Share this article on:

Visual Discomfort and Objective Measures of Static Accommodation

Chase, Chris*; Tosha, Chinatsu*; Borsting, Eric; Ridder, William H. III

Optometry and Vision Science: July 2009 - Volume 86 - Issue 7 - p 883-889
doi: 10.1097/OPX.0b013e3181ae1b7c
Original Article

Purpose. Accommodation insufficiency and fatigue have been associated with near work visual discomfort symptoms, but clinical measures of accommodation amplitude suggest insufficiency is uncommon and often not weak enough to cause symptoms. However, recent studies show that the clinical push-up test used to measure amplitude overestimates accommodative function. This study uses an open-field autorefractor to measure accommodative stimulus-response functions objectively in college students with and without near work induced discomfort symptoms.

Methods. Using a Grand-Seiko WAM 5500 autorefractor, 2 min recordings were made each at five viewing distances (0 to 5 D) to measure an accommodative response function. Visual discomfort symptoms were assessed using the Conlon survey.

Results. A strong and positive correlation between accommodative lag and visual discomfort symptoms was found under near work conditions. The prevalence of accommodative insufficiency was much higher than estimated by clinical measures.

Conclusions. Accommodative insufficiency and fatigue should be defined and described by objective methods using extended viewing times to assess function.

*PhD

OD, MS, FAAO

OD, PhD, FAAO

Colleges of Optometry and Biomedical Sciences, Western University of Health Sciences, Pomona, California (CC), Jules Stein Eye Institute, University of California, Los Angeles, Los Angeles, California (CT), and Southern California College of Optometry, Fullerton, California (EB, WHR).

This research was supported by grants from the National Eye Institute (R15EY015922) and the College of Vision Development.

Portions of the study were presented as a paper at the College of Optometrists in Vision Development meeting in October of 2008.

Received October 26, 2008; accepted December 9, 2008.

Visual discomfort symptoms are prevalent among college students. Symptoms occur after prolonged near work, such as reading or using a computer. Complaints include impaired reading performance, headaches, asthenopia, light sensitivity, blurred text, diplopia, and perceptual distortions involving letter movement and fading. Using a self-report survey developed by Conlon et al.,1 17% of students from selective, small liberal arts colleges in the United States reported moderate-to-severe symptoms,2 but prevalence can be much higher. Conlon et al.1 found 47% of students from an Australian university were symptomatic.

Rasch analysis in each of these studies has shown the Conlon survey has a single dimension1,2 that accounted for almost 75% of the sample variance. Rasch item statistics suggest that the most common complaints involve reading problems and light sensitivity. As symptom severity increased, headaches, asthenopia, blur, and diplopia occurred. In more severe cases, students reported perceptual distortions. For students with moderate or severe discomfort, 60% reported symptoms occurred after they have been reading for less than a half-hour, and 30% reported symptoms occurred after less than 15 min of reading whereas only 25% of students with low symptoms reported symptoms after a half-hour of reading. Sixty-nine percent of high discomfort students reported their symptoms affected grades received on assignments every few weeks or more often compared with 15% of low discomfort students. The severity of their symptoms positively correlated with the degree of their academic problems (r = −0.78, p < 0.0001).3

Although the Rasch analyses suggest that visual discomfort symptoms are caused by a single condition, identifying the source has proven to be elusive and created some confusion about classification. In the early 20th century, ophthalmology and optometry referred to these symptoms as visual fatigue, based on the notion that prolonged near work fatigued ciliary muscle.4 However, others proposed the fatigue was in the retina5 or sources were to be found in the central nervous system.6 Berens and Sells7 found fatigue could be induced in a masked eye, supporting Hofstetter hypothesis that the cause is not in peripheral sensory processes. More recently, visual discomfort has been called visual stress or the Meares/Irlen syndrome by those who treat the condition with colored transparencies or colored lenses.8,9 Wilkins et al10 found pattern glare sensitivity is associated with visual discomfort, and based on a similar finding with migraines, proposed a noisy visual cortex model, where interference between neural columns disrupts processing and causes asthenopia.

