Overview of nail disorders

INTRODUCTION — The human nail shields the distal digit from harm, assists in the picking up of small objects, improves fine touch, and enhances the aesthetic appearance of the hand [1]. Aesthetically displeasing nails and nail-associated symptoms, such as pain or throbbing, are common factors that contribute to a patient's decision to seek medical attention.

This topic will discuss the clinical features, diagnosis, and treatment of common acquired nail disorders. Ingrown nails, nail dermoscopy, nail surgery, nail biopsy techniques, and hereditary and acquired nail disorders in children are discussed separately.

●(See "Ingrown nails".)

●(See "Nail disorders in infants and children: Acquired nail diseases".)

●(See "Nail disorders in children: Congenital and hereditary nail diseases".)

●(See "Dermoscopy of nail pigmentations".)

●(See "Nail avulsion and chemical matricectomy".)

●(See "Principles and overview of nail surgery".)

●(See "Nail biopsy: Indications and techniques".)

●(See "Pachyonychia congenita".)

●(See "Nail-patella syndrome".)

ANATOMY AND PHYSIOLOGY OF THE NAIL UNIT — The nail unit is composed of the nail matrix, the nail bed, the proximal and lateral nail folds, and the hyponychium (picture 1 and figure 1) [2,3].

The nail matrix is the germinative epithelium from which nail matrix keratinocytes differentiate to ultimately form the nail plate [2-4]. Most of the nail matrix is hidden beneath the proximal nail fold, but the distal third is sometimes visible through the proximal portion of the nail plate as a half-moon-shaped structure called the lunula (picture 1) [2-4].

Maturation and differentiation of the nail matrix occurs along a diagonal axis oriented distally (figure 2). Thus, the keratinization of the distal matrix cells forms the ventral portion of the nail plate, whereas the keratinization of the proximal matrix cells forms the dorsal portion of the nail plate [2-5]. Nail plate abnormalities typically result from pathologic processes involving the nail matrix or space-occupying lesions involving the overlying nail fold.

On average, fingernail regrowth takes approximately 6 months, and toenail regrowth takes approximately 12 months (up to 18 months for the great toenail) [6]. Nail growth slows with advancing age and vascular disease and can be partially or completely interrupted by systemic illness, trauma, or medications (eg, antimitotic drugs) [6].

The nail bed dermis lies directly beneath the nail plate and is believed to contribute some epithelial cells to the ventral surface of the nail, allowing the nail to grow continuously while adhering to the nail bed [2]. For this reason, nail bed surgery may be complicated by mild onycholysis and, rarely, by permanent nail dystrophy [2]. (See 'Onycholysis' below.)

The dermoepithelial interface of the nail bed is composed of longitudinal rete ridges and papillary body ridges. Each papillary body ridge contains three to five longitudinally oriented capillaries, explaining the longitudinal orientation of splinter hemorrhages [2-4]. (See 'Splinter hemorrhages' below.)

The proximal and lateral nail folds are collectively known as the paronychium (picture 1) [2,3]. The nail folds serve to protect the nail plate and direct nail plate growth in the correct orientation. The hyponychium, located at the distal free edge of the nail plate just proximal to the distal groove, is contiguous with the volar skin. The hyponychium functions to seal and protect the distal nail unit from the environment. Disruption of the hyponychium may result in onycholysis and allow the penetration of pathogens that are unable to digest keratin. The paronychium plus the hyponychium and nail bed is called the perionychium.

The surgical anatomy and blood and nerve supply of the nail apparatus are discussed separately. (See "Principles and overview of nail surgery", section on 'Surgical anatomy of the nail unit'.)

NAIL EVALUATION

Patient instructions — Before the visit, patients should be instructed to bring all nail care products, including cosmetics and instruments, to the appointment. Nail polish, lacquer, or other topical substances should be removed before the appointment to allow for the examination of all nails.

History — Important aspects of the history in a patient with a complaint of nail problems include the time of onset of the disease, occupation, hobbies, nail care habits, topical substance exposure, medical history, medication history, and family history of nail disorders [7,8]. Some clinicians ask their patients to complete a detailed nail questionnaire before the visit (table 1) [7].

Nail examination — All nails should be examined under adequate lighting without glare and with magnification [7,8]. Natural sunlight is preferred over artificial light. Transillumination of the distal phalanx by using a penlight may help in localizing an abnormality. To detect subtle changes of the nail plate surface, alcohol or acetone can be used to cleanse the surface, remove any adherent substances, and reduce glare.

Digits should be relaxed and not pressed against a surface during the examination, since any alteration in the hemodynamics of the nail bed can change the nail appearance. Each component of the nail apparatus, including the nail plate, nail bed, proximal and lateral nail folds, and hyponychium, should be evaluated for any abnormalities.

The nail plate is assessed for discoloration; detachment from the nail bed; and changes in thickness and surface texture, including pitting, ridging, and longitudinal and transverse grooving. (See 'Surface texture abnormalities' below and 'Nail color changes' below and 'Defects of nail plate attachment/nail shedding' below and 'Nail thickening' below.)

The nail bed, nail folds, and hyponychium should be assessed for discoloration, erythema, growths, scale, cuticle attachment, and vascular abnormalities. If a subtle change in pigmentation is found, squeezing the tip of the digit may be helpful in identifying vascular lesions, which become less visible with the application of pressure.

The pattern of nail involvement must be identified. In cases where only one or a few nails are affected, infection, trauma, tumor, or circulation disturbances are possible etiologic factors. However, dermatoses such as lichen planus or psoriasis may affect only one nail.

The skin should also be examined for any concurrent findings [7,8]. Appropriate referral is indicated for patients with nail signs associated with systemic diseases. (See 'Nail signs of systemic diseases' below.)

SURFACE TEXTURE ABNORMALITIES

Brittle nails — Brittle nails are characterized by roughness and dullness of the surface of the nail plate. A lamellar exfoliation (onychoschizia) or splitting (onychorrhexis) may be present at the distal margin of the nail plate.

Overview — Brittle nails are a common nail disorder that predominantly affect women over the age of 50 years, with an estimated prevalence rate of approximately 20 percent in the general population [9-11]. It presents with roughness of the surface of the nail plate, fragility, and peeling. Ultrastructurally, brittle nails show a disorganized protein and lipid structure with a dishomogeneous orientation of keratin filaments.

Causes of nail brittleness include [12]:

●Aging

●Pregnancy

●Associated dermatoses (eg, eczema, lichen planus, alopecia areata)

●Systemic disorders (eg, peripheral arterial disease, iron deficiency anemia, endocrine disorders)

●Environmental exposures (eg, wet work occupational exposure to chemicals [solvents, alkalis, acids], use of nail cosmetics [nail polish, nail polish removers, nail sculpturing, application of acrylic gels])

●Repeated microtrauma (occupational, onychotillomania, onychophagia)

●Drugs (eg, retinoids, chemotherapeutic agents)

●Idiopathic

The treatment of brittle nails involves uncovering and treating the underlying condition; avoiding contact with detergents, solvents, external irritants, and trauma (eg, using gloves for wet work and chores); avoiding aggressive manicures with excessive buffing and filing; and moisturizing the nails with emollients. Although biotin supplementation is often used for the treatment of brittle nails, evidence for its efficacy is lacking.

