Jagdish Dukre

Introduction

Glaucoma is characterized by loss of

retinal ganglion cells and their axons i.e.

retinal nerve fiber layer (RNFL).

Several studies have shown that

changes in optic nerve head (ONH) and

retinal nerve fiber layer (RNFL) precede

the visual field loss by several years.

Thus, RNFL examination helps in early

diagnosis of glaucoma.

Principle

Scanning laser polarimetry (SLP) is designed

to quantitatively assess the thickness of the

peripapillary RNFL.

It is based on the measurement of a physical

property called retardation of an illuminating

laser beam passing through the birefringent

RNFL.

Birefringence in the nerve fiber layer arises

from the parallel arrangement of

microtubules within the axons of this layer.

Light polarized in one plane travels more

slowly through the birefringent RNFL than

light polarized perpendicularly to it.

This difference in speed causes a phase

shift (retardation) between the perpendicular

light beams.

Like the RNFL, the anterior segment (the cornea and

lens) is also birefringent.

There are three types of corneal compensation:

Fixed Corneal Compensation (FCC)

Variable Corneal Compensation (VCC)

Enhanced Corneal Compensation (ECC)

Intersubject variability of measured RNFL

thickness of normal eyes was reduced,

the facility to discriminate between normal and

glaucomatous eyes was improved, and

the correlation of RNFL thickness values with

visual function was greater.

GDx VCC

Adjust the table or patient chair to

allow the patient to lean forward and

place his/her face into the face

mask.

Make sure that the patient’s

forehead are centered and the

patient is comfortable.

For proper eye positioning, align the

eyes ( the canthus) with the eye-

level indicators on the face mask.

The patient should be comfortably

seated so that their head remains

stationary; any head movement

interferes with the quality of the

acquired image.

Patient Fixation

With the patient in position, instruct him/her that

he/she will see a field of thin red horizontal lines.

A fixation target is located on one side of the

field. For the right eye, the target will be on the

left side of the red field; for the left eye it will be

on the right side.

When the target fixation is enabled, the patient

will see the fixation target as two small, bright,

fast-blinking, red horizontal lights, similar to a

small equal (=) sign.

A 780-nm diode confocal scanning laser with

an integrated polarimeter is focused on the

retina.

The backscattered light that doubly passes

through the RNFL shows retardation that is

measured by a polarization detection unit.

The total data acquisition takes 0.7 seconds.

Three images of each eye are obtained.

Each image measures 20 × 20 degrees and

contains (256 × 256) pixels.

A reflectance image of the scanned image is

produced.

These are displayed in a color-coded map. Areas of high retardation are displayed in yellow and areas of low retardation displayed in blue.

The operator outlines the optic disc margin and retardation values are automatically generated along a 10 pixel-wide ellipse, concentric with and 1.75 times larger than the disc diameter.

The thickness values along the perimeter of the ellipse are then plotted as a cross-sectional graph (TSNIT).

Patient’s identification data

Image quality score : Scores of 7 or

higher are considered to be of good

quality, while scores less than 7 should

be interpreted with caution.

Fundus Image

The Fundus Image is a

reflectance image depicting

a 20° x 20° image of the posterior pole.

The GDx utilizes more than 16,000 data points

from the scan area to produce and display the

Fundus Image showing the optic nerve head.

This image allows the initial quality evaluation of

the scan to determine if it is adequate for further

analysis and is used for centering the ONH

ellipse

Nerve Fiber Layer Map The Nerve Fiber Layer Map is a color map

depicting the different RNFL levels in the 20° x 20° area surrounding the optic nerve head (ONH).

This image presents the phase shift generated by RNFL thickness and its structural organization.

RNFL is represented using a color scale, with dark blue representing smaller RNFL values (smaller phase shift) and generally bright red representing larger RNFL values (greater phase shift).

A typical normal pattern is characterized by

bright yellows and reds (thicker) in the superior

and inferior sectors, and greens and blues

(thinner) in the nasal and temporal sectors.

