Phototherapy is a second-line treatment modality for the most common
dermatoses that is safe and effective. Most
phototherapy regimens denote the use of ultraviolet (UV) radiation of different wavelengths in the management of several
dermatoses. Currently, irradiations with broadband UVB (290-320 nm), narrowband UVB (311-313 nm), 308 nm
excimer laser, UVA 1 (340-400 nm), UVA with
psoralen (PUVA), and
extracorporeal photochemotherapy (
photopheresis) are being used. Beneficial effects of UV radiation are far from being completely understood.
Dermatoses that may benefit from such approach are numerous, with
psoriasis,
parapsoriasis,
atopic dermatitis,
cutaneous T-cell lymphomas,
morphea, and
vitiligo vulgaris as main indications. UVB radiation primarily acts on cells at the epidermis and the epidermodermal junction, while UVA radiation affects epidermal and dermal components, especially blood vessels. UVradiation has im- mediate and delayed effects. Immediate effects are the formation of
DNA photoproducts and DNA damage leading to apoptosis of heratinocytes, Langerhans cells, activated T-lymphocytes, neutrophils, macrophages, NK cells, fibroblasts, endothelial cells, and mast-cells, cell membrane damage by lipid peroxidation, and isomerization of chromophores such as
urocanic acid. Delayed effects include synthesis of
prostaglandins and
cytokines that play important roles in immune suppression. Systemic and local immune suppression, alteration in
cytokine expression (induction of
interleukin-1 (
IL-1) receptor antagonist, decrease in IL-2, increase in IL-IO, IL-15), and cell cycle arrest may all contribute to the suppression of disease activity. PUVA is a form of controlled and repeated induction of phototoxic reactions which uses UVA light to activate chemicals known as
psoralens. The conjunction of
psoralens with epidermal
DNA inhibits
DNA synthesis and causes cell apoptosis. PUVA also causes an alteration in the expression of
cytokines and
cytokine receptors.
Psoralens interact with
RNA,
proteins and other cellular components and indirectly modify
proteins and
lipids via
singlet oxygen-mediated reactions or by generating of
free radicals.
Psoralens and UV radiation also stimulate melanogenesis with variable effects in patients with
vitiligo vulgaris. Extracorporealphotopheresis is treatment modality used in management of erythrodermic
cutaneous T-cell lymphomas. It is very potent in induction of lymphocyte apoptosis. Despite the introduction of numerous potent bioengineered systemic medications in the field of dermatology,
phototherapy remains established, and often preferred, option for the most common
dermatoses.