Open Access
Refractive Intervention  |   January 2025
Comparison of Unaided and Aided Visual Acuity in Adults With Down Syndrome
Author Affiliations & Notes
  • Lauren V. Schneider
    The Ohio State University College of Optometry, Columbus, OH, USA
  • Jason D. Marsack
    University of Houston, College of Optometry, Houston, TX, USA
  • Ruth E. Manny
    University of Houston, College of Optometry, Houston, TX, USA
  • Heather A. Anderson
    The Ohio State University College of Optometry, Columbus, OH, USA
  • Correspondence: Lauren V. Schneider, The Ohio State University College of Optometry, 338, W 10th Ave., Columbus, OH 43210, USA. e-mail: [email protected] 
Translational Vision Science & Technology January 2025, Vol.14, 30. doi:https://doi.org/10.1167/tvst.14.1.30
  • Views
  • PDF
  • Share
  • Tools
    • Alerts
      ×
      This feature is available to authenticated users only.
      Sign In or Create an Account ×
    • Get Citation

      Lauren V. Schneider, Jason D. Marsack, Ruth E. Manny, Heather A. Anderson; Comparison of Unaided and Aided Visual Acuity in Adults With Down Syndrome. Trans. Vis. Sci. Tech. 2025;14(1):30. https://doi.org/10.1167/tvst.14.1.30.

      Download citation file:


      © ARVO (1962-2015); The Authors (2016-present)

      ×
  • Supplements
Abstract

Purpose: Individuals with Down syndrome (DS) have reduced visual acuity (VA), even when wearing refractive correction. The relationship between refractive error and VA in adults with DS is explored.

Methods: Thirty adults with DS (age = 29 ± 10 years) were enrolled in a trial comparing clinical and objectively determined refractions. Monocular VA was recorded unaided and aided with best refraction. Vectors M, J0, and J45 were calculated from unaided wavefront aberration measures at the habitual pupil size. The square root of the sum of the squared vectors was calculated providing a single positive vector length representing unaided refractive error. Residual refractive error was determined after applying the best performing refraction. Linear regression determined correlation between refractive error and VAs.

Results: Unaided and aided VAs ranged from 0.22 to 1.42 logMAR and 0.06 to 0.82 logMAR, respectively. Unaided and residual refractive error represented as vector length ranged from 0.68 diopters (D) to 13.76 D and 0.05 D to 1.87 D, respectively. Unaided refractive error and VA were significantly positively correlated (r2 = 0.776, P < 0.001), but not residual refractive error and VA (r2 = 0.005, P = 0.721).

Conclusions: There was a positive correlation between unaided VA and refractive error magnitude in adults with DS; however, unaided VA was better than expected given the high levels of refractive error. Aided VA and residual refractive error were not correlated, despite overall low levels of remaining residual refractive error, suggesting that factors in addition to optical quality may be limiting VA in this population.

Translational Relevance: Understanding the relationship between refractive error and VA in individuals with DS may provide clinicians clearer expectations for the acuity end points before and after correction for this patient population.

