The distribution or extension of a dermatosis must always be emphasized. The disease may be localized (single, isolated lesion) (Fig. 10.1), segmental or regional (Fig. 10.2), diffused or generalized (Fig. 10.3), or universal (Fig. 10.4).

Certain distributions have a remarkable aspect and their identification requires experience. A particular distribution may reflect the exposure of uncovered skin to a pathogen present in the environment, or to the sun. Therefore, these dermatoses will be located mainly on exposed areas: face, neck, neckline, and back of the hands and/or forearms (Figs. 10.5, 10.6, and 10.7). It may be difficult to distinguish a dermatitis caused by an airborne agent from a photoinduced dermatitis. Certain signs may be of help: the airborne agent will tend to accumulate in the exposed cutaneous folds such as the palpebral or retroauricular folds which are naturally photo protected. The airborne dermatitis will thus affect these areas which are not involved in photoinduced dermatitis. Likewise, the anterior cervical area located below the chin is naturally protected from sun exposure, i.e., the submental triangle, which will remain clear in photosensitive dermatitis (Fig. 10.8). Distribution also gives an indication about the underlying mechanism of a lesion: a very sharp demarcation, as if traced using a ruler, is typically found in phototoxic dermatitis (Fig. 10.9), while imprecise demarcation is characteristic of photoallergic dermatitis and photoaggravated disorders (Figs. 10.5 and 10.8).