|
| |
 |
|
| ORIGINAL ARTICLE |
|
| Year : 2022 | Volume
: 71
| Issue : 3 | Page : 191-198 |
|
Age- and gender-based morphometric variation of macula in indian population using optical coherence tomography
Pareesa Rabbani1, Shalini Kumar1, Taskin Khan2, Shayama K Razdan1
1 Department of Anatomy, HIMSR, New Delhi, India
2 Department of Ophthalmology, HIMSR, New Delhi, India
| Date of Submission | 12-Dec-2021 |
| Date of Decision | 16-May-2022 |
| Date of Acceptance | 05-Jun-2022 |
| Date of Web Publication | 20-Sep-2022 |
Correspondence Address:
Dr. Shalini Kumar
Department of Anatomy, Hamdard Institute of Medical Sciences and Research (HIMSR), Hamdard University, New Delhi - 110 062
India
 Source of Support: None, Conflict of Interest: None
DOI: 10.4103/jasi.jasi_205_21
Introduction: Histological morphometric analysis of retinal layers has inherent limitations while processing the specimen. A new in vivo technique, optical coherence tomography (OCT), has been developed that can be used to analyze and differentiate normal and pathological retina. To do a morphometric analysis of normal macula in the adult population of India and study its variations on the grounds of sex and age. Material and Methods: One hundred (200 Eyes) healthy adult subjects (18–65 years) underwent macular cube scanning using Zeiss spectral-domain OCT (SD-OCT). Macular thickness from all nine regions of the Early Treatment Diabetic Retinopathy study map was documented for each subject. Their variations for age and sex were determined manually and automatically. Statistical analysis was done by entering into an MS Excel sheet using IBM SPSS Statistics for Windows,Version 25.0. Armonk, NY: IBM Corp. (2017). The data were also analyzed using an independent t-test and analysis of variance. Results: The mean age of the subjects was 34.2 ± 13 (range, 19–65) years. The mean Central Subfield Thickness (CST) measured automatically (foveal thickness) and manually was 239.52 ± 22.9 μm and 167.75 ± 21.94 μm, respectively, while mean macular thickness was 284.73 ± 15.7 μm and 276.76 ± 14.84 μm. Males were associated with greater foveal, central foveal thickness, and mean macular thickness than females (P < 0.0001). There was no significant correlation of CST, outer and inner ring, and mean macular thickness with increasing age (>30 years). However, with respect to gender in the inner ring (parafoveal region), all the quadrants except the inferior quadrant, CST was significantly (P < 0.0001) higher in males than females while in the outer ring (perifoveal region), it was the temporal quadrant that had statistically significant higher CST in males compared to females. Discussion and Conclusion: The results will add evidence and can serve as a normal database in morphometry of macula in Indians, created and found significantly different in already fed normal comparative data in SD-OCT machines. It will help analyze morphometry of macula and understand macular pathologies in Indian eyes.
Keywords: Central subfield thickness, macular thickness, optical coherence tomography
How to cite this article:
Rabbani P, Kumar S, Khan T, Razdan SK. Age- and gender-based morphometric variation of macula in indian population using optical coherence tomography. J Anat Soc India 2022;71:191-8 |
How to cite this URL:
Rabbani P, Kumar S, Khan T, Razdan SK. Age- and gender-based morphometric variation of macula in indian population using optical coherence tomography. J Anat Soc India [serial online] 2022 [cited 2023 Mar 29];71:191-8. Available from: https://www.jasi.org.in/text.asp?2022/71/3/191/356494 |
| Introduction | |  |
The retina is a thin sheet of cells, ranging from <100 μm at its edge to a maximum of around 300 μm at the foveal rim. It lines the inner posterior surface of the eyeball, lying in between the choroid externally and the vitreous body internally, and terminates anteriorly at the Ora Serrata.[1] The total surface area of the retina is 266 mm2. It is thickest near the disc, about 0.56 mm, and thinnest in the periphery, where it is approximately 0.18 mm wide at the equator and 0.3 mm at Ora Serrata.[2] The retina is derived embryologically from the two layers of the invaginated optic vesicle. The outer layer becomes a layer of cuboidal pigment cells that separates the choroid from the neural retina and therefore forms the outermost layer of the retina: the retinal pigment epithelium (RPE). The other nine layers of the retina develop from the inner layer of the optic vesicle and form the neural retina. Significant landmarks which are seen in the retina are[3] optic disc which is a round to oval well-defined structure about 1.5 mm in diameter where retinal layers terminate except the nerve fibers which pass through the lamina cribrosa to run into the optic nerve. The center contains a depression or pit called the physiological cup. Another landmark is area centralis (Macula Lutea) or anatomical macula which is the part of the retina lying lateral to the optic disc, demarcated by superior and inferior temporal arteries with an average diameter of 5.5 mm. It corresponds to approximately 15° of the visual field where the fovea is a depression in the center. The foveola constitutes its floor surrounded by a parafoveal and perifoveal ring around it. The peripheral retina is an area which lies outside the macula. The retina comprises various epithelial, neural, and glial cell types, whose distribution is divided into ten layers.[1] Microscopic structure of the retina, as seen under light microscopy, can be distinguished into ten layers.[4],[5]
