An unusual linear plaque in a child along with nail dystrophy

A 2-year-old girl presented with an asymptomatic single brownish-black plaque on the right hand since birth. Initially the lesion had central depigmentation at the proximal end. Gradually, the lesion progressed to become blackish in color along with the development of nail dystrophy just before presentation. There was no history of similar lesions in any family members. On examination, there was an irregular, blackish well-defined solitary plaque on the dorsum of the right hand, extending throughout the middle finger in a linear or blashkoid distribution and onto the palmar aspect (Figures 1 and 2). It measured 2 cm in breadth at the widest part of the lesion at the base of the finger and 5 cm in length. The lesion had a thin, furrowed, raised, keratotic, and hyperpigmented border with central clearing and fine yellowish-white scaling. There was longitudinal ridging in the nail of the middle finger along with melanonychia. Figure 3 shows the dermoscopic findings. A 3 mm punch biopsy was obtained from the lesion (Figure 4). Dermoscopy showed a double-edged keratotic rim on a brown background, with yellowish-white fine scaling and central clearing (Figure 3). On histopathological examination, the epidermis showed acanthosis and hyperkeratosis. Columns of parakeratosis were observed which represent the cornoid lamellae (Figure 4, H&E, ×20) This was associated with invagination of the parakeratotic columns into the epidermis and hypogranulosis of the epidermis below. Keratin plugs were also seen. The dermis showed mild perivascular infiltrate with focal basal cell vacuolization and pigment incontinence (Figure 5, H&E, ×40). Porokeratosis constitutes a rare group of acquired or inherited skin disorders of unknown etiology characterized by aberrant keratinization. The subtypes that are commonly recognized in children are porokeratosis of Mibelli, linear porokeratosis, and punctate porokeratosis.1 Various theories of the pathogenesis of porokeratosis have been put forward including expansion of a mutant clone of keratinocytes (Reed's hypothesis), cutaneous mosaicism in focal or linear forms, and mutations in the genes related to the phosphomevalonate kinase pathway.2, 3 Though linear porokeratosis is frequently sporadic, family history of diffuse superficial actinic porokeratosis representing a Type 1 segmental mosaicism with an autosomal dominant inheritance pattern has also been reported.4 Linear porokeratosis is characterized by annular well-circumscribed keratotic papules or plaques arranged in a linear or blaschkoid pattern, commonly occurring over the distal extremities. The dermoscopic hallmark is a white peripheral track or keratin rim which represents the cornoid lamella on histopathology—a vertical stack of parakeratotic corneocytes within the stratum corneum resting on a shallow depression of the underlying epidermis along with focal hypogranulosis.5 Congenital forms of linear porokeratosis have been reported rarely.6 Nail changes in porokeratosis are seldom observed but when they occur they present as nail splitting, longitudinal ridging, nail dystrophy, and pterygium unguis.6 Longitudinal ridging and melanonychia were observed in our patient, which could be explained by the involvement of the nail bed and matrix by abnormal clonal keratinocytes.7 The most important diagnosis to consider in the differential diagnosis in this patient includes porokeratotic eccrine and ostial dermal duct nevus or porokeratotic adnexal ostial nevus (PEODDN), which clinically presents from birth as keratotic or spiny papules arranged in a linear fashion mainly over the palms and soles and may resemble palmoplantar punctate porokeratosis.8 They can often be differentiated only on histopathology, where PEODDN shows columns of parakeratosis resembling cornoid lamellae, but they extend into a dilated acrosyringium or hair follicles (when present in hair-bearing skin). However, the lesions in our patient showed a keratotic rim and double-edge scale which was clinically suggestive of porokeratosis. Other important entities to consider in the differential diagnosis include verrucous epidermal nevus (VEN), linear Darier's disease, and linear lichen planus. Though all these lesions occur in a linear or blashkoid configuration, the morphology is distinct. VEN presents as skin colored to brown papules with a velvety or verrucous surface; Darier's disease as yellowish-brown keratotic lesions and lichen planus presents as flat-topped, violaceous papules with Wickham's striae. On histopathology, VEN is characterized by hyperkeratosis, acanthosis, papillomatosis, and variable parakeratosis. In Darier's disease, suprabasal acantholysis, and clefting with hyperkeratosis and dyskeratosis was observed. The histopathology of lichen planus will demonstrate interface dermatitis. Though porokeratosis is benign, there are reports of squamous cell carcinoma, basal cell carcinoma, and Bowen's disease developing.9 The overall risk of malignancy was 7.5% in a series of 281 cases of porokeratosis, with the highest risk in linear porokeratosis (19%) followed by disseminated palmoplantar porokeratosis (9.5%).9 Careful observation is important as malignancy can be detected decades after the onset of porokeratosis.9 Porokeratosis can often be refractory to treatment, thus observation alone may be appropriate. Treatment options include topical salicylic acid, corticosteroids, Vit D analogs, topical retinoids, 5-flurouracil, and 5% imiquimod.5, 10 Destructive therapies such as cryotherapy, electrosurgery, dermabrasion, photodynamic therapy, ablative laser therapy, and surgical excision can be considered. Oral acitretin or isotretinoin may also be options. Recently topical lovastatin with topical cholesterol was successfully used to treat a small series of patients with porokeratosis including 2 with linear porokeratosis.11 Our patient was initiated on topical tazarotene 0.1%, but was lost to follow-up. Divya Garg, Anubha Dev, Apoorva Sharma, and Tarun Narang were responsible for the management of the patient. Divya Garg was involved in the preparation of the first draft of the manuscript. Anubha Dev, Apoorva Sharma, and Tarun Narang were involved in critical revision and finalizing the final draft. Debajyoti Chatterjee was involved in histopathological diagnosis and provided the histopathology images. The authors declare no conflicts of interest.