Physical urticaria is a distinct subgroup of the urticaria that are induced by an exogenous physical stimulus rather than occurring spontaneously. There are seven subcategories that are recognized as independent diseases. Physical urticaria is known to be painful, itchy and physically unappealing; it can recur for months to years of a persons life. Signs and symptoms
Urticaria are characterized by dermal edema (wheal, swollen) and erythema (flare, red), also known as hives. Hive lesions typically last less than 24 hours and are usually itchy (pruritic). Hives can appear anywhere on the body and they may change shape, move around, disappear and reappear over short periods of time. Types of hives
Acute urticaria (short-term): can develop suddenly and will last less than 6 weeks. About 1 in 6 people will have acute hives at one point in their life.Chronic urticaria (long-term): can develop suddenly and will persist more than 6 weeks. This type of urticaria is uncommon and occurs in only 0.1% of the population. 20% of people with chronic urticaria report still having problems 10 years after its onset. Causes
The cause of physical urticaria is unknown but it has been suggested to be an autoimmune disease.
Molsidomine and nitroprusside already contain nitrogen in the right oxidation state (+2) and liberate NO without the aid of enzymes.NO stimulates the soluble form of the enzyme guanylate cyclase in the smooth muscle cells of blood vessels. Guanylate cyclase produces cyclic guanosine monophosphate (cGMP) from guanosine triphosphate (GTP). cGMP in turn activates cyclic nucleotide-dependent protein kinase G, which phosphorylates various proteins that play a role in decreasing intracellular calcium levels, leading to relaxation of the muscle cells and thus to dilation of blood vessels.The most important effect in angina is the widening of veins, which increases their capacity to hold blood ("venous pooling") and reduces the pressure of the blood returning to the heart (the preload). Widening of the large arteries also reduces the pressure against which the heart has to pump, the afterload. Lower preload and afterload result in the heart needing less energy and thus less oxygen. Besides, NO donated by nitrovasodilators can reduce coronary spasms, increasing the hearts oxygen supply.PDE5 inhibitors block deactivation of cGMP by the enzyme phosphodiesterase-5. In combination with the increased cGMP production caused by nitrovasodilators, this leads to high concentrations of cGMP, extensive venous pooling, and potentially life-threatening hypotension. Nitrate tolerance
Nitrates exhibit development of tolerance, or more specifically tachyphylaxis, meaning that repeated application results in a fast decrease of effect, usually within 24 hours. A pause of six to eight hours restores the original effectiveness. This phenomenon was originally thought to be a consequence of depletion of thiol (–SH) groups necessary for the reduction of nitrates.