III.2
III.2.1 Definition
The term acral nevus is not uniformly defined in the literature. Some authors refer to acral nevi in broader terms as melanocytic lesions on both the volar and dorsal surfaces of hands and feet;
however, recent publications tend to limit the term “acral melanocytic nevus” only to acral vo- lar melanocytic nevus. It would be better to limit the usage this way, in consideration of re- cent genetic definition of acral melanoma by Bastian et al. [1].
In this atlas, with the above reason, an ac- quired or congenital benign melanocytic lesion exclusively on the palms and soles (volar skin) should be called an acral nevus. Subungual or ungual nevus is also included in the term acral
Chapter II.2
Acral Nevus
Masaru Tanaka, Masayuki Kimoto, Toshiaki Saida III.2
Contents
III.2.1 Definition . . . .66
III.2.2 Clinical Features . . . .66
III.2.3 Dermoscopic Criteria . . . .66
III.2.4 Relevant Clinical Differential Diagnoses . . . .69
III.2.5 Histopathology . . . .69
III.2.6 Management . . . .70
III.2.7 Case Studies . . . .70
III.2.7.1 Case 1 . . . .70
III.2.7.2 Case 2 . . . .71
III.2.7.3 Case 3 . . . .71
III.2.7.4 Case 4 . . . .72
References . . . 74
melanocytic nevus for the same reason. It is also called longitudinal melanonychia and is thought to be a variant of acral nevus. Subungual nevi are described in Chap. III.18.
III.2.2 Clinical Features
Most lesions are small (usually <7 mm in diam- eter) and flat, with light-brown to dark-brown pigmentations on the palms and soles, except congenital nevi that can be larger and often slightly elevated. The color shade is darker in the center than at the periphery of the lesion. A blu- ish central area is sometimes observed in corre- lation with histopathological dermal component in compound type of nevi. Congenital acral nevi, depending on the size of the lesion, are inclined to have a larger dermal component and blue- white structures are more often observed on dermoscopy as a background in the central area.
Most acral nevi are flat; however, compound type of congenital nevi are slightly elevated.
III.2.3 Dermoscopic Criteria
The basic dermoscopic global feature for acral
nevi is parallel furrow pattern [6, 8]. The paral-
lel furrow pattern is a pattern with linear pig-
mentation parallel to the skin markings. The
linear pigmentation is basically narrow and
confined along the furrow. There are some
modifications of parallel furrow pattern, name-
ly fibrillar pattern and lattice-like pattern
(Fig. III.2.1). The prevalence of these patterns in
Japan is reported to be 42, 33, and 19%, respec-
tively [8]; however, up to 10% of acral nevi could
show non-typical pattern [6].
Acral Nevus
Masaru Tanaka, Masayuki Kimoto, Toshiaki Saida III.2
Fibrillar pattern is a variant of parallel furrow pattern, which is often observed at pressure- loaded areas of the sole [5] or at the lateral nail ridges. This variant is attributed to the fact that the horny layer would go obliquely up with tiers of melanin in it. Figure III.2.2 demonstrates the
schematic explanation of the fibrillar pattern.
Each fibrillar pattern reveals a different angle between skin markings and linear pigmenta- tions depending on the pressure-loading sites.
Lattice-like pattern is also sometimes ob- served especially on the arch areas as a variant
Fig. III.2.1. Basic parallel patterns for acral melanocytic nevi. The basic dermoscopic global feature in acral nevus is parallel furrow pattern.
Modifications of parallel fur- row pattern include fibrillar pattern and lattice-like pat- tern. Parallel furrow pattern is further divided into four variants, namely single dotted line, single line, double dotted line, and double line variants.
Linear pigmentations in all
patterns, excluding fibrillar
pattern, tend to distribute
along the furrows. White
circles indicate eccrine pores
and form a row in the center
of each ridge
68 M. Tanaka, M. Kimoto, T. Saida
III.2
of parallel furrow pattern [5]. The lattice-like pattern might be due to some differences in skin markings at the arch area.
The parallel furrow pattern might be further divided into four variants (Fig. III.2.1), namely single or double; line or dotted line along the sulcus superficialis [8]. The line variants could be observed as a result of pressure loading and oblique melanin columns inclined to the direc- tion parallel to the skin markings. Another ex- planation of the line variants might be continu- ous lentiginous proliferation of melanocytes along the crista profunda limitans. Crista dot-
ted pattern is usually seen in combination with parallel furrow pattern, but on rare occasions is seen on its own. Congenital acral nevi that are mainly composed of dermal elements often show homogeneous pattern, sometimes with subtle remaining of parallel furrow pattern due to the presence of nevus cells at the dermal–epi- dermal junction. Congenital acral nevi some- times display a combination of parallel furrow, lattice-like, and fibrillar patterns [11], because the lesion is relatively large. Figure III.2.3 shows a congenital acral nevus exhibiting a typical combination of these three patterns.
