What test reveals that this hypopigmented patch is not vitiligo?

Dermclinic
A Photo Quiz to Hone Dermatologic Skills

Case: This 17-year-old girl presents for evaluation of a hypopigmented patch on her neck and wants to know the current therapeutic options for vitiligo. She has had the lesion for as long as she can remember. Her parents told her that it had been diagnosed as vitiligo at birth and that it had grown with her. Through Internet searches, the patient found images of similar-looking lesions, which she felt confirmed this diagnosis. She is surprised to learn that she does not have vitiligo. 

What simple test helps establish the true diagnosis?

(Answer on next page.)
 

Dermclinic—Answer

On stroking the patch, the surrounding skin flares but not the lesion, consistent with nevus anemicus

This teenage girl has a rare congenital abnormality that involves the cutaneous dermal vasculature. Her clinical presentation is typical. The morphology of nevus anemicus is as depicted in the photo—normal skin interspersed within a lighter-colored patch, the borders of which are often indistinct and irregular. In my experience, it is not unusual for these patches to go unnoticed or to be misdiagnosed, particularly in children with fair skin that burns easily (Fitzpatrick skin type I or II). Nevus anemicus persists for life and is not correctable.

Results of a skin biopsy would be interpreted as normal; only physiological testing can reveal the nevus in contrast to normal skin. Stroking the patch elicits no red flare. The photograph illustrates this phenomenon; after stroking the entire lower half of the patch, only the normal skin reacted with a characteristic erythematous response. Examination under a Wood lamp also can reveal the diagnosis: the nevus anemicus patch will not accentuate as a patch of vitiligo would. The lack of color change in the lesion is explained by an increased sensitivity to catecholamines in the blood vessels of the nevus, which inhibits vasodilation.¹

Although nevus anemicus is of no medical significance, it is important to document it to avoid confusion with other conditions in the differential diagnosis. This includes the hypopigmented patches of tuberous sclerosis (see Figure in "What's Your Diagnosis?" of this issue) and nevus depigmentosus and the amelanotic patches of vitiligo. ■

REFERENCE
1. Barber K. What about this light-colored patch over an infant’s brow rules out vitiligo? Consultant For Pediatricians. 2010;9:233, 237.