Because visual discomfort is associated with near work, early studies examined the role of accommodation dysfunction in discomfort symptoms.5,6,11,12 Results were mixed. For example, during a 15-min near work task that required sustained accommodation, Berens and Stark12 found 30.8% of the office workers they tested showed a decrease in accommodative amplitude, 28.7% showed an increase, and 40.5% showed no change. Visual discomfort symptoms were rarely reported among those who showed a decrease in amplitude. In a similar study, Hofstetter reported that when amplitude began to weaken, subjects often could improve accommodation with encouragement and renewed effort.6 However, Berens and Sells7 consistently found evidence of accommodative fatigue when subjects sustained near accommodation (25 cm) for approximately 30 min. A more recent study of computer workers showed a 0.69 diopter (D) decrease in accommodative amplitude over a 4-day work period compared with a 0.18 D decrease in controls, suggesting that a longer accommodative demand period may be needed to detect fatigue or insufficiency in asymptomatic subjects.13

Recent studies have looked for a higher incidence of visual discomfort among those who have accommodation insufficiency.13–21 Accommodation insufficiency or fatigue is characterized as inadequate near-point accommodation amplitude, adjusted for age. Accommodation insufficiency describes consistently weak performance, whereas fatigue is used to describe normal accommodative function that initially is sustained with effort but deteriorates over time.11 Studies have used slightly different diagnostic criteria, but all agree that there must be a reduction in accommodative amplitude, usually adopting the criteria of 2 D below age appropriate minimum values (e.g., Hofstetter formula of 15 − 0.25 × age). Almost every study measures accommodative amplitude using Donder push-up test.22 In this task, an individual with normal or corrected-normal distance vision observes a target as it approaches the eyes. She or he is instructed to keep the target as clear as possible and report when it just begins to blur at the edges and cannot be cleared with additional effort. The inverse of the viewing distance for the first sustained blur is the amplitude of accommodation.

As measured by the push-up test, accommodation insufficiency and fatigue appear to occur infrequently. In an adult optometry clinical sample, only 3% met criteria for accommodation insufficiency15; in a college sample, only 6.2% were found.14 In a clinic sample of patients with accommodation dysfunction, Daum23 found that 84% had insufficient accommodation, but only 1 case out of 114 had accommodation fatigue.

Several studies have examined accommodative amplitude in individuals who report visual discomfort while reading. Scheiman et al.24 found that 34% of those with moderate visual discomfort symptoms had accommodative disorders. Both Scott et al.19 and Evans et al.17 found weaker accommodative amplitude in children with visual discomfort compared with controls who had no discomfort symptoms while reading, but concluded that it was not a cause of visual discomfort symptoms due to the relatively mild weakness. In children, Borsting et al.16 found a small but significant correlation between symptom scores and accommodative amplitude (r = −0.24).

In summary, the research literature presents a confusing picture of the relationship between accommodative amplitude and visual discomfort. Near work accommodative demand appears to be associated with visual discomfort for some individuals, but studies also suggest that accommodation insufficiency or fatigue are infrequent conditions and so unlikely to be responsible for the relatively high incidence of visual discomfort found among college students.

Unfortunately, the low prevalence of accommodation insufficiency may be the result of how amplitude is measured. The push-up test requires a subjective judgment about the onset of blur. Blur detection is influenced by depth-of-field effects that are enhanced by accommodative pupil restriction. As a target is moved closer to the eye, the relative size increases, and an individual may still be able to identify the target even in the presence of a large defocus error. Recent comparisons between the push-up andobjective measures have found the push-up test overestimates accommodative amplitude,25–27 thus the prevalence and severity of accommodation insufficiency actually may be higher than estimated by the push-up test.