Fig. III.2.2. A schematic
explanation of fibrillar pat-
tern. Fibrillar pattern is often
observed at pressure-loaded
area of the soles. When the
histopathological section
is cut parallel to the streaks
of fibrillar pattern, namely
almost perpendicularly to the
skin markings, the melanin
columns in the horny layer
are observed obliquely. Nests
of nevus cells might be
observed mainly at the crista
profunda limitans. Most
pigment streaks of fibrillar
pattern would start from the
left side of the furrows and
end at the farrows
III.2.4 Relevant Clinical Differential Diagnoses
The most important differential diagnosis, of course, is melanoma. Clinically, most acquired acral nevi are small and well circumscribed, regular in shape, and usually less than 7 mm [7].
If the distribution of the color has regular ten- dency, namely dark in the center and light at the periphery, the lesion probably is a melanocytic nevus; however, if there is irregularity of color distribution, further dermoscopic and/or histo- pathological examination will be needed. If der- moscopic features show parallel ridge pattern and irregular color distribution, the findings are highly specific to the diagnosis of melano- ma. Black heel (subcorneal hemorrhage or hema- toma) is another differential diagnosis of great importance. Homogeneous areas or lacunas with red-black to red-blue color strongly suggest that the lesion is hemorrhagic [10]. Typical cases of black heel would exhibit a very unique feature on dermoscopy named “pebbles on the ridges,”
which is multiple reddish-black, pebble-like droplets with smooth margins chiefly distrib- uted on the ridges of skin markings [6, 8]. Red- dish satellites are also a helpful clue for a diag-
nosis of hematoma [10]; however, irregular pigmentation together with lacunas might be a finding of advanced melanoma with hemor- rhage.
III.2.5 Histopathology
Acral nevi tend to be more cellular than com- mon nevi with arrangements in predominantly lentiginous fashion rather than forming nests of nevus cells. The melanocytic proliferation is usually limited on the tips of epidermal rete ridges. The nevus cells proliferate mainly near the crista profunda limitans, but sometimes ne- vus cells can also be found at the crista profunda intermedia. These nevus cells at the bottom of the epidermis tend to produce melanin granules and transfer them to the surrounding keratino- cytes. Since more melanin is inclined to be pro- duced from the nevus cells near the crista pro- funda limitans, prominent melanin columns are often observed in the horny layer corre- sponding to the furrows, thus forming parallel furrow pattern on dermoscopy. The nuclei of these nevus cells are oval and uniform, but mild cellular atypism is sometimes observed. Nevus cells of congenital acral nevi tend to proliferate more deeply than those of acquired acral nevi.
The nevus cells are present in sweat ducts and glands, in vessel walls, and in the perineurium of nerves.
As acral melanocytic nevi often cause diag- nostic problems to dermatopathologists because they share histopathological features with mela- noma, the most important thing, when a speci- men has been taken, is to order a pathological dissection along a perpendicular plane to the skin markings (dermatoglyphics) [9]. Typical features of benignity, namely symmetry, cir- cumscription, and melanin columns in the horny layer, are more frequently observed in le- sions cut perpendicularly to dermatoglyphics [9]. Acral melanocytic nevi especially in chil- dren might show lentiginous proliferation, con- fluence of junctional nests, transepidermal elimination of melanocytic nests, and atypical size, shape, and location of the junctional nests [3]. Acral nevi occasionally demonstrate paget- oid spread of melanocytes, but to a minimal de-
Fig. III.2.3. A combination of parallel patterns seen in
a congenital acral nevus. This case is a congenital acral
nevus exhibiting a typical combination of three patterns,
namely parallel furrow (left side), fibrillar (right side),
and lattice-like (central part) patterns. Blue-white struc-
tures seen in the background of central part correspond
to the melanin producing dermal component of nevus
cells. Eccrine pores are well recognized as “white dots”
70 M. Tanaka, M. Kimoto, T. Saida
III.2
gree [2]. These atypical histopathological fea- tures of acral melanocytic skin are also observed in nevi on genital, flexural, and auricular re- gions, and need careful attention with regard to differential diagnosis of melanoma [4].
