Optic nerve and nerve fiber layer imaging

ByChristopher A. Girkin, MD, MSPH

Instruments to image and analyze the optic disc and nerve fiber layer are some of the most valuable tools in an ophthalmologist’s armamentarium. Among these technologies are retinal tomography, scanning laser polarimetry and optical coherence tomography (OCT).

Clinical studies of these technologies confirm how information from these tests can best be applied in clinical practice. This article reviews advances in imaging technologies and compares their efficacy for diagnosing glaucoma and for monitoring the progression of glaucoma.

Heidelberg Retinal Tomograph II

The Heidelberg Retinal Tomograph II (HRT, Heidelberg Engineering, Vista, Calif.) uses confocal scanning laser ophthalmoscopy. A laser scans the surface of the retina to measure reflectance from the vitreoretinal interface through a confocal aperture, allowing an ophthalmologist to capture serial optical slices that can be stacked together to provide a height map of the instrument’s target.

A multimodal technique that can potentially examine topography for the optic nerve and retinal nerve fiber layer integrity would be of clinical use.
— Christopher A. Girkin, MD, MSPH

Christopher A. Girkin, MD, MSPH [photo]

The current HRT software places a reference plane below the papillomacular bundle height. Above the reference plane is considered neuroretinal rim; below is considered optic cup. Retinal nerve fiber layer thickness is inferred as the distance between the reference planes. Ophthalmologists must accurately draw a contour line around the optic disc to determine the measured retinal surface height.

A new HRT technique employs several modeling approaches to examine the optic disc shape. A Gaussian curvature and quadratic function measure the surface height of the retinal nerve fiber layer and shape of the optic disc, producing a variety of parameters automatically obtained through the HRT (Figure 1). The parameters are summated through a neural network system that has been tested in a small trial to distinguish between normal and glaucoma.¹

The technique as used by Swindale and colleagues does not require a contour line and is reference plane independent.¹ In the initial study, it appeared equivalent to the Moorfield’s regression classification for glaucoma classification and is available on the current version of the HRT II and HRT III software.

Figure 1. The Heidelberg Retinal Tomograph II employs several modeling techniques to examine the optic disc..

Source: Girkin CA

Scanning laser polarimetry and OCT

Scanning laser polarimetry measures retardation or shift in polarization of the light. Light reflects from the retinal pigment epithelium to the instrument, passing through the retinal nerve fiber layer, which functions as a polarizing filter. An ellipsometer measures the amount of retardation, which is directly proportional to retinal nerve fiber layer thickness; subsequently the instrument measures the axis and magnitude of polarization shift.

One scanning laser polarimeter, the GDx- variable corneal compensator (GDx-VCC, Carl Zeiss Meditec, Dublin, Calif.), scans the macula to produce an estimate of anterior segment birefringence. By subtracting the anterior segment estimate from the measurement of peripapillary birefringence, the GDx-VCC gives the ophthalmologist an accurate measurement of the retardation induced by the peripapillary nerve fiber layer (Figure 2).

OCT uses a Michelson inferometer to measure the echo time delay and backscattered light. The technique is similar to ultrasound, only measuring light instead of sound. The end product is high resolution cross-sectional images of the retina. Ophthalmologists can use OCT for optic nerve head imaging and peripapillary imaging.

Figure 2. Accurate measurements of the nerve fiber layer can be taken using GDx-VCC.

Source: Girkin CA.

Comparisons

When examining the nerve head, HRT, OCT and an ophthalmologist’s subjective assessment return similar cup:disc ratios. HRT tends to yield a smaller measurement of cup:disc ratio, whereas OCT yields a larger measurement; the difference is caused by how the instruments define cup:disc ratio.

Greaney and colleagues compared qualitative assessment of optic nerve head stereophotographs by glaucoma specialists and original versions of HRT, OCT and GDx to distinguish normal eyes from those with early-to-moderate glaucomatous visual field defects.² They found similar efficacy for glaucoma diagnosis using the best parameters of each technique. Sensitivity ranged from 82 with OCT to 94 for photographic analysis. Specificity ranged from 84 with OCT to 90 with HRT.

In a similar study, Medeiros and colleagues, using the most recent version of the instruments, found GDx, HRT and OCT detected glaucoma with similar efficacy.³

Based on this study, all the instruments provide important information about the glaucomatous process, but each provides different information; therefore, a multimodal technique that can potentially examine topography for the optic nerve and retinal nerve fiber layer integrity would be of clinical use.

HRT, OCT and GDx provide quantitative reproducible information that can assist ophthalmologists in monitoring glaucomatous progression. However, although all of the instruments provide quantitative, reproducible information about the optic nerve and peripapillary nerve fiber layer, ophthalmologists require more longitudinal studies comparing HRT, OCT and GDx in the same cohort to best determine how to use these instruments in detecting progression in glaucoma.

References

Swindale NV, Stjepanovic G, Chin A, Mikelberg FS. Automated analysis of normal and glaucomatous optic nerve head topography images. Invest Ophthalmol Vis Sci. 2000;41:1730-1742.
Greaney MJ, Hoffman DC, Garway-Heath DF, et al. Comparison of optic nerve imaging methods to distinguish normal eyes from those with glaucoma. Invest Ophthalmol Vis Sci. 2002;43:140-145.
Medeiros FA, Zangwill LM, Bowd C, Weinreb RN. Comparison of the GDx VCC scanning laser polarimeter, HRT II confocal scanning laser ophthalmoscope and Stratus OCT optical coherence tomograph for the detection of glaucoma. Arch Ophthalmol. 2004;122:827-837.