Near-point Convergence (NPC) is the nearest point at which the individual can maintain the clear Binocular Single Vision (BSV). The primary purpose is to measure the convergence amplitude. It is the most common measure which is assessed by Optometrists to diagnose Convergence Insufficiency (CI); however, it is not the only measure upon which the diagnosis of CI depends upon.

As Scheiman and Wick (1) cited, a remote NPC is the most common criterion used by optometrists to diagnose CI. Additionally, corroborative data from other accommodative and convergence parameters are necessary to confirm the diagnosis of CI.

Measurement of NPC:

Traditionally, the measurement of NPC is done in a metric system (cm) and is recorded as Break (a point at which the individual no longer can fuse the target using his/her convergence) and Recovery (a point at which the individual regains the fixation at the target). It is performed by moving the target close to the patient until the point where he/she appreciates a double vision (subjective measurement) / the examiner spots one of the eyes losing fixation (objective measurement). The target is then moved slowly away from the patient until he/she reports a single image or when the examiner sees the refixation of the deviated eye. A crucial aspect is to instruct the patient to keep the target clear all the time.

Targets used for measuring NPC.

Let us now discuss the targets used to measure the NPC. Targets that we can choose to measure NPC are Accommodative Target (AT), Red Lens (RL), and Penlight Red Green Light (PLRG). Scheiman and Wick suggest using an AT in a routine examination and repeat the measurement with PLRG.

In a paper published by (2), he cites an anecdotal report which commented on using  PL target to diagnose the condition of CI, the reason to which was given that the dissociation created by the red lens in front of the right eye makes the test more sensitive to detect subtle convergence anomalies. Not only that, but Capobianco also further suggests that the PL target can be used to monitor the progression in the treatment of CI. The paper further cited other authors, which discussed using different targets for measuring NPC; however, it states a lack of agreement on one target. For this purpose, the author demonstrated and addressed the question of using an appropriate target.

The study compared the Normal group with the CI group in which they had measured the NPC using each of the targets. They found positively skewed measurements for normal subjects with a cumulative percentage of > 90% of subjects having NPC measurements in the range of 0.5 – 3cm. In comparison, the CI group had relatively distributed data across the number line. The additional modification that they did was repeating the measurement ten times in each group. This was based on an assumption/theory that in a CI suspected case/symptomatic cases, the NPC recedes when the measurement is repeated more than thrice, as cited by (2). Moreover, as expected, the mean Break of NPC in CI patients was 12.1cm on the first measurement, which receded to 15.7cm after ten repetitions, similarly for recovery values and measurement of 17.7cm receded to 21.9cm after ten repetitions.

The study agreed with the cut-off value of >6cm suggested by Hayes et al. for the clinical diagnosis of CI. As a principle, we know that the NPC involves, patient’s ability to use accommodative

convergence, proximal convergence, and fusional convergence; for this reason, an accommodative target provides the most precise measurement as it exerts excessive accommodative demand. Thus, the study suggests, a clinical diagnosis can be made using any of these targets, but the AT attains the precision.  Furthermore, the difference between AT and PL was minimal clinically (<1cm) but was statistically significant. The authors also speculate that due to excessive accommodative demand exerted by the AT, it may overestimate the data, whereas if we look at the PLRG target (the difference that the authors found to be maximum in relation to AT), not only dissociates the eyes by the red and green lenses but also reduces the accommodative efforts thereby making the target sensitive towards more subtle changes in symptomatic and CI patients.

Thus, the results suggest using an accommodative target in a routine clinical examination and repeating the measurements using a PLRG target in symptomatic or CI suspect patients to diagnose accurately. Another study done by (3) evaluated different targets to be used in the measurement of NPC and found that the CI patients exert excessive accommodative convergence while using an AT (which may falsely give normal NPC measurements) as compared to the PLRG target, which in turn was in agreement with the results of (2).

