3 Optical Coherence Tomography of the Optic Nerve


Andrew Williams and Jullia A. Rosdahl


Assessment of the optic nerve is a critical component of glaucoma evaluation. Imaging technologies have evolved to provide quantitative measurements of optic nerve head parameters and to detect changes in measurements over time that may suggest glaucoma progression. Optical coherence tomography (OCT) in particular provides reproducible, reliable, and accurate measurements of optic nerve parameters, such as thickness of the retinal nerve fiber layer, and has become integral to clinical practice to guide physicians in glaucoma diagnosis and management. We describe in this chapter a systematic approach to OCT interpretation in clinical practice with attention to its limitations and potential artifacts.

3 Optical Coherence Tomography of the Optic Nerve

3.1 Introduction

3.1.1 Glaucoma Imaging

Glaucoma is characterized by progressive loss of retinal ganglion cells that causes structural changes to the optic nerve and thinning of the retinal nerve fiber layer (RNFL) with subsequent corresponding visual field defects. Prompt recognition of early structural changes is critical to mitigate permanent vision loss from glaucoma. Visual field defects may not become apparent until 40% of retinal ganglion cells are lost, suggesting that perimetry alone may not capture early disease. Careful stereoscopic examination of the optic nerve remains a hallmark in detection and management of glaucoma, but subjective interpretation and two-dimensional documentation can limit the ability to identify subtle changes. Along the same lines, stereoscopic photographs of the optic nerve and monochromatic photographs of the RNFL are limited by only fair agreement in interpretation between glaucoma specialists and the lack of quantitative information.

Imaging modalities have been developed to aid ophthalmologists in optic disc assessment to monitor structural signs of glaucoma. Technologies such as the Heidelberg Retina Tomograph (HRT, Heidelberg Engineering, Heidelberg, Germany) emerged to provide quantitative information about the topography of the posterior fundus using confocal laser scanning microscopy (Fig. 3‑1). Similarly, the GDx scanning laser polarimeter estimated RNFL thickness by measuring its birefringence using polarized light (Fig. 3‑2). However, the detailed and precise imaging of the optic nerve possible with optical coherence tomography (OCT) is the most common computerized imaging of the optic nerve in current clinical practice. Commercially available OCT devices include Cirrus HD-OCT (Carl Zeiss Meditec, Dublin, CA), Spectralis (Heidelberg Engineering, Heidelberg, Germany), RTVue-100 (Optovue, Freemont, CA), 3D OCT-2000 (Topcon Medical Systems, Oakland, NJ), and RS-3000 Advance (Nikon, Tokyo, Japan). These different commercial devices utilize similar OCT acquisition principles but vary in scanning protocols and segmentation algorithms (discussed in depth in Chapter 7).

Fig. 3.1 Heidelberg retina tomographs of normal and glaucoma eyes. Heidelberg retinal tomograph (HRT) confocal images of the optic nerve are shown. (a) A healthy eye and
(Continued) (b) a glaucomatous eye. (Courtesy of Dr Teresa Chen of Massachusetts Eye and Ear Infirmary.)
Fig. 3.2 GDx scanning laser polarimetry of normal and glaucoma eyes. GDx scanning laser polarimetry of the optic nerves are shown. (a) Healthy eyes and
(Continued) (b) glaucomatous eyes. (Courtesy of Dr Teresa Chen of Massachusetts Eye and Ear Infirmary.)

3.1.2 Optical Coherence Tomography

OCT noninvasively acquires a real-time image of the ophthalmic structures in optical cross section. Compared to previous models of time-domain OCT technology, spectral domain OCT (SD-OCT) produces higher resolution images by utilizing spectrally separated detectors and taking Fourier transform of broad spectral information (Fig. 3‑3). Using a near-infrared super-luminescent diode light, SD-OCT acquires 26,000 to 85,000 axial scans per second for an axial resolution of about 5 μm and transverse optical resolution of about 14 μm. Images are acquired either in raster scans (parallel frames), radial scans across the optic nerve, or as concentric circles to measure peripapillary RNFL thickness in a 3.46-mm scan circle centered on the optic nerve head (ONH). Software can now delineate Bruch’s membrane opening (BMO) as standard reference for defining the region of the ONH, while previous device iterations required manual centration of the scan circle. Additionally, eye motion tracking recently has been incorporated to reduce noise and artifacts in acquired images. Software programs also contain a normative database of measurements from healthy subjects to aid interpretation of patient’s measurements by comparing them to normal ranges.

