Dermoscopy of pigmented lesions of the palms and soles

INTRODUCTION — In populations with darkly pigmented skin, melanoma occurs most frequently in acral areas, with a particular predilection for the soles of the feet. In Black people, acral melanomas account for around 80 percent of all melanomas [1]. In Japanese individuals, almost one-half of cutaneous melanomas occur in acral areas, and approximately 30 percent affect the sole [2]. The prognosis of acral melanoma is generally poor, mainly as a consequence of a delay in diagnosis [3,4]. Biologic aggressiveness of this subtype could be another reason [5,6].

Dermoscopy, a noninvasive technique performed by a handheld instrument called a dermatoscope, increases the clinician's diagnostic accuracy for pigmented lesions of the palms and soles and may help in the recognition of acral melanoma at an early, curable stage [7-9].

This topic will review the dermoscopic features of melanocytic and nonmelanocytic pigmented lesions of the palms and soles and the dermoscopic criteria for differentiating benign melanocytic nevi from early melanoma. The principles of dermoscopy and the use of dermoscopy for the evaluation of lesions located on the nonglabrous skin, face, mucosal surfaces, and nails are discussed separately. Dermoscopic algorithms for skin cancer triage are also discussed separately.

●(See "Overview of dermoscopy".)

●(See "Dermoscopic evaluation of skin lesions".)

●(See "Dermoscopy of facial lesions".)

●(See "Dermoscopy of mucosal lesions".)

●(See "Dermoscopy of nail pigmentations".)

●(See "Dermoscopy of nonpigmented nail lesions".)

●(See "Dermoscopic algorithms for skin cancer triage".)

HISTOLOGIC FEATURES OF PALMOPLANTAR SKIN — The palmoplantar skin is anatomically and histologically unique. It is characterized by a thick, compact, cornified layer and by the presence of dermatoglyphics, consisting of ridges and furrows (sulci) that run on the surface in a parallel fashion and form loops, whorls, and arches in highly individualized patterns. Hair follicles are absent, but eccrine sweat glands, whose ducts open in the center of surface ridges, are well developed.

The pattern of the epidermal rete ridge is characteristic. In a tissue section cut perpendicularly to the surface skin markings, two types of rete ridges can be identified: the crista profunda limitans, situated under the surface furrow, and the crista profunda intermedia, situated under the surface ridge (picture 1) [7]. On scanning electron microscopy, these rete ridges appear as longitudinal, parallel rows protruding into the dermis (picture 2) [10]. Transverse ridges bridging the longitudinal ridges may also be seen. They are generally short and thin but are more prominent in the peripheral areas of the palms and soles and in the foot arch.

The assessment of the distribution of melanin granules and melanocytes in relation to the rete ridges is critical in order to differentiate acral nevi from early acral melanoma [11-13]. In acral nevi, melanocytes arranged in nests are predominantly located in the crista profunda limitans, although some melanocytes may also be detected in the crista profunda intermedia [13]. Melanin granules appear as regular columns situated in the cornified layer underneath the surface furrows, but they are usually absent under the surface ridges (picture 3). In contrast, in early acral melanoma, melanocytes arranged as solitary units are mainly present in the crista profunda intermedia underlying the surface ridges, although a few melanocytes can also be seen in the crista profunda limitans (picture 4).

DERMOSCOPIC FEATURES OF ACRAL MELANOCYTIC LESIONS — Melanocytic lesions of the palms and soles exhibit unique dermoscopic patterns that are significantly different from those seen in nonglabrous skin due to the distinctive histologic characteristics of the acral skin (picture 1). (See 'Histologic features of palmoplantar skin' above.)

Major patterns — The main pigmentation patterns of acral melanocytic lesions are as follows (figure 1) [7,14,15]:

●Parallel furrow pattern – Linear pigmentation along the furrows of the skin markings

●Lattice-like pattern – Pigmented lines along and across the furrows

●Fibrillar pattern – Fine, fibrillar or filamentous pigmentation usually arranged in the direction crossing the parallel skin markings

●Parallel ridge pattern – Band-like pigmentation located on the ridges of the skin markings

The first three patterns are typically seen in benign acquired nevi, whereas the parallel ridge pattern is the hallmark of acral melanoma. Since early melanoma and benign melanocytic nevi on the palms and soles may have a similar appearance on naked eye examination, the recognition of these specific pigmentation patterns by dermoscopy is of great help to the clinician in determining whether a lesion should be biopsied or not.

