Differential Diagnosis of Annular Lesions -- American Family Physician

www.aafp.org/afp AMERICAN FAMILY PHYSICIAN 289 believed to be caused by invasive worms. The main causes belong to three genera: Trichophyton, Microsporum and Epidermophyton. The most common cause of tinea corporis in the United States is infection with Trichophyton rubrum, Trichophyton tonsurans, Trichophyton mentagrophytes and Microsporum canis. All dermatophytes are aerobic, and they share the ability to assimilate keratin, thus allowing them to penetrate the keratinized layers of the stratum corneum. Humans may become infected with tinea corporis through close contact with infected persons, animals or soil. Occasionally, autoinoculation from infected nails, scalp or feet may occur. The peak incidence of infection is after puberty, although patients may present in the preadolescent period. There is no sexual predilection. Climate and personal habits are the main determinants of these infections. Fungal infections are facilitated by warm, moist environments (such as public showers and swimming pools) and the sharing of towels, clothing and toiletries. Prolonged use of systemic corticosteroids may render a person more vulnerable to fungal infections such as this. Patients present with well-demarcated, erythematous papules or plaques on the skin. Patients may note that these lesions have gradually enlarged over time. The borders A nnular lesions are extremely common and striking in appearance but can also be misleading. The term “annular” stems from the Latin word “annulus,” meaning ringed. The lesions appear as circular or ovoid macules or patches with an erythematous periphery and central clearing. The most common cause of annular lesions encountered in the adult population is dermatophytosis, which may be successfully diagnosed without a biopsy. However, other conditions may present with much the same appearance (Table 1). The clinician must exclude other diagnoses, especially if the patient has failed previous treatment for dermatophytosis.