TASSINARI, JOHN T. OD, FAAO, FCOVD
The accommodative response to a near target may exceed, equal, or be less than the accommodative stimulus. The difference between accommodative response and accommodative stimulus is termed lead, a negative number, when response exceeds stimulus. An accommodative response that is less than the accommodative stimulus is a lag and is represented by a positive number. Lag and lead are absent when the accommodative response equals the accommodative stimulus of the target, and the numerical value of this manifestation of accommodative response is zero.
The accommodative lag or lead that an individual habitually adopts may be influenced by several factors, not all of which are understood. A small accommodative lag of approximately +0.25 D may simply reflect the minimum accommodation required for adequate resolution of the target. Because of depth of focus, accommodative response does not have to exactly equal the stimulus. A depth of focus of ±0.40 D is typical and is inversely related to pupil size. 1,2 Alternatively, a small 0.25 D lag may be a purposeful error that provides a steady-state stimulus to accommodation to replace decaying innervation to accommodation. 3 Rutstein et al. 4 agree with both of these factors and summarily state, “The lag of accommodation is primarily a result of the control properties of the steady-state accommodative system and the depth of focus of the eye.”
Other potential influences on the manifestation of accommodative response as lag, lead, or no lag are age, accommodative convergence, convergence accommodation, proximal accommodation, tonic state of accommodation, refractive status, medication side effects, systemic disease, and ocular disease. 5–8 One study showed that accommodative lag/lead changes with vision therapy, 9 and another study showed a change after vision therapy that was not statistically significant. 10
Determination of accommodative lag or lead is an important test in the clinical evaluation of accommodation, vergence, and overall visual function, 11 and a slight lag of +0.33 D is considered to be normal. 6 Accommodative lead is a hallmark finding in the diagnosis of accommodative excess 12–14 and is associated with exophoria and other accommodative and binocular dysfunctions. 10,12,14–17 A momentary shift in accommodative lag toward less lag/increased lead may reflect an increase in cognitive demand. 18 In the Optometric Extension Program case analysis method, an accommodative lead, sometimes called minus projection, is thought to result from a visual system that has deteriorated significantly from nearpoint visual stress. 19
An accommodative response that manifests as an excessive lag of accommodation may indicate latent hyperopia 6 or be associated with accommodative insufficiency, 12,14,20 esophoria, 12,14,21,22 and other disorders of accommodation 23 and binocular vision. 12,14,16 An excessive lag of accommodation may be a stimulus for axial elongation/myopia, 7,8,24 and an increased lag found on the binocular cross cylinder test has been shown to precede juvenile onset myopia in emmetropic individuals. 25 There is some evidence suggesting that an excessive accommodative lag and a tonic/resting state of accommodation that is more remote than normal occur together. 20,26,27
Monocular estimate method (also known as monocular estimation method) dynamic retinoscopy (MEM) is a widely used objective clinical test for accommodative lag and lead developed by Haynes. 20 It is specifically recommended as a useful clinical test in a recent American Optometric Association Clinical Practice Guideline 11 and by others. 5,20 MEM has been shown to be valid, 28 and to have satisfactory interexaminer reliability. 29,30 The intraexaminer test-retest reliability of the MEM is not known. Griffin 31 and others 32,33 caution that the supplementary lenses used during MEM may alter accommodative response and lead to inaccuracies.
An important normative study of MEM was conducted by Rouse et al. 6 more than 15 years ago. In this study of 721 schoolchildren in kindergarten through sixth grade, a mean accommodative lag of +0.33 D (±0.35) was found with a median of +0.25 D. A total of 8.6% of this population had accommodative lead, and 5.5% had accommodative lag that exceeded +0.75 D. The investigators concluded that the range of normal values for MEM is 0.00 to +0.75 D. There were three characteristics to the study population that could limit the application of their normative data to a clinic population. Schoolchildren who were older sixth grade and kindergarten through sixth-grade children who failed a Modified Clinic Technique (MCT) screening were excluded from the study. Ostensibly, patients older than sixth grade and patients who fail an MCT screening are likely to comprise a significant percentage of a clinic population. A third characteristic of this study population is that the subjects were not necessarily wearing lenses that compensated for their full ametropia when MEM was administered.
