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ORIGINAL ARTICLE
Year : 2014  |  Volume : 5  |  Issue : 1  |  Page : 5-10

Analysis of retinal nerve fiber layer thickness using optical coherence tomography in normal South Indian population


1 Department of Ophthalmology, Father Muller Medical College, Kankanady, Mangalore, Karnataka, India
2 Sankara Eye Hospital, Guntur, Andhra Pradesh, India
3 Department of ENT, Kasturba Medical College, Mangalore, Karnataka, India

Date of Web Publication15-Mar-2014

Correspondence Address:
V Sowmya
Department of Ophthalmology, Father Muller Medical College, Kankanady, Mangalore - 575 002, Karnataka
India
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/0975-9727.128933

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  Abstract 

Introduction: The high resolution optical coherence tomography (OCT) is a new technique, which allows precise measurement of retinal thickness as well visualization of intraretinal layers, particularly the retinal nerve fiber layer (RNFL). RNFL is a very sensitive structure, which gets damaged in various disease processes. Spectral domain OCT has been recently introduced in India and the normative profile of various measurements has not been established for the Indian population. Purpose of this study is to use OCT to evaluate the peripapillary variation in RNFL thickness in normal South Indian population. Materials and Methods: The study groups included 60 eyes of 30 normal individuals who underwent RNFL analysis using Topcon three-dimensional-OCT 2000. Descriptive statistical analysis has been carried out in the present study and significance was assessed at 5% level of significance. Paired t-test was used to obtain the P value. Results: In this study, average nerve fiber layer thickness along the 3.4-mm-diameter circle around the optic nerve head was 111.75 ± 4.83 μm. RNFL thickness was found to be more in the inferior quadrant followed by superior, nasal and temporal quadrant, suggesting that ISNT rule does apply to this subgroup of Indian population. Conclusion: This study concludes that RNFL thickness can be measured effectively using spectral domain OCT and should be considered while evaluating patients for diagnosis and follow-up of glaucoma. The normative data provided by this study may assist in identifying changes in RNFL thickness in glaucoma and other optic nerve diseases in South Indian population.

Keywords: Optic nerve diseases, optical coherence tomography, retinal nerve fiber layer, South Indian population


How to cite this article:
Sowmya V, Venkataramanan V R, Vishnu Prasad K P. Analysis of retinal nerve fiber layer thickness using optical coherence tomography in normal South Indian population. Muller J Med Sci Res 2014;5:5-10

How to cite this URL:
Sowmya V, Venkataramanan V R, Vishnu Prasad K P. Analysis of retinal nerve fiber layer thickness using optical coherence tomography in normal South Indian population. Muller J Med Sci Res [serial online] 2014 [cited 2021 Dec 2];5:5-10. Available from: https://www.mjmsr.net/text.asp?2014/5/1/5/128933


  Introduction Top


The retinal nerve fiber layer (RNFL) (nerve fiber layer, stratus opticum) is formed by the expansion of the fibers of the optic nerve. The nerve fiber layer is thickest at the nasal edge of the disc, where it measures 20-30 μm. The temporal part of the optic disc receives nerve fiber only from a small part of the retina because of which the thickness of the nerve fiber layer is reduced to 10 μ here. [1] According to histological studies, the peripapillary RNFL contour follows a double hump pattern whereby the RNFL thickness is greatest at the inferior and superior poles of the optic disc and thinnest at the temporal and nasal disc margins. [2],[3]

RNFL is a sensitive structure. Certain processes can excite its natural apoptosis. [4] Factors such as age, [5],[6],[7] gender, [8] axial length, [9],[10],[11] size of the optic disc, refractive status of the eye, [9],[10],[11] ethnicity/race [12] are shown to affect the RNFL thickness, as well as situations such as raised intra ocular pressure (IOP), intraocular inflammation, vascular diseases and any kind of hypoxia can damage the RNFL. [13] Landmark studies by Sommer et al. and Quigley et al. have shown that RNFL defects precede visual field loss and therefore, examination of the RNFL yields important diagnostic information. [13],[14] Several studies by Sommer et al. demonstrated that visible atrophy of the NFL preceded visual field loss by as much as 5 years and was at least as accurate in predicting further damage as optic disc examination. [15],[16]

Although RNFL can be directly visualized and imaged in vivo, using red free fundus ophthalmoscopy and photography, these techniques allow, so far, subjective and qualitative evaluation of the RNFL but not quantitative thickness measurement, also these measurements are variably reproducible. There is wide inter-observer and sometimes, intra-observer variability in between different examinations. [17] With the introduction of the optical coherence tomography (OCT) objective measurement of the RFNL thickness may be indirectly estimated in vivo.

OCT is a non-invasive, non-contact imaging technique which produces cross-sectional images with millimeters penetration (approximately 2-3 mm in tissue) and micrometer scale axial and transverse resolution of not only the retina and optic nerve, but also the anterior segment of the eye. The high resolution of OCT allows precise measurement of retinal thickness as well-visualization of intraretinal layers, particularly the RNFL. [18],[19] The technique was first demonstrated in 1991 with ~30 μm axial resolution. With advancement in technology, today we have the conventional OCT with an axial resolution of ≤10 μm and the three-dimensional OCT (3D-OCT) with a much higher resolution of 5 μm.

