a healthy macula offers the most acute vision and is crucial to tasks with high visual demands . any disorder  a macular hole or macular edema , for instance  reduces visual acuity .
the success of the treatment for these conditions is often determined by the presenting thickness and depends on posttreatment thickness .
the conventional practice of assessing the macula for its thickness , involving slit - lamp biomicroscopy , fundus photography , and fluorescein angiography , is less sensitive to subtle changes and is only qualitative,1 and is not a common practice even in developing countries such as nepal where a more accurate and sophisticated instrument , optical coherence tomography ( oct ) , is used .
oct , an optical analog of ultrasound,2 is unique on account of its combined features : an objective method of quantitatively determining the macular characteristics,3 ability to produce high resolution and cross - sectional images accurately and precisely,4,5 and yet purely noninvasive ; all of which enable diagnosis , management , and monitoring of patients with retinal diseases .
decision making while comparing the normative population database information stored in the oct software with results obtained from the population under examination in nepal has the probability to be flawed , as we might have different values for the macular variables compared with the normative database in the software , and hence puts the reliability of the test in question . as a unique study , this aims to determine the normal macular thickness and volume measurements in normal nepalese eyes .
this cross - sectional , quantitative , and hospital - based study comprised 126 eyes of 28 male and 35 female subjects ( mean age , 21.176.72 years ; age range , 1037 years ) . they were recruited between february and december 2013 from the department of ophthalmology , institute of medicine in nepal .
ethical clearance approval was obtained from the institutional review board at the institute of medicine .
the study fully adhered to the tenets of the declaration of helsinki . before their inclusion
, informed verbal consent was sought from the subjects who were 18 years or over and from the parents or attendants when the subjects were under 18 years .
sixty - three subjects ( 126 eyes ) diagnosed as having healthy normal eyes following a complete anterior and posterior segment evaluation and refraction , and not having diseases and conditions ( diabetes mellitus ; hypertension ; transplant ; autoimmune disease ; high intraocular pressure , ie , greater than 21 mmhg ; and refractive error , ie , greater than 0.25 d spherical equivalent ) underwent a fast mode macular scanning with the commercially available spectral domain ( sd ) oct ( spectra lis hra + oct ; heidelberg engineering , inc . , heidelberg , germany ) immediately after retinoscopy was carried out by an optometrist when the pupils were still dilated ( > 5 mm diameter ) .
the basic working principles have already been explained in great detail.6,7 the scan was performed over a 66 mm area in the posterior pole to achieve a high quality image .
the center point of each scan direction represented minimum foveal thickness ( central minimum thickness , or foveola).8 a traditional early treatment diabetic retinopathy study ( etdrs ) grid which contains three concentric rings of diameters 1 , 3 , and 6 mm , and two reticules to divide the macula into nine sections was employed .
axial length measurements were taken using an ultrasound a - scan biometer ( axis - ii pr ; quantel medical , inc .
central subfield thickness ( cst ) , also known as foveal thickness , was defined as the average thickness of the macula in the central 1 mm etdrs grid.9,10 average macular thickness was defined as the mean of thicknesses in nine sections.11 macular volume was defined as the sum of all volumes of all nine sections .
all data were entered into epidata v3.1 ( the epidata association , odense , denmark ) and , then for analysis , were exported to spss version 20 ( ibm corporation , armonk , ny , usa ) .
the shapiro  wilk test ( p>0.05 ) was used to test the normality of the distribution.12,13 a student s t - test and one - way analysis of variance ( for approximately normally distributed data ) and mann  whitney u - test and kruskal  wallis test ( for skewed data ) were used to generate p - values between groups .
a regression model was used to assess any correlation between 1 ) age and axial length , 2 ) age and macular thickness , 3 ) age and macular volume , 4 ) axial length and macular thickness , 5 ) axial length and macular volume , and 6 ) macular thickness and macular volume .
this cross - sectional , quantitative , and hospital - based study comprised 126 eyes of 28 male and 35 female subjects ( mean age , 21.176.72 years ; age range , 1037 years ) . they were recruited between february and december 2013 from the department of ophthalmology , institute of medicine in nepal .
ethical clearance approval was obtained from the institutional review board at the institute of medicine .
