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   Table of Contents      
ORIGINAL ARTICLE
Year : 2004  |  Volume : 52  |  Issue : 3  |  Page : 199-204

Macular thickness evaluation using the optical coherence tomography in normal Indian eyes.


Dr. Rajendra Prasad Centre for Ophthalmic Sciences, All Institute of Medical Sciences, New Delhi, India

Date of Submission05-Jun-2003
Date of Acceptance06-Jan-2004

Correspondence Address:
Hem K Tewari
Dr. Rajendra Prasad Centre for Ophthalmic Sciences, All Institute of Medical Sciences, New Delhi
India
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Source of Support: None, Conflict of Interest: None


PMID: 15510458

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  Abstract 

PURPOSE: To determine the normative values for macular thickness and volume by Optical Coherence Tomography (OCT 3) in healthy Indian subjects. METHODS: The macula of 170 consecutive, randomly selected normal subjects was imaged on OCT 3 in this cross-sectional study. OCT parameters of macular thickness were analysed with baseline variables including age, gender, axial length and refractive error. RESULTS: The average foveal thickness in the population under study was 149.16 +/- 21.15 micro. Macular thickness and volume parameters of OCT correlated significantly (Pearson's Correlation coefficient) with age (r=0.23, P<0.01), but not with gender, axial length and refraction. CONCLUSIONS: The macular thickness and volume parameters have a significant correlation with age. This normative database of macular thickness by OCT in Indian eyes may be a useful guideline for management and further research in diseases of the macula and glaucoma.

Keywords: Macular thickness, optical coherence tomography


How to cite this article:
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

How to cite this URL:
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 [serial online] 2004 [cited 2019 May 22];52:199-204. Available from: http://www.ijo.in/text.asp?2004/52/3/199/14591



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High resolution and reproducible measurement of the macular thickness are needed for both medical and surgical management of macular diseases. Recently it has also been found useful in diagnosis and management of glaucoma. Optical coherence tomography (OCT) is a new diagnostic technology for high-resolution, cross-sectional, quantitative imaging of the retina.[1] OCT is a non-invasive non-contact technique which uses near infrared low coherent light passing through a Michelson interferometer to obtain two dimensional images of the retina and optic nerve head. The resolution of OCT 3 is approximately 10 µ and 20 µ in the axial and lateral planes respectively.[1], [2] There are few large studies on the normative data for macular thickness using the OCT. The macular thickness measurement for diagnostic function may differ with the population used as a database. Thus it is desirable that measurements derived from the normative population be as close as possible to the population for which the instrument is to be used. To the best of our knowledge there is no reported normative database for macular thickness measurement by OCT in normal Indian eyes (Medline search). This study was done to establish the normal macular thickness and volume parameters using OCT 3 in Indian eyes.


  Materials and Methods Top


The subjects were healthy volunteers, non-blood related patient attendants and patients undergoing evaluation for refractive errors, dry eyes, presbyopia, etc. 340 eyes of 170 subjects were included in the study from March 2003 to August 2003. Informed consent was obtained from all the volunteers. All subjects underwent anterior segment evaluation. Slitlamp biomicroscopy using a +90D lens was performed to exclude any posterior segment pathology. Patients with a history of glaucoma, trauma, laser therapy or intraocular surgery, posterior segment pathology, media opacity, family history of macular diseases or glaucoma, unexplained visual loss and history of concurrent systemic disease were excluded from the study. In all the subjects, refraction (Humphrey Auto Refractor and Model 599, Zeiss-Humphrey, Dublin, CA) and axial length (Humphrey Ultrasonic Biometer 820, Allergan-Humphrey, San Leandro, CA) measurements were performed.

All subjects underwent macular scanning using commercially available Optical Coherence Tomographer 3 (model 3000, Humphrey-Zeiss medical system, San Leandro, CA) with A 2.0 version software. Detailed descriptions of the principles of OCT have already been published. [3],[4],[5],[6],[7],[8],[9],[10],[11],[12] The OCT interferometer electronically detects, collects, processes and stores the echo delay patterns from the retina. With each scan pass, the OCT captures from 128 to 768 A scans. Each A scan consists of 1024 data points over a 2 mm depth, thus the OCT 3 integrates from 131,072 to 786,432 data points to construct a cross-sectional image (tomogram) of retinal anatomy. It displays the tomograms in real time using a false colour scale that represents the degree of light backscattering from tissues at varying depth in the retina.

