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Pattern of Joint Involvement
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In addition to categorizing the type of pain a child is experiencing,
it is critical to determine how many joints are affected. The potential causes
of a monoarticular process differ significantly from those of polyarticular
conditions, so careful examination of all joints is mandatory even
when the complaint involves only a single location. It is also important
to determine the type of onset (sudden or gradual), duration of
symptoms, and any associated systemic features such as fever or
rash. Distinct differential diagnoses must be considered for monoarticular
and polyarticular processes, as well as for joint complaints associated
with fever or other extra-articular signs and symptoms (see Table 199-3).
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The potential etiologies of a monoarticular process may be narrowed
down by consideration of the nature of onset and duration of symptoms.
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When a monoarticular process involving pain and swelling of a
single joint starts acutely, bacterial infections must be excluded. Unlike
most types of inflammatory arthritis, where delaying the diagnosis
by several days is unlikely to have long-term implications, treatment
of septic arthritis must not be postponed. In fact, a history of
fever associated with a single red, swollen, painful, or hot joint
necessitates arthrocentesis for cell count and culture. Conversely,
in an afebrile child, traumatic injury is a more likely explanation
for acute joint symptoms. It is helpful if there has been clearly
documented antecedent trauma, but this may be difficult to elicit in
young children who are unable to verbalize specifics of the history.
In patients with an underlying bleeding disorder such as hemophilia,
routine daily activities may cause hemarthrosis.
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Once these possibilities are excluded, postinfectious or “reactive
arthritis” must be considered in the case of an acutely
symptomatic joint. This process may involve one or many joints, but it
characteristically causes less inflammation than an acute infection.
Thus, postinfectious arthritis does not usually cause erythema overlying
the joint, and although it may be uncomfortable, excruciating pain
is uncommon. Reactive arthritis generally responds well to nonsteroidal
anti-inflammatory agents, and is typically transient. Lyme disease may
also be difficult to distinguish clinically from septic arthritis,
though generally it causes more indolent symptoms.9
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Diagnostic considerations of an isolated, chronically swollen
joint differ from those related to an acute arthritis. Bacterial
infections are far less likely, whereas lower grade infections,
especially Lyme disease in endemic areas, must be excluded. Chronic monoarthritis
also may be caused by Mycobacterium tuberculosis,
particularly in immunocompromised children. Also within this category
are chronic forms of synovitis, especially pauciarticular juvenile
arthritis and psoriatic arthritis. Rarer inflammatory arthropathies, such
as arthritis due to sarcoidosis, may also cause monoarthritis. Primary
tumors of the cartilage and synovium, although extremely rare, are
also most likely to present as discomfort in a single joint. The
most common of these, pigmented villonodular synovitis (PVNS), typically
causes a chronically painful and swollen knee. A nontraumatic arthrocentesis
that yields bloody fluid is suggestive of an articular tumor.
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When several joints are involved, rheumatologic conditions rise
to the top of the differential diagnosis. Most common among these
is polyarticular juvenile arthritis, although other autoimmune diseases
such as systemic lupus erythematosus and vasculitis typically involve
multiple joints as well. Infections are instead progressively less
likely as more joints are involved, with the exceptions of gonococcal arthritis
in sexually active or abused children, and salmonella arthritis
in immunocompromised patients. Arthritis associated with systemic
conditions, such as inflammatory bowel disease or cystic fibrosis,
must also be considered. Usually, extra-articular involvement (such as
a new murmur in rheumatic fever or hives in serum sickness) offers
a clue to these conditions. The pattern of joint involvement may
also be suggestive: rheumatic fever, vasculitis, and serum sickness
characteristically cause a migratory polyarthritis, whereas most
other conditions cause additive or fixed involvement of multiple
joints. In general, children with polyarthritis are most likely
to benefit from consultation with a pediatric rheumatologist.