Definition:

Erythema nodosum is a form of panniculitis, which is an inflammatory condition that specifically targets the subcutaneous fat. It appears on the anterior shins as painful red nodules. Occasionally, they affect the thighs and forearms.

Etiology:

Erythema nodosum is a type of hypersensitivity reaction that occurs in approximately 55% of patients, and its etiology is currently unknown. In some situations, there is an association to an infection, medication, inflammatory disease, or cancer.

Pharyngitis: caused by streptococcal bacteria or viral pathogen.
Yersinia infection: can lead to symptoms such as diarrhea and stomach pain.
Chlamydia infection.
Fungal infections: histoplasmosis and coccidioidomycosis.
Parasitic infection: amoebiasis and giardiasis.
Viral: such as herpes simplex, viral hepatitis and human immunodeficiency virus (HIV) infection.

Clinical features:

Erythema nodosum is characterized by the presence of painful, red, and swollen nodules that appear under the skin. These nodules can range in size from 3 to 20 cm and typically develop over a period of one to several weeks. The symptoms include fever and joint pain. Swelling and pain in the ankle occur in 50% of cases and might last for many weeks. It may also affect other joints, such as the knees.

The nodules are primarily located on the anterior lower legs, knees and arms.
Their shape can be described as ill-defined, warm, oval, round, or arciform, and they do not exhibit ulceration.
The nodules exhibit an initial color ranging from bright to deep red.
Erythema contusiformis is a term used to describe the development of violaceous, brownish, or yellowish bruise-like marks that dissolve on their own within eight weeks.
Erythema nodosum does not result in permanent scarring.

Diagnosis:

Erythema nodosum is primarily diagnosed through clinical examination and confirmed by laboratory tests and histology. Erythema nodosum pathology primarily manifests as inflammation of the septa between subcutaneous fat lobules in the absence of vasculitis.

Differential diagnosis:

Panniculitis can manifest as either mainly septal, characterized by inflammation between lobules, or lobular, characterized by inflammatory cells within subcutaneous fat lobules. Septal and lobular inflammation can coexist.

The nodules caused by mostly septal panniculitis include:

-Different types of scleroderma
-Medium vessel vasculitis, such as that caused by polyarteritis nodosa, is characterized by the presence of sensitive subcutaneous nodules accompanied by ulceration, necrosis, livedo racemosa, fever, joint discomfort, myalgia, and peripheral neuropathy.
-Necrobiosis lipoidica
-Eosinophilic panniculitis
-Rheumatoid nodule.

The nodules caused by mostly lobular panniculitis include:

-Connective tissue illness, such as panniculitis linked with cutaneous lupus erythematosus.
-Erythema nodosum leprosum.
-Pancreatic panniculitis is a condition characterized by the development of subcutaneous nodules that may become ulcerated or fluctuant. The laboratory tests reveal increased levels of lipase, amylase, and trypsin.
-Traumatic panniculitis
-Nodular vasculitis, also known as erythema induratum, is a condition characterized by the presence of ulcerated and draining nodules that specifically affect the posterior calves.
-Lipodermatosclerosis is a condition that occurs as a result of venous insufficiency.
-Subcutaneous fat infection caused by bacteria, mycobacteria, or fungus, resulting in the formation of ulcerated, fluctuant, and draining abscesses.

Treatment:

Treatment for erythema nodosum is determined by the primary cause of the condition. It is necessary to address any underlying infection.

Methods for pain management may involve prolonged periods of rest, administration of colchicine, use of NSAIDs, and application of venous compression therapy.
After infection, sepsis, and cancer have been ruled out, a prescription for systemic corticosteroids (1 mg/kg daily) may be prescribed until the erythema nodosum resolves.

Written by:

Mashael Alanazi, Medical Student.

Revised by:
Maee Barakeh, Medical Student.

Resources:
DermNet