Postinflammatory hyperpigmentation is one of the most common causes of dark marks on the skin, yet it is one of the hardest to treat. It can affect both the epidermis (the outermost barrier layer of the skin) and the dermis (the deeper tough layer that gives strength and elasticity to the skin).
The skin is formed from two layers of skin - the outer epidermis forms a barrier and the the inner dermis gives strength and elasticity to the skin. The pigment producing cells 'melanocytes' are located at the junction between the epidermis and the dermis. Melanocytes are interspersed between the keratinocytes that make the skin barrier and send tree-like projections to supply pigments (melanin) to the keratinocytes. In the normal healthy skin there is no melanin pigment or melanocytes below the superficial layer of the skin (epidermis).
Any form of injury, inflammation or rash can lead to damage to the cells at the junction of the epidermis and the dermis. This causes melanocytes to 'fall' deeper within the skin the level of the dermis. It can also cause increased pigment production by melanocytes remaining within the dermis.
Post inflammatory pigmentation will generally improve slowly with time as immune cells eliminate the melanocytes within the dermis. The most important element of treatment is to identify the cause of inflammation - for example acne, eczema or contact allergy - and to treat this aggressively. Assiduous photoprotection is essential with the use of sunscreen on a daily basis. Depigmenting creams such as hydroquinone can help, however since the pigment can be deep within the skin these often have minimal effect. Other treatment options include the Q-switched Nd:YAG (Revlite) laser.