Introduction 

Lower limb swelling is a common Emergency Department (ED) presentation. Bilateral limb swelling is frequently due to venous insufficiency. However, unilateral limb edema poses a diagnostic challenge in the ED, often overshadowed by common causes such as cellulitis, deep vein thrombosis (DVT) or a ruptured Baker’s cyst. This case report describes a middle-aged lady who presented with progressive left calf swelling and pain. She was initially misdiagnosed as a ruptured baker’s cyst. Subsequent follow up and admission revealed a septic arthritis with a large calf abscess. 

 

Case presentation

A healthy 63-year-old Chinese lady, presented to the ED with a two-week history of progressive left calf pain and swelling. An initial ultrasound from a private clinic, performed 9 days prior to her visit, reported a ruptured baker’s cyst and no DVT was detected. She denied any fever, trauma or insect bites to the lower limb.

 

On examination, patient was afebrile and non-toxic. Her left calf was swollen and tense. Left knee examination was unremarkable and had no palpable popliteal bursa. She had no signs of compartment syndrome and her lower limb pulses were well felt with a capillary refill time < 2 seconds. Blood investigations showed a white blood cell (WBC) count of 9.83 x 10^9/L with 88% neutrophils. Her C-reactive protein (CRP) was elevated at 148mg/L. She was discharged from ED with analgesics and orthopedic follow up. Impression then was a ruptured baker’s cyst. 

 

Persistent symptoms during her orthopedic follow up 2 week later prompted admission for further evaluation. Examination on admission was similar except for pain on passive knee movement. The knee otherwise was not warm to touch.

 

Repeat bloods showed persistently elevated inflammatory markers (CRP 93.5mg/dl and ESR 115mm/hr) thought her procalcitonin was 0.3ug/l. Patient remained afebrile and systemically well. Knee aspiration yielded minimal straw-colored fluid and X-rays were unremarkable for any osteomyelitis or gas gangrene. However, Magnetic Resonance Imaging (MRI) of the left calf, performed 2 days after admission, reported a large enhancing fluid collection in the posterior calf worrisome for an abscess. Small intramuscular abscess and a left knee effusion with thick enhancing synovium concerning for septic arthritis were also noted.

 

Patient underwent a knee joint and posterior calf surgical drainage with wound debridement. Intra-operative findings included 3mls of sero-purulent joint fluid in the knee with an inflamed and sloughy synovium. Posterior calf had an extensive collection of pus with unhealthy and sloughy underlying fascia. The abscess did not extend into the distal femur or proximal ankle. Intra-operative cultures came back positive for Methicillin-Sensitive Staphylococcus Aureus (MSSA). Notably, during her 1-month inpatient stay, patient was afebrile except for a low-grade fever for 2 days post operatively. She had no systemic signs of infection and blood tests to screen for immunocompromised state (e.g. diabetic or HIV) were negative. Infectious disease specialists assisted with antibiotics titration. Patient was transitioned from intravenous Augmentin to intravenous Cloxacillin, to complete a total of 6 weeks duration via the  Outpatient Parenteral Antimicrobial Therapy (OPAT) Service.

 

Discussion

Septic arthritis associated with extra-articular abscess is rare. This case underscores the importance of re-evaluating seemingly benign diagnosis when clinical symptoms are persistent especially with significantly elevated inflammatory markers. Even in the absence of classical signs (e.g. fever, sepsis, immunocompromised host), an elevated CRP level should prompt further evaluation. When CRP is paired together with an elevated ESR it increases sensitivity for joint infections to 98%. Early recognition and imaging can significantly alter patient outcomes by enabling timely surgical and antimicrobial intervention.