How can I differentiate between herpes and eczema in a patient with skin lesions or rashes?

Differentiating Herpes from Eczema

Herpes simplex virus infections present as vesicles or ulcerations in specific distributions (nasolabial, genital, or rectal areas), while eczema presents as pruritic, scaling patches in characteristic locations like skin creases with general dry skin. 1

Key Clinical Distinguishing Features

Herpes Simplex Characteristics

• Vesicular morphology: Look for grouped vesicles containing clear fluid with thousands of infectious viral particles that burst to form shallow ulcers or erosions, eventually crusting and healing without scars in less than 10 days 1
• Distribution patterns: Herpes simplex typically involves nasolabial areas, genital regions, or rectal mucosa, while herpes zoster presents in a dermatomal distribution 1
• Pain predominates over itch: Cutaneous pain is characteristic of herpes infections, contrasting with the intense pruritus of eczema 2

Eczema Characteristics

• Pruritic scaling patches: Eczema presents with itchiness in skin creases, general dry skin, and chronic inflammatory changes rather than acute vesiculation 3
• Chronic relapsing course: Eczema typically has onset in early childhood with chronic adult presentation, unlike the acute episodic nature of herpes 3
• Dry, scaling skin: Look for xerosis and lichenification rather than fluid-filled vesicles 4

Critical Diagnostic Pitfall: Eczema Herpeticum

The most dangerous diagnostic error is missing eczema herpeticum—a herpes simplex superinfection of pre-existing eczema that constitutes a dermatologic emergency. 5

Recognize Eczema Herpeticum by:

• Monomorphic dome-shaped vesicles and pustules arising within areas of pre-existing eczematous skin 6
• Punched-out erosions or grouped vesiculation in a patient with known atopic dermatitis 7
• Severe systemic illness with high fever, cutaneous pain, and rapid progression despite antibiotic therapy 2, 8
• Failure to respond to standard eczema treatment: When infected eczema does not improve with antibiotics, consider herpes superinfection 8

High-Risk Populations:

• Patients with severe atopic dermatitis, particularly those with early onset and elevated IgE levels 6
• Importantly, over 75% of eczema herpeticum cases occur in patients NOT using topical corticosteroids, contradicting the common misconception that steroids predispose to this complication 6

Definitive Diagnostic Testing

Obtain vesicle fluid specimens for immunofluorescence antigen testing, culture, or PCR to confirm herpes virus infection and differentiate between simplex and zoster. 1

Rapid Diagnostic Methods:

• Tzanck smear: Presence of giant cells is diagnostic for herpes virus infection, though it cannot distinguish between simplex and zoster 1
• Electron microscopy: Takes only minutes and can rapidly identify viral particles 8
• Viral culture: Often positive within 24 hours 8
• PCR and immunofluorescence: Provide speciation between HSV-1, HSV-2, and varicella-zoster virus 1

Collection Technique:

• Open vesicles with a sterile needle, collect vesicle content with a swab, and either apply to a microscope slide for immunofluorescence or introduce into transport media for culture or PCR 1

When Clinical Diagnosis Alone Is Insufficient

Laboratory confirmation should always be sought because clinical differentiation of genital HSV from other causes of genital ulceration is difficult, and exclusive reliance on clinical diagnosis leads to both false positive and false negative diagnoses. 1

Differential Diagnosis Requiring Laboratory Confirmation:

• Other infectious causes: Treponema pallidum (syphilis), Haemophilus ducreyi (chancroid), or bacterial superinfection with Staphylococcus aureus 1, 7
• Non-infectious causes: Inflammatory bowel disease (Crohn's), Behçet syndrome, fixed drug eruption, Stevens-Johnson syndrome, or contact dermatitis 1, 2
• Note: HSV and T. pallidum can occasionally be recovered from the same lesion 1

Immediate Management Implications

If Herpes Confirmed:

• Initiate acyclovir immediately for eczema herpeticum, which is potentially life-threatening 7, 5
• Implement infection control measures, as herpes is transmissible by direct contact and fomites 1

If Eczema Without Superinfection:

• Use high-potency topical corticosteroids as mainstay therapy with intermittent breaks 4
• Replace all soaps with dispersible cream cleansers and apply emollients liberally 4
• Never use topical steroids on facial skin if perioral dermatitis is suspected, as this causes steroid-induced atrophy and worsens inflammation 7