Since times immemorial retinal studies were being done by fundus photography, angiography, B-Scan ultrasonography, and various histological techniques, but all these techniques have inherent limitations like dehydration of tissue and retinal detachment. Moreover, histological techniques are not in vivo studies and also there are limited cadaveric studies on human and primate eyes. Hence, these layers can be seen in vivo using a newly emerged technique known as optical coherence tomography (OCT), which uses backscattered light to visualize layers by differences in their optical scattering properties. Each distinct retinal layer can be visualized by OCT that corresponds well to histological studies[6] On OCT, the retinal layers can be seen with different color coding as well. The hyperreflective lines are seen as bright (red or white). Those layers with minimum reflectivity are shown as dark (blue or black), and intermediate reflectivity is green. OCT scan identifies the following hyper-reflective layers which are nerve fiber layer, inner plexiform layer, outer plexiform layer, external limiting membrane, RPE, ellipsoid zone, inter digitation zone. The hyporeflective layers which can be seen are ganglion cell layer, inner nuclear layer, outer nuclear layer, Henle's nerve fiber layer at the macula, outer segments of photoreceptors, myoid zone, chorio capillaries, and inner and outer choroidal layers as shown in [Figure 1]a, [Figure 1]b, [Figure 1]c. | Figure 1: (a) Representative OCT image from a healthy subject of the right eye. (b) Layers seen on SD-OCT, These hyperreflective layers should be identified while evaluating an OCT scan: NFL, IPL, OPL, ELM, RPE, Ellipsoid zone, and Inter digitation zone. The hypo reflective layers are GCL, INL, ONL and Henle's nerve fiber layer at the macula, OSPR, and ISPR and myoid zone–Chorio capillaries and inner and outer choroidal layers. (c) Standard ETDRS. This is a representation of standard ETDRS map showing map diameters centered on the fovea and nine standard ETDRS Map ETDRS regions: Outer and inner superior quadrant, outer and inner inferior quadrant, outer temporal quadrant, inner temporal quadrant, outer nasal quadrant and INQ. The macular thickness was defined as the distance between the ILM and the inner boundary of RPE in each of the nine regions. OCT: optical coherence tomography, SD-OCT: Spectral-domain optical coherence tomography, NFL: Nerve fiber layer, IPL: Ganglion cell layer Inner plexiform layer, OPL: Outer plexiform layer, ELM: External limiting membrane, GCL: Ganglion cell layer, INL: Inner nuclear layer, ONL: Outer nuclear layer, OSPR: Outer segments of photoreceptors, ISPR: inner segment of photoreceptors, ETDRS: Early treatment diabetic retinopathy study, INQ: inner nasal quadrant, RPE: Retinal pigment epithelium, ILM: Internal limiting membrane
Click here to view |
The most important landmark of the retina where images are formed is known as the anatomical macula or macula lutea. It has the highest concentration density of cones responsible for color vision and central vision. Several retinal diseases can affect the macula, leading to vision loss if they are not treated earlier. Therefore, to lower down morbidity by analyzing diseases such as diabetic retinopathy and macular degenerations earlier assessment of change in macular anatomy is necessary and can be assessed using OCT.[7] Whereas the light microscopic structure of macula has the outermost layer of RPE,consisting of a single layer of hexagonal cells as in the retina elsewhere but in the macular area, they are taller and denser. The outer nuclear layer contains the nuclei of cones at the fovea centralis as rods are absent. Here the cones are covered by an internal limiting membrane (ILM). Outer Plexiform Layer is thickest at the macula, formed by inner fibers of cones known as Henle's layer , which are arranged obliquely.The Ganglion cell layer in the macula is about 6-8 layers thick, seen only at the edge of the foveola. This layer is absent at the foveola and optic disc.[8]
Variations have been reported in macular structure concerning age, gender, ethnicity, and various ocular diseases and systemic comorbidities like diabetes and hypertension before the development of retinopathies These variations are perhaps because of the normative database that has been created and developed in different OCT machines based on studies conducted mostly on Caucasians.[9] Hence, it is imperative to assess and overcome the inherent limitations faced during the preparation and processing of slides. It is necessary to have normative data based on Indian eyes to distinguish between normal anatomical variants from macular pathology in vivo. However, a normative database for variations in normal macular anatomy on OCT remains lacking on Indian eyes.[10] The present study serves as a baseline for OCT data of normal macular morphology in healthy Indian eyes.
