RETINAL NERVE FIBRE LAYER THICKNESS AND OPTIC NERVE HEAD SIZE MEASURED IN MYOPES BY OCT: DOES MAGNIFICATION PLAY A ROLE?.

Kaavya Hanuraj Kolalapudi , K.Venkateswarlu and V A N P Kumari Oruganti. Junior Resident, Ophthalmology, Andhra Medical College, Visakhapatnam. ...................................................................................................................... Manuscript Info Abstract ......................... ........................................................................ Manuscript History Received: 04 December 2018 Final Accepted: 06 January 2019 Published: February 2019


Inclusion Criteria:
Patients attending GREH with refractive power of spherical equivalent > -6D for test group i.ehighmyopic group. Patients attending GREH with refractive error of spherical equivalent of 0.50D to +0.50D for control group Exclusion criteria: Patients with intra ocular pressure of more than 21mm of Hg Patients with hazy media Presence of any optic nerve disease, peripapillary area atrophy, disc abnormalities History of intra cranial diseases History of usage of any medications like steroids etc History of previous ocular surgery

Procedure:
Patients attending to the OPD were examined under slit lamp. The refractive error of the patients was measured by autorefractometer andor cycloplegicretinoscopy,and then subjective verification was done for these patients. Depending upon the refractive error they were divided into two groups: the test group and the control group.IOP was measured by applanationtonometer.In both the groupsindividually axial length was measured for each patient by A Scan.
The OCT examinations were done by CIRRHUS HD OCT, after the instillation of mydriatic, in dim light.The optic nerve head(ONH) and retinal nerve fiber layer thickness(RNFL) parameters were measured by CIRRHUS HD OCT with the optic disc cube 200x 200 scan protocol. The values were then adjusted for magnification.
To determine the actual size of an object on the fundus of the living eye, Littmann recommended adjusting the size of a retinal feature observed by fundus photography.
The relationship between the measurements obtained by an imaging system and actual fundus dimension measurements can be expressed as t = p x q x s 1 , wheret is the actual fundus dimension, p is the magnification factor for the imaging system, q is the magnification factor related to the eye and s is the measurement obtained by an imaging system Kang et al adjusted the Littmann formula of the thickness t = p x q x s for area into t 2 = p 2 x q 2 x s 2 which was used for optic nerve head to calculate the disc and rim area 2 1. The OCT system has a p-value of 3.382 2 , 2. The formula for obtaining theq-value(magnification factor)for the eye is q = 0.01306 x (AL-1.82) 2 . 3. The recommended magnification adjustments for RNFL thickness and disc and rim area were given in Table:1

Results:-
There was no significant statistical difference between age of the test and control groups.There was significant difference between spherical equivalence and axial length between the two groups (Table -2) OCT measurements after magnification adjustments in high myopic eyes and control eyes were greater than those without magnification adjustment in the two groups (p < 0.001)(TABLE -3)  Magnification-adjusted OCT measurements of the global and temporal RNFLs were significantly thicker in myopic eyes than in control eyes (p< 0.001), nasal RNFL was significantly thinner in myopic eyes than in control eyes (p<0.001). Superior and inferior RNFL thickness measurements did not differ significantly between the two groups (p > 0.001). ( Table-4) Disc and rim areas were significantly larger in myopic eyes than in control eyes (p < 0.001). (Table -5)

Discussion:-
Certain anatomical changes that occur in longer axial length eyes such as globe elongation, scleral widening, and subsequently enlargement of the lamina cribrosa, result in larger disc areas in such eyes. The use of magnificationadjustment can correct disc and rim areas measurements in those eyes In this study after magnification the disc and rim areas of optic nerve head were significantly larger in highmyopes than in control eyes. In a study done by Sheng yaohsu et al, OCT measurements with magnification adjustment found that highly myopic eyes have larger ONH size than in normal eyes. This study shows that magnification adjusted RNFL thickness were significantly thicker in global and temporal quadrants in myopic eyes compared to non-myopic eyes .There was no significant difference between superior and inferior RNFL thickness between the two groups.
In highly myopic eyes, only the nasal nerve fibre layer is subnormal, whereas the superior and inferior RNFL thicknesses are normal. In a study conducted by Lueng et al, without magnification adjustment, they reported that highly myopic eyes have significantly lower RNFL thickness globally and temporally 4 . In a study done by Sheng Yaohsu et al, OCT measurements with magnification adjustment found that highly myopic eyes have thicker temporal and global RNFL thicknesses than in control eyes 3 . In another study by Hyun Bae et al, disc area and RNFL thickness decreased in high myopic eyes, they increased after adjustment for the magnification effect 5 Based on this, the ONH size and RNFL measurements were influenced by the magnification effect. Hence, prior to diagnosis of glaucomatous or non-glaucomatous optic atrophy in highly myopic patients where there is thinning of RNFL thickness, the magnification effect should be considered.