Unilateral systematized linear porokeratosis: a report of a rare case.

Sir,

A 14-year-old boy was brought to us with slowly progressive linearly arranged keratotic papules and plaques over the left half of the body. These lesions started at the age of 6 months. They first appeared on the left thigh and left dorsum of the hand and then progressed linearly to involve the leg, foot, forearm and arm of the same side. Each lesion started as an itchy erythematous papule and became brownish hyperkeratotic over a period of 2 weeks. There was no history of erosions, fissures, vesicobullous lesions pustules or comedo-like papules. The patient was born of nonconsanguineous marriage after a full-term normal delivery. There was no history of similar lesions in the family. On cutaneous examination, the left half of the body was involved in the form of linearly arranged multiple discrete to coalescing hyperkeratotic papules and annular plaques along the Blaschko's line [Figure 1]. On the upper limb, the linear streak extended from the lower third of the arm, running across the radial aspect of the forearm, to reach the tip of the thumb and second finger without affecting the nails. On the lower limb, lesions were distributed in two streaks. One of the streaks extended from the medial aspect of the knee to the medial malleolus and another streak extended from the gluteal region, running across the popliteal fossa, calf, heel and lateral aspect of the foot to reach the tip of the last toe without affecting its nail. Few lesions of same morphology were also seen on the left side of the trunk. Isolated plaques were oval to annular with hyperpigmented and atrophic center and hyperkeratotic ridge containing thread-like grooves [Figure 2]. General physical examination and systemic examination were unremarkable. Histology of skin biopsy specimen from the foot and thigh revealed similar features of basketweave hyperkeratosis with a cornoid lamella located adjacent to an acrosyringium with perivascular mononuclear infiltrate in the dermis [Figure 3]. Based on the cutaneous examination and histological findings, a diagnosis of unilateral systematized linear porokeratosis was made.

Figure 1

Linear porokeratosis involving the trunk, left upper and lower limb

Figure 2

Linearly arranged papules and plaques with an atrophic center and hyperkeratotic ridge

Figure 3

Photomicrograph showing cornoid lamella (H and E, ×40)

Porokeratosis is a disorder of keratinocyte growth and differentiation that presents as sharply demarcated scaly papules with a distinctive hyperkeratotic ridge containing thread-like grooves. Subtypes include porokeratosis of Mibelli, disseminated superficial actinic porokeratosis, punctate porokeratosis, porokeratosis palmaris et plantaris disseminata and linear porokeratosis.[1] Linear porokeratosis can further be classified as localized, zosteriform, systematized or generalized. The distribution of all these variants follows the Blaschko's lines, which may be explained by cutaneous mosacism.[2] It usually appears at birth or in childhood as an eruption of papules or plaques with a thread-like hyperkeratotic rim that expands peripherally and leaves an atrophic center.[3] Histologically, the hyperkeratotic ridge shows a keratin-filled invagination of the epidermis. In the center of this keratin-filled invagination rises a parakeratotic column, the so-called cornoid lamella, representing the most characteristic feature of porokeratosis.[4] In our patient, porokeratosis presented in a unilateral pattern involving the upper limb, the lower limb and the trunk. This unilateral systematized distribution of porokeratosis is a rare subtype; a PubMed search located only three case reports between 1966 and 2010.[5-7] Hence, we are reporting this case for its clinical rarity.