Chapter 89. Biology of Nails and Nail Disorders

The nail apparatus consists of a horny “dead” product, the nail plate, and four specialized epithelia: (1) the proximal nail fold, (2) the nail matrix, (3) the nail bed, and (4) the hyponychium (Fig. 89-1A). The nail apparatus develops during the 9th embryonic week from the epidermis of the dorsal tip of the digit as a rectangular area, the nail fold that is delineated by a continuous groove.1 The proximal border of the nail fold extends downward and proximally into the dermis to form the nail matrix primordium. By the 15th week the nail matrix is completely developed and starts to produce the nail plate, which will continue to grow until death. The nail apparatus lies immediately above the periosteum of the distal phalanx. The intimate anatomic relationship between the nail and the bone is responsible for the common occurrence of bone alterations in nail disorders and vice versa. The shape of the distal phalangeal bone also determines the shape and the transverse curvature of the nail.

Fingernails usually present a longitudinal major axis and toenails a transverse major axis. The ratio between length and width is important for the aesthetic appearance of the nails. The size of the nails varies in the different digits. The biggest nail is that of the first toe, which covers approximately 50% of the dorsum of the digit.

Nails have numerous functions. Fingernails not only contribute to the pleasing appearance of the hands, but are very important in protecting the distal phalanges and enhancing tactile discrimination and the capacity to pick up small objects. They are also widely used for scratching and grooming and are an efficient natural weapon. Toenails protect the distal toes and contribute to pedal biomechanics.

Nail Plate

(See Fig. 89-1A)

The nail plate is a fully keratinized structure that is continuously produced throughout life (see Fig. 89-1B). It results from maturation and keratinization of the nail matrix epithelium and is firmly attached to the nail bed, which partially contributes to its formation. Proximally and laterally the nail plate is surrounded by the nail folds, which cover its proximal third and lateral margins. At the tip of the digit, the nail plate separates from the underlying tissues at the hyponychium. The nail plate is rectangular, translucent, and transparent. It is curved in both the longitudinal and transverse axes, especially in the toes. The nail plate surface is smooth but frequently shows mild longitudinal ridges that increase with aging (Fig. 89-2). The pattern of these ridges can be used for forensic identification. The bottom of the nail plate shows longitudinal ridges that correspond to the rete ridges of the nail bed. The nail plate is homogeneously pink, except for its free edge, which is white. The pink color of the nail plate is due to the nail bed blood vessels.

The proximal part of the fingernails, especially of the thumbs, shows a whitish, opaque, half-moon-shaped area, the lunula that is the visible portion of the nail matrix. In this area the nail plate attachment to the underlying epithelium is loose. More than 90% of fingernails show a thin distal transverse white band, the onychocorneal band, better defined as the isthmus, which marks the most distal portion of firm attachment of the nail plate to the nail bed.2,3 This area represents an important anatomic barrier against environmental hazards, and its disruption produces nail plate detachment with onycholysis. The onychocorneal band is separated from the nail plate white free edge by a 1.0–1.5-mm pink band called the onychodermal band.

In transverse sections, the nail plate consists of three portions: (1) dorsal nail plate, (2) intermediate nail plate, and (3) ventral nail plate.4 The dorsal and the intermediate portions of the nail plate are produced by the nail matrix, whereas its ventral portion is produced by the nail bed. Above the lunula the nail plate is thinner and consists only of the dorsal and intermediate portions. There is a natural line of cleavage between the dorsal and the intermediate nail plate.

The nail plate progressively thickens from its emergence to its distal margin. The mean toenail thickness at the distal margin is 1.65 ± 0.43 mm in men and 1.38 ± 0.20 mm in women. Fingernails are thinner; the mean thickness is 0.6 mm in men and 0.5 mm in women. There is an increase in nail thickness with age, particularly in the first two decades. Nail thickness depends on the length of the nail matrix and nail bed.5 Thinning of the nails is usually a sign of nail matrix disorders, whereas nail thickening is most commonly a consequence of nail bed disorders.

Proximal Nail Fold

(See Fig. 89-1A)

The proximal nail fold is a skin fold that consists of a dorsal and a ventral portion. The dorsal portion is anatomically similar to the skin of the dorsum of the digit but thinner and devoid of pilosebaceous units. The ventral portion, which cannot be seen from the exterior and proximally continues with the germinative matrix, covers approximately one-fourth of the nail plate. It closely adheres to the nail plate surface and keratinizes with a granular layer. The limit between the proximal nail fold and the nail matrix can be histologically established at the site of disappearance of the granular layer.

The horny layer of the proximal nail fold forms the cuticle, which is firmly attached to the superficial nail plate and prevents the separation of the plate from the nail fold. The integrity of the cuticle is essential for maintaining the homeostasis of this region.

The dermis of the proximal nail fold contains numerous capillaries that run parallel to the surface of the skin and may easily be observed in vivo by capillary microscopy. This permits the observation of both the arterial and the venous limbs of the capillaries, which are arranged in parallel rows and appear as fine regular loops with a small space between the afferent and efferent limbs. The morphology of proximal nail fold capillaries is typically altered in connective tissue diseases.6–8

Nail Matrix

(See Fig. 89-1A)

The nail matrix is a specialized epithelial structure that lies above the mid portion of the distal phalanx. After elevation of the proximal nail fold, the matrix appears as a distally convex crescent with its lateral horns extending proximally and laterally.

In longitudinal sections the matrix has a wedge-shaped appearance and consists of a proximal (dorsal) and a distal (ventral) portion. Nail matrix keratinocytes divide in the basal cell layer and keratinize in the absence of a granular zone. The site of keratinization (keratogenous zone) of nail matrix onychocytes can be clearly distinguished in histological sections as an eosinophilic area where cells show fragmentation of their nuclei and condensation of their cytoplasm.1 In this area, nuclear fragments are destroyed by deoxyribonuclease and ribonuclease enzymes. In some conditions nuclear fragments may persist within the intermediate nail plate, producing leukonychia spots. However, these frequently disappear before reaching the nail-free edge, due to the persistence of active DNA and RNA lytic enzymes within the horny nail plate.

Maturation and differentiation of nail matrix keratinocytes do not follow a vertical axis, as in the epidermis, but occur along a diagonal axis that is distally oriented. For this reason, keratinization of the proximal nail matrix cells produces the dorsal nail plate and keratinization of the distal nail matrix cells produces the intermediate nail plate.

