Four types of manual IPD measurements were performed: binocular near, binocular distance, and monocular right eye and monocular left eye distance (from the bridge of the nose) using the Viktorin method as described by others 5,24,44

Each measurement was performed two consecutive times and averaged.5,24,34

The manual PD for near and distance was determined using a two-step procedure. The first step measured the binocular near IPD. The subject was asked to fixate on the examiner’s left eye. During fixation, the examiner determined the distance between the temporal limbus of the subject's left eye (Ltemporal) and the nasal limbus of the subject's right eye (Rnasal), using a millimeter ruler. The near IPD was then calculated as:

Near IPD = Subject fixating on examiner's left eye : Ltemporal → Subject fixating on examiner's left eye :Rnasal

In the second step, the binocular distance IPD was determined. Unlike the first step, the subject was asked to change fixation from the examiner's left eye, to the examiner’s right eye. Then, similarly to the near IPD measurement, the examiner determined the location of the subject's right nasal limbus and calculated:

Distance IPD = Subject fixating on examiner's left eye : Ltemporal → Subject fixating on examiner's right eye :Rnasal

Manual monocular measurements were performed with the examiner level with the subject and facing the subject. A specially marked PD ruler (Appendix 1) was placed on the bridge of the nose in the center of the subject's face, approximately in the back vertex plane of the spectacle lens. In order to measure the monocular right eye pupillary distance, the examiner closed his left eye and asked the subject to fixate on his open right eye. The examiner then noted the location of the center of the subject's right eye pupil from the corresponding mark on the PD ruler scale that was located directly below the subject's right eye. In order to measure the monocular left eye pupillary distance, the examiner opened his left eye and closed his right eye. He then asked the subject to fixate on his left eye while reading the location of the center of the subject's left eye pupil from the corresponding mark on the PD ruler scale.

Four types of automatic PD measurements were performed: binocular near, binocular distance, monocular right eye distance, and monocular left eye distance. The automatic PD measurements were performed using the Essilor pupillometer (Pupillon, Essilor, France). The Pupillon displays a reading that is the mean of four measurements. As such, only one recording with the Pupillon was measured for each condition.

The manual and automatic measurements were conducted and recorded in alternating order. For example, if a subject was first measured using the manual method, they were subsequently measured using the automatic method. The next subject was then first measured using the automatic method, and subsequently measured using the manual method, etc.

The manual method was performed by one examiner and the automatic method was performed by another examiner, both experienced optometrists, each of whom were each masked to each other’s result.

Outcome parameters were tested for normality using the Shapiro-Wilk test, which indicated that most measures were normally distributed. Therefore, nonparametric statistics were applied for subsequent analyses. The paired-samples Wilcoxon test was applied to compare between the results of the two IPD measurement methods and an Independent Mann Whitney test was applied to compare between the gender groups.

The interchangeability of the manual and automatic measurements of data obtained in the vision-screening assessment was evaluated using Spearman's rank correlation and Bland and Altman plots.45 Bland-Altman plots indicate the mean difference (Md) and 95% confidence intervals (CI) calculated as Md ± 1.96* standard deviation (SD), which specify the interval within which 95% of the differences between the measurement of the two sessions was expected to fall.45 These are known as the 95% limits of agreement.

Non-normally distributed data were analyzed using the Bland & Altman non-parametric approach.46 According to this approach, measurements are compared using a clinically acceptable interval range. Therefore, based on previous results,44 an interval range of ±1 mm for monocular measurements and ±2 mm for binocular measurements was considered the clinical range of resolution.

The inter-test repeatability for each method was assessed using the mean values of the three measurements of each session. A paired-samples Wilcoxon test was applied to compare the first session's measurements to the second session's measurements, separately for each type of measurement.

As stated above, the manual IPD was measured two consecutive times, and the mean measurement was calculated. The automatic pupillometer displays the results of four consecutive measurements, without providing the values for each individual measurement. As such, it was not possible to compare the intra-test repeatability or intra-class correlation coefficient of the two techniques.

For all statistical tests, P values lower than 0.05 were considered statistically significant.

