Differential Diagnosis of Pustules

Introduction

Pustules are superficial, circumscribed elevations of the skin containing purulent material (pus). Despite their appearance, pustules are not always infectious—many pustular dermatoses are sterile (non-infectious), representing an accumulation of neutrophils without bacterial involvement. The fundamental clinical distinction is between Follicular (centered on hair follicles) and Non-Follicular pustules, which guides the differential diagnosis.

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28.1 Follicular vs. Non-Follicular Pustules

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28.2 Follicular Pustular Dermatoses

28.2.1 Bacterial Folliculitis

• Causative Agent: Most commonly Staphylococcus aureus.
• Clinical Features:
  • Superficial: Small pustules around hair follicles (Bockhart's impetigo).
  • Deep: Furuncle (boil) = Single painful nodule. Carbuncle = Coalescence of multiple furuncles with systemic symptoms.
• Risk Factors: Occlusion, Shaving, Diabetes, Immunosuppression.
• Distribution: Scalp, beard area (Sycosis barbae), thighs, buttocks.

28.2.2 Gram-Negative Folliculitis

• Setting: Occurs during prolonged antibiotic treatment for acne.
• Mechanism: Suppression of normal flora allows gram-negative overgrowth (Klebsiella, Enterobacter, Proteus).
• Clinical Features:
  • Sudden flare of pustular acne, predominantly perioral.
  • May have "lakes" of pustules.
• Treatment: Isotretinoin or appropriate antibiotics based on culture.

28.2.3 Hot Tub Folliculitis (Pseudomonas Folliculitis)

• Causative Agent: Pseudomonas aeruginosa.
• Exposure: Inadequately chlorinated hot tubs, pools, whirlpools.
• Clinical Features:
  • Pruritic follicular papulopustules on trunk and buttocks (areas covered by swimsuit).
  • Spares face, hands, feet.
  • Onset 1-4 days after exposure.
• Course: Self-limited (resolves within 7-10 days).

28.2.4 Pityrosporum (Malassezia) Folliculitis

• Causative Agent: Malassezia species (lipophilic yeasts, part of normal flora).
• Clinical Features:
  • Monomorphic follicular papulopustules (all lesions look similar—unlike acne which has comedones, papules, pustules, nodules).
  • Distribution: Trunk (upper back, chest, shoulders).
  • Pruritic (important clue).
  • Fails to respond to standard acne antibiotics.
• Risk Factors: Occlusion, Sweating, Immunosuppression.
• Diagnosis: KOH preparation shows yeast forms.
• Treatment: Topical or oral antifungals.

28.2.5 Eosinophilic Folliculitis

A sterile folliculitis with eosinophilic infiltrate.

HIV-Associated Eosinophilic Folliculitis

• Setting: Late-stage HIV (CD4 <300).
• Clinical Features:
  • Intensely pruritic follicular papulopustules.
  • Distribution: Face, neck, upper trunk.
  • Urticarial papules may be present.
• Histology: Eosinophilic infiltrate around follicles.
• Treatment: Antiretrovirals (immune reconstitution), UVB phototherapy, antihistamines.

Eosinophilic Pustular Folliculitis of Infancy (Ofuji Disease Variant)

• Age: Occurs in infants.
• Clinical Features: Recurrent crops of sterile pustules on scalp.
• Course: Self-limited.

28.2.6 Acne Vulgaris

• Epidemiology: Adolescents and young adults. Most common skin disease.
• Pathogenesis: Sebum overproduction + Follicular hyperkeratinization + Cutibacterium acnes proliferation + Inflammation.
• Lesion Types:
  • Comedones: Open (blackheads) and Closed (whiteheads).
  • Inflammatory Lesions: Papules, Pustules, Nodules, Cysts.
• Distribution: Face, chest, upper back (sebaceous areas).
• Key Differentiator from Rosacea: Presence of comedones. Rosacea has NO comedones.

28.2.7 Rosacea (Papulopustular Subtype)

• Clinical Features:
  • Centrofacial erythema with papules and pustules.
  • Telangiectasias.
  • NO COMEDONES (distinguishes from acne).
  • Flushing exacerbated by triggers (heat, alcohol, spicy food).
• Other Subtypes: Erythematotelangiectatic, Phymatous (rhinophyma), Ocular.

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28.3 Non-Follicular Pustular Dermatoses

28.3.1 Pustular Psoriasis

Generalized Pustular Psoriasis (von Zumbusch Type)

[!CAUTION] DERMATOLOGIC EMERGENCY

• Clinical Features:
  • Sudden onset of widespread erythema studded with sterile pustules.
  • Confluent "lakes of pus" on erythematous base.
  • Systemic symptoms: High fever, malaise, leukocytosis.
  • May be triggered by: Withdrawal of systemic steroids, Infection, Pregnancy (Impetigo herpetiformis).
• Complications: Hypocalcemia, Hypoalbuminemia, Sepsis, Cardiac failure.
• Management: Hospitalization. Systemic therapy (Acitretin, Cyclosporine, Infliximab). Avoid systemic steroids (risk of rebound flare).

