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ORIGINAL ARTICLE
Year : 2017  |  Volume : 4  |  Issue : 3  |  Page : 82-88

Comparison of retinal nerve fiber layer thickness in relation to optic nerve head size and average retinal nerve fiber layer thickness using optical coherence tomography in glaucomatous optic neuropathy


Department of Optometry, College of Health Sciences, University of Buraimi, Al Buraimi, Sultanate of Oman

Date of Web Publication14-Sep-2018

Correspondence Address:
Dr. Janitha Plackal Ayyappan
P.O. Box 890, P.C. 512, Al Buraimi
Sultanate of Oman
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DOI: 10.4103/bijo.bijo_11_16

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  Abstract 


Context: This study is conducted to determine the correlation between retinal nerve fiber layer (RNFL) thickness and optic nerve head (ONH) size in normal and glaucomatous subjects by optical coherence tomography (OCT).
Objective: The objective of this study is to compare the thickness of RNFL in relation to ONH size and average retinal nerve fiber thickness in glaucomatous optic neuropathy using OCT.
Subjects and Methods: A prospective study was carried out on one hundred and fifty eyes of 75 patients with 39 normal and 36 glaucoma patients aged between 20 and 60 years.
Results: Stratus OCT, values were correlated with the data obtained by ONHs analysis. The values of RNFL thickness showed a high correlation with the ONH size. The RNFLA values were also positively correlated with the *2.27 value. Mean RNFL in glaucoma patients is 72.20 ± 18.79 (standard deviation [SD]) and in *2.27 is 63.57 ± 17.3 (SD) (calculated Z value is 2.58 and P ≤ 0.004) and is highly significant at 4% level in glaucoma patients. Besides correlation coefficient of RNFLA*2.27 in glaucoma patients found positive correspondence with the correlation coefficient 0.867. In normal subject, mean RNFL is 92.16 ± 9.80 and in *2.27, 85.16 ± 10.21 (calculated Z value is 4.34 and P ≤ 0.007) and significant at 3% level in normal subjects. A negative correlation was seen with the value of correlation coefficient − 0.21.
Conclusions: RNFL thickness *2.27 measurement has high correlation with each other as obtained by Stratus OCT; moreover, it decreased significantly with an increase in optic disc size.
Statistical Analysis Used: To test the difference between two means, Z-test was used. “Z” distribution with population variance not known at that point of time because no normative-based population data were available for Indian population to find out the correlation between two variance correlation coefficient at that point of time.

Keywords: Glaucoma, optic disc, optical coherence tomography, retinal nerve fiber layer (RNFL), retinal nerve fiber layer analysis (RNFLA)


How to cite this article:
Ayyappan JP, Khurshid F. Comparison of retinal nerve fiber layer thickness in relation to optic nerve head size and average retinal nerve fiber layer thickness using optical coherence tomography in glaucomatous optic neuropathy. Albasar Int J Ophthalmol 2017;4:82-8

How to cite this URL:
Ayyappan JP, Khurshid F. Comparison of retinal nerve fiber layer thickness in relation to optic nerve head size and average retinal nerve fiber layer thickness using optical coherence tomography in glaucomatous optic neuropathy. Albasar Int J Ophthalmol [serial online] 2017 [cited 2018 Sep 22];4:82-8. Available from: http://www.bijojournal.org/text.asp?2017/4/3/82/241120




  Introduction Top


New ocular imaging technologies such as optical coherence tomography (OCT) enables clinicians to perform accurate, objective, and reproducible measurements of the retinal nerve fiber layer (RNFL) and optic nerve head (ONH) topography.[1]

OCT is a high-resolution reproducible imaging technology that has been widely used to evaluate the RNFL thickness in patients with or without glaucoma. Images are obtained by aid of low coherence near-infrared light (850 nm) from a superluminescent diode and subsequent backscattering from the retina. Recent software enhancement also allows ONH analysis.[2]

To assess RNFL thickness, a circular scan concentric to the ONH is performed. In 1996, Schumann et al. found a circle diameter of 3.4 mm to be the most accurate in terms of reproducibility and all studies since then have used circular scans with this diameter, independent of ONH size. However, it is generally recognized that the optic disc size shows a high interindividual variability in normal eyes and its area may range between 0.8 and 6.0 mm. Similarly, a high variability has been found in other ONH parameters, such as the optic disc vertical (range 0.96–2.91 mm) and horizontal (range 0.91–2.61 mm) diameters and neuroretinal rim size (range 0.8–4.66 mm2).[3]

