|Classification and external resources|
Urticaria on the arm
Urticaria (from the Latin urtica, nettle,) commonly referred to as hives, is a kind of skin rash notable for pale red, raised, itchy bumps. Hives might also cause a burning or stinging sensation. Hives are frequently caused by allergic reactions; however, there are many nonallergic causes. Most cases of hives lasting less than six weeks (acute urticaria) are the result of an allergic trigger. Chronic urticaria (hives lasting longer than six weeks) is rarely due to an allergy.
The majority of chronic hives cases have an unknown (idiopathic) cause. In perhaps as many as 30–40% of patients with chronic idiopathic urticaria, it is caused by an autoimmune reaction. Acute viral infection is another common cause of acute urticaria (viral exanthem). Less common causes of hives include friction, pressure, temperature extremes, exercise, and sunlight.
Wheals (raised areas surrounded by a red base) from urticaria can appear anywhere on the surface of the skin. Whether the trigger is allergic or not, a complex release of inflammatory mediators, including histamine from cutaneous mast cells, results in fluid leakage from superficial blood vessels. Wheals may be pinpoint in size, or several inches in diameter.
Angioedema is a related condition (also from allergic and nonallergic causes), though fluid leakage is from much deeper blood vessels in the subcutaneous or submucosal layers. Individual hives that are painful, last more than 24 hours, or leave a bruise as they heal are more likely to be a more serious condition called urticarial vasculitis. Hives caused by stroking the skin (often linear in appearance) are due to a benign condition called dermatographic urticaria.
Acute versus chronic 
- Acute urticaria is defined as the presence of evanescent wheals which completely resolve within six weeks.:150 Acute urticaria becomes evident a few minutes after the person has been exposed to an allergen. The outbreak may last several weeks, but usually the hives are gone in six weeks. Typically, the hives are a reaction to food, but in about half the cases, the trigger is unknown. Common foods may be the cause, as well as bee or wasp stings, or skin contact with certain fragrances.
- Chronic urticaria (ordinary urticaria) is defined as the presence of evanescent wheals which persist for greater than six weeks.:150 Some of the more severe chronic cases have lasted more than 20 years. A survey indicated chronic urticaria lasted a year or more in more than 50% of sufferers and 20 years or more in 20% of them.
Acute and chronic urticaria are visually indistinguishable.
By cause 
Urticaria can also be classified by the purported causative agent. Many different substances in the environment may cause urticaria, including medications, food and physical agents.
Drugs that have caused allergic reactions evidenced as urticaria include dextroamphetamine, aspirin, ibuprofen, penicillin, clotrimazole, sulfonamides, anticonvulsants, and antidiabetic drugs. The antidiabetic sulphonylurea glimepiride (trade name Amaryl), in particular, has been documented to induce allergic reactions manifesting as urticaria. Drug-induced urticaria has been known to have an effect on severe cardiorespiratory failure.
The listed side-efects of the antibiotic, trichazole include urticaria.
Infection or environmental agent 
The rash that develops from poison ivy, poison oak, and poison sumac contact is commonly mistaken for urticaria. This rash is caused by contact with urushiol and results in a form of contact dermatitis called urushiol-induced contact dermatitis. Urushiol is spread by contact, but can be washed off with a strong grease- or oil-dissolving detergent and cool water and rubbing ointments.
Dermatographic urticaria 
Dermatographic urticaria (also known as dermatographism or "skin writing") is marked by the appearance of weals or welts on the skin as a result of scratching or firm stroking of the skin. Seen in 4–5% of the population, it is one of the most common types of urticaria, in which the skin becomes raised and inflamed when stroked, scratched, rubbed, and sometimes even slapped.
The skin reaction usually becomes evident soon after the scratching, and disappears within 30 minutes. Dermographism is a common form of chronic hives. Dermatographism is the most common form of a subset of chronic hives, acknowledged as "physical hives".