In some fingers the distal matrix is not completely covered by the proximal nail fold but is visible through the nail plate as a white half-moon-shaped area, the lunula. The white color of the lunula results from two main anatomic factors: (1) the keratogenous zone of the distal matrix contains nuclear fragments that cause light diffraction, (2) nail-matrix capillaries are less visible than nail bed capillaries because of the relative thickness of the nail-matrix epithelium.9

Nail Matrix Keratinocytes

(See Fig. 89-1B).

The nail matrix cells are able to synthesize both “soft” or skin-type and “hard” or hair-type keratins.10–12 Evaluation of keratin expression in the different constituents of the nail apparatus showed that the nail matrix is the sole site of expression of hard keratin proteins, particularly Ha1 keratin.13 Data indicate that fibroblasts derived from the nail matrix may induce hard keratin expression in nonnail-matrix keratinocytes.14

Melanocytes

(See Chapter 72).

Nail matrix melanocytes are usually quiescent and therefore not detectable in pathologic sections. However, they possess the key enzymes that are necessary for melanin production, and may become activated by a large number of physiologic and pathologic conditions.15 Nail matrix melanocyte activation produces diffuse or banded nail pigmentation and is more common in blacks and Japanese than in Caucasians. DOPA-negative (inactive) melanocytes are sparsely present in the nail matrix and in the nail bed.16 DOPA-positive, activable melanocytes are especially seen in the distal nail matrix, where they are frequently arranged in small clusters among the suprabasal layers of the nail matrix epithelium.17

Langerhans Cells

(See Chapter 10).

Langerhans cells are more numerous in the proximal than in the distal nail matrix. As in normal epidermis, Langerhans cells are predominantly found in the suprabasal layers. However, they may occasionally be seen within the basal layer of the nail matrix epithelium.

Merkel Cells

The presence of Merkel cells in the nail matrix has been demonstrated. Their density is possibly influenced by age, with these cells being more numerous in fetus than in adult nails.18

Nail Bed

(See Fig. 89-1A)

The nail bed extends from the distal margin of the lunula to the isthmus and is completely visible through the nail plate. The nail bed epithelium is so adherent to the nail plate that it remains attached to the undersurface of the nail when the latter is avulsed.

The nail bed epithelium is thin and consists of two to five cell layers. Its rete ridges, which are longitudinally oriented, interdigitate with the underlying dermal ridges in a “tongue-and-groove”-like fashion. The nail bed epithelium is a specialized epithelial structure with a horny layer that interlocks to of the inferior border of the nail plate and is responsible for the strong attachment between the two tissues. Nail bed horny layer forms the ventral nail plate, which corresponds to approximately one-fifth of the terminal nail thickness and mass.19 In pathologic sections, the ventral nail plate is easily distinguishable because of its light eosinophilic appearance. Nail bed keratinization is not associated with the formation of a granular layer. This may appear, however, when the nail bed becomes exposed after nail avulsion.1

The nail isthmus is a thin transverse distal band that represents a transitional zone between the nail bed and the hyponychium and exhibits a unique pattern of keratinization, the onycholemmal keratinization, with pale, nucleated keratinocytes. The cornified layer of the nail isthmus closely adheres to the undulating inferior surface of the nail plate preventing onycholysis; the two grow forward together.3 Keratin expression in the nail bed differs form that of the nail matrix since keratins K6, K16, and K6hf are only expressed in the nail bed.20 The isthmus differs from nail bed because of a strong suprabasal expression of K10. Transition from isthmus to hyponychium is marked by the disappearance of expression of K6hf and K6/16, and return to expression of K5/17.

Hyponychium

The hyponychium marks the anatomic area between the nail bed and the distal groove, where the nail plate detaches from the dorsal digit (see Fig. 89-1A). Its anatomic structure is similar to that of plantar and volar skin, and keratinization occurs through the formation of a granular layer. The horny layer of the hyponychium partially accumulates under the nail plate free margin. The hyponychium is normally covered by the distal nail plate, but it may become visible in nail biters. The architecture of the capillary network of the hyponychium dermis consists in regular capillary loops arranged perpendicularly to the skin, visible as red dots with dermoscopy.

Basement Membrane Zone

The antigenic structure of the basement membrane zone of the nail is identical to that of the epidermis, and there are no differences in the antigenic composition of the basement membrane zone in the different portions of the nail apparatus.21 This may explain the involvement of the nails in conditions characterized by mutations of basement membrane-associated genes as well as in autoimmune skin diseases involving the basement membrane zone antigens.22

Dermis

(See Fig. 89-1A)

The nail apparatus is devoid of subcutaneous tissue, and its dermis does not contain pilosebaceous units. The arrangement of the rete ridges varies in the different portions of the nail apparatus. The dermis beneath the proximal nail matrix consists of condensed connective tissue that forms a tendon-like structure connecting the matrix to the periosteum of the proximal phalangeal bone (posterior ligament). A small amount of subdermal fat tissue is present close to the periosteum of the base of the phalanx.23 The close connection between the lateral horns and the periosteum is possibly responsible for the nail plate's lateral convexity. The rete ridges of the dermis underneath the nail matrix are characteristically long and root-like in shape. The dermis under the distal matrix consists of a loose network of connective tissue containing numerous blood vessels and rare glomus bodies.

The dermis of the nail bed has a unique arrangement with longitudinal grooves and ridges that run from the lunula to the hyponychium.1 The longitudinal orientation of the capillary vessels within the nail bed grooves explains the linear pattern of nail bed hemorrhages (splinter hemorrhages). The nail bed dermis contains abundant connective tissue networks with connective tissue bundles radiating to the phalangeal periosteum. It contains numerous glomus bodies.

Blood and Nerve Supply

The nail apparatus has an abundant blood supply provided by the lateral digital arteries. These run along the sides of the digits and produce both branches that supply the matrix and the proximal nail fold and arches that supply the matrix and the nail bed. The nail matrix, therefore, has two different sources of blood supply. The nail bed is richly supplied (10–20 cm2) by encapsulated neurovascular structures containing one to four arteriovenous anastomoses and nerve endings. These glomus bodies are arteriovenous shunts involved in the regulation of the blood supply to the digits related to thermoregulation. The cutaneous sensory nerves, which originate from the dorsal branches of the paired digital nerves, run parallel to the digital vessels.

