Onychomycosis

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Onychomycosis means fungal infection of the nails. It represents up to 20% of all nail disorders.

This condition may affect toe- or fingernails, but toenail infections are particularly common. The most common type of onychomycosis (80-90%), caused by dermatophytes, is technically known as tinea unguium (tinea of the nails).^[1] It can result in discoloration, thickening, chalkiness, or crumbling of the nails and is often treated by powerful oral medications which, rarely, can cause severe side effects including liver failure.

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief:

• There is limited information about the historical perspective of onychomycosis.

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Onychomycosis may be classified according to clinical appearance of the nail into 5 subtypes

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Onychomycosis may be classified according to clinical appearance of the nail into 5 subtypes.^[1]

• Distal Subungual Onychomycosis : The most common form of tinea unguium usually caused by Trichophyton rubrum, which invades the nail bed and the underside of the nail plate.
• White Superficial Onychomycosis: Caused by fungal invasion of the superficial layers of the nail plate to form “white islands” on the plate. Accounts for only 10 percent of onychomycosis cases.
• Proximal Subungual Onychomycosis: Fungal penetration of the newly formed nail plate through the proximal nail fold. It is the least common form of tinea unguium in healthy people but found more commonly when the patient is immunocompromised.
• Endonyx subungual Onychomycosis : Fungal penetration through the full thickness of the nail from directly under the skin. The nail bed is not infected. Commonly found in immunocompromised conditions.
• Total Dystrophic Onychomycosis: Total destruction of the nail plate. It is the end result of any of the above four types.

Candidal Onychomycosis has been excluded as a separate type.

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The pathophysiology of onychomycosis depends on the histological subtype.

Onychomycosis is usually preceded by a dry hyperkeratotic tinea pedis.Moist environment and micro traumatic pressure on the nail unit break the hyponychial seal, which allows penetration of the dermatophyte into the nail bed. The dermatophytes producekeratinases that begin the infection between the lesser toes, spread to the hyperkeratotic sole, and gradually extend to the distal hyponychial space of micro-traumatized nail units. Once the distal nail hyponychium is breached, the dermatophytes infect the nail bed, spreading proximally as onycholysis and subungual hyperkeratosis.

The acute infection occurs in the nail bed with a low-grade inflammatory response and progresses to a chronic phase as total dystrophic onychomycosis. Onychomycosis secondarily damages the viable nail matrix and invades the overlying nail plate, detaching and distorting it over time. The nail plate becomes elevated and misaligned as the infection becomes chronic. Studies show high levels of cytokines IL-6- and IL-10-positive cells in few cases.

Distal subungual onychomycosis (DSO)is characterized by invasion of the nail bed and underside of the nail plate beginning at the hyponychium .^[1]

Proximal subungual onychomycosis (PSO) occurs when organisms invade via the proximal nail fold through the cuticle area. It migrates distally through newly formed nail plate.

White superficial onychomycosis (WSO) occurs when certain fungi invade the superficial layers of the nail plate directly. It forms well-delineated opaque white islands on the external nail plate. As the disease progresses, they coalesce and spread. The nail becomes rough, soft, and crumbly. As viable tissue is not involved, inflammation is minimal is seen. The toenails are primarily involved.

Total dystrophic onychomycosis may be the end result of any of the four main patterns of onychomycosis. The entire nail unit becomes thick and dystrophic.

Candida onychomycosis can be divided into three general categories.

(i)Infection (whitlow) begins as a paronychia .Invasion by Candida spp., unlike dermatophytic invasion, penetrates the nail plate only secondarily after it has attacked the soft tissue around the nail . After the nail matrix gets infected, transverse depressions (Beau’s lines) may appear in the nail plate. The nail becomes convex, irregular, and rough and, ultimately, dystrophic.

(ii)Candida granuloma is common in patients with chronic mucocutaneous candidiasis and immunocompromised patients. The organism invades the nail plate directly and may affect the entire thickness of the nail, resulting in swelling of the proximal and lateral nail folds, ultimately giving the pseudo-clubbing or “chicken drumstick” appearance to the digit .

(iii)Candida onycholysis can occur when the nail plate has separated from the nail bed. Distal subungual hyperkeratosis can be seen as a yellowish gray mass lifts off the nail plate, mostly in hands. The lesion resembles that seen in patients with DSO.

Histologically, the acute lesion resembles psoraisis and manifests as spongiosis, acanthosis, papillomatosis with edema, hyperkeratosis and a dense inflammatory infiltrate. At the chronic stage of the infection, there are large amounts of compact hyperkeratosis, hypergranulosis, acanthosis, onychylosis and papillomatosis with sparse perivascular infiltrate. Dermatophytosis and subungual seromas can occur.

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Common causes of onychomycosis include dermatophytes.

Dermatophytes are the fungi most commonly responsible for onychomycosis.^[1]

Two dermatophyte species, Trichophyton rubrum and Trichophyton interdigitale, cause the vast majority of onychomycosis cases worldwide.

Other related dermatophyte fungi that may be involved are Epidermophyton floccosum, Trichophyton violaceum, Microsporum gypseum, Trichophyton tonsurans, Trichophyton soudanense (considered by some to be an African variant of T. rubrum rather than a full-fledged separate species) and the cattle ringworm fungus Trichophyton verrucosum. A common outdated name that may still be reported by medical laboratories is Trichophyton mentagrophytes for T. interdigitale.

The name T. mentagrophytes is now restricted to the agent of favus skin infection of the mouse; though this fungus may be transmitted from mice and their danders to humans, it generally infects skin and not nails.

Other causal fungi include yeasts (5-17%), e.g., Candida, and non-dermatophytic moulds, in particular members of the mould genera Scytalidium (name recently changed to Neoscytalidium), Scopulariopsis, and Aspergillus.

Yeasts mainly cause fingernail onychomycosis in people whose hands are often submerged in water. Scytalidium mainly affects people in the tropics, though it persists if they later move to areas of temperate climate.

Other moulds mainly affect people over the age of 60, and their presence in the nail reflects a slight weakening in the nail’s ability to defend itself against fungal invasion.

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Onychomycosis must be differentiated from other diseases that cause discoloration and change in texture of the nail, such as psoriasis, and other causes.

The differential diagnosis includes

1. Inflammatory disorders such as psoriasis and lichen planus;
2. Genetic disorders such as pachyonychia congenita;
3. Nail malignancies,
4. Nail trauma
5. Drugs like antiretrovirals, cytotoxic drugs.
6. Bacterial infections
7. Systemic diseases like liver failure, uremia , connective tissue disorders, etc

A nail unit biopsy may be needed to confirm diagnosis.

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

The incidence/prevalence of onychomycosis is approximately 6- 8% worldwide.

The prevalence of onychomycosis is about 6-8% in the adult population.^[1].The prevalence of onychomycosis in the United States is around 12%; and increases with increasing age. Approximately 90% of cases of toenail onychomycosis is caused by Trichophyton rubrum.^[2]

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Common complications of onychomycosis include infections deeper to the nails.

Onychomycosis including adjacent skin injury may allow colonization of the organisms , thereby increasing the risk of infectious complications. Complications are more common in immunocompromised patients, diabetics and elderly people. Most common complications are cellulitis, osteomyelitis, sepsis, and tissue necrosis