The Effect of Over Minus Prescription on Negative and Positive Fusional Vergence and Amplitude of Accommodation After Watching Video at Six Meters

Introduction

Accommodation refers to the ability of the eye to adjust its focal length and focus on objects at varying distances. This process involves changing the shape and refractive power of the lens to ensure that the image falls precisely on the retina. In general, myopic individuals who have longer axial length, tend to have higher amplitudes of accommodation than hypermetropic and emmetropic individuals. However, it is important to note that excessive accommodation demands, such as prolonged near work, can lead to visual discomfort and related symptoms.¹

Myopic individuals have a higher level of accommodation lag compared to those with emmetropia. Accommodation lag is the delay or sluggishness in the ability of the eye to focus properly on a target at different distances. This lag can lead to discomfort and reduce visual efficiency, especially during prolonged near work.² During near-work activities, the accommodation system should not exhibit a lag greater than a certain threshold, as it can lead to retinal defocus and produce blurry images. In the present study, the participants were given an over correction of - 1.00 D at the spectacle plane, which resulted in an accommodative lag.³

The amplitude of accommodation varies among individuals with different refractive errors. Myopic individuals generally exhibit the highest amplitude of accommodation, while hyperopic individuals have the lowest amplitude of accommodation until the age of 44 years. The amplitude of accommodation, which is the amount of accommodation required to move the focus from the far point to the near point decreases as we age, typically from childhood to around 65 years. The amplitude of accommodation can be calculated as the reciprocal of the near point of accommodation (NPA).^4,5

Fusional vergence is a measure of the ability of the eyes' extrinsic muscles to diverge or converge without changing accommodation. Positive fusional vergence, also known as convergence, is the degree of convergence that stimulates the eyes' structures while maintaining a constant level of accommodation. It is typically measured using external base prisms. Negative fusional vergence, on the other hand, also known as divergence, is the degree of divergence that can relax the eyes while maintaining accommodation. Its measurement is made using internal base prisms. Studies have shown that myopic individuals generally exhibit larger divergence than emmetropic individuals. Accommodation-vergence interactions may change with age, which can affect the amplitudes of fusional vergence.⁶

Therefore, the purpose of this current study was to investigate the effect of - 1.00 Dioptre Sphere (DS) over minus prescription on accommodation and vergence system after watching a video for 15 minutes at 6 meters.

Materials and Methods

Study design

The current prospective cross-sectional research design received ethical approval from the Institutional Ethical Committee before being implemented. It was conducted over a four-month period from September to December 2021 at Mangala College of Allied Health Sciences and recruited college students aged between 18 and 25 years who were currently enrolled in an educational program.

Inclusion criteria

The present study recruited myopic individuals aged between 18-25 years. The study was conducted involving undergraduate college students, both male and female. To be included in the study, participants needed to have fully corrected refractive errors and be free of any systemic or ocular health problems. Both eyes of participants had a best-corrected visual acuity of 6/6. The study included individuals with refractive errors up to <-4.00DS spherical correction and <-1.00DC astigmatism. Participants were also required to have a stereopsis of 60 seconds of arc or better, to be included in the study.

Exclusion criteria

Participants who declined to participate in the study had uncorrected refractive errors or any ocular/systemic health problems, as well as those taking medications, were excluded from the study. Students with a bestcorrected visual acuity below 6/6 were also excluded from the study.

Method

The study was conducted at Mangala College of Allied Health Sciences over a period of four months, spanning from September to December 2021. The study was approved by the Institutional Ethics committee and all participants provided informed consent prior to their inclusion in the study.

Participants who agreed to take part in the study were provided with a detailed explanation of the study's purpose and procedures, and written informed consent was obtained from those who agreed to participate. The study began with the measurement of visual acuity and refractive error, and participants with fully corrected refractive errors underwent a binocular vision evaluation. Stereopsis was assessed using the Toegepast Natuurwetenschappelijk Onderzoek (TNO) test with red and green display, followed by the assessment of phoria using cover tests, the Maddox rod test, and the broad H test to evaluate ocular motility. The slit lamp bio microscopy was also conducted to check for any abnormalities in the anterior and posterior chambers. The pre-intervention procedures included the measurement of NPA, Near Point of Convergence (NPC), and accommodative amplitude (AA) using a Royal Air Force (RAF) ruler (N8 target). The AA measurement was initiated with the right eye occluded, and participants were asked to fixate on a target at 40 cm while the target was gradually moved closer until the subject reported first sustained blur. The target was then slowly moved away until the subject reported the first clear image. The measurement was repeated three times to obtain the mean, and both the first sustained blur and clear image points were recorded, but only the blur point was used for data analysis. The same steps were repeated for the left eye while occluding the right eye. The AA was also measured binocularly without occluding either eye.

During the assessment of negative fusional vergence (NFV) at 6 m, participants were instructed to fixate on a 6/60 letter target while a base-in prism was placed before the right eye. The power of the prism was gradually increased until they reported diplopia, which was recorded as the break point. The power of the prism was then decreased until the participant reported a single image (recovery point), and both outcomes were recorded. The same procedure was repeated for the near NFV test using an N36 target at a distance of 40 cm.

