Chapter 31: Nail Diseases

INTRODUCTION TO CHAPTER

The nails have several important functions. The nail plate acts as a protective shield for the fingertips; it assists in grasping and manipulating small objects. Nails are also used for scratching, grooming, and cosmetic adornment.

NAIL ANATOMY

The nail unit is composed of the nail plate, nail matrix, nail folds, nail bed, and hyponychium (Figure 31-1 A and B).¹

• Nail matrix: Forms the nail plate.

• Nail plate: Hard, translucent, keratin-containing structure covering the dorsal surface of the distal digits on the hands and feet. Formed by the nail matrix, the nail plate grows out from under the proximal nail fold. The nail usually appears pink, which is due to the underlying vasculature of the nail bed. The small, white, semi-circular structure at the proximal portion of the nail is the lunula, which is the visible portion of the nail matrix.

• Nail bed: Structure underlying the nail plate, which contributes to the nail plate's ability to attach to the finger.

• Hyponychium/Onychodermal band: Under the distal free edge of the nail. The hyponychium is the transition point between the nail and the normal skin of the digits. The onychodermal band is the point of strongest attachment between the nail and the underlying digit.

• Nail folds: Proximal and lateral. These are epithelial structures. The cuticle protects the matrix by sealing off the potential space between the nail plate and the proximal nail fold.

DIFFERENTIAL DIAGNOSIS

Nail disorders can be difficult to differentiate from one another. To determine the correct diagnosis takes practice and often laboratory studies such as fungal cultures. To add to the confusion, many nail disorders can have secondary fungal or bacterial infections.

The following are the common categories of nail disorders and examples of some specific diseases.

• Infectious: Dermatophyte, candida, mold, and bacteria.

• Papulosquamous: Psoriasis and lichen planus.

• Systemic: Yellow nail syndrome, clubbing, and Beau's lines.

• Traumatic: Habit tic, some cases of onychodystrophy or onycholysis

• Tumors: Squamous cell carcinoma, melanoma, and benign tumors.

• Aging-related changes: physiologic changes in nail plate thickness, texture, and color.

Tumors involving the nail unit are an important category of nail disorders. These are covered in other sections of this textbook (see chapters 21 and 22).

A differential diagnosis of nail disorders and clinical findings that distinguish them from one another are presented in Table 31-1.