One etiology may conceal another

Skin biopsy confirmed the diagnosis of septal panniculitis with a mixed non-granulomatous inflammatory infiltrate and no vasculitis. PCR performed on the lymph node aspirate revealed Bartonella henselae DNA. This was therefore erythema nodosum (EN) occurring concurrently with cat scratch disease, or bartonellosis, in a young patient who had been in contact with a kitten for several weeks. 

In this young patient, erythema nodosum is, as is very often the case, a reaction to a recent infection, most often group A streptococcus. 

In this case, bartonellosis, which is a rare but classic cause of EN, is more likely the causative factor than COVID, a disease that is very common during pandemic periods but only exceptionally complicated by EN.

• The diagnosis of erythema nodosum was confirmed by biopsy (indicated here given the extracutaneous context), but COVID was not retained as the etiological factor. In fact, the number of cases of post-COVID EN reported during the epidemic was very small compared with the total number of infections, and the patient's adenopathy pointed to another cause.
   
• Sweet syndrome may, as in adults, be associated with a hematologic malignancy. Sweet syndrome is a highly inflammatory neutrophilic dermatosis, more papular than indurated, but the differential diagnosis with erythema nodosum is not always obvious, and it is histology that allows the distinction to be made (here not compatible).
   
• Polyarteritis nodosa is a rare cause of vasculitis in children, but its cutaneous involvement includes specific panniculitis lesions (vasculitis of cutaneous arterioles), often associated with livedo. Presence of adenopathy is not typical.