Win-Hall et al.27 compared different measures of accommodative amplitude using the push-up test and an open-field autorefractor (Grand Seiko WR-5100K) on 20 prepresbyopic adults between the ages of 21 and 30. An autorefractor accurately and objectively measures accommodation under normal reading conditions without the confounding problems associated with the push-up test.28–30 The push-up test produced an average accommodation amplitude of 7.75 D [standard deviation (SD) = 1.75], whereas the autorefractor averaged 5.69 D (SD = 0.74). Ninety-five percent of subjects produced higher amplitude estimates with the push-up test. The differences between measures ranged from −0.39 to 4.93 D with an average of 2.06 D. The correlation between measurements was significant (p = 0.02) but only accounted for 27.9% of the variance, a very weak association for two procedures that are supposed to be measuring the same function.

Another problem with clinical accommodation tests is that sustained viewing is not required. Even though, we tend to read for long periods of time, tests usually only measure the ability to focus for a few seconds. Early researchers attempted to overcome this problem by designing ergographic devices that would record viewing distance while subjects continuously adjusted their estimates of target blur.5,6,12,31 Without continuous performance demands, testing procedures are likely to underestimate accommodation insufficiency problems and will overlook accommodation fatigue all together.

An alternative way to assess accommodation is to measure the stimulus response function. This method is similar to recording an amplitude of accommodation except that measurements are made at different viewing distances rather than for a single, minimum viewing distance. The accommodation lag, or difference between the target distance and accommodative focal point, directly measures the degree of accommodation insufficiency. The advantage of this method is that it provides information about how well an individual’s accommodative system handles increased demand and also measures performance at normal reading distances.

Objective techniques to measure accommodation have been available since the 1930s (see ref. 32 review). Three studies have looked at accommodative stimulus response functions objectively in subjects with visual discomfort. First, Ciuffreda et al.33 recorded static accommodative responses in six symptomatic subjects at 2, 3, and 4 D viewing distances using a Hartinger coincidence-optometer. Although not reported, recordings are typically brief (e.g., <5 s). Stimulus-response functions appeared normal. Second, Simmers et al.34 used a Cannon Autorefactor R-1 optometer to make continuous 10 s accommodative recordings with five symptomatic subjects over a range of viewing distances (0 to 4.5 D). Results produced normal stimulus-response functions, although symptomatic subjects produced more unstable responses than controls in the low frequency (0.3 to 0.6 Hz) range. In both studies, recording durations were short and unable to detect accommodative abnormalities that might build up over time.

Third, a recent study examined accommodative stimulus response functions in symptomatic subjects making continuous recordings with an autorefractor during a 2-min target fixation.35 Results showed a significant interaction between symptomatic and asymptomatic groups over time. Asymptomatic subjects produced stable accommodative responses, whereas symptomatic subjects showed accommodation fatigue, eventually doubling their lag over the 2-min recording.

The purpose of this study is to take a fresh look at accommodation insufficiency and visual discomfort using an autorefractor as an objective method for assessing accommodative function. A Grand Seiko autorefractor (WAM-5500) was used to measure an accommodative stimulus-response function. The WAM-5500 can make continuous recordings, sampling spherical refraction at five times a second so that accommodative function can be measured for more than just a few seconds. We hypothesize that accommodation insufficiency or fatigue is associated with discomfort symptoms and that visual discomfort scores will correlate better with objective recordings made over a prolonged period of time than with brief, clinical accommodative measures. These results will help to determine the best method for measuring accommodative dysfunction.

