In addition to the disorders already described, various other conditions can affect the nails. Some of these disorders produce characteristic nail findings that may serve as clues to unrecognized systemic disease.

Dermatoscopic examination of benign longitudinal melanonychia should reveal light to dark brown lines or bands that are parallel, regular in color, and regular in width as the band extends from the nail fold to the free edge. The borders should be clearly defined and usually of a width of less than 3 mm.

Nail biopsy: Definitive exclusion of melanoma of the nail unit is obtained with a nail matrix biopsy. There should be a low threshold for biopsy especially in elderly patients where melanonychia has appeared in a single digit.

This is not a true cyst, because it lacks an epidermal lining. It is actually a focal collection of clear, gelatinous, viscous mucin (focal mucinosis) that occurs over the distal interphalangeal joint or, most commonly, at the base of the nail.

Pressure from the lesion on the nail matrix (root) often results in a characteristic longitudinal groove in the nail plate (Fig. 22.11).

When the lesion occurs more proximally, such as over the distal interphalangeal joint, there is no longitudinal groove (Fig. 22.12).

This dome-shaped, rubbery lesion occurs exclusively in adults, particularly in women older than 50 years of age. Some myxoid cysts are believed to be a consequence of osteoarthritis, rather than trauma.

Leukonychia striata, which is also called transverse striate leukonychia, is often mistaken for a fungal nail infection.

These white lines (Fig. 22.14) result from an injury to the nail matrix that may have occurred about 2 to 3 months earlier. The underlying injury is often an antecedent illness or the repeated trauma of manicuring or liquid nitrogen therapy for periungal warts.

The nails are yellow, opaque, curved, and grow very slowly (Fig. 22.15).

This syndrome is associated with certain respiratory disorders (e.g., bronchiectasis, chronic respiratory infections, lymphedema, pleural effusion, and ascites).

Yellow nail discoloration also has been reported in patients with acquired immunodeficiency syndrome.

Increased transverse curvature is often manifested as a pincer nail deformity (Fig. 22.16).

In this deformity, the nails' normal transverse curvature increases along the longitudinal axis and becomes more pronounced at the distal edge, which results in “pinching” of the underlying skin. This condition can become quite painful and may predispose the patient to infections and ingrown nails.

Pincer nails are often congenital and may be seen in persons with the yellow nail syndrome or as a normal variant. They are also seen in women who wear ill-fitted shoes.

Acquired koilonychia, also known as spoon nails, may be seen in association with trauma to the cuticle and proximal nail folds, iron-deficiency anemia, hemochromatosis, or endocrine or cardiac disease (Fig. 22.17).

Spoon-shaped or concave nails may also be seen in early infancy and may be self-limited.

Trachyonychia or rough nails, may present as an idiopathic disorder of the nails, or it can be associated with other dermatologic conditions such as lichen planus (Fig. 22.18).

Characterized by brittle, thin nails, with excessive longitudinal ridging.

It may involve one, several or all digits. When most or all digits are involved, the term twenty-nail dystrophy is commonly used.

In childhood, it may present as twenty-nail dystrophy. In most cases of childhood trachyonychia, the nail abnormalities improve spontaneously.

Melanonychia is brown or black pigmentation of the nail unit. It is a common physiologic finding, noted frequently in dark-skinned individuals of all ages. White-skinned people are less commonly affected.

It presents as a pigmented band, or multiple bands, that extend from the nail fold (cuticle) to the free edge of the nail. It may affect a single nail or be observed in multiple nails (Fig. 22.19).

When arranged lengthwise along the nail unit, it is often referred to as longitudinal melanonychia or melanonychia striata.

The most concerning cause of melanonychia is subungual melanoma. Other causes include trauma, inflammatory disorders, fungal infections, drugs, and benign melanocytic hyperplasias.

This evenly pigmented linear nevus emanates from nests of nevus cells in the nail matrix (Fig. 22.20).

These lesions are quite common in blacks and may be multiple. They are much less common in whites.

Any smudging or leaching of pigment or variation from the original black band should prompt an immediate biopsy to rule out malignant melanoma (Fig. 22.21).

A solitary wart of the nail bed lifting the nail plate is often mistaken for a fungal infection (Fig. 22.22).

The possibility of subungual squamous cell carcinoma, basal cell carcinoma, or other neoplasm should always be considered if only one nail or periungual area is involved.

Dystrophic, thickened nail plates result from infiltration of the proximal nail folds by sarcoidal plaques (Fig. 22.23).

The proximal nail fold demonstrates periungual erythema, telangiectasias, thickening of the cuticles (“ragged cuticles”), and distal nail plate thinning (Fig. 22.24).

Terry nails are characteristic color changes in the nail associated with cirrhosis, congestive heart failure, and adult-onset diabetes mellitus. May be seen as a normal finding associated with age.

The patient shown in Figure 22.25 had a liver transplant for cirrhosis. His nail beds are white with a narrow zone of pink under the distal end of the plate (Fig. 22.25).

The proximal half of the nail is white and the distal portion retains the normal pink color.

These characteristic color changes may be seen in chronic renal failure (Fig. 22.26).

Acrylic sculptured nails and the less expensive preformed plastic artificial nails are used for nail elongation. They are attached directly to the natural nail plate, which they cover entirely. The artificial nails are glued to the natural nail plate with an acrylate-based adhesive.

Minor upward pressure on the distal tip of the artificial nail can result in significant distal onycholysis; complete nail avulsion can even result. Any space between the natural nail plate and the artificial nail may become infected (bacterial or fungal) or deformed, and this will often not be noticed until removal of the artificial nail (Fig. 22.27).