It wasn’t EN after all?
What other diseases look like erythema nodosum?
Sometimes, someone might be diagnosed with erythema nodosum when their symptoms don’t quite match up with the diagnostic criteria. The following conditions have been mistakenly diagnosed as erythema nodosum. From what I can tell, most of them are even less common than EN!
Sweet’s syndrome In 1964 a doctor named Robert Sweet described a disease that looked a lot like erythema nodosum, but occurred on the trunk, neck, hands, and face. He called it acute febrile neutrophilic dermatosis, but it’s now usually called Sweet’s syndrome. It has been seen coexisting with erythema nodosum several times (Harris and Henderson, 2011). Although biopsies can usually help decide if a rash is EN or Sweet’s, it can’t always, like in this case from 2016.
Erythema multiforme The confusion here is mostly due to disagreements in classification: dermatologists argued for about 100 years about whether or not they were the same disease. If you’re interested in skin disorder naming drama, read anything that mentions “erythema nodosum” written before 1920.
Erythema induratum / nodular vasculitis A 29-year old Vietnamese woman was treated for erythema nodosum, but she actually had a condition called erythema induratum, which is associated with tuberculosis. EI isn’t perfectly understood, but it might be a sensitivity reaction to tuberculosis antigens. (Chew et al, 2005)
Here is another case of EI mimicking EN from 2017.
Darier-Roussy (Sarcoidosis) Sarcoidosis is one of the leading causes of EN, but what if your EN is actually sarcoidosis in disguise? In 2011, three doctors in Michigan finally solved the case of a patient whose EN wasn’t getting any better. She actually had a rare kind of subcutaneous sarcoidosis that also presents as nodules. (Liu et al, 2011)
Fungal pannucilitis This is the only fatal case study involving a misdiagnosis of erythema nodosum that I have come across. The situation was complex, but basically, a woman who had a transplanted kidney (and related health issues) had a rash that looked like erythema nodosum, and she tested positive for tuberculosis. Her doctors tried treating it for three months, but she eventually got very sick and needed to be hospitalized. After two weeks in the hospital, they discovered she had histoplasmosis, which is a fungal infection, and she died of multiorgan failure.
Subcutaneous granuloma annulare A 4-year old boy in South Korea had a rash that looked very similar to EN, but a biopsy showed that it was actually an unusual presentation of a disease called subcutaneous granuloma annulare (Hyun et al, 2017).
Other conditions that might look like erythema nodosum, according to American Family Physician, are α1-antitrypsin deficiency, lymphoma, lupus panniculitis, schleraderma, and poststeroid panniculitis. You can read the full list here. Something to remember: erythema nodosum can occur at the same time as other skin conditions. Have you heard of someone being misdiagnosed with EN?
References: Concurrent Sweet’s syndrome and erythema nodosum (2011) Assessing the Patient with Arthralgia, Fevers, and a Rash (2016) Erythema induratum: a case of mistaken identity (2005) Multiple Subcutaneous Granuloma Annulare Lesions Mimicking Erythema Nodosum Observed on the Lower Extremities in a 4-Year Old Boy (2017) A case of subcutaneous sarcoidosis (Darier–Roussy) misdiagnosed as erythema nodosum (2011) Erythema induratum / Nodular vasculitis (Medscape 2016) Histoplasmosis and subcutaneous nodules in a kidney transplant recipient: erythema nodosum versus fungal panniculitis (2013) Erythema Nodosum: A sign of systemic disease (2007)