Subcutaneous Sweet's Syndrome Presenting With a Single Cutaneous Lesion on the Thigh

Sweet's syndrome (SS), also known as acute febrile neutrophilic dermatosis, manifests as tender, erythematous skin lesions such as papules, nodules, and plaques that may appear vesicular or pustular. The condition is characterized by widespread infiltrates mainly consisting of mature neutrophils, usually in the upper dermis. Erythema nodosum (EN) is a form of septal panniculitis marked by tender, erythematous lesions primarily appearing on the lower legs. Additionally, subcutaneous Sweet's syndrome (SSS) is a rare variant of SS that mainly involves the subcutaneous adipose tissue. Skin lesions in SSS generally present as tender, erythematous subepidermal nodules on the extremities, morphologically resembling EN. Both EN and SS can present with fever, malaise, gastrointestinal disturbances, lymphadenopathy, arthralgia, increased white blood cell (WBC) count with neutrophilia, elevated C-reactive protein (CRP), and elevated erythrocyte sedimentation rate (ESR), making differentiation between them often challenging. Therefore, histopathologic evaluation is necessary for an accurate diagnosis. In our case, the patient exhibited a very painful plaque measuring 20 cm in diameter on the upper thigh without significant neutrophil infiltration in the dermis, but with subcutaneous septal neutrophil infiltration. Generally, SS shows stronger leukocytosis with neutrophilia than EN does. Considering the clinical symptoms, laboratory results, and clinical progression, the clinicopathological findings aligned more closely with SSS than EN. This article describes a rare case of SSS presenting with a single cutaneous lesion on the thigh, which mimicked the histopathological features of EN.