The rank correlation coefficient, mean difference (value ± SD), the 95% CI, and the clinical range of resolution between the manual millimeter ruler and the automated pupillometer distance monocular right eye measurements are shown in Table 3andFig. 1.

Fig. 1.

Comparison of manual and automatic monocular right eye PD measurements. Bland and Altman plots representing the difference between manual and automatic monocular right eye pupillary distance measurements for all subjects (a) males (b) and females (c). The black line represents the mean difference, whereas the dashed lines show the 95% limits of agreement, and dotted lines show ±1 mm which are the upper and lower clinical range of resolution. Each data point represents one participant. All values presented are in millimeters.

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For the monocular manual and automatic right eye PD measures, the mean differences of 63%, 62%, and 63% of all participants, male participants, and female participants were within clinical range of resolution (±1 mm), respectively.

The correlation coefficient, mean difference (value ± SD), the 95% CI, and the clinical range of resolution between the manual millimeter ruler and the automated pupillometer distance monocular left eye measurements are shown in Table 4andFig. 2.

Fig. 2.

Comparison of manual and automatic monocular left eye PD measurements. Bland and Altman plots representing the difference between manual and automatic monocular left eye pupillary distance measurements for all subjects (a) males (b) and females (c). The black line represents the mean difference, whereas the dashed lines show the 95% limits of agreement, and dotted lines show ±1 mm which are the upper and lower clinical range of resolution. Each data point represents one participant. All values presented are in millimeters.

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For the monocular manual and automatic left eye PD measures, the mean differences of 74%, 76%, and 73% of all participants, male participants, and female participants were within clinical range of resolution (±1 mm), respectively.

The correlation coefficient, mean difference (value ± SD), the 95% CI, and the clinical range of resolution between the manual millimeter ruler and the automated pupillometer distance binocular IPD measurements are shown in Table 5andFig. 3.

Fig. 3.

Comparison of manual and automatic binocular distance IPD measurements. Bland and Altman plots representing the difference between manual and automatic distance binocular inter-pupillary distance measurements for all subjects (a) males (b) and females (c). The black line represents the mean difference, whereas the dashed lines show the 95% limits of agreement, and dotted lines show ±2 mm which are the upper and lower clinical range of resolution. Each data point represents one participant. All values presented are in millimeters.

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For measures of binocular IPD, the mean differences of 77%, 81%, and 75% of all participants, male participants, and female participants were within the clinical range of resolution (±2 mm), respectively.

The correlation coefficient, mean difference (value ± SD), the 95% CI, and the clinical range of resolution between the manual millimeter ruler and the automated pupillometer near binocular IPD measurements are shown in Table 6andFig. 4.

Fig. 4.

Comparison of manual and automatic binocular near IPD measurements. Bland and Altman plots representing the difference between manual and automatic near binocular inter-pupillary distance measurements for all subjects (a) males (b) and females (c). The black line represents the mean difference, whereas the dashed lines show the 95% limits of agreement, and dotted lines show ±2 mm which are the upper and lower clinical range of resolution. Each data point represents one participant. All values presented are in millimeters.

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The mean differences of 92%, 91%, and 92% of all participants, male participants, and female participants were within the clinical range of resolution (±2 mm), respectively.

A total of 30 subjects (mean age: 27.9 ± 4.54, range: 23–39, 6 male, 24 female) participated in the inter-test repeatability sub-study. The mean age, distance monocular visual acuity, stereoacuity, and all measured IPD values using the manual and automated methods for all the subjects and, separately, for males vs. females are tabulated in Table 7. There was a statistically significant difference between male and female binocular near IPD measurements.

The inter-session repeatability was assessed by calculating the mean difference between the PD values measured in the first session and the second session, as seen in Table 8. No statistically or clinically significant difference was observed between the first and the second sessions for either manual or pupillometer measurements. The mean differences were all lower than 0.6 mm. Bland and Altman analysis, summarized in Table 9 also showed no significant difference between the first and second sessions for both methods, with 70% to 100% of the observations falling within the clinical range of resolution (±1 mm).

