Discussion
PAON is characterized by small discrete scaly papules arranged linearly, which may coalesce into plaques. Usually unilateral, most common sites are the palm or sole, with occasional extension onto the dorsae of the hands and feet, although more generalized eruptions have been reported.2 Lesions are typically asymptomatic. The male to female ratio is almost 1:1. 2 PEODDN is usually a congenital phenomenon, however up to 26% of cases are reported to be late-onset, with some cases noted to develop following puberty.3 PEODDN usually becomes more warty in appearance over time.
PAON is a disorder of the intraepidermal eccrine duct (acrosyringium). Histopathology is diagnostic, characterized by parakeratotic columns within an epidermal invagination appearing as cornoid lamellae. Vacuolated and dyskeratotic keratinocytes are often present and there may be loss of the granular layer. 4 Somatic mutations in GJB2 have been identified as causative in PAON, representing a mosaic form of keratosis ichthyosis deafness (KID) syndrome. GJB2 encodes the gap junction protein connexin26 (Cx26), which permits intercellular ion and macromolecule flux. 5 Previously reported cases of PAON have described coexisting problems like seizures, hemiparesis, scoliosis, deafness, developmental delay, palmoplantar keratoderma, psoriasis, hyperthyroidism, polyneuropathy, breast hypoplasia and KID.2
The differential diagnosis of PAON includes variants of porokeratosis, naevus comedonicus, epidermal naevi, viral warts, or porokeratoma. Linear or punctate porokeratotosis lack the combination of cornoid lamellae with acrosyringia or hair follicle ostia, the eccrine duct hyperplasia and the thicker epidermal invagination. Naevus comedonicus and epidermal naevus (or inflammatory linear verrucous epidermal naevus) can rarely present with porokeratosis-like features, but are not related to eccrine ostia or hair follicle. Focal hyperparakeratosis, koilocytes and haemorrhagic exudates are seen in viral warts but not in PAON. In this case eccrine duct involvement was not clear on histology, but a diagnosis of PAON was favoured following clinicopathological correlation.
Current treatment options are of limited efficacy. Reported therapies include topical corticosteroids, topical salicylic acid, topical retinoids such as tazarotene, photodynamic therapy, and carbon dioxide laser. 2,3 While the lesions tend to be refractory to treatment, they are usually asymptomatic and may not require intervention.
This case highlights a typical presentation of a rare condition whose clinical features are often initially subtle and can be easily overlooked. There is evidence that PAON is a manifestation of somatic mosaicism for KID syndrome and, although never previously reported, there is a theoretical risk of transmission of systemic disease to offspring.