Introduction
Down syndrome (DS), or Trisomy 21, is one of the most common chromosomal birth defects in the United States, with a prevalence of 17.19 per 10,000 births between 2016 and 2020.1 For individuals with DS, there is a higher prevalence of ocular conditions that require ongoing care and monitoring compared with their developmentally typical counterparts, such as higher magnitudes of both myopic and hyperopic refractive error and astigmatism, strabismus, nystagmus, and increased higher order aberrations.24 Visual acuity (VA) in individuals with DS also tends to be reduced even with refractive correction,5,6 with one study suggesting adults with DS have, on average, 6 lines worse VA compared with age-matched controls.7 These conditions, along with varying intellectual disability within this group, can make accurate refraction and VA measurements difficult during eye examinations. 
Reasons for reduced VA in individuals with DS have been explored, and recent efforts have looked to characterizing and mitigating the degrading effects of higher order aberrations on optical quality.8,9 One way to quantify the effects of higher order aberrations on image quality is the use of metrics – mathematical formulae that can be “optimized by subjective [or objective] refractions when higher order aberrations are present.”10,11 Two metrics that have been shown to have strong associations with VA are pupil fraction tessellated (PFSt) and visual Strehl ratio in the spatial domain (VSX).12,13 A recent clinical trial has described a method that optimized these image quality metrics, identifying spectacle corrections that consider and minimize the effects of residual, uncorrected aberrations.14 This clinical trial used a computer algorithm to identify refractions that optimized PFSt and VSX to dispense experimental spectacle corrections to adults with DS, resulting in equal or better acuity gains compared to a refraction derived from traditional clinical techniques such as dry and wet retinoscopy and autorefraction.14 Whereas VA showed some improvement with these optimized corrections, the gains did not match those predicted by a normally sighted control observer viewing an acuity chart that had been blurred to simulate the retinal image of an individual with DS.15 
The relationship between refractive error and VA has been explored in developmentally typical adults and children,1620 but this relationship has not been well established for the DS population. In participants without DS, some previous literature has estimated a linear reduction in uncorrected acuity of about 0.30 logMAR to 0.40 logMAR for every diopter of refractive error present,16,18,19 whereas other literature has found parabolic relationships for reduction in acuity between 0.25 logMAR and 0.35 logMAR for every diopter of refractive error present.17,20 The goal of the current study is to explore the relationship between refractive error and VA in adults with DS to ascertain if 1) trends for unaided VA reduction per diopter of refractive error reflect those found in the literature for typically sighted individuals and 2) greater magnitudes of refractive error, both unaided and the residual amount when aided with a given correction, are correlated to VA measures. Understanding the relationship between refractive error and VA in individuals with DS would provide clinicians clearer expectations for the acuity end points before and after correction for this patient population. 
Methods
This study was approved by the University of Houston Committee for the Protection of Human Subjects and adhered to the Declaration of Helsinki. For all participants with DS, parental permission was obtained from a parent or legal guardian along with participant assent when able. Participants were recruited from the clinics at the University of Houston College of Optometry and from local DS organizations in Houston, TX. In all, 30 adults with DS at least 18 years of age with no significant ocular pathologies, nystagmus, and strabismic or anisometropic amblyopia who were able to be dilated and to fixate for several seconds during imaging, were enrolled. The study was listed on clinicaltrials.gov (NCT03367793) in accordance with National Institutes of Health (NIH) policy. Primary outcomes of the study, along with detailed methods and participant demographics, have been published elsewhere.14,15,21 
The study involved an initial study visit, comprised of a comprehensive eye examination administered by an expert clinician with over 30 years of experience working with the DS patient population. Habitual refractions were measured via lensometry or considered plano (n = 9) if a participant did not present with spectacles. Measures of monocular distance VA, binocular vision, suitability to dilate, pupil diameter in dim and dark lighting, dry autorefraction, subjective refraction, and wavefront aberration using a COAS-HD wavefront aberrometer (Johnson & Johnson, Santa Ana, CA, USA) were obtained. Participants were then dilated with one drop of 1% tropicamide and one drop of 2.5% phenylephrine, separated by 5 minutes once all non-cycloplegic testing was completed. Dilated wavefront aberration was measured 30 minutes after installation of drops, followed by cycloplegic autorefraction, retinoscopy, and ocular health assessment. A final clinical refraction was determined by the expert examiner using any combination of clinical data, such as dry and wet autorefraction and retinoscopy and subjective refraction. 
After the initial study visit, three to five wavefront error measures per eye were re-sized to the individual eye's average pupil diameter in dim lighting using the method described by Campbell (2003),22 and averaged using a custom program (Spectacle Sweep; University of Houston College of Optometry Core Programming Module, Houston, TX, USA) in MATLAB (MathWorks, Natick, MA, USA) to provide a single, averaged wavefront measure per eye. Spectacle Sweep was then used to mathematically apply refractions over a range of more than 20,000 sphero-cylindrical combinations in 0.25 diopter (D) steps for sphere and cylinder surrounding the patient's habitual correction for the entire range of cylinder axes in 1-degree steps. The metric values for PFSt were then calculated and sorted such that the single refraction with the best PFSt values was identified for each eye. The process was repeated for VSX. 
The program was also used to provide a calculation of higher order root mean square (RMS) over a 4 mm pupil diameter for each participant by taking the square root of the sum of the squares of all terms in the third to tenth Zernike orders, which was compared with higher order RMS levels at the same pupil diameter in the typical population. 
The experimental refraction types – clinical, PFSt, and VSX – were fabricated in identical frames selected by each participant. Each experimental correction was dispensed for 2 months in a randomized treatment order and the order of eye testing was randomized within each individual and maintained for each subsequent follow up visit. 
After each correction's 2 months of wear, the best performing correction was identified and dispensed for an additional 6 months of wear. The refraction that performed the best was determined by an examiner masked to the treatment types who made a holistic decision by considering factors such as monocular and binocular distance acuity, near VA, ocular alignment, stereoacuity, accommodative accuracy assessed by the monocular estimation method (MEM) with retinoscopy, daily wear time measured with a temperature sensor data logger mounted to the temple of each pair of spectacles, and participant preference (to the extent that it could be elicited from the participants). After wearing the best performing refraction for 6 months, a final study visit was conducted during which monocular VAs, both unaided and aided with the best refraction, were measured. 
VA Measures
At each study visit, participants viewed a logMAR-style eye chart from a chin and forehead rest and read one letter at a time, monocularly. At the final study visit, this was done first with the best refraction and then unaided. The chart was composed of lines with five letters each of either the British Standard (D, E, F, H, N, P, R, U, V, and Z) or a restricted set (H, O, T, and V) with one repeated letter per line, along with a matching card if needed depending on the cognitive ability of the participant. The largest line (0.8 logMAR) was presented first, continuing line by line until the participant made five total mistakes. Final logMAR acuity was scored letter-by-letter such that each letter was equal to 0.02 logMAR, and the total score of correct letters (number of correct letters times 0.02) was subtracted from 0.9 logMAR. The monitor displaying the letters was positioned 12.2 feet from the forehead rest in the standard condition. To continue testing at this distance, participants were required to correctly read four of the five letters on the largest letter size displayed (0.8 logMAR). When testing participants unaided, the standard test distance was also used, except for four participants with high myopic refractive error who were unable to correctly identify 4 of 5 letters on the 0.8 logMAR line from 12.2 feet away. These individuals were moved closer to the monitor until they could successfully read the top line of letters. Acuity testing continued at the reduced test distance and acuity was later converted to reflect the adjusted logMAR score for the reduced testing distance. 
Determination of Refractive Error
Traditional sphero-cylindrical prescriptions are difficult to compare because the three components of the refraction (sphere, cylinder, and axis) are not independent of one another. Power vectors have been utilized to more easily compare refractions statistically.23 To quantify unaided refractive error, power vectors M, J0, and J45 were calculated from the second order Zernike terms (denoted as \(c_2^{ - 2},\) \(c_2^0\), and \(c_2^2\), in microns) of the uncorrected eye and the participant's habitual pupil size (equal to radius in mm, r; Equations 13). From there, a single dioptric value (D) for refractive error was calculated by the square root of the sum of the squared vector components (Equation 4), as defined by Thibos et al. (1997 and 2004).11,23 This value is referred to as “vector length” throughout for simplicity and reported without a sign because the values can only be positive.  
\begin{eqnarray} && {{M}} = \frac{{ - c_2^0 4\sqrt 3 }}{{{{r}^2}}}\quad {\textit Power\, Vector\, Component\, M\, (defocus)}\quad \end{eqnarray}
(1)
 
