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Best Free Papers This paper was judged the BEST PAPER of GLAUCOMA-I Session The Neural Canal Opening in Myopia and Its Clinical and OCT Correlates Dr. Nangia Vinay Kumar B., Dr. Priyanka Pardhi T he retinal nerve fibers converge to form the optic disc papillae and course through the Bruchs membrane opening and through the surrounding junctional sclera and the lamina cribrosa to form the optic nerve. The space between the Bruchs membrane is the Neural canal opening.1 The neural canal is often considered to be a surrogate of the optic disc margins. In essence the clinical visible optic disc margins may define the edges of the Bruchs membrane. In a previous study the authors showed the correlation of the Bruchs membrane edges with the margins of the optic nerve in non myopic eyes from a cross sectional randomly selected cohort of a population based study, the Central India Spectral Domain OCT Study. Passage of the retinal nerve fibres through the neural canal opening is an important anatomical landmark. It is also important that the orbital CSF surrounds the optic nerve in the area of the Bruch Membrane. Therefore any alteration in the neural canal opening may translate into altering the environment around the optic nerve. Since all anatomic parameters may be considered to be important around the site of optic neve damage and also since the Cerebrospinal Fluid Pressure is now considered important in the health of the optic nerve2,3 and the Translamina cribrosa Pressure Difference4 has been shown to be important in the evolution of glaucomatous damage, it was considered important to further evaluate the Bruchs membrane related neural canal opening. In this study we evaluated the neural canal opening in myopes. Myopes have significant variation in the size of the optic disc and presence of parapapillary atrophy. In addition the size of the eye ball is much larger as borne out by the axial length measurments. It was the purpose to determine the dimensions of the neural canal and their ocular correlations in a clinic based group of myopes. MATERIALS AND METHODS It was a cross sectional study conducted at Suraj Eye Institute, Nagpur. during May 2012-May 2013. 197 eyes of 100 myopic (47 males and 53 females) patients were included. The mean age was 25.76+-10.53 (8-60) years. Only those subjects with spherical equivalent on automated refractometry of myopia -1.00 D or more (197 eyes of 100 subjects) were included. Subjects with media opacities, pseudophakia, with other retinal findings, with glaucoma and previous sugery were excluded. 73rd AIOC 2015, New Delhi All patients underwent routine Ophthalmic evaluation, including Automated refraction and assessment of visual acuity, Slit lamp biomicroscopy, Applanation tonometry, Fundus evaluation with the 78 diopter lens and Indirect Ophthalmoscopy following pupillary dilatation using tropicamide and phenylephrine. Axial length and keratometry readings were determined on Intra-Ocular Lens Master-500. Colour photography using 20 degree and 50 degree views. OCT was done using Spectral domain OCT (SD-OCT). Horizontal and vertical line scan passing through disc and macula were done in enhanced depth imaging mode. Retinal Nerve fiber layer (RNFL) scans were done to assess the thickness of the layer. Manual corrections were done in the scans if they were found to had errors in automated assessment. Planimetric measurements for diameters of optic disc, peripapillary atrophy and neural canal were made using software provided with Spectralis. RESULTS The mean age was 25.76±10.53yrs, mean BCVA in Decimal Units was 0.77+0.27. The mean size of the neural canal at 180 degrees horizontally was 1501.33+-261.58 microns. (See Table 1) Table 1: Ocular Parameters in the study group. Parameters Mean and SD Range Axial length (mm) 25.54±1.7622.51-35.57 IOP (mmHg) 15.07±2.698-25 Disc area (mm2) Disc diameter Horizontal (mm) 2.38±0.711.01-6.19 1501.33±261.58587-2267 Diam of Neural Canal (horizontal) mm. 1872.24±3991347-3901 Max. Width of PPA in microns 561.28±43250-2227 PPA total number of Clock hours 6.53±3.532-12 In bivariate analyis diameter of neural canal showed a significant correlation with diameter of the