Dermatology

British Medical Journal, March 25, 2000 by Peter A Foley

Summary points

Lasers are now used for depilation and for removing tattoos without leaving scarring

Photodynamic therapy is increasingly being used to treat skin tumours

Narrowband ultraviolet B light has largely replaced other forms of phototherapy for psoriasis and severe atopic dermatitis

Kaposi's sarcoma is associated with human herpesvirus 8

Better understanding of androgenetic alopecia has allowed the development of effective treatment with finasteride

More specific immunomodulatory drugs, including those adapted from other areas of medicine, are now being used in dermatology

Dermatology is a diverse specialty. It is one of the few areas of medicine in which a specialist can be both physician and surgeon, treating malignant and non-malignant disease in a population ranging from infants to adults. It is constantly evolving. In recent years, dermatology has incorporated techniques and treatments from other disciplines, such as genetic studies and the use of immunosuppressive agents, and pioneering work in the rapidly expanding area of laser therapy has broadened the diagnostic and therapeutic horizon. Advances in the understanding of dermatological conditions have allowed dermatologists to refine treatments.

Methods

The recent advances discussed in this review are common knowledge among dermatologists. The choice of topics is personal and, after a review of the major dermatological journals, covers a selection of aetiological and therapeutic areas that are making a difference in practice now or are likely to do so in the very near future.

Lasers

Laser therapy in dermatology is expanding rapidly and can be used to treat vascular and pigmented lesions, to remove tattoos, for depilation, and for skin resurfacing. It is now possible to choose laser treatments based on a specific wavelength, pulse width, and fluency. This allows selective targeting and destruction of specific cellular and subcellular structures while the normal surrounding structures are spared thermal damage. Scarring and other complications are minimised.[1-3]

Treatments available for tattoo removal had been limited by the likelihood of scarring.[3-5] The new short pulsed laser treatments (including pulsed dye and Q switched ruby, Nd:YAG, and alexandrite lasers) which are now the preferred means of tattoo removal are based on the theory of selective photothermolysis. Thermal damage is localised by the choice of a wavelength that is selectively absorbed by the specific target chromophore and by delivering sufficient energy to damage the target within a pulse duration which is briefer than the time taken for the target to cool (thermal relaxation time).[3 4] Proposed mechanisms for removing pigment include rephagocytosis of laser altered pigment particles and transepidermal elimination via the scale crust that forms.[3] Patients may require four to eight treatments every six to eight weeks to achieve maximum improvement, and the response of professional tattoos may be slower and less complete.[1 6]

The Q switched ruby, Nd:YAG, and alexandrite lasers have also been used to treat disorders of cutaneous pigmentation such as solar lentigines, cafe au lait macules, ephelids, post-inflammatory pigmentation, melasma, and naevus of Ota.[5 6]

Although laser assisted epilation (or phototricholysis) is not considered a permanent way of removing hair, some patients will achieve a long term reduction in hair density after a single treatment.[7 8] Lasers with wavelengths of 600-1100 nm (such as the ruby, alexandrite, Nd:YAG, and diode) penetrate deeply and are absorbed by eumelanin in the hair shaft and follicle, which is thought to be the target chromophore.[1] Relatively long pulses are used to create sufficient damage to the follicles. The predominant effects are photomechanical (shock waves) with nanosecond domain Q switched pulses and photothermal (heat) with millisecond domain pulses.[8] These lasers work best on pale skin with pigmented hairs.

Photodynamic therapy

In general, surgical excision is the most effective and preferred treatment for primary skin cancer. However, photodynamic therapy is increasingly being used to manage many epithelial tumours, including those of the skin. After the selective uptake and retention of an exogenously administered or endogenously formed photosensitiser within tumour tissue, exposure to light of a specific wavelength results in intracellular activation of the photosensitiser. This leads to cell death by the formation of radical products (type I mechanism) or intracellular singlet oxygen (type II mechanism).[9-12]

Topically applied aminolaevulinic acid (converted intracellularly to predominantly protoporphyrin IX) and systemically administered photosensitisers have been used to treat cutaneous neoplasms.[10 13] Irradiation with the appropriate wavelength of light is performed when an optimal ratio of photosensitiser in tumour tissue compared with non-tumour tissue is reached. This tends to occur four to six hours after topical aminolaevulinic acid is applied or 96 hours after systemic photosensitisers are given intravenously.[10]