The principle of OCT involves a low coherence infrared (843 nm) diode light source which is divided into reference and sample paths. Reflected sample light from the subject's eye creates an interference signal with the reference beam, which is detected by fiberoptic interferometer.

OCT uses low coherence near infrared light (830 nm) from a super luminescent diode laser, which is transmitted to the retina through a fiber optic delivery system. Backscatter from the retina is captured and resolved using a fiber optic interferometer. Modulating the reference mirror allows longitudinal data to be extracted. Cross-sectional OCT images of the retina are constructed from the backscattering information provided by multiple individual axial A scans. Cross-sectional images of the retina and disc are then constructed from a sequence of signals similar that of an ultrasound B mode. Instead of sound waves, however, the OCT uses low coherence light to quantify RNFL thickness, by measuring the difference in delay of backscattered light from the RNFL inside imaged tissue. RNFL can be differentiated from other retinal layers with the algorithm that detects the anterior edge of retinal pigment epithelium and determines the photoreceptor layer position. A digitalized, composite image of the multiple A scan is produced on a monitor with a false color scale representing the degree of light scattering from tissues at different depths within the retina. [20]

Image may be acquired using either a linear or circular scanning beam. Scanning acquisition time is approximately 1 s. A circular scan of RNFL is generally performed with a diameter of 3.4 mm in order to avoid areas of peripapillary atrophy. The stratus interferometer captures, with each scan pass, between 128 and 768 longitudinal (axial) samples or "A-scans". Each A-scan has 1024 points covering 2 mm depth. Each resulting image consists of RNFL thickness measurement of 360° circular area around the optic disc. [19] A computer algorithm identifies and demarcates the signal corresponding to the RNFL and measurements of mean RNFL thickness in terms of quadrants and individual clock hours are calculated. Normal RNFL thickness is characterized by a "double-hump" appearance corresponding to the increased thickness along the superior and inferior poles of the optic nerve head. [18],[21] RNFL thickness is displayed as averaged over quadrant (superior, inferior, temporal, nasal) or over clock hour or individually. The cross-sectional structure of the nerve fiber layer is displayed "unwrapped" as a flat image on the page.

New 3D-OCT instruments (Topcon - 3D-OCT-1000, OPTOVUE - RTVue-100, OPTOPOL - SOCT Copernicus) are cutting edge ophthalmic imaging solutions that merge technology, speed, quality and clinical versatility. These devices are 50 times faster than conventional OCT. Instead of adjusting the position of the reference mirror, the Topcon 3D-OCT - 1000 OCT system records the interferometric information using Fourier domain spectrometric method, allowing for an increase in the scanning speed (~18,000 A-scan/s). Through integration with the Topcon non-mydriatic retinal camera, 3D-OCT-1000 provides high resolution cross-sectional B-scan OCT images (up to 4096 resolution) and 3D volumetric images to cover up to 6 mm × 6 mm of the retinal area. The acquired data from 3D-OCT is further viewed and analyzed by the software. 3D-OCT 2000 is a newer spectral domain OCT of Topcon, which has anterior segment imaging incorporated in it.

Knowing normal RNFL thickness is very important before interpreting OCT results. Since 3D-OCT devices are newly introduced to India with their wide application in our daily practice to diagnose as well as follow-up glaucoma and other optic nerve diseases, it is important that we develop a normative data for our population. The purpose of this study was to assess the RNFL thickness in normal south Indian population.


  Materials and Methods Top


The study was carried out at Sankara Eye Hospital, Guntur, during the period of March 2010 to February 2012. 60 eyes of 30 age and sex matched normal individuals fitting into the inclusion and exclusion criteria were selected for the study. Inclusion criteria s were either healthy patient presenting for eye evaluation to our out-patient department or patient attenders or the staff members of hospital without any eye problem in the age group of 20-40 years, with the refractive error less than ± 0.5 spherical diopter, IOP less than 21 mmHg. Those with refractive error more than ± 0.5 diopter, with any lids or adnexal pathology, glaucoma, family history of glaucoma, corneal pathologies, cataract, IOP more than 21 mmHg, pupillary abnormalities, retinal or optic disc diseases were excluded from the study.

60 eyes of 30 individuals were selected as per the above mentioned criteria. After taking informed and written consent from the subjects, they underwent detailed clinical examination which included detailed ophthalmic and medical history taking, assessment of visual acuity, auto-refraction, retinoscopy, complete anterior and posterior segment evaluation and IOP measurement using non-contact tonometry.

