Ocular dominance and its association with retinal thickness profile – A cross-sectional study

Abstract

Purpose:

The retinal thickness profile is essential for detecting ocular diseases like glaucoma and other optic neuropathies. The retinal nerve fiber layer (RNFL) thickness is affected by age, ethnicity, axial length, optic disc area, and inter-eye differences. Ocular dominance has a strong functional correlation with cerebral cortical activity. However, its relationship with RNFL thickness profile is yet to be fully established.

Methods:

A cross-sectional study was conducted in 136 healthy adults to study the association between ocular dominance and RNFL parameters measured by Spectral domain optical coherence tomography (SD-OCT) and to study the association of ocular dominance with other parameters such as handedness, intraocular pressure, average axial length, average keratometry, and refractive error. Sighting ocular dominance was detected using the Miles test, and sensory ocular dominance was detected using the fogging test. Visual acuity and refraction assessment were done, and the patients underwent ocular biometry using the Lenstar 900 machine to measure the axial length and keratometry. The RNFL thickness was measured using the Cirrus HD optical coherence tomographer.

Results:

One hundred and thirty-two (97.06%) individuals were right-handed, four (2.94%) were left-handed, 108 (79.41%) participants were right eye dominant, and 28 (20.59%) were left eye dominant. There was 100% agreement between sighting and sensory ocular dominance. The average RNFL thickness and other measured ocular parameters were comparable in the dominant and nondominant eyes. Regardless of dominance, the left eyes in the study cohort had a greater statistically significant difference in superior RNFL thickness (P < 0.05), which correlated with increased central macular thickness.

Conclusion:

Ocular dominance occurred mostly in the right eye. The RNFL thickness profile is not associated with ocular dominance in emmetropic and mild myopic individuals with normal best corrected visual acuity.

The anatomical and functional development of the right and left hemispheres of the human brain has a high degree of symmetry. However, there are differences between the two sides; each is dominant in processing specific cognitive tasks. Studies have shown that functional asymmetry for spatial orientation exists in the cerebral hemispheres.[1,2] This asymmetry of the brain may affect handedness and verbal and spatial cognition.[3,4,5,6,7,8] The ocular system exhibits laterality like the brain system, known as ocular dominance.[9] Ocular dominance is the tendency to prefer visual input from one eye over another. The dominant eye is the one that a person usually uses in monocular behavior coordination while looking through a keyhole or a telescope.[10] It has been said that eye dominance might be related to cortical hemispheric specialization to visual attention.[11] The importance of eye dominance in daily life is not clearly understood, yet it is clinically crucial in sports vision, vision therapy, and monovision treatments.[12] The functional role of the dominant eye is vital in ophthalmology as patients might suffer more significant visual handicaps if a macular disease affects the dominant eye rather than the nondominant eye. In ocular mechanisms like strabismus, the nondominants are more likely to develop amblyopia.[13] This concept must also be considered while providing presbyopia correction by refractive surgery, intraocular lens implantation, or contact lenses. Greater patient satisfaction might result from correcting the dominant eye for distance and the nondominant eye for near vision.[14,15,16] It is also known that the dominant eye is related to cerebral laterality with significantly higher cortical activation than the nondominant eye.[17,18] Earlier, detecting structural differences between dominant and nondominant was difficult, as no technology was available to utilize in situ. With the advent of optical coherence tomography (OCT), it is now possible to measure the morphologic structure of the eye. OCT is a widely used imaging tool for measuring retinal nerve fiber layer (RNFL) thickness and macular thickness parameters. The development of spectral domain OCT has resulted in faster scanning and high-resolution images to interpret RNFL. Previous studies by Choi et al. have shown the associations between ocular dominance with inferior retinal nerve layer thicknesses and macular ganglion cell–inner plexiform layer (GCIPL) which differed by few microns, which may not be clinically significant.[19,20] Hence, this study was undertaken to identify any structural differences in the retinal layers between the dominant and nondominant eyes.

