Retinal Nerve Fiber Layer (RNFL) Analysis in Glaucoma

Neil T. Choplin, MD
Eye Care of San Diego

Captain, Medical Corps, United States Navy, Retired
Adjunct Clinical Professor of Surgery
Uniformed Services University of Health Sciences

Dr. Choplin has received research and travel support from Laser Diagnostic Technologies, Inc.

Definition of Glaucoma - Rationale for Retinal Nerve Fiber Analysis

Glaucoma is a multi-factorial optic neuropathy characterized by progressive loss of retinal ganglion cells and their nerve fibers, which leads to characteristic loss of visual function. Although the exact cause for RNFL damage is not known, major risk factors for glaucoma have been identified, including intraocular pressure, heredity (family history, race), and possibly vascular dysfunction. Humans have a large reserve of retinal nerve fibers, and redundancy in the system may prevent detection of early functional damage. It has been estimated that up to 50% of fibers may be lost before a visual field defect is detectable with white-on-white perimetry. Furthermore, intraocular pressure and cup-to-disc ratio may not be sensitive or specific enough as indicators for the presence of glaucomatous optic neuropathy. Indeed, glaucoma patients may have normal intraocular pressure, and patients with elevated intraocular pressure usually do not have glaucoma. Many people with large cup-to-disc ratios do not have glaucoma (physiologic cupping), which is related to the overall size of the disc. Visual field testing may miss defects, especially early in the disease (now called “pre-perimetric glaucoma”). It makes sense, therefore, to evaluate the RNFL for the diagnosis and follow-up of glaucomatous disease, at least prior to the development of characteristic visual field loss.

A number of techniques are available for evaluation of the RNFL. These include red-free ophthalmoscopy and photography, retinal thickness analysis, optical coherence tomography, retinal topography (contour analysis) by confocal scanning laser ophthalmoscopy, and scanning laser polarimetry.

Red-free Ophthalmoscopy and Photography

Use of a red-free (green) light to evaluate the RNFL may allow the visualization of defects. This can be performed with a hand-held ophthalmoscope or at the slit-lamp with a hand-held condensing lens. A red-free filter is available for both instruments. The healthy RNFL is usually highly reflective, and defects may be seen as dark areas surrounded by the more reflective healthy tissue. These defects will connect to the optic disc, and widen peripherally, corresponding to the normal anatomy. Defects may be difficult to visualize in lightly pigmented eyes or those with media opacities. Diffuse loss of the RNFL may also be difficult to detect, unless one learns to recognize the increased clarity of the blood vessels, unmasked by the loss of the overlying RNFL.

RNFL photography requires the use of high contrast black and white film and a highly skilled photographer. Again, defects appear dark against reflective nerve fibers. This technique usually proves not to be practical in most practice situations.

Retinal Thickness Analyzer (Talia, Inc.)

This instrument was originally conceived and designed to detect retinal thickening for the diagnosis of diabetic macular edema. It projects a narrow slit (20 microns) of green laser light onto the fundus. The image is acquired on digital fundus camera and registered by a computer algorithm, providing registered thickness maps of the posterior pole and peripapillary retina. The image thus represents optical cross section of the retina, corresponding to the area between the choriocapillaris and the internal limiting membrane. Studies have suggested that loss of RNFL in the macula occurs in glaucoma. Measurement of macular thickness provides an indirect measurement of retinal nerve fiber layer thickness in the macula, since thickness of entire retina is determined. Additionally, this instrument provides some measure of optic nerve (and cup) contour.

Optical Coherence Tomography (Zeiss-Meditec, Inc.)

Retinal structures can be imaged with this instrument utilizing a near infrared 820 nm coherent diode laser light source. OCT is analogous to B-scan ultrasonography, but measures the delay of backscattered light from the tissue being imaged. Using an interferometry principle, the various layers of the imaged tissue can be differentiated. Cylindrical scans around optic nerve head can determine peripapillary retinal nerve fiber layer thickness. A normative database has been introduced to facilitate determination of RNFL abnormalities.

Confocal Scanning Laser Ophthalmoscopy

This device, typified by the Heidelberg Retina Tomograph (Heidelberg Engineering, Inc.), uses a scanning laser and a moving pinhole aperture to acquire multiple image planes. 32 planes from anterior (vitreous) to posterior (retro-laminar region) are acquired and the software then reconstructs them to create a 3-D image, which is used to determine optic disc and retinal topography. An indirect measure of RNFL thickness may be determined by measuring the height of the retinal surface above a reference plane.

Scanning Laser Polarimetry

Of the available techniques for the assessment of the RNFL in vivo, scanning laser polarimetry (SLP) is the only one to utilize a physical property of the tissue other than reflectivity (form birefringence) to provide a measure of thickness. Results utilizing SLP have been shown to be reproducible, correlate with RNFL physical properties, and to differentiate normal eyes from those with glaucoma. Form birefringence is a property of a tissue or a material that arises when the tissue is composed of parallel structures, each of which is of a smaller diameter than the wavelength of the incident light used to image it. In the case of the RNFL, form birefringence is due to the microtubules contained within the axons. A polarized light passing through a form birefringent medium undergoes phase shift. A detector in the scan head (ellipsometer) measures the retardation, which has been shown to be directly proportional to the thickness of the nerve fiber layer (approximately 7.4 microns per degree).

The currently commercially available scanning laser polarimeter is called the GDx VCC (Laser Diagnostic Technologies, Inc.). It is a tabletop unit capable of measuring retardation at approximately 16,000 points contained in a 15-degree grid centered on the optic nerve head. A proprietary compensating device is incorporated into the instrument that is designed to remove the portion of the signal which arises from birefringent structures in the eye other than the RNFL. The majority of non-RNFL birefringence comes from the cornea, and the ability to customize the compensation for an individual eye gives the instrument its name (Variable Corneal Compensation). Clinical uses of SLP may include screening for glaucoma, quantitating damage in patients with glaucoma, identifying early damage in patients at risk for glaucoma i.e. ocular hypertension, differentiating physiologic cupping from glaucomatous damage, and detecting progression (or stability) of glaucoma over time.

