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 Table of Contents  
ORIGINAL ARTICLE
Year : 2021  |  Volume : 59  |  Issue : 1  |  Page : 13-17

Association between central corneal thickness and ocular dominance in a South Indian population


1 Department of Glaucoma and Research, Mahathma Eye Hospital Private Limited, Tiruchirappalli, Tamil Nadu, India
2 Department of Glaucoma, Sathyan Eye Care Hospital and Coimbatore Glaucoma Foundation, Coimbatore, Tamil Nadu, India
3 Biostatistician, Sathyan Eye Care Hospital and Coimbatore Glaucoma Foundation, Coimbatore, Tamil Nadu, India

Date of Submission18-Aug-2020
Date of Acceptance08-Oct-2020
Date of Web Publication27-Mar-2021

Correspondence Address:
Dr. Prasanna Venkatesh Ramesh
Mahathma Eye Hospital Private Limited, No 6, Tennur, Seshapuram, Trichy, Tamil Nadu
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/tjosr.tjosr_116_20

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  Abstract 


Aims: The aim of the study is to investigate the association between ocular dominance and central corneal thickness (CCT) among normal eyes. Settings and Design: A cross-sectional study was conducted in the outpatient department of a tertiary care center in Southern India. Subjects and Methods: Participants of the study included 87 outpatients and volunteers with bilateral nonpathological eyes. This study was performed according to the tenets of the Declaration of Helsinki. To determine the dominant eye, hole-in-card test was utilized. Masked of the ocular dominance result, an independent observer measured CCT at the visual axis with specular microscopy (CEM-530, Nidek, Canada) by noncontact modality. Three readings were obtained from each eye. The right eye was first examined. Average of the three CCT readings was used for analysis. Statistical Analysis Used: Statistical methods included paired t-test (for comparison of eyes within patient) and Chi-square test to assess the association between ocular dominance and CCT. A P < 0.05 was considered statistically significant. Results: The 87 volunteers studied were aged 37.6 ± 14.8 years and 62.1% were female. CCT was 525.5 ± 28.4 μm in the right eye and 534.8 ± 29.8 μm in the left eye. Right eye was dominant in 63 (72.4%) patients. CCT in the dominant eye was 528.0 ± 29.5 μm and was significantly thinner (P < 0.001) than the nondominant eye (532.2 ± 29.3 μm). This was true in 69.0% of the patients. However, thinner cornea in a person was not indicative of a dominant eye (P = 0.535). Conclusions: As with other populations, right eye dominance was seen in South Indian population among nonpathological eyes. CCT is predominantly thinner in the dominant eye. Yet, this result cannot be applied to an individual by labeling the eye with thinner cornea as dominant with precision.

Keywords: Central corneal thickness, hole-in-card-test, ocular dominance, specular microscopy


How to cite this article:
Ramesh PV, Parthasarathi S, John RK. Association between central corneal thickness and ocular dominance in a South Indian population. TNOA J Ophthalmic Sci Res 2021;59:13-7

How to cite this URL:
Ramesh PV, Parthasarathi S, John RK. Association between central corneal thickness and ocular dominance in a South Indian population. TNOA J Ophthalmic Sci Res [serial online] 2021 [cited 2021 Apr 22];59:13-7. Available from: https://www.tnoajosr.com/text.asp?2021/59/1/13/312273




  Introduction Top


The concept of ocular dominance came into existence from 1593. It is also called ocular preference or sighting eye dominance. It is defined as the innate tendency of preferring visual input of one eye over the other.[1],[2],[3] The eye whose visual input is preferred over the other is called the dominant eye, and the fellow eye is called the nondominant eye.[1]

