INTRODUCTION. The subcutaneous tissue is a fibrofatty layer spread between skin and muscles. It functions not only as a thermal and mechanical insulator but also as an active metabolic organ. The characteristic signet ring lipocytes are organized into lobules by fibrous septa, which are continuous with the dermis and contain the blood and lymph vessels and reticuloendothelial cells.

The diagnosis of panniculitis frequently requires deep skin biopsy. The most important histologic characteristic is the location of the inflammatory process. Inflammation primarily in the septa is designated septal panniculitis, whereas inflammatory cells primarily in the fat lobules designate lobular panniculitis. The presence or absence of vasculitis further differentiates panniculitis into the four major groups.

LOBULAR PANNICULITIS WITHOUT VASCULITIS

Nodular Panniculitis-Weber-Christian Disease. Nodular panniculitis describes a group of syndromes or diseases characterized by subcutaneous nodules and inflammatory cells in the fat lobules. The term Weber-Christian disease is applied when cutaneous lesions are associated with systemic complaints.

The etiology of this group of diseases is unknown. In the early stages the fat lobules are infiltrated with polymorphonuclear leukocytes. Later, macrophages appear and ingest fat, producing the characteristic lipophagic granuloma. The lesions heal with lobular fibrosis. Modest septal vasculitis may be observed. Lobular panniculitis most commonly presents in females between the ages of 30 and 60, although cases have been reported in all age groups. The lesions begin as red, slightly tender nodules deep in the skin. They appear more or less in symmetric crops on thighs and lower legs, but lesions may also occur on arms, trunk, and face. The number of lesions may vary enormously. The lesions become firmer, less red, and less tender over a period of weeks. They heal, leaving a depressed hyperpigmented scar. Liquefying panniculitis is a variant in which the lesions become necrotic and drain an oily, yellow-brown fluid. Rothmann-Makai syndrome is a very rare variant of lobular parmiculitis, affecting children with numerous large lesions; the lesions do not liquefy, and healing usually occurs within 12 months.

Systemic nodular parmiculitis or Weber-Christian disease is a widespread proces~ affecting cutaneous and visceral fat. Patients usually present with unequivocal cutaneous nodules and arthralgias, malaise, fatigue, weight loss, and abdominal pain. Involvement of the bone marrow may produce anemia, leukocytosis or leukopenia, and bone pain. Hepatomegaly, steatorrhea, and intestinal perforation have also been reported. Inflammation may occur in other internal organs such as lungs, pleura, pericardium, spleen, kidney, and adrenal glands. Visceral involvement may be confined to the retroperitoneal space, producing abdominal pain, nausea, and vomiting. Alpha,-antitrypsin deficiency is associated with this syndrome not infrequently. Malignant histiocytosis (cytophagic histiocytic panniculitis) may present as Weber-Christian disease. The prognosis of nodular panniculitis is good in patients with only cutaneous involvement. There are frequent remissions and exacerbations of the lesions. Some patients recover after a few months, and permanent remission is usual after several years. On rare occasions visceral involvement may be fatal.

There is no specific therapy for this disease. Saturated potassium iodide, increasing from 5 drops three times daily by 1 drop per day to 30 drops three times daily, has been suggested. Hydroxychloroquine,* 200 mg twice per day, has also been advocated as treatment. High dose prednisone, 40 to 60 mg for one to two weeks with gradual tapering over six to eight weeks, has also been reported to be of value in patients with severe disease; steroids should be used only for acute attacks and for limited periods of time.

Lobular Panniculitis Associated with Pancreatic Disease. The diagnosis is made by skin biopsy, which discloses acute fat necrosis with characteristic ghost cells. These patients often have associated arthritis, ascites, and eosinophilia. Acute pancreatitis, trauma to the pancreas, chronic pancreatitis, pancreatic cysts, and pancreatic carcinoma have been reported to be associated with this syndrome. Diagnosis depends upon the histologic findings at skin biopsy and documentation of a specific pancreatic abnormality. Therapy is directed at the underlying pancreatic disease.

