Subungual hyperkeratosis is defined as the thickening of the stratum corneum of both the epithelium of the hyponychium and the nail bed, with an accumulation of keratinocytes and debris underneath the nail plate distal region.7,8 Numerous causes can lead to subungual hyperkeratosis, including inflammatory, infectious, tumoral, or various process inducing changes to the nail bed—such as trauma, drugs, or systemic diseases—and that produce onycholysis. Subungual hyperkeratosis is more common in the toenails, likely due to repetitive trauma.9

Hyperkeratosis can be diffuse or localized. It is diffuse in diseases with extensive involvement of the nail bed, such as dermatitis, onychomycosis, and psoriasis. When hyperkeratosis is localized, a tumoral process should be suspected.10

Onychomycosis often presents with onycholysis and thick—predominantly yellowish—scaling next to the nail plate ventral region11,12 (Fig. 2A and fig. B). The presence of ruin-like appearance and/or distal irregular termination at the nail distal end, due to fragility caused by fungal invasion, supports this diagnosis.13 The aerial view reveals the presence of onycholysis with sharp irregular borders, and various matte colors of the nail (the so-called Aurea borealis), etc.

Figure 2.

Content changes. A: Dermatoscopy of onychomycosis in the aerial view; onycholysis with an irregular border, dermatophytoma, and the aurora borealis sign can be seen. B: Dermatoscopy of onychomycosis in the distal frontal view with presence of onycholysis, diffuse subungual hyperkeratosis, and the ruination sign. C: Dermatoscopy in nail psoriasis showing oil drop sign and onycholysis. D: Dermatoscopy in psoriasis with presence of onycholysis with diffuse, fine, silver-white subungual hyperkeratosis.

(0.25MB).

Onycholysis with fine, silvery-white scaling—slightly more significant in the sides—is indicative of psoriasis (Fig. 2B and fig. C). Dermatoscopy may reveal elongated and dilated capillaries with irregular distribution in the hyponychium and fingertip.14,15 The clinical signs typical of psoriasis in the aerial view, such as pitting, the oil-drop sign, and vascular involvement of the nail bed, among others, can also be present. Associated onychomycosis may exist, and typical signs of fungal infection should be looked for during the physical examination.16

Diffuse hyperkeratosis is typical of periungual dermatitis, pityriasis rubra pilaris, and repetitive trauma, among other conditions.7,17

Onicoclavus, also known as subungual heloma or horn, presents as a painful hyperkeratosis with a translucent center, sometimes associated with hemorrhagic areas. Onicoclavus is often found in the nail bed middle distal region18 (fig. 3A, and fig. B). When lifting the nail, the keratotic formation with an area of bleeding can be seen. Onicoclavus is a reactive process to persistent pressure or trauma to the nail bed, often occurring as a consequence of anatomical or biomechanical changes to the foot.

Figure 3.

Content changes. A: Onychoclavus: onycholysis with pigmented spot in the nail middle distal region. B: Onychoclavus: hyperkeratosis with a hemorrhagic area in the hyponychium middle section. C: Onychopapilloma: discreet longitudinal erythronychia with splinter hemorrhage and mild distal “V” onycholysis. D: Onychopapilloma: onycholysis with focal subungual hyperkeratosis attached to the nail plate ventral region. F: Fibrokeratoma: lateral onycholysis with a hemorrhagic pigmented area. G: Fibrokeratoma: onycholysis with presence of well-demarcated subungual hyperkeratosis.

(0.38MB).

Onychopapilloma is a specific nail tumor. Hyperkeratosis is often localized, attached to the nail plate ventral region, and painless.10,19 (Fig. 3C and fig. D). The most common clinical sign is a longitudinal erythronychia band with distal onycholysis. However, it can also show as leukonychia, longitudinal melanonychia, or splinter hemorrhages.20

Warts typically appear as a papule or plaque with hyperkeratosis and black dots on the surface. Dermatoscopy reveals papillomatous areas with intracorneal hemorrhages in the papilla central region, resembling “frog eggs,” or black dots with an irregular distribution on the surface.21

Other causes of localized hyperkeratosis include conditions such as Bowen's disease, or squamous cell carcinoma.10

Hypertrophy of the distal nail fold is physiological in the early years of life and pathological at older ages. The first toe is the most commonly affected of all, and the extent of hypertrophy can only be assessed in the frontal view. The physiological form seems to be due to a lack of nail pressure on the subungual tissue (Fig. 4A and fig. B). The pathological form is common in situations where the nail is overwhelmed by the forces exerted by the lateral and distal folds (Fig. 4C and fig. D).32 The most typical clinical sign is an increased distal nail fold and fingertip stopping nail growth. Hypertrophy of the distal fold is closely associated with onycholysis, especially in its stage 5 (nail bed loss), in which the nail bed undergoes progressive size reduction.33

Figure 4.

