The differential diagnosis for rash with joint pain in children is broad and includes both infectious and noninfectious causes (Table). A detailed history with pertinent positives and negatives and a thorough exam is helpful in making the diagnosis. Some of the common differentials are discussed below.
SYSTEMIC LUPUS ARTHRITIS
Systemic lupus arthritis (SLE)1 is a multisystem autoimmune condition caused by inflammation of the blood vessels and connective tissue. Because this condition can involve multiple systems of the body and occurs in episodic flares, the periodic constellation of symptoms can make it hard to diagnose.
Generalized symptoms such as fever, weight loss, lymphadenopathy, and hepatosplenomegaly along with the classic malar rash and nonerosive symmetric arthritis should raise the suspicion for SLE. Laboratory workup is usually positive for cytopenias, transaminitis, and elevated inflammatory markers. A positive ANA titer is a very sensitive marker but not specific, and in the event that it is positive, follow-up with specific anti–double-stranded DNA (anti-ds-DNA) antibody and anti-smith (Sm) antibody should be done to differentiate SLE from other connective tissue and vascular disorders.
The patient in this case was well appearing without systemic symptoms and had both a normal blood count and a negative ANA, which made SLE less likely.
With Lyme disease,2 the pathognomic of the early localized stage (most common presentation within the first 1 to 4 weeks after a tick bite) is the erythema migrans (EM) rash, which appears as a “target-like lesion” or “bull’s-eye appearance” and can be found on the abdomen, axilla, inguinal, or popliteal areas. This is associated with systemic signs such as fever, arthralgia, and headaches.
Laboratory studies might show leukopenia or leukocytosis, elevated inflammatory markers such erythrocyte sedimentation rate (ESR), and liver function abnormalities. Early disseminated disease secondary to hematogenous spread of the bacteria presents as multiple EM, neurologic involvement including facial nerve palsy, or carditis. Lyme arthritis is the main symptom of late disseminated disease, presenting months to years after the tick bite. Lyme disease can be monoarticular or oligoarticular with small effusions and absence of fever.
No exposure to the tick and absence of the characteristic rash and laboratory abnormalities made this diagnosis less likely in this patient.
Acute rheumatic fever (ARF)3 is one of the causes of primary acquired heart diseases due to an inflammatory reaction after a streptococcal infection. It is a clinical syndrome that includes criteria as outlined in the Jones criteria.3 Laboratory evidence of a preceding group A streptococcal infection is mandatory for the diagnosis. The rash associated with ARF is erythema marginatum, which is a macular blanching rash characterized by central clearing and found mostly on the trunk and proximal extremities. Migratory polyarthritis and carditis are the most common presenting symptoms.
The rash in this patient did not match the description of the classic rash seen with ARF and her rapid strep test was negative. Since the suspicion for ARF was low based on clinical findings, a throat culture was not ordered.
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