Panniculitis

Panniculitis refers to a heterogeneous group of conditions characterized by inflammation of the subcutaneous fat. It can be a primary inflammatory process or secondary to systemic disease, infection, or trauma. Histologically, panniculitis is categorized as septal or lobular, with or without involvement of the overlying skin or vessels. The clinical presentation varies but commonly includes tender nodules or plaques in the subcutaneous tissue, most often on the legs.

Erythema nodosum is the prototypic septal panniculitis and is typically manifested as painful, erythematous nodules on the shins. It often accompanies infections such as streptococcal pharyngitis, sarcoidosis, medications, or pregnancy. Erythema nodosum usually resolves within weeks to a few months and often leaves no scarring.

Erythema induratum, also known as nodular vasculitis, is a lobular panniculitis with clinical and histologic features that may include vasculitis. It commonly affects the calves and can be chronic and relapsing. Historically it has been associated with tuberculosis, though associations vary by patient and region.

Other forms include pancreatic panniculitis associated with pancreatic disease; lupus panniculitis (lupus profundus); cold panniculitis after exposure to cold; and panniculitis at sites of intramuscular or subcutaneous injections. These entities reflect a range of etiologies from autoimmunity to metabolic disorders.

Diagnosis relies on history, physical examination, and skin biopsy to distinguish septal versus lobular patterns and to identify fat necrosis or vasculitis. Laboratory tests and imaging focus on potential underlying causes such as infection, autoimmune disease, or malignancy. Management targets the underlying condition; symptomatic therapies include NSAIDs, compression, and rest, with corticosteroids or immunosuppressants used for select inflammatory panniculitides. Prognosis varies, ranging from self-limited to recurrent or chronic disease.