Atypical Presentation of Staphylococcus aureus Septic Arthritis in an Elderly Woman

Septic arthritis, also known as infectious arthritis, is a life-threatening condition that occurs when microorganisms, most often bacteria, spread through the bloodstream and invade a joint, or when the joint becomes directly infected, as can occur following surgery or an injury. The most common sites of infection are the knees and hips, with many patients presenting with acute monoarthritis. We report the case of an atypical presentation of septic arthritis caused by Staphylococcus aureus in a 75-year-old woman with a history of chronic pain. She developed a subacute left knee effusion without systemic signs of sepsis; however, aspiration of the knee joint demonstrated methicillin-sensitive S aureus. When these results confirmed a diagnosis of septic arthritis, the seemingly benign knee joint effusion proved to be a medical emergency. The patient was transferred for urgent orthopedic joint washouts and received a prolonged course of intravenous (IV) antibiotics. This case serves as a reminder for geriatricians and medical practitioners to maintain a high index of suspicion for atypical presentations of septic arthritis in elders. It also highlights the importance of being vigilant when an elderly patient with chronic pain develops new pain symptoms.

Case Presentation

A 75-year-old woman was admitted to the geriatric unit for rehabilitation following treatment of a fracture of the right femoral neck that occurred when she fell in her bathroom. Following an epidural abscess 4 years earlier, she experienced chronic severe musculoskeletal pain localized in her back and thighs that had been managed with escalating doses of an opioid analgesic. On hospital admission, the patient was wearing fentanyl patches that delivered a total dose of 150 µg per hour to manage this pain. Her other medications included oral calcium carbonate, 600 mg daily (1 tablet); oral esomeprazole, 20 mg daily; and oral escitalopram, 10 mg daily. Her medical history also included osteoarthritis, which had required a total left knee joint replacement 6 years earlier; osteoporosis; inactive ulcerative colitis; myelodysplasia; and depression. During the first week of admission, she noticed gradual swelling and increasing pain in her left knee. There was no history of falls or trauma while in the hospital, and she did not have a fever at any time during her admission. Despite her knee issue, the patient looked well clinically and continued to participate in physiotherapy.

Physical examination demonstrated a moderately swollen left knee that was not erythematous or warm, and the range of motion was not limited compared with her right knee. Laboratory testing revealed a normal white blood cell (WBC) count at 6800/µL (normal range, 4500-11,000/µL) and an elevated C-reactive protein serum level at 137 mg/L (normal range, <5 mg/L). An ultrasound image of the left knee confirmed an effusion.

Clinically, the diagnosis of crystal arthropathy (gout or pseudogout) was suspected, given the lack of systemic signs and symptoms of sepsis. Before commencing prednisolone therapy for the pain and swelling, however, a left knee joint aspiration was performed to exclude septic arthritis. Surprisingly, microscopy did not reveal any evidence of crystals, but demonstrated a WBC count of 35,800/µL (99% polymorphs). Gram-positive cocci were detected, which were later confirmed on culture to be methicillin-sensitive S aureus. With the diagnosis of septic arthritis, which constitutes a medical emergency, the patient was transferred to the orthopedic unit and underwent urgent left knee joint washouts and received a prolonged course of IV antibiotics. In addition to experiencing a delayed return of joint function following the joint washouts, she became more depressed clinically. These physical and mental obstacles significantly slowed her rehabilitation progress.

Discussion

Septic arthritis is a medical emergency with a high mortality rate. A review of 38 adult patients with septic arthritis in the United States found an in-hospital mortality rate of 26%.¹ Other more recent reports reveal mortality rates ranging from 11% to 15%.^2,3 The most common pathogenic organisms are staphylococcal and streptococcal species.⁴

Although the true incidence of septic arthritis is not known, a European study published in 1995 estimated the incidence to be between two and five cases per 100,000 in the general population and as high as 30 to 70 cases per 100,000 in individuals with rheumatoid arthritis (RA).⁵ In addition to RA, risk factors for septic arthritis include age, with children and elders more commonly affected than other age groups; diabetes mellitus; presence of hip or knee prosthesis; joint surgery; weak immune system from a comorbidity or medication; and skin infection.^4,6,7 Because elders are more likely to have these risk factors, they are at an especially high risk of developing septic arthritis. It is also more difficult to recognize and diagnose the condition in these patients, as they often present atypically.^6,7