the study fully adhered to the tenets of the declaration of helsinki . before their inclusion
, informed verbal consent was sought from the subjects who were 18 years or over and from the parents or attendants when the subjects were under 18 years .
sixty - three subjects ( 126 eyes ) diagnosed as having healthy normal eyes following a complete anterior and posterior segment evaluation and refraction , and not having diseases and conditions ( diabetes mellitus ; hypertension ; transplant ; autoimmune disease ; high intraocular pressure , ie , greater than 21 mmhg ; and refractive error , ie , greater than 0.25 d spherical equivalent ) underwent a fast mode macular scanning with the commercially available spectral domain ( sd ) oct ( spectra lis hra + oct ; heidelberg engineering , inc . , heidelberg , germany ) immediately after retinoscopy was carried out by an optometrist when the pupils were still dilated ( > 5 mm diameter ) .
the basic working principles have already been explained in great detail.6,7 the scan was performed over a 66 mm area in the posterior pole to achieve a high quality image .
the center point of each scan direction represented minimum foveal thickness ( central minimum thickness , or foveola).8 a traditional early treatment diabetic retinopathy study ( etdrs ) grid which contains three concentric rings of diameters 1 , 3 , and 6 mm , and two reticules to divide the macula into nine sections was employed .
axial length measurements were taken using an ultrasound a - scan biometer ( axis - ii pr ; quantel medical , inc .
central subfield thickness ( cst ) , also known as foveal thickness , was defined as the average thickness of the macula in the central 1 mm etdrs grid.9,10 average macular thickness was defined as the mean of thicknesses in nine sections.11 macular volume was defined as the sum of all volumes of all nine sections .
all data were entered into epidata v3.1 ( the epidata association , odense , denmark ) and , then for analysis , were exported to spss version 20 ( ibm corporation , armonk , ny , usa ) .
the shapiro  wilk test ( p>0.05 ) was used to test the normality of the distribution.12,13 a student s t - test and one - way analysis of variance ( for approximately normally distributed data ) and mann  whitney u - test and kruskal  wallis test ( for skewed data ) were used to generate p - values between groups .
a regression model was used to assess any correlation between 1 ) age and axial length , 2 ) age and macular thickness , 3 ) age and macular volume , 4 ) axial length and macular thickness , 5 ) axial length and macular volume , and 6 ) macular thickness and macular volume .
the age distribution of participants as shown in table 1 was based on sturges formula and followed an approximately normal distribution curve ( p=0.194 and 0.333 for males and females , respectively ) .
the measures of central tendency involved the mean and/or the median , depending on the distribution of data .
the statistics of the macular thickness section - wise are shown in tables 2 and 3 .
the average macular thickness was higher in males than in females ( u=334 , p=0.031 , right eyes ; t ( 61 ) = 2.094 , p=0.04 , left eyes ) ( tables 4 and 5 ) . using the wilcoxon signed - ranks test
, we observed that the right and the left eyes differed significantly over inner temporal thickness ( p=0.007 ) , inner superior thickness ( p=0.019 ) , and outer nasal thickness ( p=0.001 ) . only total macular volume ( p=0.007 ) , inner superior volume ( p=0.000 ) , and outer nasal volume ( p=0.007 ) were different between the right and the left eyes .
also , intersex variability in the left eyes was observed to be the same as that of the right eyes for macular volume measurements ( tables 6 and 7 ) .
unless otherwise stated , the values will pertain to right eyes only . upon comparing three axial length groups ( 22.0522.70 mm , 22.7023.19 mm , and 23.1924.44 mm ) , the average macular thickness ( kruskal  wallis test , p=0.202 ) ( figure 2 ) and
macular volume measurements ( kruskal  wallis test , p=0.543 ) were not statistically different .
average macular thickness and macular volume correlated with each other ( r=0.944 , p=0.000 ) .
average macular thickness ( r=0.30 , p=0.017 ) and macular volume ( r=0.335 , p=0.007 ) negatively correlated with age ( figures 3 and 4 ) .
macular thickness and macular volume decreased by 0.556 m and 0.0156 mm , respectively , for each year of increasing age .
