Septic arthritis

by Nathan Wei, MD, FACP, FACR

Nathan Wei is a nationally known board-certified rheumatologist and author of the Second Opinion Arthritis Treatment Kit. It's available exclusively at this website... not available in stores.

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From the American College of Rheumatology

Septic arthritis is an inflammation of a joint caused by bacterial invasion. Septic arthritis can occur as a result of infection with many different types of bacteria.

Bacterial infections are the most serious infections affecting the joints. Normal joints, diseased joints, and prosthetic joints are all vulnerable to bacterial infection. Mortality rates among adults range from 10% to greater than 50%. Full recovery is possible, but poor outcome is common among those with preexisting arthritis.

The most common way septic arthritis occurs is through bacteremic seeding (infection transmitted by the blood) of the affected joint from an extraarticular (outside of the joint) site of infection such as pneumonia, pyelonephritis, or skin infection.

Less commonly, direct inoculation of the bug into a joint can occur. Examples include Pasteurella multocida infection of a finger joint as the result of a cat bite and Pseudomonas aeruginosa infection of the foot as the result of a puncture wound caused by a nail piercing through the sole of a sneaker.

The rare instance of an infection resulting from the introduction of bacteria into the joint during arthrocentesis or joint injection occurs at a rate of 0.0002%. In other words, it is very rare.

The onset of the symptoms is usually rapid with joint swelling, intense joint pain, and low-grade fever.

A normal joint is very resistant to infection compared to a diseased joint or a prosthetic joint.

An important predisposition to septic arthritis is an impaired immune system. Rheumatoid arthritis (RA), liver cirrhosis, chronic renal failure, and malignancies are often present among patients with septic arthritis. Hemodialysis patients and intravenous drug abusers are predisposed to bacterial joint infection at sites such as the sternoclavicular joint and the sacroiliac joint. Other susceptible hosts are patients with acquired immunodeficiency syndrome, hemophilia, organ transplantation, or hypogammaglobulinemia.

Risk factors include a simultaneous bacterial infection, chronic illness, diseases or medications that suppress the immune system, rheumatoid arthritis, intravenous drug abuse, sickle cell disease, artificial joint implants, recent joint trauma, age greater than 80, or recent joint arthroscopy or other surgery.

Septic arthritis may be seen at any age. In children, it occurs most often in those less than 3 years old. The hip is a frequent site of infection in infants.

Septic arthritis is uncommon from age 3 to adolescence, at which time the incidence increases again (appearing as gonococcal arthritis in females with cervical gonorrhea). Children with septic arthritis are more likely than adults to be infected with group B streptococcus and Haemophilus influenza.

Acute septic arthritis tends to be caused by organisms such as staphylococcus, streptococcus (pneumoniae) and group B streptococcus while chronic septic arthritis (which occurs less frequently) is caused by organisms such as Mycobacterium tuberculosis and Candida albicans.

Septic arthritis affects a single joint (monoarticular) 80%-90% of the time. The predilection is for a single large joint, typically the knee. Thus, in the evaluation of a patient with an acute monoarthritis, septic arthritis is always a consideration, especially if the patient is febrile, appears toxic, or has an extraarticular (non-joint) site of bacterial infection.

In the patient with an underlying inflammatory arthritis such as RA, an acute flare of joint inflammation - whether monoarticular or polyarticular - must raise the suspicion of superimposed infection complicating rheumatoid disease.

Arthrocentesis (withdrawing fluid from the joint) and synovial fluid analysis are the cornerstones for the diagnosis of septic arthritis. If the synovial fluid cell count is extremely high (eg, 100,000 white blood cells [WBC] per cubic mm or greater), treatment for presumed septic arthritis should be initiated pending culture result of the fluid.

Polyarticular infection occurs more commonly in patients with preexisting arthritis and suggests a less favorable outcome. S. aureus is the major pathogen. Rheumatoid arthritis patients with polyarticular septic arthritis had a mortality rate of greater than 50%. More than one joint should be aspirated when infection in multiple joints is suspected.

Prompt treatment will hasten recovery and cure the infection with less morbidity. Once septic arthritis is suspected and the proper samples for microbiologic studies are collected, antibiotic treatment should begin immediately.

The choice of which antibiotic agent(s) to use depends on what the Gram-stained smear of the synovial fluid shows and what the most likely causative microorganism may be based on the entire clinical picture. Narrow antibiotic coverage is provided if one suspects staphylococci and Gram-positive cocci are found on the synovial fluid smear.

On the other hand, if the Gram-stained smear is non-revealing and no clue is found after searching for an extraarticular source of infection in an elderly debilitated patient, then broad antibiotic coverage against Gram-positive cocci and Gram-negative bacilli should be given initially.

In an otherwise healthy young sexually active person who presents with tenosynovitis and swelling of a wrist following two days of migratory joint pain, and the Gram-stained smear reveals no visible bacteria, initiating monotherapy against gonococcal infection is appropriate after culturing all areas of possible infection.

Once the identity and the sensitivities of the microorganism are known, therapy should continue with the most efficacious agent that has the best safety profile and the lowest cost.

Drainage of the infected joint space must be adequate in order to relieve pain, eradicate the infection and hasten recovery of lost function. During the initial few days, immobilization of the affected joint and effective analgesic medication will ensure patient comfort. Physical therapy should be instituted as soon as the patient can tolerate mobilization of the inflamed joint.

Repeated needle aspirations may prove adequate in some patients if sterilization of the joint space can be achieved rapidly. However, if needle aspiration is technically difficult or does not provide thorough drainage of the joint, if sterilization of the joint fluid is delayed, if the infected joint is already damaged by preexisting arthritis, or if infected synovial tissue or bone needs debridement, surgical drainage may be necessary.

Tidal lavage to wash out the joint and arthroscopic procedures are intermediate steps that may benefit some patients and avoid the morbidity of an open knee drainage procedure. Involving the orthopaedic surgeon and the physical therapist early on in the course of treatment will facilitate the best choice of drainage procedure and result in the best functional outcome.

The duration of antibiotic treatment has not been well prospectively studied. For native joint infections, antibiotic treatment can be as brief as two weeks for uncomplicated infection by susceptible microorganisms. The treatment duration is typically more prolonged, between 4 to 6 weeks, for more serious infection in a compromised host.

For prosthetic joint infections, the antibiotic course is usually quite protracted. For most cases of infected joint replacement, the prosthesis is removed and antibiotic treatment is continued until the site is sterile before reimplantation is considered. Antibiotic impregnated cement or beads are sometimes employed in the reimplantation, either during multi-staged procedures or during an exchange arthroplasty.

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