Optical Coherence Tomography to Measure Retinal Nerve Fiber ...

GLAUCOMA SESSION-I239Optical Coherence Tomography to Measure Retinal Nerve FiberThickness in Normal Children of North Indian PopulationDr. Monica Gandhi, Dr. Suneeta Dubey, Dr. Julie Pegu(Presenting Author: Dr. Monica Gandhi)Retinal nerve fiber layer (RNFL) and theretinal ganglion cells (RGC) are involved invarious optic neuropathies including glaucoma.Anatomic damage in the form of RGC loss,RNFLdefect and optic disc changes usually precedefunctional damage in glaucoma. More than 40%of the optic nerve axons can be damaged beforeglaucomatous changes are detectable onachromatic perimetry. With the advent ofvarious new diagnostic modalities like HRT,OCT and GDx the RNFL thickness has beenmeasured and its diagnostic capability has beenevaluated by various studies. OCT has been usedto detect early RNFL loss in adult patients withglaucoma and to observe patients withhereditary optic neuropathy.OCT has been shown to have good structural andfunctional correlation with known parameters inadults. Also good correlation between RNFLmeasurements by OCT and histology of theretina has been demonstrated. RNFL thicknessvaries with age and is different in different races.The studies evaluating RNFL thickness havebeen conducted in adults and the normative dataavailable is of ages 18 and above. Normal Indianadult population has been studied in differentdemographic locations. There is no data availablefor below 18 years in Asian Indian populationand in this study we propose to collect data innormal subjects below 18 years so that it can beused to compare with similar age matchedindividuals with optic nerve pathologies.Materials and Methods100 participants were identified amongst patientsattending the out patient department of DrShroff’s Charity Eye Hospital, New Delhi. Theseparticipants were amongst those who had noother ocular complaints except refractive error.Written informed consent was obtained from theguardian accompanying the participant. Thisstudy was approved by the institutional ethicscommittee.Inclusion criteria: Subjects between 8-17 years ofage with normal appearing discs were includedin the statistical analysis.Exclusion Criteria: Participants with familyhistory of glaucoma, intraocular pressure of morethan 22 mm Hg, and abnormal optic disc wereexcluded. Also subjects with best-correctedvisual acuity less than 20/40, sphericalequivalent more than ±5D Sph, or history ofintraocular surgery, presence of tropia ormanifest squint were excluded.The ocular and systemic history wasdocumented. The participants underwentrefraction under dilatation with tropicamide 1%or cyclopentolate 1% (according toage/refraction/ accommodation). Intraocularpressure was recorded. A detailed slitlampexamination and fundus evaluation was donewith 90D and indirect ophthalmoscopy.The measurement of RNFL thickness wasobtained with third generation OCT (StratusOCT, 4.0.7 (0132) Carl Zeiss Meditec).For the RNFL measurement all scans were doneby one observer (MG) .The procedure wasexplained to the subject. Internal fixation wasused in all cases. The fast RNFL protocol (3.4)circle scan centered on the optic disc was used forall. 3 scans of good quality were obtained .Thequality was judged by the sharpness of the scansand signal strength of 6 and more was accepted(max 10). Good centration was also ensured.Average of 3 scans was taken.Outcome measures: The following parameterswere evaluated: Average thickness (360° measurement),temporal quadrant thickness (316° to45°), superior quadrant thickness (46°to 135°),nasal quadrant thickness (136° to 225°), inferiorquadrant thickness (226° to 315°), superior maximum(SMax), inferior maximum (IMax) andthickness in 12 clock hours.The time taken to complete the scans and thedifficulty in acquiring the scans was noted. It wasscored 0 if the scan was not possible, 1 if the scanwas possible with difficulty and 2 if it waspossible without difficulty.

