Cholinergic urticaria

# Overview
Cholinergic urticaria is subcategory of physical urticaria (aka hives) that is a skin rash brought on by a hypersensitive reaction to body heat. Symptoms follow any stimulus to sweat such as exercise (sometimes called exercise-induced urticaria), heat from the sun (which could also indicate solar urticaria), saunas, hot showers (reaction to water can also indicate aquagenous urticaria), or even stress due to blushing or anger. Some people only have symptoms during the winter months where their body temperature rises when it is unacclimatized to heat.

# Symptoms
The visible hives (sometimes called heat bumps) appear as multitude of small 2-3mm wealts typically surrounded by patches of red skin. The affected area will often feel warm and can be extremely itchy or exhibit a burning sensation. Typically the rash occurs on the upper trunk and the arms but can appear on other parts of the body. The rash may be worse in areas where clothing restricts the skin's ability to cool itself such as hat brims, waist bands, tight collars, backpack straps, etc. This can be compounded since many people with cholinergic urticaria also have pressure urticaria, in which pressure  on the skin causes a reaction.
The rash typically develops within a few minutes of a rise in body temperature but can take longer to appear visibly on the skin. The visible rash is often preceded by a general warming of the skin or itchiness. The hives last from a half an hour to several hours with a mean time of 80 minutes, with the duration often contingent on the severity of the outbreak.
Aspirin can sometimes worsen symptoms as may other drugs.

# Frequency
Cholinergic urticaria is a fairly common condition and is often comorbid with other forms of urticaria, especially chronic urticaria. The prevalence of the disorder is also higher in people who have other atopic conditions such as eczema (atopic dermatitis), allergic conjunctivitis, allergic rhinitis or asthma. These other atopic reactions can sometimes be triggered by the urticaria attack. There is some evidence that in at least some individuals the condition is hereditary.
The age of onset is anywhere from 10-30 and the condition may occur spontaneously in people with no history of the condition. People who are chronically affected by the condition will sometimes go through phases of no reactions and phases where their skin is hypersensitive. These phases may relate to the season, diet changes, or other environmental factors, but often have no obvious cause. Most people who are afflicted by this condition maintain tendency for outbreaks for several years (anywhere from 3-30 years) before it finally disappears.

# Physiology
All urticarias are caused by an elevated histamine release by the body's mast cells. With cholinergic urticaria the exact triggering mechanism for this response is unknown, but it is assumed to be related to the body's thermoregulatory response.
The named cholinergic urticaria comes from studies where some people with the disorder produce a rash when injected with the neurochemical acetylcholine; however, it is not clear how acetylcholine is involved in the reaction.

# Treatment
Cholinergic urticaria can be very difficult to treat. Most treatment plans for cholinergic urticaria involve being aware of one's triggers, but this can be difficult since there is often comorbidity with other forms of urticaria and some urticaria is idiopathic.
Often it is handled just with limiting one's exposure to triggers such as strenuous exercise or heat. For some, the reaction can be limited by making sure to wear light breathable clothing allowing the body to remain cool. Also, gradually warming the body with light exercise first can help limit the effects in some people. Since an attack can often be felt coming on, it can sometimes be halted by rapid cooling, such as applying cold water or an ice pack to the skin.

## Pharmacotherapy
Drug treatment is typically in the form of Antihistamines such as loratadine (Claritin), hydroxyzine, cetirizine and other H1 receptor antagonists. These are taken on a regular basis to protective effect, lessening or halting attacks. Loratadine has an advantage due to its minimal side effects; however it may not be as effective for some. Hydroxyzine while effective may not be well tolerated due the main side effect of sleepiness.
For some people, H2-receptor antagonists such as cimetidine (Tagamet) and ranitidine (Zantac) can also help control symptoms either protectively or by lessening symptoms when an attack occurs. When taken in combination with a H1 antagonist it has been shown to have a synergistic effect which is more effective than either treatment alone. The use of ranitidine (or other H2 antagonist) for urticaria is considered an off-label use, since these drugs are primarily used for the treatment of peptic ulcer disease (PUD) and gastroesophageal reflux disease (GERD).
Some people have shown results using a combination of both the aforementioned Tagamet (400 mg) with a low level dose of anti-cholinergic Cogentin, aka benztropine, (.5 mg). Cogentin and other anti-cholinergics are typically used to treat Parkinson's disease at higher dose levels and can have multiple side effects.
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.
None of these treatments are surefire means of controlling attacks. Some people prove to be treatment resistant, and medications can suddenly cease being as effective as they once were. In these instances, changes to a treatment plan can sometimes help. 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 since the condition is intermittent and outbreaks typically clear up without any treatment.

## Alternative and complementary medicine
While the disease is obviously physiological in origin, psychological treatments such as stress management can sometimes lessen severity and occurrence. Additionally, methods similar to psychological pain management can be used to shift focus away from the uncomfortability and itchiness during an attack.
Many people suffering from chronic conditions like urticaria use some form of alternative medicine. There are little to no data to support the effectiveness of most of these therapies. Urticaria is often intermittent and idiopathic and can be caused by stress, which amplifies the placebo effect. As such, many alternative treatments may seem to work great when in reality it is only a combination of the placebo effect and a chance remission in the disease.
Diet changes are often tried by people with urticaria in attempts to stop what is presumed to be a food allergy. Also, people often try changing their laundry detergents, shampoos, soaps, etc. While food and other allergies can cause hive outbreaks, cholinergic urticaria and other urticaria outbreaks often occur on their own with no connection to food or other allergy. Urticaria's intermittent nature can fool people into thinking that it is caused by an allergy to food or product.