Chemical Properties

(See Fig. 89-1B)

The nail plate, like hair, consists mainly of low-sulfur filamentous proteins (keratins) embedded in an amorphous matrix composed of high-sulfur proteins rich in cysteine. Other nail constituents include water, lipids, and trace elements. Nail keratins consist of 80%–90% hard hair-type keratins and 10%–20% soft skin-type keratins. Hard keratins have been identified as the acidic 44K/46K and basic 56K/60K keratins. Soft keratins have been identified as the 50K/58K and 48K/56K keratin pairs.24 Keratin filaments have a transverse orientation that is parallel to the nail surface. This explains why the nail plate is more susceptible to transverse fractures than to longitudinal fractures. Specific keratins are expressed only in some compartments of the nail unit; for instance, K6a and K6b, K16, and K17 are not expressed in the nail matrix.13,25,26 Mutations of the genes encoding for these keratins are associated with nail thickening due to nail bed hyperproliferation, as is seen in pachyonychia congenita (PC).27,28 Nail keratin content and composition, measured as the quantity of carbon (C), nitrogen (N), and sulfur (S) in the fingernails, vary between sexes and in relation to the aging process. Sulfur content is higher in female than in male nails and the opposite is for nitrogen. Carbon content is equal in the two sexes. The carbon content increases with aging, possibly due to loss of inorganic material, and the nitrogen content decreases, while the sulfur content remains stable.29

Under normal conditions, the water content of the nail plate is 18%, and most of the water is in the intermediate nail plate.30 The average water content of the nail plate is significantly lower in winter than in summer.31 and varies significantly in time, due to the high porosity of the nail plate, which allows it to be rapidly hydrated and dehydrated. Dehydration is faster when the nails are kept long. When the water content decreases below 18%, the nail becomes brittle; when it increases above 30%, it becomes opaque and soft.32

The nail contains less than 5% lipids, mainly cholesterol; the nail plate lipid content is under hormonal control and decreases after menopause.33 The nail plate also contains traces of several inorganic elements, particularly iron, zinc, and calcium. However, these do not contribute to nail hardness.

Physical Properties

The nail plate is hard, strong, and flexible. The hardness and strength of the nail plate are due to its high content of hard keratins and cysteine-rich high-sulfur proteins, whereas its flexibility depends on its water content and increases with nail plate hydration.34,35

The double curvature of the nail plate along its longitudinal and transverse axes enhances nail plate resistance to mechanical stress.36

The physical properties of the nail also depend on the arrangement and adhesion of onychocytes in the different portions of the nail plate, as well as on the orientation of the keratin filaments within the nail plate onychocytes.36 At the ultrastructural level, the corneocytes of the dorsal nail plate are flat, with their shorter diameter perpendicular to the nail plate surface. The average sizes of these cells are 34 μm in length, 64 μm in width, and 2.2 μm in height.37 Cell adhesion is strong. This portion of the nail is responsible for nail plate hardness and sharpness. The onychocytes of the intermediate nail plate show multiple interdigitations of their cell membranes. The average dimensions of these cells are 40 μm in length, 53 μm in width, and 5.5 μm in height. Cell adhesion is provided by desmosomes. This part of the nail plate is responsible for nail pliability and elasticity. The ventral nail plate is thin and consists of soft keratins. It provides adhesion to the underlying nail bed.

Nail Growth

The nail plate grows continuously in a proximal to distal manner throughout life. The nail plate is “pushed” out by two factors: (1) matrix keratinocytes proliferation and differentiation which makes a new plate, (2) the nail bed which moves slowly, parallel to the direction of the nail growth, toward the inferior border of the nail plate.3

Fingernails grow two times faster than toenails, with a mean growth rate in adults of 3.5 mm/month for fingernails and 1.5 mm/month for toenails. The 5th fingernail growth rate is significantly slower than other fingernails and the growth rate of the great toenail significantly faster than other toenails.38

Complete replacement of a fingernail requires 100–180 days (6 months). When the nail plate is extracted, it is approximately 40 days before the new fingernail first emerges from the proximal nail fold. After a further 120 days it will reach the fingertip.39 The total regeneration time for a toenail is 12–18 months. As a consequence of the slow nail growth rate, diseases of the nail matrix only become evident a considerable time after their onset and require a long time to disappear after treatment.

Nail growth rate varies among different individuals and among the different digits of the same individual. It depends on the turnover rate of the nail matrix cells and is influenced by several physiologic and pathologic conditions. Nail growth rate is slow at birth, increases slightly during childhood, and usually reaches its maximum between the second and the third decades of life. It sharply decreases after the age of 50 years.39

Conditions that have been associated with a slow growth rate include systemic illness, malnutrition, peripheral vascular or neurologic diseases, and treatment with antimitotic drugs. Nails affected by onychomycosis frequently exhibit a slow growth rate. An arrest of nail growth is a typical feature of yellow nail syndrome. Conditions that have been associated with accelerated nail growth include pregnancy, finger trauma, psoriasis, and treatment with oral retinoids or itraconazole. Accelerated nail growth may cause longitudinal ridging of the nail plate (nail beading).

Due to their slow growth rate, the nails may provide information on pathologic conditions that have occurred up to several months before the time of observation. Drugs, chemicals, and biologic substances accumulate in nails, where they can be detected and measured. Advantages of analyzing nail samples include the ease and noninvasiveness of their collection, the small sample size required for analysis, and the ease of storage at room temperature. The nail of the big toe is the best site for investigation because of its size (big toenail length of an adult: 20 mm) and slow growth rate (about 2 mm/month) permitting to obtain data on exposure to drugs and chemicals over a period of 10 months.40

(See references 1, 32, and Table 89-1)

Beau's Lines and Onychomadesis (Nail Shedding)

Beau's lines result from a temporary arrest of proximal nail matrix proliferation and appear as transverse grooves, often deeper in the central nail plate, that move distally with nail growth. Onychomadesis also results from a temporary arrest in nail matrix activity, and the proximal nail plate is detached from the proximal nail fold by a whole-thickness sulcus. Causes of onychomadesis are the same as those for Beau's lines but are more severe (Box 89-1). Multiple Beau's lines or onychomadesis in the same nail indicates repetitive insults. Measuring the distance of the groove from the proximal nail fold can date the time of the insult leading to Beau's lines. Local trauma, such as from manicures or onychotillomania, or related to local cutaneous disease, particularly dermatitis, periungual erythema, and paronychia, are causes of Beau's lines. Beau's lines or onychomadesis at the same levels in several nails suggest a systemic cause (Fig. 89-3). Most common among these are drugs (especially chemotherapy), high fever, viral illness,41,42 surgery, and peripheral ischemia. Onychomadesis in children often relates to recent coxsackievirus infection (hand-foot-mouth disease).41,43

Pitting

Pits result from small areas of abnormal keratinization of the proximal nail matrix that produce foci of parakeratotic cells in the superficial nail plate. They appear as small punctate depressions of the superficial nail plate, which progress distally and often become more evident with nail growth. Deep and irregularly distributed pits are seen in psoriasis (Fig. 89-4) and atopic dermatitis; geometric and superficial pits are typical of alopecia areata (see Chapters 14, 18, and 88).

