What information does CSLO provide?
- The surface height determined at the pixel gives the surface height map (topography) for the optic disc and surrounding retina.
- The height of each pixel is determined from the intensity of light reflected from that pixel in each optical section. The surface height approximates to the optical section with the greatest reflected intensity for that pixel.
- The topography image is color-coded, with lighter colors representing greater depth and darker colors more anterior structures.
- Once the topography image has been generated, an observer draws a contour line around the optic disc (outlining the inner margin of Elschnig’s ring). Proper care must be taken in drawing this contour line to decrease the interjection of artifact into the measurements of the RNFL surface contour. As a physician resource, quality of data used for image interpretation is essential and recognition of artifact cannot be over-emphasized. The colored wedge indicates the region on the optic disc margin from which the reference plane height is set.
Topography image from HRT
- The software inserts a reference plane. This plane is parallel to the retinal surface, but set 50 μm below the surface at the temporal disc margin. Tissue above the reference plane (including blood vessels) and within the disc margin is colored green and blue and is called “rim.” Space below the reference plane and within the disc margin is colored red and is called “cup.”
- HRT measures “stereometric parameters”, which relate to the 3-dimensional shape of the ONH.
- The software quantifies the rim and cup area and volume for the whole disc (global) and for 6 predefined sectors. The shape (steepness) of the cupping is described by the “cup shape measure”.
Stereometric Results ONH 0° – 360° |
| Disk Area | 1.978 mm |
| Cup Area | 0.946 mm |
| Cup/Disk Area Ratio | 0.478 |
| Rim Area | 1.032 mm |
| Height Variation Contour | 0.193 mm |
| Cup Volume | 0.182 mm |
| Rim Volume | 0.135 mm |
| Mean Cap Depth | 0.224 mm |
| Maximum Cup Depth | 0.504 mm |
| Cup Shape Measure | 0.074 |
| Mean RNFL Thickness | 0.117 mm |
| RNFL Cross Section Area | 0.586 mm |
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Moorfields Regression Analysis
HRT also conducts a statistical analysis called the Moorfields Regression Analysis (MRA):
- The measured rim area is compared to the rim area predicted for an optic disc of the size measured and for the patient’s age. The predicted values are derived from a normative database.
- The comparison is made for the whole disc (global; first bar) and 6 segments (in order, temporal, temporal superior, temporal inferior, nasal, nasal superior and nasal inferior).
- The sector is marked with a red cross if the rim is outside statistically normal limits (below the 99.9% prediction interval), a yellow exclamation mark if it is borderline (between the 95.0% and 99.9% prediction intervals) and a green check if it is within statistically normal limits (above the 95.0% prediction interval).
- Although disc size is taken into account, false positive results (outside statistically normal limits) are more likely in larger optic discs.
Figure: Illustration of the Moorfields Regression Analysis calculation View large
A Glaucoma Probability Score (GPS) is also produced:
- The optic disc margin does not need to be drawn for this analysis. The shape of the cupping and contour of the retina surrounding the optic disc is compared to a model for normal and glaucomatous nerves. The comparison is made for the whole disc (global; first bar) and 6 segments (in order, temporal, temporal superior, temporal inferior, nasal, nasal superior and nasal inferior). The sector is marked with a red cross if the rim is outside statistically normal limits (above 64% probability), a yellow exclamation mark if it is borderline (between 28% and 64% probability) and a green check if it is within statistically normal limits (below 28% probability).
- Although disc size is taken into account, false positive results (outside statistically normal limits) are more likely in larger optic discs.
- The GPS has similar sensitivity and specificity to the MRA. Since the GPS looks at different structural characteristics; however, the results are not always the same. The MRA assesses the health of the rim relative to optic disc size, while the GPS makes a more global assessment by utilizing information from the entire image.
Figure: Illustration of the Glaucoma Probability Score calculation View large
HRT Standard Report
The Standard report displays the results of the Moorfields Regression Analysis (MRA), and also compares the linear cup/disc ratio, cup shape measure, rim area, rim volume and RNFL (disc margin height above reference plane) values against the normative database.
A standard HRT III report is shown below:
Figure: HRT Standard Report View Large
LIMITATIONS
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