1. Septic arthritis in the immunocompetent and immunocompromised host

Septic arthritis is an orthopaedic emergency.  Timely diagnosis is important to prevent disabling joint damage. Clinical presentation may be subtle in immunocompromised patients, and an infectious aetiology for arthritis may be overlooked in patients known with a rheumatologic condition.

Arthrocentesis should be performed when there is any suspicion of septic arthritis and usually yields a purulent aspirate with predominant neutrophilia. Visualizing organisms on gram stain or culture confirms septic arthritis.

What are the risk factors for septic arthritis?

Septic arthritis most frequently arises from haematogenous seeding of microorganisms to joints, thus many of the risk factors for septic arthritis are those which increase the risk of bacteraemia. These include older age, diabetes mellitus, chronic organ failures, immunosuppression and intravenous drug use. However, preexisting joint disease remains the most significant predisposing factor for septic arthritis. Abnormal joints are more likely to become a septic focus following bacteraemia and patients with joint disease are more likely to undergo joint surgery and intra-articular injection of corticosteroids, which may also increase risk for septic arthritis.

Septic arthritis infrequently results from penetrating injury or contiguous spread from overlying skin or soft tissue infection. A fifth of patients with septic arthritis will have no predisposing clinical conditions.

What are the likely pathogens?

Historically, Neisseria gonorrhoeae was the most common cause of septic arthritis, but in the post-antibiotic era this has been superseded by Staphylococcus aureus. S aureus is a common cause of bacteraemia and the organism possesses multiple virulence factors which facilitate entry into joint tissues and bone. Methicillin resistant S aureus is becoming increasingly common. β-haemolytic streptococci are the second most commonly isolated organisms, predominantly groups A, C, F and G.

Gram negative bacilli are important in the elderly and in immunocompromised hosts. Escherichia coli is commonly found but Listeria, Klebsiella, Salmonella, Enterobacter, and Proteus species are also important. Pseudomonas aeruginosa is also significant in patients with a history of intravenous drug use.

In the immunocompromised host, Staphylococcal and Streptococcal infections remain the most important causes overall, but gram negatives and unusual or opportunistic organisms will be more common than in immunocompetent hosts.

What is the management of septic arthritis?

Management of septic arthritis consists of both joint drainage and antimicrobial therapy.

Drainage is important for source control and to limit damage to the joint and can be achieved by needle aspiration or by arthroscopy or open drainage in theatre. Surgical drainage is preferred when repeated joint aspiration is not feasible or fails to control the infection.

Empiric antimicrobial therapy should be initiated without delay. A suggested empiric regimen for community acquired septic arthritis in an immunocompetent host:

Cloxacillin 2g IVI 6 hourly

OR

Ceftriaxone 1g IVI daily 

A suggested empiric regimen in the immunocompromised or hospitalized patient (when MRSA or GNB may be suspected):

Cefepime 2g IVI 8 hourly

AND

Vancomycin 1g IVI 12 hourly 

Antimicrobial therapy must be de-escalated as soon as an organism is identified and directed therapy should be continued parenterally for a minimum of 2 weeks, followed by oral therapy for an additional 1 to 2 weeks. Hard to treat pathogens, persistent immunosuppression or concomitant osteomyelitis or endocarditis may necessitate a longer duration of therapy.