All OCT measurements were performed following pupillary dilation. Internal fixation was chosen because of better reproducibility than external fixation.[13] In all patients, the macular thickness map scan protocol was used, which includes six radial lines through a common central axis centered on the fovea. This protocol consisted of radial scan length of 6 mm at equally spaced angular (30°) orientation [Figure - 1]. Measurement of retinal thickness at selected points on the tomographs was obtained automatically by means of a computer algorithm, which assumes that the first highly reflective band corresponds to the vitreoretinal interface and the second corresponds to the retinal pigment epithelium. Thus retinal thickness measurement was made by evaluating the displacement between anterior surfaces of these two interfaces. The retinal thickness volume tabular analysis protocol was used in this study. This provides the retinal thickness and volume data table that includes thickness and volume quadrants, averages, and ratio among the quadrants. The volumetric study is based on the rationale that thickness measurements represented an average thickness in nine regions and the weighted average thickness of the nine regions multiplied by scanning area provide volume estimate [The weighted average thickness (pr2) = volume].[14] The macular thickness and volume map was divided into nine sections and displayed as three concentric circles including a central circle, an inner ring and an outer ring with diameter of 1mm, 3mm, 6mm respectively, each ring being divided into four quadrants. All OCT parameters were stored in an MS Excel 2000 spreadsheet. Statistical analysis was performed with SPSS 11.0 software for Windows (SPSS Inc. Chicago: IL).


  Results Top


One eye was randomly selected from each patient (n=170) for the final analysis [Table - 1]. The mean age of the subjects was 35.54 years (range 10-78 years). Of the 170 eyes, 83 (48.8%) were emmetropic, 37 (21.7%) were hypermetropic and 50 (29.4%) were myopic. The range of refractive error in the present study population was -8.0 D to +5.8 D. The study population comprised 67 (39.4%) females and 103 (60.58%) males. All eyes had best corrected visual acuity of 6/9 or better.

The unpaired t-test was used to compare the macular thickness parameters in males and females [Table - 1] and differences between the majorities of values were not found significant [Table - 2]. Both the minimum foveal thickness and average macular thickness had a normal distribution in the sample population [Table - 2] and [Table - 3]. Age was significantly correlated with minimum foveal thickness (r=0.23, P<0.01) (Pearson's Correlation coefficient) and with total macular volume (r=-0.26, P<0.01). There was no correlation between axial length and refraction with OCT parameters. Most of the macular thickness and volume parameters were found to have good correlation with each other.


  Discussion Top


Various modalities used for assessing macular thickness include slitlamp biomicroscopy, stereoscopic fundus photography and fundus fluorescein angiography. Interpretation by all these methods are subjective and semi-quantitative.[13] Optical Coherence Tomography offers an objective method of high-resolution cross-sectional imaging of the retina utilising near infrared light to detect relative changes in reflection at optical interfaces.[1] In several studies, measurements from OCT have been found accurate, precise, reproducible and repeatable.[12],[13],[15],[16],[17]

OCT has been found to be useful for detecting early diabetic macular abnormalities and in monitoring the effect of laser treatment on macular oedema.[18],[19],[20],[21],[22],[23],[24] Foveal thickness is a strong and independent predictor of clinically significant macular oedema (CSME).[24] Macular thickness changes have shown to be well correlated with changes in visual function and retinal nerve fibre layer (RNFL) structure in glaucoma, OCT macular volumes are said to correlate significantly with glaucoma status. [25],[26],[27],[28]

A summary of macular thickness evaluation by OCT reported so far in the literature by various authors is shown in [Table - 3].[6],[9],[12],[13],[17],[22],[26],[28],[29],[30],[31],[32] Our results showed a minimum foveal thickness of 149.16 ± 21.15 µ using the macular thickness scan protocol and retinal thickness/ volume tabular analysis protocol. Many studies[6],[9],[13],[17],[22],[28],[29],[30],[31],[32] have shown macular thickness at around 150 µ, two studies[12],[15] at approximately 175 µ, and one study[26] at 210 µ.The cause for this discrepancy had not been addressed earlier but might be related to the ethnicity of the study group, the OCT model, the scan (e.g. radial vs linear) and analysis protocol. This discrepancy highlights the importance of paying attention to the above variables before making comparisons.

In our study, age had a significant positive correlation with minimum foveal thickness but not with average foveal thickness. Axial length and refraction were not significantly correlated with minimum and average foveal thickness. No significant difference was seen in average foveal thickness and minimum foveal thickness in males as compared to females. Some of these results differ from recent studies, which may be due to differences in ethnicity of study subjects or differences in study design. Gobel et al[31] found that retinal thickness was not dependent upon age or length of the eye, no corrections were necessary when analysing pathological retinal thickening, such as in diabetic retinal disease. Kanai et al showed that retinal thickness is reduced with age at all points in the retina except the fovea.[33] They also found that there was a significant correlation between attenuation of parafoveal retinal thickness and age. However, no change in the retinal thickness at the foveola was observed with increasing age. Wakitani et al found that retinal thickness in the three circular areas did not correlate with the axial length and refraction of the eye.[34] Massin et al showed that the axial length of the eye did not influence average thickness in the temporal peripapillary retinal area, when the magnification of the optical system and the optical dimension of the eyes were taken into consideration.[23] In their study, mean macular thickness was not affected significantly by age or laterality, but it was significantly greater in men than women (P=0.0139). [23]

In conclusion, our study provides a normative database for macular thickness and volume parameters in Indian eyes by Optical Coherence Tomography. This could be useful in diagnosis, management and further research in macular disorders and glaucoma.



 
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    Figures

  [Figure - 1]
 
 
    Tables

  [Table - 1], [Table - 2], [Table - 3]


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