Antonella Tosti - Onychomycosis: An Illustrated Guide to Diagnosis and Treatment

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Part I
Is Onychomycosis a Disease or a Cosmetic Problem?

Springer International Publishing Switzerland 2017

Antonella Tosti , Tracey C. Vlahovic and Roberto Arenas (eds.) Onychomycosis 10.1007/978-3-319-44853-4_1

1. Fungi and the Nails

Jeremy Brandon Freedman 1

(1)

Department of Dermatology and Cutaneous Surgery, University of Miami, Miami, FL 33136, USA

Jeremy Brandon Freedman

Email:

Antonella Tosti (Corresponding author)

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Key Features

• Fungal nail infections are incredibly common.
• The most common fungal pathogens of the nails are dermatophytes, but non-dermatophytic molds and yeasts can be implicated as well.
• Both the fungal species and their mechanism of invasion are major determinants of clinical presentation.
• The most common pattern of fungal invasion is distal lateral subungual onychomycosis (DLSO).
• Keratin invasion is an important part of initiating and establishing a fungal nail infection. Unfortunately, the exact mechanisms are still not fully understood.

Introduction

From a clinical perspective, nails are simply specialized keratin structures on the dorsal surfaces of our fingers and toes. To the average person, however, they represent so much more. Nails are protective, useful for grasping fine objects, perfect for scratching itches, and important to an individuals overall appearance: people spend time cutting, filing, and sometimes decorating them in order to look presentable. Understanding that, its easy to see how diseases of the nail can have substantial physical and psychosocial consequences [].

Indeed, individuals with nail dystrophies often suffer from considerable pain and discomfort, can have difficulty walking, and are at risk for significant complications, including bacterial superinfection and cellulitis [].

Although there are a variety of disorders (both cutaneous and systemic) that can affect the nails, more than half of all nail dystrophies are caused by fungal infections (onychomycosis) [].

While fungal infections of the nail are normally superficial and very rarely life threatening, many are particularly difficult to treat, and patients are often left either entirely without treatment or with incompletely cured infections and recurrences.

Since practically every patient who visits a physicians office has nails, and the prevalence of onychomycosis can climb as high as 50 % in patients over 70 years of age [], it is extremely likely that most physicians will encounter many cases of fungal nail infections during their years of practice. It makes sense, therefore, for every physician to have a good understanding of how fungus can affect the nails.

State of Art

As mentioned, onychomycosis can be caused by different species of fungi, specifically dermatophytes, non-dermatophytic molds, and yeasts [].

The vast majority of fungal nail infections are caused by dermatophytes (parasitic fungal organisms that feed on keratin) [].

Other fungal pathogens that affect the nails are non-dermatophytic molds (NDMs), the five most frequently isolated organisms of which are Scopulariopsis brevicaulis , Fusarium species, Aspergillus species, Scytalidium dimidiatum , and Acremonium species [].

Rarely, onychomycosis can be caused by yeasts [].

Fig. 1.1

Candida onychomycosis in a patient with chronic mucocutaneous candidiasis

Aside from the multiple fungal culprits, there are also multiple routes of invasion that fungi can use to infect the nail. However, in order to properly understand the different mechanisms of fungal invasion, its important to first have a solid grasp of nail anatomy and the process of nail growth.

The nail consists of the nail matrix, the nail plate, the underlying nail bed, and the nail folds. The nail plate is composed of hard, keratinized cells which grow out of the nail matrix the distal, visible part of which looks like a half-moon and is called the lunula and emerges from beneath the proximal nail fold and cuticle (which protects the nail matrix from the environment), bordered on either side by the lateral nail folds. It extends along the nail bed, and is normally cut distally, just past the hyponychium (where the skin of the fingertip meets the nail bed), at the free edge of the nail (Fig. ].

Fig. 1.2

This illustration of a sagittal section of the finger provides us with a visual of some internal and external details of nail anatomy. The nail plate ( NP ) grows out of the nail matrix ( NM ) and extends along the nail bed ( NB ). It is bordered proximally by the proximal nail fold ( PNF ) and distally by the hyponychium ( HYP )

There are five generally accepted patterns of fungal nail infections, each utilizing a slightly different mechanism of invasion. They are: distal lateral subungual onychomycosis (DLSO), superficial onychomycosis (SO), proximal subungual onychomycosis (PSO), endonyx onychomycosis (EO), and total dystrophic onychomycosis (TDO) [].

The most common form of fungal invasion of the nail is DLSO []. This occurs when fungus invades the nail bed and undersurface of the nail plate via the hyponychium (usually as an extension of tinea pedis/manuum), and spreads proximally along the longitudinally oriented rete ridges of the nail bed.

SO was previously known as superficial white onychomycosis (SWO), but that term became too narrow once SO was found to present with other features (such as deep penetration, brown/black pigmentation, etc.) depending on the organism involved [].

In PSO, the fungal elements invade the deeper, ventral aspect of the nail plate from the proximal portion of the nail and migrate distally, causing a patch or a band of leukonychia that moves distally with nail growth. PSO is a relatively uncommon subtype that can suggest the possibility of HIV infection or other types of immunosuppression []. As mentioned above, when PSO is associated with periungual inflammation, we should suspect a mold infection.

EO occurs when the fungus invades the distal nail plate directly, without any involvement of the nail bed. This is a very rare presentation, characterized by nail plate invasion without subungual hyperkeratosis (because the nail bed is spared) [].

Besides those five general classifications, there are some patients that present with features from multiple forms of nail infection, called mixed pattern onychomycosis (MPO) []. The two most common examples of this are when a nail affected with DLSO develops SO as well (especially in an area where another toe overrides it) and when SO extends under the proximal nail fold, creating an increased risk of PSO.

Eventually, if the fungal infections are allowed to progress, the entire nail will become thick and dystrophic. This end-stage nail disease is referred to as total dystrophic onychomycosis (TDO) (Fig. ].

Fig. 1.3

Total dystrophic onychomycosis. Note the severe concomitant tinea pedis

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