Pigmentation

Published on 04/03/2015 by admin

Last modified 04/03/2015

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Pigmentation

Skin colour is due to a mixture of the pigments melanin (p. 8), oxyhaemoglobin (in blood) and carotene (in the stratum corneum and subcutaneous fat). Pigmentary diseases are common and particularly distressing to those with darker skin. Disorders of pigmentation mainly involve melanocytes, but other causes are mentioned where relevant.

Hypopigmentation

Pigment loss may be generalized or patchy. Generalized hypopigmentation occurs with albinism, phenylketonuria and hypopituitarism; patchy loss is seen in vitiligo, after inflammation, following exposure to some chemicals and with certain infections (Table 1).

Table 1 Causes of hypopigmentation

Cause	Example
Chemical	Substituted phenols, hydroquinone
Endocrine	Hypopituitarism
Genetic	Albinism, phenylketonuria, tuberous sclerosis, piebaldism
Infection	Leprosy, yaws, pityriasis versicolor
Postinflammatory	Cryotherapy, eczema, psoriasis, morphoea, pityriasis alba
Other	Vitiligo, lichen sclerosus, halo naevus, scarring

Vitiligo

Vitiligo is an acquired idiopathic disorder showing white non-scaly macules. The association with thyroid disease, pernicious anaemia and Addison’s disease suggests an autoimmune aetiology in some cases. About 30% of patients give a family history of the disorder. Melanocytes are absent from affected skin on histology.

Clinical presentation

Vitiligo affects 0.5% of the population, is seen in all races and is most troublesome in those with a dark skin. The sex incidence is equal, and the onset, usually between 10 and 30 years of age, may be precipitated by injury or sunburn. The sharply defined white macules are often symmetrical (Fig. 1). The hands, wrists, knees, neck and areas around orifices (e.g. the mouth) are frequently affected. Occasionally, vitiligo is segmental (e.g. down an arm), generalized or universal. The course is unpredictable; areas may remain static, spread or (infrequently) repigment. In light-skinned individuals, vitiligo may be discernible only in summer, when the non-vitiliginous areas become sun-tanned.

Fig. 1 Vitiligo showing symmetrical involvement of the forearms in a patient with pigmented skin.

Differential diagnosis

Postinflammatory hypopigmentation is often accompanied by other skin changes (Table 1). In chemical leucoderma, a history of exposure to phenolic chemicals should be sought. The hypopigmented macules of leprosy are normally anaesthetic.