A rare case of Streptococcus sanguinis mycotic popliteal aneurysm

INTRODUCTION Mycotic popliteal aneurysms are not a common phenomenon. They can initially be easily confused with other more trivial conditions such as a Baker's cyst. We present a case of a patient presenting with a progressively worsening leg swelling which was initially misdiagnosed. Only until symptoms rapidly progressed was a popliteal aneurysm diagnosed. To our knowledge this is the only identified case of a Streptococcus sanguinis mycotic popliteal aneurysm. CASE PRESENTATION An 81-year-old gentleman presented to the surgical assessment unit with a six-week history of a painful, diffuse swelling in the left popliteal fossa. Initially, when symptoms developed a provisional diagnosis of a Baker's cyst was made. When the symptoms progressed to involve swelling of the entire lower limb, an ultrasound was arranged. Detailed Imaging revealed a popliteal aneurysm with signs of rupture. Urgent repair was performed, with high suspicion of a mycotic aneurysm intra-operatively. Cultures confirmed this, isolating Streptococcus sanguinis. Multiple investigations failed to isolate an acute infective source of this infection. The patient recovered promptly with a long course of intravenous antibiotics, being able to mobilize normally. CONCLUSION Mycotic popliteal aneurysms are not very common and can easily be confused with other benign lesions. The key to diagnosis is the presence of a pulsatile mass and further detailed imaging. This case was unique in that Streptococcus sanguinis has not been isolated from such an aneurysm until now. The most likely explanation of this case was that the aneurysm was secondary to transient bacteraemia of this organism through the oral cavity, in the absence of any cardiac involvement.