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
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.