Granuloma annulare is an uncommon, benign inflammatory papular eruption. Granuloma annulare is classified as localized, generalized (disseminated), perforating, erythematous, or subcutaneous. The first four types are cutaneous and are diagnosed and managed by dermatologists.
Subcutaneous granuloma annulare, because of its deeper location, can present a diagnostic challenge, leading to multiple referrals and imaging evaluation. Indeed, subcutaneous granuloma annulare is the most frequently biopsied benign soft tissue mass in the lower extremity of children under the age of 5, and is susceptible to inappropriate and sometimes repeated surgical intervention before the correct diagnosis is determined.
Synonyms for subcutaneous granuloma annulare include benign rheumatoid nodule, pseudorheumatoid nodule, deep granuloma annulare, subcutaneous palisading granuloma, palisading granuloma nodosum, and isolated subcutaneous nodule.
Subcutaneous granuloma annulare typically presents as a rapidly growing, firm, painless, subcutaneous mass in children younger than 5 years of age. A pretibial location is typical and lesions along the extensor aspects of forearms and feet, and within the occipital scalp are also common. Atypical presentations with pain and multiple soft lesions have also been reported.
Radiographs reveal a dense subcutaneous mass without calcifications. The lesions are hypoechoic on ultrasound. MRI reveals a subcutaneous lesion with indistinct margins and signal abnormality extending into the adjacent subcutaneous fat. The lesions are iso- or slightly hyperintense to muscle on T1-weighted images and heterogeneously hyperintense on T2-weighted images. Enhancement can be variable.
The imaging findings are nonspecific, but the age and location can suggest the diagnosis. Differential considerations include:
- Subcutaneous nodules of rheumatoid arthritis: Imaging findings can be identical, and pathological findings can be very similar.
- Foreign-body reactions:
- Post-traumatic lesions: Fat necrosis, hematoma,
- Infectious lesions: Abscess, inflammatory granuloma
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