Erythema nodosum is a painful red nodular rash that may be a manifestation of numerous systemic diseases.
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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.
Etiology
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, leukemia)
Radiographic features
Ultrasound
The sonographic features of erythema nodosum in its acute stage have been described as non-specific heterogenous hypoechogenicity of the affected subcutaneous fat (in six patients). Indeed, the appearance may be solely due to focal edema 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
A single paper has nicely shown the MRI appearances in a single patient (from 2021) 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.
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