The term panniculitis refers to a group of inflammatory disorders in which the primary site of inflammation is in the subcutaneous fat. Suspicion for panniculitis typically first arises from the physical examination

  • Inspection demonstrates a nonspecific area of erythema
  • Palpation reveals deep-seated nodules and plaques--characteristic of an inflammatory process located beneath the dermis. Associated tenderness is common.
  • Additional features such as ulceration, atrophy, or sclerosis may also be detected and help differentiate the type.

The differential for panniculitis includes:

  • Inflammatory disorders:
    • Erythema nodosum is the most common (associated itself most commonly with streptococcal pharyngitis, but also classically in IBD and Loefgrens/sarcoidosis)
    • lipodermatosclerosis (known as sclerosing panniculitis, related to venous insufficiency)
    • lupus panniculitis (related to SLE, associated with atrophy on exam, rare)
    • cutaneous vasculitis (including polyarteritis nodosa in our case today, also associated with livedo)
    • erythema induratum (presents with ulcers, associated with other infections or drugs)
  • Infections: may be caused by bacterial, mycobacterial, fungal, protozoal, or viral infections. Today, we discussed causes of nodular lymphangitis including Sporothrix schenckii, Nocardia, Mycobacterium marinum, leishmaniasis, tularemia, and systemic mycoses
  • Trauma: blunt or cold, can also been seen in factitious disorders with subcutaneous injections
  • Malignancy: cutaneous lymphoma predominantly
  • Deposition: tophi in gout, also calciphylaxis
  • Enzymatic destruction: seen in cases of pancreatitis as well as alpha-1 antitrypase deficiency

Note: deep inflammatory nodules and plaques are not exclusive to panniculitis--and disorders involving the deep dermis or fascia may need to be considered (eg. morphea,  eosinophilic fasciitis).