The authors also measured the sensitivity and specificity of all the targets and compared them; the figure above demonstrates the descriptive statistics for the same. Furthermore, as we can observe from the table, with a 95% confidence interval, the sensitivity was highest for the PL target as compared to other targets, and false negative was also found to be lowest; based on which they suggested that the RL target is more reliable than the AT.

So, the results that we discussed so far are based on the data collected from the studies done in the USA; thus, a question arises whether these facts hold for the Indian population. Let us briefly look over it as well. A population-based cross-sectional study (BAND) was done by (4)  on the Indian population. They had screened and assessed 920 normal subjects and obtained normative data for all the binocular vision tests. Talking about the NPC, the BAND study found a breakpoint at 3 ± 3cm with an AT and 7±5cm with a PL target, which agreed with the data we saw previously, as published by Scheiman.

Thus, corroborative, and reliable evidence shows that the PL/PLRG target to measure NPC is more reliable to diagnose CI clinically.

Reliability of Different Methods to Measure the Phoria: An Evidence-Based Discussion.

Heterophoria is the hidden or latent deviations of the visual axes of the eyes, which are manifested in the absence of fusion or when the fusion is deliberately broken to assess them. The phoria majorly depends upon tonic convergence, proximal convergence, and accommodative convergence. (5). He explains Esophoria as visual axes intersecting closer to the object of interest, Exophoria as visual axes intersecting beyond the object of interest, and Orthophoria as visual axes intersecting exactly at the object of interest. Assessment of heterophoria is an essential aspect in the clinical diagnosis of conditions like CI. As discussed earlier in this discussion, other than NPC, other parameters are essential to make a clinical diagnosis. The determination of heterophoria is one of the most critical parameters.

Heterophoria is objectively assessed using the Cover Test (CT). The examiner observes the eye movement; further, using a Prism Bar, the examiner neutralizes the eye movement he/she observes. Since exophoria manifests as an outward deviation (pointing to the need for positive fusional vergence to compensate), Base-In prisms are introduced in front of the eye (since phoria is a binocular phenomenon, the prisms in front of either eye has no difference on the measurements). Other subjective methods such as Von Graefe Method (VG), Modified Thorington Method (MT), Maddox Rod method (MR) have been explained in detail by (1). Here, we will be discussing the reliability of these tests.

However, before we discuss the reliability of these tests, let us briefly look at whether the room illumination/testing conditions affect the phoria measurement. In the same paper, Schroeder cites Scobee and Green., 1947 who had compared the MR test done in the dark and illuminated room and found that the amount of illumination had no significant effect on the measurement of phoria with MR. Also, it did not matter whether the Maddox was placed in front of the dominant or non-dominant eye.

The paper also cites Hirsch and Bing, who had compared different tests, including VG and MT, for test-retest repeatability and found that MT yielded repeatable results compared to VG with a correlation of coefficient 0.94 and 0.85, respectively. Furthermore, since VG involves using prisms whereas MT does not, the authors compared both methods based on yielding repeatable results as assessed by the instructor and then by a 4th-year Optometry student/intern that the coefficient of reliability for MT was 0.94. In contrast, for VG, it was found to be 0.84 thereby suggesting that MT has reliable results and is easy to perform compared to VG. So, the paper suggests that both methods can yield reliable and repeatable results.

However, because MT is easy to perform, it can be reliably used to diagnose the case of CI (for example) clinically. Scobee and Green had also discussed that the heterophoria measured using MR lacks control of accommodation and thus near heterophoria measurement may be influenced. Schroeder had found better results using VG and MT methods as compared to MR.

The information we discussed so far was investigated more than 40 years ago, so let us travel ahead in time were (1) put forward the normative data for distance and near as measured using MT method (Distance: 1±2 exophoria; Near 3±3 exophoria).

The BAND study that we discussed earlier also had assessed the phoria using the MT method and agreed upon the fact that MT is more reliable than the other tests. However, the range of phoria in the Indian population ranges from orthophoria at a distance to exophoria near (Distance: 0.02±1 esophoria; Near: 0.4±2 exophoria) and less exophoria than that reported by Morgan, as Scheiman and Wick cites. Furthermore, another study was done by (6) to determine the minimum battery of tests required to diagnose the Non-strabismic Binocular vision anomalies. The paper suggested a minimum battery of tests such as determining NPC with a PL/PLRG target, phoria for distance and near using the MT method, and monocular accommodative facility. The limitation it possessed was an extrapolation of the findings to diagnose all the Non-Strabismic Binocular Vision Anomalies.