Content changes. A: Aerial view of a child's first toes. B: Hypertrophy of the distal nail fold. C: Aerial view of an adult's first toes, with a reduced nail length. D: Hypertrophy of the distal nail fold and angle loss of the transverse axis of the nail free edge.

(0.38MB).

Hutchinson's sign is pigmentation of the periungual skin. Hutchinson's sign can be a benign or malignant clinical sign and be associated with local or systemic disease. When it appears accompanying melanonychia in adult patients, it is probably suggestive of melanoma. Conversely, it is a common finding in melanonychia in children and is often indicative of a congenital melanocytic nevus.34

Frontal view assessment of melanonychias help us identify which part of the matrix is responsible for pigmentation. If pigmentation is located in the nail plate upper region, it is related to the proximal matrix; if located in the middle-lower region, by distal matrix35 (fig. 5, fig. 5A-C). When numerous pigmentation spots are found in the thickness of the nail plate in melanonychia, it may be indicative of melanoma.

Figure 5.

Content changes. A: Aerial view of longitudinal melanonychia. B: Pigmentation affecting the nail plate ventral region. C: Aerial view of longitudinal melanonychia. D: Pigmentation affecting the nail plate dorsal region.

(0.1MB).

The term hapalonychia refers to a decreased nail thickness (< 0.5mm).5 It is often seen in fingernails with physiological aging, but it is also a typical finding of the brittle nail syndrome, specific local nail diseases (such as occupational dermatitis, trachyonychia, psoriasis, or lichen planus), and systemic diseases (such as chronic arthritis, leprosy, myxedema, peripheral neuritis, hemiplegia, and cachexia, among others).6 (fig. 6A-C). Thinning of the nail plate can be total or focal and affect one or multiple nails. If thinning is seen in a single finger and is localized, the most likely cause is a tumor, such as onychopapilloma. In Darier's disease, this partial thinning of the nail plate can be seen in multiple nails.17

Figure 6.

Thickness changes. A: Aerial view of onychorrhexis, longitudinal grooves across the entire nail plate surface. B: Irregular loss of nail plate thickness. C: Aerial view of psoriasis, total nail dystrophy. D: Hapalonychia, thinning of the nail plate.

(0.26MB).

Nail growth stop can be due to local or inflammatory causes, repetitive microtrauma, and systemic processes (such as yellow nail syndrome, for example). In these situations, the nail thickens while changes of color—turning yellow—and morphology.6,36 These changes also occur physiologically with the aging of the toenails and pathologically in retronychia, among others.37 (fig. 7A, fig. B).

Figure 7.

Morphological changes. A: Aerial view of the feet of an elderly patient with xanthonychia of the first 2 nails. B: Onychauxis, uniform thickening of the nail plate entire distal edge. C: Aerial view of the feet of a patient with a decreased transverse diameter of the nails middle distal region. D: Pachyonychia: nail of cylindrical morphology due to thickening of the nail plate middle distal region and union of both ends of the nail. E: Aerial view of onychomatricoma: discreet splinter hemorrhages and distal “V” onycholysis in the nail plate distal region. F: Localized honeycomb-shaped nail plate thickening.

(0.28MB).

Congenital pachyonychia is a genodermatosis characterized by very distinctive nail changes, along with other symptoms. The nails gradually thicken in the middle distal region at the expense of the nail plate and bed. This results in an increased transverse curvature that gives the nail a cylindrical morphology and a dark brown color. These changes can cause pain and difficulty during finger manipulation.38 In advanced stage of the pincer nails, we can observe this morphology (Fig. 7C and fig. D)

Sometimes, the nail plate increased thickness is due to a localized change to the matrix. This is called onychauxis (the nail longitudinal thickening). An example of this is onychomatricoma, a benign fibroepithelial tumor (Fig. 7E and fig. F). It shows as the nail longitudinal and transverse thickening, xanthonychia, splinter hemorrhages, and the typical honeycomb shape or distal woodworm appearance.39 The histopathological analysis of distal nail clippings could confirm the diagnosis.40