Module 12: Solar Lentigines, Seborrheic Keratoses, and Lichen Planus-Like Keratosis (LPLK)

Clinical Features

Solar lentigines are sharply circumscribed, uniformly pigmented macules located predominantly on sun-exposed areas: the dorsal hands, shoulders, and scalp. They result from hyperplasia of keratinocytes and melanocytes with increased melanin accumulation in keratinocytes, induced by ultraviolet light exposure. Unlike freckles (ephelides), solar lentigines persist indefinitely. Younger individuals who burn easily can develop lentigines after acute or prolonged UV exposure.

Clinical appearance:

• Shape: oval, round, or irregular
• Size: few millimeters to a few centimeters in diameter
• Color: most are uniform light brown; some range from dark brown to black
• Surrounding skin: often shows actinic purpura or other signs of solar damage

Histology: Club-shaped rete ridges with small nub-like extensions, increased number of melanocytes, and increased pigmentation of basal keratinocytes.

Clinical significance: Most solar lentigines are easily recognized clinically. However, some pose diagnostic challenges because their clinical appearance resembles melanoma. Dermoscopy is helpful in correctly differentiating solar lentigo from melanoma. A solar lentigo is a benign lesion that can evolve to form a pigmented seborrheic keratosis.

Dermoscopic Features of Solar Lentigo

The six key dermoscopic features are:

1. Moth-eaten border

• A sharply demarcated and irregularly curved border
• Portions of the border are scalloped, giving a moth-eaten appearance (as if fabric had been nibbled by moths)
• This is highly characteristic of solar lentigines
• The border is sharp but irregularly concave, distinguishing it from the fading, diffuse edges seen in melanocytic lesions

2. Homogeneous light brown pigmentation (Jelly sign)

• Many lesions have no structures or networks, only light brown structureless areas
• The term "jelly sign" describes the pigment quality: the pigment appears as if jelly had been smeared on the skin surface
• Under dermoscopy, the pigment can create an almost fish scale-like appearance, with U-shaped structures all oriented in the same direction, resembling jelly spread across toast

3. Pigment network (faint reticulation)

• An area of faint reticulation may be present
• Correlates histologically with melanocytes and melanin-filled keratinocytes in the rete ridges
• The network is typically thin, regular, and fading at the periphery

4. Fingerprint-like areas

• Areas consisting of fine parallel running lines of light brown to dark brown color
• Resemble the dermatoglyphics of a human fingerprint
• The lines are parallel and do not interconnect to form a true reticulated network
• Represents a very characteristic finding in solar lentigines

5. Pseudonetwork

• Seen in lentigines located on the scalp and face (special locations)
• Created when a diffusely pigmented area is interrupted by nonpigmented adnexal openings (follicular ostia)
• Shares features with pigmented melanocytic lesions in these special locations
• See Module 30 (Face) for further information on pseudonetwork patterns

6. Symmetric follicular pigmentation

• Pigment around hair follicles distributed in a symmetric fashion, creating small brown circles
• The pigment is usually light brown and matches the color of the rest of the lesion
• Some follicles may appear asymmetrically pigmented as brown crescent-shaped structures, but the color still matches the rest of the lesion

Important differential point: If the color of the pigment around the follicle (whether symmetric or asymmetric) is of a grayish hue or differs from the rest of the lesion, then melanoma (specifically lentigo maligna) must enter the differential diagnosis.

Ink Spot Lentigo

Ink spot lentigo is a distinct variant of solar lentigo with the following characteristics:

• Clinical: Jet-black color
• Dermoscopic pattern:
• Very prominent black pigmented network with an almost three-dimensional quality
• Network lines can be either thin or thick in width
• The network ends abruptly at the edge of the lesion
• The quality has been described metaphorically as resembling a "chicken-wire fence"
• Significance: Despite the alarming dark color, the regular (though prominent) network and sharp termination are reassuring features

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Clinical Scenario

A 67-year-old woman presents with a well-demarcated brown macule on the left temple with a moth-eaten border. Dermoscopy reveals a sharp, scalloped border with thin, gently curved parallel lines (fingerprint-like structures) across the lesion. The surface is smooth and flat. No comedo-like openings, milia-like cysts, or fissures are identified.