Additionally, (7) did an inter-examiner and intra-examiner repeatability and compared the reliability of four methods, namely, Prism bar alternating Cover test, Von Graefe Method, Maddox Rod method, Howell Card, and found that the minor variability in the results was yielded by the Howell card method. Howell card is based on the optical principle of the MT method. It has been reported to have reliable results in the literature. The study, however, did not compare the results of the Howell Card with the MT method. Furthermore, they also speculate a learning effect while administering the Howell Card method on the participants since two examiners did the tests on the same visit. A very important concern was addressed in this paper which is discussed further in this assignment.

Now that we have seen the most reliable test to assess and determine the magnitude of phoria let us know a little more about it. We discussed earlier in this phoria discussion that the testing condition in terms of illumination has no significant difference in the measurements. Let us address recent concerns, i.e., whether using a phoropter or trial frame would affect the measurements of phoria.  A study done by (8) compared MR, VG, MT methods done using a Trial frame with the phoropter for both distance and near. The participants were 60 visually normal with no manifesting ocular disease, strabismic or non-strabismic binocular vision anomaly. In terms of repeatability of the results yielded by each method, VG showed the poorest repeatability, whether done using a trial frame or phoropter. Best repeatability was found to be with the MT method for both distance and near conjunction with the trial frame.

Moreover, this paper contradicts the findings of Scobee and Green that said, MR when used for near heterophoria measurement, lacks control of accommodation and suggested not being used to measure the near phoria. In their paper, Casillas & Rosenfield (8) found better repeatability with the MR method when used in conjunction with a trial frame. Thus, they recommend using MR to determine heterophoria at near with a piece of valid evidence based on their findings. Overall, the authors suggested using a trial frame to measure the heterophoria.

References:

1. Mitchell Scheiman BW’. Clincial Management of Binocular Vision. Fourth. Wolters Kluwer, LIPPINCOTT WILLIAMS & WILKINS; 2014. 705 p.
2. Scheiman M. Nearpoint of convergence: Test procedure, target selection, and normative data. Optom Vis Sci. 2003;80(8):555.
3. Pang Y, Gabriel H, Frantz KA, Saeed F. A prospective study of different test targets for the near point of convergence. Ophthalmic Physiol Opt. 2010;30(3):298–303.
4. Hussaindeen JR, Rakshit A, Singh NK, Swaminathan M, George R, Kapur S, et al. Binocular vision anomalies and normative data (BAND) in Tamil Nadu: report 1. Clin Exp Optom. 2017;100(3):278–84.
5. Schroeder T et al. Reliability of and Comparison Among Methods of Measuring Dissociated Phoria. Optom Vis Sci. 1996;73(6):389–97.
6. Hussaindeen JR, Rakshit A, Singh NK, Swaminathan M, George R, Kapur S, et al. The minimum test battery to screen for binocular vision anomalies: report 3 of the BAND study. Clin Exp Optom. 2018;101(2):281–7.
7. Anstice NS, Davidson B, Field B, Mathan J, Collins A V, Black JM. The repeatability and reproducibility of four techniques for measuring horizontal heterophoria : Implications for clinical practice. J Optom [Internet]. 2020; Available from: https://doi.org/10.1016/j.optom.2020.05.005
8. Casillas EC, Rosenfield M. Comparison of Subjective Heterophoria Testing With a Phoropter and Trial Frame. 2006;83(4):237–41.

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Information Compiled by: Prathamesh H. Shasane | B. Optom | M. Optom 1^st year student.

Affiliation: Department of Optometry, Manipal College of Health Professions, Manipal Academy of Higher Education

E-mail ID: pshasane@ools.co.in