Minor (nontypical) patterns — In addition to the three major dermoscopic patterns (ie, parallel furrow pattern, lattice-like pattern, fibrillar pattern), minor patterns (formerly, collectively called "nontypical patterns") can be detected in approximately one-third of acquired melanocytic nevi of the palms and soles [16-19]. Minor patterns include:

●Globular pattern

●Acral, reticular pattern

●Homogeneous pattern

●Globulo-streak-like pattern

●Transition pattern

These patterns are described in detail below. (See 'Minor (nontypical) patterns' below.)

INFLUENCE OF AGE ON ACRAL DERMOSCOPIC PATTERNS — Several studies indicate that the prevalence of specific dermoscopic patterns of melanocytic nevi on the palms and soles varies with age [20-22]. The crista dotted pattern and the peas-in-a-pod pattern are commonly detected in acquired and congenital acral nevi of children and adolescents. (See 'Crista dotted pattern' below and 'Peas-in-a-pod pattern' below.)

In one study evaluating the dermoscopic images of 75 acral nevi in 69 patients younger than 18 years, the parallel furrow pattern and the crista dotted pattern were the most common patterns, detected in 71 and 21 percent of nevi, respectively [20]. Digital follow-up dermoscopic images obtained after a median follow-up time of 32 months showed a change in global pattern (from parallel furrow to lattice-like or fibrillar pattern) in 5 of 31 nevi and a decrease or increase in local criteria (eg, pigmentation, size, and number of globules/dots) in 20 nevi.

In another study, the peas-in-a pod pattern was observed in 20 percent of acral nevi seen in persons younger than 20 years but only in less than 1 percent of individuals older than 59 years [21]. An opposite trend was seen for nontypical patterns, detected in 36 percent of older individuals and in less than 10 percent of those younger than 20 years.

ACQUIRED MELANOCYTIC NEVI — Most melanocytic nevi detected on the palms and soles are acquired [23,24]. Approximately two-thirds of acquired acral nevi show one or combinations of the three major, benign dermoscopic patterns: the parallel furrow pattern, the lattice-like pattern, and the fibrillar pattern (figure 1) [7,14-17,19,24-26].

Parallel furrow pattern and its variants — The parallel furrow pattern results from a linear distribution of the pigment along the furrows of the skin markings, which run on the skin surface in a parallel fashion (figure 1). The basic type of parallel furrow pattern shows a single solid line of pigmentation in the furrows. Variants include the double solid line, single dotted line, and double dotted line (picture 5) [19].

The parallel furrow pattern is seen in approximately 50 percent of acral nevi in any ethnic group [8,27] and is occasionally associated with a light brown background pigmentation [19]. Orderly combinations of the parallel furrow pattern with the two other major dermoscopic patterns (lattice like and fibrillar) are also common in acral nevi (picture 6A-B) [8,19,28].

Rarely, the parallel furrow pattern may be detected in acral melanoma. However, in melanoma, this pattern is present focally or unevenly within the lesion, whereas in melanocytic nevi, it is regularly distributed across the lesion. (See 'Melanoma' below.)

Lattice-like pattern — The lattice-like pattern is formed by pigmented lines along and across the furrows of the skin markings (figure 1 and picture 7). A light brown background pigmentation may be present. This pattern is detected in approximately 15 percent of acral nevi, most often in those located on the arch of the foot or in peripheral areas of the palms and soles, where the skin markings lose the typical, parallel pattern [8,10,28]. The lattice-like pattern can be regarded as an anatomical modification of the parallel furrow pattern [19].

Fibrillar pattern

Overview — The fibrillar pattern consists of a densely packed, fine, fibrillar or filamentous pigmentation arranged perpendicularly or obliquely to the parallel skin markings (picture 8 and figure 1). It results from the oblique arrangement of the thick, cornified layer of the plantar skin due to the mechanical pressure exerted by the body weight and may be considered an artifactual modification of the parallel furrow pattern [8,19,28].

