A comparison of polarised and nonpolarised dermoscopy
Author: Dr Harmony Thompson, Medical Registrar, Waikato Hospital, Hamilton, New Zealand. DermNet Editor in Chief: Adjunct A/Prof Amanda Oakley, Dermatologist, Hamilton, New Zealand. Copy edited by Gus Mitchell. June 2020.
Dermoscopy is a non-invasive technique used to examine skin lesions with a dermatoscope. It is also known as dermatoscopy, epiluminescence microscopy, incident light microscopy, and skin-surface microscopy [1,2].
A dermatoscope usually consists of a light source, achromatic lens, contact plate, and power supply. A dermatoscope allows better visualisation of deeper skin structures, therefore improved diagnostic accuracy of skin lesions.
There are three main modes of dermoscopy [1,2]:
Polarised contact dermatoscopy
Polarised noncontact dermatoscopy
Nonpolarised contact dermatoscopy (also called unpolarised dermoscopy).
Polarised and nonpolarised dermoscopy are complementary and the combination of both methods increases diagnostic accuracy and clinician confidence.
What are the physics of dermoscopy?
To understand the difference between the different modes of dermoscopy, it is important to know the basic physics of light, refraction, and reflection.
Reflection of light
Refraction of light
A light wave is an electromagnetic wave; it can be thought of as an oscillating form that vibrates in multiple directions.
Nonpolarised light is a light wave that vibrates in more than one plane.
Polarised light vibrates in a single plane.
Nonpolarised light
Polarised light
Polarisation of light
The process of transforming nonpolarised light to polarised light is known as polarisation.
Most light sources, such as the sun, lamps, and torches, are nonpolarised. As the light hits the surface of the skin, it is absorbed, refracted, and reflected. Nonpolarised light can undergo polarisation by reflecting off nonmetallic surfaces, creating specular reflectance (glare). Glare reduces the ability of our eyes to see the underlying structures.
Nonpolarised dermoscopy uses a glass plate as a medium with a smaller refractive index to skin to minimise glare [1,2].
An interface fluid (such as alcohol, liquid paraffin, water, or ultrasoundgel) is applied for contact dermoscopy. Interface fluid increases the penetration of light and allows excellent visualisation of the superficial layers of the skin from the epidermis to the dermo-epidermal junction [2].
Polarised dermatoscopy uses cross-polarisation to view the structure. Two filters are held orthogonally at 90 degrees. The source light initially is polarised with the first filter, then the reflected light from the skin is blocked by the second filter. This technique minimises glare and allows better visualisation of the underlying structures, from the dermo-epidermal junction to the superficial dermis [1,2].
Normal light reflection off skin
Nonpolarised dermoscopy
Polarised dermoscopy
What are the clinical differences between polarised and nonpolarised dermoscopy?
Different modes of dermoscopy produce similar results. However, there are minor differences in the appearance of cutaneous structures and colours [3]. Skin structures with high concordance between polarised and nonpolarised dermoscopy (described using conventional pattern analysis) include [4]:
Effect of polarisation in dermoscopy of dermatofibroma
Nonpolarised dermoscopy of dermatofibroma
Polarised dermoscopy of dermatofibroma
Effect of polarisation in dermoscopy of superficial basal cell carcinoma
Nonpolarised dermoscopy of basal cell carcinoma
Polarised dermoscopy of basal cell carcinoma
Effect of polarisation in dermoscopy of pigmented melanoma
Nonpolarised dermoscopy of a pigmented melanoma
Polarised dermoscopy of melanoma
Nonpolarised dermoscopy has increased specificity for seborrhoeic keratosis and is better at visualising acral lesions.
Effect of polarisation in dermoscopy of seborrhoeic keratosis
Nonpolarised dermoscopy of seborrhoeic keratosis
Polarised dermoscopy of seborrhoeic keratosis
Either method can be used to diagnose most skin lesions; they provide complementary information.
References
Nirmal B. Dermatoscopy: physics and principles. Indian J Dermatopathol Diagn Dermatol. 2017;4(2):27-30. doi: 10.4103/ijdpdd.ijdpdd_13_17. Journal
Pan Y, Gareau DS, Scope A, Rajadhyaksha M, Mullani NA, Marghoob AA. Polarized and nonpolarized dermoscopy: the explanation for the observed differences. Arch Dermatol. 2008;144(6):828-9. doi:10.1001/archderm.144.6.828. PubMed
Benvenuto-Andrade C, Dusza SW, Agero AL, et al. Differences between polarized light dermoscopy and immersion contact dermoscopy for the evaluation of skin lesions. Arch Dermatol. 2007;143(3):329-38. doi:10.1001/archderm.143.3.329. PubMed
Liebman TN, Jaimes-Lopez N, Balagula Y, et al. Dermoscopic features of basal cell carcinomas: differences in appearance under non-polarized and polarized light. Dermatol Surg. 2012;38(3):392-9. doi:10.1111/j.1524-4725.2011.02205.x.PubMed
Wang SQ, Dusza SW, Scope A, Braun RP, Kopf AW, Marghoob AA. Differences in dermoscopic images from nonpolarized dermoscope and polarized dermoscope influence the diagnostic accuracy and confidence level: a pilot study. Dermatol Surg. 2008;34(10):1389-95. doi:10.1111/j.1524-4725.2008.34293.x. PubMed