Determination of the ocular dominance is important in the current clinical practice when it comes to taking crucial decisions, such as selecting the type of intraocular lens to be implanted in dominant or nondominant eye, or while selecting the profile of corneal ablation during refractive surgery. Recently, ocular dominance was evaluated for association, with ganglion cell inner plexiform layer (GCIPL) and retinal nerve fiber layer (RNFL) thickness.[4] It was reported that in dominant eyes, GCIPL and RNFL thickness in normal individuals were slightly but significantly higher.[4] It also suggested that ganglion cell distribution in the macular area differed according to the ocular dominance. Thus, the variation in the thickness of GCIPL profile in the macula is an indicator of the relative dominance of an eye, providing the evidence determining the dominant eye.[4] In this regard, we hypothesized that the central corneal thickness (CCT) analysis may also be appropriate for assessing the effect of ocular dominance. The association between corneal parameters and optic disc morphology is well documented as both corneal stroma and lamina cribrosa differentiate from a common embryological structure known as the neural crest; and also, all three parameters CCT, RNFL thickness, and GCIPL thickness are significantly thinner in glaucoma patients and are correlated with glaucomatous visual field progression and nerve fiber damage.[5],[6],[7]

To our knowledge, there were no studies reporting the association between CCT and ocular dominance or whether CCT is an indicator of relative ocular dominance of an eye; hence, this study was conducted. The purpose of this study was to investigate the association between ocular dominance and CCT among nonpathological eyes in the South Indian population.


  Subjects and Methods Top


In total, 87 consecutive consenting outpatients and volunteers with bilateral nonpathological eyes (174 eyes) were included in this study. The sample size was calculated considering an alpha error of 0.05 and statistical power of 95%. The study was conducted prospectively in a cross-sectional manner in December 2019 at a tertiary eye care institute, India. This study was performed according to the tenets of the Declaration of Helsinki.

A detailed review of the medical history followed by a full ophthalmic examination for the participants was done at the initial visit. The examination included best-corrected visual acuity (BCVA) by Snellen chart; refraction measurements; intraocular pressure (IOP) measurement using noncontact tonometry; and slit lamp bio-microscopic examination with +90D for fundus examination.

Patients were included if they had a normal anterior and posterior segment. Eyes with glaucomatous optic nerve head changes (i.e., disc hemorrhages, neuroretinal rim thinning, notching of the rim, or glaucomatous cupping) and eyes with a glaucomatous visual field defects were excluded from the study.[8] Other exclusion criteria included eyes with a BCVA of <20/40; an IOP >21 mmHg in either eye; if the difference in CCT between two eyes were >20 μm; contact lens users; pterygium; an ambiguous dominance in the hole-in-card test; a closed or occludable angle; a history of intraocular or refractive surgery; severe ocular trauma; one eyed patients; amblyopia; media opacity; evidence of diabetic retinopathy or other vitreoretinal disease; evidence of optic neuropathy in either eye; and any other eye pathology in either eye.[9],[10]

Blinding/masking

Participants underwent dominant eye testing using hole-in-card test by one investigator followed by CCT evaluation by specular microscopy by another investigator at the same visit. Both the investigators were masked from the other's result.

Dominant eye testing by hole-in-card test

To determine the dominant eye, the most common method reported in the literature, the hole-in-a-card test was utilized.[11] First, the patient was asked to hold a card with a hole centered in the middle [Figure 1]a and [Figure 1]b using both hands and was asked to view a target at 6-m distance through the hole in the card. Then, each eye was occluded alternately by the examiner to establish which eye is aligned with the hole and the distant target. The aligned eye was considered to be the dominant eye. The process was repeated one more time. Second, for confirmation, the patient was asked to move the card toward their face without losing alignment with the fixation target, until the hole was over an eye [Figure 1]c and [Figure 1]d. This eye was considered to be the dominant eye.
Figure 1: (a-d) Identification of ocular dominance by hole-in-card test

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Central corneal thickness measurement by specular microscopy

CCT was measured with specular microscope (CEM-530, Nidek, Canada) by noncontact modality [Figure 2]a. CCT values measured by specular microscope are comparable to CCT values measured by ultrasound pachymetry.[12] It also has good intraobserver and interobserver repeatability.[13] Automated settings were used and the CCT was measured by asking the patient to fixate on an internal fixation target, and three reading were obtained from each eyes [Figure 2]b. Three readings from the right eye were measured first followed by three readings from the left eye. Average of the three was used for statistical analysis.
Figure 2: (a and b) Central corneal thickness of the vertex measurement using specular microscope by using an internal fixation target

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Statistical analysis

The patient records/information was anonymized and de-identified before analysis. CCT between dominant and nondominant eye was compared using paired t-test. CCT of the eyes was labeled as “high” or “low” in comparison with the fellow eye. Ocular dominance (dominant/nondominant) and relative CCT (thick/thin) were tested for association using Chi-square test. A P < 0.05 was considered statistically significant.