Onychorrhexis (Longitudinal Striations and Fissuring)

Onychorrhexis results from diffuse defective keratinization of the proximal nail matrix. The nail plate is usually thinned and presents multiple longitudinal ridges and fissures. Onychorrhexis is a sign of severe nail fragility and typical of lichen planus (see Chapter 26).

Longitudinal Grooves

Longitudinal grooves are usually single and appear as a longitudinal depression of the nail plate (1–2 mm large) due to compression of the nail matrix by tumors of the proximal nail fold.

Trachyonychia

Trachyonychia results from multiple foci of defective keratinization of the proximal nail matrix. The nails are rough due to excessive longitudinal ridging.

True Leukonychia

True leukonychia results from defective keratinization of the distal matrix with persistence of parakeratotic cells in the ventral nail plate. The superficial nail plate is structurally normal, but the nail presents opaque white patches or striae, which often disappear before reaching the distal edge of the nail. Punctate leukonychia is due to microtrauma and is typically seen in the fingernails of children (Fig. 89-5). Striate leukonychia of fingernails is a consequence of aggressive manicure. Total or subtotal leukonychia is rare and usually hereditary.

Koilonychia

In koilonychia the nail plate is thin and spoon shaped. Koilonychia is physiologic in the toenails of children. In adults it can be a sign of iron deficiency or occupational damage to the nail plate.

Melanonychia44

Melanonychia describes a brown to black color of the nail due to the presence of melanin in the nail plate. It can be caused by activation or proliferation (benign or malignant) of nail matrix melanocytes. The pigmentation may involve the whole nail (total melanonychia) or may be banded, as in transverse melanonychia (rare) or in the most common longitudinal melanonychia (LM). LM may appear as a single band involving one digit, or as multiple bands affecting several digits, which are usually due to melanocyte activation, as is seen in dark-skinned individuals, pregnant women, inflammatory nail disorders, individuals with Laugier–Hunziker syndrome, and those taking certain medications (Box 89-2). LM has also been described in individuals with a variety of systemic disorders, particularly human immunodeficiency virus infection and Addison syndrome. In Laugier–Hunziker syndrome, melanonychia begins during adolescence, affects several digits, and is associated with the presence of lip and/or genital pigmented macules45,46 (see Chapter 78). Melanonychia due to melanocyte activation may in some cases involve a single digit, as in patients with onychotillomania, with frictional melanonychia of the 4th or 5th toenails (Fig. 89-6), with inflammatory nail diseases, such as psoriasis or lichen planus, or with nail tumors, such as Bowen's disease. A single band of melanonychia deserves a careful evaluation, since it may be a sign of a nail matrix nevus or melanoma (see Chapters 122 and 124). LM of a single nail often deserves biopsy.

Longitudinal Erythronychia47

Longitudinal erythronychia reflects a nail bed disorder and appears as a pink–red longitudinal band of various width extending from the proximal nail to the distal edge. A single band of longitudinal erythronychia is most commonly caused by an onychopapilloma or by another benign or malignant subungual tumor (Fig. 89-7). Multiple bands of longitudinal erythronychia are seen in lichen planus. In Darier's disease bands of longitudinal erythronychia alternate with white longitudinal bands and V-shaped indentations of the nail-free margin (see Chapter 51).

Onycholysis

Onycholysis is detachment of the nail plate from the nail bed and can be caused by traumatic, inflammatory, infectious, or neoplastic nail bed disorders. See Section “Onycholysis under Environmental Nail Disorders.”

Splinter Hemorrhages

Splinter hemorrhages appear as red to black small thin longitudinal lines under the nail plate. They are more commonly located in the distal nail plate and represent rupture of the longitudinally oriented nail bed capillaries (Fig. 89-8). Causes include trauma and inflammatory nail disorders, such as psoriasis.

Apparent Leukonychia

In apparent leukonychia the nails are pale white due to nail bed discoloration that fades with pressure.

Subungual Hyperkeratosis

Subungual hyperkeratosis is due to inflammatory disorders that cause an abnormal keratinization of the distal nail bed and hyponychium with accumulation of scales under the distal nail plate. The most common causes include psoriasis, onychomycosis, trauma, contact and atopic dermatitis (see Chapters 13, 14, and 18).

Paronychia

Paronychia is inflammation of the proximal nail fold and presents as painful periungual erythema, sometimes with associated purulence. Acute paronychia is usually caused by infection (see Section “Infectious Nail Disorders”). Chronic paronychia is most commonly due to mechanical or chemical factors. If the periungual area is fluctuant or shows purulence, it should be drained to avoid matrix damage. Topical and/or systemic antibiotics should be administered if bacterial infection is suspected.

Nail Pigmentation

Nail pigmentation is most commonly due to exogenous staining of the nail plate. In this case the proximal margin of the pigmentation follows the shape of the proximal nail fold. Exogenous nail pigmentation is most commonly due to occupational exposures or nail cosmetics. Nail pigmentation due to endogenous causes is rare. The proximal margin of the pigmentation follows the shape of the lunula. Possible causes include drugs, argyria, hemochromatosis, alkaptonuria, and Wilson disease.

(See reference 48 and Table 89-2)

Ectodermal Dysplasias

(See Chapter 142)

Nail changes may be associated with hypotrichosis, hypodontia, and hypohidrosis. Most commonly the nails are short, thickened, and hypoplastic.

Epidermolysis Bullosa

(See Chapter 62)

Nail abnormalities are a common feature in most subtypes of epidermolysis bullosa (EB) and have recently been included among the criteria for scoring EB severity.49 Trauma undoubtedly contributes to the development of nail dystrophy and for this reason the great toenails are more often severely affected. Pachyonychia of the toenails be the first or the only symptom of dominant dystrophic EB (DDEB) in some families (eFig. 89-8.1). Junctional and dermolytic EB may produce anonychia.50

Pachyonychia Congenita

(See Chapter 50)

PC is an autosomal dominant genodermatosis characterized by painful keratoderma, nail thickening, oral leukokeratosis, and epidermal cysts. The severity of PC can vary greatly among patients and the most problematic aspect of PC, the painful palmoplantar keratoderma (PPK), have vary in extent from focal to a severe, diffuse PPK.51 The International Pachyonychia Congenita Research Registry (IPCRR) has compared the PC phenotype with genotype in hundreds of individuals, and have found issues with the classical division into PC1 (Jadassohn-Lewandowski type) and PC2 (Jackson-Lawler). A new molecular classification has been proposed, in which subtypes of PC refer to the mutated keratin gene.52 Mutations in KRT6A account for almost 50% of known cases, while 24% have mutations are in KRT16, 23% in KRT17, and 3% in KRT6B.53 In contrast to the old classification, in which PC1 was thought to result from mutations in KRT6a or 16, and PC2 from mutations in KRT6b or 17, the clinical features of the former subtypes overlap. For example, cysts (a feature of PC2 and not PC1) most commonly occur in individuals with either a KRT17 mutation or KRT6a mutation.