Six other studies of MEM on populations of various ages found mean lags that ranged from +0.23 to +0.75 D as shown in Table 1. Published studies of accommodative response using Nott dynamic retinoscopy or an optometer show that the average result with these methods is within 0.25 D of +0.50 D. 18,21,32,33,36
The relationship between MEM and refractive status and the relationship between MEM and near phoria have not been studied extensively. There is evidence, however, that such relationships may exist. Ong and Ciuffreda, 24 based on a review of several published studies, concluded that myopes have reduced steady-state accommodative responsivity at near. It has also been shown that myopes have poorer blur sensitivity. 7,8 Reduced responsivity and blur sensitivity would predispose myopes to an increased lag of accommodation, which may be evident on MEM. One study found that the results of the binocular cross cylinder test were higher in plus in a group of emmetropic subjects who later became myopic compared with emmetropic subjects who remained emmetropic. 25 Another study found that young adult myopes had a greater lag of accommodation than emmetropes based on Nott retinoscopy. 36 No studies have investigated the use of MEM to determine the lag or lead in myopes.
Because of the accommodative convergence and convergence accommodative links, it could be predicted that exophoria would tend to influence accommodative response to manifest as lead and esophoria toward lag. Thus, the MEM result has been related to several nonstrabismic binocular dysfunctions as stated above. 12,14,16,21,22 Three recent studies investigated the relationship between accommodative response and near phoria. 17,21,22 One showed no relationship between the results of Nott retinoscopy and near phoria. 17 The other two studies found that an increased lag, as measured by an infrared autorefractor, was associated with esophoria. 21,22 No published studies considered a systematic relationship between MEM and near phoria.
The purpose of the present study was to attempt replication of the MEM central tendency measures reported by Rouse et al. 6 with a different population. For this study, a clinic-based population was used rather than a school-based population. Other population sample differences were inclusion of pre-presbyopic subjects older than sixth grade and inclusion of subjects who failed a MCT screening. In addition, all subjects wore their subjective refraction lenses. A second component in the study design was to create subgroups based on refractive status, phoria, and refractive status/phoria combinations. The accommodative lag/lead of the groups, based on MEM, was compared, and the null hypothesis of this aspect of the study was that MEM is the same regardless of refractive status or phoria. The intent of these comparisons was to add to the understanding of the nature and clinical significance of the manifestation of accommodative response as lag or lead.
The 211 pre-presbyopic subjects in this cross-sectional retrospective study were drawn consecutively from the author’s private optometry practice located in a large metropolis. The patients are culturally and ethnically diverse and from the full range of socioeconomic status. Age and gender were the only demographics recorded. In preparation for this study, it was decided that MEM retinoscopy would be included as a routine test during the comprehensive examination of all pre-presbyopic patients during the period January 1, 1998 to June 1, 1998. The subjects for this study were drawn retrospectively from this patient population according to the exclusionary criteria described below.
Subjects were excluded from this study if constant strabismus, amblyopia, or eye disease affecting visual acuity were present. Also excluded were subjects who were unresponsive to MEM. The number of subjects excluded by these criteria was not tracked. Another group of subjects was excluded if their record lacked the written MEM result or was illegible. These criteria lead to exclusion of 13 subjects.
The author was the sole examiner administering MEM, which was administered in the context of a comprehensive vision and eye health examination. MEM followed basic entrance tests, static distance retinoscopy, subjective refraction, and phorometry. Thus, the examiner was not masked to the refractive status and near phoria when MEM was performed. MEM preceded diagnostic pharmaceutical agents, tonometry, biomicroscopy, and ophthalmoscopy. The refraction took place after static retinoscopy in a 4-m examination room with a mirror that lengthened the test distance to 6 m. A binocular sphere balance followed a monocular subjective refraction. MEM was administered with the subjective refraction lenses in place even if they differed from the patient’s habitual lenses. For example, if the patient habitually wore no lenses and their refraction was OD +0.50 −0.50 × 075 and OS +0.25 −0.25 × 100, these lenses were put in a trial frame, not the phoropter, and MEM was administered.
