Psoriasis is probably the dermatosis that most commonly affects the nails. Around 3% of the population has psoriasis, and the nails are involved in almost 50% of cases. The clinical manifestations of this condition in the nails vary according to the part of the nail affected by the inflammation. Involvement of the proximal matrix, responsible for producing the most superficial part of the nail plate, gives rise to pitting.13 Histologically, pitting is due to foci of parakeratosis in the superficial layers of the nail. Psoriasis affecting the hyponychium and nail bed produces oil spots and onycholysis. The accumulation of material over the nail bed leads to ungual hyperkeratosis. Splinter hemorrhages reflect vascular changes that develop in the nail bed.14 Transitory inflammation of the matrix produces transverse striae, known as Beau lines. Onychorrhexis and longitudinal striae are signs of more prolonged involvement of the matrix.15 Histologically the changes are similar to those observed in psoriasis plaques on the skin: hyperkeratosis, parakeratosis, psoriasiform epithelial hyperplasia, dilated and tortuous vessels in the papillary dermis, and the presence of neutrophils (Fig. 4). However, in contrast to other areas of the skin, the nail commonly shows evidence of spongiosis and serous exudates, and hypergranulosis may be observed. The histological changes seen in psoriasis can be virtually identical to those provoked by fungal infection of the nail and so PAS stain must always be used to exclude the presence of fungi. A histological feature that can help to differentiate the 2 conditions is the presence of serous exudates, which are common in psoriasis and rare in onychomycosis.16

Figure 4.

(0.67MB).

Psoriasis of the nail. There is hyperkeratosis, parakeratosis, psoriasiform hyperplasia of the epithelium, and accumulation of neutrophils in the corneal layer (hematoxylin-eosin, original magnification: A ×10; B, C, and D ×20).

As with psoriasis, lichen planus can affect different parts of the nail unit, and this will determine the clinical manifestations. Focal involvement of the matrix causes partial thinning of the nail plate and produces longitudinal striae. More aggressive disease that affects the entire matrix can give rise to complete nail atrophy. The healing process can provoke pigmentary changes, or even the formation of a pterygium or fusion of the matrix with the proximal nail fold, which permanently blocks nail growth.15 Disease that only affects the nail bed will cause subungual hyperkeratosis and/or onycholysis. The histological changes observed in the nails (Fig. 5) are very similar to those of affected skin: hyperkeratosis, hypergranulosis, irregular epidermal hyperplasia, a lichenoid infiltrate with the formation of Civatte bodies, vacuolar degeneration of the basement membrane, and pigmentary incontinence. However, the nail can present certain additional features, such as parakeratosis and serous exudates. The changes described are not pathognomic of lichen planus, as they can also be observed in other situations, such as in response to trauma.

Figure 5.

(0.56MB).

Lichen planus of the nail. There is hyperkeratosis, acanthosis of the epithelium, and a lichenoid lymphocytic infiltrate in the dermis. Degeneration of the basal layer can be seen, with dyskeratotic keratinocytes and Civatte bodies (hematoxylin-eosin, original magnification: A ×10, B ×20, C ×40, and D ×20).

Eczema typically affects the periungual nail folds. If the inflammation involves the matrix it can alter the appearance of the nail plate, with changes in color, roughness, pitting, or striae. Histologically we observe foci of parakeratosis, the formation of a granular layer, spongiosis, intraepidermal vesicles, and a lymphocytic infiltrate in the dermis (Fig. 6).

Figure 6.

(0.39MB).

Eczema of the nail. Marked spongiosis and lymphocyte exocytosis. A perivascular lymphocytic infiltrate is present in the dermis (hematoxylin-eosin, original magnification ×20).

The typical changes in patients with this condition are limited to the proximal matrix and therefore give rise to pitting of the nail plate. Histologically, these pits are the same as those that develop in psoriasis, but they are shallower and present a geometric distribution.3

The majority of patients with Darier disease present nail changes, visible clinically as longitudinal white or red bands that can become V-shaped at the free border of the nail. Affected areas of the nail plate are weak and show a tendency to fracture. The histological correlate of this clinical phenomenon is the presence of acanthosis of the nail bed, with parakeratosis and the presence of multinucleated epithelial giant cells. Acantholysis may sometimes be observed in the matrix or nail bed.16

• 1.