As a consequence, using the same, fixed diameter circular scan in all eyes may result in RNFL thickness measurements performed at different distances from the ONH margin.[4],[5] Since histological studies have already demonstrated that RNFL thickness decreases with increasing distance from the disc margin, we were prompted to investigate how RNFL thickness, as measured by OCT, may be influenced by the optic disc size and thus by the distance between the circular scan and the ONH margin. RNFL examination is important in diagnosing and monitoring the progress of glaucoma. Damage to RFNL mostly precedes visual-field loss.[6] Hence, objective methods of measuring RNFL thickness may aid physicians in making an accurate and early diagnosis. Recent advancement in various imaging technologies has made assessment of RFNL possible. These methods utilize the optical property of RNFL for obtaining the quantitative RFNL thickness measurements. These techniques are rapid have good reproducibility and objectivity.[7]

OCT is a noncontact, noninvasive diagnostic technique that provides detailed structural information of the posterior segment. It shows a cross-sectional living histology of retina with high resolution (of approximately 10 μ) than most of the conventional imaging systems.[8] It also has a high reproducibility. OCT allows direct measurement of RNFL thickness by in vivo visualization of retina and RNFL. A high reflectance layer located just under the inner surface of the retina that corresponds to the RFNL is measured using a computer-fed algorithm to generate the RFNL measurement. OCT-generated morphologic findings in experimental animals have been shown to correspond very well with histological findings. RNFL has been demonstrated to have considerable inter-individual variation.[9] This variation can be age or race related. RNFL measurement varies with the technique used. The measurement may also differ with the population used as a database.[10],[11] It would therefore be preferable to use the values derived from a normative population as close as possible to the population for which the instrument is used.[12] To the best of our knowledge, there are no reported data for comparison of average RFNL thickness and RFNL thickness (2.27*disc size) in glaucoma patients and normal subjects in Indian eyes (Medline Search) [Figure 1].
Figure 1: Diagram shows a flowchart of the events seen in retinal nerve fiber layer in glaucoma

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Optical coherence tomography acquisition protocol: Retinal nerve fiber layer thickness (3.4)/fast retinal nerve fiber layer thickness (3.4)

This protocol acquires scans of three concentric circles of diameter 3.4 mm around the optic disc and measures the RNFL thickness. Average thickness can be measured. Fast protocol combines 3 RNFL thickness scans into one scan. The size or number of scans cannot be altered. The scans are used for RNFL thickness analyses protocol.[13]

Retinal nerve fiber layer thickness (2.27@disc)

This protocol acquires a single circle scan around the optic disc that is 2.27 times the radius of the aiming circle. The average diameter of optic disc is 1.5 mm and the standard circle to measure the RNFL around it is 3.4 mm in diameter. The ratio of these two is 2.27, from which the multiplication factor is derived. This scan allows variations in the optic disc size where scans can be acquired adjusting the aiming circle size.[13]

Aims

  1. The purpose of the study was to comparatively investigate and evaluate the RNFL thickness in relation to ONH size and average RNFL thickness using OCT in glaucomatous optic neuropathy patients
  2. To comparatively evaluate the RNFL thickness in relation to ONH size and average RNFL thickness using OCT in normal patients.


Objectives

  1. To comparatively evaluate the RNFL thickness in relation to ONH size and average RNFL thickness using OCT in glaucomatous optic neuropathy patients
  2. Comparing average RNFL thickness *2.27 in normal subjects
  3. Comparing average RNFL thickness *2.27 in glaucoma patients.



  Subjects and Methods Top


A total of 150 eyes of 39 normal subjects and 36 glaucomatous optic neuropathy subjects, aged between 20 and 60 were enrolled in this study. Each subject was informed of its purpose and provided written consent to participate. All subjects underwent basic eye examinations such as distance visual acuity testing, ocular motility evaluation, anterior segment slit lamp examination, Goldman applanation tonometry, gonioscopy and fundus examination with plus 90D lens, retinal examination using indirect ophthalmoscope with help of +20.00 diopter condensing lens, automated refraction (Retinomax 2 Autorefractor, Nikon Corp., Japan) and no significant ocular disease found by routine ophthalmological examination, and/or systemic diseases with possible ocular involvement, such as diabetes mellitus. In addition, all glaucoma patients undergone reliable measurements of visual-field examination with (fixation loss <20%, false negative and false positive <25%, mean deviation and corrected pattern standard deviation within 95% normal limits, and a glaucoma hemifield test result within normal limit) performed using the central 24-2 or 30-2 program of the frequency doubling technology advance version of Humphrey visual field analyzer (Allergen-Humphrey, San Leandro, CA, USA).