Poststeroid Lobular Panniculitis. Children who receive large doses of steroid for a short period of time, followed by abrupt discontinuance, may develop lobular parmiculitis. Lesions may occur in the viscera, and a fatal case has been reported. Physical Lobular Panniculitis. Physical trauma of any kind and cold injury, especially in children, can produce lobular panniculitis. A unique traumatic panniculitis occurs in the obese breasts of women in their 50's. Injection of silicones or other foreign materials into female breasts or buttocks and into the male genitalia may induce a granulomatous foreign body nodular panniculitis. Similar inflammatory lesions may be seen following injections of Talwin.

Lobular Panniculitis Associated with Systemic Disease. Lupus erythematosus, sarcoidosis, granuloma annulare, Sweet's disease, acute sudden weight loss from gastrointestinal surgery, and infections including deep fungi and pyogens may present as lobular parmiculitis. Lymphoma or leukemia may also present as panniculitis; histologically, these lesions demonstrate malignant cells in the fat lobules. Lupus erythematosus confined primarily to the fat is known as lupus profundus. The skin may be exclusively involved, or the panniculitis may be associated with systemic disease.

LOBULAR PANNICULITIS WITH VASCULITIS.

This category of disease includes nodular vasculitis and erythema induratum. The eruption consists of recurring, tender, painful nodules on the calves, which often ulcerate and heal with scarring. It is much more common in females than in males. Increased erythrocyte sedimentation rate and hypertension have been associated with this syndrome. Bazin gave the name erythema induraturn to this disease when histologic examination revealed caseation necrosis and the lesions were associated with tuberculosis. There is no specific therapy. Most patients have remission of lesions with bed rest. Severe cases have been successfully treated with nonsteroidal anti-inflammatory drugs, dapsone, and prednisone. In the very rare case of nodular vasculitis associated with tuberculosis, appropriate antituberculous therapy is indicated.

SEPTAL PANNICULITIS WITHOUT VASCULITI

This histology in a patient with nodular, painful, tender lesions, especially on the anterior leg, is diagnostic of erythema nodosum, which is discussed in Ch. 534. A chronic disease similar to erythema nodosum clinically and histologically except that the lesions spread peripherally over months, forming rings, is called subacute migratory panniculitis. This disease responds to therapy with increasing doses of saturated *This use is not listed in the manufacturer's directive.

potassium iodide as described for nodular panniculitis. Septal parmiculitis without vasculitis can also be seen in scleroderma, eosinophilic fasciitis, dermatomyositis, and necrobiosis lipoidica diabeticorum.

SEPTAL PANNICULITIS WITH VASCULITIS

Thrombophlebitis may present with subcutaneous nodules. Histology reveals inflammation of veins with adjacent parmiculitis (see Ch. 57). Cutaneous polyarteritis is a chronic, recurring, painful nodular eruption, primarily of the legs. There is often an associated mottled livedo vascular pattern. Cutaneous polyarteritis is associated with myalgias, arthralgias, and increased erythrocyte sedimentation rate. Histologic examination demonstrates leukocytoclastic vasculitis of medium-sized arterioles. This disease is usually not associated with systemic involvement. It has a benign course, but lesions may recur for years. Therapy includes nonsteroidal anti-inflammatory agents and short courses of corticosteroids. Cutaneous polyarteritis associated with granulornatous bowel disease has responded to short courses of Cytoxan.

LIPOATROPHY. Loss of subcutaneous tissue can occur as a consequence of healing in almost any of the parmiculitides described previously. The most common diagnosable cause of lipoatrophy is recurrent insulin injection. Insulin lipoatrophy is usually associated with repetitive injections of high doses of insulin in exactly the same location in females. Talwin injections may also produce panniculitis and severe lipoatrophy.

Total lipoatrophy associated with diabetes may occur in children and adults. The clinical picture is dramatic, and there is almost complete loss of subcutaneous fat. Partial lipoatrophy usually begins in children or young adults. It is five times more common in females than in males. Patients often lose the fat in the face and the upper half of the body. In some cases, there is hypertrophy of the fat on the lower half of the body. Patients with partial lipodystrophy often develop progressive mesangiocapillary glomerulonephritis and hypocomplementernia. Diabetes develops in one third of these patients. Retinitis pigmentosum has also been reported with this disease. The prognosis depends upon the severity of the renal disease.