25 Feb - Fever and Rash

     Fever and rash in an otherwise healthy male patient with pneumonia.  When working up fever and rash, "Its probably a virus" will be correct much of the time, but there are some never miss life threatening diagnoses to remember.  Unfortunately, the differential diagnosis of fever and rash is only slightly less broad than that of fever alone.  Of all the rashes, petechial / purpuric rash with fever is the most ominous.  Think meningococcemia, RMSF, endocarditis, and fulminant gram negative sepsis.  Ask about the spleen!  I think I will make this article from ID Clinics of North America (Vol 10:1, 1996) required reading for the ID rotation.  It gives a few ways to lump the diagnoses by appearance and determine the treatable, untreatable, and non-infectious possibilities.
     In this patient, the prominent maculopapular rash sparing the palms and soles in conjunction with pneumonia suggest mycoplasma as a diagnosis.  Though generally benign, mycoplasma infections can be severe and even fatal, especially if encephalitis, erythema multiforme, aplastic anemia, or DIC occurs.  This article by Schalock in International Journal of Dermatology 2009,48, 673–681 is a great review of mycoplasma infections covering all the systemic manifestations and focusing on the dermatologic findings.  Up to 1/3 of mycoplasma cases are complicated by an exanthematous eruption according to their review. 
     Finally, when working up a patient with suspected acute retroviral syndrome (fever, rash, sore throat, adenopathy), the plasma viral load is the test of choice.  p24 antigen testing used to be the correct answer, but viral load testing is now preferred.  Low titer viral loads (1000 or less) probably represent false positive tests in this setting.  You can expect a Step 3 / ABIM question on this topic.