Nail abnormalities are a constant feature and develop during infancy to early childhood, although a late-onset variety of PC has been described.54 In typical cases the 20 nails are thickened, very difficult to trim, darkened, and with an increased transverse curvature. Nail thickening is a consequence of nail bed hyperkeratosis and is more evident on the distal half of the nails, which have an upward angling. Recent evidence indicates that PC may also present with very subtle nail changes and that there is not a good correlation between mutations detected at molecular level and clinical phenotype. Severity may even vary among family members with the same gene mutation.55,56

Nail Patella Syndrome

Nail abnormalities in nail patella syndrome may involve all fingernails or may be limited to the thumbs, which are always the most severely affected digits (Fig. 89-9). Nail hypoplasia is usually more marked in the medial portion of the nail. The shape of the lunula is typically triangular.48

Congenital Malalignment of the Hallux

Congenital malalignment of the hallux is the most common cause of ingrowing (ingrown) nails and is usually diagnosed when the child starts to walk. The great toenail shows a lateral deviation that produces embedding of the lateral side of the nail plate (Fig. 89-10). The digit is painful and the toenail often shows Beau's lines and onycholysis. Congenital malalignment may be unilateral or bilateral. Spontaneous improvement may occur, and most children do not have symptoms by the age of 2 years.57 Inflammation due to lateral ingrowing can be managed by daily massaging of the lateral nail fold with creams containing steroids, antibiotics, and urea. Surgical treatment may be necessary if nail symptoms are severe and do not subside with growth.58,59

(See Table 89-3)

Acute Paronychia

Acute paronychia is most commonly due to Staphylococcus aureus infection and typically affects a child's fingernail. Predisposing factors include nail biting or sucking and occupational traumas. The proximal nail fold is painful, erythematous, and swollen. Pus may be discharged after pressure (see Chapter 176). The differential diagnosis includes herpes simplex virus infection (see Chapter 193) and Hallopeau's acrodermatitis (see Chapter 21), both of which have a typical relapsing course. Suspicion of Herpes simplex-virus infection should arise when the pain intensity is disproportionate to the clinical symptoms and the disease is recurrent. The treatment of choice depends on the extent of the infection. If diagnosed early, acute paronychia without obvious abscess can be treated nonsurgically, often with topical antibiotics alone. If an abscess has developed, incision and drainage must be performed. Oral antibiotics with Gram-positive coverage against S. aureus, such as cephalexin, amoxicillin with clavulanic acid, and clindamycin, are effective.

Green Nails

Bacteria are not capable of attacking a healthy nail plate. The Gram-negative bacterium Pseudomonas aeruginosa may colonize the dorsal or ventral nail plate under propitious conditions, such as chronic paronychia or onycholysis. The presence of Pseudomonas is revealed by characteristic green–black nail pigmentation due to pyocyanin staining. Topical application of a few drops of diluted bleach or chlorhexidine solution two or three times a day clears the pigmentation in a few weeks. Administration of systemic antibiotics is unnecessary.

Onychomycosis60,61

For a detailed description of the onychomycoses, see Chapter 188. Onychomycosis is a common toenail disease, and its prevalence increases with age.

Clinical features depend on type of nail invasion (Table 89-4). The possibility of mold onychomycosis should be suspected when proximal subungual onychomycosis (PSO) is associated with periungual inflammation, or when white superficial onychomycosis is severe and involves the entire nail plate (Fig. 89-11). Nondermatophytic onychomycoses are becoming more frequent worldwide and represent a clinical problem, because they usually respond poorly to systemic treatment.

Nail Fragility62

(Box 89-3)

With nail fragility, the nails are brittle and show distal lamellar splitting (onychoschizia) (Fig. 89-12). Several fingernails are usually affected. The nail plate margin is irregular due to distal splitting. Idiopathic nail fragility usually affects middle-aged women who are exposed to water and chemicals that dehydrate the nail plate. Aging also is associated with increased nail fragility. The risk of nail fragility from trauma is increased by manicures, onychotillomania, and certain occupations, especially those that involve frequent exposure to water and chemicals. Fragile nails can be a feature of several dermatologic disorders, such as lichen planus, alopecia areata, psoriasis, and onychomycosis. In addition, nutritional deficiency, peripheral neuropathies, peripheral vascular disease, and use of certain medications increase the risk of nail fragility. Management includes protection of the hands by the use of cotton gloves under rubber gloves and frequent application of topical moisturizers.61 Oral biotin, 5 mg/day, can be helpful.63

Chronic Paronychia64–66

Chronic paronychia is an inflammatory disorder that almost exclusively involves the fingernails of adult women. Mechanical or chemical traumas damage the cuticle and permit penetration of irritant and allergenic environmental substances under the proximal nail fold, causing an inflammatory reaction of the nail folds and matrix. Secondary colonization with Candida sp. and/or bacteria occurs in most cases, causing self-limited episodes of painful acute inflammation (see Chapter 189). Chronic paronychia most commonly affects the first, second, and third fingers of the dominant hand. Clinically, the proximal and lateral nail folds show mild erythema and swelling, and the cuticle is absent. The nail plate may show superficial abnormalities and green discoloration due to Pseudomonas invasion. Hand protection from the environmental hazards is mandatory for remission of chronic paronychia, which can be considered cured only when the cuticle has regrowth. Systemic antifungals are not effective. Chronic paronychia should be treated as contact dermatitis, with topical steroids or tacrolimus associated with topical antiseptics to prevent secondary microbial colonization.

Idiopathic Onycholysis65

Idiopathic onycholysis usually affects the fingernails of women and is a consequence of mechanical and chemical damage of the nail bed isthmus (Box 89-4). The detached nail plate is white due to the presence of air and frequently presents areas of green–brown discoloration due to bacterial colonization (Fig. 89-13). The use of sharp tools to clean the nail plate free margin produces roller coaster onycholysis (manicure onycholysis). Traumatic onycholysis of the fingernails is usually occupational and is more commonly observed in butchers, slaughterhouse workers, chicken processing workers, and workers lifting heavy bags. Traumatic onycholysis of the toenails most frequently affects the big toe, often bilaterally. It is usually a consequence of anatomic abnormalities (overlapping of the second toe on the first toe) or poorly fitting shoes. Subungual hematoma is frequently associated. Secondary onycholysis is seen in several dermatologic disorders. When only one nail is affected, it is important to explore the nail bed after cutting the onycholytic nail to rule out a nail tumor. Patients with idiopathic onycholysis should be instructed to keep the nail dry, trim the onycholytic nail plate, and wear cotton gloves under rubber gloves. Application of topical 4% thymol in chloroform solution on the exposed nail bed can accelerate cure.

