Recognizing different types of nail fungus
Fungal infections of the nail plate or nail bed are collectively called onychomycosis or tinea unguium . These infections account for approximately one-half of all nail disease, and one-third of all fungal infections of the skin.1,2 Onychomycosis is becoming increasingly common worldwide, particularly in Western countries; approximately 10 % of the general public suffer from nail fungus. This increases to 50 % of individuals over the age of 70, and up to 1 in 3 individuals with diabetes.3 Elderly and diabetic individuals are both at increased for toenail fungal infections due to decreased blood supply to the feet and weakened immune systems.4,5 While not usually life-threatening, these infections can cause embarrassing disfigurement and pain, to the point of interfering with the quality of daily life. Fungal infections of the toenails also have the potential to spread to other parts of the body.
Understanding nail anatomy can help in understanding the different ways fungus can infect the nail. The growth center of the nail is called the matrix; the distal part of the matrix is visible as the light-colored half-moon at the bottom of the nail (the lunula). The remainder of the visible nail is called the nail plate.The nail plate grows by extending over the nail bed, the supportive layer of connective tissue underneath the nail. The most distal part of this nail bed is called the hyponychium.Toenails grow at a rate of only 1 mm per month, so it typically takes between 12 and 18 months to completely replace a great toenail that has been disfigured by fungal infection, and between 4 and 6 months for a little toenail.7 The rate of growth is slowed even further with increasing age; between the ages of 25 and 100, the rate of growth slows approximately 1% every two years,5 possibly due to decreased blood flow and decreasing circulating levels of human growth hormone (HGH). Given this slow rate of growth, prompt in the early stages of infection can significantly decrease the amount of time and money required to regain the appearance of a healthy nail.
Four main types of onychomycosis (nail fungus)
Toenail fungal infection is divided into four main types. The first three, distal subungual onychomycosis, proximal subungual onychomycosis and white superficial onychomycosis, are based primarily on where the infection starts in the nail. This can be at the nail tip (distal), at the base near the cuticle (proximal) or on the surface of the nail (superficial). The fourth subtype, candidal onychomycosis, is based on the infecting organism, candida. The below table helps to summarize the key characteristics of the four main types of toenail fungus.
Distal subungual onychomycosis
The most common type of toenail fungus is distal subungual onychomycosis (DSO) and distal subungual lateralonychomycosis (DLSO) . This infection is usually caused by Trichophyton rubrum, a fungus that also causes athlete’s foot and jock itch; most patients with distal toenail fungus have an accompanying athlete’s foot infection (also called tinea pedis). Individuals with a family history of toenail fungus may be more likely to develop this distal infection; researchers have discovered that follows an autosomal dominant (form of genetic inheritance) pattern within families.8
The fungus invades the nail bed under the nail, beginning at the distal portion (the hyponychium) and gradually growing inward toward the nail matrix at the base of the nail.2,9 The infection can also enter via the lateral folds on the side of the nail. As the infection spreads, the associated inflammation worsens. This can result in onycholysis, or separation of the nail from the nail bed. The fungal infection also results in thickening of the tissue under the nail, the subungual tissue and will often appear as yellow and “crumbly”. This thickened tissue serves as a breeding ground for bacteria and molds, resulting in the typical yellow or brown appearance.10
The chart above depicts the appearance of a distal fungus infection of the toenail with lateral entry. The affected portion of the nail is thickened and yellow-brown in color, with irregular erosion of the nail tip. If left untreated, the infection is likely to spread throughout the entire nail.
Proximal subungual onychomycosis
Proximal subungual onychomycosis (PSO) is a relatively uncommon form of nail fungus in the general population. As opposed to distal nail infections, proximal toenail fungus starts near the cuticle, invades the newly formed nail, and grows outward toward the tip of the nail. The fungus Trichophyton rubrum is also the most common agent responsible for proximal nail infections, although the association with concurrent athlete’s foot is not as strong. Proximal fungal infection can be seen in the setting of trauma (damage) to the cuticle or proximal nail.11 Unlike distal toenail fungus infections, the surface of the nail usually remains smooth and regular. However, because the infection involves the proximal nail near the matrix, or source of new nail tissue, severe proximal fungal infections have the potential to destroy the entire nail unit.
While proximal nail fungus infections are uncommon in the general population, they are seen commonly in patients with HIV/AIDS. In one study of patients with both AIDS and onychomycosis, nearly all (88.7%) had the proximal subungual subtype.12 The association is so strong that many doctors consider proximal nail fungus infection as an early sign of HIV infection, especially in younger patients. PSO can also occur in those who have other factors that lead to a compromised immune system, such as diabetics and those taking antibiotics over long periods.
The above table depicts proximal fungal infection in a toenail. An area of whitish or yellowish discoloration develops in the proximal nail in the region of the lunula (half-moon). The nail remains fairly thin and regular.
White superficial onychomycosis
White superficial onychomycosis (WSO) accounts for approximately 10% of nail fungus cases.9 The most common causative agent is a fungus called Trichophyton mentagrophytes var interdigitale. These fungi directly invade the superficial layers of the nail. Because the infection predominantly involves the superficial nail and not the underlying connective tissue, the degree of associated inflammation is relatively mild.7
There is a less common presentation of WSO consisting of a diffuse (involving the entire toenail) and deep infection of the nail. This is usually associated with infection by more aggressive molds including Fusarium sp. and Aspergillus sp, in otherwise healthy adults, or with infection by Trichophyton rubrum in children and immunocompromised individuals with HIV/AIDS.13 This subtype is no longer considered a superficial infection, and has the ability to progress to total dystrophic onychomycosis (see below).
