A 4-year-old male with painful lesion on left knee

Issue: July 2011

A 4-year-old male presented to the clinic with the chief complaint of a painful lesion on his left knee. The onset was 3 days earlier, beginning as an erythematous pustule that grew in size, accompanied by subjective fever at home the day before admission. There was no history of injury or unusual activity. His past medical history was positive for mild asthma, but otherwise, it was that of a previously healthy child with no other history of significant medical or surgical problems. His immunizations were up-to-date for his age and his family history was unremarkable, with no history of any sick contacts, including skin sores. There’s no animal or insect exposure and no recent travel.

James H. Brien

Examination revealed a healthy-appearing 4-year-old male who would not bear weight on his left leg, with fever of 102·F and a normal exam except for his left knee, which was a bit swollen and painful on range of motion testing and palpation. There is also a large blister-like lesion over the anterior aspect of the knee in the area of the tibial tuberosity (Figures 1-4).

Figure 1: The only positive findings were the lesion on his lower lip and chin and a hemangioma on the lower part of the chin, well away from the area of erythema.

Figure 1: There is a large blister-like lesion over the anterior aspect of the knee in the area of the tibial tuberosity (Figures 1-4).

Figure 2

Figure 3

Figure 4

A lateral radiograph of the left knee is shown in Figure 5. After admission and on the advice of the orthopedic consultant, an MRI of the left knee was obtained (Figures 6-8). Lab tests included an erythrocyte sedimentation rate (ESR) of 21 mm/hour, a C-reactive protein (CRP) level of 18 mg/L and a CBC revealing a WBC count of 10,200 cells/mcL. Because of his fever and refusal to bear weight, the orthopedic surgeon performed an arthrocentesis, revealing normal synovial fluid. The lesion was then aspirated of some yellow cloudy fluid. The GRAM stain of the “blister” fluid showed GRAM-positive cocci, but the synovial fluid was negative. Cultures of both are pending.

Figure 5: A lateral radiograph of the left knee is shown.

Figure 6: After admission and on the advice of the orthopedic consultant, an MRI of the left knee was obtained (Figures 6-8).

Figure 7

Figure 8

What’s Your Diagnosis?

Superficial staph abscess
Prepatellar bursitis
Erysipelas
Osteomyelitis of the tibia

This child turned out to have a superficial abscess with surrounding cellulitis that grew methicillin-sensitive Staphylococcus aureus (MSSA). It was unclear how this happened or why it looked so unusual, but there may have been some minor, unwitnessed injury that created a port of entry for the organism. Staph abscesses that occur as a part of a cellulitis usually don’t have such a sharp line of demarcation, but rather an abscess pocket within the infected soft tissue with more surrounding soft tissue swelling. The MRI showed normal bones and joint, only the mild soft tissue swelling and the fluid-filled lesion. This also helped rule out osteomyelitis, which almost always reveals the site of infection, as shown in another patient in Figure 9 (T1 MRI image) with osteomyelitis of the distal femur. Also, soft tissue manifestations of an underlying bone infection would more likely be diffuse erythema with painful swelling, as shown in Figure 10, a patient with osteomyelitis of the distal tibia and septic arthritis of the ankle.

Figure 9: The MRI showed normal bones and joint, only the mild soft tissue swelling and the fluid-filled lesion.

Figure 10: Soft tissue manifestations of an underlying bone infection would more likely be diffuse erythema with painful swelling.

Prepatellar bursitis occurs right over the top of the patella, with a fairly characteristic pattern of localized swelling (Figure 11), without knee joint involvement. It is usually associated with some penetrating injury, but again, it may go unnoticed. Treatment is drainage and antimicrobials, usually directed against S. aureus, but should be based on culture results.

Figure 11: Prepatellar bursitis occurs with a fairly characteristic pattern of localized swelling.

Figure 12: Swelling with a raised edge and a sharp line of demarcation.

Lastly, erysipelas is a superficial cellulitis, involving the lymphatic vessels in the superficial layers of the skin, and usually due to group A strep. It will have a pattern of erythema and swelling that has a raised edge and a sharp line of demarcation; occasionally with superficial blisters (Figure 12). Treatment is with just about any penicillin or cephalosporin. In newborns, erysipelas may be secondary to group B strep and should be treated aggressively with ampicillin and gentamicin until sensitivities to penicillin are available.

James H. Brien, DO, is Vice Chair for Education at The Children’s Hospital at Scott and White and is the Associate Professor of Pediatrics at Texas A&M University, College of Medicine, Temple, Texas. email: jhbrien@aol.com. Disclosure: Dr. Brien reports no relevant financial disclosures.

Follow the PediatricSuperSite.com on Twitter.