|Year : 2019 | Volume
| Issue : 4 | Page : 285-288
Correlation between retinal nerve fiber layer thickness and central corneal thickness in ocular hypertension
Aditya Mahadevan, Kirti Nath Jha, Srikanth Krishnagopal, G Ezhumalai
Department of Ophthalmology, Mahatma Gandhi Medical College and Research Institute, Sri Balaji Vidyapeeth University, Puducherry, India
|Date of Submission||07-Jun-2019|
|Date of Decision||07-Oct-2019|
|Date of Acceptance||25-Oct-2019|
|Date of Web Publication||26-Dec-2019|
Dr. Kirti Nath Jha
Department of Ophthalmology, Mahatma Gandhi Medical College and Research Institute, Pillaiyarkuppam, Puducherry - 607 402
Source of Support: None, Conflict of Interest: None
Aim: The aim of the study is to determine correlation between the retinal nerve fiber layer thickness (RNFL) and central corneal thickness (CCT) in patients with ocular hypertension (OHT) and age-matched control. Materials and Methods: This was a prospective, observational study, done between January 2016 and June 2017 at a rural tertiary care eye center. Participants diagnosed with OHT, defined as an intraocular pressure (IOP) of 21 mm Hg or higher, with normal optic disc and visual field, in the absence of any other ocular disease, were included in the study. The control group included age- and sex-matched participants with normal IOP, optic discs, and visual fields. Patients with OHT were subdivided into thin (CCT ≤555 μm) and thick (CCT >555 μm) corneas. RNFL was measured with spectral-domain optical coherence tomography and CCT with ultrasound pachymetry. Statistical Analysis: Mean, percentage, and standard deviation were used for descriptive values. Mean, standard deviation, and 95% confidence interval were utilized for the analysis of IOP, CCT, and RNFL. Mean was compared using the independent t-test. P < 0.05 was taken as statistically significant. Results: We examined 65 eyes of 35 OHT patients and 73 eyes of 40 controls. The mean IOP was 23.48 ± 2.47 mmHg, mean CCT was 553.81 ± 38.3 μm, and the mean RNFL was 102.12 ± 12.28 μm in the OHT group. Mean RNFL in OHT was 101.14 ± 10.68 μm in thin corneas, 103.21 ± 13.92 μm in thick corneas, and 104.23 ± 8.37 μm in the controls. There was no significant difference in the average (P = 0.502) or quadrant-wise (superior, nasal, inferior, and temporal) RNFL, comparing the two (thin and thick cornea) OHT groups (P = 0.247, 0.882, 0.897, and 0.551) or the control group. Conclusion: There is no correlation between CCT and RNFL thickness in patients with OHT. Mean retinal nerve fiber thickness in OHT and control revealed no difference.
Keywords: Central corneal thickness, ocular hypertension, retinal nerve fiber layer thickness
|How to cite this article:|
Mahadevan A, Jha KN, Krishnagopal S, Ezhumalai G. Correlation between retinal nerve fiber layer thickness and central corneal thickness in ocular hypertension. TNOA J Ophthalmic Sci Res 2019;57:285-8
|How to cite this URL:|
Mahadevan A, Jha KN, Krishnagopal S, Ezhumalai G. Correlation between retinal nerve fiber layer thickness and central corneal thickness in ocular hypertension. TNOA J Ophthalmic Sci Res [serial online] 2019 [cited 2020 Jul 10];57:285-8. Available from: http://www.tnoajosr.com/text.asp?2019/57/4/285/273978
| Introduction|| |
Glaucoma has been recognized as an important cause of blindness and is the second-most common cause of preventable blindness after cataract. The global prevalence of the condition is estimated to be around 3.54% (64.3 million) in the age group of 40–80 years. Over 11 million Indians, more than 40 years of age, are estimated to be affected with glaucoma, with primary open-angle glaucoma (POAG) said to affect 6.48 million. Of all the risk factors, raised intraocular pressure (IOP) has been recognized as a major factor associated with the development and progression of glaucoma. Ocular hypertension (OHT) is defined as a condition where the IOP is 21 mm Hg or more, with normal optic disc and visual fields. OHT Treatment Study (OHTS) suggests that, among others, thin cornea is a risk factor for the progression of OHT to glaucoma. Without treatment, a proportion of OHT cases progress to POAG; topical antiglaucoma therapy reduces conversion to glaucoma.,
Early diagnosis of POAG requires the detection of preperimetric changes. Preperimetric diagnosis of glaucoma involves imaging the optic nerve and quantitative analysis of the peripapillary retinal nerve fiber layer (RNFL) thickness.,
In this prospective study, we aim to study the RNFL thickness and the central corneal thickness (CCT) in OHT and age-matched controls.
