Differentials
Septic arthritis
SIGNS / SYMPTOMS
Pain is more severe, and the child often cannot walk. Patient may appear systemically ill. Any hip motion is painful, even through a small arc of motion or micromotion.[15]
Destruction of the articular cartilage begins quickly and is secondary to proteolytic enzymes and impairment of intra-capsular vascular supply.
A delay in treatment can have devastating consequences, with near-total destruction of the hip.[7][8]
INVESTIGATIONS
Four independent clinical predictors have been identified to differentiate between septic arthritis and transient synovitis: fever >38.5°C (>101.3°F); non-weight-bearing; erythrocyte sedimentation rate >40 mm/hour; and serum WBC count >12,000 cells/mm³. The predicted probability of septic arthritis was summarised as <0.2% for zero predictors, 3.0% for 1 predictor, 40% for 2 predictors, 93.1% for 3 predictors, and 99.6% for 4 predictors.[9]
When this paradigm was applied prospectively by either the same institution or others, the predicted probability did decrease, but its use as a clinical indicator is still extremely helpful.[10][11]
If joint aspiration is performed, white cell count >50,000 cells/mm³ would suggest that septic arthritis.
Lyme arthritis
SIGNS / SYMPTOMS
A late manifestation of Lyme disease, it usually manifests as monoarticular or oligoarticular arthritis, commonly involving large joints. Large knee effusions are common, usually resolving in a few weeks to a few months if untreated.
INVESTIGATIONS
Serology can confirm previous history of Lyme disease.
Osteomyelitis
SIGNS / SYMPTOMS
Osteomyelitis of the hip presents with hip pain. The child may appear ill, have a fever, and have pain with ambulation. Joint motion will be less restricted.
Osteomyelitis of the pelvis would present with hip or pelvic pain. Acute pain with hip motion should be less significant.
INVESTIGATIONS
X-ray may show bony changes if late in the disease process: for example, periosteal elevation and cortical or medullary lucencies. MRI will demonstrate increased signal intensity on the T2 images in the femoral head and metaphysis early in the disease process, and has been shown to be a reliable method to assess the periarticular bone for osteomyelitis.
Legg-Calvé-Perthes' disorder
SIGNS / SYMPTOMS
Children, more commonly boys, will present typically with a limp that occurs over weeks to months.
INVESTIGATIONS
Early x-rays may show only subtle signs of joint space widening; more obvious signs, such as femoral head sclerosis or collapse, will be evident weeks to months later. Labs will be normal.
Pyogenic sacroiliitis
SIGNS / SYMPTOMS
This would typically present with pain and/or tenderness over the sacroiliac joints.
INVESTIGATIONS
An MRI of the pelvis would show changes in the sacroiliac joint, such as increase in T2 signal intensity.
Juvenile idiopathic arthritis
SIGNS / SYMPTOMS
For a diagnosis to be made the child must have had objective arthritis in the joint for at least 6 weeks. There may be stiffness on waking or after periods of inactivity.
In certain sub-types the child may have a spiking fever, a salmon-coloured rash over the trunk and proximal extremities, uveitis, and rheumatoid nodules on extensor surfaces of tendons.
INVESTIGATIONS
Diagnosis is predominantly based on clinical signs and symptoms. Laboratory tests such as erythrocyte sedimentation rate, CRP, antinuclear antibody, and rheumatoid factor are mainly used to determine the sub-type. Imaging of the joints involved may aid diagnosis but is not specific. X-rays are usually normal in early disease but may be used to monitor disease activity.
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