In recent years, science began to bring the ultraviolet laser into use in treatment of psoriasis.

Lasers that emit light in wavelengths from ultraviolet to infrared have wide use in dermatology. Their use in treatment of psoriasis is very specific within the spectrum of ultraviolet radiation.

Lasers: What They Are
The word LASER is an acronym: Light Amplified by Stimulated Emission of Radiation. In the simplest terms, stimulated emission is a principle of optical physics that amplifies light (or other electromagnetic radiation) by (1) segregating light of a single wavelength or very narrow band of wavelengths, and (2) "pumping" the light to high intensity.

The first laser was developed in 1960 using a ruby to segregate visible light in a narrow red wavelength. Since then lasers have been developed across the electromagnetic spectrum from very short ultraviolet to long infrared wavelengths, using a variety of substances to separate radiation of the selected wavelength. These substances include gases (carbon dioxide, argon, etc.), metals and metal vapors (copper), crystals (ruby, alexandrite, etc.), and dyes.

The atoms or molecules of the substances that seperate selected wavelengths of radiation are said to be excitable. When "pumped" by an electrical, chemical or optical energy source, they become pumped-up. Then, when hit by one unit of radiation (a photon) each excited atom or molecule is stimulated to emit another identical photon. Mirrors are used to bounce the photons back and forth, stimulating emission of more photons with each pass. At a given point in the process, the highly selected, extremely bright, monochromatic (single wavelength) radiation is allowed to emit from the laser as a coherent beam. The beam is highly controllable, allowing precise direction of radiation energy into a target area. The beam exceeds the intensity of any natural radiation source.

Electromagnetic wavelengths are measured in fractions of a meter. The wavelengths of lasers used in dermatology range from long infrared (10,600 micrometers—millionths of a meter) to short ultraviolet (193 nanometers—billionths of a meter). As new lasers are developed, the list of instruments and wavelength applications will grow.

Photochemical Interactions
Photochemical interactions are the interactions principally involved in the phototherapy of psoriasis. Ultraviolet (UV) light induces biologic reactions in skin cells that can result in clearing of psoriatic lesions. The UV radiation of sunlight contains all ultraviolet wavelengths. Phototherapy investigators have shown that UV wavelengths from 300 to 313 nanometers (nm) are most effective in clearing psoriasis. UV lamps used in phototherapy are selective in this narrow band of wavelengths.

Excimer lasers in the 300 nm to 313 nm spectrum are now being used and/or investigated in the treatment of psoriasis. An advantage of the UV laser over a UV lamp is the ability to put the coherent laser beam on a very restricted area—e.g., elbow or knee—avoiding UV exposure of larger areas of skin. It is likely that these lasers will be of benefit in helping to clear small, localized plaques of psoriasis, resistant to traditional therapy. Since the use of lasers in the treatment of psoriasis is a new treatment, information on effectiveness and side effects is still being collected.

Some Terminology
This laser terminology is helpful in understanding laser applications:

Continuous wave laser beam: Laser light emitted in an uninterrupted beam.

Excimer laser: Excimer is a compound word describing an excited dimer laser. A dimer is a compound formed by two identical molecules; the excited dimer is "pumped" by an energy source to become excited to emit photons of a selected wavelength.

Photon: A unit of light. A photon has the unique ability to behave as a particle and as a wave. Although it is identified as a particle, its energy level is defined as wavelength.

References

AAD Dermatology Insight, Spring 2000, P. 4

Asawanonda P, Anderson RR, Chang Y, Taylor CR. 308 nm excimer laser for the treatment of psoriasis: a dose-response study. Arch Dermatol 2000; 136:619-624.