Phototherapy

Excerpt

Phototherapy is a major effective therapeutic treatment modality in dermatology and has influenced the treatment of various skin diseases dramatically. It consists of a controlled administration of non-ionizing radiation to the skin in various dermatoses, which involves the ultraviolet part of the electromagnetic spectrum and commonly includes ultraviolet A (UVA) spectrum, ultraviolet A-1 (UVA-1) spectrum, UVA spectrum with a psoralene sensitizer (PUVA), and ultraviolet B (UVB) spectrum, i.e., broad-band (BB)-UVB, or narrow-band (NB)-UVB.

The UVA spectrum is usually between 320 and 400nm, whereas UVA-1 (340-400nm) has longer wavelengths of UVA.. The UVB spectrum is between 280-320nm, which may be delivered as full-spectrum (broad-band UVB lamps 270-350nm, along with the shorter wavelengths from UVA spectrum) or delivered as small-spectrum (narrow-band UVB lamps 311-313nm).

Few specialized types of phototherapy exist, that include lasers, photodynamic therapy (PDT), bath-PUVA, and extracorporeal photochemotherapy. Phototherapy is still the most frequently used treatment modality for different skin diseases, including parapsoriasis, psoriasis, pityriasis lichenoides chronica, eczema, atopic dermatitis, vitiligo, photodermatitis, polymorphous light eruption, actinic prurigo, hydroa vacciniforme, cutaneous porphyrias, Mycosis fungoides, and many others.

UVB phototherapy has different properties like anti-inflammatory, immunosuppressive, and cytotoxic. The mechanisms of its action are unclear but include the induction of cis-urocanic acid, Langerhans cell depletion, altered antigen presentation, decreased activity of natural killer(NK) cells, and apoptosis of T lymphocytes and keratinocytes. The mechanisms of action of PUVA include the cross-linking of DNA through psoralen photoadducts, DNA replication inhibition, depletion of Langerhans cell, and immunosuppressive effects on T-lymphocyte function and migration.UVA-1 phototherapy penetrates deeper into the dermis, and it induces interstitial collagenase and cytokines, resulting in the softening of the sclerotic skin. Photopheresis results in dendritic cells acquiring antigen from apoptotic lymphocytes, which elicit a specific immune response without causing systemic immunosuppression.