role of oct in glaucoma

1. Role of OCT in glaucoma
Dr samarth mishra

2. Various imaging techniques
• Anterior Segment:
• AS-OCT
• UBM
• Posterior Segment:
• OCT
• HRT
• GDx

3. • OCT is a well known and frequently used technology to image the
posterior segment.
• Izatt et al. published the first report of OCT imaging of the cornea in
1994.
• In October 2005 Carl Zeiss Meditec, Inc, Dublin, CA released the
Visante®, an anterior segment OCT.
• Using time domain OCT technology (TD-OCT), the Visante® creates
cross-sectional images of anterior segment structures .
• It also provides measurement tools to document and follow changes in
the cornea, angle, and anterior chamber.
• AS-OCT is a non-contact procedure and is more user-friendly when
compared to ultrasound biomicroscopy (UBM)

4. • Although optic nerve damage and a progressive loss of the visual field are the
final common pathways of the glaucomas, the configuration of the anterior
segment is one of the most important things the clinician must assess.
• A determination of anterior segment anatomy (such as the angle grade, iris
configuration, and lens position) provides essential guidance to proper clinical
therapeutic decisions.
• The various ways in which physicians evaluate the anterior segment, aside from
a detailed examination and gonioscopy, include technologies such as
Scheimpflug photography, scanning slit-lamp systems, ultrasound biomicroscopy,
and anterior segment optical coherence tomography (AS-OCT).
• Although Huang et al first described optical coherence tomography of the eye in
1991, Izatt et al described its use in the anterior segment in 1994.

5. • Areas in which anterior segment OCT is showing promise include :
-evaluation of the angle in glaucoma patients;
-evaluation of the cornea in refractive surgery patients—particularly LASIK
patients;
-measuring the anterior chamber for certain phakic intraocular lenses; and
-as an adjunct in other operations involving the cornea, such as PTK and
corneal transplants.

6. • Optical coherence tomography has become a powerful tool for analyzing the
retina and other ocular structures.
• Automated software segmentation algorithms are able to outline the retinal nerve
fiber layer with much precision, which is relevant in glaucoma since this layer is
thinned as ganglion cells are lost.
• OCT became widely popular in 2002 with the release of Stratus OCT, a time-
domain technology (TD-OCT) that was well-studied and validated for use in
glaucoma and retina and went on to become a standard structural imaging test.
• four years later, several companies started to release the next generation
technology, spectral-domain OCT (SD-OCT), fourier-domain OCT, which
improved upon TD-OCT by capturing more data in less time at a higher axial
image resolution, around 5 µm.
• Ultrahigh speed swept source OCT, ultrahigh resolution OCT, polarization
sensitive OCT, and adaptive optics OCT are all on the horizon.

7. • Currently, the most common four commercially available SD-OCT devices are:
Cirrus HD-OCT (Carl Zeiss Meditec, Dublin, CA, USA),
RTVue-100 (Optovue Inc., Fremont, CA, USA),
Spectralis OCT (Heidelberg Engineering, Heidelberg, Germany), and
Topcon 3D-OCT 2000 (Topcon Corporation, Tokyo, Japan).
• Each machine has different glaucoma scan patterns, proprietary software
segmentation algorithms, and display outputs.

8. AS-OCT (For glaucoma diagnosis)
• Documents parameters such as the angle’s width, the iris’ thickness, and
anterior chamber depth.
• In addition, it permits physicians to view dynamic images of the angle’s
configuration under different lighting conditions.
• AS-OCT is therefore a useful adjunct in the diagnosis and treatment of
glaucoma.
• Two common flaws in the performance of clinical gonioscopy include the
placement of pressure on the cornea and the use of excessive amounts of light.

9. • Both produce the illusion of an open angle in a patient who otherwise may
have narrow or even appositionally closed angles.
• AS-OCT provides little light artifact.
• It can actually dynamically show how an angle imaged in bright light may
appear open but look narrow when assessed in the dark (Figure).
• The noncontact test also eliminates the problem of corneal compression.

10. Figure 2. AS-OCT allows the clinician to use a caliper tool to measure spaces precisely,
here the angle recess.
AS-OCT of the angle in an eye with the room’s lights
on (A) and then off (B). Note the appositional closure in the
dark.

11. • Although AS-OCT provides important information and various clues, plateau
iris, or any ciliary body mediated posterior pushing of the peripheral iris (such
as a cyst or tumor) cannot be diagnosed with this technology alone.
• AS-OCT is unable to image any tissues posterior to the iris due to its
pigmentation.
• AS-OCT is able to show an obliterated sulcus space and angle closure in a
patient with a myopic shift after routine cataract surgery and elevated IOP.
• The technology is unable to image the ciliary processes or through the
pigmented iris, it can show the suprachoroidal space and effusions, another
potential cause of angle closure.

