Evaluation of preperimetric
glaucoma
CHAIRPERSON -Dr. H T Venkategowda
MODERATOR - Dr. Spoorthy S
PRESENTER : Dr. Malvika Suresh
Definition
POAG- A chronic, progressive, anterior optic neuropathy that is accompanied by a
characteristic cupping and atrophy of the optic disc with corresponding visual field loss.
Preperimetric glaucoma (PPG) is defined as the presence of characteristic
glaucomatous changes in the optic disc and increased vulnerability to damage in the
retinal nerve fiber layer (RNFL), without the presence of visual field defects
detectible with standard automated perimetry.
NEED FOR EARLY DIAGNOSIS
Studies have proved that structural changes in glaucoma (ONH, RNFL)
preceed functional changes
Glaucoma is a continuum.
Optic nerve maybe abnormal WITHOUT glaucoma
This has a great potential value in delaying and avoiding progression of the
disease
Indications for early diagnosis
1.
2.
3.
4.
5.
Family history
Raised IOP
Increase in size of cup
Difference in size of cup
Associated disease state - diabetes mellitus
Pre perimetric Glaucoma diagnosis &
monitoring require 3 main tests
1.
SCANNING LASER POLARIMETRY (GDX)
2.
CONFOCAL SCANNING LASER OPHTHALMOSCOPY (HRT)
3.
OPTICAL COHERENCE TOMOGRAPHY
1. Scanning laser polarimetry
●
Scanning laser polarimetry is an imaging technology to
measure peripapillary RNFL thickness
●
GDX is a device that uses this technology
●
Latest generation - GDX VCC (variable corneal
compensator)
●
Evaluates the site of damage before patient experiences
visual loss
●
Total chair time less than 3 minutes for both eyes
●
Undilated pupils work best, no drops required
●
Painless procedure, safe,
●
Principle - BIREFRINGENCE
●
Phenomenon of double refraction where a ray of light when incident on a
birefringent material is split by polarization into 2 ways taking slight different
paths.
●
Main birefringent intraocular tissues - cornea, lens and 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
●
Technique -
●
780nm DIODE laser
●
The scan is obtained by the use of an 3.2mm fixed ellipse centered over the optic
disc as seen in reflectance map
●
Data is displayed as color coded grids which displays a ‘heat map’ - representing
different levels of retardation and therefore RNFL thickness.
●
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 control for measurement of
corneal birefringence
GDX PRINTOUT
●
PATIENT DATA: patient data and quality score - the patient’s name, date of
birth, gender and ethnicity are reported. An ideal quality score is from 7-10.
●
FUNDUS/ REFLECTANCE IMAGE -useful to check 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 no. of factors including - well focussed, evenly
illuminated, optic disc well centered and ellipse is properly around the ONH
●
The calculation circle is the area
found in between the two concentric
circles, which measure the TSNIT and
Nerve fibre indicator (NFI)
parameters.
●
By resizing the calculation circle and
the ellipse, the operator is able to
measure beyond a large peripapillary
atrophy area
RNFL thickness map
●
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
●
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
TSNIT map
●
TSNIT displays the RNFL thickness values along the calculation circle in relation
to normal range
●
Normal eye - typical “double hump” pattern
●
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 2
curves will have less overlap
Green and pink shaded areas of normality, over which a black line is
drawn to demonstrate patient’s RNFL thickness
TSNIT symmetry graphR & L superimposed on
each other
Deviation map
●
Reveals the location and magnitude of RNFL defects over the entire thickness map.
Color coded pixels indicate amount of deviation.
●
Compared to the age matched normative database
●
Dark blue squares - RNFL thickness is below 5th percentile of the normative
database
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Light blue squares - deviation below the 2% level
●
Yellow - deviation below 1%
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Red- deviation below 0.05%
Nerve fibre indicator
●
An indicator of likelihood that an eye has glaucoma
●
Calculated using an advanced form of neural network, called a support vector
machine (SVM)
●
Output values range from 1-100
1-30 = low likelihood for glaucoma
31-50 = glaucoma suspect
51+ = high likelihood of glaucoma
Quality assesment
●
Appropriate focusing and illumination of retina.
