Optic Nerve and RNFL: I Want My Imaging

Optic Nerve and RNFL:  I Want My Imaging
Coverage of the the Hawaiian Eye Meeting, January, 2009

Comments in italics are the responses of the author to the information presented.

Dr. Joel Schuman of the University of Pittsburgh presented his thoughts on optic nerve and retinal nerve fiber layer (RNFL) imaging in glaucoma at the Hawaiian Eye Meeting, January, 2009. He began with an introduction of the different technologies used in glaucoma imaging, including confocal scanning laser ophthalmoscopy (eg; HRT), scanning laser polarimetry (eg: GDX), and optical coherence tomography (OCT).

The participants in the Ocular Hypertensive Treatment Study (OHTS) were analyzed at baseline with the HRT 2 and these characteristics analyzed to see if they were predictive of which individuals progressed to glaucoma (Zangwill et al, Archives of Ophthalmology, 2005). The positive predictive value for an HRT classification “outside normal limits” was 14%, and 40% for a Moorfields Regression Analysis for superior temporal quadrant abnormality.

Another study by Lalezary et al (American Journal of Ophthalmology, 2006) studied the baseline OCT measurements to predict the development of glaucomatous change in glaucoma suspects. Their conclusions were that thinner OCT RNFL measurements at baseline were associated with development of glaucomatous change in glaucoma suspect eyes. RNFL thinning was an independent predictor of glaucomatous change, even when adjusting for optic nerve assessment, age, intraocular pressure, central corneal thickness, and pattern standard deviation on visual field.

One must interpret the findings of these tests carefully and in context, however. For example, Dr. Schuman presented a case of a 40 year old African American male with an IOP=23 mmHg, CCT=555 and a large cup to disc ratio of approximately 0.8. The HRT shows this to be outside normal limits by the Moorfields Regression Analysis in 5 out of 6 sections, with the remaining nasal section borderline. At face value, this may be a patient that many would begin treating. However, the visual field is normal, and the OCT RNFL shows a full RNFL. On closer examination of the HRT, the optic disc area is 3.95mm2, well above the normal range. This case represents one of physiologic cupping with a large optic disc resulting in a large optic cup and high C/D ratio.

While the OCT parameters of RNFL thickness can be followed for change over time in order to track glaucoma progression, he cautioned that validated, robust software for this purpose is not yet commercially available. Longitudinal studies are currently being performed (such as the Advanced Imaging in Glaucoma Study: see aigstudy.net) in order to identify those parameters that are best able to diagnose glaucoma and follow progression in the various technologies.

He then showed a case study of a patient with relatively normal looking optic nerves followed over time. On the red free photography, one can appreciate a RFL defect in the left eye infero-temporally. On GDX VCC testing, the NFI is OD=8-10, OS=18-20, but careful analysis of the color coded thickness map reveals a possible focal thinning in this area. Similarly, the HRT 2 shows a normal Moorfields Regression Analysis and Glaucoma Probability Score, but the TCA Overview Report shows some highlighting for change in this area. The Stratus OCT exhibits a full RNFL map on TSNIT analysis, but the RNFL thickness deviation map does show the RNFL defect, which lies mostly outside the scanning circle.

Newer Fourier Domain OCT scanners are developing software to examine not only the peripapillary nerve fiber layer, but also the macular RNFL. Macular scanning was found before with time domain OCT to not be useful in the diagnosis of glaucoma. However, by segmenting the retina into layers and establishing the Ganglion Cell Complex (comprised of the nerve fiber layer, ganglion cell layer, and inner plexiform layer), macular OCT scans may significantly aid in the detection of glaucomatous damage. Dr. Schuman presented examples from this case study showing the superior detection of this nerve fiber layer defect on the macular thickness scans, especially on the Ganglion Cell Complex analysis.

He concluded by remarking that “imaging technologies standardize interpretation of ocular structure at an expert level, allowing detection of subtle abnormalities and changes. By using structural imaging plus functional measures, a subtle defect or change can be confirmed, eliminating the need for repeated confirmatory retesting. Thus, optic nerve and RNFL imaging enable earlier detection of disease and progression.”

Optic nerve and retinal nerve fiber layer imaging are certainly valuable in the diagnosis of glaucoma. However, the technology is still improving, as is the ability to follow patients over time and accurately detect progression. At this time, imaging remains an important resource to the physician treating glaucoma, but it is not of itself able to detect and follow disease. The data need to be put into context, and balanced with functional testing (visual field) and clinical impression to truly practice the art of glaucoma treatment.

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