MAPLES, WILLIS C. OD, MS, FAAO; HOENES, RICHARD MA
The near point of convergence (NPC) is a basic visual measure performed by eye care clinicians.1–6 It defines the amplitude of convergence (punctum proximum of convergence), or the closest point in space where the patient can hold fusion, and therefore, see one target.1 It is a simple test. The examiner slowly moves (at about 1 to 2 cm/s) a small accommodative target toward the patient. The test is performed on the subject’s midline, as the subject tracks the target with his eyes. The endpoint is measured and recorded when the subject reports seeing two targets (preferable response), or when the examiner sees one eye deviate from the target. The recovery to binocularity is then performed by slowly moving the target away from the subject until the subject reports one target, or when the examiner sees the eyes regain triangulation on the target.
The test has a long history of clinical use because of its simplicity of administration and its value as a diagnostic tool. Although literature contains some variations in the normative values reported, these differences can be largely explained by differences in administration techniques.4–6 Target design does not appear to be a significant factor in NPC results.4 There is evidence that the NPC mean measures recede from the subject as a factor of age, but not of gender.7 When the NPC was repeated on the same adult subjects, the test has been found to have good to excellent agreement between means for both intra-examiner and inter-examiner reliability.6 The NPC is proposed to be a major, but not exclusive, finding in the diagnosis of convergence insufficiency.8–13
Convergence insufficiency (CI) is the most prevalent of the binocular dysfunctions.8,10–12 Its prevalency is reported to be anywhere from 1 to 33%. The NPC is one of the diagnostic tests often performed to help determine the presence of CI. NPC measures were found to differ between normal subjects, (2.4 break; 4.4 recovery) and CIs (9.3 break; 12.5 recovery) in adults.4 A measure of more than 5 cm for the break and a recovery of more than 7 cm on the recovery has been suggested in an adult sample of subjects, because 85% of a test sample fell at or below these values.4 Hayes et al., has written that a 6-cm break for the NPC be considered the upper acceptable limit for the NPC, and that the recovery should be a maximum of 3 cm more than the break finding.5 Although 9 cm, or less, would then be the expected recovery limit, because of the variability of the recovery findings, a limit of 12 cm was suggested for the recovery value. Elementary school norms vary from the adult norms.5,7 Children are suspected of exhibiting CI, however, with a NPC break finding of more than 6 cm or a recovery finding >12 cm. A receded NPC does not in itself, constitute a diagnosis of CI.
The College of Optometrists in Vision Development Quality of Life (COVD-QOL) instrument is a 30-item survey.14–18 The higher the numerical score on this test, the greater the number and/or severity of symptoms one exhibits. This checklist has been shown to have acceptable test–retest reliability.14 The COVD-QOL scores were found to be almost double in attention deficit disorder/attention deficit hyperactivity disorder (ADD/ADHD) children, when compared with a control group, even though these children’s attention deficits had been deemed controlled by medications for at least 1 year.15 The COVD-QOL was used to compare pre- and postvision therapy results.16 Significant improvement was found overall and with each of the 30 items, in this study. A short form of this instrument has shown significant inverse correlations to standardized academic test scores (Stanford IX).18
The aim of this study was to investigate normative NPC behavior for elementary school children (6 to 9 years) and to ascertain if this finding might predict higher scores on the COVD-QOL and, therefore, more symptoms.
