Residency · Residency · Dermatology
Dermoscopy Fundamentals: Algorithms and Pattern Analysis
Principles of Dermoscopy
Physics and Optics
Dermoscopy (also called dermatoscopy or epiluminescence microscopy) eliminates surface light reflection to reveal subsurface structures invisible to the naked eye. Standard magnification is 10x, though devices range from 6x to 100x. Non-polarized dermoscopy requires a liquid interface (immersion oil, ultrasound gel, or alcohol) and excels at visualizing superficial structures such as milia-like cysts and comedo-like openings. Polarized dermoscopy requires no liquid interface and provides superior visualization of deeper structures including crystalline structures, shiny white lines, and blood vessel morphology. Using both modalities is recommended for the most comprehensive evaluation.
Histopathologic Correlates of Dermoscopic Structures
Each dermoscopic structure has a corresponding histopathologic basis. The pigment network corresponds to the rete ridge pattern with melanin in basal keratinocytes and melanocytes. Globules represent melanocyte nests, while dots reflect small melanin aggregates. Blue-white veil results from an acanthotic epidermis overlying dense dermal melanin with fibrosis. Streaks (pseudopods and radial streaming) correspond to confluent junctional nests at the periphery of a melanocytic lesion. Regression structures encompass white scar-like depigmentation (reflecting fibrosis) and peppering or granularity (reflecting melanophages). Crystalline or shiny white structures, visible only in polarized dermoscopy, correspond to collagen bundles. Vascular structures, best appreciated with polarized light, show morphological patterns that correlate with specific diagnoses.
<image>Side-by-side comparison of non-polarized and polarized dermoscopy of the same melanocytic lesion, highlighting structures best seen with each modality</image>
The Two-Step Algorithm
Step 1: Is the Lesion Melanocytic?
The first step determines whether a lesion is melanocytic by looking for pigment network, aggregated globules, streaks, homogeneous blue pigment, or a parallel pattern on acral skin. If melanocytic features are absent, non-melanocytic diagnoses should be considered. Basal cell carcinoma is recognized by arborizing vessels, leaf-like areas, spoke wheel structures, and blue-grey ovoid nests. Seborrheic keratosis shows milia-like cysts, comedo-like openings, fissures and ridges, sharp demarcation, and "fat fingers." Dermatofibroma characteristically displays a central white scar-like patch surrounded by a peripheral delicate pigment network. Vascular lesions exhibit red-blue lacunae and red-purple homogeneous areas. If the lesion remains unclassifiable after Step 1, biopsy is recommended.
Step 2: Is the Melanocytic Lesion Benign or Suspicious?
Once a lesion is confirmed as melanocytic, a structured algorithm is applied to determine whether it warrants biopsy.
Major Dermoscopic Algorithms
Pattern Analysis (Pehamberger)
Pattern analysis is the original and most comprehensive dermoscopic method. It evaluates global patterns (reticular, globular, cobblestone, homogeneous, starburst, parallel, multicomponent, or nonspecific) alongside local features. Benign lesions typically display a symmetric pattern with orderly arrangement of structures. Suspicious lesions show asymmetry of pattern and structures, multiple colors, irregular network, irregular dots and globules, regression, or blue-white veil.
ABCD Rule of Dermoscopy (Stolz)
The ABCD rule is a semiquantitative scoring system. Asymmetry is scored 0 to 2 (in zero, one, or two axes) and weighted by 1.3. Border is scored 0 to 8 based on abrupt cutoff in segments and weighted by 0.1. Color is scored 1 to 6 for each color present (white, red, light brown, dark brown, blue-grey, black) and weighted by 0.5. Differential structures are scored 1 to 5 for each structure present (network, structureless areas, dots, globules, streaks) and weighted by 0.5. The resulting total dermoscopy score (TDS) is interpreted as follows: below 4.75 is benign, 4.75 to 5.45 is suspicious, and above 5.45 is highly suspicious.