| Material and Methods | | |
A cross-sectional study was conducted on the subjects reporting to the department of ophthalmology outpatient department in the age group of 18–65 years. After getting ethical approval from institutional ethical committee and informed written consent from the study subjects. A complete ophthalmic examination was done including best-corrected visual acuity, slit-lamp examination, and dilated fundus examination to rule out any ocular disease. The subjects who had clear ocular media with best-corrected visual acuity of 20/20, a refractive error with the spherical equivalent of less than or equal to ±6D, intraocular pressure of <21 mm Hg, and without any signs of glaucoma or any other ocular and systemic disorder that would affect eye were included in the study. Those who had undergone ocular surgeries or corneal transplantation and subjects with maculopathies or retinopathies were excluded from the study. The sample size taken was 100 subjects (200 eyes), the study duration was 1 year.
The selected subjects underwent dilated macular imaging using OCT spectral-domain OCT (SD-OCT) (Carl Zeiss, Primus 200).
Macular thickness was defined as the distance between the ILM and the inner boundary of RPE in each of the nine regions as defined by Early Treatment Diabetic Retinopathy Study[11] using Carl Zeiss SD-OCT. The inner and outer rings were segmented into 4 quadrants, with radii of 1.5 mm and 3 mm, respectively. Foveal thickness was defined as the average thickness in the central 1000-μm diameter of the Early Treatment Diabetic Retinopathy Study layout. Central foveal thickness was defined as the mean thickness at the point of intersection of the 6 radial scans. Automatic measurements were taken in all nine regions and were documented. The anatomical landmarks of the macula on OCT imaging were manually determined based on the International Nomenclature for OCT Panel[12] based on relative reflectivity by the structures The manually determined central foveal thickness measurements were compared with the values generated by the software. The macular thickness of the central foveal area and all four quadrants (superior, inferior, nasal, and temporal) of the outer and inner rings were evaluated. Statistical analysis of the data collected was done using SPSS version 25. The data were also analyzed using an independent t-test and statistically tested using appropriate tests of significance.
| Results | | |
The total number of subjects enrolled in the study was 100 (200 eyes). The number of males and females was 50 each (total of 200 eyes). The age of the subjects ranged from 18 to 65 years (mean 34.2 ± 13 years).
Average morphometric parameters of macula of 100 subjects (200 eyes) in the study are represented in [Table 1]. | Table 1: Average morphometric parameters of macula of 100 subjects (200 eyes)
Click here to view |
As shown in [Table 1], the mean foveal thickness (measured automatically) was 239.52 ± 22.9 μm, while measured manually, the central foveal thickness was found to be 167.75 ± 21.94 μm, 72 μm less than automatically measured data. The mean thickness in both parafoveal region (inner ring) and perifoveal region (outer ring) of Macula, the nasal quadrant was found to be thickest, with values of 311.7 ± 20.6 micrometer and 293.29 ± 18.6micrometer ,respectively,while the superior quadrant of the two regions it was 309.81 ± 19.5 micrometer and 276.73 ± 16.91 micrometer.
