Nail mycosis: what do we know about it?

Onychomycosis, the medical term for fungal nail infection, is the most common nail disorder worldwide.

Although its overall prevalence is estimated at 5.5%, it is strongly correlated with age. While it affects few children, it concerns more than 20% of adults over the age of 60. This infection is not merely a cosmetic inconvenience; it can cause local pain, discomfort when walking, and a significant social impact. Nail fungus usually develops slowly and insidiously. Unlike trauma, which causes a sudden haematoma, onychomycosis gradually alters the structure of the nail. The most common symptoms include:

• Change in colour : The nail loses its natural transparency and takes on a whitish, yellowish or brownish hue. In rarer cases, green or black spots may appear.

• Thickening, that is to say hyperkeratosis : The nail bed accumulates keratinous debris, making the nail plate abnormally thick and difficult to cut.

• Fragility : The nail becomes brittle, crumbles at the edges, or splits into layers.

• Onycholysis : The nail gradually detaches from its underlying nail bed, creating an empty space where debris can accumulate.

To reliably distinguish a fungal infection from another condition, such as eczema, psoriasis or a repetitive trauma, which is common in athletes, dermatologists now use dermoscopy, also called dermatoscopy. For this, they use a dermatoscope, an illuminated magnifier that enlarges the image 10-fold and allows a better view of the nail. This non-invasive examination makes it possible to observe very specific signs, such as a “saw-tooth” proximal border in the detached area, or coloured longitudinal streaks. A particularly suggestive sign is the dermatophytoma, which appears as a band or opaque spot within the nail itself, indicating a dense cluster of fungi.

In certain cases, a sample of the nail may be taken for analysis, in order to rule out other conditions that mimic fungal infection.

Contrary to certain common misconceptions, onychomycosis is not the result of poor hygiene.

This condition is caused by the invasion of the nail plate by opportunistic micro-organisms. These pathogens possess specific enzymes, such as proteases and keratinases, capable of degrading keratin – the rigid protein that forms the structure of nails – in order to feed on it and spread through the tissues. Three main groups of fungi are responsible for fungal infections of the fingernails and toenails:

• Dermatophytes : These are the main causative agents. The most common by far is Trichophyton rubrum, involved in more than half of infections, followed by Trichophyton mentagrophytes. These fungi show a particular preference for the toes, often following an untreated athlete’s foot.

• Non-dermatophyte moulds : Species such as Aspergillus or Fusarium can colonise the nail, often following a prior traumatic injury.

• Yeasts : The genus Candida, particularly Candida albicans, is frequently involved. Yeasts more commonly affect the fingernails, often in individuals whose hands are regularly immersed in water.

A major scientific discovery has recently shed light on infection resistance: the formation of biofilms. Rather than remaining isolated, fungi organise themselves into structured, tightly connected communities, enveloped in a protective matrix. This process is extremely rapid. In only 72 hours, the biofilm is fully formed and firmly anchored to the nail plate. This biological shield acts as a true fortress that prevents antifungal molecules from penetrating effectively and allows the fungi to evade the immune system. It is this structure that explains why fungal infection is so persistent and why relapses are common if treatment is not continued until the nail has completely regrown.

Onychomycosis is a highly contagious condition whose spread is promoted by genetic, medical and environmental factors. Once the infection has managed to become established on this favourable ground, the risk of transmission to other family members is estimated to be nearly 45%, through the sharing of towels or bath mats.

Onychomycosis does not always present in the same way. Dermatologists classify the infection into several subtypes according to the part of the nail that is initially affected and the pattern of its progression.

• Distal and lateral subungual onychomycosis : This is the most commonly encountered form. In this situation, the fungus infiltrates beneath the nail, beginning at the free edge or from the sides. The infection then progresses towards the nail root, causing the gradual detachment of the nail plate and an accumulation of yellowish or brownish debris. It is this form that gives the nail its typical appearance of a thickened, brittle horn-like structure.

• White superficial onychomycosis : This form is characterised by a direct attack on the upper surface of the nail, without spreading from underneath. It presents as small white, opaque, chalky-looking spots that may eventually cover the entire surface. Unlike other forms, the fungus remains on the surface. The spots can moreover be easily scraped or filed off during a pedicure treatment. It is found almost exclusively on toenails.

• Proximal subungual onychomycosis : This condition is rarer and, paradoxically, begins at the nail root, beneath the cuticle, then spreads towards the tip. It presents as a white patch that appears at the lunula, the pale, crescent-shaped area located at the base of the nail.

• Endonyx onychomycosis : This is a particular variant in which the fungus directly invades the interior of the nail plate, without infecting the underlying nail bed. Visually, the nail takes on a milky discoloration and may show splitting or lamellar layering. Notably, in this form the nail remains firmly attached to the skin and does not show the typical thickening seen in more classic types of fungal nail infection.

• Total dystrophic onychomycosis : This represents the end stage of the infection when it has not been treated in time. At this point, the entire nail apparatus is colonised and destroyed by the fungus. The nail becomes extremely thick, completely opaque, and breaks up into friable fragments until it crumbles away entirely. At this stage, the nail matrix may be permanently damaged, making the regrowth of a healthy nail much more uncertain.

Treating a fungal nail infection is a long-distance race that depends directly on how fast the nail grows back. Since the nail on a hand grows at roughly twice the speed of that on a foot, both the duration and the treatment strategy vary according to the area affected. Before starting any treatment, it is strongly recommended to consult a healthcare professional, so that they can confirm the diagnosis and, if necessary, take a mycological sample to send to the laboratory. This makes it possible to confirm the presence of a fungus and to identify its exact type, as some treatments are more effective against particular strains. Once the diagnosis has been made, several options are generally available.

For mild to moderate infections, particularly when less than 50% of the nail is affected and the matrix is spared, topical treatments are preferred. These consist of medicated nail lacquers containing amorolfine or ciclopirox olamine, to be applied regularly. These products penetrate the nail plate to eradicate the fungus in situ. For superficial white onychomycosis, simple scraping of the nail surface in combination with the lacquer may be sufficient. Patience is essential here: treatment must be continued without interruption until a completely healthy nail has regrown, i.e. around 6 months for fingernails and 9 to 12 months for toenails.

When the fungal infection is deep, reaching the nail matrix or affecting several nails, topical treatments are no longer sufficient because they do not penetrate far enough. The doctor will then prescribe oral antifungal medicines, most often terbinafine. These tablets travel through the bloodstream to reach the nail matrix and become directly incorporated into the newly forming keratin. This treatment is regarded as the standard of care for severe cases, with high cure rates. However, it requires medical monitoring to detect any potential side effects, particularly those affecting the liver.

In certain cases, laser sessions are offered. Although expensive and not reimbursed, laser therapy is an effective treatment for fungal nail infections, and works by heating the fungus to destroy it. It is often used as an alternative for individuals who cannot take oral treatment.

Advice : Additionally, it is strongly recommended to treat your shoes with antifungal powders. This helps to prevent recurrences, as fungal spores can survive for very long periods in enclosed environments.

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• TOSTI A. & al. Onychomycosis: Old and new. Journal of Fungi (2023).