●
Ellipse must be centered over ONH
Review of GDx
LIMITATIONS
STRENGTH
●
No pupil dilatation
●
Rapid data on RNFL
●
Large normative database
●
Sensitivity and specificity to detect
glaucoma >80%
●
●
●
●
Only RNFL information
Corneal surgery will induce error (if
no VCC)
Macular pathology is a hinderance
NFI -proprietary value
2. Confocal scanning laser ophthalmoscopy (HRT)
●
CSLO is a rapid, non-invasive, non contact imaging
technology
●
Heidelberg Retina Tomography is the major
commercially available instrument that uses this
technology
●
3 generations - HRT, HRT 2, HRT 3
●
Gives 3-D images- high resolution
●
Principle - spot illumination and spot detection
●
Stimulus - 670 micron DIODE laser
●
Uses laser light instead of a bright flash of white light to illuminate the retina
●
Conjugated pinholes are placed in front of the light source and light detector and allow
only light originating from a determined focal plane to reach the detector.
●
Sequential sections are obtained by moving through the whole depth of tissue - in this
case the optic nerve.
●
Oscillating mirrors in the HRT redirect the laser beam to the x and y axis (which is
perpendicular to the optic axis). A bi-dimensional image is obtained at each focal plane.
What the HRT does
●
Once the patient is positioned, HRT 2 automatically performs a pre-scan
through the optic disc to determine the depth of the individual’s optic nerve
●
Next, it determines the number of imaging planes to use (range of scan depth
1-4mm)
●
Each successive scan plane is set to measure 0.0625mm deeper
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Automatically obtains 3 scans for analysis
●
Aligns and averages the scans to create the mean topography image
●
A series of 32 confocal images each 256x256
pixels is obtained in a duration of 1.6 secs
●
Computer converts 32 confocal images to a
single topographic image in approximately 90
secs.
●
Once image is taken, operator indicates the optic nerve contour line over the
reflectance or topography image.
●
Reference plane - located 50 microns posterior to the mean height along a 6deg arc
of contour line at temporal inferior sector
●
HRT 3 provides ONH stereometric analysis without manual delineation of disc.
●
Stereometric parameters are calculated by the machine and analysed by A1 classifier - Relevance Vector
Machine(RVM). Then a Glaucoma Probability Score is created.
HRT REPORT
HRT III
HRT II
PATIENT DATA
●
Provides information on exam type (initial report/baseline/follow up), patient
demographic information ( patient name, age, gender, ethnicity, etc) and basic
image information including image focus position and whether astigmatic lenses
were used during acquisition,
TOPOGRAPHIC IMAGE
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On left upper corner
●
A false color image
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Color coded representation of disc
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Superficial areas- darker
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Deep areas - lighter
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Red - cup
●
Blue, green - NRR
Blue - sloping rim
Green - non sloping rim
REFLECTION IMAGE
●
On right upper corner in u/l report, below topographic image in OU report
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Brighter areas - higher reflectance - cup
●
Darker areas - less reflectance
●
Valuable in locating and drawing the contour line around the disc margin
●
In the reflectance image the optic nerve head is divided into 6 sectors
●
Depending on this patient’s age and overall disc size the eye is then statistically
classified as :
HORIZONTAL and VERTICAL HEIGHT PROFILE
●
Height profile along the white horizontal
line in the topography image
●
Height profile along the white vertical line
in the topography image
●
The subjacent reference line (red) indicates
the location of the reference plane
(separation between cup and NRR)
●
The 2 black lines perpendicular to the
height profile denote the borders of the
disc as defined by the contour line.
MEAN CONTOUR HEIGHT GRAPH
●
After the contour line is drawn around the border of the optic disc, the software
automatically places a reference plane parallel to the peripapillary retinal surface
located 50 um below the retinal surface
●
The reference plane is used to calculate the thickness and cross-sectional area of the
RNFL.