Correlation, agreement, mean difference, and percentage of observations within the clinical range of resolution between sessions for all comparisons.

One mm for monocular, and 2 mm for binocular PD measurements are reported to be acceptable inter-examiner variability as well as the acceptable tolerance for differences in IPD in spectacles.44 Differences between measurements that are smaller than 1 mm for monocular and 2 mm for binocular PD measures in this study were considered clinically insignificant. There is more clinical significance in IPD measurement errors for patients with higher refractive errors, with visual consequences such as asthenopia.47 Many subjects are unable to tolerate horizontal prismatic deviations larger than 2 prism diopters48 According to Prentice's formula, the deviation of the image in prism diopters is equal to the distance of the image from the optical axis in centimeters multiplied by the focal power of the lens in diopters.49 Based on this, a prismatic deviation of 2 prism diopters is obtained with an IPD decentration of 2 mm for refractive errors of 10 diopters and above.

Manual measurements examine the anatomical location of the pupil, whereas the automatic measurements examine the physiological corneal reflex.1,23 The physical pupil, and specifically, the iris, are dynamic structures. As such, the corneal reflex has an intrinsic imprecision that cannot be eliminated.50 Despite this limitation, our findings show that the two measurements do not differ clinically in the majority of subjects. Nonetheless, our findings show that between 20%-30% of the subjects do differ clinically between these measurement methods.

The mean manual binocular distance IPD of the study participants ranged between 53 mm and 69 mm. The mean measurement was 60.6 ± 3.2 mm (female mean:60.0 ± 3.0, range: 53–67 mm, male mean: 61.9 ± 3.4, range: 55–69 mm).

The mean automatic binocular distance IPD of the study participants ranged between 52 mm and 68.5 mm. The mean measurement was 59.3 ± 3.4 (female:58.6 ± 3.1, range: 52.0–66.5, male: 61.0 ± 3.7, range: 53.5–68.5 mm).

The mean difference between the manual and automatic binocular distance measurements was lower than 2 mm for all participants, the female subgroup and the male subgroup. As such, these differences cannot be considered clinically significant.

As seen in Table 10, our IPD values are lower than reported in other studies around the world, with the exception of the female participants, who had distance IPDs similar to Saudi Arabian females.

The mean manual binocular near IPD of the study participants ranged between 51 mm and 65 mm. The mean measurement was 57.1 ± 3.1 mm (female mean: 56.5 ± 2.8 mm, range: 50.5–63.0, male mean:58.6 ± 3.2 mm, range: 51.5–65.0 mm).

The mean automatic binocular near IPD of the study participants ranged between 48.5 and 65 mm. The mean measurement was 56.3 ± 3.3 mm (female mean:55.6 ± 2.9 mm, range: 48.5–63.5 mm, male mean: 58.0 ± 3.6 mm, range: 51.0–65.0 mm).

The mean difference between the manual and automatic binocular near measurements of the cohort were lower than 2 mm for all participants, the female subgroup and the male subgroup. As such, these differences cannot be considered clinically significant.

As seen in Table 10, these measurements are lower than reported in other studies around the world, with the exception of the male participants who had near IPD similar to Turkish males.

Similar to some studies,1,23 but in contrast with another,38 the present study found wider binocular IPD measurements using the manual as opposed to the automatic measurement method. These differences, though not clinically significant, have been attributed to the fact that automatic and manual measurements are based on different points of reference.1 Automatic measurements are based on the locations of the corneal reflexes, which essentially measure the visual axes.51 The corneal reflex generally is offset nasally with respect to the pupillary center.3 Manual measurements are based on anatomical locations, of either the pupil or the limbus. As such, differences between the two methods are to be expected.

