Snapshot A 33-year-old man presents with a 1-week history of a painful rash on his shins. He recently attended a music festival in the desert near California. He reports developing a rash a few days after returning back to New York. He also reports having joint pain throughout his body, particularly his knees and elbows. Physical exam reveals multiple, symmetrically distributed 1-3 cm erythematous and warm subcutaneous nodules on his anterior shins. Introduction Clinical definition a panniculitis characterized by painful subcutaneous nodules typically located on pretibial surfaces Epidemiology incidence the most common panniculitis demographics female > male peak incidence between 20-30 years of age risk factors infection autoimmune disease leprosy Pathogenesis some evidence suggests type IV delayed hypersensitivity response immune complex deposition in venules of connective tissue or subcutaneous fat, causing subcutaneous nodules Associated conditions infection coccidioidomycosis (desert bumps) histoplasmosis tuberculosis streptococcus leprosy Behcet syndrome Sweet syndrome inflammatory bowel disease sarcoidosis Prognosis erythema nodosum typically resolves spontaneously and fades over 1-2 months Presentation Symptoms sudden onset painful symmetric rash often preceded by inflammation/infection may be preceded by prodrome fever, malaise, and fatigue arthralgia Physical exam warm and erythematous subcutaneous nodules that are 1-10 cm in diameter tender to palpation poorly demarcated margins location typically symmetrically on anterior shins can also affect forearms, thighs, trunk, head, or neck Studies Labs ↑ inflammatory markers erythrocyte sedimentation rate C-reactive protein Biopsy indication only if diagnosis is unclear clinically Histology septal panniculitis without vasculitis septa is thickened and has inflammatory infiltration Differential Scleroderma Granulomas Treatment Management approach treat underlying cause Medical nonsteroidal anti-inflammatory drugs (NSAIDs) indication for symptomatic relief as needed Complications Secondary bacterial infection