Onychoschizia — Onychoschizia, also called lamellar dystrophy, is characterized by lamellar splitting of the free edge of the nail due to impairment of intercellular adhesive factors of the nail plate (picture 2). It is often caused by exposure to external factors that alter the intercellular adhesive factors of the nail plate (wet work, chemicals, trauma).

Onychorrhexis — Onychorrhexis occurs when superficial grooves in the nail plate lead to a distal split (picture 3).

Transverse grooves (Beau lines) — Beau lines result from a temporary arrest of proximal nail matrix proliferation and appear as transverse grooves that move distally with nail growth (picture 4A-B). The time of the insult leading to transverse grooving can be dated by measuring the distance of the groove from the proximal nail fold (approximately one month for every millimeter from the proximal nail fold).

Causes of transverse grooving include [13-16]:

●Local trauma (eg, manicure, onychotillomania [habit tic deformity], ill-fitting footwear)

●Local cutaneous disease (eg, dermatitis, paronychia)

●Drugs (eg, retinoids, chemotherapy agents)

●Viral infections (eg, hand, foot, and mouth disease) (see "Hand, foot, and mouth disease and herpangina", section on 'Coxsackievirus A6 HFMD' and "Hand, foot, and mouth disease and herpangina")

●Pemphigus

●Kawasaki disease (see "Kawasaki disease: Clinical features and diagnosis", section on 'Extremity changes')

Longitudinal grooves — Longitudinal grooves may result from focal compression of the nail matrix from tumors or myxoid pseudocysts located in the proximal nail fold or nail bed (picture 46D).

Median nail dystrophy, also called median canaliform dystrophy of Heller, is a distinctive form of longitudinal groove characterized by a paramedian canal or split in the nail plate of one or more nails (picture 4C) [17]. Small cracks or fissures that extend laterally from the central canal or split toward the nail edge give the appearance of an inverted fir tree. The condition is usually symmetric and most often affects the thumbs. The condition results from a temporary defect of matrix function of unknown etiology. Trauma, including habitual nail picking, has been implicated as a causative factor in some cases. (See "Skin picking (excoriation) disorder and related disorders", section on 'Nail picking disorder'.)

Longitudinal grooves must be distinguished from physiologic furrows and ridges, which are accentuated in lichen planus, rheumatoid arthritis, peripheral vascular disease, older age, and Darier disease. (See 'Darier disease' below.)

Pitting — Nail pitting results from focal areas of abnormal keratinization of the nail matrix that produce foci of parakeratotic cells in the dorsal nail plate as it grows beyond the cuticle (picture 5A-B). Nail pitting is seen in patients with psoriasis, alopecia areata, and eczema. (See "Nail psoriasis", section on 'Clinical manifestations' and "Alopecia areata: Clinical manifestations and diagnosis", section on 'Nail abnormalities'.)

Trachyonychia — Trachyonychia is a nail plate abnormality characterized by roughness, excessive longitudinal ridging, pitting, thickening of the cuticle, and distal brittleness (picture 5C). Trachyonychia results from multiple foci of defective keratinization of the proximal nail matrix.

Trachyonychia involving most or all nails is also called "twenty nail dystrophy" (picture 6). It occurs predominantly in children and may be idiopathic or associated with other skin diseases, most commonly psoriasis, lichen planus, alopecia areata, or atopic eczema [18]. Histology shows a psoriasiform/lichenoid infiltrate or a spongiotic infiltrate [19]. In approximately 50 to 80 percent of children, the disease resolves spontaneously over several years [20,21].

NAIL COLOR CHANGES

Leukonychia — "True leukonychia" results from defective keratinization of the distal matrix with persistence of parakeratotic cells in the ventral nail plate. The nail has opaque, white patches or striae that do not change upon applying pressure on the nail plate and move distally as the nail grows (picture 7) [22].

In contrast, "apparent leukonychia" is due to abnormal vasculature of the nail bed, disappears with pressure, and does not disappear as the nail grows.

Leukonychia punctata — Leukonychia punctata is the most common type of leukonychia. It occurs most often in children and presents as small, 1 to 3 mm, white spots in the nail plate that move distally as the nail grows. It is most frequently caused by trauma.

Transverse leukonychia (Mees' lines) — Transverse leukonychia, also called leukonychia striata or Mees' lines (picture 8), is characterized by a white discoloration of the nail plate in bands or striae 1 to 2 mm wide that run parallel to the nail base. They are caused by abnormal keratinization of the nail matrix due to repeated trauma, infections, drugs (systemic retinoids, taxanes), systemic disease, or exposure to arsenic and thallium [23-30].

Muehrcke's lines — Muehrcke's lines typically appear as a couple of transverse, white bands that run parallel to the lunula across the entire width of the nail. First described in patients with hypoalbuminemia and nephrotic syndrome, Muehrcke's lines are reported in association with systemic diseases (eg, liver disease, malnutrition, organ transplant, HIV infection) and chemotherapy [31]. They are a type of "apparent leukonychia," likely due to vascular changes in the nail bed. On examination, they will disappear when pressure is applied to the nail plate.

Longitudinal white lines — Longitudinal white lines can be seen in most patients with two rare inherited disorders: Darier disease (picture 9) and Hailey-Hailey disease. (See "Darier disease" and "Hailey-Hailey disease (benign familial pemphigus)".)

Half-and-half nails — Half-and-half nails (also termed "apparent leukonychia" or "Lindsay nails") present with a dull, white, ground-glass appearance involving the proximal portion of the nail due to underlying nail bed changes (picture 10). They are predominantly seen in patients with chronic renal insufficiency and uremia [32,33]. (See 'Nail signs of systemic diseases' below.)

Terry's nails — Terry's nails are characterized by leukonychia that involves more than the proximal two-thirds of the nail plate, whereas the distal third appears red (picture 11A-B). They are seen in patients with liver cirrhosis and other systemic diseases. (See 'Nail signs of systemic diseases' below.)

Total leukonychia — In the rare inherited total leukonychia (MIM #151600), a nonsyndromic, isolated nail disorder due to a mutation in the phospholipase C, delta-1 (PLCD1) gene, all nails are milky and porcelain white (picture 12) [34].

Longitudinal melanonychia — Longitudinal melanonychia is a banded, brown to black pigmentation of the nail due to the presence of melanin in the nail plate. Longitudinal melanonychia may appear as a single band involving one nail or as multiple bands affecting several nails. The latter form is most commonly seen in individuals with darkly pigmented skin (picture 13). Longitudinal melanonychia is discussed separately. (See "Longitudinal melanonychia".)

