Principles of dermoscopy
|Description||This chapter describes the principles of dermoscopyThe examination of [skin lesions] with a 'dermatoscope'. This traditionally consists of a magnifier (typically x10), a non-polarised light source, a transparent plate and a liquid medium between the instrument and the skin, and allows inspection of skin lesions unobstructed by skin surface reflections. Modern dermatoscopes dispense with the use of liquid medium and instead use polarised light to cancel out skin surface reflections. including polarized and non-polarized lightThis glossary term has not yet been described. dermoscopy, as well as the differences of between the two modalities.|
|Responsible author||Alon Scope → send e-mail|
|Status update||January 2, 2020|
|Status by||Ralph P. Braun|
The clinical "naked eye" examination is important in the assessment of the gross morphological features of the lesion, such as size, shape, colorsThis glossary term has not yet been described., contours, and surface topography.
Since the refractive indexA dimensionless number that describes how light propagates through a medium of the stratum corneum (surface layer of the skinThis glossary term has not yet been described.) is higher than that of air, much of the incident light is reflected off the surface of the skin and causes the surface glareDifficulty seeing in the presence of light that is reflected from the skin surface, 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 structuresThis glossary term has not yet been described. of the deeper layers of the epidermis and the superficialThis glossary term has not yet been described. dermis.
This figure shows the clinical examinationThis glossary term has not yet been described. of a pigmented lesion showing the surface glare. The interpretation of the lesion is very limited and difficultneeding much effort or skill to accomplish.
Non polarizied dermoscopes
A dermoscope (or dermatoscopeThis traditionally consists of a magnifier (typically x10), a non-polarised light source, a transparent plate and a liquid medium between the instrument and the skin, and allows inspection of skin lesions unobstructed by skin surface reflections. Modern dermatoscopes dispense with the use of liquid medium and instead use polarised light to cancel out skin surface reflections.) is a handheld deviceA piece of equipment designed to perform a special function, equipped with a magnification lens and a light source. It enables the visualization of the subsurface morphologyThis glossary term has not yet been described. 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 accuracyThis glossary term has not yet been described. and confidence level of experienced users, for both pigmented and non-pigmentedThis glossary term has not yet been described. 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 immersionThis glossary term has not yet been described. 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 glareDifficulty seeing in the presence of light that is reflected from the skin surface is eliminated by matching the refractive indexes of the NPD's glass plate, the immersion medium, and the skin.
- SuperficialThis glossary term has not yet been described. 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 cystsThis glossary term has not yet been described.) 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.
Globular nevus as it appears clinically (insert) and on NPD:
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 lightThis glossary term has not yet been described. 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 structuresThis glossary term has not yet been described. from the DEJ and superficial dermis.
Shiny white streaks, a feature only visible on PD:
ImagesA representation of a person, animal or thing, photographed, painted or otherwise made visible. for keyword "shiny white streaksWhite lines that appear parallel and perpendicular to each other on 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 dermoThis glossary term has not yet been described.-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, rosettesFour bright white dots or clods arranged together as a square (or a four leaf clover)) 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 vesselsare the part of the circulatory system, and microcirculation, that transports blood throughout the human bodyThis glossary term has not yet been described. and pink colorColor (American English) or colour (Commonwealth English) is the characteristic of human visual perception described through color categories, with names such as red, yellow, purple, or blue. (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 neviThis glossary term has not yet been described.) 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 accuracyThis glossary term has not yet been described.
The differences between PD and NPD may impact the diagnostic accuracy and diagnostic confidence level. For example, PD can increase sensitivityThis glossary term has not yet been described. 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 specificityThis glossary term has not yet been described. 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 diagnosisis the identification of the nature and cause of a certain phenomenon. Diagnosis is used in many different disciplines with variations in the use of logic, analytics, and experience to determine "cause and effect". In systems engineering and computer science, it is typically used to determine the causes of symptoms, mitigations, and solutions 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.
Toggeling between loparized light and non polarized lightThis glossary term has not yet been described. will reveal the differences between both even better. This has been described as the blink signWhile toggling between polarized and non polarized light, the structures that are more visibile with either one type of light will be enhanced and though blink at you because while toggeling the structures will blink at you.
Differences between polarized and non polarized light as seen in dermoscopy