What is your diagnosis and key dermoscopic findings?

Solar lentigo (flat seborrheic keratosis precursor)

The moth-eaten border is highly characteristic of solar lentigo -- it represents a sharply demarcated, scalloped edge created by the variable extent of melanocyte proliferation along the DEJ. Fingerprint-like structures (thin, curved parallel lines) correspond to early gyri formation and are specific to solar lentigines and early flat seborrheic keratoses. As the lesion matures and thickens, the fingerprint structures may evolve into the fissures and ridges of a seborrheic keratosis. The absence of comedo-like openings and milia-like cysts confirms this is still an early, flat lesion in the lentigo-to-SK continuum.

Clinical Features

Seborrheic keratoses are benign epithelial lesions appearing on any body site except mucous membranes, palms, and soles. They are prevalent in people older than 30 years. Their etiology is unclear -- UV light may drive some (they can evolve from solar lentigines), but many develop in UV-protected areas such as inframammary folds.

Clinical progression:

• Early: Light to dark brown oval macules with sharply demarcated borders (indistinguishable from solar lentigo)
• Mature: Transform into plaques with a waxy or "stuck-on" appearance, warty and keratotic surfaces, and follicular plugs scattered over the surface
• Size: Few millimeters to a few centimeters

Histology: Papillomatous epidermal hyperplasia of uniform and monotonous keratinocytes with pseudocysts. Several distinct histologic subtypes exist.

Clinical significance: While most SK are diagnosed clinically with ease, deeply pigmented variants can simulate melanoma. Irritated or traumatized SK can mimic melanoma or squamous cell carcinoma. Skin cancer can develop within an SK, justifying biopsy of atypical-appearing lesions.

Nine Classic Dermoscopic Features of Seborrheic Keratosis

Early (flat) SK share features with solar lentigines: moth-eaten borders, fingerprint-like structures. As they thicken, the classic SK features emerge:

1. Milia-like cysts

• White-to-yellow, round structures that appear very bright against dark brown or black surroundings
• Multiple milia-like cysts in a pigmented SK create a "stars in the sky" appearance
• Histologic correlate: Intraepidermal, keratin-filled cysts
• "Cloudy" vs. "Starry" quality:
• In SK: cysts appear "cloudy" -- larger and hazier in appearance
• In melanoma and congenital nevi: cysts appear "starry" -- small, bright, and sharp
• Diagnostic value: If a lesion is nonmelanocytic and not a BCC, the presence of milia-like cysts is diagnostic of SK, especially if more than three are present
• Polarization note: Milia-like cysts are more conspicuous with nonpolarized dermoscopy and are often difficult to visualize with polarized dermoscopy. When SK is suspected, examine with nonpolarized dermoscopy if available.
• Milia-like cysts can also be seen in BCC, congenital nevi, and (rarely) melanoma

2. Comedo-like openings (crypts)

• Round to ovoid craters containing black or brown comedo-like plugs
• Histologic correlate: Keratin-filled invaginations of the skin surface
• Highly characteristic of SK

3. Fissures (sulci) and ridges (gyri)

• Fissures are comedo-like openings that are not round but rather linear
• Appear as dark brown to black linear to curvilinear structures within the lesion
• Histologic correlate: Deep invaginations of the epidermis, filled with keratin
• Numerous fissures and ridges can create network-like structures or produce a cerebriform pattern

4. Network-like structures

• Interlacing gyri (ridges) and sulci (fissures) can create an appearance of network-like or negative network-like structures
• Key differentiation from melanocytic network: The grid in SK is significantly larger than the pigment network grid seen in melanocytic nevi
• However, at times the network-like structure in SK can look very similar to that of a nevus
• Clinical tip: Examining the lesion via side lighting can make the ridges more evident, helping differentiate network-like ridges in SK from the pigment network of melanocytic lesions
• Clonal SK can reveal features that mimic a negative network

