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septic arthritis with 100,000 to 300,000 white blood cells (WBCs) per cubic
millimeter (see Chapter 101 Rheumatologic Emergencies ).
In the absence of a clear history of a tick bite, arthralgia secondary to early,
localized Lyme disease may be a challenging diagnosis to establish because only
about 40% to 70% of children have the characteristic erythema migrans rash,
constitutional symptoms may be mild, and serologic tests will be normal in the
early stages of disease. However, serologic testing will be confirmatory in
patients with Lyme arthritis secondary to disseminated disease. Among those with
monoarticular arthritis, it may be difficult to distinguish septic arthritis from
arthritis associated with Lyme disease on clinical grounds alone, with or without
analysis of synovial fluid. There are clinical features that favor the diagnosis of
Lyme disease such as knee involvement, absence of recent fever, and lower
inflammatory markers, but these characteristics may be shared with a subset of
patients who have septic arthritis. Complicating this further is that synovial fluid
analysis may not be definitive. There is a wide range of synovial WBC counts in
children with Lyme disease and although median values are lower than those
typically seen in patients with septic arthritis, there is considerable overlap. Given
the difficulties of distinguishing Lyme disease from septic arthritis among
children presenting with monoarticular arthritis, management of cases where
suspicion for septic joint is significant may require an approach that includes
analyzing synovial fluid (including Lyme PCR) and performing Lyme serology
testing, especially if one practices in a region where Lyme disease is endemic.
Transient (also called toxic) synovitis is a poorly understood inflammation of
large joints, afflicting children 3 to 6 years of age. The diagnosis is typically
made on clinical grounds, and this self-limited disease does not result in joint
destruction. When the hip is involved, the challenge for clinicians is to distinguish
transient synovitis from Lyme (in endemic areas) or septic arthritis.
Reactive, or postinfectious, arthritis is probably more common than septic
arthritis. Arthritis following various enteric infections is not rare in children, and
joint complaints after parvovirus B19 infection are seen among adolescents.
Chlamydia trachomatis infection of the genitourinary tract should be considered

in any sexually active adolescent with new-onset arthritis. With postinfectious
arthritis, antimicrobial treatment does not modify the disease course.
Traumatic injuries to a joint may cause periarticular swelling or an effusion
indicative of a hemarthrosis. In addition, ligamentous or tendon injuries will
result in joint pain and impaired range of motion. Serum sickness and Henoch–
Schönlein purpura (HSP) are marked by characteristic rashes.

EVALUATION AND DECISION