How do you differentiate between psoriatic nail changes and fungal infections?

Differentiating Psoriatic Nail Changes from Fungal Infections

The most reliable way to differentiate between psoriatic nail changes and fungal infections is through laboratory confirmation with KOH preparation or fungal culture, as clinical features alone are insufficient for accurate diagnosis. 1

Clinical Features to Observe

Psoriatic Nail Changes

• Pitting: Irregular, deep pits on the nail surface (hallmark sign) 1, 2
• Oil drop sign: Salmon-colored patches visible through the nail plate 2
• Onycholysis: Separation of nail from nail bed with erythematous border 2
• Subungual hyperkeratosis: Thickening under the nail 2
• Red spots in lunula: Reddish discoloration of the half-moon area 3
• Crumbling: Destruction of nail plate 3
• Splinter hemorrhages: Thin linear bleeding under the nail 2
• Distribution pattern: Often affects multiple nails symmetrically 2
• Associated findings: Look for psoriatic skin lesions elsewhere on the body 3

Fungal Nail Infections (Onychomycosis)

• Distal and lateral subungual onychomycosis (DLSO): Most common form, starts at distal/lateral edges 1
• Superficial white onychomycosis (SWO): White, crumbly patches on nail surface 1
• Proximal subungual onychomycosis (PSO): Starts at proximal nail fold, rare except in immunocompromised patients 1
• Total dystrophic onychomycosis (TDO): Complete destruction of nail plate 1
• Distribution pattern: Often asymmetric, may affect isolated nails 1
• Color: Usually yellow-brown discoloration without erythematous border 1

Diagnostic Approach

1. Visual examination is insufficient:

   • Up to 50% of nail dystrophies are non-fungal in origin 4
   • Psoriatic nails can be clinically indistinguishable from onychomycosis, especially in toenails 3

2. Laboratory confirmation is essential:

   • KOH preparation: Direct microscopic examination of nail clippings 1, 4
   • Fungal culture: Takes 2-6 weeks but identifies specific pathogens 4
   • PCR testing: Higher sensitivity but may detect non-pathogenic fungi 4

3. Proper sampling technique:

   • Take material from discolored or dystrophic parts of the nail 4
   • Cut affected nail as far back as possible 4
   • Include crumbly material from under the nail 4

Important Considerations

• Coexistence is possible: Psoriatic patients have similar prevalence of onychomycosis as the general population (13.1% vs 7.9%), but are more susceptible specifically to dermatophyte infections 5, 6

• Diagnostic pitfalls:

  • Assuming all nail changes in psoriatic patients are due to psoriasis 5
  • Treating empirically without laboratory confirmation 1, 4
  • Failing to consider secondary fungal infection in psoriatic nails 5, 6

• When to suspect both conditions:

  • Asymmetric nail involvement in a patient with psoriasis 2
  • Poor response to psoriasis treatment 6
  • Change in clinical appearance of previously stable psoriatic nail disease 3

Management Implications

• Correct diagnosis is crucial as treatment approaches differ significantly:

  • Psoriatic nail disease: Topical corticosteroids, vitamin D analogs, systemic treatments including biologics 2
  • Fungal infections: Antifungal medications (topical or oral) 1, 4
  • Combined disease: May require both anti-inflammatory and antifungal therapy 6

• Misdiagnosis leads to:

  • Delayed appropriate treatment
  • Unnecessary medication exposure
  • Progressive nail damage and potential permanent deformity 3, 2

When in doubt, laboratory confirmation should always be obtained before initiating treatment, as clinical features alone cannot reliably distinguish between these conditions.