Differential Diagnosis of Exanthemas

Introduction

An exanthem (from Greek: "efflorescence") is an acute, widespread, symmetric eruption typically associated with a systemic illness—most commonly viral infection or drug reaction. The term "rash" is often used colloquially, but exanthem specifically denotes a generalized cutaneous eruption of relatively sudden onset that may be the presenting feature of a systemic process.

The diagnostic approach to exanthems requires systematic evaluation of:

1. Morphology: Morbilliform, scarlatiniform, vesicular, petechial/purpuric
2. Distribution and Spread Pattern: Cephalocaudal, centrifugal, acral
3. Prodromal Symptoms: Fever, cough, coryza, conjunctivitis
4. Mucosal Involvement: Oral, conjunctival
5. Associated Symptoms: Lymphadenopathy, arthralgia, systemic toxicity
6. Drug History: Recent medications
7. Epidemiological Context: Vaccination status, contacts, travel

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19.1 Classification by Morphology

19.1.1 Morbilliform (Measles-like) Exanthem

The morbilliform pattern is the most common morphology of exanthems. It is characterized by erythematous macules and papules that may coalesce into patches but leave areas of uninvolved skin between them.

Drug-Induced Morbilliform Eruption (Most Common Cause in Adults)

• Timing: Typically 7-14 days after starting a new medication (shorter if prior sensitization).
• Morphology: Symmetric, blanching erythematous macules and papules starting on trunk and spreading to extremities.
• Pruritus: Usually prominent (unlike viral exanthems which are often non-pruritic).
• Mucous Membranes: Spared (if involved → consider SJS/DRESS).
• Common Culprits: Beta-lactam antibiotics (penicillins, cephalosporins), sulfonamides, anticonvulsants, allopurinol, NSAIDs.
• Resolution: Self-limited. Resolves within 1-2 weeks of drug discontinuation, often with desquamation.

[!IMPORTANT] Red Flags for Severe Drug Reaction (SJS/DRESS)

• Mucosal involvement (oral erosions, conjunctivitis)
• Skin pain or tenderness out of proportion to appearance
• Facial edema
• Lymphadenopathy
• High fever
• Target lesions with central necrosis
• Eosinophilia or atypical lymphocytosis

Measles (Rubeola)

• Prodrome (Catarrhal Phase): 2-4 days of high fever (up to 40°C), the "3 Cs": Cough (barking), Coryza (rhinitis), Conjunctivitis (non-purulent with photophobia).
• Koplik Spots: PATHOGNOMONIC. White-gray papules on bright red buccal mucosa opposite molars. Appear 1-2 days before rash, fade as rash appears.
• Exanthem: Erythematous maculopapular rash beginning on face (behind ears, hairline) and spreading cephalocaudally (downward) over 3-4 days. Lesions may become confluent. Rash fades in same order.
• Complications: Pneumonia (giant cell), encephalitis, subacute sclerosing panencephalitis (SSPE - years later), otitis media.
• Prevention: MMR vaccine.

Rubella (German Measles / 3-Day Measles)

• Prodrome: Mild or absent. Low-grade fever.
• Lymphadenopathy: CHARACTERISTIC. Tender posterior auricular and suboccipital lymph nodes (appear before rash).
• Exanthem: Pink maculopapular rash starting on face, spreading rapidly to trunk and extremities. Lesions do NOT coalesce (unlike measles). Fades quickly (2-3 days - hence "3-day measles").
• Forchheimer Spots: Petechiae on soft palate.
• Complication: Congenital Rubella Syndrome (CRS) if infection in first trimester (cataracts, heart defects, deafness).

Roseola Infantum (Exanthem Subitum / Sixth Disease)

• Causative Agent: Human Herpesvirus 6 (HHV-6), less commonly HHV-7.
• Age: 6 months to 2 years. Classic infant viral exanthem.
• Clinical Course:
  • Febrile Phase: Abrupt onset of high fever (39-40°C) for 3-5 days. Child may appear surprisingly well despite high fever. Febrile seizures may occur.
  • Exanthem Phase: "Fever breaks, rash erupts." As fever resolves by crisis (sudden defervescence), a rose-pink maculopapular rash appears on trunk and spreads to neck and extremities. Rash is non-pruritic and fades within 1-2 days.
• Nagayama Spots: Ulcers at uvulopalatoglossal junction.

Erythema Infectiosum (Fifth Disease / Slapped Cheek Disease)

• Causative Agent: Parvovirus B19 (targets erythroid progenitors).
• Age: School-aged children (5-15 years).
• Clinical Phases:
  • Phase 1 (Prodrome): Mild fever, malaise, headache. Viremia present - patient is contagious.
  • Phase 2 ("Slapped Cheek"): Bright red, confluent erythema on both cheeks with perioral pallor. Fever has resolved by this stage.
  • Phase 3 (Lacy Rash): Reticulated (lacy), maculopapular rash on trunk and proximal extremities. May wax and wane with sunlight, heat, or bathing for weeks.
• Complications:
  • Transient Aplastic Crisis: In patients with hemolytic anemias (sickle cell, thalassemia).
  • Hydrops Fetalis: If pregnant woman infected (especially 2nd trimester).
  • Arthropathy: Symmetric polyarthralgia in adults (especially women).

