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 Table of Contents  
ORIGINAL ARTICLE
Year : 2019  |  Volume : 20  |  Issue : 2  |  Page : 55-62

Correlation between central corneal thickness and retinal nerve fiber layer thickness in open-angle glaucoma


1 Department of Ophthalmology, Port Said Ophthalmology Specialized Hospital, Port Said, Egypt
2 Department of Ophthalmology, Menoufia University, Shebeen El-Kom, Egypt

Date of Submission20-Dec-2018
Date of Acceptance18-Mar-2019
Date of Web Publication24-Jul-2019

Correspondence Address:
Ahmed I Basiony
Department of Ophthalmology, Menoufia University, Shebin El-Kom 72311, Menoufia
Egypt
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/DJO.DJO_69_18

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  Abstract 


Background Central corneal thickness (CCT) is an important risk factor in the development and progression of primary open-angle glaucoma (POAG). The retinal nerve fibers respond to high intraocular pressure (IOP) with irreversible loss. So, there is a great interest to study the correlation between these two factors: CCT and retinal nerve fiber layer thickness (RNFLT) using the spectral-domain optical coherence tomography (SD-OCT).
Aim The aim of this study was to correlate the CCT and the RNFLT in POAG patients by using SD-OCT.
Patients and methods This study was conducted in Menoufia University Hospitals, Egypt, from October 2017 to October 2018. The study included 116 eyes of 61 patients with POAG receiving antiglaucomatous eye drops with controlled IOP. CCT and peripapillary RNFLT were measured by using SD-OCT (Optovue RTVue-100). Then, we performed a correlation between them.
Results The study included 61 (40 males and 21 females) patients. The mean age was 51.5±6 years. A significant positive relationship was found between the CCT and the RNFLT (r=0.6). This means that patients with thick cornea have thicker nerve fiber layer than those with thinner corneas.
Conclusion CCT is an important risk factor and predictive factor for both diagnosis and prognosis of POAG. So, it is mandatory to be measured to modify the IOP measurements and to correlate with the nerve fiber layer thickness. This helps us to tailor the plan of treatment.

Keywords: central corneal thickness, intraocular pressure, primary open-angle glaucoma, retinal nerve fiber layer thickness, spectral-domain optical coherence tomography


How to cite this article:
Mohamed EY, Basiony AI, Ibrahim A. Correlation between central corneal thickness and retinal nerve fiber layer thickness in open-angle glaucoma. Delta J Ophthalmol 2019;20:55-62

How to cite this URL:
Mohamed EY, Basiony AI, Ibrahim A. Correlation between central corneal thickness and retinal nerve fiber layer thickness in open-angle glaucoma. Delta J Ophthalmol [serial online] 2019 [cited 2019 Sep 23];20:55-62. Available from: http://www.djo.eg.net/text.asp?2019/20/2/55/263417




  Introduction Top


Glaucoma is the leading cause of irreversible blindness worldwide [1]. Therefore, glaucoma is a global cause of preventable blindness, and early detection is a mainstay of vision preservation. Better structural imaging methods hold the potential for improved early detection, because structural damage that can be imaged objectively and quantitatively, often precedes irreversible vision loss [2],[3].

Glaucoma is an optic neuropathy that manifests as a typical pattern of optic nerve damage owing to death of retinal ganglion cells, their axons, and adjacent glial cells. This damage leads to cupping of the optic nerve head and thinning of the retinal nerve fiber layer (RNFL) and ganglion cell layer [4], resulting in a characteristic visual field loss. In recent years, the emphasis for glaucoma management has shifted to earlier diagnosis and treatment, which has been largely facilitated by the introduction of newer diagnostic tools to image the optic nerve head and the RNFL, such as the Heidelberg retina tomograph and optical coherence tomography (OCT) [5],[6],[7].

With the advent of spectral-domain optical coherence tomography (SD-OCT), the diagnosis of glaucoma was revolutionized [8]. This noninvasive diagnostic facility has been used to detect structural glaucomatous damage, which is thought to precede the functional damage. The device measures the peripapillary retinal nerve fiber layer thickness (RNFLT), which is believed to be thinned as a result of glaucoma [9],[10],[11],[12],[13].