12. AS OCT (IN GLAUCOMA SURGERY)
• AS-OCT in glaucoma surgery include the imaging of subconjunctival blebs in
trabeculectomy; not only can the internal ostium be visualized but also the
internal bleb structure itself.
• In tube shunt surgery, the position of the device in the anterior chamber, its
proximity to the corneal endothelium, and obstructions in the lumen or
irregularities in the course of the tube can be readily visualized.
• AS-OCT allows visualization of the distended Schlemm’s canal postoperatively.
• Distal intrascleral aqueous veins are sometimes visible on AS-OCT.
• AS-OCT technology can image the suprachoroidal space, thus permitting the
evaluation of device success as the gold suprachoroidal microshunt . (Solx
Gold Shunt; Solx, Inc., Waltham, MA).
• This device is designed to provide a conduit by which aqueous humor can exit
the anterior chamber into the suprachoroidal space and thereby lower the IOP

13. • AS-OCT has limitations, most notably its inability to image through
pigmented tissue.
• It is therefore inadequate for the assessment of the ciliary body, zonules,
posterior chamber, or anterior vitreous.

14. Posterior segment OCT Software analyzes:
 Macular Thickness
 Retinal Nerve Fibre Layer (RNFL)
 Optic Nerve Head (ONH)
Optic nerve
head
Peripapillary nerve
fibre layer
macular thickness

15. Macular Thickness Analysis
• Ganglion cells- 30-35% of total retinal thickness at macula
upto 50%of ganglion cells in macula
glaucoma preferentially involves the ganglion cell complex (GCC).
Normal
Glaucoma with thinner GCC

16. SD-OCT Parameters
• There are three main parameters relevant to the detection of
glaucomatous loss: retinal nerve fiber layer, optic nerve head, and the
“ganglion cell complex.”
• The numeric values for all parameters are shaded as white, green,
yellow, or red, with the yellow and red representing, < 5% and < 1%,
respectively compared to the normative database.
•

17. Retinal Nerve Fiber Layer
• Glaucoma is a group of many conditions sharing a final common
pathway characterized by accelerated death of retinal ganglion cells
and their retinal nerve fiber layer (RNFL) axons .
• It results in characteristic visual field defects and corresponding optic
nerve head anatomical changes.
• SD-OCT can directly measure and quantify RNFL thickness by
calculating the area between the internal limiting membrane (ILM) and
RNFL border.
• The Cirrus RNFL map represents a 6 x 6 mm cube of A-scan data
centered over the optic nerve in which a 3.4 mm diameter circle of
RNFL data is extracted to create what is referred to as the TSNIT map
(temporal, superior, nasal, inferior, temporal).

18. • It is displayed as a false color scale with the thickness values referenced to a
normative database.
• The TSNIT map displays RNFL thickness values by quadrants and clock hours.
• The RNFL peaks give a sense of the anatomic distribution of nerve fiber axons
represented by the superior and inferior bundles that emanate from the optic
nerve. ( figure ).

20. • A normal RNFL curve will have the characteristic “double hump”
appearance with the superior and inferior RNFL being thicker than the
nasal and temporal RNFL.
• The RNFL curve is drawn as a black line on a graph featuring thickness
in microns and different areas of peripapillary RNFL.
• The RNFL curve is drawn over a background of color coded areas
representing RNFL thickness classification according to normative
database.

21. RNFL analysis
Circular scanning around ONH at a radius
of 1. 73mm.
Three scans are acquired and data is
averaged and compared with normative
data base of age matched subjects
Scan begins temporally

22. RNFL thickness average analysis printout -7 zones
• Zone -1-Pt. I.D
• Zone -2-TSNIT with age matched normative data base
• Zone-3-TSNIT overlap of two eyes
• Zone -4-circular scan-quadrant/clockwise
• Zone-5-DATA TABLE-ratio/average
• Zone-6-RED FREE PHOTOGRAPH-position
• Zone-7-PERCENTILE COLOR CODING

23. 7
6
5
4
3
2
1

24. Optic Nerve Head (ONH)
• SD-OCT also automatically outlines the optic nerve head, optic cup, and disc
borders .
• Also calculates more objective measurements such as optic disc area and
neuroretinal rim area in addition to the classic clinician-subjective average
and vertical cup-to-disc ratios.
• Cirrus does this by defining the edge of the disc as the termination of Bruch’s
membrane and then finds the shortest perpendicular distance to ILM,
minimum band distance, to define the inner cup margin in each slice in a
spiral around the optic disc cube data until a center is located.
• The calculated ONH parameters, except for disc area, are then compared to
a normative database (figure).
• Radial line scans through optic disc provide crosssectional information on
cupping and neuroretinal rim area.