See reference 67.

Tumors of the nail usually affect one digit and are associated with symptoms that depend on tumor localization: nail plate furrows or grooves are due to tumors in the proximal nail fold, while onycholysis or subungual nodule are a consequence of nail bed tumor. The nail plate may be altered both in its shape and thickness and in color.

Benign Tumors

Fibroma/Fibrokeratoma

Fibroma/fibrokeratoma appears as a nodular or filiform growth that often has a keratotic surface. Most fibromas originate in the proximal nail fold and extend to the nail plate surface, where the tumor presents as a longitudinal furrow or groove. Subungual lesions are uncommon. Tuberous sclerosis should be considered, especially if multiple lesions are present (see Chapter 140).

Subungual Exostosis68

Subungual exostosis appears as a subungual hard nodule that detaches and lifts the nail plate. The toenails of young adults are usually affected, and a history of trauma is not unusual. Radiography confirms the diagnosis.

Myxoid Cyst69

Myxoid cyst appears as a nodule of the proximal nail fold associated with a longitudinal groove in the correspondent nail plate. Because the cyst often drains spontaneously, the shape of the groove is irregular. Myxoid cysts usually affect the fingernails of middle-aged women and are associated with osteoarthritis of the interphalangeal joints.

Glomus Tumor

Glomus tumor may hardly be seen, appearing as a small red patch under the nail plate and usually affecting the hand. The minimal clinical appearance is disproportionate to the intense pain, which is usually accentuated by cold and radiates to the limb. Due to the small size of the tumor, preoperative assessment with MRI or high-variable frequency ultrasound is advisable to improve the out come of surgery.

Onychomatricoma70

Onychomatricoma is a rare benign fibroepithelial tumor that originates from the nail matrix and produces typical clinical features: the whole or part of the nail is thickened, overcurved, with a yellow–white discoloration and multiple longitudinal tunnels (hollows) that end in the distal nail producing a beehive appearance of the free margin.

Nail Matrix Nevi71,72

Nail matrix nevi produce a band of LM and usually appear in childhood. The color and width of the band are variable, and modification of the pigmentation (fading or darkening) with time is common (eFigs. 89-13.1 and 89-13.2). Pigmentation of the periungual tissues (Hutchinson's sing) is commonly associated with congenital nevi. Clinical management of nail matrix nevi in children is often problematic as lesions frequently show signs (Box 89-5) that are indicative for nail melanoma in adults.73,74 Excision of the lesions that show rapid increase in size is probably the best option, to exclude the possibility of a melanoma, which is rare but can occur.

Malignant Tumors

Squamous Cell Carcinoma

(See Chapter 114).

In situ squamous cell carcinoma (Bowen's disease) usually manifests in fingernails, with a lesion that clinically, closely resembles a wart. Associated melanonychia or paronychia may be a diagnostic clue (Fig. 89-14). Human papillomavirus (HPV) 56 has been detected in tumoral cells of cases of Bowen's diseases associated with LM, indicating that the virus may be involved in the carcinogenesis of these cases.75 Squamous cell carcinoma presents as a slowly growing subungual nodule that eventually ulcerates (Fig. 89-15) or a warty periungual growth. The underlying bone is commonly involved. It is more common in the fingernails and after the fifth decade of life, and the diagnosis is often delayed, since the tumor simulates other benign nail lesions and is frequently not recognized until it ulcerates.76 Oncogenic HPV may be isolated from fingernail lesions. Surgical excision with Mohs surgery is the best treatment for squamous cell carcinoma without bone involvement.

Melanoma.77–79

(See Chapter 124)

Nail melanoma is an uncommon form of acral melanoma that arises within the nail matrix or bed. The incidence for acral melanomas is similar worldwide, but the proportion is higher in dark-skinned individuals. It represents about 2% of cutaneous melanomas in Caucasians, and up to 25% in Africans and 10% in Japanese. Nail melanoma most commonly affects the thumb or great toe of middle-aged or elderly patients and is usually an acral lentiginous melanoma. Melanoma of the nail matrix presents as a band of LM, usually dark in color and with irregular border. Periungual brown–black pigmentation (Hutchinson nail sign) indicates superficial spreading of the tumor and is a diagnostic clue (Fig. 89-16). Although dermoscopy is increasingly utilized in the evaluation of nail pigmentation, the experience in this field is still limited and there are no data showing that dermoscopy is superior to clinical evaluation in early detection of nail melanoma. Recent evidence indicate that dermoscopy should not be considered a substitute for pathology in the differential diagnosis of doubtful cases of LM and an excisional biopsy is recommended in all cases of LM showing suspicious features (Box 89-5).74

Up to 33% of subungual melanomas are amelanotic, and they are often misdiagnosed as pyogenic granuloma or squamous cell carcinoma, because the tumor appears as a nail bed growth that first detaches the nail plate and then destroys the epithelium with erosion and bleeding. The low survival rate of patients with nail melanoma is related mainly to the delay in diagnosis.

(See Chapter 18)

Up to 50% of patients with psoriasis have concurrent nail psoriasis, which can occur in the absence of skin lesions. Up to 30% of patients with skin psoriasis also have psoriatic arthritis and of these, approximately 80% have nail disease. It has been recently understood that the close proximity of the nail unit to the distal phalanx and the joint has important functional and pathological consequences.80 The fibers of the extensor tendon of the digit insert to the periosteum and then are directed to the nail matrix, which they envelop and link to the bone. The collateral ligaments of the digit anchor the lateral sides of the nail to the interphalangeal joint. Thus, any acute inflammatory process affecting the interphalangeal joint necessarily affects the nail and vice versa.

Psoriasis limited to the nails can be easily diagnosed when it produces typical signs, usually detectable only in the fingernails: psoriatic pitting, onycholysis with erythematous border and salmon patches of the nail bed.81 Psoriatic pits are large, deep, and irregular (see Fig. 89-4), and represent psoriatic involvement of the proximal nail matrix. Onycholysis is actually the most common manifestation of nail psoriasis and may affect both fingernails and toenails. In fingernails the presence of an erythematous border along the onycholytic area is diagnostic for nail psoriasis (Fig. 89-17). In toenails, onycholysis is usually combined with subungual hyperkeratosis and may closely resemble onychomycosis (Fig. 89-18). Salmon patches (oil drop sign) appear as yellow–red areas of discoloration in the center of the nail or bordering an onycholytic area. Rarely, nail psoriasis may produce severe nail plate abnormalities such as trachyonychia or crumbling. Other common but rather aspecific signs include splinter hemorrhages and paronychia.