  Results Top


The study participants (n = 87) with bilateral nonpathological eyes were aged 37.6 ± 14.8 years, ranging from 15 to 77 years. Majority (62.1%) of them were female. Mean CCT of the eyes was 530.1 ± 29.4 μm. 60% had CCT ≤540 μm, 24% had CCT between 540 and 560 μm, and 16% had CCT ≥560 μm [Figure 3]a. Ocular dominance was identified in the right eye in 63 (72.4%) participants. Overall, CCT in the dominant eye was 528.0 ± 29.5 μm and was significantly lower (P < 0.001) than the nondominant eye (532 ± 29.3 μm). The identified dominant eye had relatively low CCT in 60 (69%) participants when compared to the other eye. The distribution of CCT in the dominant eye and nondominant eye are shown in [Figure 3]b and [Figure 3]c. Two pairs (two patients) had no difference (2.3%), and the rest (28.7%) had higher CCT in the dominant eye [Figure 4]. Relative CCT is not significantly associated with identified ocular dominance (P = 0.535), i.e., eye having thinner cornea could not be identified as the dominant eye.
Figure 3: (a) Overall central corneal thickness categories of the par ticipant's eyes. (b) Central corneal thickness distribution in the dominant eye. (c) Central corneal thickness distribution in the nondominant eye

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Figure 4: Relative central corneal thickness and ocular dominance

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  Discussion Top


Central corneal thickness

CCT is a quite commonly measured parameter in clinical practice. Although ultrasound pachymetry has been the gold standard for the measurement of CCT, development of newer modalities such as specular microscope has widened the options and introduced further accuracy. There are two centers for cornea [Figure 5]. They are the corneal vertex and the corneal apex. Corneal vertex is slightly nasal to the corneal apex. Whenever pupillary center is used as a landmark for measuring CCT (optical axis is aligned), corneal apex CCT is measured; e.g., CCT measurement by ultrasound pachymetry. Whenever a fixation target is used for measuring CCT (visual axis is aligned), corneal vertex CCT is measured; e.g., CCT measurement by specular microscope. Corneal vertex (visual axis) was chosen as the center for measuring CCT over corneal apex (optical axis) as the mean difference between CCT measured between the two different corneal centers was clinically <1 μm and specular microscope was noncontact, fully automated with excellent repeatability.[14],[15] The mean CCT of this study was 530.1 ± 29.4 μm which was similar to previous Indian studies.[16] 60% of the participants had CCT ≤540 μm, 24% had CCT between 540 and 560 μm, and 16% had CCT ≥560 μm [Figure 3].
Figure 5: Centers of the cornea

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Ocular dominance

Ocular dominance is defined as the relative input or preference of neurons throughout the visual system, especially primary visual cortex.[4] The dominant eye performs the majority of seeing. The nondominant eye assists the fellow dominant eye during visual functions.[1] The dominant and nondominant eye of an individual complement each other. They do not affect the visual consciousness with equal force.

