Non-Strabismic Binocular Vision Dysfunction Among Contact Lens Wearers

Binocular vision dysfunction (BVD) refers to a range of anomalies that disrupt the coordinated binocular function of both eyes, resulting in symptoms of eyestrain, headache, blur vision, and increased difficulties with near activities. These disorders are prevalent among the general population, about 20 to 30% of individuals experience some form of accommodative or binocular dysfunction [1]. The proportion of BVDs among young adults, across different refractive groups ranges from 13 to 40% [2] [3] [4] [5]. Contact lenses are widely used for refractive error correction, providing superior visual comfort and cosmetic appearance over conventional spectacles. However, the relationship between contact lens wear and BVD has gained clinical interest. A recent study reported that 25% of myopic, nonpresbyopic adult contact lens wearers had BVD, the most prevalent was exo-related vergence disorders, especially convergence insufficiency [6]. This proportion may be influenced by myopic contact lens wearers requiring greater accommodative and convergence demand at near compared when spectacles are worn [6]. Though controversial [7], many commercially available single-vision contact lenses are present with negative spherical aberration [8] [9] that showed reduced accommodative lag [10] [11] by increasing accommodation among myopes wearing contact lenses [12]. The coexistence of BVD among contact lens wearers can exacerbate visual discomfort. There may be overlap of symptoms such as dry eye sensation and visual fatigue, making the clinical presentation more complicated. Rueff et al., [13] reported a significant correlation between dry eye symptoms and BVD in contact lens wearers, suggesting that individuals experiencing contact lens discomfort might have underlying binocular vision anomaly that may lead to contact lens discontinuation in some proportion of wearers due to BVD [6]. This study aims to investigate the prevalence and types of Non-strabismic binocular vision dysfunction (NSBVD) among contact lens wearers.

METHODS

1. Recruitment and Enrollment

Participants were recruited from our institute outpatient department of optometry. The study was reviewed and approved by the Institutional Ethical and Scientific Committee and followed the tenets of the Declaration of Helsinki. Informed consent was obtained from each participant after the nature, possible consequences, and procedures of the study were explained to them. Eligible participants aged between 18 and 30 years and had between -0.50 D to -12.00 D of spherical myopia with ≤ 0.75 D of astigmatism. Participants were also required to be wearing single-vision contact lenses with a minimum of 6 months wearing experience, with prescription of at least 15 days old. Participants had a best-corrected acuity of at least 0.10 logMAR (6/7.5) in each eye at 4 m. The exclusion criteria were any systemic or ocular conditions that may adversely affect contact lens wear, no ocular surgery including extraocular muscles, not strabismic and not pregnant.

