GDxDR SHYLESH B DABKEGLAUCOMA FELLOW

ARAVIND EYE HOSPITAL

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INTRODUCTION• GDX measures thickness of RNFL based on birefringent property

of the tissue.• GDX is: - Simple to use and easy for both the patient and operator. - Near Infra-red wavelength(780 nm). - Measurement time is 0.7 seconds. - Total chair time less than 3 minutes for both eyes. - Undilated pupils work best. - Painless & safe procedure.

• The GDx :

- Maps the RNFL and compares them to a database of healthy,glaucoma-free patients.

- Analyses the RNFL thickness around the optic disc.

• Sensitivity of 89% and a Specificity of 98%.

Why RNFL Assessment??• RNFL defects - earliest sign of glaucoma.

• 88% of ocular hypertensives who converted to glaucoma had RNFL defects along with VF defects. 60%of these converters had RNFL defects 6 years prior to VF defects (Sommer et al).

• RNFL changes are detected more frequently than ONH changes in eyes converted from ocular hypertension to glaucoma (Quigley et al).

• RNFL damage occurs earlier than VF defects and ONH damage.

PRINCIPLE - Scanning Laser Polarimetry• Scanning laser polarimetry is an imaging technology that is utilized to measure

peripapillary RNFL thickness.

• Based on the Principle of Birefringence.

• Main birefringent intraocular tissues are the cornea, lens and the retina.

• In the retina, the parallel arrangement of the microtubules in retinal ganglion cell axons causes a change in the polarization of light passing through them.

• The change in the polarization of light is called Retardation.

• The retardation value is proportionate to the thickness of the RNFL

RNFL - highly ordered parallel axon bundles

Axons - microtubules, cylindrical intracellular organelles with diameters smaller than the wavelength of light.

Highly ordered structure of microtubules are the source of RNFL birefringence

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Birefringence: Splitting of a light wave by a polar material into two

components

These components travel at different velocities which creates a relative phase shift

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Retardation

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Amount of phase shift is ≃ RNFL thickness

Polarized light passes through the eye and is reflected off the retina. Because the RNFL is a birefringent, the two

components of the polarized light are phase shifted relative to each other (retarded). The amount of retardation is captured by a detector, and converted into thickness

(microns).

Scanning Laser Polarimetry• Amount of retardation is captured by a detector and

converted into thickness (microns)

• Raster scan - 40° horizontally and 20° vertically and includes both peripapillary region and macular region

• Anterior segment birefringence - cornea and lens.

• The total retardation is the sum of the cornea, lens and the retina.

• Compensation of anterior segment birefringence is needed to isolate RNFL birefringence.

• GDx VCC(variable corneal compensation) measures and individually compensates for each eye.

• VCC stands for variable corneal compensator, which was created to account for the variable corneal birefringence in patients

• Uses the birefringence of Henle’s layer in the macula as a control for measurement of corneal birefringence

GDx VCC

GDx VCC Printout1.Fundus Image

2.Thickness map

3.Deviation map

4.TSNIT map

5.TSNIT parameters

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Patients information• Patient data and quality score:

- Patient’s name- Date of birth- Gender- Ethnicity - An ideal quality score is from 7 to 10

FUNDUS IMAGE• The fundus image is useful to check for

image quality:• Every image has a Q Score representing

the overall quality of the scan• The Q ranges from 1-10, with values 8-10

representing acceptable quality.• This score is based on a number of factors

including : -Well focused, - Evenly illuminated, - Optic disc is well centered, - Ellipse is properly placed around the ONH.

• The operator centers ellipse over the ONH

• The fixed sized default calculation circle is a centred on ONH.

• Inner diameter of the band is 2.4mm, outer diameter of the band is 3.2mm & the band is 0.4mm wide.

• The calculation circle is the area found between the two concentric circles, which measure the temporal-superiornasal-inferior-temporal (TSNIT) and nerve fiber indicator (NFI) parameters

• By Resizing The Calculation Circle And Ellipse, the Operator is able To Measure Beyond A Large Peripapillary Atrophy Area

RNFL Thickness Map• The thickness map shows the RNFL thickness

in a color-coded format from blue to red.

• Hot colors like red and yellow mean high retardation or thicker RNFL

• Cool colors like blue and green mean low retardation / thinner RNFL

Healthy RNFL Glaucomatous RNFL

• A healthy eye has yellow and red colors in the superior and inferior regions representing thick RNFL regions and blue and green areas nasally and temporally representing thinner RNFL areas.

• In glaucoma, RNFL loss will result in a more uniform blue appearance

Deviation Maps• Reveals the location & magnitude of RNFL defects over the entire thickness map

• RNFL thickness of patient is compared to the age-matched normative database

• Each square represents a “Super Pixel”• RNFL thickness at each super pixel is

compared to age matched normative database

• Super pixel that fall below normal range is flagged by color squares based on probability of normality

• Dark blue squares RNFL thickness is below the 5th percentile of the normative database• Light blue squares deviation below the 2% level• Yellow deviation below 1%• Red deviation below 0.5%.

TSNIT Map • Displays the RNFL thickness values along the calculation circle

• In a normal eye the TSNIT plot follows the typical ‘Double Hump’ pattern, with thick RNFL measures superiorly and inferiorly and thin RNFL values nasally and temporally

• In a healthy eye, the TSNIT curve will fall within the shaded area which represents the 95% normal range for that age

• When there is RNFL loss, the TSNIT curve will fall below this shaded area, especially in the superior and inferior regions

• In the center of the printout at the bottom, the TSNIT graphs for both eyes are displayed together.• Healthy eye there is good symmetry

between the TSNIT graphs of the two eyes and the two curves will overlap• In glaucoma, one eye often has more

advanced RNFL loss and therefore the two curves will have less overlap

Parameters Table• The TSNIT parameters are summary measures based on RNFL thickness values within the calculation circle• Normal parameter values are

displayed in green• Abnormal values are color-coded

based on their probability of normality.colours are similar to deviation maps.

• TSNIT SD : Captures the amount of modulation (peak to trough difference) in TSNIT graph.

In normal eye there is high modulation.

In glaucoma eye there is RNFL loss in Sup & Inf region which results in low modulation & a low TSNIT SD value.

• Inter-eye Symmetry Values nearing 1 – Good SymmetryValues nearing 0 – Poor Symmetry

The Nerve Fiber Indicator (NFI) • Global measure based on the entire RNFL thickness map• Calculated using an advanced form of neural network,

called a Support Vector Machine (SVM)• Not colour coded• Output values range from 1 –100

– 1-30 -> low likelihood of glaucoma– 31-50 -> glaucoma suspect– 51+ -> high likelihood of glaucoma

Clinical research has shown that the NFI is the best parameter for

discriminating normal from glaucoma

Serial AnalysisDetecting RNFL Change Over Time

• Serial Analysis can compare up to four exams

Early Glaucoma

Moderate Glaucoma

Advanced Glaucoma

GDx VCC and Visual Fields are Highly Correlated

Abnormal SAP

Abnormal GDx VCC OD

Normal SAP

Glaucoma is Detected Early with GDx VCC

Normal SAP Abnormal GDx VCC

Glaucoma is Detected Early with GDx VCC

Normal SAP

GDx VCC Abnormal OS

Normal SAP

CONCLUSIONS• The ability to detect early glaucomatous structural changes has

great potential value in delaying and avoiding progression of the disease

• Should not be regarded as replacing the skilled ophthalmologist’s capacity to evaluate all aspects of the patient’s diagnosis.

• But they can definitely aid in the complicated decision-making process