\begin{eqnarray} && {{J}_0} = \frac{{ - c_2^22\sqrt 6 }}{{{{r}^2}}}\quad {\textit Power\, Vector\, Component\, }J_0 \left( 180/90\, {\textit astigmatism} \right) \end{eqnarray}
(2)
 
\begin{eqnarray} && {{J}_{45}} = \frac{{ - c_2^{ - 2}2\sqrt 6 }}{{{{r}^2}}}\quad {\textit Power\, Vector\, Component\, }J_{45}\,\left( {\textit oblique\, astigmatism} \right) \end{eqnarray}
(3)
 
\begin{eqnarray} && D = \sqrt {{{M}^2} + J_0^2 + J_{45}^2} \quad {\textit Vector\, Length}\quad \end{eqnarray}
(4)
 
In a similar fashion, residual refractive error with optical correction was calculated by first representing the best performing spectacle correction as second order Zernike coefficients (\(c_2^{ - 2},{\rm{\ }}c_2^0\), and \(c_2^2\)). This Zernike representation of the correction was then added to the average second order Zernike representation of the unaided eye. The second order Zernike coefficients resulting from the addition represent the residual lower order aberrations of the correction/eye combination. These residual second order aberrations, along with the third to tenth order aberrations of the unaided eye represent the aberrations experienced by the patient when they wear their spectacle correction. 
Data Analysis
Calculations of power vectors and vector length were performed using Excel (Microsoft, Redmond WA, USA). Linear regression was performed using SPSS version 28 (IBM, Armonk, NY, USA) to determine the correlation between refractive error and VAs. For simplicity, only values for the right eyes are used throughout the analyses except for the four subjects who had a constant right esotropia; for these individuals, data from the left eye was included in the analysis to avoid the possibility of including an eye with an amblyogenic factor. 
Different models of expected VA based on magnitude of refractive error have been reported in the literature. The model proposed by Raasch (1995) is the only one that uses vector length; therefore, the expected VA from the Raasch model Equation 7 (logMAR = 0.48 + (1.07*[Log(Vector)] + (0.46*[Log(Vector)]2) is compared to the actual VA obtained from the participants with DS.17 Rates of change in VA per diopter of defocus listed in other literature sources are compared with the current cohort's regression analysis. 
Results
Thirty adults with DS were enrolled in the study, with a mean age of 29 ± 10 years (range = 18–52 years). Twenty-nine of the participants were tested using the British Standard set and one participant was tested with the restricted letter set for VA. The range of refractive errors as one would describe clinically (i.e. not in vector length) based on dry autorefractor readings was large (−15.25 D to +6.00 D sphere) with all but 4 participants having at least one eye with −1.00 D cylinder power or more (range = −5.25 D to −0.25 D cylinder). There were 12 myopic eyes, 9 hyperopic eyes, 8 eyes with mixed astigmatism, and 1 eye with emmetropia (emmetropia defined as both principal meridians falling between −0.50 and +1.00 D with less than 0.50 D of cylinder). Accommodative accuracy was successfully measured for 29 of 30 participants with a mean (SD) of +1.29 (±0.46) D for the eye reported in this analysis. All participants demonstrated a lag of accommodation with a minimum of +0.50 D and a maximum of +2.25 D. For the last 6 months of the trial, 7 (23%) participants were dispensed their clinical correction, 9 (30%) were dispensed their PFSt correction, and 14 (47%) were dispensed their VSX correction (Table 1). Additional demographic information has been reported previously.14,15 
Table 1.
 
