Disorders of skin color are caused by disturbances of the four types of pigments in the skin, namely, oxyhemoglobin causing red skin color, reduced hemoglobin causing blue skin color, carotenoids/bile pigments producing yellow color and melanin responsible for brown to black color.
The main color determinant of skin color is melanin, the dark pigment produced by melanocytes in the skin layers. The type of melanin, the amount and distribution of melanin pigment is responsible for the three skin color variations of human race: the black, the brown and the white.
These colors are genetically determined, meaning individuals inherit skin colors from their parents. The normal skin color can be increased or darkened by exposure to ultra violet rays. Depending upon the darkening of the skin to sunlight UV radiation, the skin has been classified into 6 phototypes by Fitzpatrick in 1975. This skin phototype is a very important marker for skin cancer risk, where darker the color, lesser the risk.
Disorders of Skin Color
The main disorders of skin color depend upon the quantity and distribution of melanin pigment. It may either increase the pigmentation resulting in dark skin, known as hypermelanosis or hyper pigmentation; or decrease the pigmentation leading to skin lightening known as hypomelanosis or hypo pigmentation.
Causes of Dark Skin Discoloration
Increase of the melanin pigment causing dark brown or black skin spots can be due to two reasons:
- Hyper pigmentation caused by increased number of melanocytes in the epidermal layer of the skin. A lentigo or sun spot is an example for this type of melanocytic hypermelanosis.
- Dark skin discoloration caused by increased production of the melanin pigment, known as melanotic hypermelanosis. The classic example for this type of skin darkening or dark pigmentation is melasma or solar melanosis.
The hyper pigmentation causing dark brown or dark skin spots can be the result of following factors:
- Genetic: As in birth marks and moles.
- Hormonal: As in Addison’s disease, where it is caused by an increase in the amount of a circulating pituitary melanotrophic hormone in response to adrenocortical insufficiency.
- Solar: Due to the ultraviolet radiation, as in sun tanning.
Causes of Loss of Skin Color or Skin Lightening
Decrease of melanin pigment in the skin is known as hypomelanosis or hypo pigmentation. When there is complete loss of melanin pigment it is called depigmentation, as in vitiligo.
- Skin lightening caused by a decrease in the number or absence of melanocytes. This is called melanocytopenic hypomelanosis, the classic example being vitiligo.
- When the loss of skin color is caused by a reduction in the melanin production within the melanocytes, this is called melanopenic hypomelanosis. Example is albinism.
Diseases causing loss of skin color may be classified as:
- Genetic, as in albinism.
- Autoimmune, as in vitiligo.
- Post inflammatory leukoderma: as in leprosy, psoriasis, pityriasis alba, polymorphous light eruption or tinea versicolor.
Most of these disorders of skin color are treatable (melasma) or partly treatable (vitiligo); while some are not treatable (albinism).
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Reference
- Fitzpatrick's Color Atlas & Synopsis of Clinical Dermatology, Fifth Ed, 2005
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