Stain

Circumscribed, sharply or indistinctly defined, differently sized and shaped lesion located at skin level and therefore not palpable, which differs from the surrounding skin only in its color, but otherwise has the surface texture of the surrounding skin. A spot can be acquired or congenital. It can be primary or secondary to another skin disease or its healing.

As a rule, spots are circular, possibly oval, striped or jagged. The edges of the spots can be in a particularly striking color contrast to the adjacent skin area, resulting in a sharp border. In other spots, the color at the periphery gradually merges into that of the surrounding skin, which is why such edges are also referred to as "blurred" or "fuzzy". The size of the individual spots varies enormously from pinprick size to palm size etc. In Anglo-American literature, patches are subdivided into "maculae" < 1.0 cm and"patches" > 1.0 cm. In the following, both terms are used synonymously. However, this distinction can be made in descriptions of findings.

• Dark spot (yellow, green, red, blue, brown, black):
  • Dye deposits, melanin, hemosiderin, carotene, bile pigment or foreign substances: coal, powder, tar, bismuth, silver, gold, etc.
  • Increased vascular filling
  • Increased vascularization, e.g. nevus flammeus.
• Light spot:
  • Pigmentation reduced or completely absent: pityriasis alba, leukoderma, vitiligo
  • Constricted blood vessels: e.g. in digitus mortuus.
  • Disturbed blood vessel dynamics: e.g. naevus anaemicus.

• Dark spot (yellow, green, red, blue, brown, black):
  • Dye deposits, melanin, hemosiderin, carotene, bile pigment or foreign substances: coal, powder, tar, bismuth, silver, gold, etc.
  • Increased vascularization and blood filling, e.g. nevus flammeus.
• In the case of red spots, it is useful to distinguish between anaemic and non-anaemic spots for differential diagnostic reasons. A non-anaemizable red spot (apart from a red tattoo) is caused by an accumulation of blood in the dermis. An anaemic spot, on the other hand, an erythema, is caused exclusively by an (inflammatory) dilation of the dermal vessels.
• A dark (not red) spot appears as a contrast to its surroundings and can only be perceived as such. A clearly visible lentigo maligna is easy to diagnose as a morphological pattern because the brown-black spot is a striking feature in an otherwise normal environment. It becomes more difficult when the skin discoloration affects the entire integument evenly. A generalized yellow coloration will be immediately recognizable in good light, as the yellow hue is a non-physiological coloration. The situation is different with the physiologically occurring color, e.g. brown. A generalized brown spot, such as in Addison's disease, in which a homogeneous brown discoloration of the skin occurs, will not initially be perceived as such in purely morphological terms. The initial diagnosis may be "healthy tanning". Only the anamnestic data will lead further.
• Light spot:
  • Decreased pigmentation: Depigmentation, leukoderma
  • Constricted blood vessels: e.g. in digitus mortuus, steroid skin
  • Disturbed blood vessel dynamics: e.g. naevus anaemicus.
• A bright spot forms as a contrast to the surrounding area. If there is a loss of pigment cells or their dysfunction in circumscribed areas of the skin, a light spot appears in the surrounding healthy skin. If islands of unchanged healthy skin remain as "nappes claires" in a surrounding pathologically reddened skin (e.g. nappes claires in erythroderma), these islands of healthy skin are interpreted as apparently pathological light spots although they are unchanged skin. A similar mechanism can be observed in psoriasis that has healed after dithranol treatment. The formerly lesional skin stands out as a light patch against the surrounding area pigmented by dithranol. Here too, the change takes place in the surrounding skin (brown coloration of the horny layer).
• A universal light patch of skin, as in the various forms of albinism or in extensive vitiligo, will not be perceived as a light patch because there is no differentiation from normal skin. Thus, even in the case of extensive vitiligo, it is not the bright spot as such that is disturbing, but the often bizarre islands of normally pigmented or repigmented skin.
• In principle, the following scenarios for a light spot can be derived: Light spots are caused either by a circumscribed disturbance of blood vessel dynamics due to vasoconstriction (anaemic nevus) or by a reduction or complete absence of pigment cells (depigmented scar) or as a negative contrast in a color-altered environment (dithranol discoloration in psoriasis treatment).
• A final possibility of light discoloration arises in the case of sclerotic skin changes. Superficial sclerosis of the dermis leads to a frosted glass effect; existing vessels are rarefied, deeper vessels cannot show through the sclerotic layer, resulting in a whitening effect (e.g. lichen sclerosus et atrophicus).

In addition to the circumscribed patches, diffuse discoloration of the skin is also found. These can affect larger areas but also the entire integument. They are called dyschromia (older term: dyschromasia) if they are due to non-melanogenic deposits of color pigments(icterus, argyrosis, etc.). A change in skin coloration can also be caused by a change in blood circulation (erythema fugax, anxiety pallor, cutis marmorata).

1. Altmeyer P (2007) Dermatological differential diagnosis. The way to clinical diagnosis. Springer Medicine Publishing House, Heidelberg
2. Nast A, Griffiths CE, Hay R, Sterry W, Bolognia JL. The 2016 International League of Dermatological Societies' revised glossary for the description of cutaneous lesions. Br J Dermatol. 174:1351-1358.

3. Ochsendorf F et al (2017) Examination procedure and theory of efflorescence. Dermatologist 68: 229-242