All subjects underwent evaluation of RNFL analysis of both eyes using Topcon 3D-OCT - 2000 3D-OCT, PC software edition version 4.0×. Following pupillary dilatation with 1% tropicamide and 5% phenylephrine. Single experienced observer captured images with patient fixating at the internal fixation target. After acquiring the best possible fixation and clear retinal video image following scans of each eye was imaged. RNFL analysis was carried out using fast-RNFL-thickness 3.4 scanning protocol, which automatically records three circular scans of diameter 3.4 mm around the center of the optic disc for 256 points along the scanning circle with the fixed scan resolution of 1024. The RNFL is identified as a red colored high reflectivity zone adjacent to the optically zero reflective vitreous. These scanning and analysis tools are part of the Topcon 3D-OCT 2000 PC software edition version 4.0× provided in the 3D-OCT 2000. Quality of scanned image was assessed on the basis of signal-to-noise ratio (SNR) and accepted A-scan percentage values. As a guideline, an A-scan was considered good if the maximum of each A-scan in the image is at least 6 dB above the noise floor in 95% of axial scan length or has a SNR greater than 31 db. Finally, the quality of scan image was subjectively assessed to notice the richness of red and yellow color, which if high, suggests a good scan. The scans were repeated until we obtained good image quality based on the above criteria. Mean RNFL thickness in micrometers along the whole circle circumference, four quadrants, 12 clock h and at 256 A-scan lengths were obtained. The sectors were defined in degrees, where in 0° was temporal horizontal point and the 360° measurements along the circle were clockwise in the right eye and anticlockwise in the left eye. Superior quadrant was from 45° to 135°, nasal from 135° to 225°, inferior from 225° to 315° and temporal quadrant was from 315° to 45°. Twelve 30° sectors were also defined in clockwise order for right eye and in counterclockwise order for the left eye. Maximum RNFL thickness in superior and inferior quadrants was also analyzed. Scans with good centration on the optic disc and signal strength more than 5 were accepted [Figure 1].
Figure 1: Retinal nerve fiber layer thickness analysis print out

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Descriptive statistical analysis has been carried out in the present study. Results on continuous measurements are presented on mean and standard deviation (Min-Max) and results on categorical measurements are presented in number (%). Significance is assessed at 5% level of significance.


  Results Top


A randomized cross-sectional study of 60 eyes of 30 normal individuals underwent evaluation of RNFL thickness in the age group of 20-40 years using spectral domain OCT. There were 10 individuals in 20-25 years age group (33.3%), 10 in 26-30 year group (33.3%), 8 in 31-35 year group (26.7%) and 2 in 36-40 year group (6.7%) [Graph 1]. Study group comprised of 16 males (53.3%) and 14 females (47.7%).



The average RNFL thickness was 111.8 ± 4.8 μ. The quadrantic assessment of the showed inferior quadrant thickest (147.5 ± 7.9 μ) followed by superior (135.4 ± 9.9 μ), nasal (89.0 ± 6.7 μ) and then temporal being the thinnest (74.9 ± 8.0 μ) [Table 1]. There was no difference noted between males and females as well between different age groups [Table 2].
Table 1: Descriptive statistics of RNFL thickness

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Table 2: Age and gender distribution

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  Discussion Top


Several instruments and techniques are used for the analysis of the optic nerve head, with the idea of detecting glaucomatous damage in its early stages, even before the functional field loss is detectable. [23] OCT provides an assessment of RNFL thickness by passing a near - infrared illumination (840 nm) beam into the eye and studying its reflectivity patterns by computer-assisted software. No reference plane is required to calculate RNFL thickness because OCT provides an absolute cross-sectional measurement of the retinal substructure, from which the RNFL thickness is calculated. [23] RNFL is seen as red-colored high-reflectivity zone adjacent to optically zero reflective vitreous. [24],[25] RNFL thicknesses in previous studies have shown the high reproducibility [19],[26],[27] and reliability [25] with the new OCT machines and software, thus making it an important tool in glaucoma diagnosis and management. Spectral domain OCT has been recently introduced in India and the normative profile of various measurements has not been established for the Indian population.

In our study, average nerve fiber layer thickness along the 3.4 mm-diameter circle around the optic nerve head was approximately 111.75 ± 4.83 μm. RNFL thickness was found to be more in the inferior quadrant followed by superior, nasal and temporal quadrant, suggesting that ISNT rule does apply to this subgroup of Indian population. This corresponds to the double hump pattern of RNFL as described previously. [2],[27],[28],[29] However, the study by Kanamori et al. [22] and Ramakrishnan et al. [30] found the superior quadrant was more thicker than inferior. Overall measurements produced a double hump pattern curve previously reported with OCT and on histopathology. [31]

We noticed a significant difference in the average RNFL thickness between our group and other groups [Table 3]. Mistlberger et al., [32] Bowd et al., [33] Mok et al., [34] Sony et al. [28] and Ramakrishnan et al., [30] reported a lower average nerve fiber layer thickness, whereas Soliman et al., [35] Carpineto et al., [25] and Guedes et al., [36] showed higher values compared with our study. This difference could be attributed to ethnicity, different population studied, difference in sample size, different types of OCT machines used and changing OCT machine parameters.
Table 3: Comparison of average RNFL thickness in various studies

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  Conclusion Top


Spectral domain OCT is a non-invasive, non-contact technique helpful in measuring the RNFL thickness with accuracy and is a good additional mode of investigation in early diagnosis of glaucoma. In normal subjects, variation in RNFL thickness is significant in different quadrants. The normative data provide by this study may assist in identifying changes in RNFL thickness in glaucoma and other diseases in South Indian population.

 
  References Top

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