Deviation Map

The Deviation Map shows how the patient’s RNFL measurements compare with values derived from the normative database.

Small color-coded squares indicate the amount of deviation from normal at each given location and are presented over a black-and-white fundus image to provide a visual frame of reference.

Symmetry Analysis report, the TSNIT

(Temporal-Superior-Nasal-Inferior-

Temporal) nerve fiber layer graph displays

the normal range (shaded area) and

patient’s values of RNFL developed from

the measurement data obtained along the

Calculation Circle.

The green plot displays the right eye (OD), and the purple plot displays the left eye (OS).

The left side of the graph starts the plot from the Calculation Circle, beginning at the temporal side of the retina.

As the map progresses to the right it plots the RNFL values obtained by tracing around the Calculation Circle, passing through the Temporal, Superior, Nasal, Inferior, and then back to the Temporal positions.

Parameters Table

It presents parameters

computed from the

Calculation Circle and they

are compared to values

from the normative

database.

Values are color- coded to

indicate deviation from

normal, as in the Deviation

Map.

TSNIT Average :This parameter evaluates the

average RNFL (μm) in the Calculation Circle.

( Normal 46 -68 μm)

Superior Average: This is the average of all

pixels (μm) in the superior 120 degrees of the

Calculation Circle. ( Normal 55 - 85 μm)

Inferior Average : This is the average of all

pixels (μm) in the inferior 120 degrees of the

Calculation Circle.

( Normal 40 - 75 μm)

TSNIT Std. Dev. (Standard Deviation) : This

number represents the standard deviation of

the values contained in the Calculation Circle.

The higher the number, the greater the

modulation of the double-hump pattern.

Inter-Eye Symmetry

This is the correlation of corresponding points in

the TSNIT data for right and left eyes.

The closer the ratio is to 1.0, the more

symmetric the nerve fiber layer.

If only one eye is evaluated, this value is not

shown.

The Nerve Fiber Indicator

(NFI) for GDx is an

algorithm that analyzes the

entire RNFL profile.

The NFI is an indicates the

likelihood that the

polarimetric retinal nerve

fiber layer map is abnormal.

A higher number is more

likely to be related to

abnormality, but is not

definitive NFI (Nerve Fiber

Indicator)

Advantages

Easy to operate

Does not require pupillary dilation

Comparison with age matched

normative database

Good reproducibility

Does not require a reference plane.

Limitations

Affected by anterior and posterior segment

pathologies.

Does not measure actual RNFL thickness

Limited use in moderate/advanced glaucoma.

Difficult in nystagmus, very small pupil and media

opacities.

Requires wider database for Indian population.

Young patients database not available.

Backward compatibility not present.

THANK YOU

Figure 12 is an example of a healthy eye, demonstrating the typical eye double hump curve, representing a cross section of the two arcuate bundles, with the nasal, usually thinner, area in the center and the temporal area on the sides.

13 is an example of a glaucomatous eye with thinning of the superotemporal neuroretinal rim with a corresponding inferonasal visual field defect.

Retardation is markedly attenuated and the double hump curve is significantly depressed in the superotemporal quadrants.

Quality parameters (listed in Table 4-1) for each image are displayed on the IMAGE CHECK screen. Review the scan Quality Scores and messages for both eyes.

A good quality image can be characterized as follows:

•OK’s for Alignment, Fixation, Refraction, and Other parameters

•well focused

•well aligned

•even illumination

•minimal eye movement

Potential causes of image quality problems include:

•image out of focus indicating an incorrect refraction setting

•ONH ellipse not centered (too close to edge) indicating improper fixation

•uneven illumination indicating incorrect alignment

•an overexposed or black image indicating that the patient blinked during acquisition.

•black borders along one or more sides of the image indicating patient eye movement during acquisition.• a scan Quality Score of 7 or above • improper ellipse placement