**9. Determining progression using OCT**

Glaucoma is a progressive disorder and monitoring progression forms a quintessential part of glaucoma management. Quantitative assessment of progression can be done in a predictable manner using OCT as compared to the qualitative and subjective assessment of the optic nerve head using optic disc photographs. There exists an inherent variability of each machine which is calculated in each machine by repeated measurements preferably the same day. Any change amounting to two to three times the standard deviation of the machine is taken as a real change in terms of progression. In Stratus OCT, the measurement variability is markedly lower for RNFL scans as compared to ONH analysis thereby making RNFL scans a better method in determining changes over time or what is termed as true progression.

Glaucoma progression is either event based or trend based according to statistical analysis. In event analysis, a threshold is determined and true progression is said to have occurred when a follow-up measurement exceeds this preestablished threshold. Any change below this threshold is considered to be due to natural age-related loss or measurement variability. Event analysis thus, is intended to identify a gradual change over time crossing the threshold or development of a sudden event that falls above the predetermined threshold. On the contrary, a trend analysis identifies progression by monitoring the behavior of a parameter over time. This method is therefore, less sensitive to sudden change and the variability among consecutive tests.

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**Figure 9.**

*Role of Optical Coherence Tomography in the Evaluation and Management of Glaucoma*

Progression with time domain OCT mostly utilizes the RNFL thickness measurements as these have shown to discriminate well between normal and glaucomatous eyes [16, 17]. Both diffuse and localized glaucomatous RNFL defects in the peripapillary area have shown to have good reproducibility with low intra-test and inter-test variability and can be utilized in determining progression [18–24]. Based on the published data on the repeatability of mean RNFL thickness measurements, any decrease in thickness exceeding 6.4–8 μm can be considered to be abnormal and beyond the limits of test-retest variability with 95% tolerance [25]. These values are used only for mean peripapillary RNFL thickness and not for quadrants and clock hours as the variability is significantly higher in these areas owing to the shifts in scan locations. The guided progression analysis (GPA) is a trend-based analysis that uses a linear regression to report change in overall mean RNFL thickness over time and also provides the significance of this change. The point of concern with the GPA analysis of Stratus OCT is that statistical significance reported doesn't take into consideration the rate of normal age-related loss. Therefore, some normal age-related changes may be reported as significant even though they do not represent true disease progression. The average age-related RNFL loss is expected to be between

The substantial increase in SD-OCT scanning speed over TD-OCT makes scans less prone to eye movement artifacts. Studies have reported excellent intra-visit and inter-visit measurement reproducibility for SD-OCT [29–34], superior to TD-OCT [35–37]. This makes the SD-OCT a potential tool in monitoring glaucoma progression.

*Cirrus SD-OCT RNFL-guided progression analysis (adapted Carl Zeiss Meditec, Dublin, CA).*

*DOI: http://dx.doi.org/10.5772/intechopen.84202*

**9.1 Progression with time domain OCT**

0.16 and 0.31 μm/year [26–28].

**9.2 Progression with spectral domain OCT**

*Role of Optical Coherence Tomography in the Evaluation and Management of Glaucoma DOI: http://dx.doi.org/10.5772/intechopen.84202*

### **9.1 Progression with time domain OCT**

*A Practical Guide to Clinical Application of OCT in Ophthalmology*

under 5% of the normal population is labeled as borderline with a yellow color and thickness under 1% is labeled as outside normal limits and has a red color. A green

Glaucoma is a progressive disorder and monitoring progression forms a quintessential part of glaucoma management. Quantitative assessment of progression can be done in a predictable manner using OCT as compared to the qualitative and subjective assessment of the optic nerve head using optic disc photographs. There exists an inherent variability of each machine which is calculated in each machine by repeated measurements preferably the same day. Any change amounting to two to three times the standard deviation of the machine is taken as a real change in terms of progression. In Stratus OCT, the measurement variability is markedly lower for RNFL scans as compared to ONH analysis thereby making RNFL scans a better method in determining changes over time or what is

Glaucoma progression is either event based or trend based according to statistical analysis. In event analysis, a threshold is determined and true progression is said to have occurred when a follow-up measurement exceeds this preestablished threshold. Any change below this threshold is considered to be due to natural age-related loss or measurement variability. Event analysis thus, is intended to identify a gradual change over time crossing the threshold or development of a sudden event that falls above the predetermined threshold. On the contrary, a trend analysis identifies progression by monitoring the behavior of a parameter over time. This method is therefore, less sensitive to sudden change and the variability among

color means above the 5% of normal population.

**9. Determining progression using OCT**

*GCC map displays (Optovue: RTVue Version A4).*

termed as true progression.

