Viral Exanthems

Viral Exanthems - Rash Basics & Beyond

• Viral exanthems: widespread skin eruptions caused by viral infections; often associated with systemic symptoms.
• Rash characteristics:
  • Morphology: Maculopapular (most common), vesicular, petechial.
  • Distribution: Centripetal (trunk-to-limbs), centrifugal (limbs-to-trunk), or generalized.
  • Progression: Cephalocaudal (head-to-toe) spread is classic for some (e.g., measles).
• Prodrome: Fever, malaise, coryza often precede rash.
• Diagnosis: Clinical; serology/PCR for confirmation if needed.

⭐ Koplik's spots (buccal mucosa opposite second molars) are pathognomonic for measles, appearing 1-2 days before the rash.

Morbilliform Duo - Measles & Rubella Rumble

• Measles (Rubeola)

  • Virus: Paramyxovirus
  • Prodrome: 3 Cs (Cough, Coryza, Conjunctivitis); high fever (>39°C)
  • Pathognomonic: Koplik spots (buccal mucosa, pre-rash)
  • Rash: Brick-red, maculopapular; cephalocaudal, coalesces; lasts 5-7 days
  • Complications: Pneumonia, Otitis Media, SSPE (Subacute Sclerosing Panencephalitis - late)

• Rubella (German Measles)

  • Virus: Togavirus
  • Prodrome: Mild fever; lymphadenopathy (postauricular, suboccipital)
  • Signs: Forchheimer spots (soft palate, non-specific)
  • Rash: Pink, maculopapular; cephalocaudal, discrete; rapid spread (body in 24h); lasts ~3 days
  • Complications: Arthritis (adults); Congenital Rubella Syndrome (CRS). 📌 CRS Triad: Deafness, Eye (cataracts), Cardiac (PDA).

⭐ Measles: Koplik spots (bluish-white on buccal mucosa opposite 2nd molars) appear 1-2 days before rash and are pathognomonic.

Vesicular Villains - Chickenpox & HFMD Showdown

• Chickenpox (Varicella)
  • Agent: Varicella-Zoster Virus (VZV). Highly contagious.
  • Prodrome: Fever, malaise.
  • Rash: "Dew drop on a rose petal". Centripetal (trunk > limbs), pleomorphic (all stages co-exist). Intensely pruritic.
    • 
  • Diagnosis: Clinical; Tzanck smear (multinucleated giant cells).
  • Complications: Pneumonia (adults), encephalitis, Reye's syndrome (aspirin).
  • Rx: Symptomatic; Acyclovir for high-risk/severe.
• Hand, Foot, and Mouth Disease (HFMD)
  • Agent: Coxsackievirus A16 (common); Enterovirus 71 (neuro complications).
  • Prodrome: Low-grade fever, sore throat.
  • Rash: Painful oral ulcers; greyish vesicles on palms, soles, buttocks.
    • 
  • Rx: Symptomatic (analgesia, hydration).

⭐ Chickenpox lesions are typically centripetal (trunk/face first, then limbs), sparing palms/soles, unlike HFMD where palmar/plantar involvement is characteristic.

Childhood Classics - Slapped Cheeks & Sudden Roses

• Erythema Infectiosum (Fifth Disease)
  • Agent: Parvovirus B19.
  • Clinical: "Slapped cheek" rash (spares nasolabial folds), followed by lacy reticular rash on trunk/extremities.
  • Prodrome: Low-grade fever, malaise.
  • Complications: Aplastic crisis (e.g., sickle cell), hydrops fetalis in pregnancy.

    ⭐ Parvovirus B19 is a common cause of acute symmetrical polyarthropathy in adults, mimicking rheumatoid arthritis.

• Roseola Infantum (Exanthem Subitum / Sixth Disease)
  • Agent: Human Herpesvirus 6 (HHV-6) > HHV-7.
  • Clinical: 3-5 days of high fever, then abrupt defervescence followed by maculopapular rash (trunk → neck/extremities).
  • Age: Typically 6 months - 2 years.
  • Complication: Febrile seizures common due to high fever.
  • 📌 Mnemonic: "Fever first, Rash follows" for Roseola.

High‑Yield Points - ⚡ Biggest Takeaways

• Measles: Koplik's spots (pathognomonic), maculopapular cephalocaudal rash. Complication: SSPE.
• Rubella: Milder rash, Forchheimer spots. Congenital Rubella Syndrome (cataracts, deafness, PDA).
• Erythema Infectiosum: Parvovirus B19; "slapped cheek" appearance, then lacy reticular rash. Risk: aplastic crisis.
• Roseola Infantum: HHV-6/7; high fever for 3-5 days, then rash as fever breaks.
• HFMD: Coxsackie A; painful oral vesicles, rash on hands/feet/buttocks.
• Varicella: VZV; pruritic vesicles in crops ("dew drop on rose petal"), all stages co-exist.