There are various theories on ocular dominance. Best possible explanation for ocular dominancy is the theory of functional lateralization. Functional lateralization occurs in most of the paired organs of the body, such as hands, legs, and cerebral hemispheres.[17] Just as how the two hands are not equal in response, both from a motor and from a sensory point of view, so are the eyes. They can either be left eyed or right eyed, similar to how a person can be either right handed or left handed.[1]

Determination of dominant eye is dependent on the test used in determining it along with the gaze angle.[3],[18],[19],[20] The most common and effective method reported in the literature is the “hole-in-a-card” test. It determines the ocular dominance by using hole in a card. Previously, palm of both the hands were joined in such a way to create a triangular opening, instead of hole in a card test, through with sighting eye dominance was found. However, with hole in a card, the question of hand dominance interfering with the result can be avoided.[21]

Ocular dominance was identified in the right eye in 63 (72.4%) participants in this study. Unanimously, results published in the literature reported right eye as the dominant eye in majority of the patients.[3],[9] The best possible explanation is as follows. Although there is no physiological evidence linking the dominant hand with the dominant eye, speculation is that there is a functional advantage of having ipsilateral hand–eye dominance. In many sensorimotor coordination, it would be useful to have the dominant eye aligned with the dominant hand, such as in aiming a target or throwing a ball. A rather lyrical theory would be the preponderance of individuals with a dominant right hand to have a dominant right eye, due to natural selection. For example, a right-handed warrior would predominantly carry his weapon in his right hand with his shield in his left hand. This arrangement would provide maximal defensive protection to the heart to increase his survival advantage. Further, a warrior with a dominant eye on the same side as his dominant hand would be more accurate in the placement of his weapon in offensive actions.[2]

Ocular dominance and central corneal thickness

Overall, CCT in the dominant eye was 528.0 ± 29.5 μm and was significantly lower (P < 0.001) than the nondominant eye (532 ± 29.3 μm). The distribution of the CCT in the dominant and nondominant eye is shown in [Figure 4]. The identified dominant eye had relatively low CCT in 60 (69%) participants when compared to the other eye. Two pairs (two patients) had no difference (2.3%), and the rest (28.7%) had higher CCT in the dominant eye [Figure 4].

The exact reason why majority of the patients had thinner CCT in the dominant eye is not known. The postulated reason for this is as follows. It is well known that premature babies have thicker corneas than term babies.[22] And also, there is reduction in the keratocyte density and quiescence, due to the expression of TKT/ALDH1A1 aiding in corneal transparency soon after birth in the postnatal period.[23] Normally, the reduction of CCT happens in both eyes in the postnatal period. The hypothesis here is that the CCT reduction seems to be exaggerated in dominant eye to a greater degree than the nondominant eye. The possible reason for this could be the overexpression of TKT/ALDH1A1 in dominant eye than in nondominant eye. However, further molecular and genetic studies are needed to support this theory.

Relative CCT is not significantly associated with identified ocular dominance (P = 0.535), i.e., eye having thinner cornea could not always be identified as the dominant eye. The most possible explanation is as follows. Although, embryologically, most of the eye structures (such as corneal stroma and lamina cribrosa) and the cerebrum have a common embryological derivative (neural crest), there seems to be little functional association between them in terms of association between ocular dominance and CCT. A number of investigators have attempted to test directly the relationship between eye dominance and cerebral dominance and did not find any association.[24] These procedures took into account the fact that each hemi-retinae projects to only one cerebral lobe. They used an ambiguous apparent movement task and measured whether the observer resolved the movement in favor of the dominant visual field (i.e., cerebral hemisphere) or whether an entire eye was favored. Jasper and Raney found that observers display both behaviors, whereas Spreen et al. were forced to conclude that no relationship exists between visual field dominance and eye dominance. An alternative approach has involved tachistoscopic recognition tasks. Experimenters using these tasks have also found no relationship between visual field superiority and the sighting dominant eye.[2] As there is no relationship between eye dominance and cerebral dominance, there may be no relationship between eye dominance and CCT. However, this theory is still primitive and further functional studies are needed to support this.

Limitation

Major limitation of this study should be acknowledged. Only nonpathological eyes were involved in the study. To compensate for this, future studies should be done on glaucomatous eyes so that it can be applied in clinical glaucoma practice.


  Conclusions Top


As with other populations, right eye dominance was seen in South Indian population among nonpathological eyes. CCT is predominately thinner in the dominant eye when compared to nondominant eye - contrary to the GCIPL and RNFL thickness findings.[4] In an individual, the eye with thinner cornea cannot be labeled as the dominant eye, with precision.

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent forms. In the form the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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