Differential Diagnosis

The differential diagnosis of nail psoriasis is summarized in Box 89-6.

Pathology of nail clipping can be helpful for diagnosis and to rule out onychomycosis.82 When typical symptoms are not present, diagnosis of nail psoriasis may rely on videodermoscopy of the hyponychium (magnification 40×), which shows dilated, tortuous, elongated, and irregularly distributed capillaries, a finding typical of nail psoriasis (eFig. 89-18.1). The capillary density positively correlates with disease severity and decreases with the response to treatment.83

Treatment

Treatment of psoriasis affecting only the nails is often unsatisfactory and should be limited to patients who experience functional impairment or severe cosmetic problems. It is important to instruct patients to avoid trauma and to refer the patient to a rheumatologist if digital pain is described.

Systemic treatments for skin and joint psoriasis are generally effective for nail psoriasis (methotrexate, cyclosporine A). Since the advent of biologic therapies for severe skin and joint psoriasis, their effects on nails symptoms has been investigated and infliximab 5 mg/kg appears to be the most effective to date. Statistically significant mean percent improvement in the Nail Psoriasis Severity Index (NAPSI) score over placebo was obtained at both week 10 and week 24.84 Phototherapy is not effective.

Intralesional steroids (triamcinolone acetonide 2.5–5.0 mg/mL in saline) are the best treatment for nail matrix psoriasis limited to a few fingernails, for which they can be injected in the proximal nail fold every 4–8 weeks. Acitretin at low dosages (0.2–0.3 mg/kg/day) for 4 to 6 months is an effective option in severe nail psoriasis.85 In nail bed psoriasis, topical treatment with calcipotriol, combination of calcipotriol and betamethasone, or tazarotene may be effective after removal of the detached nail plate.86–88

(See Chapters 18 and 21)

The diagnosis of Hallopeau’ acrodermatitis is suggested by a clinical history of relapsing periungual and subungual pustules with onycholysis (Fig. 89-19). The patient is most commonly seen when the acute episode has subsided; at this time the affected digit shows onycholysis with nail bed and periungual erythema and scaling. Low-dose acitretin (0.3 mg/kg/day) may be useful.

(See reference 81 and Chapter 26)

Nail lichen planus is not rare, and nail lesions may occur in the absence of cutaneous or mucosal involvement. Nail localization of lichen planus should be taken seriously, because it may destroy the nails. Therefore, it is important to diagnose and treat the disease as soon as possible. Although lichen planus often affects both the nail matrix and the nail bed, clinical suspicion should be aroused by nail matrix signs, particularly nail thinning with longitudinal ridging and fissuring (Fig. 89-20). Dorsal nail pterygium formation is not common. It results from nail matrix destruction and appears as a V-shaped extension of the skin of the proximal nail fold that adheres to the nail bed. Idiopathic atrophy of the nails is a rare variety of nail matrix lichen planus characterized by acute and progressive painless nail destruction leading to diffuse nail atrophy with and without pterygium. Onycholysis is quite frequent in both fingernails and toenails. Severe toenail involvement causes features that resemble yellow nail syndrome, with thickened, yellow–brown toenails. More rarely, nail lichen planus may present with erosive lesions of the nail bed and periungual tissues. Other possible clinical presentations include trachyonychia and nail plate thickening.

Differential Diagnosis

(Box 89-7)

In patients with brittle nails, the nail abnormalities are milder. The nails are thin and ridged, but fissuring at multiple sites is not observed. Onychoschizia is frequently associated. Trauma that produces damage to the matrix in an area wider than 3 mm causes permanent thinning and fissuring. Only one nail is usually affected, and it may show a single fissure. Nail abnormalities may be the first sign of systemic amyloidosis, with nail thinning, ridging, and fissuring closely resembling mild nail matrix lichen planus. In amyloidosis, nail bed hemorrhages are common. Diagnosis depends on nail biopsy. Lichen striatus may manifest as nail thinning and fissuring limited to one or two adjacent digits. Skin changes that course from the affected nail in a linear configuration are typical. Inherited (EB) and acquired (e.g., bullous pemphigoid) bullous diseases can produce pterygium. Skin and mucosal lesions are usually present. Digital ischemia is distinguished in that the digit is cold and shows skin signs of impaired vascular skin supply.

Treatment

Nail matrix lichen planus requires oral or intramuscular treatment with systemic steroids, which induce remission of the disease in 2/3 of the cases (Figs. 89-21 and 89-22). Intralesional (vs. systemic) corticosteroid injections should be considered in patients with involvement of fewer than three digits. Relapses are not uncommon, but usually respond to treatment.

Dorsal pterygium is not reversible and when it is the sole manifestation, should not be treated. Solitary nail lichen planus presenting as trachyonychia does not lead to nail scarring and therefore does not necessarily require treatment.

Parakeratosis pustulosa is a rather common condition that occurs only in children. The disease is limited to one nail in most cases (usually the thumb or index finger) with a clinical picture that closely resembles that of psoriasis. Characteristics are distal onycholysis, fingertip desquamation, and mild subungual hyperkeratosis. Spontaneous regression usually occurs after puberty.

Trachyonychia (twenty-nail dystrophy) is a nail sign that can be caused by several inflammatory disorders that produce a mild disturbance of nail matrix keratinization. These include alopecia areata, psoriasis, lichen planus, and eczema. The nail is rough and opaque due to excessive longitudinal ridging (Fig. 89-23). The disease occurs most commonly in children. Trachyonychia does not produce nail scarring, even in cases due to lichen planus.

(See reference 1 and Chapter 51)

The nail abnormalities noted in Darier disease are diagnostic but are not seen in all patients). Key features are longitudinal erythronychia, longitudinal leukonychia associated with distal nail plate nicking (V shaped), and subungual hyperkeratotic papules.

(See Chapter 88)

Nail involvement is seen in approximately 20% of adults and 50% of children with alopecia areata and is most common in male patients with severe involvement. Geometric pitting is most typical. Pits are small, superficial, and regularly distributed in a geometric pattern along longitudinal and transverse lines. Trachyonychia is quite common in children affected by alopecia totalis or universalis. Other nail abnormalities include punctate leukonychia, mottled lunulae, and acute onycholysis. Nail abnormalities may improve with systemic steroid treatment or spontaneously; they respond poorly to application of topical medications.

