Psoriasis is a chronic scaly skin condition that affects primarily the top layer of the skin, or epidermis. The cause of psoriasis, while not entirely understood, is due in part to an individual's inherited genetic background and its interactions with the immune system and skin cells. Within psoriatic skin, there is an exaggerated immune response, which leads to overproduction, or hyperproliferation, of skin cells in the epidermis. Overactivation of the immune system in psoriasis also leads to inflammation, and together these phenomena are manifested by the thick, red, scaly skin that characterizes psoriasis.
Patients with widespread psoriasis or psoriasis that has not been responsive to topical treatments are often treated with ultraviolet light therapy, or phototherapy. Phototherapy for psoriasis usually consists of three visits per week to a dermatologist's office, during which the patient stands for several minutes in a specialized light booth that emits ultraviolet light onto the skin. There are two main types of phototherapy currently used for the treatment of psoriasis, narrow-band ultraviolet B (NB-UVB) and psoralen with ultraviolet A (PUVA). Ultraviolet light is actually divided into three groups, UVA, UVB, and UVC, based on the wavelength of the light rays involved. UVC from the sun does not reach the earth's surface, however, due to the presence of the ozone layer, and therefore our skin is only exposed to UVA and UVB, which can be produced artificially by special bulbs. With PUVA, the patient takes pills by mouth that contains psoralens, a potent photosensitizer, before each treatment. This increases the penetration of the light into the skin, making it more effective.
Light therapy acts to suppress both the hyperproliferation as well as the immune response that occurs in psoriasis. The ultraviolet light is absorbed by DNA in the skin cells, and this leads to a cascade that slows DNA synthesis and in turn decreases the production of new cells. Ultraviolet light also decreases the number of specialized immune cells, called Langerhans cells, in the skin, and therefore effectively suppresses the immune system within the skin.
You may wonder why, in a time when office visits can be very costly because of insurance co-pays, it is necessary to see a dermatologist rather than visiting a local tanning salon for treatments. There are several reasons for this, but perhaps the most important is that the light booths used in medical offices emit different wavelengths of ultraviolet light than those in commercial tanning salons. In NB-UVB booths, the light is composed mostly of rays from a very narrow portion of the UVB spectrum, usually 311-313 nanometers in length. In PUVA, only rays of a longer wavelength, which fall in the UVA spectrum, are used. Although the ultraviolet light used in tanning salons varies depending on the type of booth and the bulbs used, in general they are primarily UVA with a small percentage of UVB. Exposure of the skin to certain wavelengths of UVB can greatly increase one's risk of developing skin cancers, while exposure to UVA can cause aging of the skin, leading to premature wrinkles, sunspots, and other signs of aged skin. Thus, it is best to avoid unnecessary exposure to ultraviolet light, and going to a tanning salon may result in getting much more than you bargained for.
Another reason to see your dermatologist for light therapy is so that the light treatments can be done as safely as possible and with close supervision. There are medically developed protocols that are followed to minimize side effects of phototherapy, most commonly burning (which mimics a sunburn), while still treating the psoriasis effectively. As skin tends to become tolerant to ultraviolet light, doses of light and time spent in the light booth are slowly increased with each visit to allow for continued response. Such adjustments are best left in the hands of trained technicians who have experience operating the equipment and treating patients with psoriasis.