December 2000
A 10-year-old boy was admitted to the hospital for evaluation and treatment of a painful, red and swollen left knee. It began the day before while riding his bicycle. Because it was worse the next day, he was brought to the clinic for evaluation. There was no history of him complaining of an injury, but it did appear while playing outside on his bike. He had neither fever nor other complaints. His past medical history and family history were unremarkable. His immunizations were up to date. He has no allergies and was taking no medications. The only animal exposure is to a family cat and dog and various farm animals, such as chickens, cows and rabbits (typical country boy). There's no history of bites or scratches, which is probably not reliable in a 10-year-old boy.
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Examination reveals an obese male (140 lbs) with normal vital signs. The only significant finding is the left knee, showing swelling over the anterior aspect of the joint. There also appears to be a shallow puncture wound over the same area, in spite of his history of no injury (figures 1-3).
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There is no significant pain on range of motion testing or weight bearing, but he was noted to have a slight limp when he walked. There was some pain noted on palpation over the area of swelling, which also reveals some fluctuance without induration.
A complete blood count revealed a white blood cell count of 13,500 with 79% granulocytes. Figures 4 and 5 are plain radiographs of the left knee.
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The answer is #3, prepatellar bursitis, which is a fairly uncommon condition in children, but which is typical in the presentation here. Patients with this condition will usually have full range of motion of the joint with little to no pain except to palpation over the infected area. There may or may not be fever, and the white blood cell (WBC) count may be normal.
However, when erythrocyte sedimentation rates (ESR) are measured, they are typically elevated. It was not done in this patient because it would not have contributed to the establishment of the diagnosis or management. The patient was seen by orthopedics and the bursa was aspirated, revealing turbid, blood-tinged fluid with rare gram-positive cocci. This fluid grew Staphylococcus aureus that was penicillin- resistant but sensitive to methicillin.
The patient was treated with IV nafcillin initially then switched to oral cephalexin suspension (because he could not swallow capsules) just prior to discharge about 48 hours later. He was treated for a total of about two weeks with complete resolution. It should be noted that his knee was essentially back to normal within one week. While some experts recommend long courses of therapy, others (see below) have reported good results with very short courses of antibiotics. Complications are rare, especially with adequate drainage.
One may rarely see Streptococcus pyogenes (group A strep) cause this alone or with staph, but staph by far is the most common cause. Non-septic bursitis may also occur, mostly in adults or athletes who put various stresses on the joint. One can usually distinguish between septic and non-septic bursitis on clinical grounds alone. Septic bursitis is almost always associated with some injury, even if it went unnoticed, as in this case. Lastly, radiographs are typically normal or show only the soft tissue swelling.
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There should be little confusion between septic bursitis and septic arthritis. As shown in the patient presented above and in another patient (figures 6-8), the swelling of septic bursitis is on top of the knee and not in the joint. The swelling of septic arthritis would be diffuse as shown in figures 9 through 11. Also, range of motion and weight bearing would be very painful with septic arthritis. The radiographs may also be abnormal with arthritis.
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Figures 12 and 13 show a patient with septic arthritis of the left knee, revealing the distended joint capsule. In these same figures one can also see the abnormal proximal tibia revealing the results of coexisting osteomyelitis as well. While septic arthritis may not be this obvious in all cases, it is usually easy to distinguish from bursitis by examination, and is also more likely to have associated fever and abnormal lab tests. When in doubt, get an orthopedic consult and/or aspirate the joint (figure 14).
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Imaging techniques such as MRI
scanning may be helpful in questionable cases. A bone scan may also be useful,
especially if there is a question of an adjacent osteomyelitis. There should
always be a sense of urgency when evaluating and treating possible septic
arthritis as significant damage can occur to the joint if drainage is delayed,
even with adequate antibiotic therapy. This is especially true of infections of
the hip as shown in figures 15 and 16. These radiographs were taken three weeks
apart, and one can easily see the damage to the joint. The patient had
initially been diagnosed with toxic synovitis. In terms of frequency, the knee
leads the list of septic joints, followed by the hip.
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We have seen several children with septic ankles in the last couple of years (figures 17-19). One patient presented with a septic ankle (figure 20) who was bacteremic with community-acquired methicillin-resistant Staphylococcus aureus (MRSA). The port of entry appeared to be an injury to his knee (figure 21).
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One may also see a similar picture occur with a puncture wound to the sole of the foot. If this is the case, remember to consider Pseudomonas aeruginosa in the differential of causes, especially if the child was wearing sneakers (figure 22).
Again, never believe an active young boy when he says there has been no injury. When I was about 7 years old, I was playing Superman and jumped off the cab of one of my father's trucks (a drilling rig). I quickly realized that you really do need to be from Krypton in order to fly. When I hit the ground, I came down on a nail that went through my shoe and well into my foot. A couple of days later, when I could not get my shoe on to go to school because of the swelling, my mother took me to the doctor. Of course I played dumb because I was not supposed to be playing around my dad's equipment anyway. I finally admitted to my mother what had happened when I was about 45 years old and she was about 72. I felt like it was safe enough to come clean after all those years, but no, she spanked me anyway. There appears to be no statute of limitations with my mother.
Reactive
arthritis is an immunologic disorder causing one or more joints to become
mildly inflamed and sore. It is usually associated with other inflammatory
conditions such as inflammatory bowel disease, genitourinary or
gastrointestinal infections, or drug reactions (serum sickness reactions) as
shown in figure 23. If fluid is aspirated from the joint, it is typically clear
or serosanguinous and sterile.
Therapy usually involves treating the underlying cause if possible and nonsteroidal anti-inflammatory agents with lots of reassurance. The ESR or C-reactive protein may be elevated as well as the peripheral WBC count with this condition, but usually not as high as one would expect with septic arthritis. Again, when in doubt, aspirate and culture the fluid, and consider an MRI of the joint and/or a bone scan.
Cellulitis in this area should cause more diffuse swelling with painful erythema. Also, unless it is complicated by an underlying abscess, there should be no fluctuance, only induration. Cellulitis and septic bursitis or septic arthritis may occur together. So when you see cellulitis over the joint, be suspicious of an underlying arthritis or bursitis.
To read more about septic bursitis in children, I recommend a report of 10 cases out of Denver General Hospital published by John W. Paisley in the Journal of Pediatric Orthopedics (1982; 2[1]:57-61).
Acknowledgment: I would like to thank Michele M. Lopez-Glynn, MD, a first-year pediatric resident here at Scott & White for her help with this case.
My family and I wish you all Happy Holidays and a safe and prosperous New Year.
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