Understanding nail disorders

ARTICLE

Diagnosing nail dystrophies and understanding their pathogenesis is best accomplished if the origin of the disorder is first established.

Exogenous. In this group, the nail plate is altered by exogenous factors and is the site of primary involvement: periungual tissues may be affected secondarily.

Endogenous. In this group, the nail plate is altered secondarily by changes occurring in the matrix, proximal nail fold, nail bed, hyponychium or underlying bony phalanx.

(Key words: nail apparatus, nail unit, onychomycosis.)

Diagnosing nail dystrophies and understanding their pathogenesis is best accomplished if the origin of the disorder is first established [1].

Exogenous. In this group, the nail plate is altered by exogenous factors and is the site of primary involvement: periungual tissues may be affected secondarily.

Endogenous. In this group, the nail plate is altered secondarily by changes occurring in the matrix, proximal nail fold, nail bed, hyponychium or underlying bony phalanx (Fig. 1).

Exogenous disorders

On average, fingernails form over a period of six months. During this time, environmental exposure subjects the nail plate to many types of physical and chemical injury. Some eventually weaken the nail plate by altering intercellular bonding of onychocytes or by altering onychocyte intracellular integrity. Over the course of months, cumulative injury leads to nail plate delamination, splitting and chipping as well as to nail surface deformities. Injury to the undersurface of the nail plate and its adjacent hyponychium (the area immediately distal to the nail bed) leads to onycholysis (separation of the nail plate from the nail bed). Injury to the proximal nail fold and cuticle also leads to deformities. Normally, the cuticle adheres to the underlying nail plate, effectively "sealing" the nail unit. When the cuticle/proximal nail fold is injured, foreign material enters the cleft between the nail plate and fold/cuticle causing paronychia (erythema and oedema of the proximal nail fold).

Trauma

Occupational trauma, (from repeatedly grasping steel files or lifting heavy plastic bags) may cause wear of the nails, "usure des ongles".

Physical activities, such as fast walking or jogging, may cause subungual blistering leading to detachment of the nail plate. Sports associated with quick starts and stops may lead to subungual haematoma of the great toe, tennis or "sportsman's toe".

Chemicals

Three types of chemical alterations are possible:

Irritant effects

Alkalis, acids and strong solvents may alter the nail unit after only one exposure. Onycholysis, koilonychia (spooning) and transverse grooves represent irritant chemical alterations.

Allergic reactions

Formaldehyde, a chemical used in nail hardeners, may produce subungual inflammation, hemorrhage, onycholysis and pain. Cyanoacrylate used to repair cracked nails or to attach false nails not uncommonly produce similar nail unit changes.

Nail plate discoloration

Inadvertent exposure to chemicals such as silver nitrate and potassium permanganate change the color of the nail plate and the discoloration often follows the shape of the proximal nail fold. The application of colored nail lacquers may produce a yellow discoloration of the nail plate surface. Because the discoloration is exogenous in origin, the color can be removed by light scraping to remove pigment from the surface of the nail plate.

Onychomycosis

According to Zaias, there are four types of onychomycosis [2]:

Distal subungual onychomycosis (DSO),

Proximal subungual onychomycosis (PSO),

White superficial onychomycosis (WSO),

Candida onychomycosis.

In addition to overlapping among the first three types of onychomycosis, additional clinical changes appear in onychomycosis [3].

Subungual hyperkeratosis, onycholysis and paronychia

Distal subungual hyperkeratosis often causes the nail plate to separate from the nail bed with resulting onycholysis. This association frequently occurs in Trichophyton rubrum infections. Scytalidium dimidiatum infections also arise distally and spread proximally under the nail to produce onycholysis and sometimes paronychia. Some forms of Candida onychomycosis also lead to a combination of distal subungual onychomycosis and onycholysis (DLSO).

Superficial onychomycosis

WSO is caused not only by T. mentagrophytes but also by some non-dermatophyte molds such as Acremonium sp, Aspergillus sp and Fusarium spp. Color variants also occur. Superficial black onychomycosis is caused by T. rubrum and Scytalidium spp. Interestingly, T. rubrum may also produces WSO.

This presentation occurs in: 1. children; 2. AIDS patients; and 3. patients whose overlapping toe occludes the nail plate of the corresponding (underlying) toe. Onycholysis may accompany T. rubrum - associated WSO. We have also encountered concomitant DLSO, WSO and PSO in a patient infected with T. rubrum. When PSO occurs in conjunction with WSO, it may be preferable to regard the infection as primarily one in which the proximal infection (PSO) extends onto the surface of the nail plate (WSO).