Patients with septic arthritis are classically described as being septic and unwell, and they typically present with high fever, leukocytosis, and an inability to bear weight fully on the affected joint⁸; however, a 10-year review of 21 patients older than 60 years with septic arthritis found that most were afebrile.⁶ Also, the majority of these patients did not have leukocytosis at the time of admission. Without timely diagnosis and treatment, the outcome of septic arthritis is poor for elderly patients. Even when promptly diagnosed, there is a risk of potential complications, including osteomyelitis, secondary osteoarthritis, and delayed return of joint function.^6,7

Case reports of atypical presentations of septic arthritis in elderly patients are scarce. Most reported cases in the literature describe patients with septic arthritis secondary to systemic bacteremia, with most patients appearing unwell or presenting with septic shock. Ota and colleagues⁹ reported a case of septic polyarthritis as a result of group B streptococcal bacteremia in a 77-year-old woman. Chew¹⁰ described a case of an elderly man who had septic shock on presentation and was found to have septic arthritis and osteomyelitis following Klebsiella pneumoniae urosepsis.

Despite appearing well, our patient was at risk for significant morbidity and mortality. Her comorbidities, including ulcerative colitis and myelodysplasia, were stable and quiescent; thus, she was not heavily immunosuppressed from a medical disorder, which would predispose her to developing septic arthritis. She also was not receiving any immunosuppressive treatment that placed her at higher risk for infection. In addition, because her knee replacement occurred more than 6 years before she developed the knee effusion, and she had no previous episodes of septic arthritis, we were surprised by her diagnosis. In patients with a history of a total knee joint replacement, S aureus prosthetic joint infection is a serious and devastating complication that can be life-threatening.^11,12 As highlighted by our patient’s case, she underwent multiple joint washouts and required a prolonged course of IV antibiotics. It was fortunate that the sepsis could be controlled without requiring more invasive procedures, such as total removal of the prosthetic joint. An infected orthopedic prosthesis could result in the removal of the prosthetic joint, lifelong antibiotic therapy, and potential loss of independence and function for the patient.

Importance of the Differential Diagnosis
Our case report highlights the importance of maintaining a high index of suspicion for septic arthritis in elderly patients presenting with joint pain and effusion, even when they appear clinically well. The differential diagnosis of atraumatic joint pain in an elderly patient includes
osteoarthritis; inflammatory arthritis, such as RA; crystal arthropathy, including gout and pseudogout; and septic arthritis.¹³ Septic arthritis in elderly patients may be easily confused with gout or pseudogout, which was initially suspected in our case patient. This is a common situation because gout and pseudogout have manifestations similar to those of septic arthritis, such as pain, inflammation, and, occasionally, other constitutional symptoms.¹⁴ The work-up of elderly patients with atraumatic joint pain and effusion must include sterile joint aspiration to exclude septic arthritis, especially in those with new joint pain and effusion on presentation. Even if crystals are found in the synovial fluid, which would indicate gout or pseudogout, the presence of leukocytosis must be evaluated, and a Gram stain and culture should be performed.¹⁴

Conclusion

In this case report, the consequences of misdiagnosing septic arthritis and initiating steroid therapy for presumed crystal arthropathy could have been devastating. Clinicians must keep in mind that septic arthritis can present atypically, especially in elders; thus, septic arthritis must be included in the differential diagnosis of any patient presenting with joint pain and effusion until this diagnosis is ruled out by sterile joint aspiration.

Our patient’s chronic pain had been managed with high-dose opioid analgesia—fentanyl patches with a total dose of 150 µg per hour—which equates to more than 500 mg of oral morphine daily. Despite such a high dosage of analgesia, the patient developed new pain in the left knee, which differed from her chronic back and thigh pain. This circumstance serves to highlight the point that elderly patients with chronic pain must be assessed carefully when they develop new sites of pain or worsening pain, because these symptoms can point to new sites of pathology.

The authors report no relevant financial relationships.

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