The mean difference between the manual distance and near IPD measurements in the present study is 3.5 mm for females and 3.3 mm for males. The mean difference between the automatic distance and near IPD measurements is 3.0 mm for females and 3.1 for males. These differences between distance and near IPD are substantially smaller than the 5.0 mm differences reported by Esomonu et al.24 and 4.4 mm differences reported by AlAnazi et al.,38 and larger than the 2.0 mm differences reported by Osuobeni & Faden.4,52 However, the differences in our cohort are similar to the 3.0 mm differences reported by Osunwoke et al.53 and 3.0–3.5 mm differences reported by Everklioglu et al.54 Discrepancies between the present study and Esomonu et al.24 could be attributed to the distance IPDs. As stated in the introduction, the vergence demand is dependent on the IPD,22 and their subjects had a much larger distance IPDs than our cohort. The discrepancy with the results of AlAnazi et al.38 can be attributed to the large variation in their subjects. They reported that the differences between distance and near IPDs in their subjects varied from 2.2 mm to 6.6 mm.

As indicated earlier, monocular PD measurements are indicated in cases of craniofacial surgery43 unequal pupillary locations due to facial asymmetry,42 progressive lenses,18,39 and spectacles with prisms.40 However, to the best of our knowledge, monocular PD is not commonly studied and is rarely reported in the literature. Therefore, the present study also compared the monocular PD determined by the manual and automatic methods.

The mean manual right distance PD of the study participants ranged between 27 mm and 35 mm. The mean measurement was 30.4 ± 1.7 mm (female mean:30.1 ± 1.6, range: 27.0–34.8 mm, male mean: 31.4 ± 2.0, range: 27.5–35.0 mm).

The mean automatic right distance PD of the study participants ranged between 25.5 mm and 36.0 mm. The mean measurement was 29.6 ± 1.9 mm (female mean: 29.4 ± 1.7, range: 25.5–36.0 mm, male mean: 30.4 ± 2.1, range: 26.5–34.5 mm).

The mean difference between the manual and automatic right eye measurements is less than 1 mm for the entire cohort as well as the female sub-group. The mean difference of the male sub-group was 1 mm. These mean differences are considered insignificant clinically.

The mean manual left distance PD of the study participants ranged between 25.5 mm and 35 mm. The mean measurement was 29.9 ± 1.5 mm (female mean:29.7 ± 1.5, range: 25.5–34.0 mm, male mean: 30.4 ± 1.6, range: 27.0–35.0 mm).

The mean automatic left distance PD of the study participants ranged between 25.5 mm and 34.5 mm. The mean measurement was 29.7 ± 2.0 mm (female mean: 29.2 ± 1.8, range: 25.5–34.0 mm, male mean: 30.7 ± 2.0, range: 26.5–34.5 mm).

The mean difference for all the comparisons is lower than 1 mm and considered clinically insignificant.

The monocular PDs of the right and left eyes, as measured with both methods, were slightly asymmetric. Specifically, the PD of the right eye of 114 participants (57%) was wider than the PD of the left eye. Facial asymmetries between the two halves of the face are not uncommon.58

There was a significant difference between the mean age of the female (mean age: 23.7 ± 4.6, range: 19−46) and male participants (mean age: 25.5 ± 5.0, range: 19−53) of this study. The mean difference in age was 1.8 years. Although IPD has been shown to vary with age24,38,54 data typically are grouped in intervals of 10 years, and both the females and males in our cohort would have been grouped within the same age interval. Additionally, the IPD typically stabilizes in adults within the second decade of life.59,60 Therefore, we can assume that variations in IPD between the two sub-groups are linked with gender and size more than they are linked with age differences.

In the present study, male and female distance binocular manual and automatic measurements differed by 1.9 mm and 2.4 mm respectively. The near binocular manual and automatic measurements differed by 2.1 mm and 2.4 mm respectively. Given the clinical range of agreement of 2 mm for binocular measurements, aside from the distance manual measurements, the mean differences between the female and male participants are clinically significant. As seen in Table 10, the differences between female and male IPDs reported in this study are similar to studies from other regions around the world.