| Criterion | Score Range | Weight | Description |
|---|---|---|---|
| Asymmetry | 0–2 | x 1.3 | Assessed in 0, 1, or 2 axes |
| Border | 0–8 | x 0.1 | Abrupt cutoff in 8 segments |
| Color | 1–6 | x 0.5 | White, red, light brown, dark brown, blue-grey, black |
| Differential structures | 1–5 | x 0.5 | Network, structureless areas, dots, globules, streaks |
| TDS Score | Interpretation |
|---|---|
| < 4.75 | Benign |
| 4.75–5.45 | Suspicious |
| > 5.45 | Highly suspicious |
7-Point Checklist (Argenziano)
This algorithm assigns two points each for major criteria (atypical pigment network, blue-white veil, atypical vascular pattern) and one point each for minor criteria (irregular streaks, irregular dots and globules, irregular blotches, regression structures). A score of 3 or more indicates biopsy is recommended. The method offers high sensitivity of approximately 95 percent but lower specificity.
| Criteria Type | Feature | Points |
|---|---|---|
| Major | Atypical pigment network | 2 |
| Major | Blue-white veil | 2 |
| Major | Atypical vascular pattern | 2 |
| Minor | Irregular streaks | 1 |
| Minor | Irregular dots and globules | 1 |
| Minor | Irregular blotches | 1 |
| Minor | Regression structures | 1 |
Menzies Method
In the Menzies method, a lesion must have both negative features (single color and symmetry of pattern) to be classified as benign. If either negative feature is absent, nine positive features are evaluated, and the presence of any single one renders the lesion suspicious. These positive features include blue-white veil, multiple brown dots, pseudopods, radial streaming, scar-like depigmentation, peripheral dots and globules, five or six colors, multiple blue-grey dots, and broadened network.
CASH Algorithm
The CASH algorithm scores Color (1 to 6), Architectural disorder (0 to 2), Symmetry (0 to 2), and Homogeneity/heterogeneity (1 to 7). A total score of 8 or more warrants biopsy.
Chaos and Clues (Rosendahl) - Revised Pattern Analysis
This pragmatic approach begins by asking whether the lesion shows chaos (asymmetry of structure and color). If chaos is present, the examiner looks for specific clues of malignancy: thick reticular lines, grey or blue structures, black dots or clods at the periphery, radial lines at the margin, white lines, polymorphous vessels, reticular or branched lines, and negative network. Any clue present in a chaotic lesion warrants biopsy. This method is also applicable to non-melanocytic lesions.
<image>Illustrated flowchart of the two-step dermoscopy algorithm showing decision pathway from initial assessment to melanocytic versus non-melanocytic classification and subsequent evaluation for malignancy</image>
Dermoscopy of Common Melanocytic Lesions
Benign Nevi Patterns
Benign nevi display several recognizable global patterns. The reticular pattern shows a regular, uniform pigment network with thin lines and small holes. The globular pattern consists of aggregated round-oval brown structures, commonly seen in compound nevi in children. The homogeneous pattern features diffuse, uniform brown-blue pigmentation. The cobblestone pattern shows large, closely packed, angulated globules characteristic of dermal nevi. The starburst pattern displays symmetric streaks or pseudopods at the periphery, as seen in Spitz and Reed nevi.
Melanoma Dermoscopic Features
Melanoma produces a range of alarming dermoscopic findings. An atypical network appears as an irregular meshwork with thick lines and variably sized holes. The negative network consists of serpiginous hypopigmented lines surrounding elongated, curved globules. Irregular dots and globules vary in size, shape, and distribution across the lesion. Irregular streaks are asymmetrically distributed pseudopods or radial streaming. Blue-white veil presents as irregular blue pigmentation with an overlying white ground-glass haze. Regression combines white scar-like areas with blue-grey peppering. Atypical vessels are polymorphous, encompassing dotted, linear-irregular, hairpin, and milky-red patterns. Shiny white structures (crystalline structures) are best appreciated in polarized dermoscopy.
Dermoscopy of Non-Melanocytic Lesions
Basal Cell Carcinoma
BCC shows arborizing (tree-like) vessels that are bright red, sharply focused, and branching. Leaf-like areas appear as brown-grey bulbous extensions in pigmented BCC. Blue-grey ovoid nests are well-circumscribed large ovoid structures. Spoke wheel areas display radial projections from a central axis. Superficial BCC may show short fine telangiectasias. Ulceration, erosion, and shiny white structures (crystalline structures in polarized light) are additional features.
Seborrheic Keratosis
Seborrheic keratoses show milia-like cysts (white-yellow round structures corresponding to intraepidermal horn cysts), comedo-like openings (dark, round-oval invaginations), fissures and ridges (creating a brain-like or cerebriform pattern), and fat fingers (bulbous crypts). Sharp demarcation with well-defined borders and moth-eaten borders with concave scalloped edges are characteristic.
