Septic Arthritis
Also known as infectious arthritis, pyogenic septic arthritis is a medical emergency. It arises when bacteria invade a joint and cause the synovial lining to become inflamed. If the organisms enter the joint cavity, effusion and pyogenesis follow, with eventual destruction of bone and cartilage.
The disorder usually affects a single joint. It most often develops in a large joint but can strike any joint, including the spine and small peripheral joints. Migratory polyarthritis sometimes precedes localization of joint inflammation.
Septic arthritis can lead to ankylosis and even fatal septicemia. However, prompt antibiotic therapy and aspiration or drainage of the joint cure most patients.
Causes
In most cases of septic arthritis, bacteria spread from a primary site of infection, usually in adjacent bone or soft tissue, through the bloodstream to the joint. Common infecting organisms include:
four strains of gram-positive cocci—Staphylococcus aureus, Streptococcus pyogenes, Streptococcus pneumoniae, and Streptococcus viridans
two strains of gram-negative cocci—Neisseria gonorrhoeae and Haemophilus influenzae
various gram-negative bacilli, including Escherichia coli, Salmonella, and Pseudomonas.
Rarely, fungi or mycobacteria cause the infection. Anaerobic organisms such as gram-positive cocci may infect adults and children older than age 2.
Pediatric pointerH. influenzae most often infects children younger than age 2.
Various factors can predispose a person to septic arthritis. Any concurrent bacterial infection (of the genitourinary or upper respiratory tract, for example) or serious chronic illness (such as cancer, renal failure, rheumatoid arthritis, septic lupus erythematosus, diabetes, or cirrhosis) heightens susceptibility. Consequently, alcoholics and elderly people run an increased risk of septic arthritis.
Susceptibility also increases among patients who have immune system depression or a history of immunosuppressant therapy. Abuse of I.V. drugs can also lead to septic arthritis. Other predisposing factors include recent articular trauma, joint surgery, intra-articular injections, and local joint abnormalities.
Complications
Septic arthritis may cause infection of the bone (osteomyelitis) or other adjacent structures and a loss of joint cartilage that leads to joint destruction.
Assessment
The patient may have a history of a known infection outside the involved joint, an immunosuppressive condition, or I.V. drug abuse. He may complain of an abrupt onset of intense pain in the affected joint. He may also have fever and chills if he has a systemic infection. These findings can help differentiate septic arthritis from other types. (See Other types of arthritis: Characteristics and treatments).
Inspection may show that the patient prefers to keep the affected joint flexed. This position eases pain by minimizing intra-articular pressure. You may observe redness and edema over the affected joint and severely reduced range of motion (ROM), active and passive.
On palpation, you’ll usually note warmth and extreme tenderness over the involved joint.
Diagnostic tests
Arthrocentesis allows the collection of a synovial fluid specimen.
Synovial fluid analysis shows gross pus or watery, cloudy fluid of decreased viscosity, typically with 50,000/μl or more white blood cells (WBCs) containing primarily neutrophils. It may also show a lower glucose level than a simultaneous 6-hour postprandial blood glucose level.
Gram stain or fluid culture—or a biopsy of the synovial membrane—confirms the diagnosis and identifies the causative organism.
Blood cultures may be positive and confirm the diagnosis even when the synovial culture is negative.
