Dermoscopy is a non-invasive technique by which structures at the sub surface of the skin can be examined and analyzed for the detection of melanoma. It involves using a hand held magnification device on an oily and illuminated skin surface. Till recently, the light that was used to illuminate the skin surface was non-polarized light, which required the use of a liquid interface as well as direct contact between skin and the device. However, newer varieties of the technique use dermoscopes which use cross polarized light and the technique is called polarized lighting (PD). These newer dermoscopes are capable of showing the subsurface morphology either using direct skin contact and a liquid interface as in polarized contact dermoscopy (PCD) or without using direct skin contact and liquid interface as in polarized non contact dermoscopy (PNCD).

What is Non-Polarized Light Dermoscopy?

NPD as a more conventional technique essentially requires a liquid interface with the refraction index equal to that of skin and direct contact between skin and instruments to able to show the sub surface structures of the skin. This mechanism reduces the amount of light reflected, refracted or diffracted at the skin surface considerably and is thus extremely effective in revealing the structures beneath the corneum strata. The liquid interface corresponds optically to the stratum corneum with the help of plate glass mounted on the dermoscope. This technique is used in standard dermoscopy training and courses. The images taken with these dermoscopes are used in textbook and manuscript illustrations.

What is Polarized Light Dermoscopy?

PD can be of two types. Polarized Non Contact Dermoscopy (PNCD) where no liquid interface or direct contact with the skin is required. Whereas in Polarized contact dermoscopy (PCD) where a liquid interface and direct contact with skin surface becomes necessary. Though most images during the last 30 years were obtained using NPD, the use of the PD lighting technique is fast gaining popularity.

In polarized dermoscopy, the polarized light is obtained with the use of filters. There are sets of double filters used in this process and the mechanism is known as cross polarization. One of the filters is situated between the source of illumination and the skin, while the second one is situated between the skin and the light detector.

The first filter produces a polarized light, which reaches the skin. A part of this polarized light reflects at the stratum corneum while part of it enters the skin and gets scattered back from deeper layers. The reflected part of the polarized light maintains polarization while the backscattered part loses polarization due to the innumerable scatterings beneath the skin surface. The second filter blocks the superficially polarized light while the backscattered light reaches the detector and the eyes. So by using PD techniques one can select only backscattered light from the deep sub surface areas of the skin and block the reflected light that only gives a shiny appearance of the skin surface.

However, dermatologists using PNCD and PCD techniques should keep in mind the minute differences in vascular structures, color variation, pigment distribution and dermoscopic features between cross polarization and NPD. These differences are vital while detecting and diagnosing and reducing the chances of misdiagnosis.

Difference of Colors, Structures and Features

The colors are sharper and less distorted with use of traditional NPD as against PD. Colors like brown and blue look darker under PD and this technique shows a variety of brown and blue sheds for melanin scattered in the skin compared to NPD. With PD, blue nevi show as darker and have various shades of blue with less blue-white veil type areas as against NPD. Red areas are better seen under PD especially with PNCD. Also, PNCD does not result in skin blanching as compared to the other two contact devices enabling the dermatologists to observe blood vessels.

So, it can be assumed that PNCD is effectively better in its ability to show skin vascular structures and improves the evaluation of the shape of the skin’s blood vessels. In melanocytic lesions, which look problematic in clinical observations, the presence and shape of vessels in NPD can be used to confirm malignancy. The presence of a vascular blush seen only with PNCD should be a pointer towards the detection of skin cancer.

NPD permits improved visualization of surface structures such as milia like cysts, which are less visible with PD technique. The presence of milia like cysts and comedo like openings aids in the detection of seborrheic keratosis. Also peppering, which is a sign of regression in pigmented lesions, is more visible with NPD. So the absence of these features during PD examination may affect the clinical diagnosis of seborrheic keratosis and regression of lesions. However, polarized devices permit better viewing of vascular structures and pigment distribution and are vital for diagnosis of skin tumor.

So, PD and NPD yield overall similar patterns and images but have some differences that provide complementary information and both techniques are to be used in the diagnosis of melanomas.