Indian Journal of Ophthalmology

ORIGINAL ARTICLE
Year
: 2006  |  Volume : 54  |  Issue : 1  |  Page : 11--15

Retinal nerve fibre layer thickness measurements in normal Indian population by optical coherence tomography


R Ramakrishnan, Saurabh Mittal, Sonal Ambatkar, Mohideen A Kader 
 Aravind-Zeiss Centre of Excellence for Glaucoma, Aravind Eye Hospital, Tirunelveli, Tamil Nadu, India

Correspondence Address:
R Ramakrishnan
Aravind-Zeiss Centre of Excellence for Glaucoma, Aravind Eye Hospital, Tirunelveli, Tamil Nadu - 627001
India

Abstract

Purpose: To obtain retinal nerve fibre layer thickness measurements by optical coherence tomography (OCT) in normal Indian population. Materials and Methods: Total of 118 randomly selected eyes of 118 normal Indian subjects of both sex and various age groups underwent retinal nerve fiber layer thickness analysis by Stratus OCT 3000 V 4.0.1. The results were evaluated and compared to determine the normal retinal nerve fiber layer thickness measurements and its variations with sex and age. Results: Mean + standard deviation retinal nerve fiber layer thickness for various quadrants of superior, inferior, nasal, temporal, and along the entire circumference around the optic nerve head were 138.2 + 21.74, 129.1 + 25.67, 85.71 + 21, 66.38 + 17.37, and 104.8 + 38.81 Ám, respectively. There was no significant difference in the measurements between males and females, and no significant correlation with respect to age. Conclusion: Our results provide the normal retinal nerve fiber layer thickness measurements and its variations with age and sex in Indian population.



How to cite this article:
Ramakrishnan R, Mittal S, Ambatkar S, Kader MA. Retinal nerve fibre layer thickness measurements in normal Indian population by optical coherence tomography.Indian J Ophthalmol 2006;54:11-15


How to cite this URL:
Ramakrishnan R, Mittal S, Ambatkar S, Kader MA. Retinal nerve fibre layer thickness measurements in normal Indian population by optical coherence tomography. Indian J Ophthalmol [serial online] 2006 [cited 2020 Sep 25 ];54:11-15
Available from: http://www.ijo.in/text.asp?2006/54/1/11/21608


Full Text

Optical coherence tomography (OCT) is a new diagnostic computerized technique, which is used for generating in vivo images of retinal nerve fiber layer (RNFL) thickness which is reproducible, quantitative, and objective.[1] OCT works on the principle of low-coherence interferometry, which generates retinal tomographs with P P P 20% and false-positive or false-negative ratio > 33%.[6]

Selected cases (118 subjects-61 males and 57 females, 21-74 years) underwent OCT (Stratus OCT 3000; Carl Zeiss Ophthalmic Systems-Humphrey Division, Dublin, CA, USA) evaluation for both eyes, following pupillary dilatation with 1% tropicamide and 5% phenylephrine.[4] Single experienced observer captured images with the patient fixating at the internal fixation target.[7] A rough setting of subject's refraction (spherical equivalent) was recorded into the machine. After acquiring the best possible fixation and clear retinal video image, RNFL of each eye was imaged 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.

Measurements were then assessed using RNFL thickness average analysis protocol. These scanning and analysis tools are part of the Stratus OCT 3000 Application Version V4.0.1 software provided in the OCT 3000. 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.[8] The scans were repeated till we obtained good image quality based on the above criteria.

Mean RNFL thickness in micrometers along the whole circle circumference, four quadrants, twelve clock hours, and at 256 A-scan lengths were obtained. The sectors were defined in degrees, wherein 0║ was temporal horizontal point and the 360║ measurements along the circle were clockwise in right eye and anticlockwise in 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 ([Figure 1]: 1-superior-nasal, 2-nasal-superior, 3-nasal, 4-nasal-inferior, 5-inferior-nasal, 6-inferior, 7-inferior-temporal, 8-temporal-inferior, 9-temporal, 10-temporal-superior, 11-superior-temporal, and 12-superior). Maximum RNFL thickness in superior and inferior quadrants was also analyzed. Other parameters such as Smax/Imax, Smax/Tavg, Imax/Tavg, Smax/Navg, and max-min, provided with RNFL thickness average analysis protocol are dependent on the above parameters, and were thus not studied.

Statistical analysis

Previous reports estimate the mean RNFL thickness[3],[9],[10],[11]ranging between 85.8 and 127 Ám and their standard deviation[9],[10],[11]ranging between 11 and 14.17 Ám on OCT analysis. Expecting to get RNFL thickness results with significant difference ( d ) of 10 Ám, power of 80% (Za=0.842) and significance level of 0.01 (Zβ=2.576) was used to determine the sample size. The sample size was determined using the formula n= 2 ( Za+Zβ) 2 S 2 /d 2 . Considering the standard deviation ( S ) of 14.17, the minimum sample size calculated was 84.

