Cholinergic urticaria (CU) presents with tiny very itchy wheals and small bumps on a reddish background.It is a type of physical urticaria (or hives) that appears when a person is sweating or their core body temperature increases. Symptoms
Cholinergic urticaria typically presents with a number of small, short-lasting hives but may also involve cutaneous inflammation (wheals) and pain which develops usually in response to exercise, bathing, staying in a heated environment, or emotional stress. Although the symptoms subside rapidly, commonly within 1 hour, cholinergic urticaria may significantly impair quality of life, especially in relation to sporting activities. Causes
Sweat hypersensitivity
Acquired anhidrosis and/or hypohidrosis
Idiopathic
Opioid use
Cholinesterase inhibitors

Subtypes
Sweat hypersensitivity
This subtype of CU refers to those who are hypersensitive to their own sweat. Diagnosis
Diagnosis is made by injecting autologous (the persons own) sweat into the skin. Features
The hives are observed to coincide with perspiration points of sweating. Pathophysiology
Tanaka et al. found that the sweat hyper-sensitivities of CU and atopic dermatitis seem to be virtually the same, and therefore, the sweat-induced histamine release from basophils may also be mediated by a specific IgE for sweat in atopic dermatitis as well as CU. Treatment
Proposed first-line treatment: Rapid desensitization protocol using autologous sweat. Non-pharmacological treatment: Forced perspiration by excessive body warming (hot bath or exercise) used daily may reduce the symptoms through exhaustion of inflammatory mediators. This non-pharmacological treatment is contraindicated in those with CU as a result of hypohidrosis (see below).
Antihistamines are a commonly prescribed first-line treatment for conventional urticaria, but its effectiveness in the treatment of CU is rather limited in most cases. Some research suggests that first-generation antihistamines with anticholinergic properties such as diphenhydramine are most successful at treating CU. Treatment(s) with mixed success: omalizumab (anti-IgE therapy), danazol (synthetic androgen), propranolol (beta blocker), zileuton (antileukotriene). Acquired anhidrosis and/or hypohidrosis
This subtype of CU refers to those who have abnormally reduced sweating. Diagnosis
Sweat is readily visualized by a topical indicator such as iodinated starch or sodium alizarin sulphonate. Both undergo a dramatic colour change when moistened by sweat. A thermoregulatory sweat test evaluates the bodys response to a thermal stimulus by inducing sweating through the use of a hot box ⁄ room, thermal blanket or exercise. Failure of the topical indicator to undergo a colour change during thermoregulatory sweat testing can indicate anhidrosis and/or hypohidrosis (see Minor test).A skin biopsy may reveal cellular infiltrates in sweat glands or ducts. Features
Severe heat intolerance (e.g., nausea, dizziness, and headache), and tingling, pricking, pinchy or burning pain over the entire body on exposure to hot environments or prolonged exercise which improve after cooling the body. Occurs in the absence of any causative skin, metabolic, or neurological disorders. Pathophysiology
The wheals, hypohidrosis, and pain seems to result from the low expression levels of acetylcholinesterase (AchE) and cholinergic receptor, muscarinic 3 (CHRM3) in the eccrine gland epithelial cells.