  Melanonychia is the term used to describe brown or black discoloration of the nail plate caused by the presence of melanin. This pigmentation typically presents as longitudinal melanonychia,48 which is a longitudinal band of pigment that arises from the proximal border of the nail and advances linearly along the longitudinal axis of the nail to its free border. Longitudinal melanonychia can be caused by a number of underlying conditions: systemic diseases such as Laugier-Hunziker syndrome,49 drugs,50,51 benign melanocyte activation or proliferation, and malignant melanoma. The likelihood that longitudinal melanonychia is due to a malignant melanocytic lesion is greater in the presence of atypical clinical features—appearance in white or elderly patients, lesions in the thumb or great toe, width greater than 3mm—and biopsy should be performed in these cases.52

• 2.

  Melanocyte activation is a very common phenomenon in black individuals and, to a lesser extent, in Asians. In cases of melanocyte activation, the band of longitudinal melanonychia is caused by excess melanin in the nail plate due to hyperactivity of the melanocytes in the nail matrix. There is no melanocyte proliferation, as can be more clearly seen with specific immunohistochemical stains (melanA or MITF [microphthalmia-associated transcription factor], preferably with a red chromogen),4 which highlight the melanocytes and facilitate calculation of their density in the matrix epithelium. Immunostain with protein S100 is less sensitive and specific for the detection of melanocytes in intraepidermal lesions and should not be first-choice in this tissue analysis.53

• 3.

  In lentigo, there is a slight increase in the number of individual melanocytes (10-31melanocytes/mm2),54 but they do not show pagetoid growth or cellular atypia.

• 4.

  Melanocytic nevus has the same clinical presentation as the 2 lesions described above. Histologically, the epithelium of the matrix shows an area of melanocyte proliferation that most frequently presents as a typical junctional nevus (Fig. 15). Typical compound nevus, Spitz nevus,55 and blue nevus have also been reported at this site.56 Nevi of the nail arise as lentiginous proliferations at the dermoepidermal junction and progress to form nests of melanocytes in the more advanced stages. These nests can be irregular and confluent and they may be situated in epithelial layers slightly above the basal layer, particularly in children. However, migration to the upper layers of the matrix should make us suspect a malignant lesion.

  
  

  Figure 15.

  (0.63MB).

  Melanocytic nevus. There is benign melanocytic proliferation in the nail bed (hematoxylin-eosin, original magnification: A ×10, B ×40, C ×20, and D ×20).
• 5.

  Melanoma is a tumor that can produce nail dystrophy and/or atypical pigmentation of the nail plate. Clinical presentation is as longitudinal melanonychia in 76% of cases.4 However, in contrast to benign lesions, the band is usually irregular and more than 3mm wide, has heterogeneous pigmentation, and shows rapid growth. Another warning sign is spread of the pigmentation into the proximal nail fold or into the hyponychium (Hutchinson's sign). Melanoma of the nail apparatus most commonly arises in the thumb or great toe.57 As occurs in other melanomas, the diagnosis of malignant lesions of the nail requires evaluation of a large number of histological criteria, including the margins of the lesion, confluence and irregularity of the nests, extension of the nests into the more superficial layers of the epithelium, cellular atypia, mitoses, and altered maturation of the intradermal component. The most common histological subtype in the nail is acral lentiginous melanoma (Fig. 16), though other subtypes, such as nodular, superficial spreading, or desmoplastic melanoma, have also been reported.58 The invasive melanocytes show a spindle-shaped morphology (Fig. 17) in the majority of cases. The observation of osteoid material associated with the tumor, a very rare finding in other melanomas, is more common in melanomas of the nail.

  
  

  Figure 16.

  (0.45MB).

  In situ melanoma. Proliferation of atypical melanocytes in a lentiginous pattern and in nests occupying the matrix and nail bed (hematoxylin-eosin, original magnification: A ×10, B ×20, and C ×40).
  
  

  Figure 17.

  (0.85MB).

  Subungual malignant melanoma. A, Acanthosis and hyperkeratosis of the epithelium associated with an exophytic tumor with abundant melanin (hematoxylin-eosin, original magnification ×4). B, Detail showing the spindle-shaped morphology of the invasive melanocytes and the lentiginous proliferation of atypical melanocytes in the adjacent epithelium (hematoxylin-eosin, original magnification ×10). C and D, High-power view showing the atypical melanocytes and mitotic activity (hematoxylin-eosin, original magnification ×20).

In conclusion, we have reviewed the anatomy of the nail apparatus and the histology of the most common diseases affecting this unit. It is important for the dermatologist to know when it is necessary to perform nail biopsy and which part of this complex anatomical region should be sampled.