In addition, within the past several years, large glaucoma and ocular hypertension multicenter studies, such as the Ocular Hypertension Study and the European Glaucoma Prevention Study, have been established to determine significant risk factors and predictors for the development of open-angle glaucoma. One of the risk factors that had been shown to be a powerful predictor of glaucoma development is the central corneal thickness (CCT) [14].

CCT is considered an important variable in glaucoma. Despite its effect on intraocular pressure (IOP) measurement, studies revealed that the populations with thin CCT are at an increased risk of glaucoma development, whereas those with thicker corneas are having less glaucoma [15],[16]. Although this relationship has been examined in glaucomatous eyes and eyes with ocular hypertension, there is little information on the relationship between the RNFLT and the CCT in glaucomatous patients [17].

This study aimed at evaluating the relationship in glaucomatous eyes between CCT and RNFLT, which we believe is of value in early detection and follow-up of patients with primary open-angle glaucoma (POAG).


  Patients and methods Top


This was a cross-sectional observational study that was carried out on 116 eyes of 61 patients. Patients were diagnosed as having POAG and were receiving antiglaucomatous treatment. The participants were recruited from Port Said Ophthalmology Specialized Hospital in the period between October 2017 and March 2018. Informed consents that match Helsinki Declaration were obtained from all of them. The study was approved by the Local Ethical Committee of Menoufia University.

The study included patients from either sex, aged between 40 and 60 years, who were diagnosed as having POAG on the basis of high IOP more than 21 mmHg at the time of diagnosis, with open angle of the anterior chamber. They had glaucomatous optic disc changes like increased cup/disc ratio (cupping) and retinal nerve fiber thinning. They also had characteristic visual field defects.

Patients with signs of secondary glaucoma or a nonglaucomatous cause for the optic neuropathy; patients with known history of corneal diseases or trauma, history of any ocular surgery (corneal or retinal), or laser surgery; patients with other retinal diseases like RNFL thinning in high myopia, hypertensive, and diabetic retinopathy; patients with optic disc anomalies like optic disc drusen, papilledema, and optic disc pit; and patients with dense cataract obscuring the fundus view were excluded from the study.

Patients were evaluated through detailed history taking, slit-lamp examination including gonioscopy, peripapillary RNFL examination by red-free light, and IOP measurement by applanation tonometry. Measurement of CCT was conducted by a 6-mm diameter pachymetry map by anterior segment OCT (Optovue RTVue-100). OCT optic nerve imaging and measurement of peripapillary RNFLT were performed by Optovue RTVue-100 SD-OCT (software version A5, 1, 0, 90; Optovue Inc., Fremont, California, USA). Average thickness of RNFL was documented and was correlated with the CCT.

Statistical analysis

Demographics of the study population were described as mean, median, and range. Mean values of study variables were compared using Student’s t-test for normally distributed data. Pearson’s correlation analysis was used to correlate IOP, CCT and RNFLT, in addition to multivariate regression analysis. P value less than 0.05 was considered statistically significant.


  Results Top


The study included 116 (56 right and 60 left eyes) eyes of 61 patients with POAG. They included 40 males and 21 females, with a mean age of 51.5±6 years. The duration of the disease ranged from 1 to 12 years. Some patients received one type of antiglaucomatous eye drops, whereas others received combined forms. [Table 1] illustrates the demographic characteristics of the study population.
Table 1 General characteristics of the studied patients

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The IOP ranged from 10 to 18 mmHg. The average CCT was 525.5±40.7 μm. The average thickness of all quadrants of RNFL was 86.2±18.3 μm ([Table 2]). There were no significant differences between right and left eyes regarding CCT, IOP, and RNFLT (P>0.05).
Table 2 Clinical data of the patients and relation between both eyes

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Pearson correlation between CCT and RNFLT showed a positive significant correlation between them (r=0.621, P≤0.001), as shown in [Table 3]. Correlation between CCT and RNFLT is shown in [Table 4] and [Figure 1]. In addition, there was a positive correlation between CCT and IOP (P<0.05).
Table 3 Pearson correlation between central corneal thickness and retinal nerve fiber layer thickness

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Table 4 Correlation between central corneal thickness, retinal nerve fiber layer thickness, and intraocular pressure

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Figure 1 Scatter plot correlation between central corneal thickness and retinal nerve fiber layer thickness.