Inclusion criteria

The inclusion criteria were normal subjects and glaucomatous patients aged between 20 and 60 years, with refractive error between −4 and +4 diopter of sphere or between −2 and +2 diopter of cylinder with normal intraocular pressure <21 mmHg, normal appearance of the optic disc, as well as glaucomatous disc.

Exclusion criteria

ONH size <1.5 mm, high myopia >4.00 diopter, high hyperopia >4.00 diopter, high astigmatism >4.00 diopter, systemic diseases (hypertension and diabetes); patients older than 60 were intentionally excluded to minimize the influence of age and lens opacities on RNFL thickness measurement.

Optical coherence tomography measurements

OCT 3000 (Stratus OCT, software version 4.06; Carl Zeiss Meditec, Inc., Dublin, CA, USA) was used to measure both the thickness of the RNFL and ONH size [Figure 2]. Each eye was dilated with Tropicamide 1% before recording the images, and scans were performed with a minimum pupillary diameter of 5 mm. The internal fixation target was used owing to its higher reproducibility, and the examination was performed under mydriasis by two experienced operators. Fast RNFL thickness protocol, as well as RNFL thickness (2.27*disc size) was used. It consists of three circular scans each of 3.46 mm in diameter centered on the optic disc. This diameter has been shown to be optimal and reproducible for RNFL thickness analysis. Mean RNFL thickness was calculated using the inbuilt RNFL thickness average analysis protocol.[13] Retinal thickness was measured using the location of the vitreoretinal interface and the retinal pigment epithelium defining the inner and outer boundaries of retina, respectively. These are seen as sharp edges with high reflectivity. The boundaries of RNFL were defined by first determining the thickness of the neurosensory retina; various parameters were employed for the evaluation of RNFL thickness. The important ones included RNFL average thickness over the entire cylindrical section and RNFL thickness (2.27*disc size). Eyes that fulfilled both exclusion and inclusion criteria were selected for analysis; if both eyes fulfilled the criteria, only the eye with better image quality and higher signal to noise ratio was used for analysis.
Figure 2: Screen image showing the optic nerve head analysis

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Stratus OCT employs low coherence interferometry to generate cross-sectional images of the retina, [Figure 3] RNFLA thickness] optic disc and RNFL with ≤10 μm axial resolution and transverse resolution of 20 μm. The instrument contains an interferometer that resolves posterior pole structures by measuring the echo delay time of light that is reflected and backscattered from different layers in the retina and optic disc. The RNFL thickness algorithm searches for the RNFL in a two-pass process. It looks first for the highest rates of changes in reflectivity at the vitreoretinal interface and then for reflectivity above a threshold value in its adjacent highly reflective layer. The threshold is individually determined for each scan as a multiple of the local maximum reflectance to adjust for variations in optical alignment or drying of the corneal surface or changes in pupil size.
Figure 3: Screen image showing the retinal nerve fiber layer thickness

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The nerve fiber layer thickness is defined as a multiple of the number of pixels between the anterior and posterior edges of the RNFL.

To assess RNFL thickness, measurements were made along a circle concentric with the optic disc at a radius of 1.73 mm, using a scanning mode that samples 512 data points (RNFL Thickness 3.4 acquisition protocol). Three measurements were performed for each eye, and the mean values were recorded. For each eye, we studied the average RNFL thickness and RNFL thickness (2.27*disc size), all automatically calculated by OCT using the existing software.

ONH evaluation consisted of six radial scans centered on the ONH that were spaced 30° apart (Fast Optic Disc acquisition protocol). Each radial scan included 128 points. The machine automatically defined the edge of the optic disc as fit to circle to fill the gaps between scans. The resultant image could be manually corrected when the machine did not identify the edge correctly. A straight line connected the edges of the retinal pigment epithelium/choriocapillaris and a parallel line was constructed 150 μm anteriorly. Structures below this line were defined as the disc cup and above this line as the neuroretinal rim. Among the measurements given by the OCT ONH analysis, the following were examined: disc area, cup area, rim area, cup/disc vertical and horizontal ratios, and vertical and horizontal disc diameters [Figure 4].[12],[13]
Figure 4: Diagram showing optic nerve head

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The OCT data were exported to a personal computer, and left eye data were converted into the right eye format. Only good quality OCT data as judged by the appearance of the RNFL and the optic disc pictures were used for further analysis. Images with artifacts, missing parts, or showing seemingly distorted anatomy were excluded from the study. Considering RNFL measurements since the position of the circular scan with respect to the optic disc is crucial, we included only images where the ONH was well centerd by the scan. In the case of ONH analysis, if necessary, the manual option was used to correct a displaced ONH margin.


