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New Treatments for Septic Arthritis

Septic arthritis. You get it the same way you get osteomyelitis, in childhood. Hematogenous, delivery of direct implantation, or you get spread into a joint from a contiguous focus of joint infection. Typical presenting feature is fever, limp, refusal to move.

Causes; what's in the differential diagnosis besides bacterial infection in the joint space? Well there are a number of causes of viral arthritis, including varicella, adenovirus, hepatitis, rubella, parvovirus, Ross River fever, O'nyong-nyong fever, and a few others. The table goes on and on in Dr. Cherry's book. It's interesting to realize that during epidemics of Hepatitis B that arthralgia and arthritis are well described, and varicella has been found in joint fluid.

Mycobacterial arthritis, TB can certainly get into joint space, fungal arthritis - cocci is a good example - cellulitis overlying a joint could certainly be confused for arthritis as well, and reactive arthritis, the so-called toxic synovitis, is probably the one that we stub our toe.

Most of these patients are afebrile and in general you don't see a sed rate higher than 30. So you can usually differentiate toxic synovitis from septic arthritis by doing plain films, a CBC and a sed rate. Those are the usual elements of the standard evaluation in this setting. Joint hemorrhage in those with hemophilia or post trauma, even in a child who doesn't have hemophilia or other voiding diathesis, serum sickness, HSP, a number of metabolic diseases.

Organisms; most of these are gram positive. Both staph aureus, group A Streptococci as well as Pneumococci. Overall that comes up in younger children to be about 90% to 95% of disease. H. flu was fairly common before we succeeded in largely eliminating it as a cause. In newborns and adolescents Gonococci need to be considered, and a classic finding in adolescence.

So how do we evaluate these? Radiographs primarily to rule out other causes of pain on movement. Bone scan if needed. If the bone scan is done to try and differentiate with bone disease, osteomyelitis, you will typically see increased uptake in the second phase in the 30 minutes after uptake. But then that increased uptake with disappear and the bone will look normal on a late phase follow-up done at 2-3 hours post injection. Blood culture is really not very sensitive in this setting either. Typically only a third to half the cases, probably reflecting the fact that the bacteremia was transient and was cleared.

Surgical drainage is essential for the hip and the shoulder, and increased pressure on the joint will compromise the circulation with the possibility of aseptic necrosis developing. The treatment is typically focused on gram positive disease unless there are other specific considerations; because you are looking at an immunocompromised child, or there is reason to suspect an unusual organism.

With septic arthritis and osteomyelitis there are three different ways that they tend to emerge. Hematogenous dissemination of organisms; the child gets bacteremic, the organisms float around in circulation and lodge in a bone, or lodge in a joint space. Direct inoculation; child kneels on a needle or in the case of bone infection, gets nailed by stepping on a nail. Spread from a contiguous focus; you have an overlying focus of infection. Why is osteomyelitis so common in children? Well, Hovoback in the 1890's examined the circulation of bone in children and found that, in rabbits - and this was subsequently shown to be true in children as well - that in the metaphyses you have these areas where the capillaries terminate in this large plexus ... well, back up a little. Nutrient artery supplying the circulation to the metaphysis.

Here you have the growth plate and here you have the epiphysis. The pathogenesis of osteomyelitis as we know it, is that you have trauma in this area which leads to a collection of blood and transient bacteremia allows organisms to set up shop in this area of vascular stasis. In other words, it is somewhat of an opportunistic infection. We have increased likelihood of trauma to this metaphysis in the rapidly growing bone, and with transient bacteremia an poor host defenses, characteristic of early childhood, you are more likely to have an infection establish as a local focus of cellulitis which then takes off. So the presentation will be somewhat dependent on how old the child is. If you are an infant you will have this cellulitis of the metaphysis rupture out through the periosteum and spill into the soft tissues above, so that a child will typically present with a great big fat leg or arm or an area that looks like a large area of cellulitis. In the older child, typically preschool-age child, periosteum is tighter, more tightly applied, infection stays a little better contained.

Differential diagnosis; fracture, septic arthritis from rheumatic fever, sepsis causing long bone pain has been well described, overlying cellulitis, osteogenic sarcomas including Ewing's sarcoma, leukemia, thrombophlebitis described by Dr. Cherry and a couple of other authors, and bone infarction seen not only in sickle cell disease but also in Gaucher disease. In immunologically normal children the most common pathogens are now gram positive, staph aureus, group A Streptococci. H. flu disease has largely disappeared from the osteomyelitis scene. What seems to be replacing it in recent reports is Kingella.

How do you diagnose osteomyelitis? Well, it can be diagnosed as simply as good common sense and thinking about the disease, a point radiograph and a big needle. The last case of osteomyelitis that I've seen, in fact, was a child that I think I saw with Jim Corb. The kid presented with pain and erythema over his distal fibula. The orthopedist walked into the room, looked at the films and said, "You know, fever for one day, erythema, doesn't want to move his leg, this looks like osteomyelitis." Boom, up to the operating room, little incision one centimeter long, aspiration, gets pus, the child is back in the room and next day he's playing video games and goes home in a week. Osteomyelitis can be done that simply when you have obvious manifestations. The plain film is obtained really to make sure that the ... the plain films should be normal in most cases at the time of presentation and you are trying to rule out fracture and other explanations for a local bone pain.

Now this is a plain radiograph a week to two weeks into illness. I have another slide of this child but I thought why am I going to show a plain radiograph that's normal? Because initially the most that you expect to see is some soft tissue swelling. One to two weeks later you start to see periosteal elevation, which you see very nicely up here, on the shaft of the proximal femur of this two year old who is not febrile, but was limping. Because he had no fever and this soft tissue swelling seen on a plain film, this child also got a bone scan. And the bone scan from the same kid shows this increased area of uptake.

Typically that's all you need to establish a diagnosis. MRI however is now being used increasingly and an adjunct and hopefully to get away from the use of bone scans. But bone scans are far from out the door. MRI is useful because it will show you the placement of the fat in marrow with the water of the inflammatory fluid. The edema that you see in marrow replacing the fat content. CT has been used a lot but it really is useful.

Now that same case where I also showed you a plain film with periosteal elevation, a bone scan that was abnormal, that child also got an MRI that looks like this. Showing over here on the left side an area of high signal reflecting increased water content in the metaphysis of the femur.

Osteomyelitis in non-neonates, because that's where you are typically going to see this disease. Most cases result from hematogenous spread, as I've said, and it's usually gram positives. So starting with a gram positive agent is appropriate. Oxacillin and cefuroxime were typically used. A lot of folks still like to use cefuroxime because we used it a lot in the H. flu days and cefuroxime is not a red-hot antibiotic for meningitis but it's a good antibiotic for bone and joint infections.