MATERIALS AND METHODS
Three near point of convergence measures (break and recovery) were made in the course of the completion of a prospective, multicenter, longitudinal study of visual skills of elementary school children (I-SERVE).14–17 A total of 539 school children (first through third grades) were evaluated periodically over a 3-year period. Evaluations, including multiple NPC measures, were performed on the same children, once in the fall and once in the spring for three consecutive years. Three NPC breaks and recoveries were attempted at each testing on those children that were not absent or otherwise unavailable, yielding 2544 NPC results. The analysis of this data should allow verification of normative performance, particularly because the NPC is considered to be one significant measure employed in the diagnosis of CI.2,3,6–13
Each child that volunteered for the study was tested by the same licensed optometrist. The examiner did not have access to previous data on any subject. All testing was performed in the respective schools, in a vacant classroom, with ambient lighting. A small, highly reflective sphere attached to a very thin rod was presented on the midline of the child’s body at about 50 cm from the child. The instructions required that the child attend the sphere and attempt to keep their reflection in the sphere as clear as possible and at the same time report when the target became two. The examiner slowly (1 to 2 cm/s) moved the rod with the attached sphere toward the child until the child reported diplopia, or when one eye was seen to deviate from binocularity. The distance from the bridge of the nose (just below the brow on the midline of the face) to the break point was carefully measured with a ruler to the nearest centimeter. The sphere was then moved away from the child until either one target was again reported, or until the eyes were seen to make a triangulation movement to regain binocularity, as judged by the symmetry of the corneal reflexes. This distance was also carefully measured with a ruler and recorded in the same manner, to the nearest centimeter. If the measure was <1 cm, it was recorded as 1 cm. This technique closely mimics the procedures recommended by Scheiman et al. for their NPC accommodative target technique.4
We compared the NPC results from the first evaluation in the fall of the first year of the study (n = 539) by gender, race, and age. No significant differences were found between genders and there was no significant difference between white and Native Americans. There was a significant difference found when the blacks were compared. Because the black sample was small (n = 15), their findings were excluded from the data. Subjects aged 5 and 10 were also excluded because of a small sample size, (1 and 2, respectively).
When the breaks and recoveries of the three trials for the assessment were compared with one another, it was found that each of the succeeding measures significantly receded (p = < .001) from the previous measure for both break and recovery measures. The break and recovery finding for the three trials can also be found in Table 1. The three measures for both the break and recovery became increasingly receded. There was less than ½ cm difference between the break findings and 1 cm difference between the recovery findings. These differences were considered clinically insignificant and the three measures for both the break and recovery were averaged for all subsequent calculations (Table 1).
Table 2 contains the mean, standard deviation, minimum, median, and maximum values by age for both the break and recovery. Because only one 5-year-old and two 10-year-olds were included in the first through the third grade, these individuals were excluded from further analysis.
We analyzed the distribution of data and found it to be skewed. We, therefore, were unable to use parametric statistics in our analysis. Table 3 contains the mean, standard deviation, median, and lower/upper quartiles for ages 6 through 9 years. We found no significant difference in either the break or recovery findings for these ages.
We then performed two analyses of the data. We first compared those whose break was equal to, or less than, 5 cm to those whose break was >5 cm, along with those whose recovery was equal to, or less than, 7 cm to those whose recovery was >7 cm.4 Our sample found 88% of the children’s break findings were 5 cm or less. Only 57% of the sample scored at 7 cm or less for the recovery.
The second comparison used a more liberal criteria (=/<6 cm break; =/<12 cm recovery).6 These calculations revealed that 90% scored equal to, or less than, 6 cm break. When we compared the recovery findings, we likewise found that the percentage had increased. A total of 85% scored 12 cm or less.
We had asked both teachers and parents to complete a quality of life checklist (QOL) each time we performed an evaluation.15–18 A total of 330 QOL evaluations by the teachers were completed. The mean symptom score for the checklist was 14.8 with a standard deviation of 14.3. Previous research has suggested that a score of 20 or greater would be indicative of a visual problem.14–16,18
We made four comparisons (5 and 6 cm criteria for the break and 7 and 12 cm criteria for the recovery) between those individuals that scored on NPC break and/or recovery, or less, to those who scored higher than the chosen criteria. Children who scored 5 cm or less on the NPC break had a QOL score of 14.24, whereas those who scored higher than 5 cm on the NPC break had a significantly higher QOL score (20.41; p = 0.03). No other finding was predictive of symptoms.
Odds ratios were calculated for the significant findings (5 cm break). For those scoring 20 or more on the QOL, the odds ratio of having a break >5 cm was 2.2.
We were unable to find any previous reports that showed a comparison between gender or race. Developmental skills have sometimes indicated differences in both race and gender, but we found no differences with either of these variables.17
We compared the three break and recovery findings with one another and found that with each consecutive test, the mean break and recovery significantly receded in this elementary school population (Table 1). This is in agreement with what has been reported concerning adult subjects.4 Although the three different measures were significantly different from one another, they were not judged to be clinically different. There was only about 0.5 cm mean difference between first and third break finding and just over 1 cm between the first and third recovery finding. This amount is well within the accepted test–retest reliability limits.5
The NPC mean breaks and recoveries are reported not to change appreciably with age. Hayes had reported that kindergarten, third and sixth grade children changed slightly, but these findings, again, were not deemed clinically significant. Our study also demonstrates slight changes between younger students that were not clinically significant. The widest variation for the break between ages was just under 1 cm (age 6 2.67; age 9 3.30) seen in Table 2. Likewise the largest difference in the mean recovery between ages was just under 1 cm (age 8 6.93; age 7 7.86).