The fibrillar pattern is detected in 10 to 20 percent of plantar nevi and rarely in palmar nevi. When the fibrils composing the fibrillar pattern are very short, differentiation from the parallel furrow pattern may be difficult (picture 9). The authors' tentative criterion for the fibrillar pattern is as follows:

●The pigment fibrils typically cover at least the whole width of one surface ridge (picture 8).

●If the fibrils starting on a furrow do not reach the neighboring furrow, the pattern is classified as parallel furrow pattern (picture 9).

This classification is arbitrary and may bring some confusion because, in some cases, these two findings are simultaneously detected. However, it does not impact lesion management because both the regular fibrillar pattern and the parallel furrow pattern are benign patterns.

In some cases, particularly in children, a regular fibrillar pattern can be visualized as a parallel furrow pattern by advancing the cornified layer horizontally with the probe of a contact dermatoscope. The oblique view dermoscopy performed with a noncontact dermatoscope can change the fibrillar pattern to the parallel furrow pattern (picture 10) [29,30].

Regular versus irregular fibrillar pattern — The regular fibrillar pattern typically seen in melanocytic nevi should be differentiated from the irregular fibrillar pattern occasionally detected in acral melanomas (picture 11). As this differentiation is very important, detailed criteria for regular and irregular fibrillar patterns are described here:

●Criteria for regular fibrillar pattern (picture 8) [8,30]:

•The fibrils constituting the pattern are evenly distributed throughout the lesion and are mostly the same in color and thickness.

•The fibrillar pattern is, not infrequently, combined with the parallel furrow pattern and/or changes to the parallel furrow pattern at the periphery.

•The endpoints (deeper color ends) of the fibrils tend to line up on the sulci of the skin markings.

•In most cases, the oblique dermoscopy demonstrates that the fibrillar pattern is originally the parallel furrow pattern (picture 10).

●Criteria for irregular fibrillar pattern (picture 12) [30]:

•The fibrils constituting the pattern are unevenly distributed and variable in color and thickness.

•Within the lesion, the parallel ridge pattern is not infrequently detected at least focally.

•The endpoints of the fibrils are arranged more or less randomly.

•By oblique dermoscopy, the pattern does not change to the parallel furrow pattern but, though rare, to the parallel ridge pattern.

It should be noted that, although rare, a very early lesion of acral melanoma in situ could exhibit the regular fibrillar pattern. In this case, the fibrils composing the pattern are very thin, and the surface sulci are spared from the fibrillar pigmentation (picture 13) [30].

Minor (nontypical) patterns — In addition to the three major dermoscopic patterns (ie, parallel furrow pattern, lattice-like pattern, fibrillar pattern), minor patterns (formerly, collectively called "nontypical patterns") can be detected in approximately one-third of acquired melanocytic nevi of the palms and soles [16-19]. Minor patterns include:

●Globular pattern – Dots and globules arranged in a nonparallel fashion, often accompanied by diffuse, light brown background (picture 14A)

●Acral, reticular pattern – Reticulated pigmentation similar to the pigment network of the nonglabrous skin (picture 14A)

●Homogeneous pattern – Light brown, mostly structureless pigmentation with no other distinctive feature

●Globulo-streak-like pattern – Brown globules attached to linear or curvilinear streaks without relation to the skin markings (picture 14B)

●Transition pattern – Pigment network on the nonglabrous side and parallel furrow pattern or lattice-like pattern on the glabrous side of the lesion (picture 15)

CONGENITAL MELANOCYTIC NEVI — Medium (1.5 to 20 cm) and large (>20 cm) congenital melanocytic nevi on the palms and soles are easily diagnosed based on their larger size and clinical history of presence since birth. Small congenital nevi (≤1.5 cm) of the palms and soles may be an important clinical differential diagnosis of acral melanoma. Dermoscopic features typically detected in small congenital melanocytic nevus of the palms and soles include the parallel furrow pattern, crista dotted pattern, and peas-in-a-pod pattern, as described below [31].