Back to Top | Article Outline

METHODS

Participants

Students from the Claremont Colleges Consortium (Claremont McKenna, Harvey Mudd, Pitzer, Pomona, and Scripps) participated in this study. These five private, undergraduate collegescollectively have more than 6000 students. The Conlon Visual Discomfort Survey was distributed across different college campuses and approximately 650 responses were collected. Participants completed informed consent before filling out the survey, using procedures that conformed to the tenets of the Declaration of Helsinki and were approved by the institutional review board committee. From this sample, 42 participants with high discomfort scores and 46 participants with low to moderate scores were screened for further study. High discomfort participants were defined as one SD above the sample mean (mean = 15.3, SD = 10.2) or a raw score of 25. The normal discomfort group fell within 0.5 SD from the mean (Conlon raw score range from 5 to 20) and were randomly selected with the constraint that the group average equaled the sample mean. These recruitment procedures stratified the study sample to provide roughly equal numbers of symptomatic and asymptomatic students. The sample was not representative of the student population but instead provided more diversity in symptom severity so that students with a broad range of visual discomfort symptoms could be included for study.

A total of 88 students (57 female, 31 male) were screened for this study. The higher prevalence of visual discomfort symptoms among women had previously been reported.2 Exclusionary criteria trimmed this sample to 75 students who received optometric evaluations as described below. The sample provided a broad range (3 to 52) of Conlon visual discomfort scores with a mean of 20.8 and SD of 12.0. The average age was 20 years old (range, 18 to 22). From this group, 25 students (6 male, 19 female) completed objective accommodation testing using the autorefractor. Incomplete data further trimmed the sample to 23.

Back to Top | Article Outline

Subject Criteria

Participants were screened for possible concomitant conditions that might cause visual discomfort or interfere with reading. Exclusionary criteria included English as a second language, reading disabilities, medical conditions that might cause uncomfortable visual symptoms (e.g., epilepsy, head trauma, migraines), and taking certain medications that can affect oculomotor function. Participants with systemic diseases known to affect accommodation and vergence, such as multiple sclerosis, Graves Thyroid disease, myasthenia gravis, diabetes, or Parkinson disease were excluded from the study.

Participants also were screened for normal visual acuity (20/25 or better in each eye at distance and near), had no strabismus, normal stereopsis (appreciation of random dot targets and stereoacuity of 70 s of arc or better), no significant uncorrected refractive error (myopia of >0.50, astigmatism and anisometropia of >1.00 D, and hyperopia >1.50 D), and no significant ocular pathology, including color deficiency (pass the Ishihara plates and the L’Anthony D-15 test).

Back to Top | Article Outline

Procedures

Visual Discomfort Symptoms

Symptoms were measured using the Conlon Visual Discomfort Survey described above. Based on Rasch scale analysis (see ref. 2), four discomfort scores were calculated: soreness and headache, blur or diplopia, moving or fading, and total discomfort score. Researchers who conducted the optometric examination and measured accommodative stimulus-response functions did not know the Conlon Visual Discomfort scores of participants.

Back to Top | Article Outline

Optometric Examination

A comprehensive assessment of accommodation and vergence was made using standard clinical procedures.36 Vergence was evaluated with five tests: (1) a cover test at 3 meters and 40 cm with prism neutralization; (2) prism bar negative fusional vergence and positive fusional vergence at 40 cm; (3) near point of convergence; (4) vergence facility; and (5) fixation disparity. Accommodation was evaluated with five tests: (1) posture of accommodation using the monocular estimation method; (2) negative relative accommodation and positive relative accommodation; (3) amplitude of accommodation; (4) accommodative facility (2.00 D ± flipper test: OD, OS, and OU); and (5) amplitude-scaled facility.

Back to Top | Article Outline

Accommodative Stimulus-Response Functions

In a separate testing session, stimulus response functions were recorded using a Grand-Seiko WAM-5500 autorefractor for a subset of the sample. Participants were dark-adapted for 5 min before testing. Recordings were made in a well-lit room (85 cd/m2 luminance for a white piece of paper) from the right eye with a patch worn over the left eye for the duration of the experiment. Viewing distance and eye position was controlled by chin and forehead rests. Spherical refraction was continuously recorded for 2 min at a 5-Hz sampling rate while subjects fixated on a star symbol that was 2 cm high. Target luminance was 31 cd/m2 with a Michelson contrast of 0.79. The target was presented at five viewing distances in the following fixed order: 300 cm (0 D), 50 cm (2 D), 33 cm (3 D), 25 cm (4 D), and 20 cm (5 D). The viewing angle of the target for each distance was 0.38, 2.29, 3.47, 4.58, and 5.73°, respectively. Students were instructed to keep the target in clear focus through the recording period. Between each recording, students took a 1-min break to view the top line of a Snellen eye chart positioned at 6 meters distance.