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**Figure 8.**

consecutive tests.

Progression with time domain OCT mostly utilizes the RNFL thickness measurements as these have shown to discriminate well between normal and glaucomatous eyes [16, 17]. Both diffuse and localized glaucomatous RNFL defects in the peripapillary area have shown to have good reproducibility with low intra-test and inter-test variability and can be utilized in determining progression [18–24]. Based on the published data on the repeatability of mean RNFL thickness measurements, any decrease in thickness exceeding 6.4–8 μm can be considered to be abnormal and beyond the limits of test-retest variability with 95% tolerance [25]. These values are used only for mean peripapillary RNFL thickness and not for quadrants and clock hours as the variability is significantly higher in these areas owing to the shifts in scan locations.

The guided progression analysis (GPA) is a trend-based analysis that uses a linear regression to report change in overall mean RNFL thickness over time and also provides the significance of this change. The point of concern with the GPA analysis of Stratus OCT is that statistical significance reported doesn't take into consideration the rate of normal age-related loss. Therefore, some normal age-related changes may be reported as significant even though they do not represent true disease progression. The average age-related RNFL loss is expected to be between 0.16 and 0.31 μm/year [26–28].

#### **9.2 Progression with spectral domain OCT**

The substantial increase in SD-OCT scanning speed over TD-OCT makes scans less prone to eye movement artifacts. Studies have reported excellent intra-visit and inter-visit measurement reproducibility for SD-OCT [29–34], superior to TD-OCT [35–37]. This makes the SD-OCT a potential tool in monitoring glaucoma progression.

#### **Figure 10.**

*RNFL change analysis (adapted: Optovue, Fremont, CA).*

Ganglion cell complex diagnostic accuracy for detecting glaucoma has been shown to be similar to that of peripapillary RNFL thickness making it potentially valuable for monitoring glaucoma progression [38–42].

Progression analysis is currently available with Cirrus HD-OCT (Carl Zeiss Meditec, Dublin, CA) and RTVue (Optovue, Fremont, CA). Of these two devices, statistical analyses in form of event- and trend-based mechanism for progression detection is available only in the Cirrus HD-OCT. Data sampling of the RNFL is obtained from the 3.4-mm diameter peripapillary circle and the software also displays RNFL thickness changes from baseline for each pixel in the scanned area. Possible or likely RNFL thickness loss is reported if change exceeds the expected test-retest variability in a single or two consecutive followup examinations, respectively. In addition, linear regression is performed to determine the rate of change, confidence limits, and statistical significance of the trend (**Figure 9**).

The RTVue offers progression analysis that includes side-by-side RNFL thickness measurements and overlay of the RNFL profiles for the consecutive scans. The RT Vue also provides similar analysis for ganglion cell complex thickness along with thickness change plots (**Figure 10**). However, a formal statistical analysis of change over time is not currently included in the latest version of the software for this device (version 6.1) (**Figure 11**).

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the anterior segment.

**Figure 11.**

*Role of Optical Coherence Tomography in the Evaluation and Management of Glaucoma*

*DOI: http://dx.doi.org/10.5772/intechopen.84202*

**10. Anterior segment OCT (AS-OCT)**

*Ganglion cell analysis showing progression (adapted: Optovue, Fremont, CA).*

**10.1 Qualitative assessment using AS-OCT**

Anterior segment OCT (AS-OCT) imaging initially described by Izatt et al. using the same wavelength of light as retinal OCT [43]. However, the 830 nm wavelength was found to be suboptimal for imaging the angle due to limited penetration through scattering tissue such as the sclera. Hence, a newer OCT imaging of the anterior segment with a longer wavelength of 1310 nm was developed that had the advantages of better penetration through sclera [44]. Currently, there are two dedicated anterior segment devices commercially available. The SL-OCT (Heidelberg Engineering) and the Visante (Carl Zeiss Meditec, Inc.) Newer Fourier domain anterior segment OCT devices however have been developed, and these allow rapid three-dimensional cube scanning of

An important landmark to identify when interpreting AS-OCT images is the scleral spur. This is visible as an inward projection of the sclera at the junction between the inner scleral and corneal curvatures. Studies have shown that scleral spur may not be visible in 25% cases [45]. The apposition between the iris and the inner corneo-scleral wall has been used in several studies as a qualitative method

*Role of Optical Coherence Tomography in the Evaluation and Management of Glaucoma DOI: http://dx.doi.org/10.5772/intechopen.84202*

**Figure 11.** *Ganglion cell analysis showing progression (adapted: Optovue, Fremont, CA).*