however , cst and foveal volume ( r=0.216 , p=0.09 ) did not significantly correlate with age ( r=0.243 , p=0.055 ) .
macular thickness and volume increased with axial length ; however , not all sections had a significant correlation .
nevertheless , the correlation study showed that the average macular thickness ( r=0.254 , p=0.044 ) , cst ( r=0.363 , p=0.003 ) , and foveal volume ( r=0.387 , p=0.002 ) increased with axial length .
the upper limits of the average macular thickness and cst never overshot 330 and 287 m , respectively .
the thinnest region was within the central section and measured only 180 m , while the thickest region was the inner superior section , which measured as high as 372 m .
the outer sections were significantly relatively thin compared with inner sections . for both the right and left eyes , the central maximum thickness carried the smallest p - value to show the difference in thickness between males and females . except for the inner nasal volume in the inner macula , and the outer nasal volume and outer inferior volume in the outer macula ,
used as a diagnostic and monitoring tool for vitreomacular disorders , sd - oct on account of increased scan resolution and reliability , is becoming increasingly useful .
unlike time domain - oct ( td - oct ) with an axial resolution of ~10 m , sd has an increased axial resolution of ~5 m .
more information on axial resolution and image acquisition protocols of various commercially available oct instruments can be found elsewhere.14 the increase in resolution , which is a cutting - edge technology , makes possible the visualization of even imperceptible pathologic changes and helps with much better clinical use .
similar previous study results10,11,1420 involved either sd - oct or td - oct or both in healthy eyes with refractive error .
this study is unique because 1 ) it involved only healthy emmetropic eyes undergoing sd - oct ( spectralis hra + oct , heidelberg engineering inc . ) and 2 ) it is of a maiden kind
. we can not therefore claim to accurately compare the findings from similar studies with those from the current study .
td - oct marks the inner  outer segment interface as the posterior retinal boundary , while sd - oct generally marks the retinal pigment epithelium as the posterior surface . in light of this
, td - oct underestimates the thickness and volume measurements by 5060 m.21 in the same vein , the type of sd - oct used in this study uses the bruch s membrane as the posterior boundary , and hence there is an additional increased 20 m thickness compared to sd - oct in general.15,16 other sources of discrepancy include , but are not limited to , ethnicity variation and scan ( radial versus linear ) .
this , therefore , calls for exercising caution while comparing the data obtained from different studies .
we tabulated data of right and left eyes because we can not deny that anatomical differences may exist between two eyes , and we believe normative data of right and left eyes separately help in comparing corresponding eyes .
in addition , this study serves as a pool of data for the nepalese population , which will be used for comparison with findings from future studies to be conducted in nepal and abroad .
this study is in line with an earlier study17 that reported 1 ) a significant correlation between age and minimum foveal thickness and 2 ) no significant correlation between age and cst .
cst bears no correlation with age.18 on the contrary , studies22,23 reported a relationship of cst with age .
the finding from a study22 that foveola does not thin with age is countered by our study while the same study had a finding of parafoveal attenuation consistent with ours . similar to previous studies,11,19,2427 the average macular thickness was significantly greater in males compared to females . in contrast , studies9,15 reported no difference in retinal thickness between males and females .
the fact that thickness and volume measurements were almost always greater for males than for females could be explained by the smaller , thinner physique of females .
further studies are required of the nepalese eyes to demonstrate differences , if any , in ocular biometry between male and female eyes .
average macular thickness observed in this study was different from other studies , with measurements ranging from 258 to 300 m reported by studies.10,18,28 based on studies,15,19,20,26,27,2931 our understanding is that ethnicity has an effect on macular thickness , for we have observed higher values for almost every macular region .
we side with chauhan and marshall32 on the effect of much darker pigmentation of the retinal pigment epithelium on the light signal , which is rendered attenuated , leading to reduced retinal thickness in african americans . in line with one study,11 where the male foveae measured significantly thicker than the female foveae ,
our study findings are consistent with previous findings that inner regions are thicker than outer ones , which are thicker than cst .
nevertheless , there is a debate over which of the inner four sections measures thickest .
studies33,34 reported maximal thickness in the superior and inferior regions , which could be attributed to the papillomacular bundle course along these regions .