Methods

This cross-sectional study consisted of patients between 18 and 35 years of age with a best corrected visual acuity of 6/6, who attended the ophthalmology outpatient department in a tertiary care center in South India between January 2021 and July 2022. The sample size was calculated assuming the proportion of dominance as 85%, with respect to the study of Samarawickrama et al. in 2008, with an alpha error of 0.05, a beta error of 0.2, and a power of 80%.[21] Both eyes of the patient were considered for the study, and the total sample size was 136. Patients with high hyperopia (axial length [AL] <21 mm) and high myopia (AL >27 mm) were excluded as short and long ALs play a role in increasing or decreasing the average AL. Patients with retinopathies, diseases affecting the optic nerve, including glaucoma, chronic smokers, alcoholics, patients with past intraocular surgery, and those with trauma were excluded. The dominant hand used for writing was noted.

Measurement of ocular dominance

Sighting ocular dominance was determined using Miles test, where the participant was asked to extend both arms in front of the body and both hands together to create a small triangle between the thumb and the first knuckle. With both eyes open, the participant was asked to view a distant object (on a single 6/12 letter at 6 m) through the triangle. Then, the participant was asked to alternate closing of the eyes to determine which eye was viewing the object, and this eye was considered the dominant eye [Fig. 1]. Sensory ocular dominance was determined by the fogging test, where the subject was asked to fixate at a 6/12 letter at 6 m with both eyes open. A +/- 1.50 D lens was alternated in front of each eye for a few seconds. The dominant eye was the one in which the subject reported more blurred vision with the positive lens under binocular conditions. Then the visual acuity, with and without correction, was determined by the Snellen acuity chart and converted into log of minimum angle of resolution (logMAR) values. After collecting ocular and systemic history, slit-lamp examinations were conducted for patients to look for anterior segment pathology. Patients underwent cycloplegic refraction and dilatation to check for lens opacity and fundus examination. The optical biometer was used for ocular biometry (Lenstar 900; Haag-Streit, Koeniz, Switzerland). The Lenstar LS 900 was calibrated before obtaining the measurements, and a minimum of five readings were taken with a standard deviation (SD) of <0.2. Parameters studied using the biometer were average AL and average keratometry (K) values. Optic nerve head and ganglion cell complex evaluation by the Cirrus HD OCT 500 (Carl Zeiss Meditec, Dublin, CA, USA) was performed by a single imaging technician after cycloplegia. The optic disc cube and macular cube protocols were done for the SD-OCT to provide RNFL and Ganglion cell complex (GCC) thickness measurements. Images with a signal strength ≤6 were excluded.

Figure 1.

Clinical photograph showing the Miles test for ocular dominance when viewed through the dominant eye

Results

The study included 136 individuals between 18 and 35 years of age; the mean age was 24.4 ± 3.08 years. Sixty-seven (49.26%) of them were males and 69 (50.73%) were females. In the study population, 132 (97.06%) individuals were right-handed and four (2.94%) were left-handed, of which 108 (79.41%) participants were right eye dominant and 28 (20.59%) were left eye dominant [Fig. 2]. There was 100% agreement between sighting and sensory ocular dominance. Among the right-handed individuals, 105 (79.54%) were right eye dominant and 27 (20.45%) were left eye dominant. Among the left-handed individuals, three (75%) were right eye dominant and one (25%) was left eye dominant. Among the study participants, 109 individuals were emmetropic with an uncorrected visual acuity of 0 in the logMAR chart in both eyes and 27 individuals were myopic with vision better than 0.1 in the logMAR chart in both eyes. The myopic patients had a mean spherical equivalent of 0.61 +/- 1.40 D for the right eye and 0.62 +/- 1.39 D for the left eye among right eye-dominant participants and 0.56 +/- 1.33 D for the right eye and 0.48 +/- 1.12 D for the left eye among left eye-dominant participants. The baseline intraocular pressure, cup–disc ratio, average AL, average K (Avg K), average RNFL, Ganglion cell layer- inner plexiform layer (GCL-IPL), and central macular thickness (CMT) data for all study participants (N = 136) were comparable between the right and left eyes [Table 1]. In our study, the right eye RNFL parameters, except the superior one, were slightly higher than those of the left eye among the right eye-dominant participants, but this was not statistically significant [Table 2]. Similarly, the, left eye RNFL parameters were slightly higher than those of the right eye among the left eye-dominant participants [Table 3]. Overall, the superior quadrant RNFL thickness of the left eye was higher in both right eye- and left eye-dominant individuals, which was statistically significant [Fig. 3] and correlated with the high CMT.

Figure 2.

(a) Distribution of percentage of handedness and (b) percentage of ocular dominance in the study population

Table 1.

