Differentials
Septic arthritis
SIGNS / SYMPTOMS
Usually presents with single joint involvement, high fevers, severe pain, and/or erythematous joints.
INVESTIGATIONS
Synovial fluid analysis demonstrates the presence of bacteria, elevated white blood cells, and positive cultures.
Radionucleotide bone scintigraphy shows increased uptake in the affected joint.
MRI shows synovial enhancement and effusion and marrow edema of the adjacent bone in cases of accompanying osteomyelitis. Significant synovial thickening is more likely to be associated with inflammatory arthritis.
Synovial fluid may be sterile in cases where reactive effusions are present secondary to juxta-articular osteomyelitis.
Osteomyelitis
SIGNS / SYMPTOMS
Usually presents with high fevers, severe pain, and/or focal tenderness.
INVESTIGATIONS
Synovial fluid analysis may demonstrate the presence of bacteria, elevated white blood cells, and positive cultures when there is involvement of the joint. Synovial fluid may be sterile in cases where reactive effusions are present secondary to juxta-articular osteomyelitis.
Radionucleotide bone scintigraphy shows increased uptake in the bone.
MRI shows abnormal diffuse marrow signal, which is best seen on T1-weighted MRI images. However, it should be noted that false-negative MRI results may be observed early in the course of infection.
Malignancy
SIGNS / SYMPTOMS
Signs and symptoms consistent with bone tumors, leukemias, or neuroblastoma.
INVESTIGATIONS
CBC and film, urine catecholamines, and imaging consistent with malignancy.
Reactive arthritis
SIGNS / SYMPTOMS
Asymmetrical oligoarticular arthritis, beginning 1 to 4 weeks after genitourinary or gastrointestinal infection. May be associated enthesitis, dactylitis, conjunctivitis, iritis, and rash.
INVESTIGATIONS
Stool or genitourinary cultures may detect causative organism, although cultures are often negative by the time arthritis symptoms develop.
Acute rheumatic fever
SIGNS / SYMPTOMS
Usually presents as an acute, migratory arthritis that responds well to nonsteroidal anti-inflammatory drugs (NSAIDs). Features include continuous fever, cardiac involvement, and/or erythema marginatum rashes.
INVESTIGATIONS
Throat cultures positive for group A streptococcus; positive rapid streptococcal antigen test and/or elevated or rising streptococcal antibody titers.
Systemic lupus erythematosus (SLE)
SIGNS / SYMPTOMS
Usually presents with nonerosive polyarthritis, malar rashes, renal involvement, and/or photosensitivity. Serositis and central nervous system involvement are also suggestive of SLE.
INVESTIGATIONS
High-titer antinuclear antibody and other autoantibodies (such as anti ds-DNA, SS-A, SS-B, Smith, ribonucleoprotein) may be present.
Urinalysis may be abnormal, showing blood and/or protein.
Low levels of complements C3 and/or C4.
CBC may show leukopenia, thrombocytopenia, and (autoimmune hemolytic) anemia.
Elevated erythrocyte sedimentation rate in the absence of elevated C-reactive protein.
Juvenile dermatomyositis
SIGNS / SYMPTOMS
Usually presents with muscle weakness, muscle pain, and/or characteristic rashes such as Gottron papules, linear extensor erythema, or heliotrope rash.
Polyarticular nonerosive arthritis is usually present, especially in early stages of disease. The polyarthritis usually responds to treatment of the underlying myositis.
INVESTIGATIONS
Abnormal muscle-derived enzymes such as lactose dehydrogenase, aspartate transaminase, alanine transaminase, creatinine kinase, and aldolase may be present.
Kawasaki disease
SIGNS / SYMPTOMS
Usually presents with high persistent fevers that do not normalize for several days, polymorphous rash, involvement of lips and conjunctiva, edema of extremities, and/or desquamation.
INVESTIGATIONS
Demonstration of coronary artery enlargement or aneurysms is suggestive of Kawasaki disease.
Pigmented villonodular synovitis
SIGNS / SYMPTOMS
Usually presents with recurrent, painless swelling in one knee, ankle, or tendon sheath. Usually associated with slow, progressive destruction of cartilage with bone erosion.
INVESTIGATIONS
Synovial fluid analysis shows blood-stained, dark brown fluid.
MRI shows low signal density on T1- and T2-weighted studies.
Synovial hemangiomas
SIGNS / SYMPTOMS
There is usually no morning stiffness and treatment with nonsteroidal anti-inflammatory drugs (NSAIDs) does not result in improvement.
INVESTIGATIONS
MRI may show absence of synovial fluid and vascular elements and enhancement.
Osteochondritis dissecans
SIGNS / SYMPTOMS
Usually presents with activity-related pain, occasional recurrent bland effusions, and/or localized tenderness on examination.
INVESTIGATIONS
X-rays may show subchondral fractures.
MRI may demonstrate cartilaginous separation and can also be used in lesion staging.
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