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Multivariate regression analysis was used to assess the relationship between RNFLT and the examined risk factors (age, sex, disease duration, type of treatment, and CCT), as shown in [Table 5]. Only the CCT was the statistically significant predictor for the RNFLT (P˂0.05). [Figure 2],[Figure 3],[Figure 4],[Figure 5],[Figure 6],[Figure 7] shows the CCT and RNFLT of the same eyes.
Table 5 Multivariate analysis

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Figure 2 Central corneal thickness of right and left eyes.

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Figure 3 Retinal nerve fiber layer thickness of right and left eyes.

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Figure 4 Central corneal thickness of right and left eyes.

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Figure 5 Retinal nerve fiber layer thickness of right and left eyes.

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Figure 6 Central corneal thickness of right eye.

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Figure 7 Retinal nerve fiber layer thickness of right eye.

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  Discussion Top


CCT is considered an important variable in glaucoma. Despite its effect on IOP measurement, literature has highlighted its effect on glaucoma-related factors with controversies. In the present study, there was a positive significant correlation between CCT and IOP; patients with thick cornea had higher IOP than those with thinner corneas. In addition, there was a positive significant correlation between CCT and RNFLT. Both right and left eyes were equally affected by the studied variants. CCT can be considered as a predictive factor in POAG. It is used to modify measured IOP, detect the effect of high IOP on RNFLT, and therefore determine the plan of therapy. Similarly, in the study by Wangsupadilok and Orapiriyakul [18], there were significant correlations between CCT and average RNFLT. However, in the study of Mansouri et al. [19], the CCT had no significant correlation with RNFLT as measured by OCT. However, when the RNFLT was measured by enhanced corneal compensation algorithms of scanning laser polarimetry, the CCT was found to have a significant relation with RNFLT. Similarly, the relationship between these two variables was also not found significant in another study implying OCT [20]. In a study on patients with pseudoexfoliation syndrome, the CCT was found to have a significant relation with RNFLT. We thus deduce that better and accurate evaluation of RNFLT with modern diagnostic modalities like enhanced corneal compensation algorithms of scanning laser polarimetry shows more thinning of RNFL in thin corneas, highlighting more glaucomatous damage. These findings, though, need further verification by clinical trials [21].

Kaushik et al. [16] included a normal subset for comparison to patients with POAG in their OCT study of 35 healthy eyes from 35 patients. After stratifying their data between CCT less than or equal to 555 μm and CCT more than 555 μm, they found no significant difference in average, inferior average, or superior average RNFLT between the two CCT groups. The correlation between CCT and the three RNFLT parameters and cup-to-disc area ratio was all found to be nonsignificant in their normal subset, except for the overall average RNFLT, which had a Pearson correlation coefficient of 0.482, with a P=0.003. This agrees with this study finding of a positive relationship between CCT and overall average RNFLT as measured by OCT.

Sarfraz et al. [22] carried out a study on 60 eyes of 30 patients, with a mean age of 43.13±7.54 years. Of 30 patients, 19 (63.33%) were males and 11 (36.67%) were females. Correlation of CCT and RNFLT was significant, and there was a positive correlation between these two variants. It is similar to the results of this study.

In studies concerning correlation between CCT and RNFLT in healthy population, the study by Mumcuoglu et al. [23] enrolled 109 healthy participants from the Advanced Imaging in Glaucoma Study. All patients had a standard clinical examination, including visual field and CCT measured by an ultrasonic pachymetry. A linear mixed-effects model was used to assess the relationship between RNFLT and CCT. They found that the CCT was not statistically significantly related to RNFLT in healthy eyes.

Limitations of this study included the sample size that was relatively small owing to the weakness of the financial and social possibilities for some patients. Another reason is that patients with ocular hypertension, normal tension glaucoma, and angle-closure glaucoma were not included. Comparison between subtypes of glaucoma can provide more meaningful analysis in this regard.


  Conclusion Top


There is a significant positive correlation between CCT and RNFLT in patients with POAG. The correlation of CCT with different parameters signifying glaucomatous damage like RNFLT showed the importance of this measurement in patients with glaucoma. It not only helps in accurate estimation of IOP but can further provide insights into type and stage of glaucomatous damage. So, we can tailor the plan of treatment according to it.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
  References Top

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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7]
 
 
    Tables

  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5]



 

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