Infectious Mononucleosis (EBV)

• Causative Agent: Epstein-Barr Virus (EBV), transmitted by saliva ("kissing disease").
• Classic Triad: Fever + Pharyngitis (exudative tonsillitis) + Lymphadenopathy (especially posterior cervical).
• Exanthem: Occurs in ~5-10% of untreated cases. HOWEVER, ~80-100% develop a morbilliform rash if treated with ampicillin or amoxicillin (this is NOT a true allergy but an immune complex reaction).
• Other Signs: Splenomegaly (palpable spongy spleen - avoid contact sports), hepatomegaly, palatal petechiae.
• Diagnosis: Atypical lymphocytosis (>10%), Heterophile antibodies (Monospot test), EBV serology.

Primary HIV Infection (Acute Retroviral Syndrome)

• Timing: 2-6 weeks after HIV exposure.
• Presentation: Often mistaken for mononucleosis. Fever, pharyngitis, lymphadenopathy, headache, myalgia.
• Exanthem: Morbilliform rash on trunk and face. May have mucosal ulcers (oral, genital).
• KEY: HIGH viral load, LOW or ABSENT HIV antibodies (window period). Diagnosis requires HIV RNA PCR.
• Importance: Early diagnosis allows early ART and reduces transmission.

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19.1.2 Scarlatiniform (Scarlet Fever-like) Exanthem

Characterized by confluent, fine, "sandpaper-textured" erythema giving a diffuse red appearance.

Scarlet Fever

• Causative Agent: Group A Streptococcus (GAS) producing erythrogenic toxins (SpeA, SpeB, SpeC).
• Prodrome: Abrupt onset of fever, sore throat, headache, vomiting.
• Pharyngitis: Red, edematous pharynx with exudates.
• Tongue Changes:
  • Initial: "White strawberry tongue" (white coating with prominent red papillae).
  • Later: "Red strawberry tongue" (coating sloughs, leaving beefy red tongue with prominent papillae).
• Exanthem:
  • Fine, diffuse erythema with sandpaper texture. Blanches with pressure.
  • Appears first on neck, axillae, groin, then generalizes within 24 hours.
  • Pastia's Lines: Linear petechiae or accentuated erythema in skin folds (antecubital, axillary, inguinal) that persists even after general rash fades.
  • Circumoral Pallor: Sparing of skin around the mouth contrasts with flushed cheeks.
• Desquamation: Fine desquamation of trunk (branny) and sheet-like peeling of palms/soles occurs 1-3 weeks after rash subsides.
• Complications: Rheumatic fever, Post-streptococcal glomerulonephritis.
• Diagnosis: Rapid strep test, Throat culture.

Staphylococcal Toxic Shock Syndrome (TSS)

• Causative Agent: Staphylococcus aureus producing TSST-1 (superantigen).
• Risk Factors: Menstruation (tampon use - now rare due to awareness), surgical wounds, nasal packing.
• Clinical Features:
  • High fever, hypotension (shock), multi-organ involvement.
  • Diffuse, blanching, "sunburn-like" erythema.
  • Strawberry tongue, conjunctival injection.
  • Late Desquamation: Characteristic late (1-3 weeks) desquamation of palms and soles.
• Diagnosis: Clinical criteria (fever, rash, hypotension, multi-organ involvement).

Streptococcal Toxic Shock Syndrome (StrepTSS)

• Similar to Staph TSS but caused by GAS.
• Often associated with necrotizing fasciitis or severe soft tissue infection.
• May have less prominent rash but more severe tissue destruction.

Kawasaki Disease (Mucocutaneous Lymph Node Syndrome)

• Epidemiology: Children <5 years. Boys > Girls. Higher incidence in Asian populations.
• Etiology: Unknown (likely infectious trigger with genetic susceptibility).
• Diagnostic Criteria (Fever + 4 of 5):
  1. Fever ≥5 days (often high, remittent, unresponsive to antibiotics).
  2. Bilateral conjunctival injection (non-purulent, limbic sparing).
  3. Oral changes: Strawberry tongue, cracked red lips, pharyngeal erythema.
  4. Extremity changes: Erythema of palms/soles, edema of hands/feet, late periungual desquamation.
  5. Polymorphous rash: Morbilliform, scarlatiniform, or erythema multiforme-like. Groin accentuation common.
  6. Cervical lymphadenopathy: Usually unilateral, >1.5 cm.
• CRITICAL COMPLICATION: Coronary artery aneurysms (15-25% untreated). Leading cause of acquired heart disease in children in developed countries.
• Treatment: IVIG + Aspirin. Echocardiography essential.

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19.1.3 Vesicular Exanthem

Primary lesions are vesicles (small fluid-filled blisters <1cm).

