What is Panniculitis?
Panniculitis is an inflammation of the subcutaneous fat tissue that presents clinically as tender subcutaneous nodules, plaques, or swelling, and is classified histologically based on whether the inflammation primarily affects the fibrovascular septa (septal panniculitis) or the adipose lobules (lobular panniculitis). 1, 2
Clinical Presentation
Panniculitis typically manifests as:
- Subcutaneous nodules or plaques located in the fatty tissue beneath the skin
- Tender, painful lesions that may be red, hot, or violaceous in appearance
- Variable distribution depending on the specific type, though lower extremities are commonly affected
- May progress to ulceration, drainage, or atrophy of subcutaneous fat in certain forms 2, 3
Classification Framework
The traditional diagnostic approach relies on histopathologic examination of deep skin biopsies, which distinguishes:
Septal Panniculitis
Inflammation primarily in the fibrovascular septa between fat lobules:
- Erythema nodosum (most common type) - presents as raised, tender, red or violet nodules 1-5 cm in diameter, typically on anterior tibial areas 4, 5
- Nodular thrombophlebitis
- Periarteritis nodosa (with vasculitis) 2
Lobular Panniculitis
Inflammation primarily within the fat lobules themselves:
- Lupus panniculitis
- Pancreatic panniculitis - characterized by pathognomonic "ghost" adipocytes with saponified fat necrosis 6
- Alpha-1 antitrypsin (AAT) deficiency-associated panniculitis - necrotizing form with spontaneous ulcerations draining clear, yellow, oily, odorless sterile fluid 7
- Subcutaneous panniculitis-like T-cell lymphoma 8
Important Clinical Distinctions
Erythema nodosum is the most frequently encountered panniculitis and can often be diagnosed clinically without biopsy when presentation is typical. Histology shows non-specific focal panniculitis 4, 5.
AAT deficiency-associated panniculitis is a distinctive, potentially lethal form that typically begins as painful, hot, red, tender nodules on thighs/buttocks in young adults (mean age 40 years), with equal sex distribution. Trauma precipitates disease in approximately one-third of cases. The panniculitis can be fatal, particularly when associated with other AAT deficiency complications like cirrhosis or emphysema 7.
Diagnostic Approach
Deep excisional skin biopsy with substantial tissue is essential for accurate diagnosis, as superficial biopsies are inadequate 7, 2. Submit portions for:
- Histopathologic examination
- Microbiological analysis to exclude infectious causes
- T-cell clonal expansion studies if lymphoma suspected 2
Common Pitfall
The majority of panniculitides show mixed septal and lobular infiltration rather than clear-cut distinction, making purely histologic classification challenging. Clinical correlation is essential, and the precise cause may not be established initially, requiring follow-up and potentially repeat biopsy 9, 2.
Associated Conditions
Panniculitis occurs as a manifestation of numerous underlying diseases: