Infection Flashcards
What is septic arthritis? What are its causes?
Septic arthritis is a bacterial (and less commonly fungal or mycobacterial) infection of a joint. It can be caused by hematogenous spread (most common), direct inoculation (ie: trauma, during surgery) or contiguous spread from infected periarticular tissue.
The most common site is the knee, but the wrist and hip, among others, can be affected.
Soft tissue infections of the tendons and discs are considered separately. Also, the condition in pediatrics has a different pattern of presentation (eg the hip is often the site of infection in infants) and predicted course.
Most cases of acute septic arthritis are caused by staphylococcus. Certain sites (foot puncture wounds = Pseudomonas) and certain patients (those with sickle cell disease = salmonella) are susceptible to other characteristic organisms.
Infections can be iatrogenic (as suggested below by a glove-less injector, who probably did not wash his hands either)
Septic arthritis may not cause systemic effects. The patient will have pain especially on motion of the joint
Note that the hip has so little room for joint fluid build up, it may be very painful even without motion.
The diagnosis is made by aspirating the fluid and sending it to the lab for cell count and culture / gram stain.
Elevated risk for septic arthritis is seen in patients with joint implants, known infection elsewhere, a history of drug abuse, immune suppression and chronic diseases (such as diabetes or rheumatoid arthritis.
It is possible to get an autoimmune arthritis after a systemic infection; but in that case the joint space itself is sterile. This condition is termed “reactive arthritis” and is grouped under the rubric of rheumatological, not infectious, diseases.
How is septic arthritis diagnosed definitively? What are the temporal limitations regarding our ability to diagnose definitively? How do we get around that?
Septic arthritis is definitively diagnosed by culturing bacteria from synovial fluid. Yet diagnosis by this method is delayed due to the amount of time it takes to grow out the bacterial culture.
Waiting is bad as the damage begins once the white cells hit the joint.
While waiting for culture results, we can use a clinical decision rule to make a good guess if infection is present: COUNT THE WHITE CELLS AND ASSUME THAT A HIGH COUNT MEANS INFECTION.
(A positive gram stain may help, but it is specific, not sensitive; and even seeing “bugs” does not tell you which bug is present)
How is septic arthritis treated? Why must septic arthritis be treated expeditiously?
Septic arthritis is treated removal of the joint fluid and IV antibiotics.
The key consideration is that the body’s own response to the infection may inflict horrible damage on the sensitive articular cartilage. Joint fluid with high white counts is toxic! So even if we don’t think the patient is at risk for getting systemically sick from a joint infection, we must “wash it out” to save the joint itself. Whether you believe this should be done by serial needle aspirations or by surgery says more about your guild membership (surgeon vs non-surgeon) than about your scientific acumen.
Both gout and septic arthritis can cause acute pain and swelling of a joint. How can they be distinguished?
Symptoms of both gout and septic arthritis include redness and swelling present in just one joint and pain that is worse with movement.
The key distinction is the results of a culture of the joint fluid obtained by aspiration: a positive culture defines an infection.
One could consider the history (patients with known gout are of course more likely to have gout as the cause, as compared to patients without that history; patients with immune suppression are of course at higher risk for infection).
One could also measure the concentration of white blood cells in the aspirate: the number of cells is higher in cases of infection. The problem is that there is no cut off that perfectly segregates the two.
One can also examine the aspirate under the microscope: crystals suggest gout, and bacteria indicate infection. Yet crystals can be seen in cases of infection as well (where both conditions are present simultaneously).
Because culture results are not known immediately, all of the information listed above should be considered, especially when urgent treatment is needed.
What is osteomyelitis? What are its causes? Although bone is part of the vascular system (and therefore fractures can cause hemorrhage and metastatic cells can lodge in the skeleton) why might IV antibiotics have trouble reaching areas of infected bone?
Osteomyeltis is an infection of bone. Osteomyelitis can be caused by hematogenous spread, contiguous spread from adjacent soft tissues or joints, or direct inoculation (ie: during trauma or surgery)
In addition to bacteria, the focus of osteomyelitis contains dead bone, local inflammation and granulation tissue.
Microbiology most often reveals Staph aureus infection, although Staph epidermis and gram negative rods are implicated in a minority of cases.
Bone is normally resistant to infection. In order to overcome the normal immune system, bacteria must be present in large number (high inoculum), be able to adhere to the bone surface or bone ischemia must be present to inhibit to a local immune effort. Bone trauma, especially open fracture (thus exposing bone to environmental pathogens), establishes these conditions necessary for chronic infection. The site is most frequently the tibia, as this is the most common site of open fractures. (Non-union of the fractured bone, possibly associated with recurrent infection, is a major complication.)
Typical symptoms include local bone pain, erythema, swelling as well as systemic symptoms such as fever. A classic syndrome of cyclic pain correlating with the progression of bone necrosis has been described, although it is not a sensitive finding.
On exam, signs of local infection may be seen, possibly including small recurrent sinus tracts draining to the skin (actually communicating with underlying infected bone).
Abscesses form within the necrotic bone, expanding contiguously into bone and soft tissue, Sinus tracts may develop, which can lead to the skin and be seen as superficial orifices on exam.
Bone grows over the infection, producing a “sequestrum”— the island of bone shown below. The infected bone effectively gets walled off from the circulation.
The sequestering of the sequestrum is a positive adaptation in evolution: it keeps that bad segregated from the good. Evolution did not anticipate that we would have medicine to deliver, and today this wall-it-off strategy is a problem. Antibiotics can’t reach their target. As such, surgical debridement is often needed.
Poor nutrition and excess alcohol intake are thought to predispose at-risk patients to developing post-traumatic osteomyelitis. Additionally, patients with diabetes may develop vascular insufficiency, which would also put them at higher risk for developing osteomyelitis in the right setting.
Imaging: Plain films may show characteristic changes after 10-14 days (at which point necrotic changes, including the involucrum may be evident); three phase bone scan is high sensitivity (>90%) but low specificity ( Some form of rigid fixation had been shown in animals to improve bone union following debridement, and there is evidence supporting both internal and external methods of fixation. At the time of debridement, antibiotic-containing beads may be implanted in this space for up to 30 days (and subsequently removed).
Systemic antibiotics are used as adjuncts to surgery to help prevent infection in the healing of the operative wound and to prevent spread. The antibiotic used is based on culture results.
