Porokeratosis: a differential diagnosis to consider in benign lichenoid keratosis

Porokeratosis is a disorder of keratinization with many clinical variants. The histological hallmark feature of porokeratosis is a cornoid lamella. Other accompanying features include lichenoid inflammation, atrophy towards the centre of the lesion, dermal cytoid bodies, and adjacent lichenoid changes. Lichenoid keratosis is a benign cutaneous condition, thought to largely represent a degenerating seborrheic keratosis or solar lentigo. The classical histologic appearances are characterized by parakeratosis, epidermal acanthosis, and a dense band of lichenoid lymphocytic infiltrate. Since a lichenoid inflammatory reaction pattern can be seen in porokeratosis it has the potential to be misdiagnosed as a lichenoid keratosis if the cornoid lamella is not identified or missed due to sampling selection. We critically review 104 cases of benign lichenoid keratosis to establish whether any of these cases had features to support a diagnosis of porokeratosis. With 9.6% of cases considered for re-classification, we review clues to reaching this histologic diagnosis.