Parallel furrow pattern — The parallel furrow pattern, a major dermoscopic pattern most frequently detected in acquired acral nevi, is also frequently seen in acral congenital melanocytic nevi. In a dermoscopic study of 24 congenital nevi, six showed the parallel furrow pattern [31]. (See 'Parallel furrow pattern and its variants' above.)

Crista dotted pattern — The crista dotted pattern consists of dots/globules of pigment regularly distributed on the ridges of the skin markings (picture 16). In a series of congenital acral nevi described by the authors, this pattern was observed in 3 of 24 lesions [31]. The crista dotted pattern results from the adnexocentric distribution of nevus cells, which is one of the histopathologic characteristics of congenital nevi. The dots/globules on the ridges correspond to nests of nevus cells surrounding the upper portion of eccrine ducts opening in the center of the surface ridges.

Peas-in-a-pod pattern — The peas-in-a-pod pattern is a combination of the parallel furrow and the crista dotted patterns (picture 17). This pattern, seen in 8 of 24 nevi in the authors' series, is the most prevalent dermoscopic pattern of acral congenital melanocytic nevi [31].

Other features — Congenital nevi of the palms and soles may also show [8,31-33]:

●Combinations of the three major dermoscopic patterns seen in acquired melanocytic nevi (ie, parallel furrow pattern, lattice-like pattern, and fibrillar pattern)

●Features similar to the parallel ridge pattern found in melanoma (picture 18) or other minor (nontypical) patterns, such as the globular and globulo-streak-like patterns (picture 14A-B)

●A blue-gray background pigmentation mostly seen in the central areas of the lesions, reflecting the presence of pigmented nevus cells in the dermis (picture 19)

●Enlarged, pink ridges, seen in central areas of the lesions [34]

The symmetric distribution of dermoscopic features and an even pigmentation support the diagnosis of congenital nevus. Elements of the clinical history (eg, presence since infancy, stable course over time) may be additional clues to the diagnosis. However, lesions with equivocal or suspicious dermoscopic features should be biopsied for histopathologic evaluation.

MELANOCYTIC NEVI OF THE GLABROUS/NONGLABROUS SKIN TRANSITION ZONE — Melanocytic nevi located on the transitional zone between glabrous and nonglabrous skin (ie, peripheral areas of the palms and soles, lateral aspects of the fingers and toes) (picture 20) may show unusual dermoscopic features:

●Transition pattern – The so-called transition pattern consists of a typical pigment network in the nonglabrous portion of the lesion and a parallel furrow pattern or lattice-like pattern in the glabrous portion (picture 15) [17].

●Dense, reticular/branched pattern – Nevi situated in the interdigital web spaces or on the lateral aspects of the fingers or toes may show another unusual dermoscopic pattern composed of a densely arranged, reticular or branched pigmentation (picture 21).

On histology, melanocytic nevi located on transition zones often show a prominent proliferation of melanocytes arranged as solitary units within the epidermis that mimics melanoma in situ (picture 22) [35]. However, the symmetric, orderly, intraepidermal distribution of melanocytes and the absence of nuclear atypias differentiate a benign nevus from melanoma.

MELANOMA — The parallel ridge pattern (figure 1) and an irregular, diffuse pigmentation are highly sensitive and specific features of early and advanced acral melanoma, respectively. Advanced melanoma of the palms and soles may also show dermoscopic features characteristic of melanoma of nonglabrous skin, including irregular dots/globules, irregular streaks, blue-white veil, regression structures, and polymorphous vessels (picture 12) [7,15]. (See "Dermoscopic evaluation of skin lesions".)

Parallel ridge pattern — The parallel ridge pattern consists of a band-like pigmentation, tan to black in color, located on the ridges of the skin markings (figure 1 and picture 23A). It is highly characteristic of melanoma of the palms and soles and reflects the preferential proliferation of melanocytes in the crista profunda intermedia during the early horizontal growth phase (picture 4) [7,14,15,25,26]. This preferential proliferation could be explained by the location of the niche of melanoblasts around sweat ducts in the crista profunda intermedia of the acral skin [36].