IOP, CDR, average AL, Avg K, average RNFL, GCL-IPL, and CMT data for all study participants (n=136)

Characteristic		Right eye Mean±standard deviation		Left eye Mean±standard deviation
IOP (mmHg)		13.45±2.16 (10–18)		13.73±1.88 (10–18)
CDR		0.33±0.07 (0.2–0.5)		0.32±0.07 (0.2–0.5)
Average AL (mm)		23.94±1.19 (21.97–26.95)		23.96±1.20 (21.98–26.94)
Avg K (D)		44.09±1.62 (39.73–48.19)		45.60±1.67 (40.29–48.16)
Average RNFL thickness (μm)		93.18±7.42 (78–113)		93.40±9.25 (67–126)
Average GCL-IPL thickness (μm)		81.60±5.17 (65–91)		81.88±5.13 (62–92)
CMT (μm)		240.83±20.48 (193–272)		242.80±18.70 (207–276)

AL=axial length, Avg K=average keratometry, CDR=cup–disc ratio, CMT=central macular thickness, IOP=intraocular pressure, RNFL=retinal nerve fiber layer

Table 2.

Comparison of RNFL parameters of the right eye and the left eye among the right eye-dominant participants (n=108)

Characteristic		Right eye		Left eye		Mean difference		P
Inferior RNFL thickness (μm)		118 (109.25–128.00)		118 (106.25–130.00)		0.00		0.274**
Superior RNFL thickness (μm)		119.78±17.22		123.33±14.51		-3.55		<0.001*
Nasal RNFL thickness (μm)		70.73±9.45		69.45±9.45		1.27		0.196*
Temporal RNFL thickness (μm)		61 (56.25–69.50)		61 (55.00–67.00)		0.00		0.100**
Average RNFL thickness (μm)		93 (88.00–98.00)		92.50 (88.00–99.00)		1.50		0.420**

Values are given as mean±standard deviation/median (IQR). *Paired t-test for parameters that followed a normal distribution. **Wilcoxon signed-rank test for parameters that did not follow a normal distribution. IQR=interquartile range, RNFL=retinal nerve fiber layer

Table 3.

Comparison of RNFL parameters of the right eye and the left eye among the left eye-dominant participants (n=28)

Characteristic		Right eye		Left eye		Mean difference		P
Inferior RNFL thickness (μm)		120.21±12.18		120.68±13.18		−0.464		0.815*
Superior RNFL thickness (μm)		118.61±14.38		122.43±15.32		−3.821		0.048*
Nasal RNFL thickness (μm)		70.32±8.47		70.93±10.43		−0.607		0.652*
Temporal RNFL thickness (μm)		57 (56.00–70.00)		58.50 (56.00–67.00)		−1.500		0.599**
Average RNFL thickness (μm)		92.43±5.92		93.64±7.46		−1.214		0.128*

Value in bold shows statistical significance, Values are given as mean±standard deviation/median (IQR). *Paired t-test for parameters that followed a normal distribution. **Wilcoxon signed-rank test for parameters that did not follow a normal distribution. IQR=Interquartile range, RNFL=Retinal nerve fiber layer

Figure 3.

Bar graph showing the average and quadrant retinal nerve fiber layer thickness comparison between the right eye and the left eye in right eye dominance and left eye dominance

