HIP INFECTION
(PEDIATRIC SEPTIC ARTHRITIS)
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What is a Hip Infection (Pediatric Septic Arthritis)?
Septic arthritis is a bacterial infection within a joint, and it most commonly occurs in a child's hip (about 40% of all cases), although it can occur in any joint. Over 50% of cases occur in children under 2 years old.
An infection in the joint is a serious concern because an infection will trigger a war between the bacteria and our own immune cells which causes significant inflammation within the joint. The release of inflammatory proteins like matrix metalloproteinases has the potential to damage the cartilage (think of this as collateral damage in a war)
Unfortunately, our body is not good a regrowing cartilage so anything that gets injured will not grown back normally. Arthritis is caused by the loss of cartilage within a joint, so kids can potentially develop an arthritic hip (like a 70 year old lady) if the infection is not treated.
The cause of an infected joint can be from the spread of bacteria within the blood stream after an infection somewhere else (like a bad lung infection, aka pneumonia), or it can be caused by a cut or splinter that directly enters the joint (less common), or it can be caused by an infection within the bone (called osteomyelitis) that spreads directly into the joint (this can occurs in most joints, except the wrist and knee because the joint capsule covers the metaphysis, which is the site of osteomyelitis in kids).
Staph and strep are the most common bacteria to cause septic arthritis, although gonorrhea (a sexually transmitted disease) can also be the culprit in sexually active adolescents.
Diagnosing an Infected Hip (Pediatric Septic Arthritis):
When a doctor sees a child limping from a painful hip, they think of many things that could cause these symptoms. An inflammatory reaction (like transient synovitis or juvenile rheumatoid arthritis), a broken bone, septic arthritis, Perthes disease can all cause hip pain in children. The most common imitator of septic arthritis is transient synovitis (which is inflammation of the hip after a child has a viral upper respiratory infection).
An infection in the hip is critical to diagnose quickly, but it can often be challenging. Doctors look at blood tests and x-rays to begin their diagnosis of this condition (and to rule out other things). The Kocher Criteria has been established as a way to identify true hip infections. It looks at 4 variables to give the probability of infection: 1) blood tests that shows an elevated white blood cell count (indicating the body is mounting an immune response to something); 2) another blood test that shows an elevated inflammatory marker (ESR >40); 3) a fever over 101.3; and 4) a hip too painful to bear any weight. If only one of these factors is positive then the risk of septic arthritis is <3%, if two are positive, then the risk goes up to 40%, three are positive the risk reaches 90% and if all four are positive the risk is almost 99%. Now this is not a perfect system, doctors can't just check off boxes alone to make a decision about treatment, but it certainly helps. Based on the criteria, if a doctor is concerned for septic arthritis, further tests are ordered, like an ultrasound of the hip to look for increased fluid (an infection will cause the hip to be inflamed and swollen), or an MRI (to look for an infection in the bone surrounding the joint, aka osteomyelitis).
With all of this information, a doctor can make a fairly accurate and rapid diagnosis.
Treating an Infected Hip (Pediatric Septic Arthritis):
Injury to the cartilage due to an infection within a joint begins around 8 hours, so its important for a doctor to act quickly.
If doctors diagnose the hip as infected (or have a high suspicion for infection) the hip needs to be "washed out" (cleaned) with sterile fluid in the operating room (its a small surgical procedure). Cultures of the joint fluid would be taken at this time to identify the bacteria causing the infection, so that antibiotics can be tailored to attack this bug. The child should then be started on IV antibiotics, usually clindamycin (which is great at fighting the common types of bacteria), or a cephalosporin type antibiotic. IV antibiotics (meaning medicine goes directly into the blood stream, instead of being delivered as a pill) are given during the hospital stay, and the infection is monitored closely for 48 hours (on average, but sometimes longer).
Repeat blood tests (like the white blood cell count and inflammatory markers) are ordered to determine if the infection is improving with treatment. Occasionally the infection is severe and the child will need a second or third trip to the operating room to re-wash the joint to remove any last bacteria.
Once its believed the infection is being treated successfully, the child can go home on oral antibiotics (a pill or syrup) for a total of 2-3 weeks (which is shorter than a true bone infection, which requires 6 weeks of IV antibiotics).
What is the long term outcome?
The patients do great when the infection is identified quickly and treated expediently. Delayed diagnosis (typically greater than 4 days) can lead to worse outcomes, with damage to the cartilage, or the growth plate, or the bone itself. Other predictors of poor outcome include associated bone infections (osteomyelitis), and joint infections in kids under 6 months old.
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References
1) Papathanasiou I et al. The catabolic role of toll-like receptor 2 (TLR-2) mediated by the NF-KB pathway in septic arthritis. J Ortho Res 2011. 29: 247-51. full article. cause of cartilage damage.
2) Sucato DJ, Schwend RM, Gillespie R. Septic Arthritis of the Hip in Children. JAAOS 1997; 5: 249-260. full article.
3) Curtiss PH et al. Destruction of articular cartilage in septic arthritis: II. In vivo studies. JBJS 1965; 47: 1595-1604. full article. cause of cartilage damage.
4) Caird MS et al. Factors distinguishing septic arthritis from transient synovitis of the hip in children. JBJS 2006; 88: 1251-7. full article. diagnosis factors.
5) Kocher MS et al. Differentiating between septic arthritis and transient synovitis of the hip in children: an evidence-based clinical prediction algorithm. JBJS 1999; 81: 1662-70. full article. diagnosis factors.