disc (p<0.001; r=0.235), with the BCVA (P<0.001;-0.521), with increasing myopia (P<0.001; r=-0.752), increasing axial length (P<0.001;0.652), with the presence of PPA (P<0.001; r=-0.411), with the extent of PPA in clock hours (P<0.001; r=0.621), with maximum width of PPA at any clock hour position (P<0.001; r=0.698), with width of PPA temporally (P<0.001; r=0.566), staphyloma involving the macula (P<0.030; r=0.154) and staphyloma involving the optic disc (P<0.001; r=0.513). Paired T Test showed the neural canal diamater to be significantly larger than the disc diameter Multivariate analysis was performed using the neural canal dimensions as a dependant variables and diameter of disc, BCVA, Axial length, maximum Best Free Papers width of PPA, staphyloma involving the optic disc and total number of PPA clock hours on infra red imaging. (See Table 2). Table 2: Showing Multivariate Analysis with Neural Canal dimensions as the dependant variable Parameters Diameter of Disc Horizontal P Values 95% CI <0.001 0.665,0.937 BCVA 0.750-125.76,174.11 Axial length 0.006 PPA Maximum width <0.001 .573,.866 PPA Total no. of clock hrs <0.005 7.73,41.478 0.213 -46.708,207.33 Staphyloma involving the disc 11.54,66.798 DISCUSSION The neural canal opening (NCO) is the space between the ends of the Bruchs membrane on all sides of the optic disc. The NCO is significant because the retinal nerve fibers converge on the optic disc and pass through the NCO to form the optic nerve as we know it. While it has been shown that the neural canal opening coincides generally in emmetropes with the margins of the optic disc, studies showing such a correlation in myopia were required. The neural canal diameter was found to be significantly larger than the optic disc diameter (P <0.001). The neural canal was also found to correlate significantly with increasing axial length both in univariate and multivariate analysis. (P=0.001). The neural canal correlates with the parapapillary atrophy (PA) width and circumferential extent in multivariate analysis. The space between the border of the Bruchs membrane and the optic disc has been recently described as the Gamma Zone.5 PA is also found to be associated with increasing axial length. Further on analyzing the colour photographs which demonstrate the PA, and correlating them to the OCT Enhanced Depth Images, it was apparent that the neural canal opening margins in myopes with PA was representative to a significant extent of the borders of the PA. In essence this raises the importance of the PA which now becomes an indicator of the neural canal opening. While in emmetropes the nerve fibers pass through neural canal opening, in myopes the nerve fibres pass between the inner borders of the intermediate scleral tissues (scleral flange) which lie between the normal sclera and the lamina cribrosa. It is apparent that such anatomic alterations in myopes would subject the optic nerve to an orbital milieu that may be different from that found in the emmetropes. It may not only alter the vascular dynamics but may also alter the mechanisms of the translamina cribrosa pressure differential enabling a wider area for pressure differences across the scleral flange, thereby increasing the 73rd AIOC 2015, New Delhi susceptibility of the parapapillary retinal nerve fibers to the Trans Para Papillary (scleral flange) pressure differential. REFERENCES 1. Strouthidis, NG, Yang H, Fortune B, Downs C, and Burgoyne C. Detection of Optic Nerve Head Neural Canal Opening within Histomorphomorphometric and Spectral Domain Optical Coherence Tomography Data Sets. Invest. Ophthalmol Vis Sci. 2009;50:214-23. 2. Berdahl J, Allingham R R, Johnson DH. Cerebrospinal Fluid Pressure Is Decreased in Primary Open Angle Glaucoma. Ophthalmology 2008;115:763-8. 3. Ren R, Jonas JB, Tian G, Zhen Yi, et. al. Cerebrospinal Fluid Pressure in Glaucoma. A prospective Study. Ophthalmology 2010;117:259-66. 4. Jonas JB, Nangia V, Wang N, Bhate K. et. Al. Trans-Lamina Cribrosa Pressure Difference and Open Angle Glaucoma. The Central India Eye and Medical Study. PLoS ONE 8 (12):e82284, doi: 10.1371/Journal.pone.0082284. 5. Jonas JB. Jonas S, Jonas R, Holbach L, et. al. Parapapillary Atrophy: Histological Gamma Zone and Delta Zone.