Final Correction Type and Range of Refractive Error
Table 1.
 
Final Correction Type and Range of Refractive Error
Unaided and aided VA ranged from 0.22 to 1.42 logMAR, and 0.06 to 0.82 logMAR, respectively. Unaided and residual spherical equivalent refractive error ranged from –13.75 D to +4.92 D and –0.93 D to +1.87 D, respectively. Unaided and residual refractive error represented by vector length ranged from 0.68 D to 13.76 D and 0.05 D to 1.87 D, respectively. The average higher order root mean square based on a 4 mm pupil diameter for all eyes was 0.20 ± 0.07 microns, and was, on average, higher than that of age-matched individuals without DS (average RMS from normative database = 0.094 ± 0.04 microns24). 
There was a significant positive linear relationship between the unaided refractive error and unaided VA (Fig. 1A; r2 = 0.77, P < 0.001). There was no significant linear relationship between the residual refractive error and aided VA (Fig. 1B; P = 0.80). 
Figure 1.
 
Correlations between VA and unaided refractive error (A) and residual refractive error (B).
Figure 1.
 
Correlations between VA and unaided refractive error (A) and residual refractive error (B).
Unaided refractive error as vector length was related to the participant's pupil diameter in dim lighting such that those with higher magnitudes of refractive error tended to have smaller pupils (Fig. 2). However, at lower magnitudes of refractive error, there was a wide range of pupil diameters. 
Figure 2.
 
Relationship between unaided refractive error as vector length (diopters [D]) and pupil diameter in dim lighting (mm).
Figure 2.
 
Relationship between unaided refractive error as vector length (diopters [D]) and pupil diameter in dim lighting (mm).
Given that the vector length calculations do not reflect whether the refractive error is myopic or hyperopic, we also sought to compare VA with spherical equivalent measures. When looking to individual participants’ unaided and aided VA measures, there was variation in the magnitude of improvement, as illustrated in Fig. 3, where participants with large levels of myopic refractive error showed large gains in VA when wearing their correction, but many participants with lower levels of refractive error (both myopic and hyperopic) had small gains in VA with correction. 
Figure 3.
 
Unaided and aided VA by spherical equivalent refractive error.
Figure 3.
 
Unaided and aided VA by spherical equivalent refractive error.
Due to ocular accommodation, hyperopic refractive error in young, typically developed individuals is less tightly correlated to VA compared with those with myopic refractive error.16,20 A similar observation is made for the present study population in that the linear correlation between myopic refractive error and unaided VA was stronger for those with myopic refractive error than those with hyperopic refractive error (Fig. 4). However, this spherical equivalent analysis does not address the elevated levels of astigmatism or higher order aberrations present in this study population, which may present additional factors. 
Figure 4.
 
Correlation of unaided spherical equivalent refractive error and unaided VA for myopes and hyperopes.
Figure 4.
 
Correlation of unaided spherical equivalent refractive error and unaided VA for myopes and hyperopes.
Comparison to Correlations of VA and Refractive Error in the Typical Population
In Figure 5, unaided refractive error as vector length and actual VA measures are shown along with the predicted VA based on Equation 7 from Raasch (1995)17 that was derived for normally sighted individuals without DS (see Fig. 5). The actual VA for a given refractive error is significantly different (P < 0.001) than would be suggested by the Raasch (1995) prediction, with the actual VA consistently better than predicted. In addition, the slope of the current data's linear regression (b[1] = 0.09) is shallower compared with the rates of change proposed in other literature (Table 2), indicating the participants with DS had better acuity than would be expected with their level of refractive error based on these published models. 
Figure 5.
 
Comparison of actual participant VA and predicted VA based on the study by Raasch (1995).
Figure 5.
 
Comparison of actual participant VA and predicted VA based on the study by Raasch (1995).
Table 2.
 
Relationship Between Rate of Change in Visual Acuity Per Diopter of Refractive Error for the Developmentally Typical Population From the Literature
Table 2.
 