●
The parameters of area and volume of the NRR and optic cup are also calculated
based on the location of the reference plane. CUP = area of the image that falls below
the reference plane. NRR = above the reference plane
●
Normal contour - DOUBLE HUMP - humps correspond to the superior and inferior
NFL, which are normally thicker
●
Green contour line should never go below red ref. Plane. If it does, then contour like
likely not in proper position
●
The graph depicts from left to right T-TS-NS-N-NI-TI-T
STEREOMETRIC ANALYSIS
●
Hrt 2- 14 parameters
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HRT 3- 6 parameters
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Each value is designated as
MOORFIELDS REGRESSION ANALYSIS (MRA)
Glaucoma probability
●
New software included in HRT 3
●
Based on construction of a 3-D model of ONH and RNFL by using 5 parameterscup size & depth, rim steepness, horizontal & vertical RNFL
●
Employs artificial intelligence: relevance vector machine and derives probability
of glaucoma of scanned eye after comparing it to a predetermined model
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- probability </= 28% - WNL
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- probability >/= 28% - BL
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-probability >/= 64% -ONL
HRT can differentiate between normal
and early glaucomatous eyes with a
sensitivity of 79% to 87% and
specificity of 84% to 90%.
Unlike the MRA, the GPS utilizes the
whole topographic image of the optic
disc, including the cup size, cup depth,
rim steepness and horizontal/vertical
RNFL curvature whereas the MRA uses
only a logarithmic relationship between
the NRR and optic disc areas
Quality indicators
●
Even luminance
●
Sharp borders of topography and reflectance
11-20= very good
images
21-30= good
●
Good centration of the disc
31-40= acceptable
●
Manufacturer’s classification by standard
41-50= poor
deviation -->
●
<10 = excellent
>50 =very poor
Review of HRT
LIMITATIONS
STRENGTH
●
No pupil dilatation
●
Rapid 3D data on ONH
●
Software available for longitudinal
change analysis
●
●
●
●
Operator draws contour line (except
in HRT 3)
Reference plane affects data
outcome
Blood vessels often included in rim
area
Less useful for analysis of macula or
RNFL
3. Optical coherence tomography (OCT)
●
It is an imaging technology that measures intensity and echo-time delay of back
scattered and back reflected light from scanned tissues.
●
Developed in 1991
●
High resolution cross sectional imaging of ONH, RNFL, macula
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Generations - OCT 1,2,3 (stratus), Spectral
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Light source - 820 or 850nm IR DIODE laser
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PRINCIPLE- low coherence interferometry & the ability to differentiate retinal
layers depending on the different time delay of their reflections
●
High reflectivity tissues (white and red colors) - NFL, RPE and choriocapillaries
●
Low reflectivity tissues (blue and black colors) - photoreceptor layer, choroid,
pockets of fluid
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Advantage- can scan RNFL, ONH and macula
●
Resolution - 8-10 microns
OCT REPORT
Healthy eye
(RE)
Glaucomatous
eye (LE)
Peripapillary RNFL scan
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3.4 mm circular scan around ONH
●
RNFL curve starts with temporal quadrant & continues clockwise in RE,
counterwise in LE.
●
Thickness values are provided for 4 quadrants & each clock hours
●
Classification based on normative database is color coded
- green = WNL
- yellow = borderline
- red = ONL
●
Average RNFL is also established.
ONH SCAN
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6 linear scans (length=4mm) ina spoke fashion obtained
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Automatically defines ONH margins at the end of RPE layer (blue cross).
Straight line is drawn joining the blue crosses.
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Reference plane - parallel line drawn 150 microns anterior to previous line
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Tissues above reference plane - rim (Red)
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Tissues below - cup (contour -green & edge - yellow)
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One radial scan is yellow denoting the axis of cross-sectional image in the
printout
Ganglion cell complex (GCC) analysis
●
Measurement of retinal thickness at the macula in an attempt to
detect early stage glaucomatous damage
Review of OCT
LIMITATIONS
STRENGTH
●
Highest axial resolution
●
image can be compared with histology
slides
●
Rapid data on RNFL, ONH and macula
●
ONH margin determined by device
●
No need for pupil dilation
●
Detects different pathologies
●
Easy to operate, safe
●
●
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Limited normative database
Data originate from only 1 set of
scans
Limited programs for longitudinal
evaluation glaucomatous
progression
Conclusion
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●
●
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Proven advantages in early diagnosis and monitoring of glaucoma
Limitations :
Disability to capture all nuances of appearance as instereo- photographs
Cannot discriminate glaucoma in difficult optic discs (high myopia , tilted
optic disc)
● Hence , these modalities cannot replace the gold standards of functional and
structural tests but can definitely compliment them in clinician’s decision
making process.