Dermatology practitioners are generally familiar with the clinical diagnosis of panniculitis. The specific type, however, is not always clinically apparent. When the presentation of subcutaneous leg nodules is acute, strongly favoring the anterior legs, and associated with recent history of infection, pregnancy, or drug use, the diagnosis of erythema nodosum is generally made easily, and a biopsy may not be necessary unless the course of lesions is atypical. All other types of panniculitis require histological confirmation of an adequate biopsy specimen that contains abundant fat.

Panniculitis is traditionally divided into disorders in which the infiltrate primarily involves the lobules of fat and those in which the infiltrate favors the subcutaneous fibrous septae. The prototype of septal panniculitis is erythema nodosum and the prototype of lobular panniculitis is erythema induratum, also referred to by some as nodular vasculitis . The latter favors the posterior legs and lesions tend to liquefy and ulcerate. At present, most cases of erythema induratum are idiopathic. Tuberculosis was a common cause when the disorder was initially described several decades ago.

The remaining types of panniculitis are generally rare. Pancreatic panniculitis is usually easily suspected and confirmed. It occurs in patients who generally have known pancreatic disorders or who may have signs and symptoms that point to a pancreatic disorder. Lesions of pancreatic panniculitis often favor the joints. Alpha-1 anti-trypsin deficiency panniculitis is extremely rare, as is eosinophilic panniculitis. Lupus panniculitis is rarely limited to the legs. Most often, it involves the proximal arms and adjacent skin of the shoulders and back, and is almost always followed by a deep indentation at the site of lesions due to atrophy and fibrosis of the subcutaneous fat, which results in “pulling the skin inwards.”