Longitudinal erythronychia — Longitudinal erythronychia is a pink to red, longitudinal streak in the nail plate that corresponds to a band of thinned, more transparent nail plate [35,36]. Longitudinal erythronychia is caused by a focal reduction of function in the distal matrix. The nail bed underlying the band is less compressed than the adjacent portions so that blood pools and the engorged tissue swells and fills the ventral groove of the nail plate.

Splinter hemorrhages are commonly found in longitudinal erythronychia. The thinned portion of the nail plate extends to the distal free margin and is easily traumatized during daily activities, resulting in splitting and V-shaped chipping. Reactive hyperkeratosis of the underlying hyponychium may also occur.

Localized (monodactylous) longitudinal erythronychia involves one nail and may be associated with benign or malignant nail tumors, including (picture 14A-B) [35-39]:

●Onychopapilloma (picture 15A-D) (see 'Onychopapilloma' below)

●Warty dyskeratoma

●Glomus tumor (see 'Glomus tumor' below)

●Squamous cell carcinoma (SCC), including SCC in situ (Bowen disease) (see 'Squamous cell carcinoma' below)

●Amelanotic melanoma (see "Longitudinal melanonychia")

Longitudinal erythronychia involving multiple nails (polydactylous longitudinal erythronychia) (picture 16) is most often associated with lichen planus or Darier disease, but systemic amyloidosis, sarcoidosis, hemiplegia, graft-versus-host disease, acantholytic epidermolysis bullosa, and idiopathic cases have also been reported [39-44]. (See "Darier disease", section on 'Nail changes'.)

The necessity of biopsy depends upon the number of involved digits and associated conditions or symptoms. Biopsy is necessary for the diagnosis of monodactylous longitudinal erythronychia. For patients with polydactylous longitudinal erythronychia and a known associated condition, biopsy is not generally required for diagnosis. Patients with polydactylous longitudinal erythronychia without associated cutaneous or systemic symptoms are generally observed over time for development of cutaneous or systemic symptoms of associated conditions [36].

Biopsy indications and techniques for longitudinal erythronychia are discussed separately. (See "Principles and overview of nail surgery", section on 'Longitudinal erythronychia'.)

Splinter hemorrhages — Splinter hemorrhages appear as red to black, small, thin, longitudinal lines under the nail plate. In general, they are more commonly located in the distal nail plate and represent rupture of the longitudinally oriented nail bed capillaries (picture 17A-B). The most common causes of splinter hemorrhages are trauma (eg, nail biting) and nail psoriasis (picture 17B and picture 18) [45].

However, splinter hemorrhages may be associated with other chronic dermatoses involving the nail (eg, lichen planus, Darier disease (picture 19)) and, rarely, with systemic illness, such as infective endocarditis (picture 20), connective tissue disease, antiphospholipid syndrome, chronic renal failure, and trichinellosis [45-50]. (See "Lichen planus", section on 'Nail lichen planus' and "Darier disease", section on 'Nail changes' and "Clinical manifestations and evaluation of adults with suspected left-sided native valve endocarditis".)

Splinter hemorrhages associated with systemic diseases are often proximal in the nail, whereas those due to trauma are usually distal. However, all splinter hemorrhages grow out distally (picture 19).

Color changes of the lunula — Erythema of the lunula ("red lunula") has been described in patients with psoriasis, alopecia areata, and connective tissue diseases [49,51-53]. A blue discoloration of the lunula has been reported in patients treated with chemotherapy agents [54-56].

DEFECTS OF NAIL PLATE ATTACHMENT/NAIL SHEDDING

Onycholysis — Onycholysis is defined as distal or distal-lateral separation of the nail plate from the underlying nail bed and/or lateral supporting structures, such as the hyponychium and lateral nail folds (picture 17B) [57,58]. The onycholytic portion of the nail plate appears white due to air beneath the nail plate.

Onycholysis may be associated with many conditions, including trauma, wet work, psoriasis, lichen planus, medications, onychomycosis, allergic or irritant contact dermatitis, or yellow nail syndrome. For cases of chronic unexplained onycholysis, subungual squamous cell carcinoma should be included in the differential diagnosis [58].

Onycholysis is a predisposing condition for secondary subungual infections from dermatophytes (eg, Trichophyton rubrum), yeasts (eg, Candida albicans), or bacteria (including Pseudomonas aeruginosa and Staphylococcus aureus). If infection is suspected, cultures should be obtained and appropriate antifungal or antibacterial treatment started.

The management of onycholysis involves the identification and treatment of the underlying condition. General nail care measures for onycholysis include [57,58]:

●Keeping nails trimmed short

●Avoiding trauma

●Avoiding contact irritants

●Keeping nails dry (avoiding wet work)

●Avoiding all nail cosmetics

●Protecting hands from cold or windy weather

Photo-onycholysis — Photo-onycholysis is a rare photosensitivity reaction resulting in the separation of the nail plate from the nail bed, most often following the intake of photosensitizing medications or in the setting of photosensitive blistering disorders, such as the porphyrias or pseudoporphyria [59]. A few spontaneous cases of photo-onycholysis have also been reported. (See "Porphyrias: An overview", section on 'Blistering cutaneous porphyrias' and "Pseudoporphyria".)

Drugs most frequently involved in photo-onycholysis include [59]:

●Tetracyclines

●Fluoroquinolones

●Antifungal agents (griseofulvin, voriconazole)

●Nonsteroidal anti-inflammatory drugs (diclofenac)

●Psoralens (psoralen plus ultraviolet A [PUVA] therapy)

●Targeted anticancer agents

●Topical aminolevulinic acid (photodynamic therapy)

●Antipsychotic drugs

Drug-induced photo-onycholysis typically manifests more than two weeks after the exposure to the photosensitizing drug and may follow a cutaneous photosensitivity reaction or be an isolated reaction. The onycholysis may involve one or more fingers and appears as a semilunar or oval area of detachment of the nail plate with variable brownish pigmentation (picture 21).

Retronychia — Retronychia is a form of incomplete nail shedding that leads to the embedding of the proximal nail plate into the proximal nail fold with subsequent inflammation (picture 22) [60-62]. It predominantly occurs in young adults with a female predominance but has also been reported in children [63,64]. In most cases, the great toes are affected, likely due to repetitive minor trauma to the free nail margin that leads to incomplete separation of the proximal nail plate from the matrix. The new nail growing from the matrix pushes the old one upward, causing inflammation of the proximal nail fold and interrupting the longitudinal growth of the nail plate.

Simple avulsion is usually helpful, but the condition can recur in the setting of chronic distal onycholysis and continued minor trauma/pressure on the distal nail plate. Mild cases may respond to conservative treatment with topical high-potency corticosteroids [65].