It stands in contrast to the linear reddening that does not itch seen in healthy people who are scratched. In most cases, the cause is unknown, although it may be preceded by a viral infection, antibiotic therapy, or emotional upset. Dermographism is diagnosed by taking a tongue blade and drawing it over the skin of the arm or back. The hives should develop within a few minutes. Unless the skin is highly sensitive and reacts continually, treatment is not needed. Taking antihistamines can reduce the response in cases that are annoying to the patient.
Pressure or delayed pressure 
This type of urticaria can occur right away, precisely after a pressure stimulus or as a deferred response to sustained pressure being enforced to the skin. In the deferred form, the hives only appear after about six hours from the initial application of pressure to the skin. Under normal circumstances, these hives are not the same as those witnessed with most urticariae. Instead, the protrusion in the affected areas is typically more spread out. The hives may last from eight hours to three days. The source of the pressure on the skin can happen from tight fitted clothing, belts, clothing with tough straps, walking, leaning against an object, standing, sitting on a hard surface, etc. The areas of the body most commonly affected are the hands, feet, trunk, abdomen, buttocks, legs and face. Although this appears to be very similar to dermatographism, the cardinal difference is that the swelled skin areas do not become visible quickly and tend to last much longer. This form of the skin disease is, however, rare.
Cholinergic or stress 
Cholinergic urticaria (CU) is one of the physical urticaria which is provoked during sweating events such as exercise, bathing, staying in a heated environment, or emotional stress. The hives produced are typically smaller than classic hives and are generally shorter-lasting.
The cold type of urticaria is caused by exposure of the skin to extreme cold, damp and windy conditions; it occurs in two forms. The rare form is hereditary and becomes evident as hives all over the body 9 to 18 hours after cold exposure. The common form of cold urticaria demonstrates itself with the rapid onset of hives on the face, neck, or hands after exposure to cold. Cold urticaria is common and lasts for an average of five to six years. The population most affected is young adults, between 18 and 25 years old. Many people with the condition also suffer from dermographism and cholinergic urticaria.
Severe reactions can be seen with exposure to cold water; swimming in cold water is the most common cause of a severe reaction. This can cause a massive discharge of histamine, resulting in low blood pressure, fainting, shock and even loss of life. Cold urticaria is diagnosed by dabbing an ice cube against the skin of the forearm for 1 to 5 minutes. A distinct hive should develop if a patient suffers cold urticaria. This is different than the normal redness that can be seen in people without cold urticaria. Patients with cold urticaria need to learn to protect themselves from a hasty drop in body temperature. Regular antihistamines are not generally efficacious. One particular antihistamine, cyproheptadine (Periactin), has been found to be useful. The tricyclic antidepressant doxepin has also been found to be an effective blocking agent of histamine discharge. Finally, a medication named ketotifen, which keeps mast cells from discharging histamine, has also been employed with widespread success.
This rare form of urticaria is triggered by the continued application of heat on the skin. Hives begin to appear within two to five minutes on the area of the skin exposed to heat. The hives, however, generally do not last more than an hour.
Solar urticaria 
This form of the disease occurs on areas of the skin exposed to the sun; the condition becomes evident within minutes of exposure. After the individual is no longer exposed to the sun, though, the condition starts to weaken within a few minutes to a few hours, and hardly ever lasts longer than 24 hours. Solar urticaria is classified into six different types, depending upon the wavelength of light involved. Since glass absorbs light with a wavelength of 320 nm and below, people suffering from solar urticaria in response to wavelengths of less than 320 nm are protected by glass.
This type of urticaria is also termed rare, and occurs upon contact with water. The response is not temperature-dependent and the skin appears similar to cholinergic form of the disease. The appearance of hives is within one to 15 minutes of contact with the water, and can last from 10 minutes to two hours. The hives that last for 10 to 120 minutes do not seem to be stimulated by histamine discharge like the other physical hives. Most researchers believe this condition is actually skin sensitivity to additives in the water, such as chlorine. Water urticaria is diagnosed by dabbing tap water and distilled water to the skin and observing the gradual response. Aquagenic urticaria is treated with capsaicin (Zostrix) administered to the chafed skin. This is the same treatment used for shingles. Antihistamines are of questionable benefit in this instance, since histamine is not the causative factor.
