Viral Exanthems

Classification Framework

The numbered "diseases of childhood" classification is historical. Modern understanding maps exanthems by virus type and morphology.

1. Measles (Rubeola) — "First Disease"

2. Rubella (German Measles) — "Third Disease"

Congenital Rubella Syndrome (CRS)

• Classic triad: Cataracts, congenital heart disease (PDA, pulmonary artery stenosis, VSD), sensorineural deafness
• Microcephaly, retinopathy, glaucoma
• Neonatal: blueberry muffin lesions (dermal erythropoiesis), hepatosplenomegaly, thrombocytopenia — Fitzpatrick's Dermatology; Jawetz Medical Microbiology

3. Erythema Infectiosum (Fifth Disease) — Parvovirus B19

• Aplastic crisis in patients with chronic hemolytic anemias (sickle cell, hereditary spherocytosis) — the virus infects erythroid progenitor cells (erythroblasts), causing transient red-cell aplasia
• Hydrops fetalis: Transplacental transmission in first 20 weeks of pregnancy → fetal anemia → high-output heart failure → generalized fetal edema; can be fatal
• Immunocompromised: chronic pure red-cell aplasia

4. Exanthem Subitum (Roseola Infantum / Sixth Disease)

1. Fever phase: Abrupt onset of high fever (38–40°C) lasting 3–7 days, often with irritability, rhinorrhea, diarrhea. Febrile seizures occur in ~10–15%. "Sleepy" appearance from palpebral edema.
2. Defervescence → rash: The exanthem appears 1 day before to 2 days after the fever breaks — this temporal pattern (fever then rash) is the hallmark distinguishing ES from most other viral exanthems where they occur simultaneously.
3. Exanthem: Rose-pink (2–5 mm) macules and papules with a white halo, distributed on the neck and trunk; less commonly face and proximal extremities. Lasts 3–5 days.

5. Hand-Foot-Mouth Disease (HFMD)

• Oral enanthem: Erosions on tongue, buccal mucosa, hard palate — painful, with erythematous halo
• Cutaneous vesicles: Begin as pink macules/papules → 4–8 mm vesicles → oval "football-shaped" erosions with gray center and erythematous halo on the palms, soles, sides of hands/feet, buttocks

6. Varicella (Chickenpox)

7. CMV — Congenital Infection

• Petechiae/purpura from thrombocytopenia
• Blueberry muffin lesions (dermal erythropoiesis): dark-blue to violaceous papules (2–10 mm) present at birth, fading over 6 weeks. Histology: nucleated red cells + nonnucleated erythrocytes in the reticular dermis.
• Jaundice from hepatitis

Comparison Table — Key Differentiators

Approach to the Morbilliform/Viral Exanthem Patient

1. Timing relative to fever: Rash with fever (measles, rubella, enterovirus) vs. rash after fever resolves (roseola, Kawasaki)
2. Distribution: Cephalocaudal (measles) | Centripetal (varicella) | Acral (HFMD, parvovirus)
3. Morphology: Vesicular/pleomorphic (varicella, HFMD) | Petechial (meningococcemia, RMSF — rule out first) | Maculopapular (most viral)
4. Mucosal involvement: Koplik spots (measles), oral erosions (HFMD), Forchheimer spots (rubella)
5. Lymphadenopathy pattern: Postauricular/suboccipital (rubella), generalized (EBV, CMV), mesenteric (adenovirus)
6. Drug eruption exclusion — Viral exanthem vs. drug eruption is a common diagnostic dilemma; drug-induced morbilliform eruptions are the leading mimicker.

Viral Exanthems in the Emergency Department

Step 1 — The First Question: Does This Rash Blanch?

Step 2 — Non-Blanching Rash: Rule Out Immediately Life-Threatening Causes

Meningococcemia (Neisseria meningitidis)

• Begins as blanching maculopapular rash → rapidly progresses to non-blanching petechiae and purpura over hours
• Characteristic: palpable purpura in axilla, trunk, extremities
• Associated: high fever, toxic appearance, meningismus, hypotension (septic shock)
• Purpura fulminans = large confluent areas of hemorrhagic necrosis — indicates DIC and imminent death
• Action: Immediate blood cultures → IV ceftriaxone without waiting for LP if patient is unstable

Rocky Mountain Spotted Fever (RMSF) — Rickettsia rickettsii

• Classic triad: fever + headache + rash — but the triad is only present in ~60% at time of presentation
• Rash begins on wrists and ankles (acral) → spreads centripetally to trunk; starts as blanching macules → becomes petechial
• Critical: rash may be absent in the first 3 days (treatment must begin before rash appears)
• Action: Do not wait for serology (sensitivity is low in acute phase); treat empirically with doxycycline

Dengue Fever

• Maculopapular or petechial rash, thrombocytopenia, severe myalgia ("breakbone fever")
• Travel history to tropical/subtropical regions is key
• Dengue hemorrhagic fever → shock, bleeding diatheses

Viral Hemorrhagic Fevers (Ebola, Marburg, Lassa)

• Rare; history of travel to endemic regions is essential
• Petechiae, ecchymoses, mucosal hemorrhage + fever
• Immediate isolation and infectious disease consultation

