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.
Toggling between polarized light and non-polarized light will reveal the differences between both even better. This has been described as the blink sign because while toggling the structures will blink at you.
Differences between polarized and non polarized light as seen in dermoscopy
| 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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