Back to Top | Article Outline

RESULTS

Optometric Tests

Optometric test results were correlated to Conlon visual discomfort scores using a forward, step-wise regression analysis to determine which measures of optometric function predicted symptoms. F-to-enter threshold was set to 3.0. Due to missing values, analysis was performed on data from 67 participants, using the visual discomfort score as the dependent variable and optometric test scores as the independent variables. Two accommodation measures, amplitude-scaled facility (standard coefficient = −0.22) and amplitude of accommodation (standard coefficient = −0.33) accounted for 9.2% of the symptom variance [F (2,66) = 4.33, p = 0.02]. None of the other tests reached significance.

Back to Top | Article Outline

Autorefractor

Autorefraction data was trimmed for outliers (i.e., eye blinks) by eliminating data points that were 3 SDs above or below the mean for each subject at each viewing distance. The average accommodation response was then calculated using the first 90 s of data to assure complete recordings for all subjects. The accommodation lag, the difference between the target distance and accommodative response, was calculated for each subject. Pearson correlations were made with 23 subjects to explore relationships between accommodation lag, clinical accommodation test scores, and visual discomfort symptoms. Because the sample of visual discomfort scores was not normally distributed and to control for the effects of outliers, Spearman rank correlations also were used where significant Pearson correlations occurred.

Pupil diameter was continuously sampled by the WAM-5500. Analysis for the 5 D target showed subjects averaged a 5.0 mm diameter during the recording period with individuals ranging from 2.8 cm to 6.8 cm. There were no significant differences in pupil size between symptomatic and asymptomatic subjects.

At viewing distances of 33 to 300 cm, analyses found no significant correlations between objective accommodation measures and visual discomfort symptoms, however, at closer viewing distances (20 or 25 cm) where accommodative demand increased significant correlations occurred. Among clinical measures, the amplitude-scaled facility test37 and left eye facility test significantly correlated with headache and soreness symptoms. Table 1 presents the Pearson correlations for all accommodation measures, and Table 2 presents the Spearman Rho correlation coefficients for measures that had significant Pearson correlations.

TABLE 1

TABLE 1

TABLE 2

TABLE 2

A scatter plot of accommodation lag at 20 cm (5 D) and Conlon scores (Fig. 1) shows that during the minute and one-half recording, accommodation function varied considerably in this sample. Eight students averaged over a 1.0 D lag from the target. Note that students with accommodation lags below 0.5 D had infrequent symptoms, symptom frequency for those with lags between 0.5 and 1 D were more broadly distributed, and those with lags above 1.0 D had more severe symptoms.

FIGURE 1.

FIGURE 1.

Clinical and objective measures produced different estimates of accommodative insufficiency. Using Hofstetter formula, amplitude of accommodation was 2 D below normal (the common criteria for insufficiency) for only three students in this study (13%). However, these diagnostic criteria did not work very well. Two had low discomfort scores (Conlon raw scores of 6 and 12) and one had a high score of 51. Only the student with frequent symptoms had a large accommodation lag as measured by the autorefractor.

Objective accommodation measures suggest insufficiency is much more common. Depth of focus, or the viewing range within which blur can be tolerated without disrupting perception, varies with conditions and subject experience. Estimates vary depending on the study but are commonly in a range between 0.5 and 1.5 D for fovea with an average between 0.6 and 0.8 D.38 Table 3 presents descriptive statistics of the average accommodative lag for 90 s with 3 to 5 D viewing distances, and Fig. 2 shows proportions of students in different lag ranges for the 3 to 5 D targets. A substantial number of students had lags above 1.0 D at normal reading distances of 33 cm or less (17% for 3 D, 17% for 4 D, and 35% for 5 D).

