What are the different types of pustular psoriasis, their classification, and how is it diagnosed?

Classification and Diagnosis of Pustular Psoriasis

Pustular psoriasis presents in three main clinical forms: generalized pustular psoriasis (GPP), localized pustular psoriasis, and acrodermatitis continua of Hallopeau, each requiring specific diagnostic approaches and management strategies. 1

Types of Pustular Psoriasis

1. Generalized Pustular Psoriasis (GPP)

• Clinical presentation: Widespread sterile pustules on an erythematous background with systemic symptoms
• Variants:
  • Von Zumbusch variant: Acute, severe form accompanied by fever and toxicity 2
  • Systemic symptoms: May include fever, malaise, dehydration, and potential multi-organ complications 2, 3
• Course: Unpredictable with periods of improvement followed by relapses 4

2. Localized Pustular Psoriasis

• Palmoplantar pustulosis (PPP):
  • Multiple sterile pustules on palms and soles 2
  • Previously termed "chronic palmoplantar pustular psoriasis" but now considered a distinct disease in up to 80% of cases 2
  • Associated with psoriasis in only about 20% of cases 2

3. Acrodermatitis Continua of Hallopeau (Acropustulosis)

• Clinical presentation: Intense pustular inflammation centered around terminal phalanges 2
• Complications:
  • Can destroy the nail plate 2
  • Often associated with destructive arthritis of adjacent joints 2
  • Patients are at risk of developing generalized pustular psoriasis 2
• Now considered: A subset of palmoplantar pustulosis by some experts 5

Diagnostic Approach

Clinical Diagnosis

1. Skin examination:

   • Look for sterile pustules on erythematous background
   • Assess distribution pattern (generalized vs. localized)
   • Evaluate for nail involvement (pitting, onycholysis, subungual hyperkeratosis, oil-drop sign) 2

2. Associated findings:

   • Check for pre-existing plaque psoriasis (may be present before, during, or after pustular episodes) 2
   • Assess for joint involvement (up to 90% of psoriatic arthritis patients have nail changes) 2

Laboratory Investigations

• Pustule examination: Confirm sterility through microbiological testing
• Inflammatory markers: Assess severity through CRP, ESR, and WBC count 3
• Streptococcal infection: In guttate forms, investigate for streptococcal infection 2

Histopathology

• Key features: Spongiform pustules of Kogoj, neutrophilic collections in stratum corneum
• Differential diagnosis: Rule out other pustular conditions through biopsy 3

Genetic Testing

• Emerging approach: Genetic screening for mutations in IL36RN, CARD14, AP1S3 genes 3
• Particularly useful: For familial and early-onset variants 6

Differential Diagnosis

The diagnosis of pustular psoriasis requires careful assessment to distinguish it from:

• Acute generalized exanthematous pustulosis (AGEP)
• Subcorneal pustular dermatosis
• Pustular drug eruptions
• Infectious causes of pustular rash 3

Common Diagnostic Pitfalls

• Misdiagnosis: Confusing palmoplantar pustulosis with palmoplantar psoriasis 2
• Delayed diagnosis: Due to rarity and heterogeneous presentation 3, 7
• Inadequate assessment: Failing to evaluate for systemic complications in GPP 4
• Treatment errors: Using TNF antagonists for palmoplantar pustulosis (may worsen condition) 2, 1

Management Considerations

Based on the diagnosis, treatment approaches vary:

• GPP: Often requires systemic therapy; acitretin is first-line with biologics (particularly IL-36 inhibitors like spesolimab) for severe cases 1, 4
• Localized forms: Topical therapies with moderately potent corticosteroids, coal tar, or dithranol 2, 1
• Acrodermatitis continua: May benefit from TNF antagonists (etanercept, infliximab, adalimumab) 2, 1

Early and accurate diagnosis is crucial, particularly for GPP, which can be life-threatening if left untreated. The diagnostic approach should be comprehensive, considering clinical presentation, histopathology, and increasingly, genetic factors.