Step 3 — Blanching Maculopapular Rash: Common Viral Exanthems in the ED

The "Rash and Fever Together vs. Rash After Fever" Rule

Measles (Rubeola) — ER Protocol Required

• Most infectious virus known; airborne transmission; infectious before symptoms begin
• ED action: Any patient with fever + cough + coryza + conjunctivitis → immediately place in airborne isolation (negative-pressure room or N95 mask)
• Koplik's spots on buccal mucosa (1 mm white lesions opposite lower molars) = pathognomonic — confirm before rash appears
• Morbilliform rash starts behind ears/hairline → cephalocaudal spread over 3 days

• ED management:
  • Supportive; vitamin A (reduces pneumonia risk and mortality)
  • Admit if: pneumonia, encephalitis, immunocompromised, severe malnutrition
  • Mandatory reporting to local/state public health before discharge
  • Log potential ED staff and patient exposures
  • Offer MMR vaccination to unvaccinated contacts identified in the ED
• Serum measles IgM confirms; may be negative in first 3–4 days of rash → repeat or use PCR of urine/throat

"Because measles is so highly infectious, EDs should have protocols for identification of measles suspects at initial point of contact based on characteristic symptoms." — Tintinalli's Emergency Medicine

Rubella

• Mild illness; posterior auricular and suboccipital lymphadenopathy preceding rash by up to 7 days is the key clue
• Fine pink maculopapular rash, face → trunk, clears in 3 days; no significant cough
• Key ED concern: Identify pregnant contacts — first-trimester exposure carries >85% risk of congenital rubella syndrome (cataracts, deafness, congenital heart disease)
• Report to public health

Roseola (HHV-6B) — Exanthem Subitum

• Classic ED presentation: infant brought in for febrile seizure → by the time of the ED visit, fever has broken and the rose-colored truncal rash has appeared
• The temporal sequence is diagnostic: high fever 3–5 days → fever defervesces → rash appears
• Reassure parents: rash signifies resolution, not worsening
• No specific antiviral needed; manage febrile seizure per standard protocol

Parvovirus B19 (Erythema Infectiosum / Fifth Disease)

• Child with "slapped cheek" (confluent facial erythema with circumoral pallor) + lacy reticular rash on extremities
• Important ED considerations:
  1. Aplastic crisis in sickle cell, hereditary spherocytosis, thalassemia patients: can present with severe anemia, pallor, tachycardia → blood transfusion may be needed
  2. Hydrops fetalis if pregnant patient or pregnant contacts are exposed during first 20 weeks
  3. By the time the rash appears, the child is no longer infectious (viremia has resolved)

Hand-Foot-Mouth Disease (Coxsackievirus A16/EV71)

• Painful oral erosions + vesicles on palms/soles in a young child in summer/fall
• Key concern: Enterovirus 71 (EV71) can cause aseptic meningitis, encephalitis, acute flaccid paralysis — admit if neurological signs
• Check hydration status: mouth pain often leads to refusal to eat/drink → dehydration
• LP if meningism: CSF shows lymphocytic pleocytosis, normal glucose; PCR confirms

Varicella (Chickenpox)

• Pruritic vesicles in multiple stages simultaneously ("dew drop on a rose petal")
• ED concerns:
  • Secondary bacterial infection (Group A strep, Staph aureus) — cellulitis, necrotizing fasciitis
  • Varicella pneumonia in adults and smokers: bilateral interstitial infiltrates, hypoxia → admit, IV acyclovir
  • Reye syndrome if aspirin given — never use ASA in children with varicella
  • Neonatal varicella: maternal illness 5 days before to 2 days after delivery → 20–30% neonatal mortality without IV acyclovir
  • Immunocompromised: IV acyclovir; disseminated varicella can be fatal
• Treatment: acyclovir (valacyclovir) within 24 hours of rash onset in adults; supportive in uncomplicated childhood cases

Dangerous Mimickers and Differential Diagnosis

ED Approach Algorithm — Fever + Rash

FEVER + RASH
    │
    ├─ NON-BLANCHING? ──── YES ──→ EMERGENCY
    │                              • Blood cultures → ceftriaxone NOW
    │                              • Consider RMSF → doxycycline
    │                              • Consider dengue, VHF (travel hx)
    │
    └─ BLANCHING (maculopapular)?
           │
           ├─ Toxic / unstable? → Consider TSS, Kawasaki, DRESS
           │
           ├─ Timing: rash WITH fever
           │    ├─ 3Cs + Koplik's → MEASLES (airborne isolation, report)
           │    ├─ Mild, postauricular LAP → Rubella (check pregnancy)
           │    ├─ Pharyngitis + sandpaper → Scarlet fever (penicillin)
           │    └─ Oral erosions + acral vesicles → HFMD (EV71?)
           │
           ├─ Timing: rash AFTER fever
           │    ├─ Infant, truncal macules → Roseola (reassure)
           │    └─ Fever >5 days, no other dx → Kawasaki (echo, IVIG)
           │
           ├─ "Slapped cheek" + lacy rash → Parvovirus B19
           │    (check CBC if hemolytic anemia; alert if pregnant)
           │
           └─ No fever, new medication → Drug eruption
                (monitor for mucosal involvement → SJS/TEN risk)

Key ED Disposition Guidance