Increased levels of melanin in the epidermis result in a state known as hypermelanosis or hyperpigmentation. Certain conditions, such as pregnancy or Addison's disease (decreased function of the adrenal gland), may cause a greater production of melanin and hyperpigmentation. Exposure to sunlight is a major cause of hyperpigmentaion, and will darken already hyperpigmented areas. Hyperpigmentation can also be caused by various drugs, including some antibiotics, antiarrhythmics and antimalaria drugs.

Lichen simplex chronicus is a skin disorder with severe itching that causes thick, dark patches of skin to develop. Lamellar ichthyosis (fish scale disease) is an inherited disease that also is characterized by darkened, scaly, dry patches of skin.

Generalised hyperpigmentation

Generalised bronze hyperpigmentation may arise due to excessive circulating melanocyte stimulating hormone (MSH) and occurs in 95% of those with Addison's disease, when it is more marked on pressure areas, in flexures, scars and buccal mucosa. It is associated with loss of pubic and axillary hair in women. Early morning cortisol is a screening blood test for Addison's disease. Generalised bronze or greyish hyperpigmentation seen in 90% of those with haemochromatosis is also primarily due to melanin. It is more prominent on the genitals, in the flexures and on sun-exposed sites, particularly the face. Generalised pigmentation can also rarely be a sign of metastatic or secondary melanoma.

Localised pigmentation

Localised pigmentation may be due to:

• Melanin (the compound produced by pigment cells or melanocytes) e.g. postinflammatory pigmentation, pityriasis versicolor, phytophotodermatitis (plant dermatitis), melasma, poikiloderma of Civatte, urticaria pigmentosa (mastocyosis)
• Keratin (the protein produced by skin cells or keratinocytes) e.g. erythrasma , acanthosis nigricans, ichthyosis, xerosis (dry skin)
• Haemosiderin (the breakdown product of haemoglobin, the red pigment in blood cells) e.g. capillaritis (pigmented purpura), bruises
• Drug eruptions, e.g. fixed drug eruption

In hyperpigmentary skin disorders, melanin deposition is increased and melanocytes present may be normal or increased. Hyperpigmentation may result from the following:

Increased melanin deposition

• Generalized – Addison’s, haemochromatosis, Grave’s, ACTH and MSH producing tumours, drugs, sun-exposure
• Localized – freckles, café-au-lait macules, Peutz-Jeghers syndrome, melasma, melanosis

Increased melanocytes

• Lentigenes – lentigo simplex, actinic lentigo, LEOPARD syndrome, Peutz-Jeghers syndrome
• Nevi – melanocytic nevi, dysplastic nevi, blue nevus

Pigmentary incontinence

Post-inflammatory hyperpigmentation, macular amyloidosis, fixed drug eruption, incontinentia pigmenti

Hyperpigmentation with epidermal changes

• Lentigo – lentigo simplex, actinic lentigo
• Reticulate pigmented dermatosis – Dowling-Degos, Kitamura’s, dyskeratosis congenital, Becker’s naevus

Reactive pigmentation and melanocytic colonization

Solar lentigo, seborrhoeic keratosis, melano-acanthoma, basal cell carcinoma, solar keratosis, adnexal tumours, clear cell acanthoma.