Annular erythema is a descriptive term used in dermatology to categorize a group of chronic skin conditions characterized by red, ring-shaped lesions. These lesions, often referred to as figurate erythemas, present diagnostic challenges due to their varied appearances and potential underlying causes. Understanding the differential diagnosis of annular skin lesions is crucial for accurate identification and appropriate management.
Annular erythema encompasses several related conditions, including erythema annulare centrifugum, erythema perstans, and erythema gyratum perstans. Erythema gyratum repens is sometimes considered separately due to its distinct characteristics and strong association with underlying malignancy. These conditions share the common feature of annular, or ring-shaped, lesions, but differ in subtle clinical features, histology, and associated systemic diseases.
The clinical presentation of annular erythema typically begins as a small, raised, pink to red papule that gradually expands outwards. As the lesion enlarges, it forms a ring or arc shape with central clearing. An inner rim of scale may be present in some cases. The lesions can grow at a rate of 2–5 mm per day, reaching diameters of 6–8 cm or even larger. While often forming complete rings, some lesions may exhibit irregular, serpiginous shapes. Patients may present with solitary or multiple lesions. The most common locations are the thighs and legs, but annular erythema can also appear on the face, trunk, and arms. Symptoms are usually minimal, with some individuals reporting mild itching or stinging.
The differential diagnosis of annular skin lesions is extensive, requiring careful consideration of various dermatological conditions that can mimic annular erythema. Conditions to consider in the differential diagnosis include:
- Tinea corporis (ringworm): A fungal infection that also presents with annular, scaly lesions. Tinea corporis is typically more pruritic (itchy) and may show more prominent scale and raised borders compared to some forms of annular erythema. Microscopic examination of skin scrapings can confirm fungal hyphae.
- Granuloma annulare: Characterized by dermal papules and plaques arranged in an annular pattern. Granuloma annulare lesions are usually skin-colored to slightly erythematous and lack scale. Histopathology shows palisading granulomas.
- Erythema migrans (Lyme disease): The hallmark rash of early Lyme disease, erythema migrans, is often annular and erythematous. A history of tick bite and potential systemic symptoms like fever, fatigue, and muscle aches are important clues. Serological testing can confirm Lyme disease.
- Psoriasis annularis: Annular psoriasis lesions are scaly, erythematous plaques with typical psoriatic features elsewhere on the body, such as nail changes or involvement of other typical psoriasis sites.
- Urticaria: While typically transient and migratory, urticarial lesions can sometimes present in annular forms. Urticaria is characterized by wheals and flares, and lesions typically resolve within 24 hours.
- Secondary syphilis: Annular secondary syphilis lesions are less common but should be considered in the differential diagnosis, especially in individuals at risk. Serological testing for syphilis is essential.
- Erythema multiforme: May present with target-like lesions, some of which can be annular. Erythema multiforme is often triggered by infections, particularly herpes simplex virus, or medications.
In many cases of annular erythema, an underlying cause cannot be identified (idiopathic). However, several associations have been reported, including:
- Infections: Bacterial (tuberculosis, sinusitis), fungal (candidiasis, tinea), and viral (secondary syphilis).
- Medications: Chloroquine, hydroxychloroquine, estrogens, penicillin, and amitriptyline.
- Foods: Blue cheese and tomatoes have been implicated in some cases.
- Systemic diseases: Recurrent appendicitis, cholestatic liver disease, and Graves’ disease.
When an underlying cause is identified and treated, the associated annular erythema often resolves.
Diagnosis of annular erythema is primarily clinical, based on the characteristic appearance of the lesions. Skin biopsy and histopathological examination can be helpful to confirm the diagnosis and exclude other conditions. Histopathology typically reveals a perivascular lymphocytic infiltrate. Further investigations may be warranted to rule out underlying systemic diseases, particularly if clinical features or history suggest an association.
Treatment for annular erythema is often unnecessary as the condition frequently resolves spontaneously over time, with an average duration of around 11 months. Topical corticosteroids may be used to alleviate redness, swelling, and itching if symptomatic relief is desired. However, there is no known curative treatment for most cases of annular erythema. Management focuses on symptomatic relief and, when possible, identifying and addressing any underlying associated conditions.
