|Year : 2016 | Volume
| Issue : 2 | Page : 136-139
Anterior and posterior segment parameters measured with Fourier domain optical coherence tomography in photopic and scotopic conditions
Nishanee Rampersad1, Rekha Hansraj2
1 Discipline of Optometry, University of KwaZulu-Natal, Durban, South Africa
2 Discipline of Optometry, School of Health Sciences, University of KwaZulu-Natal, Durban, South Africa
|Date of Submission||29-Jul-2015|
|Date of Acceptance||23-Dec-2015|
|Date of Web Publication||5-Apr-2016|
D r. Rekha Hansraj
Discipline of Optometry, School of Health Sciences , University of KwaZulu-Natal, Private Bag X54001, Durban 4000
Source of Support: None, Conflict of Interest: None
Purpose: To compare anterior and posterior segment parameters measured with the iVue-100 optical coherence tomography (OCT) in photopic and scotopic conditions. Methods: Central and peripheral corneal thickness, retinal nerve fiber layer and macula thickness were measured using the iVue-100 OCT in 47 healthy individuals at a higher education institution in photopic (958 lux) and scotopic (0.03 lux) conditions. Results: As the lighting conditions changed from scotopic to photopic, a significant change in pupil size was noted (P < 0.001). However, there was no significant difference in central corneal thickness measurements with this change in surrounding illumination with only the temporal peripheral corneal area showing a significant difference (3.44 μm thinner). No significant differences were found in the retinal nerve fiber layer thickness. A significant decrease in the reading was noted in only the inferior (P = 0.05) and temporal (P = 0.05) inner macula area. Conclusion: Change in the ambient lighting conditions does not result in a clinically significant difference in corneal, retinal nerve fiber layer, and macula thickness when measured with the iVue-100 OCT,
Keywords: Corneal thickness, macula thickness, optical coherence tomography, photopic, retinal nerve fiber layer thickness, scotopic
|How to cite this article:|
Rampersad N, Hansraj R. Anterior and posterior segment parameters measured with Fourier domain optical coherence tomography in photopic and scotopic conditions. Indian J Ophthalmol 2016;64:136-9
|How to cite this URL:|
Rampersad N, Hansraj R. Anterior and posterior segment parameters measured with Fourier domain optical coherence tomography in photopic and scotopic conditions. Indian J Ophthalmol [serial online] 2016 [cited 2019 Oct 22];64:136-9. Available from: http://www.ijo.in/text.asp?2016/64/2/136/179726
| Introduction|| |
The thickness of various anterior and posterior ocular structures is of clinical importance and, therefore, its accuracy has to be ensured. Corneal thickness measurements are useful when considering eligibility for refractive surgery, diagnosis and management of corneal diseases, interpretation of intraocular pressure measurements, and aftercare of contact lens wearers, among others. The thickness of posterior segment structures including the retinal nerve fiber layer and the macula is critical in the diagnosis and management of optic nerve head and macula anomalies, respectively.
The accuracy of thickness measurements is dependent on the reliability of the measuring device. Technological advancements have led to the introduction of optical coherence tomography (OCT) as the preferred method for thickness measurements due to its noninvasiveness, higher repeatability, and reproducibility, as well as ease of use., Fourier domain devices have been found to have good repeatability and reproducibility.,,, The iVue-100 is one of the more recently introduced OCTs. This is a Fourier domain device capable of imaging and quantifying both anterior and posterior structures.
OCT works on a principle of reflected light. A super luminescent diode emits a light beam that is split into a sample beam and a reference beam. The sample beam goes to the structure being measured, whereas the reference beam travels a known pathway to a mirror. Both beams are then reflected back, combined using optical coherence and undergo Fourier transformation to obtain the thickness.