Relationship Between Rate of Change in Visual Acuity Per Diopter of Refractive Error for the Developmentally Typical Population From the Literature
Discussion
The goals of the current study were to determine if both unaided and residual refractive error are correlated with VA in adults with DS, and if the change in unaided VA per change in refractive error matched previously reported trends in the literature for individuals without DS. With respect to unaided acuity and previous reports in the literature, this study found a significant positive linear correlation between unaided refractive error and VA, which is to be expected (see Fig. 1A). However, the rate of change in unaided VA of the individuals with DS in this study was better (0.09 logMAR/ 1 D) than would have been predicted by previous literature (see Table 2Fig. 5). Specifically, the slope of the current study's data is shallower compared to trends reported by Emsley (1952), Smith (1991), and Atchison and Mathur (2011) for typically developed individuals.16,18,19 Kleinstein et al. (2021) analyzed the relationship between unaided distance refractive error and VA in children without DS, finding a 0.5 minutes of MAR reduction in acuity for every 0.35 D of refractive error.20 Using this relationship and converting to logMAR would predict a non-linear trend in VA reduction with increasing refractive error similar to that shown by Raasch (1995). With respect to Figure 5, Raasch (1995) only included myopic individuals in their analysis, whereas we included both myopes and hyperopes. We exploratorily removed hyperopes from our analysis to see if the relationship between VA and refractive error would more closely match Raasch (1995) but found no meaningful change with the removal of the hyperopic participants. 
When looking at specific refractive errors within the current cohort, myopic refractive errors were more correlated to VA compared with hyperopic refractive errors (see Fig. 4), possibly due to accommodation as noted by Kleinstein et al. (2021)20 and others.16 It is important to consider that individuals with DS have poor accommodation,4 so the impact of hyperopic refractive error on VA may be more predictable in this group than in children with normal accommodation. We observed greater than normal accommodative lags, on average, which is consistent with previous studies demonstrating poor accommodative ability in the DS population.25,26 Another explanation for less correlation in hyperopic participants could be high levels of astigmatism producing a spherical equivalent refractive error closer to zero. 
Visual Acuity With Unaided Refractive Error
A possible explanation for the better-than-expected unaided VA in adults with DS may be prolonged exposure to blur. Where developmentally typical children generally have a narrowing of refractive error distribution as infants during emmetropization, those with DS often have a widening of refractive error distribution and increasing amounts of oblique astigmatism, resulting in high magnitudes of refractive errors early in life.27 Another study of children with and without DS in the United Kingdom found there is little change in acuity development in children with DS past the age of 5 years, with children in the DS group having, on average, 0.2 logMAR worse acuity compared with those without DS.5 In this way, those with DS may have some level of blur due to spherical and cylindrical refractive error during young ages compared with other children without DS who may only develop clinically significant refractive errors later in the second decade of life, such as childhood-onset myopia. 
With such long-term exposure to some level of blur for individuals with DS, neural adaptation could be contributing to their unaided VA measures, allowing these individuals to perform with better VA than expected. We can see evidence for neural adaptation to blur in the literature for individuals without DS. For example, when myopic refractive error is simulated in emmetropes with +1.00 D lenses, VA improved after 30 minutes of exposure to the plus lenses compared to VA after initial exposure, with binocular improvement of about 0.089 logMAR, or one line.28 In a study of emmetropes and fully corrected myopes with simulated myopia via a +2.50 D lens, the myopes had greater improvements in VA over a 2 hour exposure period compared with their emmetropic counterparts, with 11 out of the 17 myopes showing >0.2 logMAR improvement after 2 hours, especially at lower contrast levels.29 Last, participants were able to adapt to blur resulting from five different higher-order aberration patterns, with their perceived neutral point after physically adjusting focus of the image being proportional to the amount of blur present.30 These studies demonstrate that exposure to blur, even as short as 30 minutes, can result in improved VA compared to the initial decrease expected with a given blur magnitude. Accordingly, the participants with DS in the current study may have become adapted to their own refractive error, resulting in better-than-expected unaided acuity. 
Neural adaptation to the effects of higher order aberrations on optical quality have also been demonstrated in studies looking at individuals with and without keratoconus. Induction of one's own aberrations in habitual orientation versus a rotated orientation showed better acuity with one's own habitually oriented aberrations.31 When higher order aberrations are compensated for in individuals with and without keratoconus, those with keratoconus have worse acuity compared with healthy controls, despite similar retinal image quality post-correction of aberrations for the two groups.32 In contrast, when the higher order aberrations of an eye with severe keratoconus are simulated for eyes with and without keratoconus, those with keratoconus have about 0.12 logMAR better acuity compared with those without keratoconus.33 Last, those with keratoconus have shown a decreased neural sensitivity to higher spatial frequencies which are more affected by blur, and a subsequent increased adaptation to lower spatial frequencies compared with those without keratoconus,34 and the sensitivities were orientation-specific.35 Again, aberration-corrected vision was worse in those with keratoconus compared with healthy controls. These studies demonstrate that neural adaptation to degraded image quality due to higher order aberrations can result in worse acuity when these aberrations are corrected or manipulated, but this acuity is generally better than those with less aberrations who are exposed to simulated degraded optics. 
