2-week old white male presented with a 13-day history of an asymptomatic rash on the left medial thigh. Physical examination demonstrated a well-nourished infant in no apparent distress with many 2-to 4-mm pink papules, pustules, and vesicles on an erythematous base in a blaschkolinear distribution on the left flank, left medial thigh, and extending to the left medial knee (Figure 1, A and B). A punch biopsy was performed on the left medial thigh for hematoxylin and eosin staining, which showed eosinophilic spongiosis, dyskeratotic keratinocytes, and superficial and mid-dermal perivascular inflammation with eosinophils (Figure 2, A and B; available at www.jpeds.com). Clinical and histopathologic findings were diagnostic of incontinentia pigmenti. Incontinentia pigmenti is a rare X-linked neurocutaneous genodermatosis characterized by a dominant loss-of-function mutation in the inhibitor of kappa light polypeptide gene enhancer in B cells, kinase gamma gene, resulting in increased cellular apoptosis. 1-4 Incontinentia pigmenti predominantly affects female infants and is usually lethal in males in utero. Rare cases of incontinentia pigmenti have been identified in male patients, for which 3 mechanisms of survival have been proposed: 47, XXY karyotype (Klinefelter syndrome), hypomorphic mutations, and somatic mosaicism. 5,6 Somatic mosaicism, resulting from a postzygotic mutation occurring during the blastocyst stage of embryogenesis leading to incomplete nuclear factor-kB inactivation, is the likely explanation for our patient as his cutaneous symptoms were relatively mild and no extracutaneous symptoms have been observed to date. 7 Furthermore, the family history was negative for incontinentia pigmenti, suggesting a de novo mutation. Dermatologic manifestations are generally the presenting sign of incontinentia pigmenti. Lesions follow Blaschko lines and are classically divided into vesicular, verrucous, hyperpigmented, and atrophic stages. The vesicular stage occurs in approximately 90% of cases, most often during the first 2 weeks of life. 8 The lesions present as superficial vesicles on an erythematous base in a linear distribution typically sparing the face. The verrucous stage occurs in approximately 70% of patients between 2 and 6 weeks of life. 8 Nearly all patients with incontinentia pigmenti experience the hyperpigmented stage between 12 and 26 weeks, with whorls and streaks of brown to gray pigmentation following the lines of Blaschko. These lesions do not typically correlate with the location of the prior stages and do not represent postinflammatory hyperpigmentation. The hyperpigmented stage can persist for years or decades. The atrophic stage appears in approximately 28% of patients as pale, hairless, atrophic patches. 8 Less commonly, it can appear as hypopigmented patches without atrophy. This stage is commonly permanent. 8 Some of the stages may occur concurrently with others or not at all. 7 Additionally, males tend to have a more localized disease, with unilateral presentation being a distinctive...