Dermatofibroma
Dermatofibroma characteristically displays a central white scar-like patch with a peripheral delicate pigment network and ring-like globules at the periphery, creating a distinctive "total network" pattern with a central white area.
<image>Dermoscopic atlas showing characteristic features of basal cell carcinoma (arborizing vessels, blue-grey ovoid nests), seborrheic keratosis (milia-like cysts, comedo-like openings), and dermatofibroma (central white patch)</image>
Dermoscopy of Special Sites
Acral Dermoscopy
Acral dermoscopy requires recognition of site-specific patterns. The parallel furrow pattern, in which pigmentation follows the sulci, is the most common benign acral pattern. The lattice-like pattern shows pigment along sulci with cross-connections and is benign. The fibrillar pattern displays fine perpendicular lines on weight-bearing areas and is also benign. The parallel ridge pattern, in which pigmentation follows the ridges or cristae, is highly suspicious for melanoma with a sensitivity of approximately 86 percent and specificity of approximately 99 percent. A multicomponent pattern with an irregular combination of patterns is also suspicious.
| Acral Pattern | Pigment Location | Interpretation |
|---|---|---|
| Parallel furrow | Sulci (furrows) | Benign (most common) |
| Lattice-like | Sulci with cross-connections | Benign |
| Fibrillar | Fine perpendicular lines on weight-bearing areas | Benign |
| Parallel ridge | Ridges (cristae) | Suspicious for melanoma |
| Multicomponent | Irregular combination | Suspicious |
Nail Dermoscopy
In nail dermoscopy, regular lines with uniform color, spacing, width, and parallelism indicate benign melanonychia. Irregular lines with variable spacing, width, color, and parallelism raise suspicion for melanoma. The Hutchinson sign (periungual pigmentation) is concerning, and a micro-Hutchinson sign visible only on dermoscopy is equally significant. Granular inclusions may represent blood or melanin deposits.
Mucosal Dermoscopy
On mucosal surfaces, structureless blue, grey, or brown patterns may be benign in mucosal melanosis. A multicomponent pattern or irregular vessels should raise concern for mucosal melanoma.
Quality Metrics and Training
Dermoscopy increases melanoma sensitivity from approximately 70 percent with the naked eye to approximately 90 percent. It also reduces unnecessary biopsies of benign lesions by improving specificity. The number needed to excise (NNE) to diagnose one melanoma is 4 to 8 with dermoscopy compared to 15 to 30 without. There is a meaningful learning curve, with proficiency generally requiring evaluation of at least 500 to 1000 lesions. Digital dermoscopy and sequential monitoring are particularly valuable for patients with numerous nevi.
<image>Side-by-side dermoscopic images of acral lesions showing benign parallel furrow pattern versus malignant parallel ridge pattern in acral melanoma</image>
Clinical Pearls
When in doubt, the "ugly duckling" principle applies to dermoscopy as well: a lesion whose dermoscopic pattern differs from the patient's predominant nevus pattern deserves closer scrutiny or biopsy. Blue-white veil is one of the most specific dermoscopic features for invasive melanoma, correlating with heavy dermal melanin beneath an acanthotic epidermis. A parallel ridge pattern on acral skin has the highest positive predictive value for acral melanoma of any single dermoscopic criterion. In nodular melanoma, the classic network-based criteria may be absent, and the examiner should instead look for blue-white veil, atypical vessels, and symmetry of structures in a structureless lesion. Dermoscopic findings should always be correlated with clinical context, as patient age, lesion location, and history of change are essential for accurate diagnosis.
References
- Kittler H, et al. Standardization of terminology in dermoscopy/dermatoscopy. Arch Dermatol. 2016;152(6):679-687
- Marghoob AA, et al. An Atlas of Dermoscopy. 2nd Edition. CRC Press, 2012
- Rosendahl C, et al. Revised dermoscopy algorithm for the management of primary melanocytic skin lesions. Arch Dermatol. 2012;148(8):925-934
- Argenziano G, et al. Dermoscopy of pigmented skin lesions: results of a consensus meeting via the Internet. J Am Acad Dermatol. 2003;48(5):679-693
- Lallas A, et al. The dermatoscopic universe of basal cell carcinoma. Dermatol Pract Concept. 2014;4(3):11-24