This condition was first distinguished in 1980. People with exercise urticaria (EU) experience hives, itchiness, shortness of breath and low blood pressure five to 30 minutes after beginning exercise. These symptoms can progress to shock and even sudden death. Jogging is the most common exercise to cause EU, but it is not induced by a hot shower, fever, or with fretfulness. This differentiates EU from cholinergic urticaria.
EU sometimes occurs only when someone exercises within 30 minutes of eating particular foods, such as wheat or shellfish. For these individuals, exercising alone or eating the injuring food without exercising produces no symptoms. EU can be diagnosed by having the patient exercise and then observing the symptoms. This method must be used with caution and only with the appropriate resuscitative measures at hand. EU can be differentiated from cholinergic urticaria by the hot water immersion test. In this test, the patient is immersed in water at 43°C (109.4°F). Someone with EU will not develop hives, while a person with cholinergic urticaria will develop the characteristic small hives, especially on the neck and chest.
The immediate symptoms of this uncanny type are treated with antihistamines, epinephrine and airway support. Taking antihistamines prior to exercise may be effective. Ketotifen is acknowledged to stabilise mast cells and prevent histamine release, and has been effective in treating this hives disorder. Avoiding exercise or foods that cause the mentioned symptoms is very important. In particular circumstances, tolerance can be brought on by regular exercise, but this must be under medical supervision.
The most common food allergies in adults are shellfish and nuts. The most common food allergies in children are shellfish, nuts, peanuts, eggs, wheat, and soy. A less common cause is exposure to certain bacteria, such as Streptococcus species or possibly Helicobacter pylori.
Related conditions 
Angioedema is similar to urticaria, but in angioedema, the swelling occurs in a lower layer of the dermis than in urticaria, as well as in the subcutis. This swelling can occur around the mouth, in the throat, in the abdomen, or in other locations. Urticaria and angioedema sometimes occur together in response to an allergen, and is a concern in severe cases, as angioedema of the throat can be fatal.
Vibratory angioedema 
This very rare form of angioedema develops in reply to contact with vibration. In vibratory angioedema, symptoms develop within two to five minutes after contact with vibration and dissolve after about an hour. Patients with this disorder do not suffer from dermographism or pressure urticaria. Vibratory angioedema is diagnosed by holding a vibrating device such as a laboratory vortex machine against the forearm for four minutes. Speedy swelling of the whole forearm extending into the upper arm is also noted later. The principal treatment is avoidance of vibratory stimulants. Antihistamines have also been proven helpful.
The skin lesions of urticarial disease are caused by an inflammatory reaction in the skin, causing leakage of capillaries in the dermis, and resulting in an edema which persists until the interstitial fluid is absorbed into the surrounding cells.
Urticaria is caused by the release of histamine and other mediators of inflammation (cytokines) from cells in the skin. This process can be the result of an allergic or nonallergic reaction, differing in the eliciting mechanism of histamine release.
- Allergic urticaria
- Histamine and other proinflammatory substances are released from mast cells in the skin and tissues in response to the binding of allergen-bound IgE antibodies to high-affinity cell surface receptors. Basophils and other inflammatory cells are also seen to release histamine and other mediators, and are thought to play an important role, especially in chronic urticarial diseases.
- Autoimmune urticaria
- In the past decade, many cases of chronic idiopathic urticaria have been noted to be the result of an autoimmune trigger. For example, roughly one-third of patients with chronic urticaria spontaneously develop autoantibodies directed at the receptor FcεRI located on skin mast cells. Chronic stimulation of this receptor leads to chronic hives. Patients often have other autoimmune conditions, such as autoimmune thyroiditis.
- Hive-like rashes commonly accompany viral illnesses, such as the common cold. They usually appear three to five days after the cold has started, and may even appear a few days after the cold has resolved.