As the principles of this device are based primarily on transmittance and reflectance of light, it may be speculated that changes in the amount and/or intensity of light entering the eye will influence the measurements obtained. The amount of light entering the eye, and subsequent scattering of light, is controlled by the pupil size which in turn is dependent on the ambient lighting. Currently, the manual  accompanying the device does not specify whether image capturing should occur in photopic or scotopic conditions. A literature search has revealed only one study  that had investigated ocular parameters measured with OCT in photopic and scotopic conditions. Dacosta et al. found no significant differences in central corneal thickness (CCT) and anterior chamber depth, but a significant difference for anterior chamber angles. This study, however, was confined to the anterior segment. The current study, therefore, compared anterior and posterior segment parameters measured in photopic and scotopic conditions.
| Methods|| |
This study used an observational cross-sectional research design. Convenience sampling was used to recruit fifty participants of both genders and all races between the ages of 18 and 51 years. Data collection commenced after ethical clearance was obtained from the Biomedical Research and Ethics Committee with all participants giving written informed consent. The tenets of the Declaration of Helsinki were adhered to.
Contact lens wearers asked to discontinue lens wear for at least 1 week before the readings were taken. All participants had normal corneal topography determined by the Oculus Keratograph 3 (Oculus Optikgeräte GmbH, Wetzlar, Germany), aided or unaided visual acuity of at least 6/6 and no history of corneal injury and/or surgery. The scans were captured with the Fourier domain iVue-100 (Optovue, Inc., Fremont, CA, USA) which has a scanning rate of 26,000 A-scans/second. The axial resolution is 5 µm with a transverse resolution of 8 µm. The iVue-100 is designed to measure and image both anterior and posterior segment structures.
The OCT scanning was performed with participants seated and the chin and forehead rests used to stabilize the participant's head. During the scans, participants were required to look at the internal fixation target. The operator manually adjusted the OCT device such that all scans taken were perpendicular to the central cornea and centered on the pupil. The real-time OCT and camera images displayed on the laptop screen were used to achieve alignment of the scan. Pupil measurements, in millimeters, were also made from the camera images. All images were captured after 10:00 am to minimize the effect of overnight corneal swelling during sleep. Three scans of the cornea, retinal nerve fiber layer, and macula thickness were captured for all participants in photopic (958 lux) and scotopic (0.03 lux) conditions. Lighting levels were measured using the ZI-202 light meter (Zenith instruments). The order of the scans and lighting conditions were randomized.
Data were captured and analyzed using the Statistical Package for Social Sciences (version 21, IBM Corp, Armonk, NY) with the assistance of the faculty statistician. Only the right eye measurements were analyzed. The paired t-test was used to analyze differences in the mean thickness of the different structures in photopic and scotopic conditions. A relationship between the change in pupil size and change in thickness measurements was investigated with a Pearson's correlation.
| Results|| |
Fifty participants were enrolled in the study, however, due to incomplete data, the findings of only 47 participants were analyzed. The data of only the right eyes were considered.
Of the 47 participants, 62% (n = 29) were female and 38% (n = 18) were male. The mean age of the participants was 23.79 (±7.11) years. The majority of participants were Indian (53%), with 38% being Black, and the remainder White (9%). There was an almost equal distribution of emmetropes (57%) and ametropes (43%). The spherical equivalent of the right eyes ranged from − 8.38 to + 1.63 DS, with a mean of −1.06 D (±2.06). The corneal astigmatism of the right eyes ranged from 0.10 to 3.00 D, with a mean of 0.85 D (±0.62). Approximately, one in every four participants was a contact lens wearer.
In both scotopic and photopic conditions, thickness did not vary with gender in the central cornea (P = 0.90 and P = 0.95, respectively) nor with retinal nerve fiber layer (P = 0.35 and P = 0.43, respectively). However, in both scotopic and photopic conditions, the central foveal thickness was found to be 10.18 and 12.77 µm thicker, respectively, in males than in females but this difference was only statistically significant for photopic conditions (P = 0.03).
The mean difference of corneal thickness measured in scotopic and photopic conditions ranged from 0.29 to 3.44 µm [Table 1]. This difference was only statistically significant for the temporal peripheral cornea (P = 0.04) where the corneal was found to be 3.44 µm thicker under scotopic conditions.
|Table 1: Mean and standard deviation of corneal thickness measurements in scotopic (S) and photopic (P) conditions, together with the mean difference (S-P) and P value|
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Retinal nerve fiber layer
The thickness of the retinal nerve fiber layer was greatest in the inferior region followed by the superior, nasal, and then the temporal region in both scotopic and photopic conditions. Greater thickness values were recorded in photopic conditions for all regions, with the exception of the upper quadrant. The mean difference in the retinal nerve fiber layer thickness measured in scotopic and photopic conditions ranged from 0.01 to 0.49 µm [Table 2]. These differences were not statistically significant for any region.
