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BASIC PHOTOBIOLOGY

Photodermatology is the study of photobiology as it relates to the skin. The emphasis in this chapter will be on abnormal responses to ultraviolet radiation (UVR), the photodermatoses. To help in understanding these disorders and aspects of therapeutic photodermatology, some basic science knowledge is required.

Dermatologically relevant non-ionizing radiation

The non-ionizing radiation in sunlight that is of dermatologic relevance (Fig.17.1) lies between the ultraviolet (UV) and the infrared areas of the electromagnetic spectrum, a region that includes:

Each of these different wavelengths of radiation has different effects on the skin, and for any wavelength the effect depends on the intensity and duration of exposure.

    Short-wavelength UVR (UVC) is used in investigative photobiology and in germicidal lamps, but is absorbed by the ozone layer and is therefore of little relevance in biologic terms. Some lasers therapeutically utilize longer wavelengths (Ch. 5) but are not discussed here.

    Infrared radiation produces heat; is probably important in the causation of some skin disorders, such as erythema ab igne (see Ch.15); and is used therapeutically to eradicate some cutaneous vascular lesions (infrared coagulator); but it is not relevant in the major photodermatoses.

    The wavelengths of main dermatologic relevance are the UVA and UVB regions of the UV spectrum, and, to a lesser extent, visible light.

Figure 17.1 The electromagnetic spectrum in relation to the skin.

Effects of radiation of different wavelengths

Within the UV and visible parts of the spectrum, penetration to deeper parts of the skin increases with wavelength. The fall-off in penetration that occurs with each wavelength is in part due to scatter and reflection from different parts of the skin, but the major factor is absorption of radiation by various chemicals in the skin, known as chromophores. Some of the most relevant chromophores and their biologic effects include:

DNA, RNA, urocanic acid, keratin, and other proteins in the epidermis largely absorb shorter wavelengths below 300nm. As a result of this absorption process, UVB is about 90% absorbed within the epidermis and essentially no UVB penetrates deeper than the upper dermis, whereas about 30% of UVA at 400nm and 85% of red light penetrates into the dermis. At least some aspects of photoaging, such as wrinkles, must therefore be primarily due to longer wavelengths, as the shorter wavelengths do not penetrate significantly to the majority of the collagen of the skin. As currently available sunscreen chemicals are much more effective at blocking shorter-wavelength UVB than UVA, they will inevitably provide better protection against burning than against the development of wrinkles.

    Dehydrocholesterol absorbs maximally around 270–280nm to form vitamin D.

    Melanin absorbs maximally below 300 nm, with gradually decreasing absorption throughout the visible light spectrum and into the near infrared. This explains the greater penetration of UVR into pale skin than into more pigmented skin (Fig. 17.2).

    Oxygenated hemoglobin has a major absorption peak at 418nm, and less marked peaks at 542 and 577nm. Knowledge of absorption spectra is important in laser therapy, where the ability to target a specific skin structure depends not only on the absorption characteristics of that structure, but also on whether the laser wavelength will penetrate to the correct depth and whether there is absorption by other chromophores. In the case of hemoglobin (which is a potential laser target within blood vessels), the ideal laser wavelength appears to be at the major absorption peak of 418nm. However, laser light at thiswavelength is therapeutically ineffective, because it is significantly absorbed by melanin and does not penetrate adequately to the dermis; wavelengths between 540 and580nm are more effective.

Figure 17.2 A dramatic demonstration of the different penetration into white skin (badly burned) compared with black skin (unburned) despite the same UV dose to each area. Photosensitivity in cattle is usually the result of ingested plants that are either photosensitizers or cause liver toxicity.

Action spectrum

This is the efficacy of radiation of different wavelengths in producing a specific effect, assuming that the same dose at each wavelength is administered. The results for each wavelength are usually expressed relative to the most effective wavelength. This can be applied to clinical effects such as efficacy in producing erythema, treatment of dermatoses, treatment of neonatal jaundice, and prevention of rickets.

    Maximum erythema occurs following exposure to UVR of 300–305nm (in the UVB range), whereas the dose of UVA required to produce the same degree of erythema is at least 1000-fold greater. The dose of UVA in natural sunlight is about 100 times higher than that of UVB, due to the absorption of shorter wavelengths by the ozone layer. The net result is that UVB accounts for about 90% of erythema due to sunlight, and UVA erythema is less important.

    Similarly, the action spectrum for the treatment of many common dermatoses, such as psoriasis, demonstrates that the main effective wavelengths are in the UVB part of the spectrum. Patients with these dermatoses therefore often improve in natural sunlight or after UVB therapy, but derive limited benefit from UVA-producing sunbeds (tanning beds), which are designed to have minimal UVB output. However, burning due to UVB therapy may be a limiting factor. The TL01 lamp, which has become popular in dermatology, achieves a useful compromise: it emits a narrow band of radiation at 311 nm, which is less erythemogenic than shorter wavelengths but still therapeutically active.

Minimal erythema doses

The minimal erythema dose (MED) is the smallest dose of UVR that produces visible erythema. The minimal phototoxicity dose (MPD) is exactly the same thing, but is the term used when the production of erythema also involves a photosensitizing drug, including therapeutic use in photochemotherapy. This type of assessment is used to determine UV sensitivity before therapeutic UVR, and typically involves performing a sequence of doses to 1-cm diameter areas of skin (Figs 17.3–17.5). It is also used in diagnostic phototesting to identify individuals who have abnormal degrees of photosensitivity (discussed in more detail later).

Figure 17.3 Testing for minimal erythema dose (MED). The machine shown is for testing the MED to TL01 (311 nm) UVR. The fluorescent tube producing UVR is in the cabinet that does not transmit this wavelength (although blue-visible light can be seen). A series of graded metal grids allow a series of doses to be transmitted, with 2 increments. With knowledge of the highest dose transmitted, and by observing the number of sites that develop erythema after 24 h, the MED can be determined. Note the the arm would actually be flat over the grid apertures during testing but has been tilted to demonstrate the apparatus.

Figure 17.4 Chlorpromazine photosensitivity. This affects the dorsum of the hand and exposed area of the wrist. The distal part of the fingers is often less affected, as shown here.

Figure 17.5 Phototests for the patient shown in Figure 17.3. Four square sites were irradiated, and all have produced spreading erythema, down to a dose of about a 50th of that which would cause erythema in an average person in the absence of photosensitizing medication.

Visual assessment of minimal erythema of small test sites is inexact and varies at different body sites. When MED tests are performed prior to UV therapy, it is common to use a therapeutic dose of 50–70% of the recorded MED to ensure that more sensitive areas than the test site do not receive significantly erythemogenic doses. Erythema due to UVR has a different time course at different wavelengths, so the timing of the assessment is important:

Skin type

The sun-reactive skin type is an inexact but useful tool for estimating individual sensitivity to sunlight. Six types are described (Table 17.1), which take into account the susceptibility to sunburn and ability to tan; some types may correlate with racial phenotype, such as the olive skin and easy tanning of Mediterranean individuals, or the pale skin, freckles, red hair, and easy burning of Celts (Fig. 17.6). The skin type assessment is used by sunscreen manufacturers to advise on the potency of sunscreen required for different skin types. However, when extrapolated for the therapeutic use of UVR, this method for assessing the risk of undue erythema is limited, because there is an approximately fourfold variation in the range of MEDs within any group of individuals who have the same self-reported skin type.

Figure 17.6 Freckles on the cheek in a patient of skin type I–II, who also has red hair.

White/Cox: Diseases of the Skin, 2ed.(c) 2006, Elsevier Inc. All rights reserved.