A 7-year-old boy presented to the emergency department with fever for six days. He had been observed at another hospital for two days for his persistent high fever and joint pain. He was discharged with instructions to continue Tylenol and ibuprofen. No other records from the visit were available. He presented to our facility with persistent fever, increased left hip pain for three days, and left knee pain for one day. He had refused to walk on his right leg since the day of discharge from the other hospital. The patient had no upper respiratory infection or urinary symptoms.
The boy had a temperature of 39°C. He was tachycardic to 145 bpm, and had a blood pressure of 100/64 mm Hg. He was tachypneic but not in any respiratory distress. His exam was notable for exquisite pain with internal and external rotation of the right hip. He had no overlying skin erythema or lesions. The left hip was normal. The patient reported pain in the right knee with movement, but had full range of motion and no warmth over the joint, erythema, effusion, or lesion near the knee. Bedside ultrasound was done on the right hip. (Image 1.)
What is the differential diagnosis for this patient?
Find the diagnosis and case discussion on next page.
Diagnosis: Septic Arthritis
Septic arthritis in children is relatively uncommon and occurs in four to 10 per 100,000 children. (Infect Dis Clin North Am 2015;29:557; http://bit.ly/2L8wFE4.) Peak incidence is around age 4, according to some studies, but it is found in all ages, including neonates. It is most commonly found in the hip and knee joints, followed by the ankle. (Infect Dis Clin North Am 2015;29:557; http://bit.ly/2L8wFE4.) It can be found in other joints but with significantly lower frequency. Many studies describe it under the umbrella of acute osteoarticular infections, which include acute osteomyelitis. The metaphasis is located within the joint capsule in the hip and the knee, and acute osteomyelitis is found in conjunction with septic joints in many cases. It is often difficult to say which process led to the other.
The classic presentation of septic arthritis is a child with acute joint pain, high fever, and refusal to bear weight on the affected extremity. Patients with these classic symptoms should be considered to have septic arthritis until proven otherwise. Younger children, children earlier in their disease course, or children with an atypical site of infection might present with fever, irritability, and more nonspecific symptoms, reinforcing that septic arthritis is on the differential for any child with fever of unclear etiology. A previous visit to a physician is a component of a clinical prediction algorithm developed to identify septic arthritis. (Pediatr Emerg Care 2015;31:54; http://bit.ly/2L2G286.)
The mechanism for septic arthritis in otherwise healthy children is unclear. Children who are immunosuppressed, not vaccinated, or have had penetrating injuries are at increased risk of infection. Some authors postulated that the mechanism is seeding of the joint from a prior infection. One study suggested that a history of trauma to the joint was a risk factor. (Infect Dis Clin North Am 2015;29:557; http://bit.ly/2L8wFE4.) This is a challenging correlation because falls and trauma are so common in this patient population.
Transient or toxic synovitis is much more common than septic arthritis, and is usually associated with low fevers (<38.5°C) and recent viral infection. Other considerations include Legg-Calvé-Perthes disease, slipped capital femoral epiphysis, tumor, and inflammatory arthropathy. (Aust Fam Physician 2015;44:213; http://bit.ly/2L7LFSq.) Other infectious etiologies are osteomyelitis and abscess (especially psoas in the setting of hip pain). Because transient synovitis is a self-limited and benign process that does not require invasive procedures, distinguishing it from septic arthritis is critical in the workup of pediatric patients with acute joint pain and fever.
Patients will most frequently be maintaining the affected joint in a position of comfort that relieves as much tension on the joint capsule as possible. This is frequently slight flexion, abduction, or external rotation for the hip. The knee is usually held in slight flexion. Passive range of motion elicits significant pain, and the child will frequently resist you getting near the affected part. Presence of effusion, erythema, or warmth of the joint is frequently found, but can often be difficult to appreciate at sites like the hip. Ultrasound has been used to look for joint effusion. This is not diagnostic of septic arthritis, but the presence of an effusion can be another piece of information that drives decision-making. A fluid collection greater than 5 mm between the femoral head and the joint capsule in a child is the diagnostic criteria for joint effusion. (Pediatr Emerg Care 2015;31:54; http://bit.ly/2L2G286.)
Pediatric patients are challenging because they frequently require sedation for MRI, and subjecting them to bedside joint aspirations poses its own set of risks. No clinical prediction tool has been well-validated, but the one developed by Kocher, et al., suggests that a history of fever, non-weight-bearing, an ESR of >40, and a white blood cell count of >12,000 is strongly predictive of septic arthritis. (J Bone Joint Surg Am 2004;86-A:1629; http://bit.ly/2L4cpDw.) They found that having all of these features was 99 percent predictive of septic arthritis.
Another study found the combination of a recent health care visit, fever >38.5°C, and WBC >12,000 to be 71 percent predictive. (J Bone Joint Surg Am 2004;86-A:956; http://bit.ly/2L3GWRK.) Another prediction tool used CRP and found a number >2 mg/dL strongly associated with septic arthritis. (J Bone Joint Surg Am 2006;88:1251; http://bit.ly/2L5Dw0N.) All of these tools have significant limitations, and should not be used in isolation, but they suggest that a combination of ESR, CRP, and white blood cell count can help raise suspicion for septic arthritis. (Infect Dis Clin North Am 2015;29:557; http://bit.ly/2L8wFE4.) Blood cultures should also be collected.
Imaging should first include x-rays of the affected part. Normal plain films will rule out some of the previously mentioned disease processes, and could prevent the need for further invasive workup. MRI is the mainstay of diagnostic imaging to detect septic arthritis because of its ability to provide details of the bone and surrounding soft tissue. Some institutions sedate the patient for the MRI, and take him directly to the operating room for washout and management if it is concerning. Other institutions perform joint aspiration as the diagnostic modality of choice. The location of the infection, age of the child, and available resources at the facility all affect this diagnostic tree. Early consultation with an orthopedic surgeon is important.
Our patient had an ESR of 80. His CRP was 58 mg/L, and his white blood cell count was 8.4. He was sedated for his MRI, and based on the results (Image 2), was taken to the OR for drainage and irrigation. Immediately post-op he was started on vancomycin, and when his cultures were positive for MSSA, he was narrowed to cefazolin. At discharge, he was transitioned to oral cefalexin. He improved clinically in subsequent days, and was discharged with a walker and follow-up physical therapy.
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