Dr. Ganesh Pramod Ambekar, A20292, Dr. Nangia Vinay Kumar B, Dr. Akshay Harne, Dr. Hemangi Rathi
Introduction: The Retinal Nerve Fiber Layer Thickness (RNFLT) is an important parameter along with the optic disc to be considered while assessing, diagnosing, managing and following up a patient of glaucoma. The RNFL has for a long time being assessed clinically using red free light for evaluation or photography. With the advent of better imaging devices especially the OCT, the assessment of RNFL has become easier and more objective. The assessment of RNFL in myopia is a challenge because of the multiple variations that are found from segment to segment in eyes with optic disc and parapapillary changes particular to myopia and also due to increase in axial length, the presence of staphyloma, retinoschisis and choroidal thinning and RNFL thinning. Data for the segmental values and their classification as normal, suspicious or abnormal is not easily available. Also normative data for the myopia subset is not available. It was the purpose of this study to determine the sectoral and global changes in the classification of RNFLT in subjects with myopia, for a better understanding in evaluating such subjects and to aid in the appropriate diagnosis of glaucoma.
Methods: The study included 492 eyes of myopes (females 252). Subjects with Tilted discs, retinoschisis, glaucoma and all subjects less than 10 years of age were excluded from the analysis. 376 eyes were included (196 females). All subjects underwent an ophthalmic evaluation, including, vision, refraction, slit lamp biomicrosopy, Applanation tonometry, pupillary dilatation, indirect ophthalmoscopy, optic disc evaluation, fundus photography and spectral domain OCT. RNFL images, foveal scan and enhanced depth imaging of the optic disc was done. For high myopes, the high axial length settings were used in the Spectralis SD OCT. RNFL readings were recorded globally and for the different segments of nasosuperior, nasal, nasoinferior, temporo inferior, temporal and temporosuperior part of RNFL. The Spectralis SDOCT gives a pie chart, which is color coded. A green sector represents the range above the 5th percentile of the RNFLT distribution in normal eyes. This is considered as ‘within normal limits’A yellow sector represents the range below the 5th percentile but above the 1st percentile. This range is considered as ‘borderline’. A red sector represents the range below the 1st percentile, this is considered as ‘outside normal limits’. Thus while assessing a patient for glaucoma, we often consider RNFLT found below the 5th percentile and below the 1st percentile to be highly suspicious or confirmatory of glaucoma. All data for global and segmental values was entered into software (SPSS) for better evaluation. The RNFLT was graded based on the percentile distribution.
Results: The mean spherical equivalent was – 5.42 D Sphere +/-3.85 D. The BCVA DU was 0.83+/-0.26. The mean axial length was 25.03+/-1.65mm; IOP was 14.84+/-2.86. Peripapillary atrophy was present in 186/190 (97.5%) eyes. The maximum width of PPA was 486.02+/-373 microns and the total extent in clock hours was 5.63+/-2.94. Diameter of disc horizontal was 1514.26+/-254.47 and vertical was 1610.98+/-257.33 microns. The mean maximum gamma zone values were 385.70+/-315.64 u. The maximum normal gradingwas found for the nasal segment. Less than 1 Percentile values considered to be abnormal were found most frequently for Temporo superior (8.8%) followed by the Temporal (6.9%), Temporo Inferior 5.8%), Nasoinferior (3.9%), Nasosuperior (3.3%) and least in the Nasal segment ( 1.9%). Less than 5 Percentile values were found most commonly for The Temporal segment at 20.4% and least for the Nasal segment 2.2%. Temporo superior segment it was seen in 10.5% and Temporo inferior in 9.6%. See Table 1.
Discussion: The grading of the RNFLT on the basis of percentile values is the obvious choice in determining if the RNFL is normal. However this makes the assessment most simplistic, since we are comparing a different subset of subjects with a completely different anatomical architecture with a ‘standard subset of subjects’. This is specially so when comparing myopes with the normative dataset. However RNFL grading does tell us that there is suspicious or abnormal degree of thinning. That may turn our attention to further clinical assessment and decision making The current normative database ( provided by Heidelberg Engineering for their Spectralis SDOCT) includes subjects with a mean age of 48.2+-14.5 years with a refractive error ranging from +5 D to -7.00 D.(1) An attempt to compare healthy myopic eyes with eyes having myopia and glaucoma has been made (2) and the authors found that the myopic database had a higher specificity for detecting RNFL abnormalities in eyes with myopia and glaucoma than the normative database. Even though we excluded subjects with staphyloma, retinoschisis, and glaucoma, still many anatomical variations remain. The presence of the gamma zone is one such parameter the influence of which on the RNFL is still not fully understood. The influence of the shape and size of the optic nerve also remains not well analyzed. The changes in the vasculature may also have some influence and finally the thinning of the choroid may also affect the RNFL thickness.
Our study found, after excluding the nasal and temporal segments, that the most common site of suspicious ( less than 5 percentile) and abnormal grading ( less than 1 percentile) was the temporo superior ( 10.5% and 8.8%) followed by the temporo inferior segment ( 9.6% and 5.8%). Table 1. This indicates that when there is thinning in these segments, one must pay attention to the other clinical indicators that may point towards the presence of glaucoma and at the same time caution must be taken for not considering all such subjects to have glaucoma and undergoing unnecessary treatment.
The strengths of the study include the large number of cases, and the care taken to assess each patients RNFL on the OCT, to correct the segmentation errors and the many other variables that have been documented. Such a study however would benefit even further by having an even larger number of cases because of the variability in such eyes. The influence of various other parameters yet remains to be determined. The study is clinically significant since it alerts the ophthalmologist to the presence of reduced values that may be considered significant in eyes that have not glaucoma and therefore may not need treatment but only close follow up.
References:
- Spectralis OCT user manual. V. 6.0. Heidelberg Engineering. August 2014.
- Biswas S, Lin C, Leung CK. Evaluation of a Myopic Normative Database for Analysis of Retinal Nerve Fiber Layer Thickness. JAMA Ophthalmol. 2016 Sep 1;134(9):1032-9.
Table 1. Showing number of eyes,( with Percentage) that showed normal, suspicious and abnormal RNFLT grading with the SDOCT.
| Grading | Naso Superior | Nasal | Naso Inferior | Temporo inferior | Temporal | Temporo Superior | Global |
| 1 | 325 (89.5%) | 348 (95.9%) | 329 (90.6%) | 307 (84.6%) | 265 (72.7%) | 293 (80.7%) | 301 (82.9%) |
| 2 | 26 (7.2%) | 8 (2.2%) | 20 (5.5%) | 35 (9.6%) | 74 (20.4%) | 38 (10.5%) | 43 (11.8%) |
| 3 | 12 (3.3%) | 7 (1.9%) | 14 (3.9%) | 21 (5.8%) | 25 (6.9%) | 32 (8.8%) | 19 (5.2%) |
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