5. Cerebriform pattern

• Multiple fissures (sulci) and ridges (gyri) produce a pattern resembling the sulci and gyri of the brain (brain-like appearance)
• Generally associated with the acanthotic subtype of SK
• The pattern is unmistakable once recognized and is essentially pathognomonic for SK

6. Fat fingers

• Linear and wide dermoscopic structures corresponding to ridges
• Appear as short sausage-shaped structures
• Colors vary from tan/brown to blue; can also be hypopigmented
• Named because shapes resemble:
• A straight finger (linear)
• A bent finger (curvilinear)
• A fingertip (oval-circular)
• A helpful clue when other SK features are subtle

7. Sharply demarcated borders

• SK characteristically have sharp, well-defined borders (the "stuck-on" appearance correlate)
• Matches the clinical impression

8. Typical hairpin blood vessels with white halo

• Hairpin vessels can appear as perfect U-shaped vessels or as U-shaped vessels twisted upon themselves
• Key feature: Typical hairpin blood vessels have a whitish halo around the vessel, corresponding to surrounding keratin
• Differential from melanoma: Some melanomas also have hairpin vessels, but these generally have a pink halo (not white)
• Similar hairpin vessels on a pink background can also be seen in irritated SK

9. Wobble sign (dynamic test)

• A dynamic test performed only with a contact dermoscopy device
• Technique: Press the contact plate firmly against the lesion (vertical pressure), then move it slightly back and forth in the horizontal plane (parallel to the skin surface)
• SK result: The lesion appears to stick to the glass plate and moves en bloc (slides back and forth)
• Intradermal nevus result: The lesion does not move en bloc but rather rolls back and forth (wobbles)
• Useful for differentiating raised SK from intradermal nevi

SK Subtypes and Their Dermoscopic Patterns

Flat (early) SK:

• Moth-eaten borders
• Fingerprint-like structures
• Features overlap with solar lentigo (these lesions exist on a continuum)
• Earliest milia-like cysts or comedo-like openings may begin to appear

Acanthotic SK:

• Prominent cerebriform pattern (gyri and sulci)
• Milia-like cysts
• Comedo-like openings
• Sharply demarcated borders

Hyperkeratotic SK:

• Thick surface keratosis may obscure underlying structures
• Comedo-like openings visible through scale
• Fissures and ridges
• Hairpin vessels may be seen at periphery

Clonal SK:

• Can reveal features confused with negative network
• Network-like structures that mimic melanocytic lesions
• Biopsy may be warranted when dermoscopy is equivocal

Irritated (traumatized) SK:

• Hairpin vessels on a pink background (without the classic white halo)
• Regression-like structures may appear
• Can mimic melanoma or SCC
• History of trauma or presence of typical SK criteria elsewhere in the lesion may be reassuring
• A biopsy is justified for atypical-appearing irritated SK

Melanoacanthoma (heavily pigmented SK):

• Dense dark brown to black pigmentation
• Comedo-like openings and milia-like cysts may still be identifiable
• Blue-white veil-like areas possible
• Sharply demarcated borders

Important warning: Skin cancer can develop within a seborrheic keratosis. A biopsy is justified for any atypical-appearing SK, particularly when dermoscopic features include regression structures, blue-white veil, or asymmetric features not explained by SK criteria alone.

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Check Your Understanding

What are the hallmark dermoscopic features of seborrheic keratosis?

The hallmark features include milia-like cysts (white-yellow round globular structures), comedo-like openings (dark, sharply circumscribed round structures), fissures/ridges (brain-like or cerebriform pattern), hairpin vessels with a whitish halo, and a sharply demarcated border. These features reflect the exophytic, epidermal nature of the lesion.

Clinical Scenario

A 54-year-old man presents with a well-demarcated, darkly pigmented, waxy plaque on the trunk with a "stuck-on" appearance. Dermoscopy reveals multiple comedo-like openings (dark brown plugs in round craters), numerous milia-like cysts (white-yellow round structures with a "cloudy" appearance), thick curvilinear fissures creating a brain-like cerebriform pattern, and looped (hairpin) vessels with a white halo at the periphery. A blue-white veil-like area is present in the most heavily pigmented portion, but within it, comedo-like openings are still visible.

