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Clinical predictors of the optimal spectacle correction for comfort performing desktop tasks Clin Exp Optom 2008; 91: 6: 530–537 Christopher T Leffler* MD MPH Byrd Davenport† MD Jodi Rentz*§ OD Amy Miller*§ OD William Benson* MD * Department of Ophthalmology and † School of Medicine, Virginia Commonwealth University, Medical College of Virginia Campus, Richmond VA, USA § Eye Clinic, McGuire Veterans Affairs Medical Center, Richmond VA, USA E-mail: [email protected]

Submitted: 17 May 2007 Revised: 29 February 2008 Accepted for publication: 6 March 2008

DOI:10.1111/j.1444-0938.2008.00288.x Background: The best strategy for spectacle correction of presbyopia for near tasks has not been determined. Methods: Thirty volunteers over the age of 40 years were tested for subjective accommodative amplitude, pupillary size, fusional vergence, interpupillary distance, arm length, preferred working distance, near and far visual acuity and preferred reading correction in the phoropter and trial frames. Subjects performed near tasks (reading, writing and counting change) using various spectacle correction strengths. Predictors of the correction maximising near task comfort were determined by multivariable linear regression. Results: The mean age was 54.9 years (range 43 to 71) and 40 per cent had diabetes. Significant predictors of the most comfortable addition in univariate analyses were age (p < 0.001), interpupillary distance (p = 0.02), fusional vergence amplitude (p = 0.02), distance visual acuity in the worse eye (p = 0.01), vision at 40 cm in the worse eye with distance correction (p = 0.01), duration of diabetes (p = 0.01), and the preferred correction to read at 40 cm with the phoropter (p = 0.002) or trial frames (p < 0.001). Target distance selected wearing trial frames (in dioptres), arm length, and accommodative amplitude were not significant predictors (p > 0.15). The preferred addition wearing trial frames holding a reading target at a distance selected by the patient was the only independent predictor. Excluding this variable, distance visual acuity was predictive independent of age or near vision wearing distance correction. The distance selected for task performance was predicted by vision wearing distance correction at near and at distance. Conclusions: Multivariable linear regression can be used to generate tables based on distance visual acuity and age or near vision wearing distance correction to determine tentative near spectacle addition. Final spectacle correction for desktop tasks can be estimated by subjective refraction with trial frames.

Key words: clinical predictors, near tasks, spectacle corrections

Presbyopia describes the universal agerelated diminution of the lens capacity to accommodate to focus on near targets. Although several theories of accommodation exist, current data support the

Helmholtz theory that resting zonular tension maintains lens flattening. During accommodation, ciliary muscle contraction reduces zonular tension, the lens undergoes elastic recovery and its curva-

Clinical and Experimental Optometry 91.6 November 2008

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ture increases.1 Presbyopia occurs due to a progressive decrease in the ability of zonular tension to increase the focal length of the lens. By approximately age 58 years, the lens focal length © 2008 The Authors

Journal compilation © 2008 Optometrists Association Australia

Clinical predictors for near spectacle corrections Leffler, Davenport, Rentz, Miller and Benson

does not change in response to zonular tension.1 Some basic aspects of refractive correction of presbyopia with spectacles require further work. Hanlon, Nakabyashi and Shigezawa2 reported that near spectacle correction was the most common reason for return visits for unacceptable spectacles and was the basis for 17 per cent of such visits. The current study on the prescription of near spectacles relates to: 1. physiologic variables that predict the final correction 2. practical strategies for determining a tentative correction 3. optimal strategies for determining the final correction. Identification of the physiologic variables that predict the near spectacle correction can shed light on the underlying physiology and optics. Numerous texts emphasise the importance of physiologic variables in the determination of the tentative near addition. The classic model is that the tentative addition should be the dioptric working distance minus one half of the accommodative amplitude.3–5 Accommodation can be measured by placing variable lenses in front of the eye or by moving the target to determine the range of lenses or distances that the patient can tolerate before blurring. For patients with normal vision, the working distance is close to 40 cm (2.5 dioptres) but might vary depending on anthropometric measures, the type of task and patient preference. Kestenbaum’s rule6 stipulates that for low vision patients to read standard newsprint, the working distance in dioptres should be the same as the visual acuity expressed in minutes of arc.7 For example, for a Snellen visual acuity of 6/18 or minimum angle of resolution of three minutes of arc, the working distance should be 18/6 = 3 dioptres = 1/3 metre. If the patient has no accommodation, then a three dioptre reading addition is required. Worsening distance visual acuity in the elderly is thought to require the increased magnification, which may be brought about with a shorter working distance and a higher reading addition.8 The classic physiologic model is:

tentative addition = maximum ( 2.5 D, 1 visual acuity ) - 1 2 (accommodative amplitude) . Other physiologic factors may be associated with the required addition. For instance, the pinhole effect of small pupils might increase the depth of field and decrease the spectacle addition requirement.9 Convergence and accommodation are neurally linked. The near response is limited by the maximum vergence response.10 Fusional vergence amplitude might be associated with working distance and required addition. Hyperopia early in life has been shown to be weakly associated with an earlier need for reading glasses.11 Myopic astigmatism might place one astigmatic focal line closer to the retina in near or intermediate viewing, thereby increasing the depth of focus.12 The relative importance of these factors in prediction of the final near spectacle correction has not been determined through multivariable analysis. A related question regards practical strategies to determine a tentative near spectacle correction. One strategy is refraction with a near target at a standard or patient-specified distance.2,4,5,8,13,14 Techniques for refraction can include near duochrome testing,14,15 dynamic retinoscopy14,15 and near cross-cylinder refraction.2,8,14,15 Classic work3,5,16,17 has suggested the importance of accommodative amplitude and working distance. Calculation based on accommodative amplitude is considered to be a ‘more exact’ method than subjective refraction.5 Traditionally, age-based nomograms have been frowned upon compared with accommodative amplitude measurements, due to inter-individual variation in accommodative strength during middle age,16,17 however, a tentative addition based on age might be more accurate than one based on accommodative amplitude.2,14,15 Clinical factors such as diabetes are associated with decreased accommodation.17 The relative importance of physiologic and clinical variables in predicting the optimal near spectacle correction is unknown.

© 2008 The Authors Journal compilation © 2008 Optometrists Association Australia

The tentative addition is a starting point that must be refined to determine the best final addition. The addition must be useful for a variety of tasks at various working distances. It is recommended that the range of clear vision farther than the working distance is greater than the range of clear vision closer than the working distance in absolute terms4 but that these ranges of clear vision are equal when measured in dioptric terms.5 A measure of dioptric symmetry might predict the optimal addition. In addition, other clinical or physiologic factors besides patient preference and range of clear vision might be important determinants of the optimal addition. A reference standard defined exclusively by patient comfort during task performance can help answer these questions. The current study sought to determine which physiologic and clinical variables predict the optimal spectacle correction for performance of near tasks at a desk. The reference ‘gold’ standard was the addition preferred by the patient wearing trial frames while performing the prototypical desktop tasks of reading, writing and counting change. METHODS Patients at the Nelson eye clinic were asked to volunteer and grant written informed consent. The study protocol was approved by the Office of Research Subjects Protection at the Virginia Commonwealth University, Medical College of Virginia Campus. Manifest refractive error was determined for all patients, ensuring they received the maximum plus tolerated without reducing visual acuity. Interpupillary distance was measured with the phoropter. Distance visual acuity was determined with the manifest refraction and Snellen projection charts and was scored per letter.8 Near vision was determined with the distance correction at 40 cm with the NearPoint Rotochart #12 (American Optical) with luminance 250 cd/m2. Vision and visual acuities were converted to minimum angle of resolution on a per-letter basis.8 A pupil gauge was


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used to measure pupil diameter to the nearest 0.5 mm with a distant nonaccommodative target under the luminance conditions used to measure near vision.8 Subjective accommodation was measured by moving the near card towards the patient from 40 cm until the monocular near point. During this test, a +2.50 D lens was placed over the distance correction and the patient focused on the smallest line read at 40 cm. The endpoint was defined by the patient’s report of blurring of this line. The range of clear vision with the addition expected for age was defined similarly, by moving the near card away and closer, until the patient reported that the smallest line read at 40 cm was blurred. These ranges were converted to dioptres. A ‘range symmetry ratio’ was defined as ‘near dioptres/far dioptres’, while a ‘range symmetry difference’ was defined as ‘near dioptres—far dioptres’. Single estimates of each measure were made and therefore, are subject to uncertainty. Relative vergence was determined by viewing a stationary target at 40 cm as Risley rotary prisms were placed before each eye and rotated to determine the fusional vergence amplitudes.16 Patients stated their height. Arm length was measured from the acromioclavicular joint to the wrist of the outstretched arm. Preferred working distance was measured from the eyes to a sheet of paper as the patient held the paper with the eyes closed. Preferred addition was determined by trial and error in the phoropter at 40 cm and at the preferred distance and in trial frames at the preferred distance, using the smallest addition with which the patient was comfortable. Full aperture trial lenses were used. No patients had manifest strabismus. Prototypical near tasks included reading two sentences printed with Microsoft Word in 10-, 12- and 14-point sizes (six sentences total, corresponding approximately with visual acuity at 40 cm of 6/18, 6/21 and 6/27), copying two sentences with a pen on white paper, and counting $1.93 in change. All tasks were performed at a desk 80 cm high, while seated on a stool, the height of which was adjusted by the patient for comfort. The tasks were

performed four times for each patient with spectacle correction in trial frames with additions selected in random order. The sentences read and written were randomly determined and varied each time. The four additions included the following: the patient preferred addition in trial frames while reading and the addition based on an age-related nomogram. Two other powers were selected to ensure a range of additions up to +3.00. The agerelated nomogram was one used clinically by the authors: for ages 43 to 44, +1.25; for ages 45 to 47, +1.50; for ages 48 to 49, +1.75; for ages 50 to 52, +2.00; for ages 53 to 58, +2.25; for ages greater than 58, +2.50.