The mean macular thickness was 284.73 ± 15.7 μm for automatically fed data while on manual evaluation; it was 276.76 ± 14.84 μm as shown in [Figure 2]. | Figure 2: Descriptive statistics of mean macular thickness (μm) (automatic measurement) and mean macular thickness (μm) (manual measurement) of study subjects
Click here to view |
The mean central subfield foveal thickness was found to be 239.52 μm when measured automatically and manually it was found to be 167.75 μm, 72 μm less than automatically fed data as shown in [Figure 3]a. According to Bland–Altman plot of Central Subfield Thickness, the mean was found to be 71.77 as shown in [Figure 3]b. | Figure 3: (a) Comparison of central subfield thickness between automatic and manual measurement. (b) Bland-Altman plot of central subfield thickness to compare measurements of automatic and manual
Click here to view |
The overall nasal quadrant was thickest followed by superior and inferior quadrant as shown in [Figure 4]. Furthermore, it was seen that the temporal quadrant was thinnest in the perifoveal (outer) ring. | Figure 4: Descriptive statistics of temporal (μm), nasal (μm), superior (μm) and inferior (μm) quadrants of outer ring of study subjects
Click here to view |
[Table 2] shows the macular parameters evaluated on the grounds of gender. The mean foveal thickness in males was 248.54 μm, higher as compared to females, 230.5 μm. The mean central foveal thickness in males was 172.51 μm and in females, it was 162.99 μm. The mean macular thickness measured automatically was higher in males, 290.16 μm compared to females, 279.31 μm. These values are statistically significant (P < 0.001). In the perifoveal region, only the temporal quadrant showed higher thickness (being higher in males) that was statistically significant, the superior, inferior, and nasal quadrants did not show any statistically significant difference. While in the parafoveal region, all the quadrants except inferior quadrant was statistically significant (P < 0.001).
As shown in [Figure 5], the mean macular thickness (both automatic and manual) was higher in males as compared to females.
As shown in [Table 3], the mean foveal thickness, central foveal thickness, macular thickness, parafoveal, and perifoveal thickness did not correlate significantly with increasing age.
| Discussion | | |
OCT has emerged as a useful imaging technique by providing new high-resolution images of retina. It is clinically relevant for the diagnosis of a variety of diseases such as glaucoma, macular degeneration, and macular edema.
It is important to have a normative data to be able to distinguish between normal anatomical variants from macular pathology. The study of normal morphology of each distinctive layer of retina using OCT is important to analyze variations and correlate it clinically such as in cases of retinopathies, maculopathies. It is an important noninvasive, rapid, in vivo, technique that helps in determining anatomical changes in retina by producing cross-sectional images. Each distinct layer of retina can be visualized on OCT that corresponds well to histological appearance.[3]
A number of studies have found variations in normal macular anatomy with regard to age, gender, and ethnicity besides also in various ocular diseases.
Natung et al. in 2016,[10] assessed macular thickness in healthy Indian subjects aged 18–65 years and found that foveal thickness was more in males than females while no significant correlation was found on grounds of age. The foveal thickness was 240.40 ± 18.26 μm, and mean macular thickness was 287.87 ± 18.07 μm. Males had greater foveal thickness and mean macular thickness compared to females (P < 0.05). There was statistical association of foveal and mean macular thickness with sex (P < 0.05) but not with age. Liu et al., 2011,[13] assessed 192 eyes of subjects aged 20–90 years using Cirrhus OCT. They found that foveal thickness never exceeded 308 μm in healthy controls. The macular thickness did decrease with age but foveal thickness did not change significantly with age similar to the results seen in the present study. The values were higher in males as compared to females. They stated that CST ranged from 216.8 μm to 308 μm in normal eyes. These results are consistent with our study where foveal thickness never exceeded 239.52 ± 22.9. The mean foveal thickness varied statistically significantly with sex (P < 0.0001) but not with age. The macular thickness was 284.73 ± 15.7 μm reported higher in males (290.16 ± 15.2) as compared to females (279.31 ± 14.28) but had no statistical relation with age. The findings of a study by Gupta et al. in 2013,[14] differ from our findings as they stated that macular thickness correlates with age and it was thinner in older people, this could be because the mean age in their study was 53.17 years and maximum age was 80 years while in the present study the average age is 34.2 years and maximum age of the subject is 65 years.