In addition to neural adaptation, another potential explanation for better-than-expected unaided visual acuity that could be explored is pupil diameter. Particularly for eyes with elevated aberrations, a smaller pupil diameter may be advantageous to reduce the effect of aberrations on the retinal image and provide a greater depth of field. In this study, those with higher magnitudes of refractive error tended to have smaller pupils in the dim lighting of VA testing conditions (see Fig. 2). Two previous studies have reported smaller pupil diameter on average in individuals with DS compared with controls.36,37 However, our participant population had a wide range of pupil diameters with lower levels of refractive error, making the relationship among refractive error, pupil diameter, and VA less straightforward. 
Another potential explanation for better-than-expected VA could be related to the higher magnitudes of astigmatism observed in this group. When representing refractive error as a spherical equivalent or total vector length, the contribution of astigmatism to that value is masked, making it difficult to determine whether a given individual has a large magnitude of astigmatism or not. However, Kleinstein et al. (2021) demonstrated that VA reduction for a given spherical equivalent was not impacted by higher levels of J0 astigmatism.20 
Visual Acuity With Refractive Correction
Based on their refraction leaving the clinical trial, approximately 75% of the participants with DS in the current study were tested with metric optimized corrections that account for the effects of higher order aberrations,38 and the other 25% were wearing refractions determined by an expert clinician. Overall, aided acuity was better than unaided acuity and there was relatively little residual refractive error with corrections. Even with the aided VA gains observed, there was still a range of aided VA with low levels of residual refractive error that was not normalized to 0.0 logMAR (20/20; see Fig. 1B). Similarly, although there were greater gains in VA for those with greater magnitudes of refractive error corrected (see Fig. 3), it was not to the levels expected from observations in the typical population. The current cohort's level of aided VA is consistent with previous measures of VA in adults with DS7 and was better than that found recently by Martin-Perez et al. (2023) who compared young adults with DS with those without, although their experimental corrections were only determined using typical clinical techniques.6 
The range of aided VA observed suggest that other non-optical factors may be contributing to limited best-corrected VA. One factor may be bilateral amblyopia. All the participants in the study were prescribed spectacles during the study due to clinically significant refractive error; however, nine entered the study without habitual corrections. This presentation alone suggests that some may not have been wearing corrections that were needed and it is certainly possible that some individuals may have had bilateral amblyopia. However, the VA range in this study was not atypical of what one sees for individuals with DS who have received early intervention with spectacle corrections.5,39,40 Other factors may include sensory deficits at the level of the retina41 or visual cortex42,43 shown previously in those with DS with optical coherence tomography (OCT) analysis, and visually evoked potential (VEP), and Vernier acuity measurements, respectively. Further study is needed to fully elucidate the effects of these sensory deficits on VA measures. 
This study had some limitations. Generalizability of the results is limited because we only included adults with DS who were mostly 30 years of age or older, whereas many studies of VA and refractive error have been conducted for children with and without DS. In addition, the inclusion of older adults makes it more difficult to eliminate long-standing bilateral amblyopia as a factor in the VA outcomes of this study. Further study of children with DS is needed to fully assess the potential acuity gains to be expected from metric-optimized corrections and the relationship between residual refractive error and VA explored in Figure 1B. Last, our eligibility requirements included the ability to participate in logMAR VA tasks. Not all individuals with DS are able to complete letter VA tasks and thus our sample may be biased toward those with higher cognitive functioning, which could also impact the relationship between refractive error and VA. 
Conclusions
There was a positive correlation between unaided visual acuity and refractive error magnitude in adults with DS; however, unaided VA was better than expected given the high levels of refractive error. Possible explanations include neural adaptation from prolonged exposure to blur and degraded optics from the presence of higher order aberrations. Pupil size may also play a role in the better-than-expected VAs observed. Aided VA and residual refractive error were not correlated, despite overall low levels of remaining residual refractive error, suggesting that factors in addition to optical quality may be limiting VA in the presence of refractive correction in this population. Further study is needed to examine VA and refractive error relationships and the influence of sensory factors on VA in the DS population. 
Acknowledgments
The authors thank Hope Queener for development of the Spectacle Sweep software used in this study. 
Supported by grants from the National Institutes of Health (NIH) R01 EY024590 (H. Anderson) and NIH P30 EY07551 (L. Frishman). 
Disclosure: L.V. Schneider, None; J.D. Marsack, None; R.E. Manny, None; H.A. Anderson, None 
References
Stallings EB, Isenburg JL, Rutkowski RE, et al. National population-based estimates for major birth defects, 2016–2020. Birth Defects Res. 2024; 116: e2301. [CrossRef] [PubMed]
Knowlton R, Marsack JD, Leach NE, Herring RJ, Anderson HA. Comparison of whole eye versus first-surface astigmatism in Down syndrome. Optom Vis Sci. 2015; 92: 804–814. [CrossRef] [PubMed]
da Cunha RP, Moreira JB. Ocular findings in Down's syndrome. Am J Ophthalmol. 1996; 122: 236–244. [CrossRef] [PubMed]
Cregg M, Woodhouse JM, Pakeman VH, et al. Accommodation and refractive error in children with Down syndrome: cross-sectional and longitudinal studies. Invest Ophthalmol Vis Sci. 2001; 42: 55–63. [PubMed]
Zahidi AA, Vinuela-Navarro V, Woodhouse JM. Different visual development: norms for visual acuity in children with Down's syndrome. Clin Exp Optom. 2018; 101: 535–540. [CrossRef] [PubMed]