Statistical analysis Refractive values were analysed in the plane of the trial lens. Refractions were converted to the power vector components described by Thibos, Wheeler and Horner18 and Thibos and Horner.19 In this system, refractions are considered to be the sum of three components: the spherical equivalent, a Jackson cross-cylinder oriented at 180 degrees (denoted J0) and a Jackson cross-cylinder at 45 degrees (denoted J45). In this article, J0 is referred to as quantifying ‘with-the-rule’ astigmatism (although if J0 is negative the patient has ‘against-the-rule’ astigmatism), and J45 is referred to as quantifying oblique astigmatism. If the astigmatic portion of a refraction includes a cylinder of power C with axis q, the cross-cylinders are calculated:18,19

J0 = - (C 2) cos (2θ ) J45 = - (C 2) sin (2θ ) For instance, if a refraction is +5.00/ +3.00 ¥ 100, then C = +3.00, q = 100, J0 = 1.41 D, and J45 = 0.51 D. Predictors of the preferred spectacle correction during near task performance were determined by univariate and multivariable linear regression. The unit of analysis was the patient, not the eye. Multivariable analysis was conducted using independent variables, which were significant in univariate analysis or bivariate analysis with age. The analysis based on physiologic vari-


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ables considered: interpupillary distance, height, arm length, pupil diameter, relative vergence, refraction, visual acuity, accommodative amplitude, range symmetry, pseudophakia and working distance variables. The analysis for the tentative addition considered the physiologic variables plus age and addition expected for age, sex, and presence and duration of diabetes. The final addition model included all physiologic and tentative addition variables plus the preferred addition variables. Variables were eliminated by stepwise backwards regression analysis if not individually significant at the 0.05 level. Statistica (v. 7, Statsoft, Tulsa OK) was used for statistical analysis. Most statistical analysis of visual acuity is based on log(MAR). However, for distance visual acuity, MAR was used for multivariable analysis due to Kestenbaum’s theory. RESULTS Thirty subjects were studied. The mean age was 54.9 years (SD 7.9), range 43 to 71 (Table 1). Fifty-three per cent were male, and 40 per cent had diabetes. Three patients (10 per cent) were pseudophakic in the right eye only.

Univariate predictors Univariate predictors of the optimal spectacle correction were: age, duration of diabetes, addition assumed for age, interpupillary distance, relative vergence with base-out prism, distance visual acuity, near vision wearing distance correction, the preferred addition in the phoropter and the preferred addition in a trial frame at the distance selected by the patient with eyes closed (Table 1). As only two patients had diabetic retinopathy, meaningful statistical analysis of this subgroup was not possible. Sex, height, arm length, pupil diameter, subjective accommodative amplitude, refraction, range symmetry and pre-task apparent working distances were not significantly predictive of near spectacle correction (Table 1). Based on r2 and p values, MAR and log(MAR) distance visual acuity models had equal predictive ability (Table 1). In both univariate (slope = 0.417, Table 1) © 2008 The Authors

Journal compilation © 2008 Optometrists Association Australia


Mean

(SD)

Age (years) 54.9 (⫾7.9) Male sex 53% Diabetes 40% Pseudophakia, right eye only 10% Duration of diabetes (years) 2.6 (4.7) Interpupillary distance (mm) 64.5 (3.9) Height (cm) 171.7 (9.7) Arm length (cm) 74.4 (4.6) Pupil diameter ambient light (mm) 4.1 (1.2) Addition assumed for age (D) 2.1 (0.5) Relative vergence (prism D) base out prism 12.9 (4.8) base in prism 12.7 (5.5) Manifest refraction, mean OU (D) spherical equivalent -0.5 (2.2) cylinder power 0.6 (0.3) -0.08 (0.3) with-the-rule astigmatism (J0) 0.02 (0.1) oblique astigmatism (J45) Vision, worse eye, with distance correction distance, MAR (minutes) 1.20 (0.58) distance, log(MAR) 0.051 (0.14) near, log(MAR) 0.61 (0.16) Range symmetry, lens for age, at desk ratio 0.99 (15) difference (D) 1.3 (1.2) Accommodative amplitude with 2.5 D lens, mean both eyes (D) 2.2 (1.1) Preferred working distance (D) selected with eyes closed 2.3 (0.4) selected in trial frames 2.4 (0.4) selected reading at desk 2.3 (0.3) with addition expected for age. selected during tasks, mean 2.5 (0.3) Preferred addition (D) in phoropter at 40 cm 2.2 (0.5) in phoropter at preferred dist. 1.9 (0.5) in trial frames at preferred dist. 2.0 (0.5) in trial frames doing tasks 1.9 (0.6)

r2

p

0.33 0.01 0.07 0.00 0.22 0.18 0.00 0.05 0.08 0.39