Fig. 3.3 Optical coherence tomography (OCT) imaging in a glaucoma suspect. These are optic nerve images of a 62-year-old man who was followed as a glaucoma suspect due to increased cup-to-disc ratio. (a) Optic nerve photos from approximately 10 years prior show physiologic cupping. (b) Time-domain OCT of the retinal nerve fiber layer (RNFL) showing normal thickness in both eyes.
(Continued) (c) Spectral domain OCT of the RNFL showing normal thickness in both eyes.

3.2 OCT Output

3.2.1 Overview

Most commercial OCT platforms generate a printout to summarize quantitative and qualitative measurements of RNFL and ONH parameters. Depending on the software algorithm and technical capabilities of the device, these measurements include RNFL thickness, cup-to-disc ratio, neuroretinal rim area, and neuroretinal rim volume.

In addition, quality metrics are given to allow the interpreter to assess the validity of the data output. Patient identification and date of study are embedded in the report. Many printouts also include a quality score or indication of signal strength, as lower quality images are more prone to artifactual measurements. Additionally, algorithmic delineation of the circumpapillary RNFL is demonstrated over acquired B-scan images in order for the user to assess accuracy of the RNFL tracing.

Retinal Nerve Fiber Layer Measurements (Fig. 3‑4 and Fig. 3‑5)

Measurements of circumpapillary RNFL thickness feature prominently in the output and are given as a global average, by quadrant, and by clock-hour for each eye. Summary measurements are presented in table form, including degree of RNFL symmetry, as asymmetry between eyes can be associated with glaucoma. Thickness maps illustrate RNFL measurements around the detected ONH, with warmer colors corresponding to thicker values in a classically butterfly-shaped distribution around the optic nerve. Deviations from normal thickness are also illustrated in a deviation map to highlight potential areas of RNFL loss.

Fig. 3.4 Optic nerve head and retinal nerve fiber layer (RNFL) analysis of a glaucoma patient (Cirrus optical coherence tomography [OCT]). This is a Cirrus OCT report for a 70-year-old white woman with primary open-angle glaucoma that is moderate stage in the right eye and severe stage in the left. There is notable asymmetry in RNFL thickness between the two eyes. The superior and inferior thinning pattern on the left eye is also characteristic for glaucoma. There are superior and inferior arcuate defects on visual field testing of the left eye. The right eye demonstrates a superior nasal step consistent with moderate-stage disease, and progression analysis demonstrates thinning of the inferior RNFL quadrant over time (see Fig. 3‑8). (Courtesy of Dr Ian Conner of the University of Pittsburgh.)
Fig. 3.5 Retinal nerve fiber layer (RNFL) analysis of a patient with moderate glaucoma (Spectralis optical coherence tomography [OCT]). This is a Spectralis OCT RNFL report of a 70-year-old woman with normal tension glaucoma. There is inferior thinning of the RNFL in both eyes. Her visual fields demonstrate early nasal defect superiorly in the right eye and dense superior nasal defect in the left eye.

RNFL thickness is also graphed in a “TSNIT” plot, in which the circumferential RNFL thickness measurements are “unrolled” into linear form and presented from the temporal to inferior quadrants (the “Temporal-Superior-Nasal-Inferior-Temporal” plot). In normal eyes, this plot should demonstrate a characteristic double hump, with thickest RNFL measurements peaking at the inferior and superior regions.

Patient measurements are plotted over a normative measurement database, and interpretation of the probability of abnormal values is aided by color coding with red, yellow, green, and white shading. Red color highlights values in the lowest 1% of normative values and is considered outside normal limits. Yellow flags values from the 1st to 5th percentile and suggests borderline abnormality. Green indicates the middle 90% of normal, from 5th to 95th percentiles, and white flags values greater than the 95th percentile of normal subjects. Plots over normative values compare patient measurements with those from subjects of similar age, as age-related changes lead to RNFL thinning over time even in the absence of disease. Despite similar presentation and color coding of the TSNIT plot between devices, images cannot be directly compared from one OCT device to another because different manufacturers generate these plots from their own normative databases.

Optic Nerve Head Parameters

ONH analyses are generated by defining the boundary of the optic disc using the BMO. The area of the neuroretinal rim is measured as the minimum distance from the BMO to the internal limiting membrane (ILM), a parameter described at the BMO-MRW (Bruch’s membrane opening minimum rim width). The Spectralis device acquires radial scans across the ONH to determine BMO-MRW and thus define the neuroretinal rim (Fig. 3‑6). For the Cirrus HD-OCT, ONH data is extracted from the 200 × 200 Optic Disc Cube scan using minimum area rather than distance to define the BMO-MRW, and normative measurement limits are corrected for optic disc tilt and size. Resulting metrics include cup-to-disc ratio, neuroretinal rim area, and neuroretinal rim volume. These ONH measurements complement RNFL data to detect glaucoma or progression, but themselves have less diagnostic accuracy. However, ONH parameters can suggest nonglaucomatous optic neuropathy if there is thin RNFL in the setting of normal neuroretinal rim and cup-to-disc ratio.