The current study included 199 Israeli adults. Compared to the Bura ethnic group of North-Eastern Nigeria whose mean distance and near IPDs, measured manually, are 73.1 ± 2.8 and 67.1 ± 2.7, respectively, the IPDs of Israeli population are much narrower.61 Israeli IPDs also are narrower than reported distance IPD values for African Americans (64.1 mm12 and 66.3 mm62), Chinese males and females (64.6 mm and 61.3 mm, respectively63), Turks (62.5 ± 4.1 mm64) and European Caucasians (62.9 ± 3.455). The IPD of the Israeli population appears to be similar to the Arab population (60.9 ± 3.0 in females and 62.0 ± 2.6 in males4), which could be due to the fact that our cohort included 53 Israeli Arabs and that the two ethnic groups are genetically similar.65,66

There was no significant statistical or clinical difference between the sessions for all conditions. In the Bland and Altman analysis, 87% or more of the observations for all conditions were within the clinical range of resolution (±1 mm), except for the manual near binocular IPD. In this condition, nine of the manual near binocular IPD measurements differed by more than 1 mm between the first and second sessions. In other conditions, a maximum of four participants differed by more than 1 mm between the sessions.

This study has some limitations. First, as tabulated in Table 10, some previous studies examined larger cohorts,3,4,24,52–54 although the present study includes more participants than some of the other studies.55,56

The current study has an imbalanced gender cohort, with 58 males and 141 females. Accordingly, the IPDs of the male sub-group have higher standard deviations than the female sub-group, though the mean differences in standard deviations are all lower than 1 mm and are not clinically significant. If the standard deviations are proportional to the mean IPD, males would be expected to have slightly larger standard deviations than females. Some, though not all of the studies listed in Table 10 reported larger standard deviations for males than female IPDs.

Two outcome measures reported in this study (Table 2) did not include the entire cohort. The stereoacuity of three female participants, and the binocular near VA of four males and 22 females were missing. As mentioned in the methods, these parameters were measured as part of a vision screening. It is possible that not all participants completed all the stations. The main outcome measures of the study are the manual and automatic binocular and monocular PD measurements. These values were reported for the entire cohort. The near VAs were missing for two subjects aged 20, and two subjects aged 25. The distance decimal VAs for these subjects were 0.8, 0.67, 1.00 and 1.00, respectively. Given the fact that the subjects are all pre-presbyopic, their near VA is also expected to be within normal limits. This assumption is further supported by the fact that their stereo-thresholds were lower than 70″ (the mean stereopsis of these four participants: 44″ ± 21″). Stereopsis requires similar values of good visual acuity in the two eyes.67 Furthermore, in IPD measurements, subjects are asked to either view the examiner’s eye (manual) or a circular large target (automatic). These large visual targets do not require very precise visual acuity. As such, we can assume that the missing outcome measures do not affect our conclusions.

The distribution of IPD values in our study was found not to be normally distributed based on statistical tests. We would have expected a normal distribution of IPD values in a wide range of ages. Given the age range in our cohort (ages 19−46 for the females and 19−53 for the males), in which the IPD is expected to stabilize without much change59,60 our results are not surprising. These findings are also consistent with other studies reporting a non-normal distribution of IPD.57 Based on the non-normal distribution, the agreement of the two IPD measurement methods was assessed using the Bland and Altman analysis of non- normally distributed data.46 This analysis includes the application of a clinical range of resolution as opposed to the calculated 95% limits of agreement. In our data, the clinical range of resolution was smaller than the calculated 95% limits of agreement for all comparisons. As a result, a smaller proportion of observations were found to fall within the resolution limits. Additionally, Bland and Altman consider two measurement methods interchangeable if only three observations fall outside the limits of agreement.45 This was not the case for all conditions (monocular RE and LE and binocular distance and near IPD). We state this as a potential limitation though based on the fact that a vast majority of the observations were well within the clinical resolution limit, we concluded that the measurements were interchangeable.

Another potential limitation is the fact that the Essilor Pupillon pupilometer does not provide the individual values of the four measurements and only displays the mean of the consecutive measurements. This does not allow examination of the intra-test repeatability of the pupillometer. Furthermore, if three consecutive measurements are to be taken by the experimenter in order to assess the intra-test repeatability, in reality 12 measurements are taken. This would have potentially introduced a bias in the results (12 measurements in one instrument compared to four with the ruler). In addition, the four consecutive pupillometer measurements are time consuming, especially relative to ruler measurements, thereby affecting patient cooperation and the measurements.