White superficial toenail fungus is characterized by opaque white patches on the nail. Each of these white patches represents a separate colony of fungi.13 As the infection progresses, the patches can grow together and involve the entire nail, which becomes soft and crumbly.9
Candida is a blanket term for a number of related species of yeast, a type of fungus. These yeast live on the skin and mucus membranes (the moist linings of cavities such as the mouth and vagina), and typically do not cause problems. However, in some individuals, the yeast become so numerous that they cause infections, known collectively as candidiasis. Individuals with chronic (long-lasting) candidiasis of the skin and mucus membranes are at risk for developing candida infections of the nails. Candidal infection most commonly involves the fingernails, although the toenails can also be affected. Yeast infections of the nails are more common in people who frequently immerse their hands or feet in water.14
The most common form of candidal nail infection begins as an infection of the soft tissue structures surrounding the nail; this swollen, reddened pad is known as a paronychia.15 The nail becomes involved only after the infection spreads from the soft tissues into the adjacent nail plate. When the nail matrix at the base of the nail becomes infected, transverse grooves (Beau’s lines) can appear, and the nail becomes irregular and rough.7
A second form of candidal toenail infection is much less common, accounting for fewer than 1% of fungal onychomycosis.7 Like proximal subungual onychomycosis, this infection is seen primarily in immunocompromised patients (patients with poorly functioning immune systems, like those with HIV/AIDS). In this setting, the yeast can directly invade the nail plate without first involving the surrounding tissues. The affected toes can start to look rounded on the end, like a “chicken drumstick”.9
The table above depicts a candidal toenail infection, with edematous (swollen) surrounding soft tissues and a markedly thickened irregular toenail.
Total dystrophic onychomycosis
All of the four subtypes of toenail fungus can result in an end-stage disease known as total dystrophic onychomycosis, which involves the entire nail unit. In dystrophic onychomycosis, the nail matrix (source of new healthy nail tissue) may become permanently scarred, and the nail plate can be completely destroyed.
This image depicts total dystrophic onychomycosis of the great toenail. Note involvement of the entire nail, which has a shrunken, dystrophic (misshaped) appearance.
Toenail fungus: final thoughts
If you have a toenail or toenails that are thickened and irregular, or have an abnormal color, particularly yellow-brown or white, this may indicate a fungal infection.
Hopefully, this overview has given you a better idea of what causes toenail fungus, and what might be growing on your toenail. It is important to note that ignoring toenail fungus will not make it go away; fungal infections do not resolve on their own without treatment. Prompt intervention with agents such as can help to stop the insidious spread of toenail fungus, and speed the reappearance of a healthy toenail.
1. Scher RK, Coppa LM. Advances in the diagnosis and treatment of onychomycosis. Hosp Med. 1998;34:11–20.
2. Crissey JT. Common dermatophyte infections. A simple diagnostic test and current management. Postgrad Med. 1998;103(2):191–1,197–200,205.
3 Thomas J, , Narkowicz CK et al. Toenail onychomyocosis : an important global disease burden. J Clin Pharm Ther. 2010 Oct;35(5):497-519.
4. Gupta AK, Konnikov N, MacDonald P et al. Prevalence and epidemiology of toenail onychomycosis in diabetic subjects: a multicentre survey. Br J Dermatol. 1998 Oct;139(4):665-71..
5. Singh G, Haneef N, A Uday. Nail changes and disorders among the elderly. Ind J Dermatol Venerol Leprol. 2005;71:386-92
6. Clinical Courier. New strategies for the effective management of superficial fungal infections. Clin Courier. 1997;16:2–3.
7. Cohen J L, Scher R K, Pappert A S. The nail and fungus infections. In: Elewski B, editor. Cutaneous fungal infections. New York, N.Y: Igaku-Shoin Inc.; 1992. pp. 106–122.
8. Scher RK, Coppa LM. Advances in the diagnosis and treatment of onychomycosis. Hosp Med. 1998;34:11–20.
9. Elewski BE. Onychomycosis: pathogenesis, diagnosis, and management. Clin Microbiol Rev. 1998;11:415–29.
10. Cohen J L, Scher R K, Pappert A S. The nail and fungus infections. In: Elewski B, editor. Cutaneous fungal infections. New York, N.Y: Igaku-Shoin Inc.; 1992. pp. 106–122.
11. Rodgers P, Bassler, M. Treating Onychomycosis. Am Fam Physician. 2001 Feb 15;63(4):663-673.
12. Dompmartin D, Dompmartin A, Deluol A M, Grosshans E, Coulaud J P. Onychomycosis and AIDS: clinical and laboratory findings in 62 patients. Int J Dermatol. 1990;29:337–339.
13. Piraccini B, Tosti A. White Superficial Onychomycosis: Epidemiological, Clinical, and Pathological Study of 79 Patients. Arch Dermatol. 2004;140(6):696-701.
14. Evans EG. Causative pathogens in onychomycosis and the possibility of treatment resistance: a review. J Am Acad Dermatol. 1998;38(5 pt 3):S32–56.
15. Scher R K. Diseases of the nails. In: Conn H, editor. Current therapy. Philadelphia, Pa: The W. B. Saunders Co.; 1990. pp. 736–742.
16. Singal A, Khanna D. Onychomycosis: Diagnosis and management. Indian J Dermatol Venereol Leprol 2011;77:659-72
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