  Results Top


One hundred and fifty eyes of 75 patients were included in the study. Out of them, 39 were normal, and 36 were glaucoma patients. The average RNFL thickness in glaucoma patient was 67.8 ± 18.42 (standard deviation [SD]) whereas in normal subjects it was found 88.66 ± 10.57 (SD). However, there was a high negative correlation found in normal subjects having RNFLA and *2.27 with a correlation coefficient of −0.12. While in Glaucoma patient, a high positive correlation of RNFLA *2.27 was observed with a correlation coefficient of 0.86. Furthermore, no difference between RNFL A and *2.27 in glaucoma patients was found statistically.

While mean RNFL in Glaucoma patients was 72.20 ± 18.79 (SD) and in *2.27, 63.57 ± 17.3 (SD) (calculated Z value is 2.58 and P ≤ 0.004) and was highly significant at 4% level in glaucoma patients. Conversely, in normal patients, mean RNFL is 92.16 ± 9.80 (SD) and in *2.2, 85.16 ± 10.21 (SD) with calculated Z value is 4.34 and P ≤ 0.007 and significant at 3% level in normal patients. There was no statistical significance found in average RNFL thickness *2.27 in relation to ONH size; which was found to decrease in normal as well as in glaucoma patients. The Z-test was performed to establish the relationship between RNFLA and *2.27 in this study [Graph 1].



There is definite correlation shows between the RNFLA*2.27 of ONHs 1.2 mm. The mean value shows RNFLA *2.27 72.20–63.57 ± 8.63 [Graph 2].



There are definite correlation shows between the RNFLA*2.27 of ONHs 1.2 mm. The mean value shows RNFLA *2.27 89.96–74.68 ± 15.28 [Graph 3].



There is negative correlation was found in normal subjects, and the value is −0.12 [Graph 4].



There was a positive correlation was found in glaucoma patients, and the value is 0.867.

In this study, we used a comparative analysis for glaucoma patients with ONHs at 0.8, 0.9, 1.0, 1.1 mm and normal subjects with ONHs at 0.8, 0.9, 1.0, and 1.2 mm in relation to RNFLA*2.27, respectively. The result stated that definitive correlation exist between RNFLA*2.27 of ONHs 1.2 mm with a mean value 72.20–63.57 ± 8.63 (SD) in Glaucoma patient. Similarly, a definitive correlation was observed between RNFLA*2.27 of ONHs 1.2 mm with a mean value 97.53–87.46 ± 10 (SD) in normal subject.


  Discussion Top


This study shows that average RNFL thickness *2.27 measured by Stratus OCT is positively correlated with ONH size, as determined by measurements of its diameter. Such a correlation could be clearly observed in glaucoma patients as well as normal subjects, but statistical significance was not achieved. More data need to be collected for each observation in glaucoma patients as well as normal subjects to collect a normative population of data in Indian eyes, and more studies should be done to evaluate the correlation between each parameter. From a clinical point of view, it is important to observe that if larger discs really do contain more retinal ganglion cell axons, they may benefit from a higher anatomic reserve capacity in progressive optic neuropathies. Second, we may consider our findings in a different perspective and interpret them as an artifact of the OCT methodology, owing to the fact that the circular scan has a fixed diameter of 3.4 mm, as suggested by the previous studies.[2] It is likely that the positive correlation between the ONH parameters and RNFL thickness depends on the distance between the OCT circular scan and the ONH margin. RNFL thickness, in fact, has been shown to decrease at increasing distances from the ONH [Figure 2].[2] If a fixed diameter circular scan is employed, the distance between the scan and the ONH margin will obviously be reduced in the presence of a large ONH. Such an artifact may lead to an overestimation of RNFL thickness in patients with large ONHs since the measurements would be made closer to the optic disc edge. Should future studies confirm this hypothesis, it might become necessary to individually adjust the analysis of RNFL thickness according to ONH size; in other words, the diameter of the circular scan should be customized according to optic disc area or diameter. As an alternative, any patient undergoing RNFL analysis by OCT for clinical purposes should be compared to a control group made up of individuals matched for ONH size. Finally, the validity of current OCT databases may be subject to challenge and the data from many, if not all, studies performed using OCT should, in any case, be carefully reevaluated. Vazirani et al. (2015) demonstrated that despite the reproducibility of RNFL measurement taken in four different quadrants, there exists substantial variation from the value of the mean RNFL thickness. They proposed that while assessing the structural progression of advanced stages of glaucoma patients, the average RNFL thickness should be taken into account.

