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Hives are red, itchy, slightly elevated swellings. Itching may be severe. Hives have clearly defined borders and may have a pale center. Typically, crops of hives come and go. One hive may remain for several hours, then disappear, and later, another may appear elsewhere. After the hive disappears, the skin usually looks completely normal. The swelling is caused by the release of chemicals (such as histamine) from mast cells in the skin, which cause fluid to leak out of small blood vessels temporarily.
Hives may occur with angioedema, which, like hives, involves swelling. However, the swelling caused by angioedema is under the skin rather than on its surface. Sometimes angioedema affects the face, lips, throat, tongue, and airways. It can be life threatening if the swelling interferes with breathing.
Hives and angioedema are often allergic reactions.
Hives may occur when certain chemicals are inhaled, consumed, injected, or touched. These chemicals can be in the environment, foods, drugs, insects, plants, or other sources. They are harmless in most people. But if people are sensitive to them, these chemicals (called triggers or allergens) can cause an allergic reaction. That is, the immune system overreacts to the chemicals.
However, hives are not always part of an allergic reaction. For example, they may result from autoimmune disorders. In these disorders, the immune system malfunctions, misinterpreting the body's own tissues as foreign and attacking them. Also, some drugs cause hives directly without triggering an allergic reaction. Emotional stress and some physical conditions (such as heat or light) may cause hives for reasons that are not well understood.
Hives usually last less than 6 weeks and are classified as acute. If hives last more than 6 weeks, they are classified as chronic.
Acute hives are most commonly caused by
Allergic reactions are often triggered by foods, particularly eggs, fish, shellfish, nuts, and fruits, or by insect stings. Eating even a tiny amount of some foods can suddenly cause hives. But with other foods (such as strawberries), an allergic reaction occurs only after a large amount is eaten. Many drugs, particularly antibiotics, may cause hives. Immediate allergic reactions may also occur when a substance comes into direct contact with the skin (such as latex), after an insect bite, or as a reaction to a substance that is inhaled into the lungs or through the nose.
Nonallergic causes of hives include infections, some drugs, some physical stimuli (such as pressure or cold), some emotional stimuli (such as stress), and some food additives.
Even though acute hives usually have a specific cause, the cause cannot be identified in about half of cases.
Chronic hives are most commonly caused by
Sometimes the cause is easily overlooked, as when people repeatedly consume a food not known to be a trigger, such as a preservative or dye in foods or penicillin in milk. Often, despite the best efforts, the cause remains unidentified.
Chronic hives can last for months or years, then sometimes go away for no apparent reason.
Not every episode of hives requires immediate evaluation by a doctor. The following information can help people decide whether a doctor's evaluation is needed and help them know what to expect during the evaluation.
Certain symptoms and characteristics are cause for concern:
Swelling of the face, lips, throat, tongue, or airways (angioedema)
Difficulty breathing, including wheezing
Hives that are deeply colored, that become open sores, or that persist more than 48 hours
Fever, swollen lymph nodes, jaundice, weight loss, and other symptoms of a bodywide (systemic) disorder
People should call an ambulance if
People should go to an emergency department or a doctor's office as soon as possible if
People should see a doctor if
If children have hives that appear suddenly, disappear quickly, and do not recur, an examination by a doctor is usually unnecessary. The cause is usually a viral infection.
Doctors first ask questions about the symptoms and medical history. Doctors then do a physical examination. What they find during the history and physical examination often suggests a cause and the tests that may need to be done (see Table: Some Causes and Features of Hives).
Doctors ask the person to describe each episode of hives in detail and any other symptoms that occurred (such as itching, difficulty breathing, or swelling of the face and tongue). They ask about the person’s activities before and during the episode and about possible exposure to substances that can trigger allergic reactions, including drugs being taken. The person is also asked about specific symptoms that could suggest a cause (see Table: Some Causes and Features of Hives), past allergic reactions, and recent travel.
The trigger is not always clear from the history, often because the trigger is something that may have been tolerated previously.
During the physical examination, doctors first check to see whether the lips, tongue, throat, or airways are swollen. If there is swelling, they begin treatment right away. Then doctors note how the hives look, determine which parts of the body are affected, and check for other symptoms that may help confirm the diagnosis. Doctors may use various physical stimuli to see whether they can trigger the hives. For example, they may apply light pressure, heat, or cold to the skin or stroke the skin.
Some Causes and Features of Hives
Usually, testing is not needed for a single episode of hives unless symptoms suggest a specific disorder that requires treatment (such as some infections). But if hives have unusual characteristics, recur, or persist, tests are usually done.
Typically, tests include a complete blood cell count and blood tests to measure levels of electrolytes, sugar (glucose), and thyroid-stimulating hormone and to determine how well the kidneys and liver are functioning. Skin tests, such as a skin prick test, are done by an allergist (a doctor who specializes in allergic disorders) to identify specific allergens. Imaging and other blood tests are done based on results of the history and physical examination. If results suggest that the cause is a bodywide disorder, a thorough evaluation is needed to identify the cause.
A skin biopsy is done if the diagnosis is unclear or if hives last more than 48 hours.
Hives often go away on their own after a day or two. If the cause is obvious or if the doctor identifies a cause, people should avoid it if possible. If the cause is not obvious, people should stop taking all nonessential drugs until the hives subside.
Bathing and showering with only cool water, refraining from scratching, and wearing loose clothing may help relieve symptoms.
Antihistamines taken by mouth are used for hives. These drugs partially relieve the itching and reduce the swelling. To be effective, they must be taken regularly, rather than as needed. Several antihistamines, including cetirizine, diphenhydramine, and loratadine, are available without a prescription. Diphenhydramine is an older drug that is more likely to cause drowsiness than the other two. Other antihistamines include desloratidine, fexofenadine, hydroxyzine, and levocetirizine. Antihistamine creams and lotions are not used because they may sensitize the skin and worsen itching.
Corticosteroids taken by mouth (such as prednisone) are used if symptoms are severe and other treatments are ineffective. They are given for as short a time as possible. When taken by mouth for more than 3 to 4 weeks, corticosteroids have many, sometimes serious, side effects (see Corticosteroids: Uses and Side Effects). Corticosteroid creams do not help.
Epinephrine is given to people who have severe reactions or angioedema who then are admitted to the hospital. People who have these severe reactions should carry a self-injecting epinephrine pen (epinephrine autoinjector) and, if a reaction occurs, use it immediately.
In about half the people with chronic hives, the hives disappear without treatment within 2 years. In some adults, the antidepressant doxepin, which is also a potent antihistamine, helps relieve chronic hives. Omalizumab, a monoclonal antibody, may be used by people whose chronic hives have continued to occur despite other treatments.
Older people are more likely to have side effects when they take the older antihistamines (such as hydroxyzine and diphenhydramine). In addition to drowsiness, these drugs can cause confusion and delirium and can make starting and continuing to urinate difficult. Usually, older people should not take these drugs for hives.
Hives may or may not be an allergic reaction.
If hives have lasted less than 6 weeks, the cause is usually an allergic reaction to a specific substance, an acute infection, or a nonallergic reaction to a specific substance.
If hives have lasted 6 weeks or more, the cause usually cannot be identified (is idiopathic) or is an autoimmune disorder.
People should call an ambulance if they have difficulty breathing or if their throat feels as if it is closing up.
People with mild symptoms should avoid any known or suspected triggers and can take antihistamines to relieve symptoms.
People who have severe reactions should carry a self-injecting epinephrine pen and, if a reaction occurs, use it immediately.
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