Diagnosing Onychomycosis

Onychomycosis is caused by dermatophytes, yeasts, and molds. Dermatophytes such as Trichophyton rubrum and Trichophyton mentagrophytes cause 91.3% of cases; yeasts such as Candida albicans cause 0.7% (usually fingernail infections); and molds such as Scopulariopsis, Scytalidium, Acremonium, and Fusarium cause 8% of cases. (Elewski, 1997)

There are four types of onychomycosis:

• Distal and/or lateral subungual onychomycosis affects the nail bed and nail plate.
  
  
• Proximal subungual onychomycosis affects the proximal nail fold, with infection extending distally under the nail plate.
  
  
• Superficial white onychomycosis affects the top of the nail plate.
  
  
• Candidal onychomycosis affects the nail, skin, and mucous membranes.
  (Elewski et al, 1996)

Onychomycosis may present clinically in a manner that is similar to other nail disorders (see Differential Diagnosis) making it essential to differentiate such nail disorders from fungal infections. The correct diagnosis of onychomycosis is essential for appropriate antifungal therapy.

A KOH test is the most direct, cost-effective, and simplest technique to diagnose onychomycosis. (Mehregan et al, 1999) Another approach is fungal culture, in which subungual debris is cultured on Sabouraud's dextrose agar containing cycloheximide to inhibit the growth of nondermatophytes. Genus and species identification of pathogens are determined from colonial morphology. Dermatophyte Test Medium can be inoculated with subungual debris. A color change in the medium in 7 to 10 days indicates the presence of a dermatophyte. (Jaffe, 1998)

If required, a nail biopsy, in which a piece of the nail is sectioned and stained with periodic acid-Schiff (PAS), can be performed.

Clinical conditions that can resemble onychomycosis include (Ashtion et al, 1998)

• Thickening of the nail bed or plate, which may be caused by conditions other than onychomycosis, such as lichen planus, pachyonychia congenita, pityriasis rubra pilarus, and psoriasis. All of these disease states involve hyperkeratosis that results in thickened nails.
  
  
• Separation of the nail plate, also called onycholysis. Conditions other than onychomycosis that produce onycholysis include anemia, proximal candidiasis, carcinoma of the lung, circulatory disorders, endocrine disease, iron deficiencies, minor trauma, pregnancy, psoriasis, and syphilis.
  
  
• Discoloration of the nails, which may be due to other conditions. Black to dark brown nails may be produced by melanoma, nevi, or trauma. Green coloration may be due to Pseudomonas infection, but may also be due to Aspergillus or Blastomyces infection. White discoloration may have many causes, including autoimmune, cardiac, gastrointestinal, or renal diseases, metabolic disorders, neoplasia, psoriasis, stress, or trauma. Yellow nail syndrome may be associated with bronchitis, diabetes, arthritis, or hyperthyroidism.
  
  
• Nail pain may be produced by trauma, inflammatory diseases (such as gout, herpes, and paronychia), and vascular disorders (such as acute ischemia and systemic sclerosis).
  
  
• Transverse layering/lamellar splitting, also called onychoschizia, can occur as the result of repeated cycles of nail hydration and drying.
  
  
• Ridging of the nail plate (longitudinal and transverse) gives a beaded appearance. Longitudinal grooves may be caused by Darier-White disease, lichen planus, peripheral circulatory disorders, or rheumatoid arthritis.
  
  
• Pitting of the nails may be found in alopecia areata, eczema, or psoriasis, and in some forms of lichen planus. Pitting is usually confined to the fingernails, occurring rarely on the toenails.

There are several systemic disorders that may present clinically in a form that resembles onychomycosis:

• Argyria is a slate-gray or bluish discoloration of the skin due to deposits of silver albuminate that occurs after long-term administration of medicinal soluble silver salts.
  
  
• Drugs that may affect the nails include bleomycin, doxorubicin, 5-fluorouracil, retinoids, captopril, chlorpromazine, chloramphenicol, tetracyclines, psoralen phototherapy agents, thiazide, diuretics, and quinine. Intermittent cycles of chemotherapy may cause multiple transverse lines called Beau's lines.
  
  
• Clubbing of the nails may result from underlying pulmonary and other visceral diseases.
  
  
• Koilonychia, which causes the nail to have a concave appearance, is associated with anemia and may also be seen in onychomycosis.
  
  

A KOH (potassium hydroxide) test is the most direct, cost-effective, and simplest technique to diagnose onychomycosis. (Mehregan et al, 1999) Conduct the test in the following manner:

1. Cleanse the nail with alcohol to eliminate any bacterial contamination.
2. Using a small curette or No. 15 scalpel, remove any subungual debris.
3. Firmly scrape the undersurface of the affected nail plate, cutting as close to the leading edge as possible, to obtain a sample of the fungal debris.
4. Place specimen (if nail clippings, first break into small pieces) on a slide with 1 drop of 10% to 20% KOH solution. Note: Stain such as Chlorazol black E (specific for chitin) may be added to help accentuate hyphae.
5. Wait 30 minutes.
6. Examine under microscope on low power and reduced light.
7. Test is positive if hyphae with a "boxcar" or "bamboo" appearance are present; the hyphae are generally larger and wider than skin cell membranes and will traverse several cells.

If you are sending a specimen to a lab for a KOH test, complete steps 1 through 3, and then follow the lab's instructions for shipping.

Most managed care organizations require a KOH test. If the test is negative and onychomycosis is still suspected, a nail culture should be performed.