What is your diagnosis and key dermoscopic findings?

Heavily pigmented seborrheic keratosis (melanoacanthoma)

This case demonstrates the classic dermoscopic features of SK: comedo-like openings (keratin-filled invaginations), milia-like cysts (intraepidermal keratin-filled pseudocysts), and the cerebriform pattern (multiple fissures and ridges resembling brain sulci and gyri). The "cloudy" quality of the milia-like cysts is characteristic of SK (versus the "starry" quality seen in melanoma). The hairpin vessels with white halos correspond to capillary loops in dermal papillae surrounded by keratinized rete ridges. The blue-white veil-like area could raise concern for melanoma, but two features confirm SK: (1) comedo-like openings are visible within the veil (melanoma BWV would not contain comedo-like openings), and (2) no melanoma-specific structures (streaks, atypical network, angulated lines, negative network) are present.

Key Takeaways

• Comedo-like openings (keratin-filled invaginations) and milia-like cysts (intraepidermal horn cysts) are the hallmark features of seborrheic keratosis, best seen under NPD.
• Fissures and ridges (gyri and sulci pattern) create a cerebriform surface that is nearly pathognomonic for SK when combined with other classic features.
• Fingerprinting pattern (parallel thin ridges) is characteristic of flat seborrheic keratosis and solar lentigo, corresponding to elongated rete ridges.

Clinical Features

LPLK is a relatively common skin lesion on actinically damaged skin. It is believed to represent an immunologic or inflammatory regression of a pre-existing lesion -- most commonly a seborrheic keratosis, solar lentigo, or actinic keratosis.

Clinical appearance:

• Usually a solitary lesion with sharply demarcated borders
• May be macular or slightly raised
• Color: brown, tan-brown, violaceous, or red-brown
• A pink variant exists that resembles basal cell carcinoma
• Clinical diagnosis is often difficult because LPLK can mimic cutaneous malignancies

Histology: Shares features with lichen planus, including band-like lichenoid inflammatory infiltrate, basal cell degeneration, and melanin incontinence (responsible for the granular dermoscopic appearance).

Dermoscopic Features of LPLK

Dermoscopy can sometimes help differentiate LPLK from other lesions; however, it does not reliably differentiate most pink or skin-colored LPLK from BCC and melanoma, because these pink lesions tend to be structureless (featureless). LPLK often reveals crystalline structures, which can also be seen in BCC and melanoma.

The three key dermoscopic patterns are:

1. Diffuse granular pattern

• Diffuse granularity (peppering) of brown, gray, bluish-gray, or white-gray colors
• The granules tend to be coarse and scattered throughout the lesion in a homogeneous fashion
• Represents melanin incontinence with melanophages in the upper dermis
• This pattern indicates complete regression of the original lesion
• The diffuse, homogeneous distribution is characteristic (as opposed to the focal, asymmetric regression seen in melanoma)

2. Localized granular pattern

• Grayish-brown granularity (peppering) localized to one focal area within the lesion
• Because regression is only occurring in part of the lesion, the typical dermoscopic features of the original lesion (SK, solar lentigo, or actinic keratosis) can still be found in the remaining portion
• This represents an intermediate stage of regression
• The coexistence of SK/SL features alongside granularity is a helpful diagnostic clue -- it reveals the lesion "in transition"

3. Pink to orange macule with shiny white lines

• A fairly structureless pattern that may reveal focal granularity, vessels, and shiny white structures (including blotches, lines, and rosettes)
• Critical limitation: This pattern is impossible to differentiate from superficial BCC and amelanotic melanoma based on dermoscopy morphology alone
• Biopsy is warranted when this pattern is encountered

Early vs. Late LPLK Patterns

Stage	Dermoscopic Features	Interpretation
Early LPLK	Features of the precursor lesion (SK or SL) still dominant; focal granularity beginning in one area	Regression just beginning; the original lesion is partially preserved
Intermediate LPLK	Localized granular pattern coexisting with residual SK/SL features (e.g., milia-like cysts + granularity; moth-eaten border + peppering)	Active regression in progress; both components recognizable
Late LPLK	Diffuse granular pattern throughout; no residual features of the precursor lesion; or pink structureless with shiny white lines	Complete regression; precursor lesion obliterated by inflammation

