What is achromic naevus?
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. 
What is the cause of achromic naevus?
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]
What are the variants of achromic naevus ?
- Isolated achromic naevus
- Segmental achromic naevus, also called segmental depigmentation disorder, has midline demarcation and poorly-defined lateral borders. It is considered the hypopigmented form of “segmental pigmentation disorder”
- Linear or ‘systematised’, achromic naevus has cutaneous findings that overlap with hypomelanosis of Ito.
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.
How is achromic naevus diagnosed?
Coupe identified diagnostic criteria for achromic naevus in 1976: 
- The patch of pale skin is present at birth or early in life
- It remains in the same site throughout lifetime
- There is no alteration in texture or change in sensation in the lesions
- There is no dark border around the affected skin
What is the treatment for achromic naevus?
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.