|Table 2: Mean and standard deviation of retinal nerve fiber layer thickness measurements in scotopic (S) and photopic (P) conditions, together with the mean difference (S-P) and P value|
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Irrespective of lighting conditions, the temporal area was found to be the thinnest followed by the inferior, superior, and then the nasal area. The mean difference of macula thickness measured in scotopic and photopic conditions ranged from 0.04 to 1.48 µm [Table 3]. Macula thickness was found to be consistently greater under scotopic conditions with the exception of the central foveal area. This difference was only statistically significant for the inferior inner and temporal inner macula (P = 0.05 and P = 0.05, respectively).
|Table 3: Mean and standard deviation of macula thickness measurements in scotopic (S) and photopic (P) conditions, together with the mean difference (S-P) and P value|
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Effect of change in pupil size on parameter measurement
A significant increase in pupil size was found in scotopic conditions compared to in photopic conditions (P < 0.001). Positive correlations, between the change in corneal thickness and the change in pupil size, were noted in all regions within the corneal mid-periphery. In contrast, inverse correlations were noted in all regions outside of the corneal mid-periphery with only the temporal peripheral cornea reaching statistical significance (r = −0.31; P = 0.04).
For the retinal nerve fiber layer, the only significant correlation with change in pupil size was found in the nasal region (r = −0.30; P = 0.04). The change in pupil size was not correlated with the change in macula thickness readings for all regions.
| Discussion|| |
Based on the operating principles, the OCT measurement is dependent on the amount and intensity of light reflected from the structure being measured. When lighting conditions changed from scotopic to photopic, a statistically significant decrease in pupil size was found. At the outset, pupil size changes were not expected to affect the corneal thickness measurements since the cornea lies anterior to the pupil, and therefore, theoretically would not alter the sample beam. However, as the posterior segment parameters being measured lie behind the pupil, a change in pupil size was expected to influence the measurements.
Thicker corneal measurements were obtained under photopic conditions for the mid-peripheral cornea with the exception of the temporal region; however, these differences were not statistically significant. These findings are similar to that of Dacosta et al. who found the CCT measurements to remain unchanged in photopic and scotopic conditions. Clinically, an average change of 1.44 µm, as found in the current study, for the central cornea is not expected to impact on the accuracy of intraocular pressure measurements with Goldmann applanation tonometry which requires a 10 µm change in CCT to impact a 0.12–0.18 mmHg change in IOP., In contrast, thinner corneal measurements were obtained under photopic conditions for all regions outside the corneal mid-periphery. Only the temporal peripheral cornea was significantly thicker by 3.44 µm in scotopic conditions. This equated to a 0.64% change in corneal thickness. It has been suggested by Alario and Pirie  that a change in corneal thickness < 1.5% is clinically negligible. However, this difference may need to be considered in refractive surgeries and the diagnosis and monitoring of peripheral corneal diseases.
The retinal nerve fiber layer thickness followed the “ISNT” rule  in both photopic and scotopic conditions. As with the central cornea, thicker readings were obtained in all regions in photopic conditions although statistically insignificant. In contrast to the trends noted with corneal and nerve fiber layer thickness, thinner macula measurements were obtained in photopic conditions for all regions with the exception of the central foveal area. Even though statistically significant differences were found in the inferior and temporal inner macula areas, these differences were approximately 1 µm which may be clinically insignificant.
Interestingly, the differences in corneal thickness measurements as pupil size changed were, on average, greater than the differences in the posterior segment parameters. The current study, unfortunately, utilized a relatively small sample size and did not measure the anterior chamber parameters which would have allowed for further comparison with the study by Dacosta et al. Nevertheless, the study concludes that no clinically significant differences in anterior and posterior segment parameters measured with OCT are expected irrespective of ambient lighting conditions. The iVue-100 OCT, therefore, can be used in either scotopic or photopic conditions and should be determined based on which ambient lighting condition allows better visualization and, therefore, alignment of the camera images.
| Conclusion|| |
The measurement of corneal, retinal nerve fibre layer and macula thickness with the iVue-100 OCT, is not affected by ambient lighting conditions.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Table 1], [Table 2], [Table 3]