Fig. 3.6 Optic nerve head analysis (Spectralis optical coherence tomography [OCT]). The Glaucoma Module Premium Edition from Spectralis provides an analysis of the optic nerve head. This is an example from a patient who does not have glaucoma. The report includes analyses of minimum rim width (MRW) and retinal nerve fiber layer (RNFL) thickness at a diameter of 3.5 mm. MRW measurements at nine cross sections are displayed with arrows, colored green (normal), yellow (borderline), or red (outside normal limits). Temporal-superior-nasal-inferior-temporal (TSNIT) plots of the MRW and the RNFL thickness are shown. (Courtesy of Drs Felipe Medeiros, Eduardo Mariottoni, Alessandro Jammal, and Eric Cabezas at the Duke Eye Center.)

3.2.2 OCT Interpretation

As with perimetry, clinicians should interpret OCT output systematically in order to avoid overlooking information that could influence decision-making and to identify artifacts or anatomic variants that affect measurement values. 1 Although there is no standardized system for interpretation, several important components of the output summary should be regularly assessed for accuracy and reliability (Fig. 3‑7 and Table 3‑1).

Fig. 3.7 Interpreting optical coherence tomography (OCT) of the optic nerve for glaucoma. Although there is no standard assessment for interpreting OCT measurements of the optic nerve, it is important to take a systematic and consistent approach in order to avoid misinterpretation of the data. An example of a systematic approach to interpreting Cirrus (a) and Spectralis
(b) OCT outputs is demonstrated. Step 1: Ensure the correct patient name and age. Step 2: Check the signal strength (Cirrus), as diminished signal can yield erroneous measurements. Step 3: Assess segmentation of the retinal nerve fiber layer (RNFL), as improper segmentation will result in incorrect measurements of RNFL thickness. Step 4: Compare the optic nerve tracing to fundus photographs or clinical examination. Accurate tracing can be affected by parapapillary atrophy or optic nerve tilt. Ensure proper centration. Step 5: Check the alignment of the temporal-superior-nasal-inferior-temporal (TSNIT) plots to be sure that the position of the superior and inferior peaks is generally aligned with those in the normative database. Step 6: Look for asymmetry in RNFL measurements between the two eyes. Step 7: Evaluate the pattern of RNFL thickness to assess whether it fits a pattern for glaucoma. In these examples from different patients, there is superior and inferior RNFL loss, which is consistent with a glaucoma diagnosis. Steps are described in more detail in Table 3‑1.

optical coherence tomography (OCT) interpreting output Table 3.1 Interpreting OCT output
Item Description
1. Patient’s name and age Make sure age was entered correctly, as OCT measurements are compared to age-matched nomograms
2. Signal strength On Cirrus OCT, values of 7 and above are preferred and values of 3 or below are quite poor For Spectralis OCT, a quality score of 15 or greater is preferred
3. Segmentation Ensure B-scan tracings accurately outline the RNFL; rectangle-shaped areas of complete RNFL loss on thickness and deviation maps are likely segmentation artifact
4. Optic nerve tracing Compare the delineation of the optic disc and optic cup to examination or photographs, as misidentified borders will skew measurements
5. TSNIT plot Ensure that the superior and inferior RNFL peaks align with normative values and look for any area of complete loss, as values of RNFL thickness <40 μm are likely artifactual
6. Asymmetry Looks for discrepancy in average or quadrant RNFL thickness between eyes, as asymmetry can be suggestive of glaucoma
7. Thickness pattern Does the RNFL thickness pattern makes sense for glaucoma? Do thin areas on OCT correspond to findings on clinical examination and perimetry?
8. Progression Examine trends over time, including raw values, to assess disease progression
9. Refractive error and axial length Be aware that myopic eyes are more prone to algorithm error due to increased axial length and optic disc tilt
10. Prior OCT scans Make sure scan location is consistent across time points and be alert of artifacts from prior studies that could affect progression detection
Abbreviations: OCT, optical coherence tomography; RNFL, retinal nerve fiber layer; TSNIT, temporal-superior-nasal-interior-temporal.

Confirm Patient’s Name and Age

Ensure that the output file is from the correct patient and confirm that age was put correctly, as measurements are made in reference to age-matched nomograms.

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Apr 30, 2022 | Posted by in OPHTHALMOLOGY | Comments Off on 3 Optical Coherence Tomography of the Optic Nerve
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