(a) Amplitude versus facility Melville and Firth (30) investigated the relationship between positive fusional vergence ranges and vergence facility. They found no correlation between these values and suggest that this indicates that the two tests assess different aspects of the vergence system. A more recent study by McDaniel and Fogt (31) also found a lack of correlation between the two test findings and concluded that patients with vision-related asthenopic symptoms who have normal compensating disparity vergence ranges should undergo vergence facility testing. Because it is possible to have normal fusional vergence amplitudes and vergence facility problems, both aspects should be evaluated with a symptomatic patient. We suggest using vergence facility testing when a patient presents with symptoms characteristic of a binocular disorder and other testing does not reveal any problems. Such a patient may have normal fusional vergence amplitudes but reduced facility.

(b) Strength of prism to use and target to use Until fairly recently, there had been a lack of systematically gathered normative data and little consensus in literature about the strength of the prism that should be used for this test. Buzzelli (4) recommended the use of 16 base-out and 4 base-in. Another common recommendation (3) was 8 base-out and 8 base-in. Gall et al. (7) performed the first systematic study of vergence facility and found that the magnitude of choice is 3Δ base-in/12 Δ base-out. This combination of prisms yielded the highest significance for separating symptomatic from nonsymptomatic subjects. They also found that this combination of prisms produced repeatable results (R=0.85) when used for near vergence facility testing.

In another study, Gall et al. (8) compared the use of three different vertically oriented targets for vergence facility testing. The targets tested were a vertical column of 20/30 letters, a back-illuminated anaglyphic target, and the Wirt circles oriented vertically. The study was designed to determine whether it is important to use a target with a suppression control for vergence facility testing. They found that vergence facility is nearly independent of the target and that a simple vertical row of 20/30 letters is an appropriate target.

3. Expected values Based on the work of Gall et al. (7), the expected finding for vergence facility, using values of 3 Δ base-in/12 Δ base-out, is 15 cpm at near (Table 1.4).

1. Purpose The purpose of the NPC is to assess the convergence amplitude. A remote NPC was found to be the most frequently used criterion by optometrists for diagnosing convergence insufficiency (32).

(a) Target to be used Different targets have been suggested for NPC testing. Recommendations vary, including an accommodative target, a light, a light with a red glass before one eye, and a light with red/green glasses. Some suggest that a variety of targets should be used to determine whether there are differences with various targets. We recommend repeating the NPC twice—first using an accommodative target and then using a transilluminator or penlight with red/green glasses.

(b) Does repetition yield additional useful clinical data? The NPC test traditionally is performed by slowly moving a target toward the eyes until the patient reports diplopia or the examiner notices a break in fusion (33). This is recorded as the breakpoint. The target is then slowly moved away from the patient until fusion is reported or the examiner notices realignment of the eyes, signaling recovery of fusion.

Several modifications to this traditional approach have been suggested in the literature to make the test more sensitive. Wick (22) and Mohindra and Molinari (34) recommend that the NPC test be repeated four to five times. Their suggestions are based on the claim of Davies (35) that asymptomatic patients manifest little change in the near point with repeated testing, whereas symptomatic patients have significantly less convergence with repeated testing. Thus, this recommendation is designed to improve the diagnostic sensitivity of the break of the NPC test. Scheiman et al. (11) found a recession of the NPC after repetition in both normal subjects and convergence insufficiency patients. In the subjects with normal binocular vision, however, the amount of recession was small, less than 1 cm. In the convergence insufficiency group, the amount of recession was 1.5 cm after 5 repetitions and about 4 cm after 10 repetitions (11). These findings suggest that the NPC test would have to be repeated about 10 times to yield useful clinical information. Maples and Hoenes (36) also investigated the changes in the NPC after repetition and found that the NPC break and recovery do not change appreciably with multiple repetitions of the test.

(c) Does the use of the red glass or red/green glasses yield any additional useful clinical data? Another criterion utilized for assessment of convergence ability is the recovery point, or the point at which an individual regains fusion (after fusion has been lost) during the push-up convergence testing. Capobianco (37) reported that a recovery point greatly different from the break indicates greater convergence problems. She also suggested repeating the test with a red glass before one eye. She stated that greater recession with the red glass suggests a more significant convergence problem. Several authors (22,34,38,39) have suggested that this procedure be part of the standard assessment of convergence amplitude.

Scheiman et al. (11) found a statistically significant difference between the break and recovery with an accommodative target and the results with a penlight and red/green glasses in patients with convergence insufficiency. For convergence insufficiency subjects, the mean break with an accommodative target was 9.3 cm and, with a penlight and red/green glasses, the mean break was 14.8 cm. The recovery finding with the accommodative target was 12.2 cm, and with a penlight and red/green glasses it was 17.6 cm. For both the break and recovery, therefore, there was a difference of about 5.5 cm between the accommodative target and penlight and red/green glasses. Statistically significant differences were not found for an accommodative target compared to a penlight or a penlight compared to a penlight and red/green glasses.

In the subjects with normal binocular vision, there were no significant differences for any of the conditions just described. The mean break was between 2.4 cm and 2.9 cm, and the mean recovery was between 4.2 cm and 5 cm.

(d) The value of assessing convergence ability using a jump convergence format Pickwell and Stephens (40) described another method of assessing convergence ability, which they termed jump convergence. In this procedure, the subject first fixates a target at 6 cm and then changes fixation to a target at 15 cm. Pickwell and Stephens (40) and Pickwell (41) reported that this jump convergence test appears to have more clinical significance and is a more sensitive way of determining the presence of convergence problems than the NPC. In the original study (41), the authors compared the effectiveness of the standard near point test (pursuit convergence) and the jump convergence procedure in a group of 74 subjects with inadequate convergence; 50 of the 74 showed normal pursuit convergence but reduced jump convergence. Only five subjects passing the jump convergence test failed the pursuit convergence procedure. The authors concluded that “this evidence clearly suggests that the jump convergence test is more likely to detect inadequacy of convergence than the measurement of the NPC.” In a second study, Pickwell and Hampshire (10) found that in a sample of 110 subjects with inadequate convergence, poor jump convergence was more frequently associated with symptoms than was poor pursuit convergence. One problem with the jump convergence test is the lack of expected values for this test. In their 2003 study, Scheiman et al. (11) found a mean of 30 cpm (standard deviation = 10) for subjects with normal binocular vision and 23 cpm (standard deviation = 11) for subjects with convergence insufficiency (11).

3. Expected values Although this test is commonly used to diagnose convergence insufficiency, there had been no normative data for children or adults until recently. Hayes et al. (42) studied 297 schoolchildren and recommended a clinical cutoff value of 6 cm. Maples and Hoenes (36) reported a similar value with a cutoff value of 5 cm. Scheiman et al. (11) studied an adult population and suggested that when using an accommodative target, a 5-cm cutoff value should be used for the break and a 7-cm cutoff value should be used for the recovery. Using a penlight and red/green glasses, the cutoff value for the break is 7 cm and that for the recovery 10 cm.