Principles of dermoscopy (full text)
Clinical examination
The clinical "naked eye" examination is important in the assessment of the gross morphological features of the lesion, such as size, shape, colors, contours, and surface topography.
Since the refractive index of the stratum corneum (surface layer of the skin) is higher than that of air, much of the incident light is reflected off the surface of the skin and causes the surface glare, as the reflected light overwhelms the retina and precludes the observer from visualizing the light reflected from the deeper layers of the skin. Thus, clinical "naked eye" examination mainly allows the assessment of morphological features of the surface layer of the skin (stratum corneum), and to a much lesser extent, the colors and structures of the deeper layers of the epidermis and the superficial dermis.
In body areas with a thick stratum corneum (e.g. on acral surfaces), there is a greater back-scatter of the light and the skin appears more opaque. In body areas with a thin stratum corneum (e.g., eyelids), the skin appear more translucent.
Non-polarized dermoscopy
Non-polarized dermoscopes
A dermoscope (or dermatoscope) is a handheld device, equipped with a magnification lens and a light source. It enables the visualization of the subsurface morphology of cutaneous lesions, down to the depth of the superficial dermis.It reveals colors and structures that are normally not visible to the unaided eye and improves the diagnostic accuracy and confidence level of experienced users, for both pigmented and non-pigmented skin lesions.
Non-polarized dermoscopes (NPD) are equipped with a magnification lens and light-emitting diodes to provide illumination. They require direct contact of the glass plate with the skin surface, and the presence of a liquid interface with a refractive index equal to or closely matching that of the skin. Different immersion liquids can be used: 70% alcohol, gel (i.e., ultrasound gel, antibacterial gel), water or mineral oil. Air bubbles between the dermoscope’s glass plate, the immersion liquid, and the skin surface create a skin–air interface. The interface causes back-scatter of light and precludes the observer from visualizing structures below the stratum corneum. Thus, air bubbles should be eliminated.
NPD allow visualization of subsurface structures located in the epidermis and the dermal-epidermal junction (DEJ), but can hardly visualize structures deeper than the DEJ.
Optical properties of light during the use of NPD
- The Surface glare is eliminated by matching the refractive indexes of the NPD's glass plate, the immersion medium, and the skin.
- Superficial penetrating light is the main source of contrast when using NPD. The light that enters the skin is absorbed (e.g. by melanin) or reflected back (e.g. by keratin in milia-like cysts) at the layers of the epidermis and the DEJ. The superficial penetrating light undergoes minimal scattering events and is therefore the main source of light that reflects back to the NPD lens.
- Deep penetrating light contributes only a small fraction of back-reflected light detected with NPD. This is due to the decay of light by multiple scattering events, as it goes deeper into the skin.
Melanoma, as it appears clinically (insert) and on NPD:
Globular nevus as it appears clinically (insert) and on NPD:
Polarized dermoscopy
Polarized dermoscopes
Polarized dermoscopes (PD), like non-polarized dermoscopes, contain light-emitting diodes to provide illumination and are equipped with a magnification lens. However, PDs use two polarized filters to achieve cross-polarization (see below). Hence, they do not require direct contact with the skin, and do not require the use of immersion liquids.
PD allow visualization of subsurface structures located at the dermal-epidermal junction (DEJ) or superficial dermis, and they are nearly "blind" to the skin's surface and to structures in the superficial epidermis (e.g., comedo-like openings).
Some PD devices allow the user to opt between non-contact PD and contact PD. Under contact PD, the use of an immersion fluid (e.g. 70% alcohol) can enhance image quality, probably by allowing more source light to enter through the stratum corneum.
Optical properties of light during the use of PD
- Light emitted from the dermoscopy unit (source) passes through a polarizer, resulting in the generation of polarized (unidirectional) light.
- Light reflecting back toward our eye (detector) must first pass through a cross-polarized filter whose direction is perpendicular (orthogonal) to that of the source polarizer.
- Polarized light cannot pass through the cross-polarizing filter unless the light changes its direction by 90°, which occurs if the original polarized light undergoes sufficient scattering events in the skin that change its direction ("randomization of polarization").
- Surface glare: maintains its original polarization, and thus cannot pass through the cross-polarized filter. Therefore, PD is "blind" to surface glare.
- Superficial light: does not undergo enough scattering events to result in randomization of polarization. Therefore, PD is also "blind" to back-reflected light from the superficial layers of the epidermis.
- Deep penetrating light: reaches the level of the DEJ and superficial dermis, undergoing multiple scattering events that result in randomization of polarization. Thus, back-reflected "deep penetrating light" can pass through the cross-polarization filter and enter our eye, allowing the visualization of dermoscopic structures from the DEJ and superficial dermis.
Shiny white streaks, a feature only visible on PD:
Pigmented basal cell carcinoma
merkel cell carcinoma
Shiney white lines under polarized and non polarized dermoscopy
Melanoma with shiny white streaks
Shiny white structures schematic
Differences between polarized and non polarized dermoscopy
The main difference between non-polarized dermoscopy (NPD) and polarized dermoscopy (PD) is the depth of visualized structures. While NPD is better for inspecting structures in the superficial skin layers (e.g., superficial epidermis down to the dermo-epidermal junction [DEJ]), PD is better for evaluating the deeper skin layers (e.g., DEJ and superficial dermis).
For example, both Milia-like cysts and blue-white veil are caused by superficial changes in the epidermis and are therefore better visualized with NPD. On the other hand, Shiny white structures (chrysalis/crystalline, blotches and strands, rosettes) are better visualized with PD since they are associated with increased collagen at the superficial dermis. In addition, polarized light rapidly randomizes its polarization when it encounters a birefringent structure, such as collagen.
An example of Milia-like cysts,better visualized under NPD:
An example of a blue-white veil, also much better visualized with NPD:
And finally an example of Shiny white lines/streaks, which are much better visualized with PD:
Additional difference between NPD and PD
- Since PD does not require direct skin contact, blood vessels and pink color (vascular blush) are more evident under PD (due to lack of pressure effect). In addition, blood vessels are located in the dermis that is better visualized with PD.
- In pigmented lesions with melanin at the DEJ (e.g.. junctional nevi) or superficial dermis (e.g. blue nevi), PD will show slightly darker shades of brown and blue, and sometimes more variability in pigmentation, compared with NPD.
Diagnostic accuracy
The differences between PD and NPD may impact the diagnostic accuracy and diagnostic confidence level. For example, PD can increase sensitivity for detecting amelanotic melanomas or structure-poor melanomas and basal cell carcinomas, because PD highlights the presence of blood vessels, vascular blush, and/or white shiny lines (chrysalis/crystalline). In contrast, NPD can increase specificity by allowing to correctly identify Milia-like cysts and comedo like openings in seborrheic keratoses.
”Hybrid” dermoscopes toggle between PD and NPD modes. These hybrid devices can enhance dermoscopic diagnosis because PD and NPD provide complementary information. These devices should always be in direct contact with the skin and used with a liquid interface, otherwise dermoscopic structures will only be visualized in PD mode.
| Colors and structures | NPD | PD |
|---|---|---|
| Melanin | + | ++ |
| Red/pink | + | +++ |
| Blue-white due to orthokeratosis | +++ | + |
| Blue-white due to regression | +++ | ++ |
| Peppering | +++ | ++ |
| Chrysalis or white scar | +/− | +++ |
| Vessels | + | +++ |
| Milia-like cyst | +++ | +/− |
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