Optical coherence tomography (OCT) is a noninvasive and noncontact diagnostic tool that can perform cross-sectional imaging of the retina and the optic nerve. The time domain OCT system measures retinal thickness from the inner segment/outer segment junction to the internal limiting membrane. This gives a lesser thickness of the retina, whereas the spectral domain (SD)-OCT system measures retinal thickness from the retinal pigment epithelium to the internal limiting membrane, thereby giving the correct thickness of the retina.1
Different OCT systems show variations in measurements.2–4 Therefore, macular thickness measurement values using one OCT system are different from another. The devices are not interchangeable because the macular thickness absolute value differs for each device.4 Moreover, there are reports of variations in macular thickness in different races.5–7
The Zeiss SD-OCT is one of the most commonly used OCT systems in the world, including India. Therefore, it is pertinent to know the normative data of the Zeiss SD-OCT system in Indian eyes.
There are 2 reports of normative data of macular thickness in healthy Indian eyes.8,9 The first study was on a Zeiss Stratus OCT; therefore, the thickness measurements were lesser.8 The second study was on a Spectralis SD-OCT, which is different from the Zeiss SD-OCT.9
To the best of our knowledge, there is no published study which reports the macular thickness in healthy Indian eyes using a Zeiss SD-OCT. Therefore, the objective of this study was to report the macular thickness in healthy Indian eyes using a Zeiss SD-OCT.
MATERIALS AND METHODS
This was a cross-sectional, observational study conducted at the department of ophthalmology of a medical college in northeast India between March 2013 and April 2014. Institute ethics clearance was obtained, and the study adhered to the tenets of the Declaration of Helsinki.
The location of the medical college includes the headquarters of many central government agencies, such as the Eastern Air Command, Indian Army, Border Security Force, Assam Rifles, Indo-Tibetan Border Police, Gurkha regiments, Bharat Sanchar Nigam Limited, Geological Society of India, and a central university. Therefore, there are many central government employees living in and around this medical college. Moreover, the All India Services officers who are posted to this area come to the medical college for annual medical checkups. Hence, the study population may be nearly representative of all India.
Inclusion criteria included subjects aged 18 years or older with best-corrected vision of 20/20, spherical refraction within ±0.5 diopters, and cylindrical correction within ±0.5 diopters.
Exclusion criteria included media opacities precluding funduscopy and SD-OCT imaging; retinopathies; glaucomatous or other optic nerve head (ONH) abnormalities; history of glaucoma, intraocular surgery, or laser treatment; and 1-eyed patients.
All the subjects underwent complete ophthalmic examination, which included visual acuity testing, refraction, slit lamp examination, and dilated funduscopic examination with a 90-diopter lens and indirect ophthalmoscope. The SD-OCT scanning was performed using a Cirrus high-definition-OCT system (Model 500; Carl Zeiss Meditec, Dublin, CA). All the scans were performed by an experienced optometrist who was well trained in using the Zeiss Cirrus HD-OCT. All the subjects underwent macular cube scanning of both eyes after instillation of artificial tears.
A total of 3 good and well-centered macular cube scans were taken in each eye, with the fovea as the centermost point. The minimum signal strength eligible was 8/10 for each scan. The scans which were clearly distinguishable, devoid of any artifacts, and which showed the full depth of the retina were included in the study (Fig. 1).
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FIGURE 1: An ETDRS macular cube scan map of one of the subjects, which is clearly distinguishable and devoid of artifacts.
As in other similar studies, macular thickness was reported in a modified Early Treatment Diabetic Retinopathy Study (ETDRS) macular map.10 The central subfield was 1 mm in diameter, and the inner and outer subfields had diameters of 3 mm and 6 mm, respectively. The OCT volume scan was performed on a 512 × 128 degree cube (Fig. 1).
The central subfield, subfields of the inner 3-mm and outer 6-mm rings, and the total macular volume were calculated. An average of 3 scans was taken for each subfield.
Statistical Analysis
Descriptive statistics was used to calculate the mean ± SD and percentages. Independent t test was used to compare values between 2 groups, such as men and women or right and left eyes. Linear regression was used to determine if there was any association of central subfield thickness (CST) with age and sex. Pearson correlation was used to determine correlation between the values of 2 eyes. A 95% limit and 5% level of significance were adopted. Therefore, a P value less than 0.05 was considered significant. Statistical analysis was performed using the SPSS software package (SPSS for Windows, version 22.0; SPSS, Inc, Chicago, IL).
RESULTS
The mean age of the subjects was 38.05 ± 12.13 years (range, 18–78 years). There were 179 men (44.8%) and 221 women (55.2%). The mean ages in men and women were 39.19 ± 12.16 and 37.13 ± 12.05 years, respectively (P > 0.05) (Table 1). The mean CST of all subjects was 240.40 ± 18.26 μm, and mean macular thickness was 287.87 ± 18.07 μm (Table 2). The central subfield was the thinnest area. In the 3-mm ring, the superior subfield was the thickest, followed by the nasal, inferior, and temporal subfields. In the 6-mm ring, the nasal subfield was the thickest, followed by the superior, inferior, and temporal subfields. Overall, the nasal quadrant was the thickest, followed by the superior, inferior, and temporal subfields (Table 2).