TABLE 3

TABLE 3

FIGURE 2.

FIGURE 2.

Figs. 3 and 4 present accommodative stimulus-response functions for low and high symptomatic students, respectively. Among asymptomatic subjects (Fig. 3), small differences were often observed with 4 to 5D targets, as shown in these two cases (24 and 68). Symptomatic students (Fig. 4) also varied in a similar manner, but in general showed larger lags with closer targets (case 1). Occasionally, a symptomatic student would show a more pronounced accommodative insufficiency, where responses were relatively weak for any near target (case 19).

FIGURE 3.

FIGURE 3.

FIGURE 4.

FIGURE 4.

Back to Top | Article Outline

DISCUSSION

This study demonstrates a strong and positive association between accommodation insufficiency and visual discomfort symptoms under near work conditions. First, in a large sample, only two clinical accommodation measures predicted symptoms: amplitude-scaled facility and amplitude of accommodation. Second, at viewing distances of 4 D or closer, those commonly used in reading, significant correlations were found between objective measures of accommodation lag and symptoms. Third, accommodation lag was strongly correlated with symptoms of headache, soreness, blur, and diplopia, but not text distortions. As described earlier, the Rasch item analysis of the survey showed text distortions to be a relatively infrequent symptom associated only with severe complaints of visual discomfort. In this high functioning college sample, there were few students with severe symptoms, and so correlations were not expected to be found. Finally, the positive correlations, as seen in Fig. 1, showed that as accommodation lag increased, visual discomfort symptoms got worse.

Average pupil diameters ranged from 2.8 mm to 6.8 mm during autorefractor recordings at 5 D. Diameters less than 2.0 mm have been found to increase depth of focus, but in this study pupil diameters were in a range that produces fairly stable blur sensitivity.39 Consequently, differences between clinical and objective measures of accommodation probably are not due to variation in pupil diameter.

Monocular recordings were made to control for binocular dysfunctions that also could contribute to visual discomfort symptoms, such as convergence insufficiency. However, by eliminating a convergent signal, accommodation may have been weakened for some individuals who were more dependent on convergence to resolve blur. Thus, it is possibility that this study overestimated the size of accommodative lag for some students who would do much better under binocular conditions. Future research should undertake both binocular and monocular measures to explore this possibility and provide a more accurate estimate of accommodative lag under normal (binocular) reading conditions.

These results strongly recommend the use of objective measures to assess accommodative function. Only two of the clinical accommodation tests predicted visual discomfort symptoms in the large sample, and correlations were quite small. In the small sample, the amplitude-scaled facility and left eye facility tests produced positive correlations with symptoms of headache and soreness, but appeared to be somewhat insensitive to other symptoms and the total score from the Conlon survey. In a large sample, amplitude of accommodation did account for a small proportion of symptom variance, but in a small sample it was a poor predictor of visual discomfort and also did not correlate with accommodation lag. Consistent with previous studies, the clinical amplitude of accommodation test underestimated near accommodation insufficiency.

Future studies should describe accommodation insufficiency by objective means. In particular, what accommodation lag criteria should be used and should discomfort symptomatology be included in the definition? In addition, to accurately assess fatigue extended viewing time is an important component of objective tests. Is insufficiency a disease condition or part of normal or age-related variation in accommodative function? The fact that so many students had a large accommodation lag suggests the possibility that insufficiency may be part of normal variation. Finally, which components of static accommodation are malfunctioning to cause insufficiency? Research on age-related changes suggest tonic accommodation, amplitude, adaptation, AC/A, or CA/C are the most likely candidates for further study, but the presence of accommodation fatigue also suggests possible involvement of the slow adaptive component.39

Back to Top | Article Outline

ACKNOWLEDGMENTS

We thank Lawrence Stark and two anonymous reviewers for their helpful comments and insights.