III.2.6 Management
A small lesion (<7 mm) of long duration with typical parallel furrow pattern needs no follow- up. The similarly typical, but recently developed, lesions would require only a short-term dermo- scopic follow-up, for about 6 months. Then, if there are no changes in color or size, further fol- low-up will not be required. Small lesions, but with irregular pigmentation, irregular structure, or any features of concern about malignancy, must be carefully followed up for a long period for any changes in color or size. Digital dermos- copy follow-up is especially recommended in this case. Parallel ridge pattern with irregular shape or pigmentation needs an excisional bi- opsy and should be histologically assessed [7].
III.2.7 Case Studies III.2.7.1 Case 1
A 19-year-old Japanese man noticed a brownish macule on his left sole about 2 years previously (Fig. III.2.4). He did not know when the lesion had appeared. As the lesion was gradually in- creasing in size, he worried about change. Clini- cally, the brownish macule was 4.5¥4.0 mm in size, and not irregular in shape and color. He had no history of melanoma.
Dermoscopically, this lesion showed a typical parallel furrow pattern of double dotted line variant (Fig. III.2.5). There were five parallel pigmented lines at the furrows, and each line was composed of double rows of dots and glob- ules. There were no criteria to suggest that this might be a high-risk lesion; however, the lesion was totally excised and a histopathological diag- nosis of melanocytic nevus was established. Me- lanocytic proliferation was found mainly near the basal layer of the epidermis with some nest formation around the crista profunda limitans.
Melanin granules were seen throughout the horny layer forming melanin columns. The horny layer below the furrows lacked melanin granules, which might explain the formation of double dotted line variant.
Comments
The double dotted line variant of parallel fur- row pattern on dermoscopy often corresponds to histopathological findings of melanocytic nest formation along the shoulders of the crista profunda limitans that is situated beneath the furrow of skin markings.
Fig. III.2.4. Clinical picture of case 1. A dark-brown macule of 4.5¥4.0 mm in size, which has been noticed for 2 years, is seen on the right plantar arch of 19-year-old Japanese man
Fig. III.2.5. Dermoscopy of case 1. A typical double-dot- ted-line variant of parallel furrow pattern is recognized.
Dark-brown globules are set in array in two rows along
the furrows of skin markings
III.2.7.2 Case 2
A 62-year-old Japanese woman noticed a brown- ish macule on her right palm approximately 40 years previously. She felt that the lesion had been getting darker in color since a few years ago. Clinically, the brownish macule was 5.0¥4.0 mm in size, and not irregular in shape and color (Fig. III.2.6). Dermoscopically, the le- sion revealed a typical parallel furrow pattern of single dotted line variant; therefore, this lesion fits into the benign categories of acral nevi.
There are several parallel pigmented lines that consist of single rows of dots and globules at the furrow (Fig. III.2.7). The lesion was totally ex-
cised, and a histopathological diagnosis of me- lanocytic nevus was obtained. Melanocytic pro- liferation was found mainly near the basal layer of the epidermis with some nest formation at or around the crista profunda limitans, and mela- nin granules were seen through the honey layer of stratum corneum.
Comments
The histopathological findings for the parallel furrow pattern of single dotted line variant are nevus cell proliferation just beneath the furrows of skin markings, namely crista profunda limi- tans. The continuous line variant, as partly seen in the upper half of the picture in this case, might be observed when the melanin columns are obliquely distributed on the specimen cut parallel to the skin markings. This phenomenon also could be explained by the theory of fibrillar pattern.
III.2.7.3 Case 3
A 27-year-old Japanese woman noticed a small pigmented spot on the sole of her right foot 5 years previously. Since the lesion gradually became prominent in color and size during the past 3 years, she visited a dermatology out- patient clinic for examination. She had no history or family history of melanoma. Clini- cal examination revealed a 6.2¥5.2-mm dark brown, elliptical macule on her right sole (Fig. III.2.8). The overall color of the lesion seemed homogeneously brown and it was sharp- ly circumscribed. Dermoscopy demonstra- ted typical fibrillar pattern with linear short pigmentations distributed in the angle of approximately 45° to the skin markings (Fig.
III.2.9). A Surgical excision was performed and the specimen was histopathologically examined. Sections were cut 45° to the skin markings. The hematoxylin–eosin staining revealed that melanocytic proliferation was mainly found near the basal layer of the epi- dermis with some nest formation at or around the crista profunda limitans. These nevus cells were producing melanin and transferring to
Fig. III.2.6. Clinical picture of case 2. A brownish mac- ule composed of linear pigmented streaks is on the right palm of a 62-year-old Japanese woman. It has been no- ticed for 40 years
Fig. III.2.7. Dermoscopy of case 2. A typical single dot- ted line variant of parallel furrow pattern is observed.