In early melanoma, the parallel ridge pattern is evenly detected within the lesion, whereas in advanced melanoma, it is focally distributed (picture 23A-B). The sensitivity and specificity of the parallel ridge pattern for the diagnosis of melanoma (including melanoma in situ) are 86 and 99 percent, respectively [25].

Occasionally, the parallel ridge pattern is detected in a variety of benign pigmented lesions of the palms and soles, such as drug-induced pigmentations, Peutz-Jeghers syndrome, or pigmented warts. However, in most cases, these lesions can be correctly diagnosed based on clinical findings and additional dermoscopic characteristics. (See 'Peutz-Jeghers syndrome' below and 'Drug-induced acral pigmentation' below and 'Pigmented ridged wart' below.)

Irregular diffuse pigmentation — Irregular diffuse pigmentation is defined as a structureless, pigmented area, tan to black in color, which is highly characteristic of acral melanoma (picture 24) [15,25,37]. The sensitivity and specificity of irregular diffuse pigmentation are 85 and 97 percent, respectively. As expected, sensitivity is lower for melanoma in situ (69 percent), since the diffuse pigmentation reflects the florid proliferation of melanocytes in more advanced lesions [25].

Irregular fibrillar pattern — An irregular fibrillar pattern is occasionally found within an acral melanoma (picture 23B). This can be explained by the fact that the fibrillar pattern is typically detected in the pressured areas of the sole, which are also the most prevalent locations of acral melanoma [30]. The criteria for the differentiation between the irregular fibrillar pattern of acral melanoma and regular fibrillar pattern of acral nevus are described above. (See 'Regular versus irregular fibrillar pattern' above.)

Other features — Advanced melanoma of the palms and soles may show the same dermoscopic features of advanced melanoma of nonglabrous skin (picture 12) [7,15]. However, the atypical pigment network, which is a major feature of melanoma of nonglabrous skin, is extremely rare in melanomas of the palms and soles [38]. (See "Dermoscopic evaluation of skin lesions".)

Brown and brownish-black globules irregularly distributed on the ridges, reflecting transepidermal elimination of melanoma cell nests, can be a characteristic dermoscopic pattern of acral melanoma [39]. Of note, brown globules are regularly distributed on the ridges in congenital nevus [31] and Spitz nevus on the palms and soles [40].

Occasionally, dermoscopic patterns typically seen in acral melanocytic nevi (eg, parallel furrow and lattice-like patterns) can also be seen in advanced acral melanoma [41]. However, in melanoma, these patterns usually have a focal or irregular distribution within the lesion (picture 23B).

Acral melanoma may be amelanotic or hypomelanotic. In a series of 126 acral lentiginous melanomas, 34 (28 percent) were unpigmented [42]. In amelanotic or hypomelanotic melanomas, the presence of microscopic remnants of pigmentation and atypical, vascular structures are important clues to the diagnosis [43]. Lesions with these dermoscopic findings should always be biopsied and sent for histopathologic examination. (See "Dermoscopic evaluation of skin lesions".)

ATYPICAL MELANOSIS OF THE FOOT — Atypical melanosis of the foot is an unusual, plantar, pigmented lesion that has clinical and dermoscopic, but not histologic, features of early acral lentiginous melanoma [44-47]. These lesions present as large macules with irregular borders and variegated colors (picture 25).

On dermoscopy, they usually show the parallel ridge pattern typical of melanoma (picture 26) [46,47]. Although the clinical and dermoscopic features suggest melanoma, on histologic examination, only a slightly increased number of melanocytes without cytologic atypias is detected in the crista profunda intermedia (picture 27). It has been hypothesized that these lesions may represent very early or slowly evolving acral melanoma in situ [12,48,49].