- Nonallergic urticaria
- Mechanisms other than allergen-antibody interactions are known to cause histamine release from mast cells. Many drugs, for example morphine, can induce direct histamine release not involving any immunoglobulin molecule. Also, a diverse group of signaling substances, called neuropeptides, have been found to be involved in emotionally induced urticaria. Dominantly inherited cutaneous and neurocutaneous porphyrias (porphyria cutanea tarda, hereditary coproporphyria, variegate porphyria and erythropoietic protoporphyria) have been associated with solar urticaria. The occurrence of drug-induced solar urticaria may be associated with porphyrias. This may be caused by IgG binding, not IgE.
- Dietary histamine poisoning
- This is termed scombroid food poisoning. Ingestion of free histamine released by bacterial decay in fish flesh may result in a rapid-onset, allergic-type symptom complex which includes urticaria. However, the urticaria produced by scombroid is reported not to include wheals.
- Stress and chronic idiopathic urticaria
- Chronic idiopathic urticaria has been anecdotally linked to stress since the 1940s. A large body of evidence demonstrates an association between this condition and both poor emotional well-being and reduced health-related quality of life. A link between stress and this condition has also been shown. A recent study has demonstrated an association between stressful life events (e.g. bereavement, divorce, etc.) and chronic idiopathic urticaria  and also an association between post-traumatic stress and chronic idiopathic urticaria.
Chronic urticaria can be difficult to treat. No guaranteed treatments or means of controlling attacks are available, and some subpopulations are treatment-resistant, with medications spontaneously losing their effectiveness and requiring new medications to control attacks. It can be difficult to determine appropriate medications, since some, such as loratadine, require a day or two to build up to effective levels, and the condition is intermittent and outbreaks typically clear up without any treatment.
Most treatment plans for urticaria involve being aware of one's triggers, but this can be difficult, since several forms of urticaria are known and people often exhibit more than one type. Also, since symptoms are often idiopathic, a clear trigger is often unknown. If triggers can be identified, then outbreaks can often be managed by limiting exposure to them.
Allergy tests 
Physicians can rarely determine any particular cause of disease for chronic urticaria. In some cases, patients or doctors come to request regular extensive allergy testing over a long period of time in hopes of getting new insight. No evidence shows regular allergy testing results in identification of a problem or relief for people with chronic urticaria. Regular allergy testing for people with chronic urticaria is not recommended.
Tricyclic antidepressants, such as doxepin, also are often potent H1 and H2 antagonists and may have a role in therapy, although side effects limit their use. For very severe outbreaks, an oral corticosteroid such as prednisone is sometimes prescribed. However, this form of treatment is controversial because of the extensive side effects common with corticosteroids, and as such, is not a recommended long-term treatment option. For acute urticaria, some topical creams, such as hydrocortisone, fluocinonide, or desonide, can also be prescribed to relieve itching. To boost relief for severe anaphylactic urticaria, a dermatologist will also administer steroid shots intramuscularly.
As of 2008, an Australian company is performing clinical trials with an analogue of alpha-melanocyte-stimulating hormone called afamelanotide (formerly CUV1647), for the treatment of solar urticaria, a type of urticaria that develops in response to exposure to specific wavelengths of light.
A diet reduced in allergens like salicytes, grains, and the 8 most common food allergens may also help to reduce symptoms. National Institute of Health Researchers have recently drawn connections between Chronic Idiopathic Urticaria and treatment with a oligoantigenic diet. In a small 2004 study, 29 of 55 subjects placed on a allergen-restricted diet saw their urticaria decrease significantly or vanish entirely.
Acute urticaria induced by the use of trichazole only reacted to treatment by taking a very hot bath in water containing epsom salts and other vitamins and minerals known to induce exfoliation. Application of lemon juice brought temporary relief from itching and Epizone A cetomacrogol cream was the most effective topical application, after the heat treatment.
In a Phase III trial whose results were published early 2013 online in The New England Journal of Medicine, the anti-asthma drug omalizumab showed promise in relieving itchiness for patients with chronic idiopathic urticaria whose symptoms were not relieved with antihistamine treatment.
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