Chan et al. in 2006,[15] reported macular thickness measurements in 37 healthy controls using OCT 3. In their study, macular thickness was thinner at center of fovea. The temporal quadrant was found to be thinner than the nasal quadrant. Waris et al. 2015[16] assessed 20 subjects who underwent complete ophthalmic examination followed by macular assessment using OCT 3. Macular thickness measurements were thinnest at the center of the fovea, thickest within 3-mm diameter of the center, and reduced toward the periphery of the macula. The temporal quadrant was thinner than the nasal quadrant. This might be due to anatomical relationship of the converging of nerve fibers with the optic disc. The results were similar to our study where temporal quadrant was thinner than nasal quadrant. In our study, only the temporal quadrant was statistically significant on grounds of gender. In the parafoveal region, all the quadrants except inferior quadrant were statistically significant (P < 0.001) on the basis of gender.
In our study, foveal thickness was found to be 239.52 ± 22.9 μm and central foveal thickness was found to be 167.75 ± 21.94 μm which was 72 μm lower than automatically obtained values. The Bland–Altman plot as depicted in [Figure 3]a and [Figure 3]b in the present study also shows that the automatic measurements of foveal thickness obtained are higher than those measured manually. This reflects the difference in approach between the manual method and the automatic method of the OCT mapping software. The software automatically determines the mean and standard deviation thickness for the center point where all 6 scans intersect, whereas we manually located the minimum point on each separate radial scan and averaged those values. This is a significant finding while determining and interpreting normal from abnormal macular parameters when doing OCT scan by automatic method.
Massin et al. 2002[17] found that the mean macular thickness was not affected significantly by age or laterality, but it was significantly higher in men than women (P = 0.0139). It has been suspected that increasing age shows decreased macular thickness but no such correlation was found in our study. The results vary from a study done by Tewari et al.[18] who reported a positive correlation with minimum foveal thickness, but not with average foveal thickness.
Anastasia et al. (2014),[19] assessed macular layer morphology in healthy controls using high-resolution SD-OCT across ethnicities comparing Asian (i.e., Indian subcontinent) and Caucasian individuals. One hundred and thirty-three healthy volunteers (67-Asian, 66-Caucasian) underwent examination using SD-OCT. The analysis of the measurements of each retinal layer at the macula was determined using tomographs obtained by SD-OCT. They found a significant difference in macular structure in Asian and Caucasian subjects. Caucasian subjects had thicker inner segment (P = 0.015 in the central region), outer segment (P = 0.04 in the temporal region), and outer nuclear (P = 0.021 and P = 0.03 for the central and temporal regions, respectively) layers, while Asians demonstrated thicker retinal pigment epithelial layer (P = 0.004 for the temporal region). They suggested that the differences in macular morphology due to ethnicity should be taken into consideration for determining control values for diagnostic purposes, and also to assess risk and prognosis of macular diseases.
A number of studies have been reported on normative data for macular thickness using different OCT machines with varying results. This might be due to the variation in the ethnicity of the study group, the OCT model (prototype, OCT 2, OCT 3), the scan (radial vs. linear), and analysis protocol. This discrepancy concludes the importance of paying attention to the above-mentioned variables before assessing OCT parameters.
Our study provides a normative database for macular thickness and volume parameters in Indian eyes by OCT. The automatic method overestimates the mean macular thickness and central foveal thickness and it is significantly associated with gender as well. This could be beneficial in assessment, early diagnosis, and management of macular disorders and retinal pathologies.
| Conclusion | | |
Various studies have reported on normative data for macular thickness using different OCT machines. Although our results are variable from previously reported values it might be due to the variation in the ethnicity of the study group, the OCT model (prototype, OCT 2, OCT 3), the scan (radial vs. linear) and analysis protocol. This discrepancy concludes the importance of paying attention to the above-mentioned variables before assessing OCT parameters.
The present study provides a normative database for macular thickness in Indian eyes by OCT SD-OCT and compares both manual with automatic methods. It also compares the differences observed with age and gender. This could be beneficial in early assessment, diagnosis, and management of macular disorders and retinal pathologies.