APPROACH TO THE DERMOSCOPIC DIAGNOSIS OF ACRAL MELANOCYTIC LESIONS

The three-step dermoscopic algorithm — The three-step algorithm for the diagnosis and management of melanocytic lesions on the palms and soles was originally proposed in 2007 [19] and then revised in 2011 [50]. Step 1 of this algorithm is based upon the high sensitivity, specificity, and positive predictive value (86, 99, and 94 percent, respectively) of the parallel ridge pattern for early acral melanoma [19,25]. Step 2 is based on data that indicate that the sensitivity, specificity, and positive predictive value of the typical parallel furrow pattern/lattice-like pattern for melanocytic nevi are 67, 93, and 98 percent, respectively [25]. The three-step algorithm was further modified in 2022 [30]. It incorporated the asymmetric multicomponent pattern in the first step [51] and introduced a follow-up strategy. The latest version is presented here (algorithm 1) [30]:

●Step 1 – The lesion is examined for the presence of the parallel ridge pattern. If the parallel ridge pattern is found in any part of the lesion, the lesion should be biopsied regardless of the size. A lesion exhibiting the asymmetric multicomponent pattern should also be biopsied. If the lesion does not show these patterns, proceed to Step 2.

●Step 2 – The lesion is examined for the presence of one or an orderly combination of the typical, benign dermoscopic patterns (ie, typical parallel furrow pattern, typical lattice-like pattern, regular fibrillar pattern). If the whole area of the lesion shows one or an orderly combination of two or three typical, benign patterns, further dermoscopic follow-up is not needed. If the lesion shows equivocal dermoscopic features (ie, part or total absence of any typical/regular patterns) (picture 28), proceed to Step 3.

●Step 3 – The maximum diameter of lesions that do not show typical, benign patterns is measured. Lesions >7 mm should be excised or biopsied for histopathologic evaluation [52]. Lesions ≤7 mm should be monitored clinically and dermoscopically with a frequency of once or twice a year.

For the correct use of the three-step algorithm, it is important to keep the following in mind:

●Acral lesions exhibiting highly suspicious dermoscopic features, such as nodular, pigmented lesions and ulcerated/hypopigmented lesions with prominent, polymorphous vessels, should be treated as melanoma. There is no need for the application of this algorithm for these lesions.

●The algorithm has been developed for the differentiation of acquired melanocytic nevi from acral melanoma of the palms and soles and may not be appropriate for the evaluation of congenital nevi in those locations. In most cases, congenital nevi can be identified on the basis of their characteristic dermoscopic features [31] (see 'Congenital melanocytic nevi' above). However, the three-step algorithm can be used to evaluate acral nevi whose type (acquired or congenital) cannot be determined.

●In the first step, it is crucial to correctly identify the furrows and ridges of the skin markings. Their recognition can be greatly facilitated by performing the "furrow ink test" before examining the lesion under the dermatoscope [30,53,54]. The periphery of the lesion is marked with liquid ink (eg, from a fountain pen) or with a whiteboard marker pen, preferably blue or green in color. The skin surface is then gently wiped with a dry paper towel. The surface furrows retain the blue or green ink and become clearly visible on dermoscopic examination as thin, inked lines. The test will allow the clinician to assess whether the melanin pigmentation follows the ink lines, as in the parallel furrow pattern (picture 29), or is located between the ink lines, thus representing a parallel ridge pattern (picture 30). Once the examination is completed, the marker pen ink in the furrows can be easily removed by wiping the skin with a wet paper towel.

●In the second step, the clinician must assess whether the benign patterns are typical/regular. Typical parallel furrow or lattice-like patterns are symmetrically and evenly distributed across the lesion. The criteria for classifying a fibrillar pattern as regular or irregular are described above (see 'Regular versus irregular fibrillar pattern' above). Orderly combinations of benign patterns are also considered benign.

The decision not to follow-up lesions that are judged unequivocally benign in Step 2 is based upon the observation that the risk of acral melanoma developing in a pre-existing nevus is extremely low [55-57]. In digital follow-up studies of acral melanocytic nevi, a change from a benign to a malignant pattern has not been reported [18,19,58]. However, changes within benign patterns have been observed in 20 to 70 percent of cases [18,58]. Lesions that cannot be unequivocally classified as benign should be biopsied for histopathologic evaluation if they are larger than 7 mm.

●Follow-up – If a lesion with equivocal patterns increases in size >7 mm in diameter between follow-up visits, it should be biopsied. If the parallel ridge pattern and/or the asymmetric multicomponent pattern appears with time within a lesion, it should also be biopsied [30].