Onychomadesis — The detachment of the nail plate from the proximal nail fold by a full-thickness sulcus is called "onychomadesis" and results from a more severe or prolonged insult to the nail matrix (picture 23A-B). Rare idiopathic or familial cases of onychomadesis have also been reported [66,67].

NAIL THICKENING

Subungual hyperkeratosis — Subungual hyperkeratosis is due to 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 (picture 17D), onychomycosis (picture 24A), trauma, and eczema. (See 'Psoriasis' below and "Nail psoriasis" and "Onychomycosis: Epidemiology, clinical features, and diagnosis" and "Onychomycosis: Management".)

Onychogryphosis — Onychogryphosis, also called "ram's horn nail," is an acquired nail disorder usually affecting the great toenail, which appears thickened, yellow-brown in color, increased in length, and distorted (picture 25). It is common in older adults and neglected individuals [68-71]. The cause of onychogryphosis is incompletely understood and may involve peripheral vascular disease, repeated trauma from poor-fitting footwear, and self-neglect.

Onychogryphosis is a feature of some rare inherited disorders, including Papillon-Lefèvre syndrome, Haim-Munk syndrome, ichthyosis hystrix, and junctional epidermolysis bullosa (picture 26). (See "Palmoplantar keratoderma", section on 'Haim-Munk syndrome' and "Palmoplantar keratoderma", section on 'Papillon-Lefèvre syndrome' and "Keratinopathic ichthyoses", section on 'Ichthyosis hystrix Curth-Macklin and ichthyosis Lambert type' and "Epidermolysis bullosa: Epidemiology, pathogenesis, classification, and clinical features", section on 'Junctional epidermolysis bullosa'.)

Complications include ingrown toenails, paronychia, and secondary onychomycosis. Treatment involves conservative measures (eg, nail trimming/filing/grinding with nail clippers and electric burs, use of appropriate footwear). The definitive treatment is nail avulsion followed by surgical or chemical matricectomy using phenol.

Pachyonychia congenita — Pachyonychia congenita is a rare autosomal dominant disorder characterized by the triad of hypertrophic nail dystrophy, plantar keratoderma with underlying blisters, and severe plantar pain (picture 27A-B). Pachyonychia congenita is discussed separately. (See "Pachyonychia congenita".)

INFECTIONS OF NAILS AND PERIUNGUAL TISSUES

Fungal infection — Onychomycosis, a fungal infection of the nail unit, is characterized by nail discoloration, thickening, and deformity (picture 24A-E) [72]. The clinical presentation, diagnosis, and treatment of onychomycosis are discussed separately. (See "Onychomycosis: Epidemiology, clinical features, and diagnosis" and "Onychomycosis: Management".)

Bacterial infections

Green nail syndrome — P. aeruginosa, a water-borne bacterium, may secondarily infect injured or onycholytic nails [73]. The infection is characterized by a blue-greenish color of the nail plate due to the deposition of pyocyanin, a blue-green pigment produced by this bacterium (picture 28). The pyocyanin pigmentation can persist for months despite adequate treatment. (See "Pseudomonas aeruginosa skin and soft tissue infections", section on 'Green nail syndrome'.)

Acute paronychia — Acute paronychia is the most common infection of the hand and typically results from local injuries to the nail fold (picture 29A-B) [74]. Most acute nail unit infections are caused by S. aureus. Other bacteria, including Streptococcus or P. aeruginosa, may cause acute paronychia.

Acute paronychia typically involves a single digit and is characterized by the rapid onset of painful erythema and swelling of the proximal and lateral nail folds. A superficial abscess is frequently present (picture 29A and picture 30).

The management of acute paronychia is discussed separately. (See "Paronychia".)

Blistering distal dactylitis — Blistering distal dactylitis is a localized infection involving the volar pad of the distal phalanx of the digits. It occurs in children and adults. Tense, nontender, oval bullae filled with thin, seropurulent fluid usually lie over the anterior fat pad and may extend dorsally to involve the proximal or lateral nail fold (picture 31A-C). Cultures from the blister fluid most commonly grow group A beta-hemolytic Streptococcus and, less often, S. aureus [75,76].

Felon (pulp space infection) — Felon is an acute infection of the fingertip pulp space usually involving the thumb and index finger (picture 32). Abscess forms in the small compartments of the fingertip pulp separated by vertical, fibrous septa (figure 3). Penetrating trauma is the most common cause of felon, but it can also be a complication of untreated paronychia. (See "Overview of hand infections", section on 'Pulp space infections'.)

Viral infection

Warts — Human papilloma virus infection is the most common viral infection of the nail. Warts usually occur in the nail fold and, less frequently, in the nail bed (picture 33). Warts involving the proximal nail fold result in longitudinal ridging and nail plate dystrophy. Nail bed warts may cause onycholysis. Periungual and subungual warts are especially resistant to treatment, and overzealous treatment in the area of the nail matrix can cause permanent nail dystrophy [73]. (See "Cutaneous warts (common, plantar, and flat warts)".)

Herpetic whitlow — Herpetic whitlow is a herpes simplex virus infection of the hand that involves the skin or the periungual area of a finger (picture 34). The infection is acquired by direct inoculation of the virus following a minor local trauma [74,77]. Herpetic whitlow typically occurs in children who suck their finger during a primary herpetic gingivostomatitis, but it is also an occupational hazard for medical and dental personnel. (See "Overview of hand infections", section on 'Herpetic whitlow'.)

SKIN DISEASES WITH NAIL INVOLVEMENT

Psoriasis — Psoriasis is the most common dermatosis involving the nails [78,79]. (See "Nail psoriasis".)

Clinical features that suggest nail psoriasis include [78]:

●Nail plate pitting – Nail pitting is the most common sign of nail psoriasis. It results from focal areas of abnormal keratinization of the nail matrix that produce foci of parakeratotic cells in the dorsal nail plate as it grows beyond the cuticle. Psoriatic pits are typically irregular, deep, and randomly distributed within the nail plate (picture 5A).

●Nail bed salmon patches – Nail bed salmon patches (also termed "oil drops") are irregular areas of yellow or pink discoloration in the nail bed visible through the nail plate, resulting from psoriatic inflammation of the nail bed (picture 17C).

●Onycholysis – Onycholysis results from the distal separation of the nail plate from the inflamed, underlying nail bed. An erythematous border and splinter hemorrhages are often associated with onycholysis (picture 17B). Onycholysis and subungual hyperkeratosis may be the only manifestations of psoriasis of the toenails (picture 17D).

Other signs of nail psoriasis include paronychia, leukonychia (white nails), erythema of the lunula, and trachyonychia (rough, lusterless nails) [78,79]. (See 'Leukonychia' above and 'Trachyonychia' above.)

Psoriasis of the nail folds resembles chronic paronychia and is often precipitated by treatment with systemic retinoids [80].

Mycology testing is indicated to differentiate psoriatic nail disease from onychomycosis, particularly if the disease is limited to the toenails. However, onychomycosis and nail psoriasis may coexist in approximately 20 percent of patients with psoriasis [81].

