Periungual warts are most commonly seen in children and teenagers, occurring most frequently in those ranging from 12 to 16 years old. There is increased incidence in those who bite their nails, who suck their fingers, and who work in a wet environment. Often, warts disappear spontaneously [10].

Periungual warts are usually caused by HPV genotypes 1, 2, and 4. They affect fingernails more often than toenails and appear as hyperkeratotic papules with a rough surface (Fig. 14.4). Pathologically, they are characterized as having sharply demarcated hyperplasia with acanthosis, papillomatosis, and hyperkeratosis with areas of parakeratosis [10].

Initially, the warts are small in size, shiny, smooth, and translucent. Weeks to months later, they grow in size and appear rough, dirty, brownish black in color, and horny [11].

When located in the proximal nail fold, warts produce periungual hyperkeratosis forming a hyperkeratotic cuticle. Subungual warts raise the nail plate causing onycholysis and appear as a subungual nodular lesions. They may also produce a longitudinal band of onycholysis with splinter hemorrhages due to linear growth underneath the nail plate. Warts in the hyponychium of the toenails may cause distal thickening.

Warts do not affect the nail matrix, but can cause damage to it due to compression, which can cause nail plate ridging and grooving [10]. They may become fissured, inflamed, and tender and often recur after treatment [11].

Subungual exostosis is a rare, benign osteocartilaginous tumor affecting the distal phalanx of the digits, usually the toes [12]. It presents more frequently in females and most often in the second decade of life. About one-third of patients report a history of trauma or infection at the site [13].

Patients commonly present with months of pain, erythema, and onycholysis [1, 12]. It most often affects the toenails, with about 70–80 % of cases overall affecting the hallux [14].

Examination shows a firm nodule with a hyperkeratotic and smooth surface (Fig. 14.5). The lesion is located at the distal end of the nail plate, away from the epiphyseal line. It is made of trabecular bone and is capped with fibrocartilage [12].

Squamous cell carcinoma of the nail is rare [15]. It is most common in the fifth and sixth decades of life and tends to have a male predominance [16].

The etiology of subungual squamous cell carcinoma is unclear, although repeated trauma, chronic infection, radiation, tar, arsenic exposure, UV radiation, immunosuppression, and HPV infection may each play a role. HPV infection is particularly relevant as it is present in 60–90 % of cases [15].

Subungual squamous cell carcinoma most frequently presents on the hands rather than the feet [17]. It most often involves only one digit, with the thumb and hallux being the most common [15]. The right index and middle fingers are also commonly affected [18].

Patients most commonly present with a wartlike appearance of the nail bed with nail dystrophy [15]. Nail pigmentation with longitudinal melanonychia is common (Fig. 14.6) [18].

Other clinical features include subungual hyperkeratosis, onycholysis, oozing, nail plate destruction, paronychia, leukonychia, and longitudinal erythronychia [18]. Bleeding along with nodules and ulcers may also be present [15].

Nail melanoma is not common, as it accounts for only about 1–3 % of cutaneous melanomas diagnosed in the general population [19, 20]. Nail melanomas are seen more often in patients 50–70 years old and more often in men than women [19, 20]. Darker-skinned individuals are more commonly affected with this subtype of melanoma. Up to 75 % of cutaneous melanomas are localized in the nail in Africans, 10 % in Japanese, and 25 % in Chinese populations [20].

Nail melanomas originate either from the nail matrix, or the nail bed (subungual melanomas), and may involve other parts of the nail unit such as the proximal nail fold and hyponychium. Nail matrix melanoma presents as bands of longitudinal brown-black discolorations of the nail plate (longitudinal melanonychia) [20]. The pigmented band is usually wider than 3 mm and has dishomogeneous color and blurred lateral margins (Fig. 14.7) [19, 20]. Nail bed melanoma appears as a pigmented or nonpigmented subungual nodule that initially causes nail plate detachment. It gradually enlarges, eventually leading to nail plate destruction [19]. Ulceration, pain, inflammation, discharge, and surrounding discoloration are common [20].

Hutchinson sign is a characteristic feature of invasive subungual melanoma. It is defined as extension of the dark pigment into the lateral or proximal periungual folds [1].

Nail melanomas are more often found in the hands than in the feet and most commonly in the thumb and hallux [19]. Although UV radiation is a well-known risk factor for cutaneous melanoma, it is unable to penetrate the nail plate and therefore is not a risk factor for subungual melanoma [19]. Instead, direct trauma to the nail is frequently reported although there is lack of evidence to form a direct correlation [20].