Discussion

This study investigated the possible associations between ocular dominance and handedness, refractive error, and other ocular biometry parameters like average AL, average keratometry, and retinal thickness profile, including the average GCL-IPL and CMT. There was no difference in gender distribution or any specific pattern in the RNFL thickness between the genders. The study participants were predominantly right-handed and only four were left-handed. There was no association found between handedness and ocular dominance. This was comparable to the systematic review by Moreno et al.,[22] in which the relationship between hand–eye laterality and sports performance was looked into in various studies on this subject over the last decade. The dominance of the eyes has different types, like sighting, sensory, and acuity dominance.[23,24] The dominant sighting eye is the one that can swiftly move toward a target and stay fixed on it.[25] If the dominant eye has better visual acuity, it will be preferred as the predominant eye for a better quality image and becomes the acuity dominance. The dominant sensory eye is the one that has stronger vision than the other, and while testing for blur with a small plus lens, it is relatively easy to get more blurred than the nondominant eye.[26,27] The exact difference between sighting and sensory ocular dominance stays unresolved.[28] In our study, there was 100% agreement between sighting and sensory ocular dominance among the participants. Mwanza et al. and Huynh et al.[29,30] found that the superior quadrant RNFL was thicker in the left eye than in the right eye. Budenz et al. and Park et al.[31,32] found that the average RNFL of the right eye was significantly thicker than the left eye. Cehver et al. compared the macular and peripapillary RNFL thicknesses of the dominant eye and nondominant eye using OCT. They found the same results in average and superior quadrant analysis. In the other three quadrants (nasal, temporal, and inferior), RNFL thickness was more in the right eyes than in the left, but there was no significant difference.[33] Cehver et al. hypothesized that the RNFL profile might be affected by ocular dominance.[33] Still, their results showed that ocular dominance does not statistically affect the differences in interocular and intraocular RNFL thickness, and this was consistent with our study. Anisometropia refers to the condition of unequal refractive errors between the two eyes, and this occurs due to an interocular difference in ALs.[34] In anisometropia, the interocular difference in refractive errors leads to constant or intermittent defocused retinal image and, thereby, a decline in the clarity and contrast of the image in the affected eye or a difference in the size of the retinal image between the eyes. This leads to two different signals of a single object to the visual system, which causes a decline in the visual function.[35,36] Maintaining a balance between the signals from the two eyes is important in developing the visual system. So, during anisometropia development, it is believed that the neural system, part of the visual system, develops some form of asymmetry to overcome this interocular asymmetry to bring about balance. It is accepted in a few studies that ocular dominance plays a major role in adapting to this asymmetry and, thereby, it becomes important to identify the dominant and nondominant eyes.[37,38] Jhiang et al. elucidated the role of laterality, ocular dominance, and anisometropic magnitude in the more myopic eye in anisometropia. They found that when the magnitude of anisometropia was less than moderate (4 D in either eye), the neural asymmetry, probably ocular dominance, favored the more myopic eye and thereby reduced the effect of the interocular optical asymmetry, so that the signals from the two eyes were close to balance. But when the magnitude of anisometropia was beyond a particular threshold, the neural asymmetry could overcome the optical asymmetry, thereby the probability of the myopic eye being dominant dropped. Ocular dominance increased the interocular optical difference instead of overcoming it.[39] Chia et al. studied the effect of eye laterality and dominance on refraction in Singaporean children. They found that ALs in the right eyes were significantly longer, and that the dominant eyes were significantly less astigmatic. Still, these differences were small and of little clinical significance.[40] In our study, no clinically significant difference was found between the dominant and nondominant eyes in terms of AL, average K, or refractive error. Observation of various retinal layers, such as the macular GCIPL and the macular and circumpapillary RNFL, in OCT is useful in the evaluation of diseases like glaucoma. The macular GCIPL cells are primarily involved in glaucoma, so their thickness assessment becomes important. Although RNFL measurements assess almost all the axons arising from ganglion cells, the circumpapillary region is likely to be affected by high myopia.[41,42] Based on this, Shoji et al. evaluated the effect of high myopia on the SD-OCT parameters in glaucoma. They found that the circumpapillary RNFL measurements are significantly affected by refractive errors, making it inferior in detecting glaucoma in high myopia compared to emmetropia. They also found that the GCC measurements and related parameters were not significantly affected, even in high myopia.[43] Patients suffer more visual handicaps if there is a macular pathology in the dominant eye than in the nondominant eye. Cehver et al.[33] compared the macular and peripapillary RNFL thicknesses of the dominant eye and nondominant eyes using OCT and concluded that there was no significant association between both. In the Sydney Myopia Study, Samarawickrama et al. evaluated CMT and ocular dominance in children and found no statistically significant difference.[21] In our study, no significant association was found between ocular dominance, ganglion cell thickness, and CMT.

Limitations

The number of right-handed and right eye-dominant individuals in the study was lower than that of left-handed and left eye-dominant individuals. Only a single SD-OCT device was used, and error characteristics may differ in other systems. Since none of the patients had significant anisometropia, the effect of refractive error on the interocular difference in RNFL could not be evaluated.

Conclusion

We found handedness and dominance to be predominantly on the right side. Overall, the superior RNFL thickness was higher in the left eye in both right eye-dominant and left eye-dominant individuals, which correlated with increased CMT in the left eye. No other ocular characteristic studied was also found to be associated with dominance.

Financial support and sponsorship:

Nil.

Conflicts of interest:

There are no conflicts of interest.