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
| References | | |
| 1. | Standring S, Borley NR, Gray H. Gray's Anatomy: The Anatomical Basis of Clinical Practice. 41 st ed., Anniversary Edition. Edinburgh: Churchill Livingstone/Elsevier; 2008.  |
| 2. | Singleman J, Ozanics V. Retina. In: Jakobiec, editor. Ocular Anatomy, Embryology and Tetralogy. Philadelphia: Harper and Row; 1982. |
| 3. | Tripathi RC, Tripathi BJ. Anatomy of human eye, orbit and adnexa. In: Davson H, editor. The Eye. 3 rd ed. London: Academic Press; 1984. |
| 4. | Pushplatha K. Inderbir Singh's Text Book of Human Histology. 9 th ed. New Delhi: Jaypee Brothers Medical Publishers: 2019. |
| 5. | Ross MH, Pawlina W. Histology: A Text and Atlas Book, with Correlated Cell and Microbiology. 6 th ed. Philadelphia: Lippencott Williams and Wilkins; 2011. |
| 6. | Ferguson LR, Grover S, Dominguez JM 2 nd, Balaiya S, Chalam KV. Retinal thickness measurement obtained with spectral domain optical coherence tomography assisted optical biopsy accurately correlates with ex vivo histology. PLoS One 2014;9:e111203. |
| 7. | Eagle RC Jr. Optical coherence tomography: Clinicopathologic correlations – The 2016 Gordon K. Klintworth Lecture. Ocul Oncol Pathol 2018;4:203-12. |
| 8. | Khurana AK, Khurana AK, Bhawna K. Comprehensive Ophthalmology. DL: Jaypee Brothers Medical Publishers; 2015. |
| 9. | Manassakorn A, Chaidaroon W, Ausayakhun S, Aupapong S, Wattananikorn S. Normative database of retinal nerve fiber layer and macular retinal thickness in a Thai population. Jpn J Ophthalmol 2008;52:450-6. |
| 10. | Natung T, Keditsu A, Lyngdoh LA, Dkhar B, Prakash G. Normal macular thickness in healthy Indian eyes using spectral domain optical coherence tomography. Asia Pac J Ophthalmol (Phila) 2016;5:176-9. |
| 11. | Early treatment diabetic retinopathy study design and baseline patient characteristics. ETDRS report number 7. Ophthalmology 1991;98 5 Suppl: 741-56. |
| 12. | Staurenghi G, Sadda S, Chakravarthy U, Spaide RF; International Nomenclature for Optical Coherence Tomography (IN•OCT) Panel. Proposed lexicon for anatomic landmarks in normal posterior segment spectral-domain optical coherence tomography: the IN•OCT consensus. Ophthalmology 2014;121:1572-8. |
| 13. | Liu T, Hu AY, Kaines A, Yu F, Schwartz SD, Hubschman JP. A pilot study of normative data for macular thickness and volume measurements using cirrus high-definition optical coherence tomography. Retina 2011;31:1944-50. |
| 14. | Gupta P, Sidhartha E, Tham YC, Chua DK, Liao J, Cheng CY, et al. Determinants of macular thickness using spectral domain optical coherence tomography in healthy eyes: The Singapore Chinese eye study. Invest Ophthalmol Vis Sci 2013;54:7968-76. |
| 15. | Chan A, Duker JS, Ko TH, Fujimoto JG, Schuman JS. Normal macular thickness measurements in healthy eyes using Stratus optical coherence tomography. Arch Ophthalmol 2006;124:193-8. |
| 16. | Waris A, Asghar A, Yunus SM, Malakar M. Normal macular thickness measurements in normal eyes using Fourier domain optical coherence tomography. IOSR J Dent Med Sci (IOSR-JDMS) 2015;14:63-6. |
| 17. | Massin P, Erginay A, Haouchine B, Mehidi AB, Paques M, Gaudric A. Retinal thickness in healthy and diabetic subjects measured using optical coherence tomography mapping software. Eur J Ophthalmol 2002;12:102-8. |
| 18. | Tewari HK, Wagh VB, Sony P, Venkatesh P, Singh R. Macular thickness evaluation using the optical coherence tomography in normal Indian eyes. Indian J Ophthalmol 2004;52:199-204. [ PUBMED] [Full text] |
| 19. | Pilat AV, Proudlock FA, Mohammad S, Gottlob I. Normal macular structure measured with optical coherence tomography across ethnicity. Br J Ophthalmol 2014;98:941-5. |
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
[Table 1], [Table 2], [Table 3]
|
Search Pubmed for