Next, the assessment of positive fusional vergence (PFV) was conducted at 6 m using a prism bar. Participants were instructed to fixate on a 6/60 letter target while a base-out prism was placed before the right eye. The power of the prism was gradually increased until they reported diplopia, which was noted as the break point. The power of the prism was then decreased until the participant reported a single image (recovery point), and both outcomes were recorded. The same procedure was repeated for the near PFV test using an N36 target in the Snellen chart at a distance of 40 cm.

The intervention involved having participants watch a "MISTER BEAN" video for 15 minutes while wearing an added -1.00 DS on spectacle correction in both eyes at a distance of 6 meters. After the 15-minute video, the added -1.00DS was removed, and measurements were taken again for the amplitude of accommodation (AA), NFV, and PFV at both 6 m and 40 cm distances.

Statistical analysis

Descriptive statistics such as mean, median and standard deviation for quantitative variables were calculated. Paired t-test was performed for comparison between pre- and post-quantitative variables. Microsoft Excel and SPSS 16.0 versions were used to carry out the analysis for the study.

Results

The current investigation involved a total of 30 participants, with 5 (17%) males and 25 (83%) females. The mean age of the participants was 18.96±1.44 years. The results of the study indicated a statistically significant change in the accommodative amplitude (AA) of the right eye (RE) following a 15-minute over minus prescription (P <0.05), suggesting that the participants' ability to adjust their focus was significantly affected. However, no significant differences were observed in the AA of the left eye and binocularly.

Surprisingly, the study revealed no statistically significant alterations in the near point of convergence (NPC) and positive fusional vergence (PFV) after the 15-minute over minus prescription. This implies that the participants' ability to align their eyes and maintain binocular vision remained unaffected by the induced blur. In other words, despite the blurred vision, participants were still capable of maintaining a single unified image.

Overall, the study suggests that inducing an over minus prescription for 15 minutes at a 6-meter fixation distance can challenge the accommodative system but does not significantly affect the vergence system. It is important to note that the results may not be generalized to other populations, and further studies are needed to confirm these findings.

Discussion

In this study, 30 participants were involved, including 5 (17%) males and 25 (83%) females. All the participants were given a fixed amount of over minus prescription. The findings indicate that only the amplitude of accommodation in the right eye showed a significant change, while there were no significant changes observed in the left eye's amplitude of accommodation, as well as in the negative fusional vergence (NFV) and positive fusional vergence (PFV).⁷

This study measured NFV and PFV after the assessment of AA, which may have resulted in a washout effect of vergence adaptation during the few minutes between the measurements. It has been suggested that this recovery of vergence adaptation occurs within a shorter period of three minutes.⁸ Additionally, the continuous accommodation at a distant fixation for 15 minutes could also contribute to accommodative fatigue. This is because the ciliary muscle responsible for accommodation may become tired from the prolonged effort. Moreover, the over minus prescription induced in the study could cause the image to focus behind the retina, leading to retinal blur. This retinal blur may stimulate the inferior division of the oculomotor nerve, causing contraction of the ciliary muscle and an increase in the thickness of the crystalline lens. As a result, accommodation may occur when viewing at a distance to clear the blurred image.^8,9

It is possible that the lack of significant reduction in the amplitude of accommodation on the left eye could be attributed to the testing sequence used in the study. Specifically, the amplitude of accommodation for the right eye was measured first, followed by the left eye, which may have led to a potential "learning effect" for the left eye and thereby reduced the observed reduction in the amplitude of accommodation.¹⁰

Over prescription can lead to various visual discomforts such as headaches, eye strain, fatigue, and watering, which ultimately affects the visual performance and quality of life of the patient. Therefore, it is important to prescribe lenses that are comfortable and well-tolerated by the patient to ensure optimal visual function and overall satisfaction.

Conclusion

This study suggests that wearing a -1.00DS over minus lens for 15 minutes while watching a video at 6 m may result in asthenopia symptoms in young myopic individuals. The reduced accommodation system caused by the over-minus lens leads to image placement behind the retina, causing retinal blur. Consequently, the accommodation system must be stimulated to bring the image on the fovea for clear vision, even during distance fixation. Prolonged wearing of over-minus lenses can cause visual fatigue, affecting the focusing and vision system of myopic individuals. Therefore, it is crucial to conduct proper visual examination in all healthcare facilities, clinics, and opticals to avoid such issues.

Limitation

The small sample size of 30 participants may limit the generalizability of the findings. A larger sample size would provide more representative results and enhance the statistical power of the study.

Funding

Nil

Conflict of interest

The authors report no conflicts of interest, and they are responsible for the content of the article.

Acknowledgement

I would like to convey my sincere appreciation to Prof. Dr. Ganapathi P, the Medical Director of Mangala Group of Educational Institutions, and Prof. Pratijna Suhasini GR, the Dean of Mangala Group of Educational Institutions, as well as my friend Mr. Ragukumar Venugopal, a Research Associate at LV Prasad Eye Institute, and all of my lecturers who have provided me with constant support and encouragement during my academic journey. Moreover, I am grateful to all the study participants for their valuable contribution to this research.