When we compare the average break and recovery of this study to the averages reported by Scheiman et al.4 and Hayes et al.,5 we find that these averages are quite similar. The mean breaks for these two studies were 2.5 and 3.9 cm (the latter score was calculated from the paper), respectively. The break findings found in this study was 2.8 cm. This is 0.3 cm receded from the study of Scheiman et al. and 1.1 cm advanced from the calculated from the study of Hayes et al. We deem these differences to also be clinically insignificant.
A comparison of the recovery findings reported here, when compared with the one of the previous studies reporting norms for NPC, would lead one to believe that the findings are enough different to be considered clinically significant. The average recovery in this study was 7.2 cm, whereas adult study of Scheiman et al. reported an average recovery of 4.4 cm, a 2.8 cm discrepancy.4 This almost 3 cm difference is a 63% difference between the two findings. Hayes et al. on the other hand, reported an average recovery (again, calculated from the paper) of 7.9 cm for an elementary school sample.5 This is a 0.7 cm or only a 9% difference between the two recoveries.
Because both Hayes and this data are for elementary school subjects, the recovery to binocularity is better in the adult than it is in either of these two elementary school studies. The small differences between the data of Hayes et al. and this study may be caused in part by the fact that we again calculated the Hayes et al. data from their paper. When we compared our first grade mean NPC break and recovery scores with the Hayes et al., kindergarten scores and our third grade scores with their third grade findings, the mean scores were similar. Our first grade, compared with the Hayes et al. kindergarten findings was only 0.61 different (2.65 cm; 3.26 cm, respectively). The third grade findings were even closer. Our scores were 3.59 cm and theirs was 4.09 cm, a 0.50 cm difference. Recovery findings were also similar in our study and Hayes. Our first grade recovery was 7.18 cm, whereas in the study of Hayes et al., it was 7.63 cm, a 0.45 cm difference. The third grade recovery was about 1 cm different, with our recovery average being 7.79 cm, whereas in the study of Hayes et al., it was 8.73 cm (0.94 cm difference).
When NPC break frequency distributions were considered, the adult sample6 reached an 85% frequency at 4 cm and elementary school children reached the 85% frequency at 6 cm.5 Again, our study (85% reached at 4.7 cm) is 0.7 cm receded from the adult study and 1.3 cm less than the children’s study. The previous elementary school study recovery for an 85% frequency distribution was 13 cm.5 This finding is within 1 cm of the 85% frequency distribution measure of 12.3 cm that we found. Our data would generally support both Hayes et al.5 and Rouse et al.6 norms for NPC measures.
A number of studies have related visual symptoms to the individual diagnosis of CI, when compared with normal binocular subjects.13,19–25 We found significantly higher visual symptoms (COVD-QOL) when we defined CI as an inability to reach a minimum score for the NPC break (5 cm; p = 0.03).
We compared the NPC break/recovery pass criteria to those individuals scoring 20 or more (failing score) with those scoring below 20 on the COVD-QOL checklist and we again found a significant difference with the break findings. The 5-cm break successfully differentiated the CI individuals from non-CI individuals, as exhibiting more visual symptoms. The odds ratio calculation, however, showed this criteria to be only a modest predictor with the odds ratio being 2.17. Based on these calculations, we believe that the 5-cm NPC break is the best predictor of symptomatic children, if the NPC finding is the only finding available.
Based upon this data we conclude:
1. The NPC break and recovery does not change appreciably with multiple administrations of the test in the same test period.
2. The criteria for a NPC break score to differentiate the more symptomatic and less symptomatic, elementary schoolchildren on the average should be 5 cm, or less.
3. The NPC break and recovery criteria described here should be tentatively used as one of the benchmarks in the diagnosis of convergence insufficiency.
4. Another study should investigate if those children who fit the above criteria do poorer in academics than those who pass these criteria.
Willis C. Maples
Southern College of Optometry
1245 Madison Avenue
Memphis, TN 38104
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