LPLK Granularity vs. Melanoma Regression

The coarse granules in LPLK can be confused with the blue-gray dots (regression structures) seen in melanomas. The key differentiating features are:

Feature	LPLK	Melanoma with Regression
Granule size	Larger, coarser	Smaller, finer ("pepper-like")
Granule character	Often appear clumped	Finely dispersed
Distribution	Homogeneous and diffuse throughout the lesion	Typically focal, asymmetric, or involving only part of the lesion
Surrounding features	Residual SK/SL features (milia-like cysts, comedo-like openings, moth-eaten border) may be present	Atypical network, irregular dots/globules, blue-white veil, irregular streaks in other areas
Border	Sharp, regular	Often irregular, fading in some areas
Crystalline structures	May be present (also seen in BCC and melanoma)	May be present
Clinical context	Solitary lesion on sun-damaged skin; stable or slowly involuting	History of change; may be growing or evolving

Clinical decision: When dermoscopic features are equivocal between LPLK and melanoma regression, biopsy is mandatory. The consequences of missing a melanoma far outweigh the morbidity of a biopsy.

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Key Takeaways

• Solar lentigo shows a moth-eaten border with fingerprinting pattern and sharply demarcated structureless brown areas, without the comedo-like openings typical of SK.
• Lichen planus-like keratosis (LPLK) represents an immunologic regression of SK or solar lentigo and shows granularity/peppering (gray dots) corresponding to melanophages.
• LPLK can mimic regressing melanoma dermoscopically; clinical context (well-defined plaque on sun-damaged skin, no prior melanocytic lesion) is essential for differentiation.

Solar Lentigo vs. Lentigo Maligna (LM)

Both lesions occur on sun-damaged facial skin and can show a pseudonetwork pattern. Differentiating features include:

Feature	Solar Lentigo	Lentigo Maligna
Border	Moth-eaten, sharply demarcated	Irregular, ill-defined, fading at edges
Pigmentation	Uniform light brown; jelly sign	Asymmetric pigmentation; variable shades of brown, gray, black
Follicular pigmentation	Symmetric; same color as rest of lesion	Asymmetric follicular pigmentation; grayish hue differing from surrounding pigment
Pseudonetwork	Regular, uniform interruptions by follicular openings	Asymmetric pseudonetwork with variable coloring
Annular-granular structures	Absent	Gray dots and globules arranged around follicular openings
Rhomboidal structures	Absent	Gray, angulated lines forming rhomboid shapes around follicles
Obliterated follicles	Absent	Follicular openings progressively filled with pigment (late sign)
Network	Faint, regular, thin lines	Atypical, irregular, thick lines with variable spacing
Color	Light brown, uniform	Multiple shades; gray component suggests dermal melanin

Seborrheic Keratosis vs. Melanoma

Feature	Seborrheic Keratosis	Melanoma
Milia-like cysts	"Cloudy" (larger, hazier)	"Starry" (small, bright, sharp) -- when present
Comedo-like openings	Present, multiple	Absent or rare
Fissures and ridges	Present (cerebriform pattern)	Absent
Fat fingers	Present	Absent
Network	Network-like structures (wider grid, caused by ridges)	True pigment network (atypical: variable lines, irregular holes)
Hairpin vessels	With white halo	With pink halo (when present)
Border	Sharply demarcated ("stuck on")	Irregular, asymmetric
Blue-white veil	Absent (except melanoacanthoma)	Present focally (indicates dermal melanin + acanthosis)
Regression structures	Absent (unless irritated/traumatized)	Blue-gray dots, white scar-like areas
Streaks/pseudopods	Absent	Present at periphery (irregular radial growth)
Wobble sign	Slides en bloc	Not applicable (flat or wobbles if papular)
Symmetry	Generally symmetric	Asymmetric in structure and color