The treatment of nail psoriasis is discussed separately. (See "Nail psoriasis", section on 'Treatment'.)

Parakeratosis pustulosa — Parakeratosis pustulosa is a nail disorder that occurs almost exclusively in children (picture 35) [82]. In most cases, the disease is limited to one nail, usually the thumb or the index finger, and presents with distal onycholysis, fingertip desquamation, and mild subungual hyperkeratosis. Parakeratosis pustulosa begins with an acute, eczematous inflammation of the periungual skin, causing hyperkeratosis and thickening of the free edges of the nail. Scaling is more marked than pustulation. The course is often protracted, and recurrences are the rule. Spontaneous regression may occur after puberty.

Lichen planus — Lichen planus is an inflammatory condition of unknown etiology that affects the skin, mucous membranes, hair follicles, and nails. Approximately 10 to 25 percent of patients with lichen planus have nail involvement [78,79,83,84]. Lichen planus of the nails occurs more commonly in the adult population and typically affects several or most nails:

●Clinical features – The clinical features of lichen planus vary by site of involvement within the nail apparatus [78,79]. Lichen planus of the nail matrix results in longitudinal ridging, nail plate thinning, longitudinal fissuring (picture 36A), trachyonychia (picture 36B), and erythema of the lunula [78]. Postinflammatory hyperpigmentation of the proximal nail fold and longitudinal melanonychia can also occur with nail matrix involvement [85]. Nail bed lichen planus usually occurs in conjunction with nail matrix involvement and is characterized by onycholysis, with or without subungual hyperkeratosis, and a violaceous hue of the nail bed.

Lichen planus of the nails can be rapidly progressive. Permanent nail dystrophy in the forms of anonychia and dorsal pterygium (the extension and adherence of the proximal nail fold to the nail bed secondary to scarring of the nail matrix (picture 36C and picture 37)) may occur.

●Diagnosis – Lichen planus is usually diagnosed by clinical examination alone, but if the diagnosis is questionable, a 3 mm punch biopsy of the nail matrix and/or nail bed is usually conclusive. Histology reveals a band-like infiltrate of the nail matrix and nail bed dermis, with hyperkeratosis, hypergranulosis, and acanthosis of the nail matrix epithelium [86].

●Treatment – Lichen planus of the nails is generally treated with systemic or intralesional corticosteroids:

•Systemic corticosteroids are preferred in cases that are rapidly progressive or involve more than three nails. Oral prednisone 0.5 to 1 mg/kg per day (maximum 60 mg per day) is given for four to six weeks and then tapered over the next four to six weeks [87]. As an alternative, monthly intramuscular injections of triamcinolone acetonide at a dose of 0.5 mg/kg may be given for three to six months [78,79,88].

•Intralesional corticosteroid therapy is an option when less than three nails are involved. Triamcinolone acetonide diluted to 2.5 to 5 mg/mL can be injected into the proximal and lateral nail folds at monthly intervals [88]. These injections are placed intradermally in the nail folds, from which the solution can diffuse to the underlying matrix. Injection directly into the nail matrix is uncomfortable and unnecessary. Treatment is generally continued at monthly intervals until the proximal one-half of the nail appears normal, at which time injections can be tapered.

In patients who do not respond to treatment, systemic or topical corticosteroids should be stopped after five to six months. Acitretin at a dose of 0.35 mg/kg per day may be a treatment option in recalcitrant cases [88].

The treatment of nail lichen planus with topical or systemic corticosteroids has not been evaluated in randomized trials. Their use is based upon clinical experience and limited evidence from small case series.

In a study of 27 patients with nail lichen planus treated with intramuscular or intralesional corticosteroids who were followed for more than 5 years (mean follow-up 10 years), 9 patients did not respond to treatment, 18 were cured, and 11 relapsed [88]. In another study, 67 patients with histologically confirmed lichen planus of the nails were treated with systemic and/or intralesional corticosteroids for six months [86]. Complete or substantial improvement was reported in 42 patients (63 percent).

Alopecia areata — Nail involvement occurs in approximately 50 percent of children and 20 percent of adults with alopecia areata [89,90]. Nail changes may not occur at the same time as hair loss and are more common in males and severe cases [78,79,89,91,92]. Abnormalities may involve one or more nails and include mottled erythema of the lunula ("spotted lunula"), longitudinal ridging or trachyonychia (picture 5C), and geometric pitting (multiple small, superficial pits regularly distributed along longitudinal and transverse lines on the nail plate (picture 5B)). (See "Alopecia areata: Clinical manifestations and diagnosis".)

The diagnosis is usually made clinically, but a nail matrix biopsy may be performed in questionable cases. Pathology reveals a lymphocytic infiltrate with spongiosis in the proximal nail fold, nail matrix, nail bed, and hyponychium [19].

Nail abnormalities are typically stable and improve spontaneously over a period of years. Topical corticosteroids or monthly intralesional injections of 2.5 to 3 mg/mL of triamcinolone acetonide into the proximal nail fold can be administered if spontaneous clearing of the nails does not occur [79].

Darier disease — Darier disease, also termed "keratosis follicularis," is an autosomal dominant condition caused by variants in the ATP2A2 gene and characterized by greasy, hyperkeratotic papules in seborrheic regions. Approximately 90 percent of patients with Darier disease have nail involvement [79]. (See "Darier disease".)

Fingernails are affected more often than toenails. Nail matrix involvement results in onychorrhexis (nail fragility), splitting, fragility, and red and white, longitudinal streaks in the nail plate (picture 38 and picture 9). Nail bed involvement is characterized by subungual hyperkeratosis and V-shaped notches in the distal nail plate at the free edge. Keratotic papules may be present over the proximal nail fold.

Sarcoidosis — Nail involvement is rare in patients with sarcoidosis. Nails appear yellow and dystrophic, often with splinter hemorrhages, nail plate pitting and/or crumbling, subungual hyperkeratosis, and painful paronychia (picture 39) [93,94]. (See "Cutaneous manifestations of sarcoidosis", section on 'Nail sarcoidosis'.)

NAIL DISORDERS RELATED TO NAIL COSMETICS — A number of nail disorders can be induced by cosmetic procedures or products. These include nail discoloration (picture 40), superficial damage of the nail plate, traumatic onycholysis, and contact reactions (irritant or allergic) to acrylic-based gel polishes or artificial nail application [95-97]. (See "Common allergens in allergic contact dermatitis", section on 'Acrylates'.)

Contact allergic reactions may present with subungual hyperkeratosis and/or onycholysis resembling nail psoriasis. Pterygium inversum unguis (abnormal adherence of the hyponychium to the ventral surface of the nail plate) has also been described [98].

Thinning of the distal nail plate and onychoschizia ("worn down nail") may occur due to repeated trauma from application and removal of acrylic nail gel [99,100].

