CASE #1: Segmental nevus depigmentosus

Nevus depigmentosus (ND) is a well-circumscribed hypomelanosis that presents at birth or in early childhood.1 ND can present as a small isolated circular patch or may develop in a unilateral segmental distribution. It is hypothesized that ND arises during embryogenesis when a clone of cells with reduced melanogenic potential arises via a postzygotic somatic mutation.2 ND occurs sporadically with no known familial pattern of inheritance.1

The most commonly accepted clinical criteria for ND were proposed by Coupe as follows: (1) leukoderma present at birth or onset in early life; (2) no alteration in the distribution of leukoderma throughout life; (3) no alteration in texture or change of sensation in the affected area; and (4) no hyperpigmented border around the achromic area.3 The two most common presentations of ND are a small isolated round or rectangular patch and a unilateral segmental patch. These lesions do not normally cross the midline. The lesions are most commonly seen on the back and buttocks but can, and frequently do, occur anywhere on the body.2

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The term depigmentosus is a misnomer; individual lesions appear hypopigmented, in contrast to the completely depigmented appearance of vitiligo.2 Individual lesions frequently have an irregular or scalloped border with no surrounding hyperpigmentation.4 Hair within an ND may be hypopigmented,2 and there are multiple reports of increased freckling or nevi occurring within these hypopigmented lesions.5,6 The lesions most commonly occur at birth or shortly thereafter but may present later in life. ND is likely present in all patients at birth but may not be apparent for years because young children generally have untanned skin; these lesions may become more obvious once the surrounding skin darkens with exposure to the sun.1 After presentation, the lesions remain stable throughout life. ND lesions grow proportionately with the patient and may become more or less apparent depending upon how much sunlight exposure the surrounding skin receives.

ND may be confused with segmental vitiligo, hypo­pigmented patches of tuberous sclerosis, nevus anemicus, post-inflammatory hypopigmentation, and hypomelanosis of Ito.2 Hypomelanosis of Ito, also called pigmentary mosaicism, describes hypomelanosis following Blaschko lines.7 There is a subtype of ND that follows Blaschko lines, and the distinction between the two entities may be artificial; however, individuals classified with pigmentary mosaicism may be considered more likely to have systemic findings, while extracutaneous findings are very rare in patients with ND. Patients with either condition may be more simply diagnosed as having nevoid hypopigmentation with or without associated anomalies.2

If the diagnosis of ND cannot be made definitively through routine clinical exam, the clinician may find the use of a Wood’s lamp very helpful. The lesion will be significantly more prominent with this lighting. Additionally, a Wood’s lamp can be useful in distinguishing ND from vitiligo if there is any clinical uncertainty. Under Wood’s lamp examination, the lesions of ND demonstrate an off-white accentuation without fluorescence, while the lesions of vitiligo are completely depigmented and demonstrate a chalky-white accentuation with obvious bluish or yellow-green fluorescence.4,8

Histologic examination of lesional skin in ND demonstrates a markedly reduced number of melanosomes with normal or variable numbers of melanocytes.1,2,4 This finding suggests a reduced synthesis of melanosomes or transfer of melanosomes to keratinocytes, rather than a decreased number of melanocytes, as a cause for the condition. Histopathological examination is not normally utilized in the diagnosis of this condition.

There are no effective treatments for ND; the aim of most therapies is to camouflage lesions, making them less clinically apparent. This goal is best achieved though sun protection of surrounding skin to diminish the color discrepancy between the lesion and normal skin or through the use of cosmetic camouflage makeup. The hypopigmented lesional skin is also more UV-sensitive than surrounding skin and should be protected from the sun. For the presented patient, reassurance and education were provided. Sun protection was discussed, although this area is less likely to receive significant UV exposure.