Summary

In the not too distant past, the diagnosis of glaucoma was based solely upon measurement of the intraocular pressure. We can call this “tonometric” glaucoma. When we realized that not all people with elevated pressure had glaucoma, and that some people could have it with normal levels of pressure, we realized that glaucoma was a disease of the optic nerve (RNFL, actually). Loss of function, manifested as visual field loss, became the hallmark of the disease, and we can call that “perimetric” glaucoma. Now we recognize that early RNFL damage precedes visual field loss, and call that “pre-perimetric” glaucoma. In the near future, perhaps we will characterize glaucoma by the technology used to detect RNFL damage. We may have terms such as “topographic,” “interferometric,” or “polarimetric” glaucoma!

We now have promising new technologies for real-time, clinical measurement of the optic disc and RNFL that are rapid, non-invasive techniques for detecting disease. These technologies may make large-scale community based screening possible, provide quantitative information for following disease progression, and allow for greater patient comfort and acceptance (at least as compared to visual field testing) with less reliance on subjective measures. Further work is needed to evaluate the effectiveness of these various methods for detecting change over time.

Additional Reading

Bathija R, Zangwill L, Berry CC, et al. Detection of early glaucomatous structural damage with confocal scanning laser tomography. J Glaucoma 1998;7:121-7.

Chauhan BC, McCormick TA, Nicolela MT, LeBlanc RP. Optic disc and visual field changes in a prospective longitudinal study of patients with glaucoma: comparison of scanning laser polarimetry with conventional perimetry and optic disc photography. Arch Ophthalmol 2001;119:1492-9.

Choplin NT, Lundy DC. The sensitivity and specificity of scanning laser polarimetry in the detection of glaucoma in a clinical setting. Ophthalmology 2001;108(5):899-904.

Choplin, NT, Lundy, DC. Differentiating patients with glaucoma from glaucoma suspects and normal subjects by nerve fiber layer assessment with scanning laser polarimetry. Ophthalmology 1998;105:2068-76.

Dreher, AW, Reiter K. Retinal laser ellipsometry: a new method for measuring the retinal nerve fiber layer thickness distribution. Clin Vision Sci 1992;7:481-8.

Guedes V, Schuman JS, Hertzmark E, et al. Optical coherence tomography measurement of macular and nerve fiber layer thickness in normal and glaucomatous human eyes. Ophthalmology 2003;110(1):177-89.

Hoh ST, Greenfield DS, Mistlberger A, et al. Optical coherence tomography and scanning laser polarimetry in normal, ocular hypertensive, and glaucomatous eyes. Am J Ophthalmol 2001;129(2):129-35.

Lee VWH, Mok KH. Retinal nerve fiber layer measurements by nerve fiber analyzer in normal subjects and patients with glaucoma. Ophthalmology 1999;106:1006-8.

Mikelberg FS, Parfitt CM, Swindale NV. Ability of the Heidelberg Retina Tomagraph to detect early glaucomatous visual field loss. J Glaucoma 1996;4:242-7.

Mistlberger A, Liebmann JM, Greenfield DS, et al. Heidelberg retina tomograph and optical coherence tomography in normal, ocular-hypertensive, and glaucomatous eyes. Ophthalmology 1999;106(10):2027-32.

Schuman JS, Hee MR, Puliafito CA, et al. Quantification of nerve fiber layer thickness in normal and glaucomatous eyes using optical coherence tomography. Arch Ophthalmol 1995;113(5):586-96.

Schuman JS, Lemij HG, eds. The Shape of Glaucoma - Quantitative Neural Imaging Techniques. Kugler Publications, The Hague, The Netherlands, 2000.

Tjon-Fo-Sang MJ, de Vries J, Lemij HG. Measurement by nerve fiber analyzer of retinal nerve fiber layer thickness in normal subjects and patients with ocular hypertension. Am J Ophthalmol 1996;122:220-7.

Tjon-Fo-Sang MJ, de Vries J, Lemij HG. The sensitivity and specificity of nerve fiber layer measurements in glaucoma as determined with scanning laser polarimetry. Am J Ophthalmol 1997;123:62-9.

Trible J, Schultz RO, Robinson JC, RotheTL. Accuracy of scanning laser polarimetry in the diagnosis of glaucoma. Arch Ophthalmol 1999;117:1298-304.

Weinreb RN, Bowd C, Zangwill L. Glaucoma detection using scanning laser polarimetry with variable corneal polarization compensation. Arch Ophthalmol 2992;120:218-224.

Weinreb RN, Shakiba S, Zangwill L. Scanning laser polarimetry to measure the nerve fiber layer of normal and glaucomatous eyes. Am J Ophthalmol 1995;119:627-36.

Weinreb RN, Zangwill L, Berry CC, et al. Detection of glaucoma with scanning laser polarimetry. Arch Ophthalmol 1998;116(12):1583-9.

Zangwill LM, Bowd C, Berry CC, et al. Discriminating between normal and glaucomatous eyes using the Heidelberg Retina Tomograph, GDx Nerve Fiber Analyzer, and Optical Coherence Tomography. Arch Ophthalmol 2001;119(7):985-93.

Zeimer R, Asrani S, Zou S, et al. Quantitative detection of glaucomatous damage at the posterior pole by retinal thickness mapping. A pilot study. Ophthalmology 1998;105(2):224-31.