How do you differentiate and treat PUPPP (Pruritic Urticarial Papules and Plaques of Pregnancy) versus impetigo herpetiformis in a pregnant patient?

Differentiating PUPPP from Impetigo Herpetiformis

PUPPP and impetigo herpetiformis (pustular psoriasis of pregnancy) are distinguished primarily by their morphology and timing: PUPPP presents with urticarial papules and plaques starting in abdominal striae during the third trimester without systemic symptoms, while impetigo herpetiformis presents with pustules on an erythematous base with systemic manifestations and carries significant maternal-fetal risk. 1, 2

Clinical Differentiation

PUPPP Characteristics

• Lesion morphology: Pruritic urticarial papules and plaques, occasionally with small vesicles (2-4 mm) but never true bullae 3, 4
• Distribution: Begins within abdominal striae and spreads to proximal thighs and extremities; typically spares face, palms, and soles (though exceptions exist) 5, 3, 4
• Timing: Third trimester, most commonly in primigravidas 3, 4
• Systemic symptoms: Absent—no fever, malaise, or laboratory abnormalities 4
• Resolution: Clears prior to delivery or within 1 week postpartum in most cases 4

Impetigo Herpetiformis (Pustular Psoriasis of Pregnancy) Characteristics

• Lesion morphology: Sterile pustules on erythematous plaques, not urticarial papules 6, 1, 2
• Distribution: Typically begins in flexural areas and spreads centrifugally 6, 1
• Timing: Third trimester through postpartum period 2
• Systemic symptoms: Fever, malaise, hypocalcemia, and other systemic manifestations are common 1, 2
• Maternal-fetal risk: Associated with adverse outcomes including fetal growth restriction, stillbirth, and maternal complications 1, 2

Diagnostic Algorithm

Step 1: Assess Lesion Morphology

• If urticarial papules/plaques without pustules → Consider PUPPP 5, 4
• If pustules on erythematous base → Consider impetigo herpetiformis 6, 2

Step 2: Evaluate for Systemic Symptoms

• No fever, normal labs, no systemic symptoms → Favors PUPPP 4
• Fever, malaise, or systemic illness → Strongly suggests impetigo herpetiformis 1, 2

Step 3: Perform Skin Biopsy When Uncertain

• PUPPP histology: Mild nonspecific lymphohistiocytic perivasculitis without specific features 4
• Impetigo herpetiformis histology: Spongiform pustules consistent with pustular psoriasis 6, 2
• Biopsy is essential when clinical presentation is atypical or when differentiating from pemphigoid gestationis (which shows vesicles/bullae, not pustules) 3

Step 4: Laboratory Evaluation

• For suspected impetigo herpetiformis: Check calcium, phosphate, complete blood count, inflammatory markers, and renal function 1, 2
• For PUPPP: No specific laboratory abnormalities expected; if bile acids or liver enzymes are elevated, consider intrahepatic cholestasis of pregnancy instead 5

Treatment Approach

PUPPP Management

• First-line: Moderate to high-potency topical corticosteroids with oral antihistamines (diphenhydramine or chlorpheniramine) 4
• Prognosis: Benign condition with no fetal risk; spontaneous resolution expected 4
• Monitoring: No special obstetric monitoring required beyond routine care 4

Impetigo Herpetiformis Management

• First-line: Systemic corticosteroids (prednisolone preferred due to minimal placental transfer) 7, 2
• Multidisciplinary care: Requires coordinated dermatology-obstetric management due to maternal-fetal risks 2
• Monitoring: Enhanced fetal surveillance for growth restriction and prematurity 7, 2
• Adjunctive therapy: Consider azathioprine or intravenous immunoglobulin for severe cases 7

Critical Pitfalls to Avoid

• Do not assume PUPPP if pustules are present—pustules indicate impetigo herpetiformis or pemphigoid gestationis, not PUPPP 3, 6
• Do not dismiss systemic symptoms—fever and malaise with a pregnancy rash mandate evaluation for impetigo herpetiformis, which requires aggressive treatment 1, 2
• Do not confuse small vesicles in PUPPP with true bullae—small 2-4 mm vesicles can occur in PUPPP, but formation of true bullae suggests pemphigoid gestationis 3
• Do not assume PUPPP if symptoms persist beyond 2 weeks postpartum—this strongly favors pemphigoid gestationis or another diagnosis 7
• Do not delay biopsy when the diagnosis is uncertain—distinguishing these conditions is essential for appropriate treatment and prognostication 3, 2