240 AIOC 2009 PROCEEDINGSTable-1: Demographics of subjects included instatistical analysis (n=97)Age (years)Mean ± SD 11.85 ± 2.58Range 8-17GenderMale 55 (57%)Female 42 (43%)Refraction Spherical equivalent in DioptersMean ± SD -0.25 ± 1.75Range +5 to -5StatisticsOne eye of each subject was randomly selectedfor statistical analysis. In case of left eye themirror image of the clock hours was used suchthat 3 O’clock was nasal for both right and lefteye. Univariate regression analysis was used toanalyze effect of age, gender and refraction onRNFL thickness. Comparisons for various groupswere done by ANOVA F test. Unpaired studentst test was used for comparisons of variablesbetween genders and paired t test for comparisonbetween right and left eyes.ResultsDemographics: One hundred subjects wereincluded in the study. Of these 3 (3%) did notcooperate for the OCT scan. The remaining 97subjects were evaluated statistically. Afterrandom selection of one eye per subject 47 righteyes and 50 left eyes were statistically analyzed.The demographic characteristics of the subjectsare listed in Table 1. There was no statisticaldifference between male and female subject forage (P =0.5) and refraction (P= 0.39)Cooperation3 (3%) of the 100 subjects selected for the studydid not cooperate for the scan. In 3 subjects lackof cooperation permitted scan only in one eye. In21 (22%) of the 97 subjects included in thestatistical analysis, scan was possible withdifficulty, but 3 scans of good quality wereobtained. In 76 (78%) scans were possible withoutdifficulty. Chi square test of cooperation scoreshowed that the cooperation in males andfemales was not statistically different. (P = 0.21).Students t test showed that the score wasdependent on age (P=0.001)Retinal nerve Fiber Layer Thickness (RNFL)Mean Global RNFL thickness of all eyes (n=97)was 103.11 ± 9.72 µm (range 80.4-126.3) Theglobal RNFL thickness in right eyes (n=47) was103.4 ± 10.41 µm and 102.23 ± 9.06 µm in the lefteye (n=50). The difference was not statisticallysignificant (P =0.4).The RNFL was thickest superiorly, followed byinferior quadrant, thinner nasally and thinnest inthe temporal quadrant. (Table 2) The differencebetween the superior and the inferior quadrantwas statistically significant (P = 0.024).The SMAX mean for all eyes was 162.1 ± 17.9 µm(range 121-202 µm) and the Imax mean was 160 ±16.9 µm (range 121-201 µm).Regression AnalysisIn Univariate analysis, age did not have astatistically significant effect (P = 0.8, R2 =0.001)on the RNFL thickness.The effect of refraction on global RNFL wasstatistically significant (P = 0.017).DiscussionThe diagnosis of glaucoma and glaucomaprogression is relatively subjective, involving aset of characteristic optic nerve findings usuallyaccompanied by visual field changes.Assessment of optic nerve changes in glaucomaand optic neuropathies can be challenging inchildren, as visual field analysis is usuallyunreliable, time consuming and may require afamiliarization phase. The RNFL thickness isknown to decrease in various optic nervepathologies and can be potentially documentedearlier than achromatic standard perimetry.Optical coherence tomography (OCT), firstdescribed in 1991 by Huang et al, is a high-Table-2: Peripapillary RNFL thickness in µm in 4 quadrants for all ages (n=97)Superior Nasal Inferior TemporalMean ± SD 133.82 ± 17.9 83.5 ± 16.82 128.3 ± 15.43 67.98 ± 9.74Range 99-226 46-129 92-165 45-99SD= standard deviation of mean, each quadrant includes 3 clock hours

GLAUCOMA SESSION-I241resolution, cross-sectional imaging technique thatallows in vivo measurements of the retinal nervefiber layer (RNFL). The third-generationmachine, Stratus OCT (Carl Zeiss Meditec) is ableto quantify the thickness of the RNFL at aresolution of approximately 8 to 10 µm.RNFL measured by OCT has been documentedin normal adults and has been used as an adjunctin diagnosis of glaucoma in adults Inglaucomatous eyes visual field changes andRNFL show good correlation. Goodreproducibility has been reported for standardand Fast RNFL scans both in normal andglaucomatous eyes. Reproducibility in childrenhas also been studied.RNFL thickness is known to vary with race andage. Hess et al studied RNFL thickness in normaland glaucomatous children and found acorrelative decrease of 23.3% in RNFL thickness.Lack of normative data in subjects less than 18years limits its use in children. In our study weevaluated the ability and feasibility of StratusOCT in assessing the RNFL thickness in normalchildren of north Indian population.Comparing our results with the publishedliterature we found the global RNFL thicknesscomparable to other studies done with the samegeneration OCT machine. Salchow et al reportedmean global thickness of 107±11.1 µm with arange of 69.6 to 144.5 µm. In a study by Bourne etal RNFL thickness varied from 44 to 124 µm,although they included normal andglaucomatous subjects. We also found aconsiderable variation in the mean global RNFLthickness.Earliest glaucomatous damage in the RNFL hasbeen documented in the superotemporal andinferotemporal bundles. The maximum areaunder the ROC curve, demonstrating the abilityof OCT to discriminate, was also for thesuperotemporal and inferotemporalperipapillary locations.We found the superior quadrant statisticallythicker than the inferior quadrant, which iscontrary to the other studies. This variation maybe due to limited sample size and larger groupstudies may be required to establish thesignificance.We evaluated the time taken and the feasibilityof obtaining OCT scans and it was noted that inmost children good quality scans are possible ina short time period compared to other diagnosticmodalities including visual fields. This makesOCT a potential tool in children.The limitation of our study is that the subjectswere chosen from those attending the hospitalhence they may not be a true representation ofthe population and all were of Indian origin andthus the data obtained may not be comparablewith other population groups.Our study shows that Stratus OCT is a feasibleoption to obtain RNFL thickness in youngerpopulation. The data obtained can help identifythe variations in the RNFL thickness in otherchildren as an adjunct in diagnosis and followupof glaucoma and other optic neuropathies.