Martin-Perez Y, Gonzalez-Montero G, Gutierrez-Hernandez AL, Blazquez-Sanchez V, Sanchez-Ramos C. Vision impairments in young adults with Down syndrome. Vision (Basel). 2023; 7:60. [CrossRef] [PubMed]
Ravikumar A, Benoit JS, Morrison KB, Marsack JD, Anderson HA. Repeatability of monocular acuity testing in adults with and without Down syndrome. Optom Vis Sci. 2018; 95: 202–211. [CrossRef] [PubMed]
McCullough SJ, Little JA, Saunders KJ. Higher order aberrations in children with Down syndrome. Invest Ophthalmol Vis Sci. 2013; 54: 1527–1535. [CrossRef] [PubMed]
Anderson HA, Ravikumar A, Benoit JS, Marsack JD. Impact of pupil diameter on objective refraction determination and predicted visual acuity. Transl Vis Sci Technol. 2019; 8: 32. [CrossRef] [PubMed]
Cheng X, Bradley A, Thibos LN. Predicting subjective judgment of best focus with objective image quality metrics. J Vis. 2004; 4: 310–321. [PubMed]
Thibos LN, Hong X, Bradley A, Applegate RA. Accuracy and precision of objective refraction from wavefront aberrations. J Vis. 2004; 4: 329–351. [CrossRef] [PubMed]
Ravikumar A, Benoit JS, Marsack JD, Anderson HA. Image quality metric derived refractions predicted to improve visual acuity beyond habitual refraction for patients with Down syndrome. Transl Vis Sci Technol. 2019; 8: 20. [CrossRef] [PubMed]
Benoit JS, Ravikumar A, Marsack JD, Anderson HA. Understanding the impact of individual perceived image quality features on visual performance. Transl Vis Sci Technol. 2020; 9: 7. [CrossRef] [PubMed]
Anderson HA, Benoit JS, Marsack JD, et al. A randomized trial of objective spectacle prescriptions for adults with Down syndrome: baseline data and methods. Optom Vis Sci. 2021; 98: 88–99. [CrossRef] [PubMed]
Schneider LV, Mitchell GL, Marsack JD, Anderson HA. Visual acuity prediction based on different refraction types for patients with Down syndrome. Transl Vis Sci Technol. 2023; 12: 11. [CrossRef] [PubMed]
Emsley HH. Visual optics. 5th ed. London, UK: Hatton Press; 1952.
Raasch TW. Spherocylindrical refractive errors and visual acuity. Optom Vis Sci. 1995; 72: 272–275. [CrossRef] [PubMed]
Smith G. Relation between spherical refractive error and visual acuity. Optom Vis Sci. 1991; 68: 591–598. [CrossRef] [PubMed]
Atchison DA, Mathur A. Visual acuity with astigmatic blur. Optom Vis Sci. 2011; 88: E798–E805. [CrossRef] [PubMed]
Kleinstein RN, Mutti DO, Sinnott LT, et al. Uncorrected refractive error and distance visual acuity in children aged 6 to 14 years. Optom Vis Sci. 2021; 98: 3–12. [CrossRef] [PubMed]
Anderson HA, Marsack JD, Benoit JS, Manny RE, Fern KD. Visual acuity outcomes in a randomized trial of wavefront metric-optimized refractions in adults with Down syndrome. Optom Vis Sci. 2022; 99: 58–66. [CrossRef] [PubMed]
Campbell CE. Matrix method to find a new set of Zernike coefficients from an original set when the aperture radius is changed. J Opt Soc Am A Opt Image Sci Vis. 2003; 20: 209–217. [CrossRef] [PubMed]
Thibos LN, Wheeler W, Horner D. Power vectors: an application of Fourier analysis to the description and statistical analysis of refractive error. Optom Vis Sci. 1997; 74: 367–375. [CrossRef] [PubMed]
Salmon TO, van de Pol C. Normal-eye Zernike coefficients and root-mean-square wavefront errors. J Cataract Refract Surg. 2006; 32: 2064–2074. [CrossRef] [PubMed]
Anderson HA, Manny RE, Glasser A, Stuebing KK. Static and dynamic measurements of accommodation in individuals with down syndrome. Invest Ophthalmol Vis Sci. 2011; 52: 310–317. [CrossRef] [PubMed]
Stewart RE, Woodhouse JM, Cregg M, Pakeman VH. Association between accommodative accuracy, hypermetropia, and strabismus in children with Down's syndrome. Optom Vis Sci. 2007; 84: 149–155. [CrossRef] [PubMed]
Al-Bagdady M, Murphy PJ, Woodhouse JM. Development and distribution of refractive error in children with Down's syndrome. Br J Ophthalmol. 2011; 95: 1091–1097. [CrossRef] [PubMed]
Mon-Williams M, Tresilian JR, Strang NC, Kochhar P, Wann JP. Improving vision: neural compensation for optical defocus. Proc Biol Sci. 1998; 265: 71–77. [CrossRef] [PubMed]
George S, Rosenfield M. Blur adaptation and myopia. Optom Vis Sci. 2004; 81: 543–547. [CrossRef] [PubMed]
Sawides L, de Gracia P, Dorronsoro C, Webster M, Marcos S. Adapting to blur produced by ocular high-order aberrations. J Vis. 2011; 11:21. [CrossRef] [PubMed]
Artal P, Chen L, Fernandez EJ, Singer B, Manzanera S, Williams DR. Neural compensation for the eye's optical aberrations. J Vis. 2004; 4: 281–287. [PubMed]
Sabesan R, Yoon G. Visual performance after correcting higher order aberrations in keratoconic eyes. J Vis. 2009; 9(6): 1–10. [PubMed]
Sabesan R, Yoon G. Neural compensation for long-term asymmetric optical blur to improve visual performance in keratoconic eyes. Invest Ophthalmol Vis Sci. 2010; 51: 3835–3839. [CrossRef] [PubMed]
Barbot A, Park WJ, Ng CJ, et al. Functional reallocation of sensory processing resources caused by long-term neural adaptation to altered optics. Elife. 2021; 10:e58734. [CrossRef] [PubMed]
Hastings GD, Schill AW, Hu C, Coates DR, Applegate RA, Marsack JD. Orientation-specific long-term neural adaptation of the visual system in keratoconus. Vision Res. 2021; 178: 100–111. [CrossRef] [PubMed]
Aslan L, Aslankurt M, Aksoy A, Gumusalan Y. Differences of the anterior segment parameters in children with down syndrome. Ophthalmic Genet. 2014; 35: 74–78. [CrossRef] [PubMed]
Asgari S, Hashemi H, Fotouhi A, Mehravaran S. Anterior chamber dimensions, angles and pupil diameter in patients with Down syndrome: a comparative population-based study. Indian J Ophthalmol. 2020; 68: 793–797. [PubMed]
Marsack JD, Thibos LN, Applegate RA. Metrics of optical quality derived from wave aberrations predict visual performance. J Vis. 2004; 4: 322–328. [CrossRef] [PubMed]
Tomita K. Visual characteristics of children with Down syndrome. Jpn J Ophthalmol. 2017; 61: 271–279. [CrossRef] [PubMed]
Woodhouse JM, Pakeman VH, Saunders KJ, et al. Visual acuity and accommodation in infants and young children with Down's syndrome. J Intellect Disabil Res. 1996; 40(Pt 1): 49–55. [PubMed]
Nicholson R, Osborne D, Fairhead L, Beed L, Hill CM, Lee H. Segmentation of the foveal and parafoveal retinal architecture using handheld spectral-domain optical coherence tomography in children with Down syndrome. Eye (Lond). 2022; 36: 963–968. [CrossRef] [PubMed]
John FM, Bromham NR, Woodhouse JM, Candy TR. Spatial vision deficits in infants and children with Down syndrome. Invest Ophthalmol Vis Sci. 2004; 45: 1566–1572. [CrossRef] [PubMed]
Little JA, Woodhouse JM, Lauritzen JS, Saunders KJ. Vernier acuity in Down syndrome. Invest Ophthalmol Vis Sci. 2009; 50: 567–572. [CrossRef] [PubMed]
Figure 1.
 