BENIGN TUMORS

Fibroma — Nail fibromas are benign tumors of the connective tissue that most commonly originate in the nail matrix. However, they can also arise in the nail bed or in the proximal nail fold and extend to the nail plate surface [101-103]. There are several types of nail fibromas, including:

●Acquired periungual fibrokeratoma – Acquired periungual fibrokeratoma is an acquired tumor presenting as a small, asymptomatic, fleshy growth with a keratotic distal tip (picture 41C) [104]. It usually arises following local trauma.

●Dermatofibroma – Periungual dermatofibromas are rare. They present as a flesh-colored, pea-shaped growth that usually develops slowly and may involve the distal subungual area with displacement of the nail plate or the proximal or lateral nail folds (picture 41A-B).

●Koenen tumor (periungual fibroma) – Koenen tumors are periungual or subungual fibromas that develop in approximately 50 percent of patients with tuberous sclerosis during childhood or adolescence. Koenen tumors occur more commonly on the toenails than on the fingernails. They present as erythematous, polypoid, digitated growths; are often multiple; and may produce a longitudinal groove in the nail plate due to matrix compression (picture 41D). (See "Tuberous sclerosis complex: Clinical features", section on 'Dermatologic manifestations'.)

Onychomatricoma — Onychomatricoma is a rare benign fibroepithelial tumor that originates from the nail matrix. Clinical manifestations of onychomatricoma include (picture 42) [105,106]:

●Yellow, longitudinal bands of variable width

●Splinter hemorrhages of the proximal portion of the nail plate

●Prominent longitudinal ridging associated with woodworm-like cavities

●Increased transverse curvature of the nail plate

Less frequent presentations of onychomatricoma include pincer nail deformity (picture 43), cutaneous horn, melanonychia, nail bleeding, or pterygium (the extension and adherence of the proximal nail fold to the nail bed secondary to scarring of the nail matrix (picture 37)) [107-115]. Nail plate avulsion in onychomatricoma reveals finger-like projections originating from a villous tumor of the nail matrix.

It has been suggested that a diagnosis of onychomatricoma can be made by histologic examination of a distal clipping of the free edge of the nail plate [116]. The specimen is submitted for routine hematoxylin and eosin staining and sectioned in the transverse plane. The finding of increased nail thickness with cavitations filled with serous material and surrounded by a layer of epithelium suggests the diagnosis of onychomatricoma.

Onychomatricoma may mimic subungual fibroma, fibrokeratoma, or squamous cell carcinoma (including squamous cell carcinoma in situ [Bowen disease]) [112,113].

The surgical removal of onychomatricoma is discussed separately. (See "Principles and overview of nail surgery", section on 'Onychomatricoma'.)

Digital myxoid cyst or myxoid pseudocyst — A digital myxoid cyst (DMC) or myxoid pseudocyst typically presents as a translucent nodule on the dorsum of the digit between the distal interphalangeal joint and the proximal nail fold (picture 44A) [117]. DMCs are most frequently located on the radial fingers [101].

DMCs result from mucoid degeneration of the connective tissue and/or joint fluid leaking from an osteoarthritic distal interphalangeal joint by way of a communicating canal. Although DMC is commonly referred to as a cyst, on histologic examination, the collection of mucinous fluid is not surrounded by an epithelial lining. Thus, a more appropriate term is "myxoid pseudocyst."

A DMC located distally in the proximal nail fold may exert pressure on the underlying nail matrix, resulting in a longitudinally oriented, depressed groove in the nail plate (picture 44B) (see 'Longitudinal grooves' above). Occasionally, a DMC may discharge its mucinous content and reduce the pressure on the nail matrix, resulting in an irregular, longitudinal depression in the nail plate [117]. Subungual DMC variants may present as subungual tumors associated with a red lunula, transverse nail plate over-curvature, and ingrowing nail plate [101,118].

High-resolution ultrasonography or magnetic resonance imaging (MRI) may confirm the diagnosis in cases that are clinically ambiguous [101]. The communicating canal between the DMC and the distal interphalangeal joint are visible on MRI in over 80 percent of cases [119].

A wide range of therapies are used to treat DMCs, including digital compression, cryotherapy, sclerosant injection, or surgical excision. Spontaneous discharge of a DMC is a risk factor for septic osteoarthritis of the distal interphalangeal joint and is an indication for DMC removal [101]. The surgical treatment of DMCs is discussed separately. (See "Principles and overview of nail surgery", section on 'Digital myxoid cyst'.)

Pyogenic granuloma — A pyogenic granuloma is a benign vascular tumor presenting as a rapidly evolving, sessile or polypoid nodule composed of red, friable granulation tissue that bleeds easily (picture 45) [120]. Pyogenic granuloma may involve the nail fold or be subungual and penetrate the nail plate. Subungual pyogenic granulomas arise from the nail matrix and produce a localized deformity of the nail plate. Local trauma or medications have been associated with the development of periungual or subungual pyogenic granuloma. (See "Pyogenic granuloma (lobular capillary hemangioma)".)

Glomus tumor — Glomus tumor is a rare, benign tumor composed of cells resembling the smooth cells of the normal glomus body [121]. It presents as a red to purple or blue lesion under the nail plate (picture 46A-D) [121-123]. Symptoms include paroxysmal pain, cold sensitivity, and tenderness. The diagnosis is suspected based upon clinical appearance and history of paroxysmal pain and cold sensitivity [123,124]. (See "Overview of benign lesions of the skin", section on 'Glomus tumor'.)

Imaging studies with MRI or high-variable frequency ultrasonography may be helpful for confirming the clinical suspicion and assessing the size and location of the tumor preoperatively [125-127]. The treatment of glomus tumors is surgical. Histologic examination of the excised tumor is necessary to confirm the diagnosis.

The surgical treatment of glomus tumors is discussed separately. (See "Principles and overview of nail surgery", section on 'Glomus tumors'.)

Subungual exostosis — Subungual exostosis is a benign, osteocartilaginous tumor that most commonly occurs on the dorsomedial aspect of the tip of the great toe in adolescents or young adults [128]. In its early stage, the tumor typically presents as a firm, porcelain-white, telangiectatic nodule with an overlying collarette of scale that extends from beneath the distal nail, causing reddish onycholysis. With time, the tumor becomes hyperkeratotic (picture 47A-B). Pain is variable and can be absent in some patients. Plain radiographs may confirm the clinical diagnosis [128]. Treatment is surgical excision.

The surgical treatment of subungual exostosis is discussed separately. (See "Principles and overview of nail surgery", section on 'Subungual exostosis'.)

Onychopapilloma — Onychopapilloma is a rare, benign tumor of the nail bed or distal part of the nail matrix that most commonly presents as a longitudinal erythronychia (picture 15A-D) [129,130] (see 'Longitudinal erythronychia' above). Less common presentations include leukonychia, melanonychia, or splinter hemorrhages (picture 15D) [130-133]. A fissure of the distal nail plate associated with a V-shaped notch can be seen in some patients. The lesion is usually asymptomatic.