Chris Chase

Colleges of Optometry and Biomedical Sciences

Western University of Health Sciences

309 East Second St.

Pomona, CA 91766-1854

e-mail: cchase@westernu.edu

Back to Top | Article Outline

REFERENCES

1. Conlon EG, Lovegrove WJ, Chekaluk E, Pattison PE. Measuring visual discomfort. Vis Cogn 1999;6:637–63.
2. Borsting E, Chase CH, Ridder WH III. Measuring visual discomfort in college students. Optom Vis Sci 2007;84:745–51.
3. Borsting E, Chase C, Ridder WH III. Association of oculomotor and sensory measures to visual symptoms in a group of pre-presbyopic college students. Optom Vis Sci 2006;83:E-abstract 065004.
4. Watten RG. Reinvention of visual fatigue: accumulation of scientific knowledge or neglect of scientific history? Ophthalmic Physiol Opt 1994;14:428–32.
5. Howe L. On varieties of the fatigue of accommodation as registered by the ergograph. Am J Ophthalmol 1917;34:239–40.
6. Hofstetter H. An ergographic analysis of fatigue of accommodation. Am J Optom 1943;20:115–35.
7. Berens C, Sells SB. Experimental studies on fatigue of accommodation. I. Plan of research and observations on recession of near point of accommodation following a period of interpolated work on the ophthalmic ergograph. Arch Ophthalmol (Chicago) 1944;31:148–59.
8. Wilkins AJ. Visual Stress. Oxford, UK: Oxford University Press; 1995.
9. Wilkins A. Reading through Colour: How Coloured Filters Can Reduce Reading Difficulty, Eye Strain, and Headaches. Hoboken, NJ: John Wiley & Sons; 2003.
10. Wilkins A, Nimmo-Smith I, Tait A, McManus C, Della Sala S, Tilley A, Arnold K, Barrie M, Scott S. A neurological basis for visual discomfort. Brain 1984;107(Pt 4):989–1017.
11. Duane A. Subnormal accommodation. Trans Am Ophthalmol Soc 1925;23:159–83.
12. Berens C, Stark EK. Studies in ocular fatigue IV. Fatigue of accommodation, experimental and clinical observations. Am J Ophthalmol 1932;15:527–42.
13. Gur S, Ron S, Heicklen-Klein A. Objective evaluation of visual fatigue in VDU workers. Occup Med (Lond) 1994;44:201–4.
14. Porcar E, Martinez-Palomera A. Prevalence of general binocular dysfunctions in a population of university students. Optom Vis Sci 1997;74:111–3.
15. Lara F, Cacho P, Garcia A, Megias R. General binocular disorders: prevalence in a clinic population. Ophthalmic Physiol Opt 2001;21:70–4.
16. Borsting E, Rouse MW, Deland PN, Hovett S, Kimura D, Park M, Stephens B. Association of symptoms and convergence and accommodative insufficiency in school-age children. Optometry 2003;74:25–34.
17. Evans BJ, Busby A, Jeanes R, Wilkins AJ. Optometric correlates of Meares-Irlen syndrome: a matched group study. Ophthalmic Physiol Opt 1995;15:481–7.
18. Evans BJ, Wilkins AJ, Brown J, Busby A, Wingfield A, Jeanes R, Bald J. A preliminary investigation into the aetiology of Meares-Irlen syndrome. Ophthalmic Physiol Opt 1996;16:286–96.
19. Scott L, McWhinnie H, Taylor L, Stevenson N, Irons P, Lewis E, Evans M, Evans B, Wilkins A. Coloured overlays in schools: orthoptic and optometric findings. Ophthalmic Physiol Opt 2002;22:156–65.
20. Sterner B, Gellerstedt M, Sjostrom A. Accommodation and the relationship to subjective symptoms with near work for young school children. Ophthalmic Physiol Opt 2006;26:148–55.
