My 14-year-old daughter's doctor said she has "erythema nodosum." He took blood for additional tests and said he may consult an infectious disease doctor. What does erythema nodosum have to do with infectious disease? The red areas are very painful on her shins. She has also had a backache, and X-rays showed signs of mild scoliosis. She has had swollen glands in the groin area, which have gotten smaller. Does all of this go hand-in-hand?

M.G.

Erythema nodosum (EN) is an inflammatory condition of the skin and underlying fat. People with this condition develop red-purple, tender nodules on the shins, and sometimes in other areas as well. Without treatment, these lesions heal without scarring in about two months. In addition to the nodules, patients typically have fever, malaise and body aches. While EN can occur in anyone, it arises most commonly in young women, from about your daughter's age up to about age 40.

Nearly all cases of ER resolve without any aftereffects. But it is not the erythema nodosum itself that is the important illness. In many cases, there is an underlying illness, often an infection, that is thought to trigger this inflammatory reaction. In a large percentage of cases -- about a third in most studies -- patients have no identifiable triggering illness. The most frequently identified trigger in most reports is streptococcal pharyngitis (strep throat, an infection caused by certain bacteria in the Streptococcus family). If laboratory testing is performed on all patients with EN, perhaps 20 percent or more will show evidence of recent streptococcal infection. While most of these patients develop EN after a typical case of strep throat, some don't recall ever having a sore throat.

Other infections are also associated with EN. Until recently, tuberculosis was a frequent infectious trigger of EN. It is less common now, but it certainly still occurs. Other relatively common infections associated with EN are three fungal infections, coccidioidomycosis, histoplasmosis and blastomycosis; chlamydial infection (genital, eye and respiratory infections due to chlamydia bacteria); and HIV (the virus that causes AIDS).

Another common trigger of EN is an inflammatory condition called sarcoidosis, which, in the United States, usually affects those of African descent. Patients with this syndrome may develop inflammation of the lungs, enlarged lymph nodes and, sometimes, erythema nodosum. No one knows the exact cause of sarcoidosis, but it is probably not an infection. EN also can be the initial sign of three inflammatory conditions: Crohn's disease, lupus and Behcet's disease. Other cases of EN are triggered by medications or toxins.

A work-up for the common triggering conditions is in order. It is not necessary to exhaustively pursue every possible diagnosis, because often no underlying cause can be found. In such cases, treatment with anti-inflammatory medication is usually all that is necessary. Your daughter's backache and swollen glands are likely related to the erythema nodosum, but I think the scoliosis (abnormal curvature of the spine) is not related to this illness.