Diagnosis: Erythema Annulare Centrifugum
This patient has a rare skin condition called erythema annulare centrifugum (EAC). The primary lesion begins as a group of pink/erythematous papules that spread peripherally as they clear centrally, forming a “trailing scale” (Figure 3) They can enlarge at a rate of 2 to 5 mm/d, producing a plaque or patch of annular, arcuate, or polycyclic shape.1 The margin is indurated and can vary in width from 4 to 6 mm. Vesicles may also be present. Distribution is usually on the thighs and legs but can also appear on the upper extremities (Figure 4), trunk, or face.
Differential diagnosis
Other causes of similar erythematous itchy rings to consider include pityriasis rosea, tinea corporis, psoriasis, nummular eczema, atopic dermatitis, drug reaction, and erythema migrans. The physical exam is the most effective method to differentiate between the above diagnoses—specifically, the distribution and morphology of the lesions. The natural history of the lesions can further narrow the diagnosis.
Pityriasis rosea has erythematous patches distributed on the trunk and extremities. These patches have a distinctive collarette border. The “herald patch,” the first patch to appear, is often oval and about 1 to 3 inches in diameter. Lesions classically follow skin lines to create a “Christmas tree” pattern on the back. Unlike EAC, pityriasis only lasts 6 to 8 weeks.
Tinea corporis (ringworm) is a fungal infection that is often a single scaly annular patch. Like EAC, it can be erythematous and is distributed anywhere on the body. When it presents as multiple lesions, it more closely resembles EAC. Unlike EAC, however, it should show branched hyphae with septae on a KOH prep and fungal culture would show growth of dermatophytes. Furthermore, it responds well to 4 to 6 weeks of antifungal treatment.
Psoriasis may present as plaques on the extensor surfaces of extremities. “Guttate” psoriasis (meaning “like water drops”) appears like small drops on the back and abdomen. Psoriatic plaques do not have central clearing or the trailing scale characteristic of EAC. Psoriatic plaques are often more responsive to steroids than EAC.
FIGURE 3
“Trailing scale”on abdomen
FIGURE 4
Ring on inner arm