They signified that the baseline measurement of progressive glaucoma taken on the Stratus OCT can be used to follow-up the advanced stage patient of glaucoma on the Cirrus OCT.]3]

The differences in average RNFL thickness *2.27 in glaucoma patient found high-positive correlation with each other. However, RNFL thickness *2.27 in normal patients also found high negative correlation with each other and shows both parameters are not statistically significant.


  Conclusions Top


This study shows there is a good positive correlation between RNFL average and *2.27 in glaucoma and negative correlation was found in normal. Further studies with large scale of data collection of parameters should be done to evaluate both the hypothesis are correct. This can serve as a useful guideline in diagnosis, management, and research in glaucoma.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
  References Top

1.
Quigley HA, Miller NR, George T. Clinical evaluation of nerve fiber layer atrophy as an indicator of glaucomatous optic nerve damage. Arch Ophthalmol 1980;98:1564-71.  Back to cited text no. 1
    
2.
Jonas JB, Schmidt AM, Müller-Bergh JA, Schlötzer-Schrehardt UM, Naumann GO. Human optic nerve fiber count and optic disc size. Invest Ophthalmol Vis Sci 1992;33:2012-8.  Back to cited text no. 2
    
3.
Vazirani J, Kaushik S, Pandav SS, Gupta P. Reproducibility of retinal nerve fiber layer measurements across the glaucoma spectrum using optical coherence tomography. Indian J Ophthalmol 2015;63:300-5.  Back to cited text no. 3
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Soliman MA, Van Den Berg TJ, Ismaeil AA, De Jong LA, De Smet MD. Retinal nerve fiber layer analysis: Relationship between optical coherence tomography and red-free photography. Am J Ophthalmol 2002;133:187-95.  Back to cited text no. 4
    
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Blumenthal EZ, Williams JM, Weinreb RN, Girkin CA, Berry CC, Zangwill LM, et al. Reproducibility of nerve fiber layer thickness measurements by use of optical coherence tomography. Ophthalmology 2000;107:2278-82.  Back to cited text no. 5
    
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Savini G, Zanini M, Carelli V, Sadun AA, Ross-Cisneros FN, Barboni P, et al. Correlation between retinal nerve fibre layer thickness and optic nerve head size: An optical coherence tomography study. Br J Ophthalmol 2005;89:489-92.  Back to cited text no. 6
    
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Carpineto P, Ciancaglini M, Zuppardi E, Falconio G, Doronzo E, Mastropasqua L, et al. Reliability of nerve fiber layer thickness measurements using optical coherence tomography in normal and glaucomatous eyes. Ophthalmology 2003;110:190-5.  Back to cited text no. 7
    
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Bowd C, Weinreb RN, Williams JM, Zangwill LM. The retinal nerve fiber layer thickness in ocular hypertensive, normal, and glaucomatous eyes with optical coherence tomography. Arch Ophthalmol 2000;118:22-6.  Back to cited text no. 8
    
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Varma R, Bazzaz S, Lai M. Optical tomography-measured retinal nerve fiber layer thickness in normal latinos. Invest Ophthalmol Vis Sci 2003;44:3369-73.  Back to cited text no. 9
    
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Hoh ST, Greenfield DS, Mistlberger A, Liebmann JM, Ishikawa H, Ritch R, et al. Optical coherence tomography and scanning laser polarimetry in normal, ocular hypertensive, and glaucomatous eyes. Am J Ophthalmol 2000;129:129-35.  Back to cited text no. 10
    
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Mastropasqua L, Carpineto P, Ciancaglini M, Falconio G, Harris A. Reproducibility of nerve fiber layer thickness measurements using optical coherence tomography in silico ne oil-filled eyes. Ophthalmologica 2001;215:91-6.  Back to cited text no. 11
    
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Liu X, Ling Y, Zhou W, Zheng X, Liang D. Qualitative and quantitative measurement of retinal nerve fiber layer in primary open angle glaucoma by optical coherence tomography. Zhonghua Yan Ke Za Zhi 2000;36:420-4, 28.  Back to cited text no. 12
    
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Prabhudesai M. Atlas Of Optic Nerve Head Evaluation In Glaucoma. 1st ed. New Delhi, India: Jaypee Brothers Medical Publishers; 2006.  Back to cited text no. 13
    


    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4]



 

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