MEM was performed under full fluorescent room illumination (45 ft-c) using a model number 18010 Welch-Allyn spot retinoscope per the protocol described by Rouse et al. 28 and Haynes. 37 MEM targets were the Pierce MEM cards clipped to the retinoscope. Target selection was based on the grade level of the patient. The target words were usually 8-point type. For patients aged 6 years and some 7-year olds, the 12-point words on the backside of the first grade card were used. The influence of this difference in target size was not included in the data collection or analysis because the influence of letter size on accommodative response is thought to be negligible. 38 Just before MEM, the examiner handed the MEM card to the patient and requested a recitation of the printed words. The patient’s spontaneous working distance was measured while he/she read the MEM card. The instructions to the patient approximated the following, “Read aloud the words on the card and ignore the light shining into your eyes. If you don’t know a word, that’s okay, just spell the word by reading each letter in the word.” Care was taken to interpose the measuring lens for only a brief moment. It is the author’s custom to interpose the measuring lens for only one quick sweep of the retinoscope beam. Also, after each sweep of the retinoscope beam, the light is away from the patients’ pupil for a second or more. The intent of this strategy is to minimize contamination of the accommodative response by the measuring lens or the light from the retinoscope. The target distance was equal to the spontaneous working distance of the subject unless it was grossly abnormal. In those instances, MEM was done at the Harmon distance.
The retinoscopic reflex was first estimated, and then a measuring lens was used to confirm the estimate. If a neutral reflex was seen through a measuring lens, the observation was confirmed by using a measuring lens that was 0.25 D more plus with the expectation of seeing fast against motion. The most plus measuring lens (least minus) that gave a neutral reflex was the recorded result. The first two sweeps of the retinoscope were of the 180 and 90 meridian. If they appeared unequal, a rare occurrence, the astigmatism measurements were repeated and MEM attempted again. The number of subjects who required this adjustment was not tracked.
The patient’s refractive status and near phoria were also recorded for analysis. The near phoria was the result of the von Graefe method administered in the standard manner 39 at a test distance of 40 cm. The test distance for the near phoria and MEM was not necessarily the same, and this difference was not recorded or factored into the data analysis. The MEM, refractive status, and near phoria results were analyzed using descriptive and comparative statistics for the total group and the refractive status/phoria subgroups.
The sample population, 211 subjects, had a mean age of 14.5 years (SD, 7.9) and a median age of 11.0 years; the range of ages was 6.0 to 37 years. Eighty-five of the subjects were between 6 and 10 years old. Eighty-four were in the age 11 to 20 bracket, and 42 were aged 21 to 37. The gender distribution was 97 males and 114 females.
The refractive status of each subject was categorized based on the spherical equivalent of the subjective refraction of the right eye. Hyperopia was defined as a subjective refraction at least +0.75 D. The emmetropia parameters were +0.50 D through −0.25 D inclusive. Myopia was the diagnosis when the subjective refraction spherical equivalent of OD was at least −0.50 D. Fig. 1 shows the distribution of the refractive status diagnoses among the subjects. The phoria diagnosis was categorized as follows: Orthophoria was the diagnosis when the von Graefe phoria test yielded a result of 1 base-out through 2 base-in inclusive. Exophoria was the diagnosis when ≥3 base-in resulted and esophoria when ≥2 base out resulted. Fig. 1 shows a fairly even distribution of the near phoria diagnosis of 28% orthophoria, 36% esophoria, and 36% exophoria. Fig. 1 also shows the prevalence of the nine subgroups derived by combining the refractive status diagnosis with the phoria diagnosis. For example 12% (25/211) of the subjects have myopia plus orthophoria.
Central Tendency Measures
The mean MEM result for the entire study population was +0.35 D lag (±0.34) (Table 2). The SEM was 0.02 D, and the 95% confidence interval was 0.30 D to 0.39 D. The frequencies of the MEM findings are plotted in Fig. 2. The median MEM is +0.25 D. The mean MEM and SD for each of the 15 subgroups are also shown in Table 2. All of the subgroups were within 0.17 D of the group average except for the myopia/esophoria group. This subgroup of 40 subjects averaged 0.25 D greater lag than the total group. The emmetropia/orthophoria group and the emmetropia/exophoria group had the lowest lag among the subgroups.