Simultaneous DLSO and WSO may also be observed in nails infected with T. mentagrophytes.

Proximal subungual onychomycosis

Paronychia absent

PSO is usually caused by T. rubrum. Other dermatophytes as well as Candida also produce PSO in the absence of paronychia. In AIDS patients, PSO may spread rapidly over the surface of the nail plate, producing widespread WSO of the affected nail plate.

Candida paronychia

Paronychia of the nail folds may occur in conjunction with Candida-induced onycholysis ; opaque strips of onycholytic nail abut the adjacent infected lateral nail folds.

Non-dermatophyte mold paronychia

Fusarium spp, and Scopulariopsis brevicaulis produce white or buff-colored discoloration of the nail plate. Aspergillus - induced PSO produces a green or black discoloration of the proximally-infected nail plate. All may be associated with paronychia.

Endonyx onychomycosis

In this type of infection, superfical and deep penetration of the nail plate produces a characteristic lamellar splitting of the nail plate. Hyphae penetrate deeply into the nail plate in this previously-undescribed pattern of infection by organisms that normally produce endothrix scalp infections. T. soudanense and T. violaceum tend to produce this invasive type of onychomycosis.

Total dystrophic onychomycosis

1. In secondary total dystrophic onychomycosis, any of the previously-described destructive dystrophies may eventuate in total nail plate involvement and/or destruction.

2. In chronic mucocutaneous candidiasis, all components of the nail unit, including the nail folds, may be simultaneously involved. This form represents primary total dystrophic onychomycosis.

Exogenous disorders

Proximal nail fold (PNF)

Alterations in the PNF affect the nail plate that is formed under the PNF in the nail matrix.

In psoriasis, the stratum corneum of the ventral PNF may become oedematous and may produce parakeratotic and inflammatory cells. Under these conditions, the newly-formed nail plate may develop small surface pits and depressions.

Paronychia may produce changes in the nail plate. Minor trauma from nail biting or overzealous manicuring, for example, may lead to acute paronychia, an infection of the PNF; S. aureus is the most common pathogen. Antibiotic therapy and/or surgical drainage are often mandated.

Candida albicans and gram negative infections, often Pseudomonas, are commonly found in chronic paronychias. Occupation and hobbies associated with chronic exposure to wet environments commonly lead to loss of cuticles, erythema, and oedema of the PNF. Often, a cleft between the nail plate and PNF develops in chronic paronychia and this enables foreign material to enter the PNF. Continued influx of foreign substances into the PNF perpetuates a cycle of chronic inflammation and chronic paronychia. Eventually, the nail plate develops surface depressions and discoloration from pressure of the PNF on the underlying nail plate.

Osteoarthritis of the distal interphalangeal hand joints leads to the formation of digital myxoid cysts. These space-occupying cysts produce swelling of the PNF and longitudinal grooving and thinning of the nail plate.

Compulsive habit tics producing trauma to the PNF and proximal nail plate lead to transverse ridging of the middle nail plate, so-called "washboard nails". Cessation of the habit tic leads to resolution of the nail deformity.

Matrix

Beau's lines are transverse nail depressions resulting from temporarily diminished matrix activity; the involved nail, thinner than normal, reflects deficient nail formation by the matrix. Diminished nutritional supply, chemotherapy and acute febrile illness may affect matrix activity leading to this thinning. If nail matrix generation is completely curtailed, formation of the nail plate will cease and the nail will be shed. By inspecting the nail, it is possible to establish when the matrix was affected.

Because fingernails grow at the rate of approximately one mm per week, the distance in mm from Beau's line to the PNF will establish how many weeks previous the nail formation was altered - up to six months in the case of fingernails. Because the great toenail grows more slowly (one to two mm per month), depressions in the nail can reflect matrix alteration for up to two years.

When the thinning is multiple, focal ridging and splitting of the nail plate occurs. When the entire matrix is shortened, thinning of the nail plate develops in addition.

Lichen planus of the nail unit leads to injury and, often, complete destruction of the nail matrix. Permanent loss of the nail plate, either partial or complete, as well as thinning of the nail plate may occur.

Nail plate thickening arises as a result of matrix hypertrophy, a disorder that may occur at any age; in the young, trauma is the most frequent cause.

