Lasers in dermatology--a critical update.

Lasers are accepted for treating nevus of Ota, other pigmented lesions, hair removal, vascular lesions, leg veins, tattoos, and for skin resurfacing. These are photothermal treatments, in which certain skin "targets" are heated, followed by selective wound healing. Small pigmented targets such as the dermal melanocytes in nevus of Ota, are best treated with short (< 1 microsecond) laser pulses. Large targets, such as hair follicles, have long thermal relaxation times and are best treated with longer pulses. In general, the ideal pulse duration is about equal to the thermal relaxation time for pigmented targets. However, sometimes the actual target is not pigmented and is at some distance from a pigmented structure. For example the follicular stem cells, which are not pigmented, line the outer root sheath far away from the pigmented hair shaft. These cells appear to be an important target for permanent hair destruction. Pulses longer than the thermal relaxation time of the hair shaft allow heat conduction and better damage of follicular stem cells. Epidermal cooling works far better with pulses longer than about 10 ms, delivered through a cold medium (e.g., cold sapphire in contact with the skin). Thus, the combination of cooling and long near-infrared laser pulses allows safe and effective pigmented hair removal in all skin types. In contrast, epidermal protection from short pulses is best with dynamic pre-cooling (e.g., cryogen spray), for example during portwine stain treatment. A major challenge for the future of photothermal laser treatments is to develop ways of treating non-pigmented skin "targets". New uses for lasers are emerging. Diagnostic laser imaging and spectroscopy will soon emerge in dermatology. A near-infrared laser confocal microscope provides histology-like images of human skin. Imaging is painless and takes only a few minutes. Lesions including melanoma, basal cell and squamous cell carcinoma, microvascular and inflammatory lesions, dermatophytes, verrucae, etc, have distinct appearances. However, sensitivity and specificity of laser-based diagnostic imaging has not yet been compared with histopathology. Laser phototherapy is also emerging in dermatology. The 308 nm excimer laser has recently been shown to clear psoriasis faster than conventional phototherapy. Scalp psoriasis may soon be treated by fiber-optic delivery of this UV laser. The variety and utility of lasers in dermatology will probably continue to grow.