A 53-year-old female presents to the emergency room with a 1-day history of progressive pain, swelling, and inability to bear weight on her right knee. She denies any history of recent trauma but says that she twisted her knee approximately 1 month ago. She also denies previous episodes of similar pain, prior knee surgery, sick contacts, or recent travel. She reports associated fevers and chills for the past 24 hours and says that she is getting over a cold. Her medical history is remarkable for non–insulin-dependent diabetes mel- litus and hypertension. On physical examination, she is 5 feet 6 inches tall, weighs 140 lb, is febrile to 102.3°F, and has a heart rate of 112 bpm. There is a mild left knee effusion with overlying warmth and erythema, and the patient has pain with short arcs of passive flexion and extension. The ligamentous examination is stable, and her straight leg raise is intact. There are no open wounds or lesions. Serum laboratory values are notable for WBC of 18,000 cells/uL, with a differential of 96% PMNs, ESR of 63 mm/hr, and CRP of 92 mg/L; blood cultures are pending. X-rays are negative for fracture, dislocation, or foreign body but show a moderate knee joint effusion.
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What is the presumed diagnosis?▶
What tests are required to confirm the diagnosis?Vignette 39: Answer
The presumed diagnosis in this case is septic arthritis. The clues in the vignette are the acute presentation, history of non–insulin-dependent diabetes mellitus, fever, examination findings, and elevated WBC (includ- ing elevated neutrophil differential), and inflammatory markers. These key findings should shout infection to you, until proven otherwise. The main distractor here is her relatively recent history of trauma (twisting her knee). The goal is to piece the data together and presume that this patient has acute septic arthritis.
Acute septic arthritis is a surgical emergency most often caused by bacterial seeding of a joint, although viruses and fungi can alternatively be involved. The most common affected joints include the knee, hip, elbow, ankle, and sternoclavicular joint. Septic arthritis can occur in 1 of 3 ways: direct inoculation from trauma or surgery, contiguous spread from adjacent osteomyelitis, or in the setting of bacteremia (more common in immunocompromised patients and hospitalized individuals). Recently, several cases of septic arthritis following corticosteroid injection, as well as hyaluronic acid injection, have been reported. Bacterial seeding of a joint causes irreversible articular cartilage damage, often within 8 hours of inoculation, due to release of proteolytic enzymes from host PMNs and from the bacteria itself, as well as from a concomitant direct pressure effect.135,136
The most common pathogen responsible for causing septic arthritis is S aureus, which accounts for more than 50% of all reported cases. Other common pathogens include Staphylococcus epidermis and Neisseria gon-
orrhea (most common in otherwise healthy sexually active young patients, can be migratory). Less common,
but still important, pathogens to be aware of include Salmonella (often seen in patients with sickle cell disease),
Pseudomonas aeruginosa (often seen in patients with a history of IV drug use), Pasteurella multocida (encoun-
tered after an animal bite), and Eikenella corrodens (encountered after a human bite). Streptococcus species can also be seen, but they are more common in children than in adults (see www.wheelessonline.com/ortho/ bacterial_menu for complete details).
Septic arthritis will typically present as an acute, monoarticular inflammation, most commonly affecting the knee joint. The differential diagnosis in these patients includes crystalline arthropathy (gout, pseudog- out), inflammatory arthropathy (rheumatoid, psoriatic, reactive), and, less likely, tumor, occult fracture, and flare of OA. Patients with acute septic arthritis will have the acute onset of a warm, edematous, erythema- tous, and painful joint, often with inability to bear weight. Pain is typically diffusely throughout the joint (as opposed to localized to a single location); pain with passive ROM is a classic physical examination finding. X-rays are typically unremarkable but will often show the joint effusion.
Serum laboratory findings are extremely useful in the workup of patients with presumed septic arthritis. Patients will most often have an elevated WBC with a left shift, as well as elevated ESR and CRP (cutoff values depend on the laboratory because various ranges of normal exist). An elevated WBC count with a left shift is indicative of infection, but the absence of an abnormal WBC does not rule out infection. Inflammatory markers, including ESR and CRP, can be elevated in the settings of infection and inflammation. The ESR will remain elevated for up to 3 to 4 weeks, while the CRP typically normalizes within 1 week of appropri- ate treatment; however, the values can vary depending on the success of the treatment regimen and other concomitant conditions.
Aspiration of joint fluid remains the gold standard for the diagnosis of acute septic arthritis.137,138 Every
attempt should be made to aspirate the joint in an area away from any erythema or open wounds so as to avoid contaminating the joint. Joint fluid should be analyzed for cell count with differential, Gram stain, aerobic culture, anaerobic culture, and crystal analysis. If there is a clinical concern for atypical pathogens, such as Propionibacterium acnes (shoulder) or Mycobacterium tuberculosis, fluid will need to be analyzed for longer periods of time (ie, 21 days for P acnes). Aspirations consistent with septic arthritis will often show a WBC greater than 50,000 cells/uL, but lower values are possible. The Gram stain is only able to identify the organism in approximately one-third of cases and is not a cost-effective test.
Acute septic arthritis is a surgical emergency because cartilage destruction can occur within 8 hours of bacterial inoculation. After obtaining samples for cultures from the joint aspiration, broad-spectrum IV antibiotics should be initiated. Antibiotics should provide coverage for gram-positive and -negative organ- isms until antibiotic choices can be narrowed based on culture results. Regimens in young, healthy adults should include coverage for S aureus and N gonorrhea, while coverage in immunocompromised patients should include P aeruginosa. Surgical intervention with open or arthroscopic debridement and irrigation should be performed as soon as possible, with a low threshold to return to the operating room for repeat
procedures pending the clinical course.139 When arthroscopic debridement is unsuccessful, open synovec-
tomy and debridement are recommended. Patients too sick to go to the operating room may be treated with serial aspirations, although it is a less ideal form of management.
Why Might This Be Tested? Acute septic arthritis is a common diagnosis that can cause significant joint
morbidity if not quickly identified and treated. There is some clinical overlap between infectious and inflam- matory arthropathies, so understanding the key components of patient presentation, examination findings, laboratory findings, and aspirate findings is critical.
Confirm the diagnosis of septic arthritis with an aspiration (WBC > 50,000 and elevated PMNs), look for underlying risk factors, and then treat the condition (operative debride- ment and irrigation with IV antibiotics).