• Onychomycosis is classified based on route of infection & clinical presentation

Distal & Lateral Subungual Onychomycosis (DLSO)

• Most common type, 90% of infections
• Invasion occurs at distal subungual space & at the distal lateral groove
  
  • More commonly involves the toenails

• Nail plate looks opacified & hyperkeratotic, its distal portion may break away
• Splinter hemorrhages are seen secondary to mild inflammation compressing small blood vessels
• Most common cause: Trichophyton rubrum
  
  • May also be caused by: Trichophyton tonsurans, Trichophyton mentagrophytes & Epidermophyton floccosum

Superficial White Onychomycosis (SWO)

• Second most prevalent type, 10% of infections
• Dorsal surface of the nail plate is invaded
• Minimal inflammation; nail becomes soft & crumbly w/ a white-colored rough surface
  
  • Nail plate is not thickened & remains attached to the nail bed

• Typically caused by T mentagrophytes but may also be caused by non-dermatophyte molds

Proximal Subungual Onychomycosis

• Least common form in healthy individuals & may be an early indicator of human immunodeficiency virus (HIV) infection
• Toenails are more commonly infected than fingernails
• Infection enters at the cuticle & the proximal nail bed is always involved
  
  • Spreads distally so if left untreated, the entire nail plate will be affected

• Nail plate is typically white in color
  
  • Subungual hyperkeratosis & proximal onychomycosis are usually present

• Usually caused by T rubrum but can be caused by Trichophyton megnini, T tonsurans & T mentagrophytes

Candidal Onychomycosis

• Commonly found in patients diagnosed w/ chronic mucocutaneous candidiasis
  
  • More commonly affects the fingernails

• Candida invades the nail bed or plate directly or may indirectly involve the nail through infecting the nail bed, folds or hyponychium
• Entire nail thickens & turns yellow-brown; progression may lead to total dystrophy of the nails
• Usually caused by Candida albicans or other Candida sp
  
  • May have some resistance to oral antifungal agents

Total Dystrophic Onychomycosis

• Most advanced form
• Nail plate is almost completely destroyed
• May be the end result of the above four forms

Onychomycosis typically requires prolonged high-cost therapy; thus, the diagnosis should be confirmed before treatment is started

Microscopy

• Specimen is mounted in 10-15% Potassium hydroxide (KOH) soln w/ calcofluor & heated gently, viewed under light microscope, & examined for hyphae & arthrospores
  
  • Scrapings of nail bed for distal subungual onychomyosis & scrapings from the nail plate white spots for superficial white onychomycosis
  • Calcofluor may decrease risk of false negative results

• Presence of fungal infection can be detected but the causative organism is not specified

Culture

• Adequate sample is inoculated onto Saboraud’s glucose agar w/ or w/o addition of antibiotics
• Identifies organism & the specific etiologic agent so that treatment can be individualized
• Considered the mainstay of onychomycosis diagnosis

Histology

• Periodic acid-Schiff (PAS) staining w/ histologic exam of the clipped, distal, free edge of the nail & attached subungual debris
  
  • Most sensitive diagnostic method

• PAS staining of nail plate biopsy or removed nail w/ culture is required to diagnose proximal subungual onychomycosis