How to diagnose onychomycosis?

Diagnosis of Onychomycosis

The diagnosis of onychomycosis requires laboratory confirmation through mycological testing before initiating treatment, as clinical features alone cannot reliably distinguish fungal nail infections from other nail disorders. 1

Clinical Presentation and Initial Assessment

Onychomycosis presents with several characteristic clinical patterns:

• Distal and Lateral Subungual Onychomycosis (DLSO):

  • Most common presentation
  • Toenails more commonly affected than fingernails
  • Thickened and discolored nail with onycholysis
  • Fungus invades from distal/lateral margins 1

• Superficial White Onychomycosis (SWO):

  • Crumbling white lesions on nail surface
  • More common in children
  • Usually caused by T. interdigitale 1

• Proximal Subungual Onychomycosis (PSO):

  • Infection originates at proximal nail fold
  • Distal portion remains normal until late stages
  • Common in persons with AIDS (potential marker of HIV infection) 1

• Total Dystrophic Onychomycosis (TDO):

  • Advanced stage where nail plate is almost completely destroyed 1

Key Clinical Features Suggesting Onychomycosis

• Nail discoloration (yellow-brown)
• Nail plate thickening
• Subungual debris
• Onycholysis (separation of nail from nail bed)
• Brittle, crumbly nail texture 2, 3

Diagnostic Testing

Essential Laboratory Investigations

1. Direct Microscopic Examination:

   • Potassium hydroxide (KOH) preparation
   • Calcofluor white staining (preferred over KOH due to higher sensitivity) 1

2. Fungal Culture:

   • Identifies specific pathogen
   • Takes 2-6 weeks for results
   • Important for distinguishing dermatophytes from non-dermatophyte molds 1, 4

3. Histopathology:

   • Periodic acid-Schiff (PAS) staining of nail clippings
   • High sensitivity for fungal elements 3

4. Molecular Diagnostic Methods:

   • Polymerase chain reaction (PCR)
   • Faster turnaround time (<2 days)
   • Higher detection rate than culture
   • May detect non-pathogenic fungi 1, 5

Proper Specimen Collection

• Take material from discolored, dystrophic, or brittle parts of the nail
• Cut affected nail as far back as possible through entire thickness
• Include crumbly material
• Use nail drills, scalpels, or nail elevators when necessary (must be sterilized between patients)
• For superficial involvement, take nail scrapings with a curette
• If skin lesions are present, sample these as well 1

Newer Diagnostic Approaches

• Dermoscopy: Cost-effective, non-invasive method to observe microscopic features of onychomycosis 5
• Reflectance Confocal Microscopy: Allows observation of hyphae at near-histologic resolution 5

Differential Diagnosis

Several conditions can mimic onychomycosis:

• Psoriasis: Nail pitting, oil spots, onycholysis
• Chronic trauma: Distal onycholysis without fungal elements
• Lichen planus: Thinning of nail plate, dorsal pterygium
• Yellow nail syndrome: Green-yellowish pigmentation, hardness
• Bacterial infection: Green or black discoloration (especially Pseudomonas)
• Onychogryphosis: Thickened, curved nails
• Subungual malignant melanoma: Pigmented lesions
• Darier disease: Characteristic nail changes 1

Distinguishing Features

• In fungal infections, the nail surface becomes soft and friable
• In trauma, examination of the nail bed shows intact longitudinal epidermal ridges
• In bacterial infection (Pseudomonas), green discoloration is common 1

Common Pitfalls and Caveats

1. Failure to confirm diagnosis before treatment: Always perform laboratory testing before initiating antifungal therapy, as treatments are lengthy and can cause adverse effects 2

2. Misdiagnosis of non-dermatophyte molds: These require multiple confirmations and repeated sampling to establish as causative agents 6

3. Overlooking mixed infections: Combinations of dermatophytes, yeasts, and non-dermatophyte molds are more common than previously thought 6

4. Treating based on clinical appearance alone: Many conditions mimic onychomycosis, leading to inappropriate treatment 1, 3

5. Inadequate sampling: Poor specimen collection can lead to false-negative results 1

By following this diagnostic approach, clinicians can accurately diagnose onychomycosis and select appropriate treatment, improving outcomes and quality of life for affected patients.