Volume 4, Case 17
Myron H. Rosen, MD
Children's Medical Center of Dallas
University of Texas Southwestern School of Medicine
This is a previously healthy 4-year old female who is brought to the ED by her mother because of fever and right leg pain since the previous night. The patient stated that she had been swimming the previous day, and had slipped in the pool, twisting her right leg. Immediately after that, however, she was able to walk around easily without pain. Later that night she developed a fever (temperature not measured), and her right leg began to hurt. She was taken to an ED because she had difficulty walking. She was diagnosed with a hip sprain and was sent home on ibuprofen. Her fever and leg pain worsened to the point that she was no longer able to ambulate, prompting a return to the ED.
Exam VS: T40.7, P164, R40, BP 139/84. She is anxious and in obvious discomfort, although she appears alert and non-toxic. She prefers to keep her right hip abducted and externally rotated, with the right knee flexed. She refuses to bear any weight on the right leg. Range of motion of the right hip is severely limited, especially internal rotation, adduction, and extension. The overlying skin is warm but not erythematous. Examination of the remainder of the right lower extremity, the left lower extremity, and spine are within normal limits.
View the patient's general appearance.
Laboratory Results: CBC WBC 31,700 with 4% bands, 85% segs, 6% monos, and 5% lymphs. Hgb 12.4, Hct 35.4. Platelet count 394,000. CRP 8.2. ESR 39. Hip radiographs were obtained.
View hip radiographs.
Think about the most likely diagnosis at this time.
The photo of her general appearance shows her preferred position of comfort with her hip in external rotation. What radiographic abnormalities would you expect to see?
Her hip radiographs show a widened joint space in her right hip. Compare the joint space to the left hip and it is rather obvious here. [Use the "tear drop" distance method as described in Case 15 of Volume 4 (Osteoid Osteoma), to measure the joint space.] This finding is seen in the minority of patients with septic arthritis; thus, the absence of joint space widening cannot be relied upon to rule out septic arthritis.
An orthopedic consultation is obtained. Under fluoroscopic guidance, the right hip is needle-aspirated, revealing grossly purulent fluid. Laboratory analysis: 80,000 WBC's with 88% segs, 1% monos, and 11% lymphs. 16,000 RBC's. Gram stain shows many gram positive cocci in chains.
What organisms are the most common causes of septic arthritis ? Given the laboratory results, what organism is likely in our patient ? What antibiotic regimen would be appropriate for patients with septic arthritis ?
The patient is placed on IV cefuroxime and hospitalized for operative drainage of the right hip. Cultures of the aspirate grow out group A beta-hemolytic strep.
Teaching Points and Discussion:
Diagnosis
1. Early diagnosis and treatment of septic arthritis of the hip cannot be overemphasized. Fibrous deposition and lysosomal enzymes from polymorphonuclear leukocytes in synovial fluid can rapidly damage cartilage. Permanent damage will occur in only a few days if the diagnosis and treatment are delayed.
View later changes.
This radiograph of a different patient at follow-up shows the result of a poorly treated septic hip. The patient's right hip shows degenerative changes. There is flattening of the femoral epiphysis with an irregular articular surface. Early effective treatment can minimize such subsequent complications.
2. The diagnosis is particularly difficult to make in infants, who cannot voice their complaints, and especially in neonates who may not have the typical findings of fever, chills and leukocytosis. Don't be fooled by a history of trauma. Parents of toddlers will almost always recall a recent "injury" prior to the onset of symptoms. Such a history is not sufficient to rule out septic arthritis.
3. The affected extremity will typically be held in a position of slight flexion, abduction and external rotation. This maximizes the intraarticular space, thus decreasing the tension of the joint exudate. Refer to the photo of our patient in this position of comfort. Click on [Patient]
4. Laboratory findings include an elevated erythrocyte sedimentation rate and/or C-reactive protein, as well as a leukocytosis, often with a "left shift". The left shift (a high percentage of immature granulocytes) is not reliable and is noticeably absent in our patient. Blood cultures may be positive in up to 50% of cases. The most important procedure in making the diagnosis, however, is hip aspiration (arthrocentesis). If the aspirated synovial fluid contains more than 20,000 white blood cells and has greater than 75% polymorphonuclear leukocytes, infection should be strongly suspected. Synovial glucose content less than 1/2 of serum and/or protein greater than 1/3 of serum are suggestive of septic arthritis. Positive results of direct smear and cultures of synovial fluid are diagnostic. Joint fluid cultures are positive in about 60-80% of cases.
5. Don't expect plain radiographs to establish the early diagnosis of septic arthritis. Plain films may demonstrate a widened hip space, but the absence of this does not rule out septic arthritis. Lateral displacement of the proximal femur due to synovial exudate and inflammation can also be seen in the early stages. Late findings include periostitis, periarticular osteoporosis, and dislocation of the femoral head. Ultrasound is the most effective study in demonstrating the presence of a joint effusion acutely. Radionuclide bone scanning can be helpful in the early stages, but the definitive diagnostic tool remains isolation and identification of the organism by hip aspiration.
Pathogenesis
1. The most common etiologic organism is Staphylococcus aureus. Other organisms include Haemophilus influenzae type B (more common in younger patients), Streptococcus pyogenes (group A beta-hemolytic strep), Neisseria gonorrhea (more common in young adults), pneumococcus, and salmonella.
2. The infection may reach the hip joint by either hematogenous spread, or from a focus of osteomyelitis in the femoral neck or (less commonly) the ilium. Hematogenous spread is more common in children than in adults.
Treatment
1. The treatment for septic arthritis of the hip includes intravenous antibiotics and immediate surgical incision and drainage of the hip, followed by a period of immobilization. Antibiotics should be continued for at least 4-6 weeks, depending on the sensitivities of the organism and the patient's clinical status.
2. The initial choice of antibiotics should take into account the most common etiologies of septic arthritis. In the case presented above, cefuroxime was used to cover streptococci and staph aureus. Cefuroxime (similarly cefotaxime and ceftriaxone) would also cover H. influenzae type B. An anti-staphyloccocal penicillin (nafcillin, oxacillin) could also have been selected to cover streptococci and staph aureus. However, given the growing frequency of methicillin resistant staph aureus, empirically starting vancomycin pending culture results and antibiotic sensitivities may be justified as well.
Differential Diagnosis
1. Transient ("toxic") synovitis can present with intense pain, refusal to walk, and limited and/or painful range of motion. A bone scan may help to rule out a septic hip, but if septic arthritis is suspected, the hip should be tapped and the patient should be given IV antibiotics. A brief period of observation can also be helpful. If there is dramatic improvement in 24 hours, the diagnosis is most likely to be transient synovitis.
2. Osteomyelitis can sometimes cause a sympathetic effusion of the joint, without an actual infection of the joint itself.
3. Acute rheumatic fever can also present with acute arthritis of the hip. However, there is usually a migrating pattern to the arthritis. Other findings consistent with rheumatic fever may be present, such as a heart murmur suggestive of acute carditis, subcutaneous nodules, erythema marginatum, or chorea. The Jones' criteria may be helpful in making this diagnosis.
4. Hemophilia with bleeding into the joint space may be difficult to distinguish from septic arthritis. However, the patient is usually a known hemophiliac. This condition requires urgent decompression of the joint space to prevent permanent damage to the femoral head.
5. Other causes of refusal to walk associated with fever include diskitis, new-onset leukemia, peri-rectal abscess, and osteomyelitis.
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