Hives are red, itchy, slightly elevated swellings. 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. 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.
(See also Itching.)
Angioedema
Hives may occur with angioedema, which, like hives, involves swelling. However, the swelling of 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.
Causes of Hives
Hives and angioedema can be allergic reactions.
Hives may occur when certain chemicals are inhaled, consumed, injected, or touched. These chemicals can be in the environment, foods, drugs (including medications), 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, most of the time, hives are not part of an allergic reaction and an allergen (the cause of the allergic reaction) cannot be identified. 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. Some physical stimuli (such as heat, cold, pressure, friction, or sunlight) 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
If a cause can be identified, acute hives are most commonly caused by
Allergic reactions (such as foods and food additives, drugs, or insect bites)
Nonallergic reactions (such as drugs, physical stimuli, or autoimmune disorders)
Allergic reactions are often triggered by foods, particularly eggs, fish, shellfish, nuts, and fruits; food additives; drugs; or insect bites or 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 larger amount is eaten. Many medications, 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 sting, 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, and some physical stimuli (such as pressure or cold).
In more than half of cases, a specific cause of acute hives cannot be identified.
Chronic hives
Possible identifiable causes of chronic hives are the same as those of acute hives. However, in the vast majority of cases, a cause cannot be identified (is idiopathic). Most cases of chronic hives that have no identifiable cause are thought to be due to an autoimmune reaction that itself has no detectable cause. Nevertheless, all efforts should be made to identify a cause, because elimination of a cause is the best approach to treatment.
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.
Evaluation of Hives
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.
Warning signs
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 for more than 48 hours
Fever, swollen lymph nodes, jaundice, weight loss, and other symptoms of a bodywide (systemic) disorder
When to see a doctor
People should call an ambulance if
They have difficulty breathing or are wheezing.
Their throat feels as if it is closing up.
People should go to an emergency department or a doctor's office as soon as possible if
Their symptoms are severe.
They feel progressively weak or light-headed or have severe fever or chills.
They are vomiting or have abdominal pain or diarrhea.
People should see a doctor if
A bee sting triggers hives or swelling (to obtain advice about treatment if another bee sting occurs).
They have other symptoms, such as fever, joint pains, weight loss, swollen lymph nodes, or night sweats.
Hives recur without exposure to a trigger.
Symptoms last for more than 2 days.
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.
What the doctor does
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), recent infections, 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.
People themselves should not try to trigger their hives, because a severe reaction could occur.
Testing
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.
Treatment of Hives
Avoidance of triggers
Measures to relieve itching
Medications
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 medications until the hives subside.
Bathing and showering with only cool water, refraining from scratching, and wearing loose clothing may help relieve symptoms.
Medications
Antihistamine creams and lotions are not used because they may sensitize the skin and worsen itching.
Corticosteroids see Corticosteroids: Uses and Side Effects).
Corticosteroid creams do not help.
narrows blood vessels and opens airways in the lungs. It is given to people who have severe reactions or angioedema
Essentials for Older People: Hives
Key Points
Hives may or may not be an allergic reaction.
For most people, a cause cannot be identified.
If a cause is identified in hives that 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).
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.