Comparison of Groups
Each of the 211 MEM measures was cross-classified according to the patient’s refractive status and phoria. These subgroup means are presented in Table 2. The results of an analysis with a two-way analysis of variance are summarized in Table 3. Both of the main effects, refractive status (F2, 202 = 6.38, p < 0.002) and phoria (F4, 202 = 4.32, p < 0.015), were significant. The refractive status by phoria interaction effect was not significant (F2, 202 = 1.50, p < 0.20).
Post hoc tests using the Tukey honestly significant differences method were conducted for each factor at the overall alpha level of 0.05. These tests indicated that the myopia group mean was significantly different from both the emmetropia and hyperopia group means, but that the emmetropia and hyperopia group means were not significantly different. Also, the esophoria group mean was significantly different from both the orthophoria and exophoria group means, but these two group means were not significantly different.
Distribution of Accommodative Response
For purposes of assigning each MEM result to a category, criteria were used that followed the guideline that >1 SD away from the mean is abnormal. Applied strictly, this guideline resulted in a normal range of +0.01 to +0.69 D. This range was modified slightly so that normal was 0.00 to +0.70 D inclusive. Accommodative lead was <0.00 D, and an accommodative response of greater than +0.70 D was an excessive accommodative lag. Fig. 3 and Table 4 show the distribution of the three accommodative response types among the groups.
This study repeats the previously established central tendency measures for MEM 6 upon which normative data are based. This consistency in MEM central tendency measures took place despite several methodological and population differences between the two studies. The effects of refractive status and phoria on MEM were both significant, whereas the corresponding interaction effect was not. An analysis of subgroups indicates that MEM tends to be greater if myopia or esophoria are present. The null hypothesis that MEM is the same regardless of refractive status or phoria is rejected. The variation of MEM with myopia and esophoria is a preliminary conclusion based on a retrospective study that lacked examiner masking.
The greater lag among myopes found in this study is consistent with prior reports that predict an increased lag among myopes due to reduced steady-state accommodative responsivity and blur sensitivity among myopes. 7,8,24 It is important to point out that none of the prior investigations used MEM and were instead based on laboratory studies that used an infrared or laser optometer. This study provides preliminary confirmation of this relationship in a clinical setting with a clinical test. Further confirmation could be provided if a masked prospective study of the relationship between MEM and myopia yielded a similar result.
Another group with a statistically significant greater lag was the esophoria group. This result can be explained on the basis of accommodative convergence and convergence accommodation. A reduced accommodative response leading to an accommodative lag reduces the magnitude of esophoria through the AC/A link, which lowers the demand on fusional divergence. Additionally, fusional divergence to overcome esophoria stimulates negative accommodation via convergence accommodation. Negative accommodation could manifest as an increase in lag. This result confirms Bieber’s observation that “. . .a lag is most frequently found in manifest or latent esophores who use reduction of accommodative convergence to lessen the stress on negative fusional vergence.”16 It is also consistent with prior studies that have associated increased lag with esophoria. 21,22 As with the myopia/accommodative lag studies, these two were done with infrared autorefractors. This study provides preliminary confirmation of those results with a clinical test in a clinical setting. Because this study was unmasked and retrospective, this confirmation is made with reservation.
It follows that subjects with both myopia and esophoria had the highest lag of all groups. In fact, this group mean of +0.60 almost doubled the total group mean of +0.35. Moreover, 40% of the 40 myopic esophores in the study had a lag that was >1 SD away from the mean. This result is similar to the results of a prior study that found lowered accommodative response among myopic esophores using an infrared autorefractor to measure monocular accommodative response. 22 It has been show that myopes have a higher AC/A than other refractive groups. 23 When a high AC/A ratio is present, small dioptric changes in accommodation lead to meaningful vergence changes. Myopic esophores could take advantage of their higher AC/A and lag an extra 0.25 to 0.50 D to lessen the magnitude of their esophoria, thereby minimizing the demand on negative fusional vergence. Moreover, because they are less sensitive to blur, 7 there is no immediate “penalty” for borrowing from the accommodative system to pay the vergence system. In support of this explanation is the striking contrast between hyperopic esophores who averaged +0.28 D on MEM and the myopic esophores who averaged +0.60 D. Clearly, myopia is a strong predictor of increased lag.