| Materials and Methods|| |
This was a prospective observational study conducted at a rural tertiary care hospital from January 2016 to June 2017. The study was approved by the Institutional Human Ethics Committee, and the study was conducted according to the tenets of Declaration of Helsinki. Written informed consent was obtained from all participants.
- OHT group: Patients diagnosed with OHT, defined as individuals with IOP of 21 mmHg or greater, normal visual fields, normal optic discs, open angles, and absence of any ocular or systemic disorders contributing to the elevated IOP. The sample size (n) was calculated by the formula: N = (z) 2P (1 − p)/d2
- Control group: Participants with IOP <21 mmHg, normal visual fields, normal optic discs, open angles, and absence of any ocular disorder.
Cases of glaucoma, media opacities, other ocular diseases, and surgery other than for cataract were excluded from the study.
Detailed history was taken. The best-corrected Snellen visual acuity recording was followed by slit lamp examination. IOP measurement was done using Goldmann applanation tonometer. Gonioscopy was done with the Zeiss 4-mirror indentation goniolens. Visual field testing was done by static automated perimetry (AP901CTS, Appasamy Associates, Chennai, Tamil Nadu, India). CCT was measured using ultrasound pachymetry (Scan Plus, Appasamy Associates, Chennai, Tamil Nadu, India).
Following this, the patient's pupils were dilated with eye drops containing a combination of tropicamide 0.8% and phenylephrine 5%. The fundus was then examined by slit-lamp biomicroscopy with +90 diopter lens. The RNFL thickness measurements in four quadrants (superior, inferior, nasal, and temporal quadrants) were taken by Spectral optical coherence tomography (OCT)/scanning laser ophthalmoscope Optos Spectral OCT SLO™ Combination imaging system, Hialeah, FL. 33010, USA). The mean of these four values gave the mean RNFL thickness.
All the measurements were taken by a single experienced examiner. OHT eyes were subdivided into two groups: eyes with thin CCT (≤555 μ) and thick CCT (>555 μ), and the results were analyzed.
Optical coherence tomography measurements
All participants, following pupillary dilatation, were subjected to the RNFL scanning modality built into the spectral domain (SD)-OCT system. Each selected eye was individually scanned. The patient was made to sit in front of the machine, with the head of the patient placed over the chinrest. The patient was asked to focus on the internal fixation target, which was predefined so as to the map the peripapillary RNFL. In SD-OCT, once the scan starts, a real-time video also starts which helps in optimal positioning and alignment. Scans with signal strength of 7 or more were taken as reliable.
The RNFL scan uses a 3.4 mm ring, centered at the optic nerve head, to measure the peripapillary RNFL thickness. The RNFL values are plotted against a color-coded double-humped TSNIT map. The quadrant-wise and clock hour-wise values are also given. The study utilizes the quadrant measurements, and mean RNFL thickness is calculated by taking the average of the four values.
Mean, percentage, and standard deviation were used for demographic profiling. Mean, standard deviation, and 95% confidence interval were utilized for analysis of IOP, CCT, and RNFL. RNFL thickness and CCT (thin and thick corneas) were correlated within the OHT group. In addition, RNFL thickness in OHT and control groups was also correlated separately. Correlations were made using the independent t-test. P < 0.05 was considered statistically significant. All statistical analysis was carried out using SPSS version 19.0 (SPSS, IBM Corp, NY, USA) software with regression models installed.
| Results|| |
The study involved 65 OHT eyes of 35 participants and 73 eyes of 40 age- and sex-matched controls. Of the 35 OHT participants, 15 were male and 20 were female (M:F ratio = 3:4) and of 40 controls, 17 were male and 23 were female. The average age was 45.4 ± 12.21 years in the OHT group and 47 ± 10.6 years in the control group. The mean age in males was 44.47 ± 10.93 years and in females, 44.14 ± 13.15 years, in the OHT group. There was no significant difference in the age between males and females (P = 0.93).