25. Optic nerve head scans are composed
of six linear scans in a spoke pattern
separated by 30-degree intervals
centered on the ONH

28. Cup/disk ratios and cup Volumes
Disc size:
 by measuring the distance
between the terminal ends of the
choriod at the level of the pigment
epithelium (green line)
Cup:
 determined by drawing a line b/w
both sides of the cup at a point
140um above the green line.
 Area below the line is cup and
above is neuroretinal rim

29. Ganglion Cell Analysis
• The ganglion cell layer is thickest in the perimacular region and decreased total
macular thickness has been observed in glaucomatous eyes likely due to
thinning of the ganglion cell layer in this region.
• Cirrus measures its Ganglion Cell Analysis (GCA) consisting of the combined
ganglion cell layer (GCL) and inner plexiform layer (IPL).
• Optovue’s Ganglion Cell Complex (GCC) chose to include RNFL in their
combined GCL and IPL layers.
• The Cirrus macular scan map (figure) is displayed using a similar color scale and
divided into various pie sectors around the fovea.
• The calculated sectors are compared to a normative database.

31. • When using the average RNFL thickness at the 5% level compared to the
normative database (yellow coloring on RNFL deviation map), SD-OCT has a
sensitivity of 83% and a specificity of 88% compared to 80% and 94%
respectively for TD-OCT.
• When using the average RNFL thickness at the 1% level (red coloring on RNFL
deviation map), the specificity for both SD-OCT and TD-OCT was 100% but the
sensitivity was only 65% in SD-OCT and 61% in TD-OCT.
• ONH parameters have also been found to have excellent ability to discriminate
between normal eyes and eyes with even mild glaucoma.
• The parameters found to have the greatest diagnostic capability are vertical rim
thickness, rim area, and vertical cup to disc ratio.
• These ONH parameters were found to be as good as RNFL thickness
parameters in diagnosing glaucoma.

32. • Similarly, GCA parameters have been found to be comparable to ONH
and RNFL parameters.
• significant structural RNFL loss occurs prior to the development of
functional visual field loss.
• In such preperimetric disease, SD-OCT RNFL is especially useful in
helping to diagnose glaucoma prior to the onset of visual field loss.
• In the presence of perimetric disease, finding RNFL bundle loss on SD-
OCT with a corresponding abnormality in the visual field served by
those retinal ganglion cells can help confirm the diagnosis of glaucoma.

33. Use of SD-OCT in Detection of Glaucomatous Progression
• GPA, introduced in 2009 on the Cirrus, compares the RNFL thickness of
individual clusters of A-scans, referred to as pixels, between baseline and follow
up RNFL thickness maps to estimated test-retest variability.
• Local pixels exceeding such test-retest variability are coded in yellow at the first
event, and in red if the same changes are seen on three consecutive images
(figure ).
• In order to generate an overall trend plot, two baseline scans with three follow up
scans are necessary.
• This linear regression line in µm/yr, representing rate of change, is drawn with
an estimated confidence interval carried forward.

35. Progressive RNFL Thinning
• Progressive RNFL thinning measured on SD-OCT can often be used to
detect progressive disease (figure).
• The top three RNFL progression patterns are:
(A) Widening of an existing RNFL defect,
(B) Deepening without widening of an existing RNFL defect, or
(C) Development of a new RNFL defect.
• In one study, the inferotemporal quadrant was the most frequent location
for RNFL progression.

37. Floor Effect
• In early to moderate glaucoma, progressive thinning of RNFL thickness
measured by SD-OCT is a very useful tool to judge progression of
disease.
• At advanced stages however, SD-OCT is less clinically useful due to a
“floor effect” of RNFL thickness.
• With advanced loss, RNFL thickness levels off, rarely falling below 50
µm and almost never below 40 µm due to the assumed presence of
residual glial or nonneural tissue including blood vessels.
• At this level of disease, serial visual fields are more useful to judge
progression. (figure)

39. Correlation of OCT with Visual Field
• Due to the variability or possible artifacts with SD-OCT measurements,
all changes on OCT should be correlated with visual field changes before
confirming definite progression.
• When such correspondence is not found, caution should be exercised
and sources of erroneous measurements should be sought