Although esophoria predicted a statistically significant greater lag, its counterpart, exophoria, did not predict a statistically significant greater lead of accommodation. It has been established that exophoria at near can be associated with a lead 12,14 or a lag. 14,23 A lead in the presence of exophoria indicates a secondary effect of overreliance on accommodative convergence in lieu of fusional convergence to compensate for the exophoria. Moreover, fusional convergence activated to overcome an exophoria triggers increased accommodation by way of the CA/C link. This increase in accommodation could manifest as a lead of accommodation. In the case of exophoria with a lag, the exophoria may be a secondary manifestation of the primary condition—a lag. This condition is known as pseudoconvergence insufficiency. 12 The presence of exophores with a lead coupled with the presence of pseudoconvergence insufficiency exophores with an abnormally high lag would tend to make the average MEM of a group of exophores look normal. In this study, this observation is true in that 8% of the exophores had a lag >1 SD away from the mean and 9% had a lead >1 SD away from the mean.
Another consideration regarding the lack of an abnormally low lead among exophores is the nature of accommodative lead. The results of this study and others like it 6 show that accommodative lead is uncommon. Many clinicians would agree that patients with accommodative lead are highly symptomatic and have widespread visual dysfunction. 19,20 An accommodative response resulting in a lead may be a visual status to be avoided at all costs, and individuals predisposed to a lead, such as exophores with a high CA/C ratio, learn to adjust their accommodative response accordingly.
The results of this study raise the possibility that the manifestation of accommodative response as lag, no lag, or lead varies with refractive status and phoria. Thus, accommodative response may be more than a manifestation depth of focus (DOF) and steady-state control properties of accommodation. It is tempting to explain accommodative lag on the basis of DOF alone because the average DOF, ±0.4 D, is very close to the average MEM result, +0.35 D. This simplistic explanation is unlikely because refractive status and phoria status had a statistically significant effect. This study did not consider pupil size, however, and this omission is a limitation of the study. Myopes and esophores may have smaller pupils with larger DOF’s, which permit higher accommodative lags. Ignoring this limitation, a summary statement is that the difference between the accommodative response and accommodative stimulus as measured by MEM is a manifestation of DOF and steady-state accommodative control properties with an overlay of refractive status and phoria status. This overlay may dominate the response in some individuals, and the accommodative response may deviate from the DOF/steady-state control level.
It is not clear from this study whether the phoria status per se influences accommodative response or whether the binocular influence on accommodative response is related to fusional vergence ability. For example, esophores with good negative fusional vergence may have a normal lag, whereas esophores with low negative fusional vergence reserves have an abnormally high lag. Likewise, exophores with ample fusional convergence may not tend toward low lag/lead, whereas exophores without ample fusional convergence do tend toward low lag/lead. Further research that considers fusional vergence reserves could provide clarification.
A clinical implication of this study concerns the use of MEM to aid the diagnosis of nonstrabismic binocular dysfunctions. Utilization of a high lag as a finding that supports the diagnosis of basic esophoria and convergence excess as advocated by Press 12 and Scheiman et al. 14 is supported by the results of this study. An abnormally low lag as a finding in the diagnosis of basic exophoria or convergence insufficiency is not directly supported because the accommodative lag among exophores was not different than normal. However, exophores were significantly different from esophores when the two groups were compared with one another, and limitations of this study mean that the low lag/exphoria relationship cannot be discounted. As mentioned, the fusional vergence status of the subjects was not considered, nor was the exophoria magnitude. Perhaps an abnormally low lag becomes evident as the exophoria magnitude increases and/or fusional convergence ability declines. Precedent for this line of reasoning is evident in a prior study of exodeviations 40 and a recent study of esophores who did not show an abnormally high lag unless the esophoria was ≥5 D. 21
The assumption that the MEM result among a population of pre-presbyopes averages a one-third-diopter lag with a SD of ±0.34 is confirmed. Refractive status, specifically myopia, and phoria status, specifically esophoria, may influence the difference between accommodative response and stimulus as measured by MEM. The influence of these two vision conditions is in the direction of increased lag. These two conclusions are presented with some reservation because of the limitations of the study design.
Dr. Paul DeLand provided the statistical analysis for this study and helpful suggestions for writing the results section. I am also grateful for the review, guidance, and encouragement provided by Dr. Mike Rouse.
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