The nail, or nail unit, consists of the following parts:
1. The nail matrix is where nail cells multiply and keratinize (harden and form into nail material) before being incorporated into the fingernail or toenail.
Most of the matrix is not visible;
The matrix starts under the skin 5 mm below the nail fold (the area of the cuticle where the finger or toe skin meets the nail) and covers the area called the lunula, or half moon (the white half moon-shaped area at the bottom of the nail);
2. The cuticle is a fold of modified skin where the finger or toe meets the nail;
The cuticle protects the matrix from infection;
3. The nail plate is the nail itself;
4. The nail bed is the soft tissue underneath the nail, anchoring the nail plate;
The nail plate protects the nail bed.
What Are the Subtypes of Fungal Nail Infections?
1. Distal lateral subungual onychomycosis (DLSO) is the most common form of fungal nail infection.
In DLSO, the fungus generally spreads from the skin and invades the underside of the nail where the nail meets the nail bed.
Inflammation in these areas of the nail causes the symptoms of DLSO;
2. White superficial onychomycosis (WSO) is a rare infection caused by the fungi directly invading the surface of the nail plate and secondarily infecting the nail bed;
3. In proximal subungual onychomycosis (PSO), the least common subtype, the fungi invade the cuticle (the skin around the nail) and the nail fold and then penetrate the nail plate (fingernail or toenail);
4. Like DLSO, in endonyx onychomycosis (EO), the fungi reach the nail via the skin. Instead of infecting the nail bed, however, the fungi immediately invade the nail plate;
5. Fungal nail infection related to yeast (Candida) infection is a little different from fungal nail infection related to other fungal infections;
Candidal fungal nail infection has several characteristics:
Onycholysis describes the nail separating from the nail bed.
Chronic mucocutaneous disease (disease of mucous membrane and regular skin) involves the nail plate (fingernail or toenail) and eventually the nail fold (the skin fold behind the cuticle, where the nail meets the finger or toe);
6. Total dystrophic onychomycosis is not a distinct subtype of fungal nail infection;
Dystrophic onychomycosis is the term used to describe the most advanced form of any of the above subtypes, and it involves the entire nail unit;
Dystrophic onychomycosis may cause permanent scarring of the nail matrix.
What Causes a Fungal Nail Infection?
Fungal nail infection is caused by three main classes of organisms: fungi that infect hair, skin, and nails and feed on nail tissue, yeasts, and non dermatophyte molds.
All three classes cause the very similar early and chronic symptoms or appearances, so the visual appearance of the infection may not reveal which class is responsible for the infection.
Dermatophytes (including Epidermophyton, Microsporum, and Trichophyton species) are, by far, the most common causes of fungal nail infection worldwide.