On dermoscopic examination, onychopapilloma shows a homogeneous, brown or gray band in the nail plate originating in the lunula, with splinter hemorrhages and a characteristic gray, keratotic mass at the free distal edge (picture 48). (See "Dermoscopy of nonpigmented nail lesions", section on 'Onychopapilloma'.)

A biopsy is necessary to confirm the diagnosis. Characteristic histopathologic features include papillomatous acanthosis of the distal matrix and nail bed; in the distal portion of the nail bed, there are layers of hyperkeratosis containing fusiform cells with eosinophilic cytoplasm resembling those of the keratogenous zone of the matrix (matrix metaplasia of the nail bed) [129].

The differential diagnosis of onychopapilloma includes other causes of localized (monodactylous) longitudinal erythronychia, such as glomus tumor, squamous cell carcinoma, or amelanotic melanoma. (See 'Malignant tumors' below.)

Treatment of onychopapilloma is surgical excision.

MALIGNANT TUMORS

Squamous cell carcinoma — Squamous cell carcinoma (SCC), including SCC in situ (Bowen disease), is the most common malignant tumor of the nail occurring most often on the fingernails, within the nail bed, or within lateral nail grooves. Nail SCC usually affects older adults, predominantly males [134]. Trauma, radiation exposure, smoking, and infection with human papilloma virus types 16 and 18 are predisposing factors for SCC development [135]. (See "Cutaneous squamous cell carcinoma: Epidemiology and risk factors", section on 'Voriconazole'.)

The clinical features of periungual or subungual SCC are generally nonspecific, often causing prolonged delay in the diagnosis. SCC of the nail bed or lateral nail groove or nail fold may present as hyperkeratosis, persistent onycholysis, longitudinal erythronychia, verruca, paronychia, nail plate dystrophy, or a subungual mass (picture 14B-E) [136,137]. SCC should be suspected when a verrucous or keratotic lesion of the lateral nail groove or fold is persistent or recurs after cryotherapy or other treatment for common warts.

Keratoacanthoma, a rare clinical variant of SCC, typically presents with painful onycholysis, digital erythema and swelling, or painful paronychia [120]. Keratoacanthomas usually grow rapidly, and osteolysis of the bone is commonly observed on radiography. Patients with incontinentia pigmenti may develop multiple subungual keratoacanthomas at a young age [138,139].

A lesion suspicious for SCC or keratoacanthoma should be biopsied using the punch, excisional, or tangential (shave) excision techniques. (See "Nail biopsy: Indications and techniques".)

Mohs micrographic surgery is the preferred treatment for SCC without bone involvement [140,141]. Alternative treatments include wide surgical excision or digit amputation if the bone is involved. The recurrence rate is low after wide excision or amputation [142]. (See "Mohs surgery".)

Melanoma — Melanoma of the nail apparatus is a form of acral melanoma that arises from the nail matrix. Nail melanoma is rare. It accounts for 1 to 3 percent of melanomas occurring in White populations and 15 to 30 percent of melanomas occurring in individuals with darkly pigmented skin [143].

In two-thirds of cases, nail melanoma presents as a brown to black, longitudinal stripe in the nail plate, known as longitudinal melanonychia (picture 49A-B) [144]. In one-third of cases, nail melanoma is amelanotic and presents as a nail bed mass or nail plate abnormality (picture 50A-B). (See 'Longitudinal melanonychia' above.)

The clinical presentation, diagnosis, and treatment of melanoma of the nail unit are discussed separately. (See "Longitudinal melanonychia".)

NAIL SIGNS OF SYSTEMIC DISEASES — Nail abnormalities associated with systemic diseases usually involve most or all nails. Signs of temporary disturbance in nail growth, such as Beau lines and onychomadesis, may occur in association with high fever, viral diseases (eg, hand, foot, and mouth disease), or Kawasaki syndrome [13,14,145]. (See 'Transverse grooves (Beau lines)' above.)

Permanent or prolonged abnormalities of nail shape, thickness, and color associated with systemic diseases include yellow nail syndrome, digital clubbing, half-and-half nails (apparent leukonychia), and koilonychia.

Yellow nail syndrome — Yellow nail syndrome is an uncommon disorder characterized by the triad of pulmonary disease, lymphedema, and slow-growing, yellow nails without a cuticle or lunula (picture 51) [146-149]. Nails progressively thicken, becoming opaque and curved with loss of the lunula and cuticle. Swelling of the periungual tissue and onycholysis can occur. In most cases, yellow nail syndrome is sporadic, but it may be inherited in autosomal dominant or recessive fashion [150]. A number of pulmonary diseases are associated with this syndrome, including pleural effusion, bronchiectasis, and chronic sinusitis [151]. (See "Diagnostic evaluation of the hemodynamically stable adult with a pleural effusion", section on 'History and examination'.)

Terry's nails — Terry's nails are a type of apparent leukonychia that involves more than the proximal two-thirds of the nail plate, whereas the distal third appears red (picture 11A-B) [152]. Initially described in patients with alcoholic liver cirrhosis, Terry's nails have been described in a number of systemic diseases, including autoimmune hepatitis, diabetes mellitus type 2, rheumatoid arthritis, Reiter syndrome, congestive heart failure, and other systemic diseases [152,153]. (See "Cirrhosis in adults: Etiologies, clinical manifestations, and diagnosis", section on 'Extremity findings'.)

The pathogenesis is incompletely understood. Changes in nail bed vascularity may be involved [154].

Clubbing — Digital clubbing is the most common manifestation of hypertrophic osteoarthropathy [155,156]. It is characterized by increased distal fingertip mass and increased longitudinal and transverse nail plate curvature (picture 52A-C) [93,146,157,158]. The Lovibond angle, the angle between the nail plate and the proximal nail fold when viewed from the side, is >180° in clubbed nails and 160° in normal nails (figure 4). Clubbed fingers show the Schamroth sign, the obliteration of the diamond-shaped window normally visible when the dorsal surfaces of the terminal phalanges of corresponding fingers from opposite hands are placed together (figure 5) [159].

Digital clubbing can be acquired or hereditary:

●Acquired bilateral clubbing is the most common form. It usually begins in the thumb and index fingers and is most often associated with pulmonary or cardiovascular diseases, including lung cancer, interstitial pulmonary fibrosis, lung abscess, pulmonary tuberculosis, pulmonary lymphoma, congestive heart failure, infective endocarditis, and cyanotic congenital heart disease [157]. Less frequently, digital clubbing may occur in patients with extrathoracic disease, including inflammatory bowel disease, liver cirrhosis, and gastrointestinal neoplasms. (See "Approach to the adult with interstitial lung disease: Clinical evaluation", section on 'Clubbing' and "Clinical presentation and diagnosis of inflammatory bowel disease in children", section on 'Extraintestinal manifestations' and "Malignancy and rheumatic disorders", section on 'Hypertrophic osteoarthropathy'.)

