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What is Panniculitis and explain in detail?
Panniculitis is a group of diseases whose hallmark is inflammation of subcutaneous adipose tissue (the fatty layer under the skin – panniculus adiposus). Symptoms include tender skin nodules, and systemic signs such as weight loss and fatigue.
Restated, an inflammatory disorder primarily localized in the subcutaneous fat is termed a "panniculitis", a group of disorders that may be challenging both for the clinician and the dermatopathologist.: 487  The general term for inflammation of any adipose tissue is steatitis.

Signs and symptoms
Panniculitis can also be classified based on the presence or absence of systemic symptoms. Panniculitis without systemic disease can be a result of trauma or cold. Panniculitis with systemic disease can be caused by:

connective tissue disorders such as lupus erythematosus or scleroderma;
lymphoproliferative disease such as lymphoma or histiocytosis;
pancreatitis or pancreatic cancer;
sarcoidosis with cutaneous involvement (seen in up to 20 percent);
Alpha 1-antitrypsin deficiency
Crohns diseaseThis is not a complete list of possible causes.

Associated conditions
Lipoatrophy or lipodystrophy (the loss of subcutaneous adipose tissue) can occur in any of these conditions.

Diagnosis
Classification
It can occur in any fatty tissue (cutaneous or visceral) and is often diagnosed on the basis of a deep skin biopsy, and can be further classified by histological characteristics based on the location of the inflammatory cells (within fatty lobules or in the septa which separate them) and on the presence or absence of vasculitis.There are thus four main histological subtypes:
lobular panniculitis without vasculitis (acute panniculitis, previously termed Weber–Christian disease, systemic nodular panniculitis)
lobular panniculitis with vasculitis
septal panniculitis without vasculitis
septal panniculitis with vasculitis

Lobular
With vasculitis
Erythema induratum, or "Bazin disease", is a panniculitis on the back of the calves