A 46-year-old male with a history of GERD and psoriasis presented with a tender, erythematous right ankle rash. First noted ten days prior to admission, he did not improve with trimethoprim-sulfamethoxazole and sought care at a local hospital. He was treated for presumed cellulitis with vancomycin and discharged on dicloxacillin and clindamycin. He presented to our institution when his rash and fevers worsened.
Physical exam on admission revealed tachycardia without murmurs. Right knee and bilateral ankles were warm and erythematous, extending beyond the visible border drawn on the right ankle at the outside institution. No joint effusions. Initial WBC count was 12.7. Sedimentation rate and C-reactive protein were 61and 196 respectively.
Septic on presentation he was started on vancomycin for presumed cellulitis failing outpatient therapy. He remained febrile despite broadening antibiotic coverage with ampicillin/sulbactam and began to develop lesions on the contralateral ankle with discontinuous spread to the knees. Rheumatology, Dermatology and Infectious Disease were consulted. Antibiotics were broadened to linezolid and piperacillin/tazobactam. An extensive laboratory investigation for obscure infectious and rheumatologic etiologies was subsequently unrevealing. MRI was negative for deep tissue infection. Additional history revealed that he had culture confirmed streptococcal pharyngitis and completed treatment with penicillin a week prior to the initial lesion. Antistreptolysin O titer was elevated at 936 supporting a diagnosis of post streptococcal erythema nodosum (EN), confirmed by biopsy. He was started on a prednisone taper with improvement.
EN is a cutaneous manifestation of underlying disease characterized by erythematous subcutaneous nodules. Pathogenesis is poorly understood but the majority of cases are either idiopathic or due to antecedent upper respiratory streptococcal infection. Other causes include mycobacterial infection, sarcoidosis, medication reactions, and inflammatory disorders. Nodules typically develop in a pretibial location and evolve into bruise-like lesions that spontaneously resolve over the course of 2-8 weeks. It is often accompanied by fevers, polyarthralgias, malaise and positive inflammatory markers. Diagnosis is clinical, though if unclear a biopsy will show a septal panniculitis of the subdermal tissue. Treatment is supportive including NSAIDs, though in severe cases glucocorticoids can be effective.
Although cellulitis is the commonest etiology in Hospital Medicine for presentation with skin erythema, edema and warmth it is critical for the Hospitalist Physician to be mindful of potential mimickers to avoid misdiagnosis, inappropriate antibiotic use and delays in care. This case illustrates the diagnostic challenge EN can present when the rash has already evolved beyond the initial nodule at presentation as well as the importance of thorough history and maintaining a broad differential diagnosis.