Localized Pustular Psoriasis

• Palmoplantar Pustulosis (PPP): Sterile pustules on palms and soles on erythematous base. Strong association with smoking. May be considered a variant of psoriasis or a distinct entity.
• Acrodermatitis Continua of Hallopeau: Pustular eruption affecting distal fingers/toes. Nail destruction.

28.3.2 Acute Generalized Exanthematous Pustulosis (AGEP)

[!WARNING] Acute Drug Reaction

• Definition: Acute drug-induced pustular eruption.
• Onset: Rapid—typically within 24-48 hours of drug exposure (can be up to 2-3 weeks for antibiotics).
• Clinical Features:
  • Fever.
  • Diffuse erythema.
  • Hundreds of small, non-follicular pustules ("pinhead" pustules) on a background of edematous erythema.
  • Flexural and facial accentuation.
  • Leukocytosis with neutrophilia (not eosinophilia like DRESS).
• Common Culprits: Aminopenicillins, Cephalosporins, Quinolones, Macrolides, Sulfonamides, Pristinamycin.
• Histology: Subcorneal/Intraepidermal spongiform pustules. Neutrophilic infiltrate. Papillary dermal edema.
• Course: Self-limited. Resolves rapidly (1-2 weeks) after drug withdrawal with characteristic desquamation.
• Differentiation from Pustular Psoriasis: AGEP has faster onset, drug association, and resolves more quickly. Histology helps.

28.3.3 Subcorneal Pustular Dermatosis (Sneddon-Wilkinson Disease)

• Epidemiology: Middle-aged to elderly women.
• Clinical Features:
  • Flaccid, sterile pustules that coalesce into annular or circinate patterns.
  • Hypopyon appearance (pus settles to lower half of pustule due to gravity).
  • Distribution: Flexures (axillae, groin), Trunk.
• Association: IgA monoclonal gammopathy or Myeloma in some cases.
• Histology: Subcorneal pustule with neutrophils.
• Treatment: Dapsone.

28.3.4 Impetigo

• Causative Agents: Staphylococcus aureus (bullous impetigo) or Group A Streptococcus.
• Clinical Features:
  • Pustules that rupture quickly → Honey-colored crusts on erythematous base.
  • Distribution: Face (perioral, perinasal), Extremities.
  • Highly contagious. Common in children.
• Bullous Impetigo: Flaccid blisters due to staphylococcal exfoliative toxin (similar mechanism to SSSS but localized).

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28.4 Pustules as Part of Systemic Disease

28.4.1 Disseminated Gonococcal Infection (DGI)

[!IMPORTANT] STI with Cutaneous Manifestation

• Clinical Features:
  • Sparse acral pustules (often with hemorrhagic component)—may be only 5-20 lesions.
  • Migratory polyarthralgia or oligoarthritis.
  • Tenosynovitis.
  • Fever.
• At-Risk: Young, sexually active individuals. Recent menstruation.
• Diagnosis: Culture of pustule contents, Blood culture, NAAT of genitourinary specimens.
• Treatment: IV Ceftriaxone.

28.4.2 Behçet's Disease

• Pustulosis: Sterile pustules on acneiform distribution.
• Pathergy: Pustule formation at sites of minor skin trauma (needle prick).
• Other Features: Oral and genital ulcers, Uveitis, Thrombosis.

28.4.3 Sweet Syndrome (Acute Febrile Neutrophilic Dermatosis)

• Classic Triad: Fever + Neutrophilia + Tender erythematous plaques/nodules.
• Pustules may occur (pustular variant).
• Associations: Infection, Inflammatory bowel disease, Malignancy (AML), Drugs.

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28.5 Diagnostic Algorithm

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28.6 Summary Comparison Table

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28.7 Clinical Pearls

1. Monomorphic trunk pustules + Pruritus → Pityrosporum folliculitis. Treat with antifungals, not antibiotics.
2. Acne has comedones; Rosacea does NOT. This is the key differentiator.
3. AGEP: Drug 24-48 hours prior + Hundreds of pinhead pustules + Fever + Neutrophilia. Stops offending drug.
4. Generalized Pustular Psoriasis is an EMERGENCY: Lakes of pus + High fever + Sick patient. Hospitalize.
5. Palmoplantar pustulosis: Ask about smoking. May be separate entity or pustular psoriasis variant.
6. Honey-colored crusts on face in a child → Impetigo. Treat with topical/oral antibiotics.
7. Sparse acral pustules + Migratory arthralgia → Disseminated gonococcemia. Young, sexually active. Send cultures.
8. Eosinophilic folliculitis: Think HIV if intensely pruritic folliculitis on face and upper trunk.