TABLE 1: Profile of Patients
TABLE 2: Mean Macular Thickness by ETDRS Subfields
The mean CST in the right and left eyes were 240.40 ± 18.25 μm and 239.65 ± 17.73 μm, respectively (P = 0.55). There was a strong correlation of all the measured values between the right and left eyes (Pearson correlation, P = 0.87 for CST). Therefore, values of only the right eyes were taken for the most analyses. Male sex was associated with greater mean central subfield and mean macular thicknesses compared with female sex (P < 0.05 in all quadrants except in the superior, inferior, and nasal outer quadrants) (Table 3). Mean CST was associated with sex (adjusted r2 = 0.095; P < 0.05) but not with age (adjusted r2 = 0.001; P = 0.229) (Fig. 2).
TABLE 3: Mean Macular Thickness by Sex
FIGURE 2: No association of CST with age was seen.
The central subfield, inferior outer subfield, and temporal outer subfield thicknesses did not correlate significantly with age (P = 0.229, 0.233, and 0.156, respectively) but the rest of the ETDRS subfields showed significant correlation with age (P < 0.05) (Table 4).
TABLE 4: Linear Regression Analyses of ETDRS Subfields With Age
DISCUSSION
The normal macular thickness in healthy Indian eyes has been reported previously in 2 studies by Tiwari et al and Appukuttan et al.8,9 The macular thickness reported in the first study was less because the authors used a Zeiss Stratus OCT.8 The second study used a Spectralis SD-OCT.9 As studies have confirmed that using different OCT systems gives different measurements for the same macula,2–4 it can be inferred that measurements using a Zeiss SD-OCT will be different than those from a Spectralis SD-OCT.
The mean central subfield and macular thicknesses in the current study were 240.40 ± 18.26 μm and 287.87 ± 18.07 μm, respectively. These are greater than those reported by Tiwari et al as SD-OCT uses the retinal pigment epithelium as the outer retinal boundary, whereas a Stratus OCT uses the inner segment/outer segment as the outer retinal boundary. However, these values are less than the findings of Appakuttan et al. This could be because of the different SD-OCT systems. Similarly, the values are different from some other studies around the world (Table 5).
TABLE 5: Comparison of Mean Central Subfield Thickness in Different Studies
Macular thickness was thinnest at the fovea. It was thickest at the nasal subfield, followed by the superior, inferior, and temporal subfields. This is consistent with previous studies.9,11,12
The central subfield, inferior outer subfield, and temporal outer subfield thicknesses did not correlate significantly with age (P = 0.229, 0.233, and 0.156, respectively), but the rest of the ETDRS subfields showed significant correlation with age (Table 4). This is in partial agreement with the findings of Appukuttan et al, Kanai et al, and Manassakorn et al9,13,14 where significant correlation was found in all ETDRS subfields except the central subfield. However, Huang et al and Grover et al3,15 did not find a statistically significant association between macular thickness and age. This could be because of the small sample size of their study.
Men had a greater foveal thickness compared with women (P = 0.00). This is in contrast with the findings of Tiwari et al and Grover et al,8,15 but it is in agreement with Wong et al, Appukuttan et al, Song et al, and Masssin et al.7,9,16,17
In previous studies, refraction was found to have no association with macular thickness.8,9,16,17 Therefore, we did not explore this further in our study.
As the mean CST was 240.40 ± 18.26 μm, using the criteria of mean ± 2 SDs, we suggest 222.14 to 258.66 μm as the normal range for CST in the Indian population using the Zeiss SD-OCT.
Our study had certain limitations. First, the number of subjects older than age 60 was small. A majority of these subjects were excluded because of exclusion criteria such as media opacities due to cataract, history of intraocular surgeries, retinopathies, and so on. A greater number of elderly individuals would have made a stronger case for age variation in macular thickness. Second, intraocular pressure (IOP) measurements were not included in the study. As increased IOP is a definite risk factor for glaucoma, the role of IOP on macular thickness changes cannot be ruled out, although subjects with glaucomatous or ONH abnormalities or a history of glaucoma were excluded. Thirdly, retinal nerve fiber layer and ONH evaluations were not done. These would have provided objective confirmation of the absence of glaucomatous damage, which might affect macular measurements. Finally, the refractive error range in the study population was narrow. It is not very representative of the general population. It would have been interesting to see the effect of refractive error on macular thickness in our own study, although previous studies have not found any association of refractive error with macular thickness.8,9,16,17
To conclude, this study provides the normative data for macular thickness using the Zeiss SD-OCT system in healthy Indian eyes. This is important because macular thickness values vary among different OCT systems. It will serve as a baseline for diagnosing and treating macular pathologies in Indian eyes using a Zeiss SD-OCT, as there is no previous study of macular thickness in healthy Indian eyes using this OCT system.
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