40. Normative Database
• As previously stated, SD-OCT measurements are compared against an age-
matched normative database.
• The normative database for the Cirrus SD-OCT consisted on 284 healthy
individuals with an age range between 18 and 84 years (mean of 46.5 years).
• Ethnically, 43% were Caucasian, 24% were Asians, 18% were African American,
12% were Hispanic, 1% were Indian, and 6% were of mixed ethnicity.
• The refractive error ranged from -12.00 D to +8.00 D
• Due to this relatively small normative database and wide variation of distribution
of RNFL, many results obtained by SD-OCT may be flagged as abnormal
statistically in patients who are not represented in the database and thus not
necessarily representing real disease.
• One should use caution to avoid overtreating “red disease” in these situations

41. • One common special case is the myopic patient.
• As mentioned previously, high myopes were not included in the normative
database.
• Myopic eyes have thinner RNFL measurements, which can confound
comparisons to the normative database.
• Additionally, myopic eyes can have unique distributions of RNFL bundles.
• With increasing myopia, the superotemporal and inferotemporal RNFL bundles
tend to converge temporally.

42. • This may result in the temporal shift of the superior and inferior RNFL bundle
peaks of normal magnitude.
• Due to this shift, although the peaks are of normal magnitude, they can be
interpreted as thinned due to having a different distribution from the normative
database.
• Additionally, even in normal eyes, split RNFL bundles, both superiorly and
inferiorly, have been found on histologic section, thus representing a true normal
variant which may appear abnormal on SD-OCT (figure).

44. • While the limitations of the normative database may hinder the utility of
SD-OCT in diagnosing glaucoma using a single scan, serial SD-OCT
scans can be very useful to judge glaucomatous progression by setting
a baseline scan against which to judge progressive thinning on
subsequent scans.
• Therefore, each patient can be his or her own “normative database” to
diagnose glaucoma in such difficult settings as high myopia.
• Based on a longitudinal study, the age-related rate of reduction in RNFL
thickness has been estimated to be -0.52 µm/year, -1.35 µm/year, and -
1.25 µm/year for average, superior, and inferior RNFL respectively

45. Signal Quality
• When assessing the adequacy of a scan, the signal strength should
always be noted.
• The signal strength, reported on a scale of 0 to 10, is defined as the
averaged intensity value of the signal pixels in the OCT image.
• The best quality scans have signal strength greater than 8 (minimum
acceptable scan > 6).

46. Blink/Saccades
• An SD-OCT RNFL scan consists of multiple single A-scans side by
side to represent a B-scan cube.
• With eye movement or blinking, these scans do not align correctly
which can lead to an erroneous RNFL thickness measurement, which
may be misinterpreted as progressive thinning (figure).
• The new SD-OCT versions have a built-in eye tracking function which
can help compensate for eye movement by relying on blood vessel
registration or iris tracking.
• Using the eye tracker significantly improves the reproducibility of RNFL
measurements.

48. Segmentation Errors
• It is important to look at the segmentation lines produced by any SD-
OCT machine’s software algorithm to ensure that they are
appropriately placed.
• Lines should not come together (go to zero). Occasionally, one will find
that the segmentation lines are misplaced along the retina leading to
errors in the calculation of RNFL thickness (figure).
• These segmentation errors are more common in the presence of poor
signal strength, tilted discs, staphylomas, large peripapillary atrophy,
epiretinal membranes, and posterior vitreous detachments.
• A decreased incidence of such segmentation errors in SD-OCT
compared to TD-OCT.

51. Media Opacities
• The en face scanning laser ophthalmoscope (SLO) image should be
examined to ensure the absence of a media opacity, such as a posterior
vitreous detachment (PVD), within the circumpapipillary scan area.
• Areas in which data is missing due to an opacity are represented in
black on the en face SLO image (figure).
• In such a case, the overlying PVD can lead to a falsely thin RNFL
measurement in the underlying area.

53. Axial Length
• Axial length has been shown to influence SD-OCT measurements of both RNFL
thickness and ONH parameters due to axial-length induced ocular magnification.
• The longer the eye, the thinner the RNFL, and the smaller the optic disc area
and neuroretinal rim area.
• However, refractive error independent of axial length has not been found to affect
RNFL thickness measurements as long as a well focused fundus image is
obtained during scan acquisition by utilizing the Cirrus SD-OCT internal fixation
focus adjustment.
• This adjustment can account for refractive errors from -20.0 D to +20.0 D

54. Effect of Decentration

55. 1.

58. DOPPLER OCT(F-Domain)
Retinal blood flow measurement with Doppler OCT may help
us understand the role of perfusion in the causation and treatment
of glaucoma, other optic neuropathies and retinal diseases

59. • SD-OCT is a powerful objective structural assessment tool that can
greatly assist clinicians in diagnosing and managing glaucoma
(especially early disease), when used in conjunction with visual field
testing and serial clinical exams.

60. THANK YOU