Blaschkoid Epidermolytic Ichthyosis

Summary: Epidermolytic ichthyosis (bullous ichthyosiform erythroderma) is a rare genetic disorder of cornification caused by autosomal dominant mutations in KRT1 or KRT10. It is characterized by severe blistering and erythroderma at birth, with subsequent phenotypically variable degrees of hyperkeratosis as childhood progresses. Blaschkoid epidermolytic ichthyosis (ichthyosis hystrix) is caused by post-zygotic somatic mutations in KRT1/KRT10 during embryogenesis and does not cause blistering at birth. It is the only human keratin disease known to exhibit mosaicism; herein we present one such case. An 18-month-old boy with no significant past medical history presented to the outpatient dermatology clinic with a history of skin lesions noted a few weeks after birth. There was no history of blistering or erythroderma. Physical exam revealed diffuse hyperpigmented (and focally verrucous) patches and plaques on the face, trunk, and extremities along Blaschko lines. Biopsy revealed hyperkeratosis, papillomatosis, and acanthosis with areas of marked keratinocyte vacuolization and cytolysis in the strata spinosum and granulosum (epidermolytic hyperkeratosis). A diagnosis of Blaschkoid epidermolytic ichthyosis was considered most likely. The differential diagnosis includes systematized epidermal nevi, epidermal nevus syndromes, congenital ichthyoses, and vesiculobullous disorders of childhood; the histopathologic finding of epidermolytic hyperkeratosis is the key distinguishing feature. The term “epidermolytic hyperkeratosis” is now preferentially reserved for the histologic reaction pattern rather than the clinical syndromes. Patients with Blaschkoid epidermolytic ichthyosis may produce offspring with epidermolytic ichthyosis due to gonadal mosaicism; referral for genetic counselling may be appropriate.