Varicella (Chickenpox)

• Causative Agent: Varicella-Zoster Virus (VZV).
• Transmission: Highly contagious. Respiratory droplets and direct contact with vesicle fluid. Contagious from 2 days before rash until all lesions crusted.
• Prodrome: 1-2 days of low-grade fever, malaise (may be absent in children).
• Exanthem:
  • Classic "dew drops on a rose petal": Clear vesicles on an erythematous base.
  • Centripetal distribution: Starts on trunk and face, then spreads to extremities. More lesions on trunk than periphery.
  • Lesions in all stages: Macules, papules, vesicles, pustules, and crusts all present simultaneously at different sites = classic finding.
  • Intensely pruritic.
• Complications: Secondary bacterial infection, Varicella pneumonia (adults, immunocompromised), Encephalitis, Cerebellar ataxia.
• Congenital Varicella Syndrome: Limb hypoplasia, cicatricial skin lesions, CNS abnormalities.

Hand, Foot, and Mouth Disease (HFMD)

• Causative Agents: Coxsackievirus A16 (most common), Enterovirus 71 (associated with severe neurological complications).
• Epidemiology: Children <10 years. Summer/Autumn outbreaks.
• Clinical Features:
  • Fever, malaise, sore throat.
  • Oral lesions: Vesicles and ulcers on buccal mucosa, tongue, soft palate.
  • Cutaneous lesions: Vesicles (oval, grayish, surrounded by erythema) on palms, soles, and often buttocks.
• Course: Self-limited (7-10 days).

Eczema Herpeticum (Kaposi Varicelliform Eruption)

[!CAUTION] DERMATOLOGIC EMERGENCY

• Definition: Widespread HSV infection in patients with pre-existing skin disease (atopic dermatitis most common).
• Clinical Features:
  • Sudden appearance of monomorphic, punched-out erosions and vesicles on areas of eczema.
  • May have fever, malaise, lymphadenopathy.
  • Can lead to viremia, encephalitis, death if untreated.
• Diagnosis: Tzanck smear, Viral PCR.
• Treatment: IV Acyclovir urgently.

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19.1.4 Petechial and Purpuric Exanthem

Non-blanching lesions indicating extravasation of blood into the skin.

[!CAUTION] Petechial Rash + Fever = Meningococcemia Until Proven Otherwise This is a medical emergency requiring immediate IV antibiotics before investigations.

Meningococcemia

• Causative Agent: Neisseria meningitidis.
• Clinical Course: Rapidly progressive. Can evolve from mild illness to death within hours.
• Rash:
  • Early: Maculopapular (may mimic viral exanthem).
  • Later: Petechiae → Purpura → Ecchymoses. May become necrotic.
• Systemic Signs: High fever, rigors, hypotension, altered consciousness.
• Waterhouse-Friderichsen Syndrome: Bilateral adrenal hemorrhage leading to acute adrenal insufficiency.
• Management: Immediate IV Ceftriaxone. Do NOT delay antibiotics for investigations.

Rocky Mountain Spotted Fever (RMSF)

• Causative Agent: Rickettsia rickettsii. Transmitted by tick bite.
• Geographic: Americas (not just Rocky Mountains).
• Clinical Features:
  • Triad: Fever + Headache + Rash.
  • Rash: Starts on wrists and ankles (acral), then spreads CENTRALLY (centripetal). Initially maculopapular, becomes petechial.
  • Palms and soles involved (important clue).
• Complications: Multi-organ failure, DIC.
• Treatment: Doxycycline (even in children for RMSF).

IgA Vasculitis (Henoch-Schönlein Purpura)

• Epidemiology: Children (4-10 years). Peak in winter. Often follows URTI.
• Tetrad:
  1. Palpable Purpura: Non-thrombocytopenic. Lower extremities and buttocks. Symmetric.
  2. Arthralgia/Arthritis: Knees, ankles. Transient.
  3. Abdominal Pain: Colicky. GI bleeding (melena, hematochezia) possible. Intussusception risk.
  4. Nephritis: Hematuria, proteinuria. Usually self-limited but may progress to CKD.
• Histopathology: Leukocytoclastic vasculitis with IgA deposits on DIF.
• Course: Usually self-limited (weeks). Renal monitoring essential.

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19.2 Diagnostic Algorithm

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19.3 Summary Comparison Table

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19.4 Ancillary Tests in Exanthems

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19.5 Key Clinical Pearls

1. Morbilliform rash + Recent drug → Most likely Drug Eruption. However, always rule out red flags for SJS/DRESS.
2. "Fever first, rash after" (defervescence rash) → Think Roseola (HHV-6).
3. Slapped cheek + Lacy rash → Parvovirus B19. Screen for anemia in at-risk patients.
4. Ampicillin/Amoxicillin + Rash → Check for EBV infection before labeling penicillin allergy.
5. Sandpaper rash + Strawberry tongue + Pharyngitis → Scarlet Fever. Treat with penicillin to prevent Rheumatic fever.
6. Fever ≥5 days + 4 of 5 criteria → Kawasaki. Echo urgently. IVIG + Aspirin.
7. Petechial rash + Fever + Sick patient → Meningococcemia. IV Ceftriaxone IMMEDIATELY.