Knee Knots in a Young Girl: What's Your Diagnosis?

On a busy clinic day, 6-year-old Polly limped into the outpatient clinic and was admitted because of the painful lumps on her lower legs. You are asked to evaluate her. Her history reveals: a sore throat 4-5 weeks ago; red, very tender nodules over her shins that have been present for 2 weeks.

Past medical history: non-contributory. No travel. No recent immunizations. Other than the previous sore throat, no illnesses.No fever, weight loss, bruising.

Review of systems. no trauma. not taking any medications. Physical examination is significant only for 1- to 4-cm oval, reddish-brown, easily palpable firm nodules over the surface of the anterior tibia. The overlying skin is tight and starting to peel.

Note the nodular lesions on the surface of the anterior tibia and that the overlying skin is peeling.

A close-up view of some of the many tender nodules and the peeling of the overlying skin.

How about contemplating a differential diagnosis?

Differential diagnosis: First, lets eliminate a few categories…based on the history and physical examination, it is not traumatic, or an obvious infection. Could it be a tumor? Could it be an inflammatory condition? Benign and malignant tumors can cause lumps including: Fibroma, lipoma, lymphoma, osteosarcoma, sarcoma. Do any of these seem to fit?

Differential diagnosis...Could it be an inflammatory condition? Inflammatory conditions to consider include: Erythema nodosum; gout; pseudogout; osteoarthritis; psoriatic arthritis; rheumatoid arthritis; systemic lupus erythematosus; some other form of vasculitis.

Answer: Erythema nodosum. A delayed cell-mediated hypersensitivity. Respiratory etiology most common cause in children. Recent streptococcal infections most common type in children.Remember, there is a 10% recurrence rate. Red, tender nodular lesions most commonly on the pretibial surface of the legs. Not suppurative.

Erythema nodosum. Histopathologic features: lymphocytic perivascular infiltrate in dermis. lymphocytes & neutrophils in the fibrous septa; in subcutaneous fat. If you make the clinical diagnosis, however, histology is not likely to be required

Erythema nodosum: etiology. Infections: Infections: group A B-Strep, infectious mononucleosisinfluenzae, herpes, tuberculosis, histoplasmosis, coccidiomycosis, cat scratch disease, leprosy. psittacosis, lymphogranuloma venereum, measles, ascariasis, leishmaniasis. Also ulcerative colitis, pancreatitis.

Erythema nodosum etiology, continued. Malignancy: leukemia, hodgkin, non-hodgkin. Collagen-vascular: systemic lupus erythematosus, polyarteritis nodosa, sarcoidosis. Drug-induced: Sulfonamides, oral contraceptives, salicylates, thiazides, dilantin, iodides, bromides, phenacetin.

Erythema nodosum etiology: Biologicals: bacille Calmette-Guerin vaccine, diphtheria antitoxin and toxoid, pure pollen extracts, vaccines.

There are times when erythema nodosum is somewhat atypical...It is important, then, to remember that erythema nodosum falls into the category of panniculitis...

Represents infiltration of subcutaneous tissues by inflammatory and / or neoplastic cells. Apparent as deep induration or swelling of the skin. Associated signs include erythema, ulceration, drainage, warmth, and pain or tenderness.

Erythema nodusum is a septal panniculitis. Predominance of inflammation involves the connective tissue septa between the fat lobules. Lobular panniculitis: inflammation is of the fat lobules themselves.

Septal panniculitis: erythema nodosum, subacute nodular migratory panniculitis (scleroderma)Lobular and mixed panniculitis: Vasculitis and connective tissue, lupus, erythema induratum, other connective tissue. Lobular and mixed panniculitis: Metabolic: altered melting points of fat in the newborn, seen in subcutaneous fat necrosis and sclerema, pancreatic, alpha-1-antitrypsin deficiency. Traumatic. Infectious. Malignancy. Lipodystrophy.