Data at 256 A-scan lengths were analyzed to provide the average, standard deviations, 95% confidence intervals, and normal distribution percentiles of RNFL thickness at 95%, 5%, and 1% of the sample population. Student's t -test for independent variables was used to compare the results for different sex. Pearson correlation coefficient and linear regression analysis was done to determine the effect of age on RNFL measurements. Measurements of average, quadrants, and clock hours of RNFL were compared for sex and age.

 Results



Of 134 enrolled subjects, 118 eyes of 118 subjects were studied with respect to their sex and age. There were 61 (51.7%) male and 57 (48.3%) female subjects with mean age of 45.2 + 13.56 years (range: 21-74 years). There were 33 (28.0%) myopic, 54 (45.8%) emmetropic, and 31 (26.3%) hypermetropic eyes with refraction ranging from -4.50 to + 2.75 Diopters of spherical equivalent. Mean peripapillary RNFL thickness on average was 105 + 38.79 Ám, with 95% confidence interval ranging from 97.8 to 111.8 Ám. RNFL thickness for superior, inferior, nasal, and temporal quadrants were 138.2 + 21.74 (95% CI: 134.3-142.1), 129.1 + 25.67 (95% CI: 124.5-133.7), 85.71 + 21 (95% CI: 81.9-89.5), and 66.38 + 17.37 (95% CI: 63.3-69.5) Ám, respectively. The mean RNFL thickness in four quadrants and for all clock hours is provided in [Table 1]. The mean RNFL thickness was highest in the superior quadrant followed by inferior, nasal, and temporal quadrants [Table 1]. The average A-scan data at 256 points formed a wave-like pattern with the superior and inferior quadrants forming the crest, and the temporal and nasal ones forming the trough, as seen in [Figure 2]. There was no statistical difference regarding the sex [Table 1] and no statistical correlation between the age and the average RNFL thickness [Table 2]. Percentile levels at 95%, 5%, and 1% for the normal distribution are provided in [Table 3] and displayed in [Figure 2].

 Discussion



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.[12] OCT provides an assessment of the 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.[4] RNFL is seen as red-colored high-reflectivity zone adjacent to optically zero-reflective vitreous.[13],[14] RNFL thickness in previous studies have shown the high reproducibility[7] and reliability[13] of the new OCT machines and software, thus making it an important tool in glaucoma diagnosis and management. OCT has been recently introduced in India and the normative profile of various measurements is not established for the Indian population.

Average nerve fiber thickness along the 3.4-mm-diameter circle around the optic nerve head was approximately 105 Ám. RNFL thickness was found to be more in superior followed by that of the inferior, nasal, and temporal quadrant, suggesting that ISNT rule does not apply to this subgroup of Indian population [Table 1]. Overall measurements produced a double-hump pattern curve [Figure 2], similar to the one previously reported with OCT and on histopathology.[15]

We noticed a significant difference in the average RNFL thickness between our group and other groups [Table 4]. Mistlberger et al.[9] and Bowd et al.[3] reported a lower average nerve fiber layer thickness, whereas Soliman et al. ,[10] Mok et al.,[16] Carpineto et al. ,[13] and Guedes et al.[11] showed higher values compared with our study. This difference can be owing to different populations studied, difference in sample size, and changing OCT machine parameters. The comparison and summary of previous reports of RNFL thickness in normal individuals is provided in [Table 4].

There was no significant difference seen between males and females for mean, quadrant, and clockwise RNFL thickness. There was a decrease in mean RNFL thickness with respect to age, but it was not statistically significant. Decline in RNFL thickness owing to loss of ganglion cells with age[17] has been stated in the previous OCT[16] and histopathological[14],[15] studies. Study with higher sample size may be required to highlight the above change in our population.

Similar to age-matched normative data with the OCT analysis protocol, we studied the 95%, 5%, and 1% levels of RNFL thickness for average, quadrants, and clock hours among our normal population sample [Table 3]. These measurements help determining the thinnest, below 1% RNFL thickness, which is outside normal and is expected to be pathological. Measurements falling above 5% level are considered normal and represent 95% of population. Measurement falling between the 5% and 1% level comprise a borderline group and requires careful monitoring.

OCT normal database commercially available with Stratus OCT does not provide information on ethnic difference within its data groups and does not provide the information on numerical data. In our analysis of RNFL thickness in our subjects with normal optic disc and visual fields, we found that 11 (9.3%) eyes had clock hour thickness falling below 5% level on comparison with normative data provided with Stratus OCT having borderline. This indicates at the possibility of ethnic differences existing with Indian population being different form other groups. Although a larger sample size is required to find out the above difference, we recommend the percentile levels as provided in [Table 3], which can assist in identifying cases distinguished as borderline on comparison with normative data by Stratus OCT machine.

In conclusion, we have obtained estimated normal RNFL measurements in Indian population using stratus OCT 3000. These measurements may serve as a reference during glaucoma screening with OCT in Indian population.

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