Author: Daniel Jun Yi Wong, Medical Student, University of Melbourne, Australia, 2013.
Achromic naevus is an uncommon birthmark (naevus) characterised by a well-defined pale patch. This is usually several centimetres in diameter, with an irregular but well-defined border. Shape and size varies. Often, smaller hypopigmented macules arise around the edges, resembling a splash of paint. 
Achromic naevus (American spelling nevus) is also called naevus depigmentosus.
The name is not quite right, as the hypomelanotic patches of an achromic naevus is not completely white, unlike the areas of depigmentation in vitiligo, which are amelanotic, i.e., completely lacking melanin. Achromic naevi are usually solitary, in contrast to tuberous sclerosis, where multiple pale patches occur and are called ash-leaf spots.
Achromic naevus is usually noted at birth or early childhood, although lesions may not be apparent until mid-childhood in those with light-coloured skin. The naevi remain stable over time. Achromic naevus most commonly arises on the trunk, but may also arise on the limbs and elsewhere. 
Achromic naevus is a form of cutaneous mosaicism. It is caused by an altered clone of melanocytes (pigment cells) with a decreased ability to make melanin (brown pigment). It may be caused by reduced numbers of melanocytes, reduced melanin production or the inability of melanocytes in the affected area to transfer melanosomes (melanin-containing granules) to keratinocytes (skin cells). [2,3]
Occasionally, achromic naevus is associated with other neurocutaneous disorders. Colocalised lentigines have been reported and are seen in figure 3 (top right) above. They may represent a reverse mutation. The ash-leaf macules seen in tuberous sclerosis are oval-shaped hypopigmented macules and look similar to achromic naevi but usually present as multiple lesions.
Coupe identified diagnostic criteria for achromic naevus in 1976: 
Treatment of achromic naevus is often unnecessary. Cosmetic camouflage may be helpful. Some of the options that have been used to attempt repigmentation include: excimer laser, phototherapy (PUVA), and skin grafting. 
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