Abnormal keratinization of onychocytes leads to pitting and leukonychia. Onychocytes arising in the proximal matrix that fail to mature (and retain their nuclei) cause surface pitting in the nail plate. Immature, nucleated, onychocytes are less "sticky": diminished adherence of cells to one another leads to shedding from the nail plate surface. When onychocytes from the middle matrix retain their nuclei, leukonychia develops. Parakeratotic onychocytes in the mid nail reflect light and appear as white lines or specs within the nail plate.

Subungual haematomas migrate distally with ongoing growth of the nail plate. If the haematoma is extensive, it may simulate longitudinal melanonychia and subungual melanoma. Melanoma and longitudinal melanonychia do not disappear as the nail grows. When haematomas are small, they may occupy small longitudinal grooves in the nail bed and appear as tiny "splinters".

Nail bed and hyponychium

A functional, intact nail bed is mandatory for normal growth and attachment of the nail plate [4]. If the nail bed is lacerated or otherwise injured, the nail plate must be surgically reattached to the nail bed while the bed is healing. Reattachment allows the nail bed to form a new epithelium which in turn provides a site for adhesion of newly-generated nail plate.

Psoriasis and distal subungual onychomycosis often lead to hyperplasia of the nail bed and hyponychium. The hyponychium is the primary invasion site for most onychomycotic nail species. Their persistence leads to continued infection of the growing nail. Parakeratosis and hyperplastic changes are typical of psoriasis in the nail bed as well as the skin. Additional changes occurring in the psoriatic nail bed are the "oil drop" sign, collections of serum and parakeratotic cells under the nail plate, and the "salmon" macule due to dilated blood vessels in the nail bed.

Sometimes no specific cause for hyperplasia can be identified. In pachyonychia congenita, the nail appears normal at birth ; shortly thereafter, the nail bed becomes hyperplastic.

When an unexplained alteration develops in the nail bed, particularly if a single nail is involved, one should consider the possibility of a neoplasm. If the nail is tender to palpation, a glomus tumor should be considered. Squamous cell carcinoma and melanoma also develop in the nail bed or matrix. Under these circumstances, biopsy is indicated.

Phalanx

Like the nail bed, an intact bony phalanx is required for normal nail plate formation. The nail plate grows along the bony axis which provides a base that supports nail elongation. If the distal phalanx is shortened, the nail plate curves ventrally forming a "hook-like" nail rather that a flat, normal-appearing nail.

Exostoses, osteocartilagenous protrusions originating from the underlying bone may elevate the nail plate or protrude through the nail plate producing clinical nail deformity.

Occupational acro-osteolysis, destruction of the distal bony phalanx, produces pseudo-clubbing of the nail, shortened nails (racket nails), koilonychia and even nail unit destruction. It occurs in workers exposed to vinyl chloride.

References

1. Kechijian P, Salache S. Biology and disorders of nails. In: Arnd K, Leboit PE, Robinson JK, Wintroub BU (eds). Cutaneous medicine and surgery in integrated program in dermatology. WB Philadelphia: WB Saunders Co., 1996.

2. Zaias N. Onychomycosis. Arch Dermatol 1972; 105: 263-74.

3. Baran R, Hay RJ, Tosti A, Haneke E. A new classification of onychomycosis. Br J Dermatol 1998; 139: 567-71.

4. Baran R, Dawber RPR eds. Diseases of the nails and their management, 2nd ed. Oxford: Blackwell Scientific Publications, 1994.

Self-evaluation questions

A. "Usure des ongles", (worn-down nails), may be due to

1. Nail cosmetics
2. Excessive manicure
3. Occupational conditions
4. Nail scratching

B. Allergic reactions to the nail unit may be due to

1. Toluene sulfonamide formaldehyde resin
2. Cyanoacrylates
3. p-t-butyl phenol (PTBP) (chemical associated with the preformed plastic nail adhesive)
4. Tulips

C. Distal subungual onychomycosis may be associated with paronychia resulting from

1. T. rubrum
2. T. mentagrophytes
3. Scytalidium dimidiatum
4. Candida spp

D. Superficial white onychomycosis, is usually caused by:

1. T. violaceum
2. T. rubrum
3. T. mentagrophytes
4. Fusarium sp

E. Proximal subungual onychomycosis usually results from

1. T. rubrum
2. Candida albicans
3. Fusarium spp
4. Scopulariopsis brevicaulis

F. What does "beau's lines" mean?

1. Transverse grooves
2. Transverse leuconychia
3. Ridging
4. Longitudinal grooving of the nail

G. What may cause a hook-like nail?

1. Bony phalanx widening
2. Bony phalanx thinning
3. Bony phalanx shortening
4. Clubbing

Answers to the self evaluation questions of vol. 11, issue n° 1