Idiopathic Granulomatous Mastitis as an Unusual Cause of Erythema Nodosum in a Malagasy Woman

INTRODUCTION: Idiopathic granulomatous mastitis (IGM) is a rare chronic inflammatory disease. Neoplastic and infectious etiologies must be ruled out. IGM is a diagnostic challenge for countries with high tuberculosis endemicity like Madagascar since it may clinically and radiologically mimic breast tuberculosis. We report a case of IGM associated with erythema nodosum in a Malagasy. CASE REPORT: A 29-year-old primiparous woman came to a dermatological consultation for typical erythema nodosum lesions that appeared one month after a breast swelling. She had no particular medical history. Examination revealed typical erythema nodosum lesions on the legs, voluminous tender mass in the right breast. Bacteriological samples and tuberculosis test were negative. Imaging showed mastitis on the right breast with no evidence of malignancy. Histology revealed a non-caseating granulomas on the lobule of the right breast. As part of an etiological work-up, COVID-19 serology was performed with a positive IgG antibody. The diagnosis of IGM associated with erythema nodosum was evocated. The evolution was favorable under systemic corticosteroid therapy. DISCUSSION: The cause of this uncommon lesion remains obscure. The extramammary localizations such as erythema nodosum and arthralgia suggest an autoimmune origin. This pathogenesis is also reinforced by a good response to systemic immunosuppression. In our patient, the etiological assessment of the mastitis revealed a chronic infection with SARS-CoV-2. Histopathology is the gold standard for the IGM diagnosis which demonstrates a lobulocentric granulomas without caseous necrosis. Oral corticosteroid therapy is the initial choice of treatment. CONCLUSION: Now, with several cases of concomitant IGM and EN reported, dermatologists should be aware that erythema nodosum can be one of the presenting signs of IGM, since the two conditions appear to be associated. The particularity of our case lies in the incidental discovery of SARS-CoV-2 infection. Is a chronic granulomatous disease associated with SARS-CoV-2 infection, a coincidence?