21. Daum KM. Accommodative insufficiency. Am J Optom Physiol Opt 1983;60:352–9.
22. Saladin JJ. Phorometry and stereopsis. In: Benjamin WJ, Borish IM, eds. Borish’s Clinical Refraction, 2nd ed. Oxford: Butterworth-Heinemann; 2006:899–962.
23. Daum KM. Accommodative dysfunction. Doc Ophthalmol 1983;55:177–98.
24. Scheiman M, Blaskey P, Ciner EB, Gallaway M, Parisi M, Pollack K, Selznick R. Vision characteristics of individuals identified as Irlen Filter candidates. J Am Optom Assoc 1990;61:600–5.
25. Ostrin LA, Glasser A. Accommodation measurements in a prepresbyopic and presbyopic population. J Cataract Refract Surg 2004;30:1435–44.
26. Wold JE, Hu A, Chen S, Glasser A. Subjective and objective measurement of human accommodative amplitude. J Cataract Refract Surg 2003;29:1878–88.
27. Win-Hall DM, Ostrin LA, Kasthurirangan S, Glasser A. Objective accommodation measurement with the Grand Seiko and Hartinger coincidence refractometer. Optom Vis Sci 2007;84:879–87.
28. Davies LN, Mallen EA, Wolffsohn JS, Gilmartin B. Clinical evaluation of the Shin-Nippon NVision-K 5001/Grand Seiko WR-5100K autorefractor. Optom Vis Sci 2003;80:320–4.
29. Gwiazda J, Weber C. Comparison of spherical equivalent refraction and astigmatism measured with three different models of autorefractors. Optom Vis Sci 2004;81:56–61.
30. Wolffsohn JS, O’Donnell C, Charman WN, Gilmartin B. Simultaneous continuous recording of accommodation and pupil size using the modified Shin-Nippon SRW-5000 autorefractor. Ophthalmic Physiol Opt 2004;24:142–7.
31. Berens C, Stark EK. Studies in ocular fatigue. III. Fatigue of accommodation: history, apparatus, and methods of graphic study. Am J Ophthalmol 1932;15:216–23.
32. Campbell CE, Benjamin WJ, Howland HC. Objective refraction: retinoscopy, autorefraction, and photorefraction. In: Benjamin WJ, Borish IM, eds. Borish’s Clinical Refraction, 2nd ed. Oxford: Butterworth-Heinemann; 2006:682–764.
33. Ciuffreda KJ, Scheiman M, Ong E, Rosenfield M, Solan HA. Irlen lenses do not improve accommodative accuracy at near. Optom Vis Sci 1997;74:298–302.
34. Simmers AJ, Gray LS, Wilkins AJ. The influence of tinted lenses upon ocular accommodation. Vision Res 2001;41:1229–38.
35. Tosha CM, Borsting E, Ridder WH III, Holt M, Smith D, Chase C. Accommodative lag and visual discomfort. Optom Vis Sci 2007;84:E-abstract 075021.
36. Scheiman M, Wick B. Clinical Management of Binocular Vision: Heterophoric, Accommodative, and Eye Movement Disorders, 2nd ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2002.
37. Wick B, Gall R, Yothers T. Clinical testing of accommodative facility: part III. Masked assessment of the relation between visual symptoms and binocular test results in school children and adults. Optometry 2002;73:173–81.
38. Wang B, Ciuffreda KJ. Depth-of-focus of the human eye in the near retinal periphery. Vision Res 2004;44:1115–25.
39. Ciuffreda KJ. Accommodation, the pupil, and presbyopia. In: Benjamin WJ, Borish IM, eds. Borish’s Clinical Refraction, 2nd ed. Oxford: Butterworth-Heinemann; 2006:93–144.
Keywords:

visual discomfort; asthenopia; accommodation; visual fatigue; autorefractor; accommodation insufficiency; accommodation fatigue

© 2009 American Academy of Optometry