Urticaria (from the Latin urtica,nettle urere, to burn or hives) are akind of skin rash notable for dark red, raised, itchy bumps. Hives arefrequently caused by allergic reactions; however, there are many non-allergiccauses. For example, most cases of hives lasting less than six weeks (acuteurticaria) are the result of an allergic trigger. Chronic urticaria (hiveslasting longer than six weeks) are rarely due to an allergy. The majority ofpatients with chronic hives have an unknown (idiopathic)cause. Perhaps as many as 30-40% of patients with chronic idiopathic urticariawill, in fact, have an autoimmune cause. Acute viral infection is anothercommon cause of acute urticaria (viral exanthem). Lesscommon causes of hives include friction, pressure, temperature extremes,exercise, and sunlight.
Wheals (raised areas surrounded by a red base) from urticaria can appearanywhere on the surface of the skin. Whether the trigger is allergic ornon-allergic, there is a complex release of inflammatory mediators, including histaminefrom cutaneous mast cells, resulting in fluid leakage from superficialblood vessels. Wheals may be pinpoint in size, or several inches in diameter. Angioedemais a related condition (also from allergic and non-allergic causes), thoughfluid leakage is from much deeper blood vessels. Individual hives that arepainful, last >24 hours, or leave a bruise as they heal are more likely tobe a more serious condition called urticarial vasculitis. Hives caused bystroking the skin (often linear in appearance) are due to a benign conditioncalled dermatographism
- Acute urticaria usually show up a few minutes after contact with the allergen and can last a few hours to several weeks. Food allergic reactions often fit in this category. 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. It is uncommon for patients to have more than 2 true food allergies. A less common cause is exposure to certain bacteria, such as streptococcus or possibly Helicobacter pylori. In these cases, the hives may be exacerbated by other factors, such as those listed under Physical Urticarias below.
- Chronic urticaria refers to hives that persists for 6 weeks or more. There are no visual differences between acute and chronic urticaria. Some of the more severe chronic cases have lasted more than 20 years. A survey indicated that chronic urticaria lasted a year or more in more than 50% of sufferers and 20 years or more in 20% of them. Of course this does mean that in almost half the people it clears up within a year and in 80% it clears up within 20 years or less.
- Drug-induced urticaria has been known to result in severe cardiorespiratory failure. The anti-diabetic sulphonylurea glimepiride (trade name Amaryl), in particular, has been documented to induce allergic reactions manifesting as urticaria. Other cases include dextroamphetamine, aspirin, penicillin, clotrimazole, sulfonamides and anticonvulsants.
- Physical urticarias are often categorized into the following.
- Aquagenic: Reaction to water (exceedingly rare)
- Cholinergic: Reaction to body heat, such as when exercising or after a hot shower
- Cold (Chronic cold urticaria): Reaction to cold, such as ice, cold air or water - worse with sudden change in temperature
- Delayed Pressure: Reaction to standing for long periods, bra-straps, elastic bands on undergarments, belts
- Dermatographic: Reaction when skin is scratched (very common)
- Heat: Reaction to hot food or objects (rare)
- Solar: Reaction to direct sunlight (rare, though more common in those with fair skin)
- Vibration: Reaction to vibration (rare)
- Adrenergic: Reaction to adrenaline / noradrenaline (extremely rare)