Some of the most important advances in our glaucoma workup have occurred in diagnostic imaging. In just 10 years, the capabilities of optical coherence tomography (OCT) have expanded from 100 to 27,000 A-scans per second, with improved resolution and tremendous enhancements in image quality and contrast. These innovations have given us new ways to view and interpret data.
The Cirrus™ HD-OCT (Carl Zeiss Meditec, Inc., Dublin, Calif.) provides a unique view of the optic nerve head (ONH) for analysis. It captures a 6mm x 6mm data cube and automatically identifies the optic disc and cup boundaries.
This data cube is comprised of 200 B-scans with 200 A-scans per B-scan, generating more than 40 million data points and is a key to the ONH analysis. When we view the ONH with the ophthalmoscope or in photographs or other types of scans, the images are two-dimensional. This makes quantitative analysis difficult, particularly because many glaucoma patients are highly myopic and have tilted discs. The high-density, 3-D data cube generated by the Cirrus HD-OCT is automatically corrected for optic disc tilt and other anatomic anomalies, such as staphylomas, that are often present in our glaucoma patients. Thus, measurements of the optic disc and the rim area correspond to the anatomy in the same plane as the optic disc (Figure 1).
Figure 1. The Cirrus high-density 3-D data cube enables automatic correction for optic disc tilt and other anatomic anomalies. Measurements of the disc and rim areas correspond to the anatomy in the same plane as the optic disc.
The ONH and retinal nerve fiber layer (RNFL) analyses generated by the Cirrus HD-OCT provide specific calculations with key parameters displayed in table format (Figure 2). The color-coded comparison to normative data allows you to quickly determine if a nerve is normal or if it deviates from the norm. For the ONH, the patient data is matched to normal, based on disc area and age. The data from the database study show that the disc area is 1.82 mm on average but can be grouped as small, medium and large. This plays a role in our analysis, as many hyperopes or people with ischemic optic neuropathy have small discs, while some myopes have tilted, large discs. The normative database provides a basis for comparison.
Figure 2. Key parameters of ONH and RNFL thickness data are displayed as a table. Comparison to normative data is indicated with color-coding.
For many clinicians, the most valuable function of the Cirrus HD-OCT is the evaluation of the RNFL (Figure 3). An en face OCT fundus image shows the boundaries of the cup and disc and the RNFL calculation circle integrated with the RNFL thickness deviation map. The deviation map, which extends the area of evaluation across the 6mm x 6mm area, indicates variance from normal. You may detect other changes from the vasculature and the RNFL, all which may be early changes in glaucoma (Figure 4).
Figure 3. The RNFL thickness map shows the cup and disc mask in gray. The en face OCT fundus image shows the boundaries of the cup and disc and RNFL calculation circle integrated with the RNFL thickness deviation map.
Figure 4. The RNFL and neuroretinal rim thickness profiles demonstrate symmetry between the two eyes and comparison to normative data. The RNFL thickness is also compared to normative data in quadrant and clock-hour displays. Quadrants may reveal diffuse thinning. Clock hours may show localized thinning.
The neuroretinal rim thickness profile is plotted in units of micrometers for the left eye and the right eye together, and is compared to normative data. The RNFL thickness profile looks at the peripapillary RNFL thickness and shows symmetry and anomalies in the RNFL structure. The RNFL thickness values are compared to a normative database, as well. This is divided into quadrants and clock hours, which may show localized thinning and should correlate with your clinical impression in the appearance of the disc as well as visual field changes.
The combined ONH–RNFL analysis from the Cirrus HD-OCT provides various valuable pieces of information for glaucoma assessment. All analyses are based on data from the 200x200 cube scan, requiring no additional scans, and RNFL and ONH parameters are compared to normative data. I am confident that I can trust the analytics from Cirrus and rely on the measurements to be precise and repeatable.
