What is Erythema nodosum

Erythema nodosum is an inflammatory disorder that involves tender, red bumps (nodules) under the skin. In about half of cases, the exact cause of erythema nodosum is unknown. Some cases may occur with infections. Some of the more common infections are: streptococcus (most common), cat scratch disease, chlamydia, coccidioidomycosis, hepatitis B, histoplasmosis, leptospirosis, mononucleosis (EBV), mycobacteria, mycoplasma, psittacosis, syphilis, tuberculosis, tularemia, and yersinia.

Erythema nodosum may occur with sensitivity to certain medications, including: antibiotics including amoxicillin and other penicillins, sulfonamides, sulfones, birth control pills, and progestin. Erythema nodosum is most common on the shins. It may also occur on other areas of the body such as buttocks, calves, ankles, thighs, and arms.

The lesions begin as flat, firm, hot, red, painful lumps that are about an inch across. Within a few days, they may become purplish in color. Over several weeks, the lumps fade to a brownish, flat patch.

Erythema nodosum (EN) is a skin condition in which red bumps (nodules) form on the shins. Less commonly, the nodules form on other areas of the body such as the thighs and forearms.[1][2] The lesions begin as firm, hot, red, painful lumps and progress to a purplish color.[1] EN is a type of inflammatory disorder affecting the layer of fat under the skin (panniculitis).[2][3] Other symptoms that may accompany the skin findings include the following: fever, a general feeling of being ill. joint aches, and swelling of the affected area.[1] In many cases, EN is presumed to be a delayed reaction to antigens associated with various infections, drugs, and certain systemic diseases.[3] In some cases, however, EN has no identifiable cause (idiopathic); in these cases, clinical follow-up is needed to rule out certain conditions including inflammatory bowel disease, sarcoidosis, lymphoma, and Behcet’s disease.[3] Treatment may include rest, nonsteroidal anti-inflammatory drugs (NSAIDS), steroids, hot or cold compresses, potassium iodide solution, and supportive bandages or compression stockings.[1][2] Symptoms usually resolve within six weeks, but EN may become a chronic disorder lasting for months and, occasionally, for years.[1][2] Approximately 30% cases of idiopathic EN may last more than 6 months.[4]

This information is provided by the National Institutes of Health (NIH) Genetic and Rare Diseases Information Center (GARD).
https://rarediseases.info.nih.gov/diseases/9260/erythema-nodosum-idiopathic