weeks. The most common causes are foods, medications, latex, or infections, but insect bites and internal disease can be responsible too. The most common foods that trigger hives are nuts, fish, tomatoes, and fresh berries. Medications that can cause hives and angioedema include aspirin, other nonsteroidal anti-inflammatory medications such as ibuprofen, high-blood-pressure medications (ACE inhibitors), and painkillers such as codeine.
• Chronic urticaria and angioedema lasts more than six weeks. The cause of this type of reaction tends to be much more difficult to identify than the cause of the acute kind. For this reason, it is often not found. In fact, the cause of chronic urticaria remains unknown for more than 80 percent of cases.
• Physical urticaria is a case of hives caused by direct physical stimulation of the skin from exposure to cold, heat, sun, vibration, pressure, sweating, exercise, and other sources. These hives usually occur at the site of the stimulation and rarely appear elsewhere. Most of outbreaks in this class appear within one hour after exposure.

Diagnosis and treatment
The best treatment for hives and angioedema is for your doctor to identify the trigger and then for you to avoid it. Identifying triggers, however, is not an easy task. Your doctor will need to ask you many questions and perform some tests.

Because there are no specific tests for hives or angioedema, the kinds of tests ordered will depend upon your medical history and a thorough examination by one or more specialists. Skin tests may be performed to determine the substance to which you are allergic. Blood tests are done routinely to rule out a systemic illness as the cause of your body’s release of histamine.

If your condition is persistent, your doctor will probably prescribe an antihistamine to provide you with relief from the symptoms. These medications work best when taken on a regular schedule to prevent the swelling. Your doctor may prescribe oral corticosteroids if antihistamines do not work well. For severe outbreaks, an injection of epinephrine (adrenaline) or a cortisone-based medication may be needed.

While you’re waiting for the hives and swelling to disappear, doing the following might help to relieve the symptoms:

• Avoid hot water; use lukewarm water instead.
• Use gentle, mild soap.
• Apply cool compresses or wet cloths to the afflicted areas.
• Try to work and sleep in a cool room.
• Wear loose-fitting, lightweight clothes.

When to seek medical help
Most of the time, hives will go away by themselves and need no treatment. You should contact your doctor if:

• your hives last longer than a few day
• they continue to recur for a month or longer
• you have symptoms of angioedema or anaphylaxis (a life-threatening allergic response)

Go to an emergency room immediately if any of the following symptoms accompany the development of your hives or angioedema:

• dizziness
• wheezing
• difficulty breathing
• tightness in the chest
• swelling of the tongue, lips, or face

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This information is provided by the Cleveland Clinic and is not intended to replace the medical advice of your doctor or health care provider. Please consult your health care provider for advice about a specific medical condition. For additional written health information, please contact the Health Information Center at the Cleveland Clinic (216) 444-3771 or toll-free (800) 223-2273 extension 43771 or visit www.clevelandclinic.org/health/.

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