(See Chapter 14)

Nail abnormalities associated with atopic dermatitis are nonspecific in most cases. Manifestations are most commonly Beau's lines, irregular pitting, and chronic paronychia. Onycholysis and subungual hyperkeratosis are seen in severe cases, especially those triggered by occupational exposure.

Yellow nail syndrome may or may not be associated with a systemic disorder and is occasionally familial.88,89 However, it is important to refer patients to a pneumologist to exclude respiratory tract involvement. Other conditions that may be associated with yellow-nail syndrome include rheumatoid arthritis and internal malignancies. The history is the most important clue to diagnosis, because patients always claim that their nails have stopped growing (Fig. 89-24).

The nail changes may benefit from treatment with high oral dosages of vitamin E. Spontaneous improvement is possible. Topical vitamin E and systemic antifungal medications (itraconazole, fluconazole) do not appear to be effective.89–91

(Table 89-5)

In systemic diseases, nail manifestations usually involve most or all nails. The diagnosis is suggested by clinical history, morphology, and distribution of the nail abnormalities. Beau's lines located at the same level in all digits or onychomadesis occurring simultaneously in all digits is strongly diagnostic for nail matrix damage from a systemic cause.

Examination and capillaroscopy of the proximal nail fold are very important for the diagnosis of connective tissue diseases. Patients with scleroderma and dermatomyositis show enlarged capillary loops with reduced capillary density and avascular areas. In systemic lupus, erythematous, periungual erythema and telangiectasia are typical; although periungual vessels can be tortuous, the capillary density tends to be normal. Cuticular hyperkeratosis and hemorrhages are seen in systemic lupus erythematous and dermatomyositis. Pterygium inversum unguium is typical of scleroderma; in this condition the nail plate adheres to the fingertip skin, which makes nail trimming very painful.

The diagnosis of systemic amyloidosis can be suggested by nail symptoms, which sometimes precedes skin and mucosal changes. The nails are thinned, longitudinally fissured, and show subungual hemorrhages.

Mild leukonychia is often seen in normal individuals and therefore does not represent a specific sign of systemic disorders. Clinical patterns include half-and-half nails (leukonychia affects the proximal half of the nail bed), Terry's nails (leukonychia affects the entire nail bed except for its 2-mm distal margin), and Muehrcke's lines (narrow paired transverse bands parallel to the lunula).

Clubbing may or may not be associated with cyanosis (Box 89-8). In clubbing the angle between the proximal nail fold and nail plate is greater than 180 degrees. The digit has a bulbous appearance, and the nail plate is enlarged and excessively curved.

Melanonychia can be associated with a variety of systemic disorders and conditions, particularly human immunodeficiency virus infection, adrenal disease, pregnancy, and certain drugs (Box 89-9).

See reference 92.

A few nail abnormalities are highly likely to have resulted from administration of medication (see Box 89-9). Pyogenic granulomas involving several nails are an occasional side effect of treatment with retinoids, indinavir, and antiepidermal growth factor receptor chemotherapeutic agents, often leading to drug discontinuation. Retinoids have also been associated with nail fragility and paronychia. Taxanes (docetaxel and paclitaxel) frequently induce painful onycholysis with subungual hemorrhage and abscess formation (Fig. 89-25). The nail lesions resemble a subungual infection and are reversible after drug discontinuation. Both β blockers and bleomycin may lead to digital ischemia. Therapy with psoralen plus ultraviolet A light has been associated with photo-onycholysis and melanonychia. Photo-onycholysis has recently been seen after photodynamic therapy.93 and can rarely follow the intake of tetracycline derivatives, psoralens, and fluoroquinolones. It may be associated with a photosensitivity reaction in the skin and typically affects the central part of the nail plate of one or more fingernails (Fig. 89-26). Onycholysis is painful and often hemorrhagic.94

Nail Biting

Nail biting is common in children and may encourage spreading of subungual warts (Fig. 89-27). When nail biting is associated with picking and chewing of the cuticle and periungual skin, the nail plate often shows superficial abnormalities and melanonychia due to melanocyte activation.

Habit Tic Deformity (Onychodystrophia Mediana Canaliformis)

In habit tic deformity the thumb shows a central longitudinal furrow with multiple transverse parallel lines (Fig. 89-28). The nail deformity is due to the nervous tic of pushing back the cuticle and the proximal nail fold of the thumb with the index finger.

Subungual Hematoma

Subungual hematoma may be caused by a single acute trauma or by repeated microtraumas (see Chapter 99). Nail hematomas last several months because part of the blood is incorporated into the nail plate. Very dark lesions require differentiation from melanotic pigmentation. Dermoscopy shows rounded red–black globules (eFig. 89-28.1).74 Acute hematomas require immediate drainage to avoid matrix compression.

Onycholysis

Traumatic onycholysis of the great toenails is frequent in athletes, in women wearing high-heeled shoes, and in individuals with podiatric abnormalities. Nail detachment is not associated with nail bed hyperkeratosis.

Onychogryphosis

Onychogryphosis is common in the elderly and neglected individuals. The nail is thickened, distorted, opaque, and yellow–brown, and tends to have an oyster shell appearance.

Pincer Nails

Pincer nails is a painful abnormality that usually affects the toenails and may be associated with subungual exostosis. The distal nail plate is overcurved and compresses the subungual soft tissues (Fig. 89-29).

Ingrowing Toenails

Ingrowing toenails most commonly affect young adults with congenital malalignment of the great toenails. Improper nail cutting may lead to embedding of a nail edge, causing inflammation and granulation tissue formation. Hyperhidrosis is frequently associated. The aim of treatment for ingrowing toenails is to extract the nail edge that is ingrowing and prevent further penetration of nail fragments into the lateral folds. To accomplish this, the lateral nail plate can be lifted by using a cotton pack or by inserting a gutter splint along the lateral nail margin.95 The width of the nail plate can also be reduced by surgical or chemical (phenolization) removal of the lateral nail matrix.96,97

Retronychia

Retronychia describes the ingrowth of the proximal nail plate into the proximal nail fold associated with multiple generations of nail plate misaligned beneath the proximal nail.98 It involves 1 or both the first toenails and starts as an onychomadesis (posttraumatic) not followed by nail shedding. The persistence of the partially detached plate under the proximal nail fold, associated with the growth of new nail plates underneath it, results in inflammation with pain and granulation tissue formation (Fig. 89-30). Nail plate avulsion leads to a slow regrowth of a normal nail.