The BRAAFF checklist — Based upon the dermoscopic examination of 603 acral lesions (472 nevi and 131 acral melanomas, including 42 in situ lesions), a new six-variable scoring system has been developed for the diagnosis of acral melanoma [59]. This system, called the BRAAFF checklist, consists of six variables, each with a positive or negative value:

●Irregular blotch (+1)

●Parallel ridge pattern (+3)

●Asymmetry of structures (+1)

●Asymmetry of colors (+1)

●Parallel furrow pattern (-1)

●Fibrillar pattern (-1)

A total score of ≥1 is needed for a diagnosis of melanoma. The threshold of one point provided a sensitivity of 93 percent and a specificity of 87 percent [59].

ACRAL LESIONS DUE TO INCREASED MELANIN DEPOSITION — Dermoscopy is helpful in the diagnosis of a variety of nonmelanocytic benign pigmented lesions of the palms and soles, some of which may mimic acral melanoma [60]. In most cases, the correct diagnosis can be made based upon clinical history and/or associated signs and symptoms. A biopsy for histopathologic evaluation may be warranted when the diagnosis is uncertain.

Drug-induced acral pigmentation — Several anticancer drugs (eg, fluorouracil, doxorubicin, cyclophosphamide) may induce focal acral hyperpigmentation, such as pigmented macules and melanonychia [61-63]. Multiple small, brownish macules may develop on the hands or feet and are often accompanied by a linear pigmentation of the palmar and plantar creases. On dermoscopy, the hyperpigmented macules show a parallel ridge pattern similar to that seen in early acral melanoma (picture 31). Histology shows increased melanin in the basal layer of the epidermis without an increased number of epidermal melanocytes along with a few melanophages in the papillary dermis [61].

Peutz-Jeghers syndrome — Peutz-Jeghers syndrome is a rare, autosomal dominant disorder characterized by gastrointestinal polyposis in association with multiple small, hyperpigmented, mucocutaneous macules most often located on the lips and buccal mucosa and on the dorsal and volar aspect of the hands and feet (picture 32A-B). On dermoscopy, the pigmented macules located on the volar skin show the parallel ridge pattern [64]. The diagnosis is based upon the characteristic distribution of the macules, family history, and demonstration of colonic hamartomas. (See "Peutz-Jeghers syndrome: Clinical manifestations, diagnosis, and management".)

Laugier-Hunziker syndrome — Laugier-Hunziker syndrome is a rare, acquired, macular hyperpigmentation of the lips, oral mucosa, and acral skin frequently associated with longitudinal melanonychia. In contrast with Peutz-Jeghers syndrome, Laugier-Hunziker syndrome is not associated with systemic disorders. On dermoscopy, the pigmented macules typically show the parallel ridge pattern, but cases with a parallel furrow pattern have been reported [65,66]. Histology shows increased melanin in basal keratinocytes, particularly in those located in the crista profunda intermedia (ie, the epidermal rete ridges underlying the surface ridges) [66]. (See "Acquired hyperpigmentation disorders", section on 'Laugier-Hunziker syndrome'.)

Volar melanotic macules — Volar melanotic macules are solitary or multiple brownish macules found on the palms and soles of individuals with darkly pigmented skin [67-69]. On dermoscopy, they may show a parallel ridge pattern (picture 33). Histologically, volar melanotic macules are characterized by increased deposition of melanin in the epidermis; hyperpigmented, solitary, dendritic melanocytes scattered along the basal layer; and melanophages in the dermis [68].

Pigmented ridged wart — The pigmented ridged wart is an uncommon type of plantar wart associated with a cystic component, caused by the human papillomavirus type 60 [70]. On dermoscopy, it shows a parallel ridge pattern and may mimic a verrucous type of acral melanoma [71-73]. When the clinical diagnosis is unclear, a biopsy is necessary to exclude melanoma. Histology reveals hyperkeratosis; acanthosis; large, vacuolated cells in the malpighian and granular layers; and increased deposition of melanin in the epidermis without proliferation of abnormal melanocytes.