●Acquired unilateral or single-digit clubbing is commonly related to nearby vascular lesions (such as a peripheral shunt, arteriovenous fistula, or aneurysm) but Pancoast tumors, lymphadenitis, or erythromelalgia can also cause unilateral clubbing [93,146]. Single nail involvement is typically traumatic but may be congenital. (See "Superior pulmonary sulcus (Pancoast) tumors".)

●Isolated congenital digital clubbing (MIM #119900) is considered an incomplete form of primary hypertrophic osteoarthropathy (PHO) [160]. PHO is an autosomal recessive disorder due to mutations in the 15-hydroxyprostaglandin dehydrogenase (HPGD) gene [161]. It presents in otherwise healthy children with clubbing, periostosis, and skin manifestations, including thickening of the skin of the face and scalp, coarsening of facial features, hyperhidrosis, and seborrhea.

Thyroid acropachy — Thyroid acropachy is a rare manifestation of Graves' disease characterized by digital clubbing, soft-tissue swelling of the hands and feet, and periosteal reaction with new bone formation (picture 53). It is almost always associated with thyroid dermopathy and exophthalmos and usually becomes apparent after the diagnosis and treatment of hyperthyroidism. (See "Overview of the clinical manifestations of hyperthyroidism in adults".)

Half-and-half nails — Half-and-half nails (also termed "apparent leukonychia" or "Lindsay nails") are a manifestation of chronic renal insufficiency and uremia [32,33]. A half-and-half nail typically exhibits a red, pink, or brown; horizontal; distal band that occupies 20 to 60 percent of the total length of the nail [93,146,162]. The proximal portion of a half-and-half nail usually has a dull, white, ground-glass appearance due to underlying nail bed changes (picture 10). The nails may revert to normal following renal transplantation [163].

Half-and-half nails have also been reported in association with other systemic diseases (including Kawasaki disease, cirrhosis, Crohn disease, zinc deficiency, chemotherapy, Behçet disease, and pellagra) and drugs [22].

Koilonychia — Koilonychia, also called spoon nail, is the upward curving of the distal nail plate that results in a spoon-shaped nail that could hold a drop of water on the surface (picture 54A-B). Koilonychia has been associated with iron deficiency and other systemic conditions in rare case reports; however, it is more commonly seen as an occupational change in nails and may be idiopathic. Ruling out iron deficiency anemia in someone with koilonychia is the only work-up necessary in this condition. (See "Iron deficiency in infants and children <12 years: Screening, prevention, clinical manifestations, and diagnosis".)

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Lichen planus".)

SUMMARY

●Nail anatomy – The nail unit is composed of the nail matrix, nail bed, proximal and lateral nail folds, and hyponychium (picture 1 and figure 1). Acquired nail disorders may involve all the components of the nail unit. They include infections, tumors, disorders associated with skin or systemic diseases, and abnormal pigmentation. (See 'Anatomy and physiology of the nail unit' above.)

●Nail examination – The nail plate is assessed for changes in surface texture; discoloration; detachment from the nail bed; and changes in thickness, including pitting, ridging, and longitudinal and transverse grooving. (See 'Nail evaluation' above.)

●Surface texture abnormalities – Common surface texture abnormalities include brittle nails, onychoschizia (picture 2), onychorrhexis (picture 3), transverse and longitudinal grooves (picture 4A-C, 23A-B, 46D), pitting (picture 5A-B), and trachyonychia (picture 5C). (See 'Surface texture abnormalities' above.)

●Color changes – Color changes include leukonychia (picture 7), longitudinal erythronychia (picture 14A-B), longitudinal melanonychia, and splinter hemorrhages. Longitudinal melanonychia is reviewed separately. (See 'Nail color changes' above and "Longitudinal melanonychia".)

●Defect of nail plate attachment – Defects of nail plate attachment to the nail bed often present with onycholysis or distal or distal-lateral separation of the nail plate from the underlying nail bed and/or lateral supporting structures (picture 17B). Retronychia is a form of incomplete nail shedding that leads to the embedding of the proximal nail plate into the proximal nail fold with subsequent inflammation (picture 22). The detachment of the nail plate from the proximal nail fold by a full-thickness sulcus is called onychomadesis (picture 23A-B). (See 'Defects of nail plate attachment/nail shedding' above.)

●Nail thickening – Common causes of nail plate thickening include onychomycosis (picture 24A), psoriasis (picture 17D), and onychogryphosis ("ram's horn nail" (picture 25)). Extremely thickened nails are a feature of pachyonychia congenita, a rare inherited disorder (picture 27B). (See 'Nail thickening' above.)  

●Infections – Common nail infections include onychomycosis (picture 24A-E); bacterial infections, including acute paronychia (picture 29A-B), blistering distal dactylitis (picture 31A-C), pulp space infection (felon (picture 32)), and green nail syndrome (picture 28); and viral infections, such as warts (picture 33) and herpetic whitlow (picture 34). (See 'Infections of nails and periungual tissues' above.)

●Nail abnormalities associated with skin diseases – The nails may be involved in numerous skin diseases, including psoriasis (picture 5A, 17B-D), parakeratosis pustulosa (picture 35), lichen planus (picture 36A-C), alopecia areata (picture 5B-C), Darier disease (picture 38), and sarcoidosis (picture 39). (See 'Skin diseases with nail involvement' above.)

●Nail tumors:

•Benign – Benign nail tumors include fibromas (picture 41A-D), onychomatricoma (picture 42), digital myxoid cyst (picture 44A-B), pyogenic granuloma (picture 45), glomus tumor (picture 46A-D), and subungual exostosis (picture 47B). (See 'Benign tumors' above.)

Malignant – Squamous cell carcinoma (SCC), including SCC in situ (Bowen disease), is the most common malignant tumor of the nail. It presents with nonspecific symptoms, such as hyperkeratosis, persistent onycholysis, longitudinal erythronychia, verruca, paronychia, nail plate dystrophy, or a subungual mass (picture 14B-E). (See 'Squamous cell carcinoma' above.)

●Nail signs of systemic diseases – Nail abnormalities associated with systemic diseases usually involve most or all nails. Signs of temporary disturbance in nail growth, such as Beau lines and onychomadesis, may occur in association with high fever, viral diseases (eg, hand, foot, and mouth disease), or Kawasaki syndrome. Permanent or prolonged abnormalities of nail shape, thickness, and color associated with systemic diseases include yellow nail syndrome (picture 51), clubbing (picture 52A-C), half-and-half nails (picture 10), and Terry's nail (apparent leukonychia (picture 11A-B)). (See 'Nail signs of systemic diseases' above.)

ACKNOWLEDGMENT — The UpToDate editorial staff acknowledges Julie A Jefferson, MD, who contributed to an earlier version of this topic review.