Specific Viral Exanthems: Diagnostic Guide For Pediatricians

An

exanthem

is a widespread rash, usually of viral origin, accompanied by systemic symptoms such as fever, headache, and malaise. Specific viral exanthems are childhood eruptions with distinctive morphologies that allow clinical diagnosis. This article reviews their key features, aiding dermatologists and paediatricians in accurate identification.

Introduction to Viral Exanthems

Viral exanthems represent a significant proportion of paediatric consultations, often presenting with fever followed by characteristic rashes. These rashes arise from immune responses to viral antigens rather than direct cytopathic effects. Common viruses include enteroviruses, parvovirus B19, and herpesviruses. Diagnosis relies on rash morphology, distribution, and associated symptoms, with historical vaccination reducing incidence of classic exanthems like measles.

Key diagnostic principles include age of onset (typically under 10 years), prodromal fever, and rash evolution. Differentials encompass bacterial infections, drug eruptions, and autoimmune conditions. Management is supportive, focusing on hydration, antipyretics, and itch relief.

Measles

Measles, caused by the measles virus (Morbillivirus), remains a global concern despite vaccination. Prodrome includes high fever, cough, coryza, conjunctivitis (the ‘3 Cs’), and Koplik spots—pathognomonic 1–2 mm white lesions on buccal mucosa opposite molars, appearing 1–2 days before rash.

The

exanthem

starts as erythematous macules behind ears and on face, spreading cephalocaudally to trunk and extremities over 3–4 days. Lesions coalesce into blotches, blanching on pressure. Rash fades in 5–7 days with desquamation, often leaving brownish pigmentation.

Complications include pneumonia, encephalitis (1/1000 cases), and subacute sclerosing panencephalitis. Diagnosis is clinical; PCR confirms virus. Notify public health authorities. Differentials: Kawasaki disease, scarlet fever.

Rubella

Rubella (German measles), due to rubella virus, presents milder than measles. Prodrome: low-grade fever, sore throat, retroauricular lymphadenopathy (posterior triangle hallmark). Exanthem comprises discrete pink macules/papules starting on face, spreading to trunk/limbs in 24 hours. Fades in 3 days without desquamation.

Forchheimer spots (palatal petechiae) occur in 20–50%. In pregnancy, congenital rubella syndrome risks deafness, cataracts, heart defects. Serology or PCR diagnoses. Differentials: scarlet fever, enteroviral exanthems. Vaccination prevents most cases.

Roseola Infantum (Exanthem Subitum)

Human herpesvirus 6 (HHV-6) or HHV-7 causes roseola, affecting infants 6–24 months. High fever (39–40°C) lasts 3–5 days, with febrile seizures in 10–15%. As fever resolves, pink-red blanching macules/papules erupt on trunk, spreading to neck, face, extremities. Resolves in 1–2 days.

Mild upper respiratory symptoms, irritability common. Diagnosis clinical; HHV-6 PCR if atypical. Differentials: measles, Kawasaki. Supportive care; antipyretics (avoid aspirin).

Hand, Foot, and Mouth Disease

Primarily Coxsackie A16 or Enterovirus 71. Prodrome: fever, malaise, sore throat. Oral lesions: 2–5 mm grey-white vesicles/ulcers on anterior tongue, buccal mucosa. Skin: oval grey vesicles with red halo on palms, soles, buttocks; may generalize in outbreaks.

Evolves to crusts in 7–10 days; post-infectious onychomadesis (nail shedding) in 4–8 weeks. Severe EV71 cases: neurological complications. PCR from vesicles/stool confirms. Differentials: eczema herpeticum, bullous impetigo.

Pityriasis Rosea

Possibly human herpesvirus 6/7-associated. ‘Herald patch’: 2–5 cm oval salmon-pink plaque with collarette scale on trunk/abdomen. 1–2 weeks later, smaller plaques (‘Christmas tree’ pattern) on trunk. Christmas tree distribution on back with peripheral trailing scale.

Mild itch/pruritus; resolves 6–8 weeks. Oral lesions rare. Diagnosis clinical; biopsy if atypical. Differentials: guttate psoriasis, tinea corporis. Topical steroids for itch.

Fifth Disease (Erythema Infectiosum)

Parvovirus B19 causes ‘slapped cheek’ appearance: bright red confluent erythema on cheeks sparing nasolabial folds/circumoral. 1–4 days later, lacy reticular erythema on proximal extremities, trunk. Rash recurs with heat/sun/exercise for weeks.

Arthralgias in adults; aplastic crisis in sickle cell. Serology diagnoses. Avoid in pregnancy (hydrops fetalis risk). Differentials: drug eruption.

Papular Acrodermatitis (Gianotti-Crosti Syndrome)

Clinical Features

Symmetrical monomorphic papules (1–5 mm) on cheeks, buttocks, extensor limbs; trunk spared. Triggered by hepatitis B, EBV, Coxsackie. Lasts 2–10 weeks; lymphadenopathy, rarely liver dysfunction.

Investigations

Hep B serology if unvaccinated; LFTs. Biopsy shows spongiotic dermatitis.

Differential Diagnosis

• Insect bite reactions
• Scabies
• Papular urticaria
• Lichen planus

Varicella (Chickenpox)

Primary varicella-zoster virus (VZV) infection. Prodrome mild fever. Centripetal pruritic vesicles: ‘dew drop on rose petal’—papule to vesicle to pustule to crust. Successive crops over 5–7 days; 200–500 lesions.

Complications: bacterial superinfection, pneumonia (adults/smokers). PCR from swab diagnoses. Aciclovir for severe/immunocompromised. Isolation until crusted.

Herpes Zoster (Shingles)

VZV reactivation. Prodrome: dermatomal pain, paraesthesia 3–5 days. Grouped vesicles on erythematous base in dermatome (thoracic T5-L2 common). Crusts in 7–10 days; post-herpetic neuralgia risk in elderly.

PCR confirms. Antivirals (aciclovir/valaciclovir) within 72h reduce duration. Differentials: herpes simplex, contact dermatitis.

Other Viral Exanthems

Enteroviral Exanthems

Echovirus/Coxsackie: morbilliform, petechial, urticarial rashes. Summer epidemics.

Non-Specific Childhood Exanthem

Most common; post-infectious maculopapular rash.

Drug Eruptions Mimicking Exanthems

Amoxicillin common mimic.

Diagnostic Approach

History: Fever duration, contacts, vaccination, exposures.
Examination: Rash distribution/morphology, lymphadenopathy, mucosal lesions.
Investigations: Selective PCR/serology; FBC, CRP if systemic.

Exanthem	Incubation	Rash Distribution	Key Feature
Measles	10–14 days	Face to trunk	Koplik spots
Rubella	14–21 days	Face/trunk	Retroauricular nodes
Roseola	5–15 days	Trunk	Post-febrile
HFMD	3–6 days	Hands/feet/mouth	Vesicles on palms
Fifth Disease	13–18 days	Cheeks/extremities	Slapped cheek

Frequently Asked Questions (FAQs)

What causes viral exanthems?

Primarily viruses like measles, rubella, parvovirus B19, enteroviruses, and herpesviruses trigger immune-mediated rashes.

How are viral exanthems diagnosed?

Clinical features suffice for classics; PCR/serology for confirmation.

Is treatment needed for viral rashes?

Supportive: paracetamol, hydration, calamine. Antivirals for severe varicella/zoster.

Can viral exanthems be prevented?

Yes, via MMR vaccine for measles/rubella/varicella.

When to seek urgent care?

High fever >5 days, breathing difficulty, neurological signs, or immunocompromise.