Panniculitis

Panniculitis - Fat Under Fire

• Definition: Inflammation of subcutaneous fat (panniculus adiposus), presenting as deep, tender, erythematous nodules, typically on the lower extremities.
• Diagnosis: Requires a deep incisional or excisional biopsy for accurate histologic classification, which is crucial for determining the underlying cause and appropriate management.

⭐ Erythema nodosum (septal, no vasculitis) is the most common form. It represents a delayed hypersensitivity reaction to various stimuli like infections (Strep), sarcoidosis, IBD, and drugs (OCPs).

Erythema Nodosum - Shins on Fire

• Pathophysiology: A delayed-type (Type IV) hypersensitivity reaction affecting the septa of subcutaneous fat.
• Clinical: Presents as tender, erythematous, non-ulcerated nodules, classically on the anterior shins. Fever and arthralgias are common.
• Histology: Septal panniculitis without primary vasculitis. Early lesions show edema, hemorrhage, and neutrophils; later lesions have lymphocytes, histiocytes, and giant cells, including characteristic Miescher's radial granulomas.
• Associations:
  • 📌 Mnemonic: ENjoys SODA
    • Sarcoidosis / Streptococcal infection
    • OCPs
    • Drugs (sulfonamides)
    • Autoimmune (e.g., IBD)

⭐ In children, streptococcal infection is the most common trigger, whereas in adults, it's often idiopathic or linked to sarcoidosis.

Erythema Induratum - Calf Ulcer Attack

• Pathophysiology: A mostly lobular panniculitis involving a T-cell mediated hypersensitivity reaction, often with vasculitis.
• Clinical Presentation:
  • Tender, erythematous nodules primarily on the posterior calves.
  • Characteristically ulcerate, drain, and heal with scarring, distinguishing it from erythema nodosum.
• Histology:
  • Lobular or mixed septal-lobular panniculitis.
  • Necrotizing vasculitis of small to medium-sized vessels within the fat lobules.
• Associations:
  • Strongly associated with Mycobacterium tuberculosis (scrofuloderma).
  • Also linked to other infections like Nocardia and hepatitis C.

⭐ When linked to tuberculosis, it's termed Bazin's disease. A positive PPD test or IGRA is a key diagnostic clue.

Other Types - The Unusual Suspects

• Alpha-1-Antitrypsin Deficiency: A lobular panniculitis with liquefactive necrosis, leading to oily discharge and ulceration. Diagnosis requires checking A1AT levels.
• Factitial Panniculitis: Self-induced by injecting foreign substances (e.g., oils, milk). Presents with bizarre, angular lesions. Histology reveals foreign material.
• Pancreatic Panniculitis: Lobular type associated with pancreatitis or pancreatic cancer.

  ⭐ Histology classically shows "ghost cells"-anucleated adipocytes with calcified walls from saponification.

High‑Yield Points - ⚡ Biggest Takeaways

• Erythema nodosum, the most common form, is a septal panniculitis without vasculitis, typically on the anterior shins.
• Erythema induratum is a lobular panniculitis with vasculitis, strongly associated with tuberculosis.
• Pancreatic panniculitis shows ghost-like fat cells from enzymatic fat necrosis due to pancreatitis or carcinoma.
• Alpha-1 antitrypsin deficiency can cause a neutrophilic lobular panniculitis.
• Diagnosis requires a deep excisional biopsy to adequately sample the subcutaneous fat.
• The key histologic distinction is septal versus lobular inflammation.