Leg Subcutaneous Nodules

SUB CUTANEOUS LEG NODULES:

When a doctor sees a patient with subcutaneous leg nodules, he has various clinical condition in mind.  The problem is that without a biopsy it is difficult to diagnose them.  Erythema nodosum can be diagnosed clinically from the location of painful, red, slightly bruised nodules over the anterior shins and also pretibial myxoedema can generally be diagnosed from the peau d’orange appearance of mucin infiltrates on the anterior shin.  However, a lot of other subcutaneous nodules particularly on the calf, will require a deep biopsy to make the diagnosis.  Other considerations include erythema induratum, pancreatic panniculitis, lupus panniculitis, subcutaneous T-cell lymphoma, deep fungal infection and atypical bacterial infection.  Under this layout the histopathology of these various conditions and how to compare and contrast them.  Other types of panniculitis to consider are Alpha 1 antitrypsin deficiency and eosinophilic panniculitis.  Lupus panniculitis typically gives overlying skin atrophy, there is a deep indentation with fibrosis or subcutaneous fat pulling the skin in.

Most patients with infective causes are usually immunosuppressed either with HIV disease or because they are on immunosuppressive drugs or chemotherapy.

Subcutaneous T-cell lymphomas are rare, both T and B cell lymphomas do involve the leg.  The large B-cell lymphoma leg type or subcutaneous T-cell lymphoma are the commonest.  Gamma Delta T-cell lymphoma usually presents as infiltrating eroding plaques or tumours rather than subcutaneous nodules and it has a poor prognosis.
Clinical - Large purple plaques on the skin surface or deeper nodules. The plaques may ulcerate.

Erythema nodosum is usually widening of the septum with infiltrate of lymphocytes and some neutrophils around the septal vessels with mild involvement of the periphery of the fat lobules.

Erythema induratum involves medium sized vessels with vasculitis and granulomatous inflammation and necrosis.  Sometimes the vascular involvement has had this condition described as nodular vasculitis.  Previously reaction to tuberculosis was the commonest cause or erythema induratum.

Lupus panniculitis. Early lesions have lobular lymphocytic infiltrate with plasma cells and there may be lymphoid follicles formed.  Mucin is also deposited in the deep dermis and subcutaneous fat.  With time the amount of mucin decreases and collagen is deposited and ultimately develops a sclerosing panniculitis which causes the indentation of the skin in these lesions.

Cutaneous polyarteritis nodosa.   Usually a large vessel is involved and initially the infiltrates neutrophilic while later it becomes cystic cytic. 

Panniculitis due to Alpha 1 Antitrypsin deficiency shows a neutrophilic infiltrate at the fat dermal junction so it is not an obvious diagnosis.

Subcutaneous Sweet’s Syndrome may also give neutrophils.  This usually involves the fat and doesn’t give a superficial infiltrate.  The clinical lesions look much the same as passive Sweet’s.

Pancreatic panniculitis involves the lobules and gives liquid fat necrosis with ghost cells.

Eosinophilic panniculitis has the fat infiltrated with eosinophils and hence is a simple diagnosis.

Infection and foreign body reactions will usually give fat necrosis but overlying dermal involvement and a suppurative and granulomatous inflammation.  Special stains may be helpful.

Subcutaneous T-cell Lymphoma Alpha Beta type shows a lymphocytic infiltrate in the fat without any sign of vasculitis, granulomatous inflammation or necrosis.  Sometimes a rim of lymphocytes surrounding individual fat cells is seen and this points to the diagnosis.  Submitting tissue for PCR monoclonality is the best way of diagnosing this condition.

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