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.