Mean intraocular pressure, central corneal thickness, and retinal nerve fiber layer
The mean values for IOP, CCT, and RNFL in the OHT and control groups are given in [Table 1]. Based on the criteria for subdivision, the thin CCT subgroup included 34 eyes and the thick CCT subgroup included 31 eyes. The mean IOP, CCT, and RNFL values in the two subgroups are tabulated in [Table 2].
|Table 1: Intraocular pressure, central corneal thickness, and retinal nerve fiber layer thickness in the ocular hypertension and control|
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|Table 2: Mean intraocular pressure, central corneal thickness, and retinal nerve fiber layer thickness in ocular hypertension (thin and thick cornea)|
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No significant difference in the CCT, and RNFL values was present between the OHT and control groups.
Correlation between central corneal thickness and retinal nerve fiber layer
The mean and quadrant-wise RNFL in OHT subgroups (thin and thick corneas) did not show a statistical difference (P = 0.502). [Table 3] shows comparison of mean and quadrantwise RNFL thickness in OHT subgroups (thin and thick corneas).
|Table 3: Comparison of retinal nerve fiber layer thickness in ocular hypertension (thin and thick corneas)|
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| Discussion|| |
Early identification and appropriate treatment of preperimetric glaucoma are being studied extensively. With the advent of advanced imaging modalities, like the OCT, earlier diagnosis before the onset of functional damage has been possible.
In OHT, thin CCT has been found to be a risk factor for progression to glaucoma. In our study, OHT patients were grouped into two groups: those with thin (≤555 microns) and thick CCT (>555 microns). This division was based on an analysis of the OHTS. This subdivision of OHT patients into the thin and thick CCT groups was based on the cutoff value of 555 microns, obtained from the findings of the OHTS, which predominantly studied a racial profile of whites, African Americans and Hispanics. We studied a rural South Indian population. It has been shown that the CCT in adult South Indians was around 511.4 μ and that Indian eyes have thinner CCT compared to a western or Chinese population.,. However, OHT patients have thicker CCT values compared to normal in Indian eyes too. Since no other data showed the exact CCT value at which progression was documented in Indian eyes with OHT, the OHTS cutoff was used.
We investigated the correlation between CCT and RNFL, to show any difference in the RNFL thickness between the thin and thick CCT groups. Our study revealed no such correlation between the groups. A study on a larger sample size may offer different results.
The normal RNFL thickness in Indians measured with an SD-OCT was around 114 microns. Another Indian study comparing RNFL in normal and OHT individuals showed that OHT individuals had thinner values only in the temporal quadrant. This study reported a mean RNFL of 110.09 μ in OHT, which is thicker than the mean RNFL of our study (102.12 ± 12.8 microns).
In 2005, Henderson et al. studied CCT and RNFL in OHT patients, using the GDx Variable Corneal Compensator scanning laser polarimeter to measure RNFL. They found that thinner corneas corresponded to significantly thinner RNFL values. However, this was not an OCT-based study, and it has been shown that the values from these different modalities are not comparable.,
Vessani et al. conducted a similar study, where they used a CCT value of 540 microns as the cutoff value, and found that lower CCT was associated with lower RNFL thickness values. Kaushik et al. studied RNFL in an Indian population, where 51 OHT eyes were analyzed. This study used 555 microns as the subdivision value and found that the inferior and mean RNFL values were significantly thinner in the thin CCT group. However, both these studies used a time-domain OCT, whose values cannot be interchangeably used or compared with the SD-OCT, used in our study., Furthermore, the study on the Indian eyes used a higher cutoff value for the diagnosis of OHT (23 mm Hg or greater in both eyes). Higher IOP is known to be a risk factor for progression to glaucoma, and hence, maybe this difference could have led to a thinner RNFL in the OHT group in the above-quoted Indian study. There is no other study comparing CCT and RNFL in OHT patients, using an SD-OCT.
Limitations of our study include a small sample size and lack of longitudinal follow-up. The RNFL values were recorded at one point of time, and they were correlated. Glaucoma being a progressive condition, serial follow-up with multiple RNFL studies could have given a clearer picture about the degree of RNFL loss in each subset.
| Conclusion|| |
There is no correlation between RNFL thickness and CCT in patients with OHT. Mean retinal nerve fiber thickness in OHT and control revealed no difference.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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