Learning about idiopathic photodermatoses

The photo biological response will depend on two factors: the penetration of wavelength and the energetic absorption. Different wavelengths penetrate different depths of skin; the longest wavelengths will penetrate much deeply on skin. Therefore UVA (from 320 to 400 nm) radiation produces a much deeper cutaneous response than UVB (from 290 to 320 nm).

On the phenomena of photo sensibility two reactions can take place: photo-toxic reactions and photo allergic reactions. The first ones are more frequent and have a direct relation with the dose that has been absorbed, any individual can present them, it only compromises photo-exposed areas with a very precise delimitation (for instance sunburn). The photo-allergic reactions represent the acquisition of an altered reactivity to the UV radiation mediated by an immune mechanism; therefore lesions can vary from an urticaria reaction to eczematous or papular eruptions which are extended generally beyond exposed areas. These photo allergic reactions can have a well known cause (for instance it can be associated to drugs, diseases of connective tissue or porphyrias among others) or they can also be idiopathic diseases of an unknown cause.

Altogether these diseases comprise the biggest group of inflammatory disorders induced by UV radiation. With the exception of chronic actinic dermatitis, most of these conditions start to manifest very early during childhood and if they are severe they can affect tremendously the life of the child, hence the importance of knowing their main characteristics especially for professionals.

How to handle idiopathic photodermatoses

In most of the cases what is pursuit is a preventive technique, mainly focused on the control on exposition to sunlight and an exogenous protection with sun blocks and with the appropriate clothes. Sun blocks or sunscreens are preparations formulated to protect skin from UV radiation. They are divided in two categories: physical and chemical agents. Physical agents reflect and disperse the UV radiation and visible light from a layer of inert metal particles which form an opaque barrier. Because they are capable of dispersing the light, they are less variable than chemical agents.

Chemical agents act as protective filters absorbing UV radiation. Each chemical absorbs a specific range of wave and then it remits the energy as heat. The efficacy of sun blockers depends on their factor of sun protection (SPF) and of their substantivity. The SPF is a measure which represents the reason between the necessary doses of radiation of UV to produce a minimum erythema on protected skin versus the necessary doses to produce it on a portion of skin that is not protected. The effectivity of the product could be less than the expected given the value of SPF; it could happen because of perspiration, immersion on water or the variation of the intensity of sunlight versus intensity of light on laboratory.

The effectiveness of the product can also vary depending on its substantivity, or the measure of repellency to water versus resistance to water or perspiration. A sunscreen with resistance to water can maintain its capacity.