What is the medical term for toenail fungus?

Onychomycosis: The Medical Term for Toenail Fungus

Toenail fungus is medically known as onychomycosis, which is a fungal infection of the nail apparatus caused by various pathogens including dermatophytes, nondermatophyte molds, and yeasts. 1

Types of Onychomycosis

Onychomycosis presents in several clinical patterns:

1. Distal and Lateral Subungual Onychomycosis (DLSO)

   • Most common presentation (80% of cases)
   • Affects the hyponychium, often starting at lateral edges
   • Spreads proximally along the nail bed
   • Results in subungual hyperkeratosis and onycholysis
   • Toenails more commonly affected than fingernails
   • Usually caused by Trichophyton rubrum 1

2. Superficial White Onychomycosis (SWO)

   • Infection begins at the superficial layer of the nail plate
   • Presents as crumbling white lesions on the nail surface
   • More common in children
   • Usually caused by T. mentagrophytes/interdigitale 1

3. Proximal Subungual Onychomycosis (PSO)

   • Uncommon in general population
   • More common in immunocompromised patients (especially HIV-positive)
   • Infection originates in proximal nail fold or beneath proximal nail plate
   • Distal portion remains normal until late stages 1

4. Endonyx Onychomycosis

   • Fungus invades the nail plate directly
   • Nail appears milky white without significant thickening 1

5. Total Dystrophic Onychomycosis (TDO)

   • Advanced stage where nail plate is almost completely destroyed
   • Can result from progression of any other type 1

Causative Organisms

• Dermatophytes: Responsible for over 90% of toenail infections

  • Trichophyton rubrum (most common)
  • Trichophyton mentagrophytes/interdigitale
  • Trichophyton soudanense
  • Trichophyton violaceum 1

• Yeasts (Candida species):

  • More common in fingernail infections
  • Usually associated with chronic paronychia
  • More common in people with wet occupations 1

• Nondermatophyte molds:

  • Less common causative agents
  • Include Scopulariopsis, Aspergillus, Fusarium species 2

Clinical Significance

Onychomycosis is not merely a cosmetic concern. It can cause:

• Pain and discomfort
• Physical impairment
• Difficulty walking and wearing shoes
• Emotional embarrassment
• Work-related difficulties
• Damage to socks and stockings due to friction with dystrophic nails 1, 3

Diagnosis

Accurate diagnosis is essential before initiating treatment as therapy is lengthy and can cause adverse effects. Diagnostic methods include:

• Potassium hydroxide (KOH) preparation
• Fungal culture
• Periodic acid-Schiff (PAS) stain
• Polymerase chain reaction (PCR) testing 3

Common Pitfalls

1. Misdiagnosis: Not all nail dystrophy is fungal in origin. About 50% of nail dystrophies are caused by fungi, while others may be due to psoriasis, lichen planus, or trauma.

2. Treatment without confirmation: Treatment should not be instituted on clinical grounds alone, as therapy is lengthy and potentially has side effects 1.

3. Failure to identify the causative organism: Different fungi may respond differently to treatments.

4. Confusing secondary colonizers with primary pathogens: Nondermatophyte molds can be contaminants rather than causative agents 4.

Onychomycosis is a chronic fungal infection that requires proper diagnosis and targeted treatment. Understanding the specific type and causative organism helps in selecting the most appropriate therapeutic approach.