some reports10,11,16,17 mentioned the inner nasal section as the thickest region , which is in keeping with the dense ganglion nerve fiber layer in the nasal section .
quite the opposite , reports20,3537 and our study showed that the inner superior region was the thickest of all . just as a study17 had reported ,
the intersex difference in thickness in the outer regions and inner nasal region was not observed , while a significant difference was observed in relation to the rest of the regions .
this could indicate that the sex - wise variation occurs predominantly more toward the center of the macula .
this variation of thickness between males and females may account for a female preponderance in macular hole.38,39 the study40 done in nepal also reported a female preponderance .
cst obtained in our study differs by ~3 to 31 m from that obtained in the literature11,16,37 which used sd - oct , whereas cst was thicker by ~35 to 60 m upon comparing this study and the literature,10,17,20,37 which used td - oct .
our study therefore reports larger values for almost all nine sections compared to other studies with the exception of one study,11 which reports a value less by 3.68 m . minimum foveolar thickness was not statistically different between males and females , and the range of mean minimum foveolar thicknesses spanned from 149 to 182 m.10,17,20,37 however , our study reports a larger value and an intersex difference .
the studies11,16 that reported smaller macular thickness in all nine regions in comparison with our study had mean total macular volumes of 9.950.49 mm and 10.010.6 mm , respectively .
in addition , one study24 that reported all nine thinner regions except the fovea which was thicker had a mean total macular volume of 9.740.71 mm .
interestingly enough , the macular volume in our study was smaller . nevertheless , this statistics was larger than other studies.17,19 reports11,17,24 that males had greater macular volume compared to females are consistent with our study .
all inner sections , except for the inner superior section , differed between males and females.17 the same observation was observed in this study .
our study and a previous study17 agreed that outer nasal volume is greatest and foveal volume is lowest . the increase in axial length was associated with decreased retinal thickness and volume.36 in contrast , we observed an increase in thickness and volume .
such an observation could be limited to emmetropes only and once ametropia occurs , our understanding of the relationship of axial length with thickness and volume may not apply .
we therefore hypothesize that some other retinal changes might follow , which can bring about anatomical changes .
we recruited participants from an age group of 1037 years because in children under ten , biometry and oct measurements were difficult to obtain as the children remained uncooperative , while in participants over 40 , cataract ( however mild it may be ) , undiagnosed diabetes mellitus , and systemic hypertension could be present .
a small sample size should not deter us from interpreting and generalizing our findings because oct is a highly reliable and reproducible sophisticated technology .
nonetheless , a small sample size , the oct model , and acquisition protocol may have given rise to discrepancy . this study does not take into account healthy eyes with refractive error .
by providing a pool of normative data of the macular measurements , this study will help differentiate a healthy macula from a diseased one , and assist with diagnosis , monitoring , and management of macular diseases , and aid similar studies in the future .
 purposeto record the normative values for macular thickness and macular volume in normal nepalese eyes.methodsin all , 126 eyes of 63 emmetropic subjects ( mean age : 21.176.76 years ; range : 1037 years ) were assessed for macular thickness and macular volume , using spectral domain - optical coherence tomography over 66 mm2 in the posterior pole . 
 a fast macular thickness protocol was employed . 
 statistics such as the mean , median , standard deviation , percentiles , and range were used , while a p - value was set at 0.05 to test significance.resultsaverage macular thickness and total macular volume were larger in males compared to females . with each year of increasing age , these variables decreased by 0.556 m and 0.0156 mm3 for average macular thickness and total macular volume , respectively . 
 the macular thickness was greatest in the inner superior section and lowest at the center of the fovea . 
 the volume was greatest in the outer nasal section and thinnest in the fovea . 
 the central subfield thickness ( r=0.243 , p=0.055 ) and foveal volume ( r=0.216 , p=0.09 ) did not correlate with age.conclusionmales and females differ significantly with regard to macular thickness and macular volume measurements 
 . reports by other studies that the increase in axial length reduced thickness and volume , were negated by this study which found a positive correlation among axial length , thickness , and volume . 
Introduction
Materials and methods
Subjects
Statistical analysis
Results
Discussion
Conclusion