2. Baseline Examinations

Total 64 participants were enrolled and all participants attended one clinic visit wearing their habitual contact lenses. During the baseline visit, participants were verbally questioned regarding their medical history including general and ocular health, family ocular history, history of their lenses (type, replacement frequency, care solution), wearing times (average days per week, average hours per week, average comfortable wearing time per day), date of last eye examination, allergies, medication, occupation, driving, visual display unit use, smoking, and hobbies. This was followed by measuring the logMAR monocular visual acuity with habitual lenses recorded at 4 m (I Chart HD Smart, Appasamy Associates, India) and 30 cm (MNREAD Acuity Chart Card; Precision Vision, Woodstock, IL). Binocular vision was fully assessed while the habitual contact lenses were being worn. Measurements of heterophoria (phoria), amplitude of accommodation, near point of convergence, fusional reserves, and relative accommodation were repeated three times and the average value taken as the final measure. Phorias were measured by the modified Thorington technique. BC/1209F and BC/1209N Muscle Imbalance Cards (Bernell Corporation, Mishawaka, IN) were used at 3 meters and 40 cm, respectively. Phorias were expressed as negative for exophoria and positive for esophoria. Phorias at 3 meters that fell beyond the scope of the phoria card were redetermined through a trail frame with the required prism correction (base-in in the case of exophoria and base-out in the case of esophoria) in order to make the red line from the Maddox rod coincide within the range of the card. A cover/uncover procedure was used to reduce prism adaptation, and the final phoria measurement was found by adding the prism power to the remeasured phoria. The accommodative convergence/accommodation (AC/A) ratio was computed using the formula given by Scheiman and Wick [14]. Amplitude of accommodation was measured monocularly using the “push-up” technique with a Royal Air Force Ruler (Unitech Vision, India) while viewing the 0.20 logMAR (6/9.5) line. The participants were asked to keep the clarity of the letters as they were gradually moved closure. The outcome was documented as the dioptric distance at which sustained blur occurred. Near point of convergence was measured using a vertical row of 0.30 logMAR (6/12) letters tapped to a pen torch. Participants were instructed to keep the row of letters as a single image. The pen torch was gradually moved closure until the participants reported diplopia, one eye drifted outward, or convergence ceased. Its result was noted as the distance in centimeters, from the center of the brow to the point where any such events occurred. Monocular and binocular accommodative facility was evaluated by a ± 2.00 D flipper (Bernell Corporation, Mishawaka, IN) with participants perceiving the 0.20 logMAR (6/9.5) line on a Saladin Near Point Balance Card Version 1.0 (Bernell Corporation) at a reading distance of 40 cm. For the purpose of testing monocular accommodative facility, participants’ left eye was covered and instructed to make the letters clear as quickly as possible subsequent to each flip. For binocular accommodative facility, the cover was removed and instructed to make to make the letters clear and single as rapidly as possible following each flip. One cycle consisted of clearing both plus and minus lenses. The test was performed for one minute, with results measured in cycles per minute. The result was measured to the half cycle, and if neither plus nor minus could be cleared; it was recorded as 0 cycle per minute. Smooth fusional vergence (positive and negative fusional reserves) was assessed at 6 meters and 40 cm with a prism bar. At each distance, the target was positioned one line above the participant’s binocular visual acuity with habitual contact lenses. The participant was asked to report when the letters become blurred or when diplopia was experienced. Prism power was increased gradually with the investigator frequently asking if the letters were still clear and if there was only a single line of letters visible. The blur, break, and recovery points were measured at the times when first blurring of the letters by prism power was experienced, diplopia appeared, and when single vision again occurred on the reduction of prism. When blur was not reported and diplopia was reported, it was recorded as “break before blur.” Positive and negative relative accommodation was recorded at 40 cm with a trail frame with the same target employed for fusional vergence at 40 cm. The participants was instructed to keep the designated line clear and single while power was increased binocularly in 0.25 D steps. The final point was when the participant reported sustained blur. Monocular accommodative lag (right eye) was measured using “Monocular Estimation Method” (MEM) technique using a retinoscope attached with MEM card. The participant was instructed to fix binocularly at the MEM card in the plane of the retinoscope positioned at the near reading distance. The investigator observed the retinoscopic reflex and introduced spherical lenses in front of the participant’s eye until the first neutral reflex was observed.  An Appasamy slit-lamp biomicroscope, AIA-11 (Appasamy Associates, India), was used to assess the contact lens surface and lens fitting. Contact lenses were removed, and keratometric readings were measured using an IOL Master 500 (CarlZeiss Meditech, Jena, Germany). Three readings were recorded for each eye with a Sound Noise Ratio (SNR) ≥ 100 and the result was the mean of these three measurements and a balanced subjective refraction for each eye was performed.  Slit-lamp biomicroscope was used to assess the anterior ocular surface at least 10 minutes after lens removal. The original Cornea and Contact Lens Research Unit grading scale [15] was used to grade bulbar, limbal, and palpebral hyperemia, corneal and conjunctival staining, and palpebral roughness in each eye on a 0 - 4 scale in 0.5 steps. The ocular surface was assessed for the presence or absence of lid-parallel conjunctival folds and meibomian gland dysfunction. Corneal and conjunctival staining was assessed using Fluorescein Sodium ophthalmic strips (Fluostrips; 1mg fluorescein sodium). Fluorescein was used to assess corneal staining and lid wiper epitheliopathy both graded as present or absent and to measure tear breakup time (TBUT). Following this all participants were grouped according to their binocular vision status (binocular vision dysfunction or non-binocular vision dysfunction).  Each measurement of binocular vision was assessed relative to a cutoff score. This criterion was used for distance [16] and near [17] phoria, near point of convergence [18], accommodative facility [19], fusional reserves [20], relative accommodation [20], and accommodative lag [21]. Cutoff values for the calculated accommodative convergence/accommodation (AC/A) ratio were taken from published literature, [14] and the amplitude of accommodation was 2 D below Hofstetter’s equation for minimum age [22]. Sheard’s criterion [14] was also used as a cutoff for fusional reserves. The diagnostic criteria for each of the binocular vision dysfunction were determined from a literature review [2] [3] [4] [5] [6] [14] [23] [24] and are shown in Table 1.

3. Statistical Analyses

The data were entered into a Microsoft Excel spreadsheet and analyzed using SPSS version 20.0 (IBM, Somers, NY, USA) statistical software and significance was set at 5%. Measures of central tendencies including means, standard deviations, and range were calculated. To compare measurements between non-binocular vision disorder and binocular vision disorder group, a repeated measure analysis of variance (ANOVA) was conducted.

TABLE 1. Summary of diagnostic criteria for each binocular vision dysfunction

RESULTS

1. Demographic Data

Sixty-four participants were enrolled in this study, 15 (23.1%) were diagnosed with a binocular vision dysfunction. Convergence Insufficiency 5 (7.8%), were more common. The specific binocular vision dysfunctions are summarized in Table 2. The majority of participants were female 35 (54.6%). The average age of all the participants was 24.8 ± 2.6 years ranging from 18 to 27 years of age. The mean subjective spherical refraction was about -5.50 D. (Table 3)

TABLE 2. Specific type of binocular vision dysfunction and prevalence