A 14-year-old boy presented to our clinic with diffuse erythematous papules that rapidly progressed to plaques with centrifugally serpiginous spreading edges and central clearing over the chest, abdomen and back (Fig. 1). There were no pustules or vesicles. Palms, soles, mucosa, scalp and nails were not affected.
He was treated with various topical treatment modalities, however, the polycyclic lesions continued to expand progressively (Fig. 2).
KOH examination of the scales for fungal hyphae was negative. Biopsy taken from the edge of a lesion revealed hyperkeratosis, mild acanthosis and dilated capillaries in the dermal papillae. No hyphae or spores were detected on a periodic acid-Schiff stain. Routine investigations including complete blood counts, renal and liver function tests and antinuclear antibodies were negative.
A diagnosis of annular plaque psoriasis was made. Low-dose narrowband UVB phototherapy was started, which resulted in marked improvement after 4 months, when his cutaneous lesions subsided with post inflammatory hypo-pigmentation.
Annular plaque psoriasis is characterized by polycyclic, erythematous, silvery-white scales with central clearing. Differential diagnosis includes other figurate erythemas: tinea corporis, granuloma annulare, erythema annulare centrifugum, subacute lupus erythematosus and pityriasis rosea.
A detailed history, thorough clinical examination and certain simple investigations will help to distinguish these diseases from annular psoriasis. Clue for the diagnosis include the presence of more typical psoriasis lesions on the rest of the skin, nails pitting and scaly scalp.