Correlations between VA and unaided refractive error (A) and residual refractive error (B).
Figure 1.
 
Correlations between VA and unaided refractive error (A) and residual refractive error (B).
Figure 2.
 
Relationship between unaided refractive error as vector length (diopters [D]) and pupil diameter in dim lighting (mm).
Figure 2.
 
Relationship between unaided refractive error as vector length (diopters [D]) and pupil diameter in dim lighting (mm).
Figure 3.
 
Unaided and aided VA by spherical equivalent refractive error.
Figure 3.
 
Unaided and aided VA by spherical equivalent refractive error.
Figure 4.
 
Correlation of unaided spherical equivalent refractive error and unaided VA for myopes and hyperopes.
Figure 4.
 
Correlation of unaided spherical equivalent refractive error and unaided VA for myopes and hyperopes.
Figure 5.
 
Comparison of actual participant VA and predicted VA based on the study by Raasch (1995).
Figure 5.
 
Comparison of actual participant VA and predicted VA based on the study by Raasch (1995).
Table 1.
 
Final Correction Type and Range of Refractive Error
Table 1.
 
Final Correction Type and Range of Refractive Error
Table 2.
 
Relationship Between Rate of Change in Visual Acuity Per Diopter of Refractive Error for the Developmentally Typical Population From the Literature
Table 2.
 
Relationship Between Rate of Change in Visual Acuity Per Diopter of Refractive Error for the Developmentally Typical Population From the Literature
×
×

This PDF is available to Subscribers Only

Sign in or purchase a subscription to access this content. ×

You must be signed into an individual account to use this feature.

×