Erythema nodosum is a painful red nodular rash that may be a manifestation of numerous systemic diseases.

Epidemiology

Erythema nodosum is three to five times more common in female patients ¹.

Clinical presentation

Erythema nodosum appears as painful erythematous nodules over the shins approximately 1-6 cm in size and are usually distributed symmetrically. Systemic symptoms (e.g. fever) may also be present depending on the underlying condition ¹.

Pathology

The rash is caused by panniculitis (inflammation of the subcutaneous fat) and is thought to be due to a type IV hypersensitivity ¹.

Aetiology

Multiple causes of erythema nodosum have been described ^2,5:

• idiopathic: commonest

• post-infectious (most commonly Streptococcal pharyngitis)

• infection (e.g. tuberculosis)

• sarcoidosis (see cutaneous manifestations of sarcoidosis)

• pregnancy ³

• drug-related (e.g. oral contraceptive pill, sulfonamides, amoxicillin)

• inflammatory bowel disease

• paraneoplastic syndromes (e.g. Hodgkin disease, leukaemia)

Radiographic features

Ultrasound

The sonographic features of erythema nodosum in its acute stage have been described as non-specific thickening, increased echogenicity and increased blood flow on Doppler of the affected subcutaneous fat ^ref. Indeed, the appearance may be solely due to focal oedema of the affected fat ^4,6. Even after successful treatment and resolution of the lesions on physical examination, subtle subcutaneous abnormalities may remain ⁴.

MRI

Signal characteristics include ⁵:

• T1: hypointense to fat/muscle

• T2: hypo-to-hyperintense to fat and hyperintense to muscle

• T1 C+ (Gd): enhancement

• T1 fat sat/STIR: hyperintense to fat/muscle

• GRE: no susceptibility artifact

Treatment and prognosis

Typically the lesions are self-limiting and resolve within a few weeks. Underlying disorders however should be investigated for and treated ¹.