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KEY POINTS
Photosensitive disorders are cutaneous diseases caused by an abnormal reaction to ultraviolet or visible light.
The differences in quality and quantity of melanin pigmentation between skin types play a role in the prevalence of photosensitivity between racial groups.
A diagnosis of photosensitivity involves a detailed history, clinical examination of sun-exposed areas and sun-protected areas, phototesting, photopatch testing, and photoprovocation testing.
Treatment plans should emphasize photoprotection, symptomatic relief, and phototherapy or systemic medications when indicated.
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Sunlight is important for human life because it plays a key role in psychological well-being and physiologic systems.1 The solar radiation that reaches the earth’s surface is in the electromagnetic spectrum with wavelengths greater than 290 nm. This includes ultraviolet (UV) B (290 to 320 nm), UVA (320 to 400 nm), visible light (400 to 760 nm), and infrared radiation (760 nm to 1 mm). Fortunately, the more damaging higher energy short wavelengths, UVC (<290 nm), are absorbed by the ozone layer and atmosphere and do not reach the earth’s surface.2 When UV interacts with the skin, the electromagnetic waves are absorbed, reflected, or scattered.
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Photosensitivity is a term that refers to a group of skin diseases induced or exacerbated by UV or visible light. Photosensitivity is synonymous with photodermatosis, and these terms are sometimes used interchangeably. Photosensitive disorders can be classified into four categories: (1) primary or immunologically mediated disorders, (2) drug- and chemical-induced disorders, (3) photoaggravated disorders, and (4) inherited disorders with defective DNA repair or with chromosomal instability [Table 29-1].3,4
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SKIN PIGMENTATION AND PHOTOSENSITIVITY
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Skin color is dependent on the number, type, and distribution of melanin pigment granules, hemoglobin, and carotenoids.5 Melanocytes located in the basal epidermis produce melanin pigment granules within specialized organelles called melanosomes and then transfer them to neighboring keratinocytes via dendritic processes.6 The photoprotective property of melanins is attributed to their ability to absorb and scatter UV light, converting it into a less harmful form of energy, thermal heat. They are located in the supranuclear region of melanocytes ...