LPLK vs. Regressive Melanoma

Feature	LPLK	Regressive Melanoma
Granule character	Coarse, clumped	Fine, pepper-like
Granule distribution	Homogeneous, diffuse	Focal, asymmetric
Residual structures	SK or SL features (milia-like cysts, comedo-like openings, fingerprint areas, moth-eaten border)	Atypical network, irregular dots/globules, blue-white veil, streaks
Shiny white structures	May be present	May be present (nonspecific)
Vessels	Absent or minimal	Polymorphous vessels may be present
Overall pattern	Symmetric, homogeneous regression	Asymmetric; malignant features in non-regressed areas
Clinical context	Solitary; actinically damaged skin; stable	History of change or growth; may have satellite lesions

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Check Your Understanding

How can dermoscopy help distinguish a solar lentigo from lentigo maligna on the face?

Solar lentigo shows a sharp, moth-eaten border, uniform brown fingerprint-like structures, and no follicular involvement. Lentigo maligna shows asymmetric pigmented follicular openings, annular-granular structures (gray dots around follicles), rhomboidal structures, and progressive obliteration of follicular openings. The pattern of follicular pigmentation is the key distinguishing feature.

Key Takeaways

• Pigmented SK can closely mimic melanoma; the absence of a true pigment network and presence of comedo-like openings and milia-like cysts favor SK.
• The blink sign (toggling NPD/PD on hybrid dermatoscope) lights up milia-like cysts and helps confirm SK diagnosis in equivocal cases.
• Irritated SK may lose classic features and develop vessels or regression-like areas; when classic SK criteria are absent, biopsy is warranted to exclude melanoma.

Criterion	Solar Lentigo	Lentigo Maligna
Border	Moth-eaten, sharp	Irregular, ill-defined
Pigmentation uniformity	Uniform light brown	Asymmetric, variegated
Jelly sign	Present	Absent
Fingerprint-like areas	Present	Absent
Follicular pigmentation	Symmetric, same color as lesion	Asymmetric, gray hue, color differs from lesion
Annular-granular structures	Absent	Present (gray dots/globules around follicles)
Rhomboidal structures	Absent	Present
Obliterated follicular openings	Absent	Present (late sign)
Pigment network	Faint, regular, thin	Atypical, irregular, thickened
Ink spot variant	Black but regular network, abrupt edge	N/A (always consider biopsy for jet-black facial lesions)
Pseudonetwork (face)	Uniform brown, regular follicular interruptions	Asymmetric, variable pigment around follicles

Criterion	Seborrheic Keratosis	Melanoma
Milia-like cysts	Cloudy (large, hazy); >3 = diagnostic if nonmelanocytic, non-BCC	Starry (small, bright, sharp); rare
Comedo-like openings	Multiple, characteristic	Absent
Cerebriform pattern	Present (acanthotic SK)	Absent
Fat fingers	Present	Absent
Fissures and ridges	Present	Absent
Network quality	Network-like (wide grid from ridges); confirm with side lighting	True atypical pigment network (irregular lines and holes)
Hairpin vessel halo	White halo (keratin)	Pink halo
Border	Sharp, stuck-on	Irregular, asymmetric
Blue-white veil	Absent	Present
Regression	Absent (unless traumatized)	Blue-gray dots, white scar-like depigmentation
Symmetry	Generally symmetric	Multicomponent, asymmetric
Wobble sign	Slides en bloc	N/A

Criterion	LPLK	Regressive Melanoma
Granule size	Coarse, clumped	Fine, pepper-like
Granule distribution	Diffuse, homogeneous	Focal, asymmetric
Residual precursor features	SK/SL features (milia-like cysts, comedo-like openings, fingerprint areas)	Atypical melanocytic features (network, streaks, globules)
Crystalline structures	May be present	May be present
Shiny white lines	May be present	May be present
Pink structureless areas	Late LPLK pattern (indistinguishable from sBCC/amelanotic melanoma)	May be present
Polymorphous vessels	Uncommon	May be present
Overall symmetry	Symmetric regression	Asymmetric; residual malignant features
Management	Biopsy if equivocal	Biopsy mandatory

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