NONMELANOCYTIC PIGMENTED LESIONS

Hemorrhage, hematoma, and hemangioma — Dermoscopic features of hemorrhage/hematoma and hemangioma of the palms and soles are similar to those seen in nonglabrous skin. However, due to the unique, anatomical structure of the acral volar skin, some hemorrhagic lesions show characteristic dermoscopic patterns, as described below. (See "Dermoscopic evaluation of skin lesions".)

Black heel (calcaneal petechiae) — Black heel, also called calcaneal petechiae or talon noir, is an asymptomatic pigmentation of the heel secondary to intraepidermal extravasation of red blood cells caused by shear force injuries (eg, friction against shoes during vigorous sports) [74]. On naked eye examination, black heel appears as a black macule or plaque that mimics melanoma (picture 34A-B).

On dermoscopic examination, black heel shows a unique dermoscopic pattern called the "pebbles on the ridges," in which a reddish-black, pebble-like pigmentation is distributed on the ridge of the skin markings (picture 35) [7,14]. The pebble-like pigmentation corresponds to aggregation of hemosiderin in the superficial skin layers, mostly in the stratum corneum.

With more abundant blood extravasations, the pebble-like pigmentation becomes confluent and forms a band-like pigmentation (picture 36) that mimics the parallel ridge pattern seen in melanoma [75]. However, the reddish tone, sharp demarcation, and subtle segmentation of the pigmented bands differentiate black heel from melanoma.

PlayStation purpura/PlayStation fingertip — The so-called "PlayStation purpura" or "PlayStation fingertip" presents as brownish macules on the index or middle fingers, which result from subcorneal hemorrhages caused by a prolonged use of the handheld game controller device. On dermoscopy, these macules show a parallel ridge pattern [76-78]. However, the symmetric location on the index or middle fingers, the rusty/reddish hue of the color, and a history of prolonged video gaming are clues to the correct diagnosis.

Tinea nigra — Tinea nigra is a superficial fungal infection of the palms and soles that presents as a large, brownish macule (picture 37A-C). Dermoscopy reveals light brown, fine strands arranged in a reticular pattern [79]. The pigmentation does not follow the furrow or ridges of the skin markings (picture 38). The diagnosis is confirmed by potassium hydroxide (KOH) examination of scrapings from the lesion (picture 39). (See "Tinea nigra".)

Pigmentation due to chemicals — Accidental staining of the plantar skin surface with paraphenylenediamine, a derivative of aniline used in hair dyes and rubber products, can display the parallel ridge pattern on dermoscopy [60,80,81]. Palmar or plantar pigmentation caused by self-tanning products can also show the parallel ridge pattern. A detailed history, including the patient's occupation and hobbies, is important for a correct diagnosis. The pigment can be removed by shaving the superficial, cornified layer with a scalpel.

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Dermoscopy".)

SUMMARY AND RECOMMENDATIONS

●Major dermoscopic patterns seen in acral melanocytic lesions:

•Acquired acral melanocytic nevi – Most acral acquired melanocytic nevi show one of three major dermoscopic patterns (figure 1): the parallel furrow pattern (picture 5), the lattice-like pattern (picture 7), and the fibrillar pattern (picture 8). (See 'Acquired melanocytic nevi' above.)

•Acral congenital nevi – Major dermoscopic patterns seen in acral congenital melanocytic nevi are the parallel furrow pattern (picture 19), the crista dotted pattern (picture 16), and the peas-in-a-pod pattern (picture 17). In addition, congenital nevus may exhibit variegated dermoscopic features mimicking those seen in melanoma. (See 'Congenital melanocytic nevi' above.)

•Acral melanoma – The parallel ridge pattern (figure 1 and picture 23A) is highly characteristic of early melanoma of the palms and soles. Advanced melanoma typically shows irregular, diffuse pigmentation (picture 24) but may also show dermoscopic features seen in melanoma of nonglabrous skin (eg, irregular dots/globules, irregular streaks, blue-white veil, regression structures, and polymorphous vessels). (See 'Melanoma' above.)

●Approach to the dermoscopic diagnosis of acral melanocytic lesions – A three-step algorithm (algorithm 1) has been proposed for the diagnosis and management of acquired melanocytic lesions on the palms and soles. (See 'The three-step dermoscopic algorithm' above.)