Linear pigmentations with several dots/globules on the
furrows are seen over the light-brown background
72 M. Tanaka, M. Kimoto, T. Saida
III.2
the nearby keratinocytes. Keratinocytes and horny layer corresponding to the furrows have abundant melanin granules. Melanin columns are obliquely seen in the stratum corneum at the furrows of skin markings (Fig. III.2.10).
Comments
The dermoscopic fibrillar pattern is well ex- plained by the histopathological distribution of melanin granules in the stratum corneum (Fig. III.2.2). The horny layer of the furrow has scattered reflection and forms linear whitish lines on dermoscopy (Fig. III.2.9). The darker pigment streaks are distributed mainly on the right halves of the ridges of the skin markings, while minor light pigment streaks are also seen on the left halves of the ridges. More melanin granules from nevus cells at the crista profunda limitans would distribute in the stratum corne- um below the furrow corresponding to the dark pigment streaks on dermoscopy.
III.2.7.4 Case 4
A 69-year-old Japanese man had noticed a small pigmented macule on his right plantar arch. He insisted that the lesion had existed since his childhood, and had paid no attention to it; how- ever, a dermatologist worried about possible malignancy, because the lesion was irregular in shape and color. Clinical examination revealed a 5.0¥7.0-mm dark-brown, slightly elevated macule on the right plantar arch (Fig. III.2.11).
The peripheral lesion seemed slightly blurred on the right side, but had an abrupt edge on the left side. Dermoscopically, the right side of the
Fig. III.2.8. Clinical picture of case 3. A 6.2¥5.2-mm small dark-brown spot is noted on the sole of the right foot of a 27-year-old Japanese woman from 5 years previ- ously, which has been enlarged for 3 years
Fig. III.2.9. Dermoscopy of case 3.Typical fibrillar pat- tern with linear short pigmentations is distributed in the angle of approximately 45° to the skin markings. The furrows are recognized as whitish lines of random reflec- tion. Note that most dark pigment streaks are distributed on the right halves of the ridges, reaching furrows at the right end. This is explained by proliferation of nevus cells mainly near the crista profunda limitans; however, there are a few light pigment streaks that are distributed on the left halves of the ridges. This minor pigment distribution is attributed to the nevus cells near crista profunda in- termedia
Fig. III.2.10. Histopathology of case 3. Melanocytic pro-
liferation was found mainly at the crista profunda limi-
tans. Melanin columns are obliquely seen in the stratum
corneum at the furrows of skin markings
lesion revealed a parallel furrow pattern, while the left side of the lesion demonstrated irregu- lar, diffuse black pigmentation with irregular black dots and globules. The center of the lesion exhibited blue-white structures (Fig. III.2.12). A surgical excision was performed and the speci- men was histopathologically examined. Sec- tions were cut perpendicularly to the skin mark- ings. The hematoxylin–eosin staining revealed that prominent melanocytic proliferation was
found near the basal layer of the epidermis and upper dermis. Many nests of nevus cells were seen not only around the crista profunda limi- tans, but also around the crista profunda inter- media (Fig. III.2.13). These nevus cells were producing large amounts of melanin and trans- ferring them to the nearby keratinocytes. Abun- dant melanin granules were diffusely scattered in the stratum corneum partly forming melanin columns, which correspond to diffuse black pigmentation on dermoscopy.
Comment
The diagnostic clue in this case might be paral- lel furrow pattern shown on the right side of the lesion; however, diffuse irregular black pigmen- tation with irregular dots and globules are sug- gestive of melanoma occurring on small con- genital nevus. A case like this should be excised and needs to be examined histopathologically.
Multiple irregular dots and globules reflect the existence of numerous melanin granules in the stratum corneum. If melanin granules were limited to the honey layer of the furrows form- ing melanin columns corresponding to the par- allel pattern, it would satisfy benign criteria.
Fig. III.2.11. Clinical picture of case 4. A 5.0¥7.0-mm pigmented macule is seen on the right plantar arch of a 69-year-old Japanese man. The lesion has allegedly ex- isted since his childhood
Fig. III.2.12. Dermoscopy of case 4. The right side of the lesion revealed parallel furrow pattern, while the left side of the lesion demonstrated irregular, diffuse black pigmentation with irregular black dots and globules. The center of the lesion exhibited blue-white structures
Fig. III.2.13. Histopathology of case 4. Abundant mela-
nin granules were diffusely scattered in the stratum cor-
neum partly forming melanin columns
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C
