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Evaluation of factors influencing distance stereoacuity on Frisby–Davis Distance Test (FD2) in intermittent exotropia
  1. Rohit Saxena,
  2. Ashish Kakkar,
  3. Vimla Menon,
  4. Pradeep Sharma,
  5. Swati Phuljhele
  1. Squint and Neuro-Ophthalmology Services, Dr Rajendra Prasad Centre for Ophthalmic Sciences, All India Institute of Medical Sciences, New Delhi, India
  1. Correspondence to Dr Rohit Saxena, Room no 485, Dr Rajendra Prasad Centre for Ophthalmic Sciences, All India Institute of Medical Sciences, New Delhi 110029, India; rohitsaxena80{at}yahoo.com

Abstract

Aim To evaluate the factors affecting the restoration of stereoacuity after surgery in cases of intermittent exotropia.

Methods Thirty consecutive patients of intermittent exotropia X (T) scheduled for surgical intervention underwent preoperative evaluation, including complete ophthalmic and orthoptic examination. Deviation was measured for near and distance. Near stereo acuity was measured by The Netherland Organization stereotest (TNO), and distance stereo acuity was measured using the Frisby–Davis Distance (FD2) stereotest at 6 m for all cases and age-matched controls. All cases of X (T) were followed postoperatively at 1 week, 1 month, 3 months and 6 months.

Results Successful surgical alignment, defined as alignment within eight prism dioptres of exophoria, was seen in 84% of cases. The median distance stereo acuity improved from preoperative value of 50 s of arc to 17.5 s of arc, and near stereoacuity improved from 240 s of arc to 90 s of arc at 6 months postoperatively. The median distance and near stereo acuity in controls were 15 and 60 s of arc respectively. There was a significant difference in both distance and near stereoacuity between controls and cases, even after surgery. A high grade of preoperative stereoacuity was found to be a significant factor in determining the achievement of normal stereoacuity postoperatively. Age and amount of pre- and postoperative deviation were not found to have any affect. None of the above-mentioned factors had any influence on successful postoperative surgical correction.

Conclusion There is a significant improvement in both near and distant stereoacuity postoperatively in X (T); however, the achievement of normal level depends upon the preoperative sensory status of the patient.

  • Distance stereoacuity
  • FD2
  • intermittent exotropia
  • muscles
  • diagnostic tests/investigation
  • treatment surgery
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Introduction

Intermittent exotropia (X (T)) is one of the most common types of strabismus with prevalence of 1% in the general population.1 The natural history of the disease may show progression; however, some patients do remain stable or may even show improvement.2–4 The surgical success rates in these patients vary depending upon the various surgical options,5–7 and it has been reported that intervention in the form of surgery may help restore binocularity.8–10 Although the relationship between the distance stereoacuity and the fusional control is controversial, some studies have shown that decreased stereoacuity corresponds with the progression of the disease.11 12 The aim of this study was to evaluate the effect of surgery on distance and near stereo acuity in cases of intermittent exotropia and to assess the factors affecting achievement of normal levels of stereoacuity after surgery.

Subjects and methods

This study was a prospective interventional institutional based clinical study approved by the Institutional Review Board of our hospital. Thirty consecutive patients with intermittent exotropia who were scheduled for surgical intervention were enrolled after providing written informed consent. Patients suffering from amblyopia (defined as a difference of 2 or more lines in Snellen visual acuity between the two eyes or vision less than 6/18 in both eyes) or anisometropia of more than 2 dioptres were excluded from the study. Similarly, patients with a significant oblique muscle over action, with significant A-V patterns or previous history of squint surgery were excluded from the study. The age-matched control subjects were included in the study to obtain the normal threshold values for the distance and the near stereoacuity. Controls comprised asymptomatic subjects with at least 20/25 best corrected vision in either eye, exophoria <10 prism dioptre (PD), absence of esophoria or significant (>1 dioptre) anisometropia.

Preoperative evaluation included the complete ophthalmic and orthoptic examination including cycloplegic refraction and optimal correction of the refractive error. The angle of deviation was measured with the Prism Bar Cover Test (PBCT) for both near (33 cm) and distance (6 m). The PBCT was done with spectacle correction in patients who were using glasses. A patch test (occlusion for 6 h) was used whenever there was any disparity between the near and distance deviation to rule out pseudodivergence excess while measurement was repeated with the addition of +3 dioptre in cases with a high AC/A ratio. Near stereo acuity was measured by The Netherlands Organization (TNO, Clement Clarke, Harlow, UK) for an applied scientific research test with subjects wearing red–green spectacles, and distance stereo acuity was measured using the Frisby–Davis Distance (FD2, Stereotest, Sheffield, UK)13 stereotest at 6 m using the protocol provided with the test. Readings were also recorded monocularly to rule out the possibility of a subject using monocular cues. Threshold stereo acuity was recorded in seconds of arc, which was the smallest disparity for which the observer makes at least two out of three correct choices. Fusional amplitude for both convergence and divergence was measured at distance (6 m) and near fixation (33 cm) with a prism bar. Perception of diplopia was considered as the breaking point. All cases of X (T) were followed postoperatively at 1 week, 1 month, 3 months and 6 months for ocular deviation (PBCT for distance and near), fusional vergences for distance and near, near stereopsis using TNO and distance stereopsis using the FD2 stereotest.

Surgical intervention

All cases were operated by a single surgeon. Fifteen cases (50%) underwent unilateral recession-resection surgery, seven cases (23%) underwent unilateral lateral rectus recession surgery, and eight cases (27%) underwent bilateral lateral rectus recession surgery. No postoperative overcorrection was planned while deciding on the amount of surgery, and we were not expecting any significant postoperative esotropia. Surgical success was defined as postoperative alignment within 8 PD of exophoria at 6 months postoperatively.

Statistical analysis

For statistical purposes, nil stereoacuity was assumed as 400 and 960 for distance and near, respectively, and median values were used for comparison. Preoperative and postoperative parameters at 6 months were compared using the Wilcoxon signed-rank test. The Wilcoxon rank-sum (Mann–Whitney) test was used to compare postoperative stereoacuity at 6 months for both near and distance between the cases and control group. The Spearman correlation coefficient was used to find any relationship between the fusional vergence and the stereoacuity, as well as between the near and distance stereoacuity.

Results

The mean age of cases was 13.7±7 years compared with controls, which was 16.8±8 years (p=NS). There were 40% of males in cases (12 males and 18 females) and 50% of males (17 males and 17 females) in the control group.

Distance stereoacuity

The median distance stereo acuity improved from the preoperative value of 50 s of arc to 30 s of arc at 1 week postoperatively (p=0.001), which improved further to 20 s of arc at 1 month (p=0.001) and 17.5 s of arc at 3 months (p=0.02) postoperatively. Thereafter it remained stable at 17.5 s of arc till 6 months of follow-up (figure 1). The median distance stereo acuity in control group was 15 s of arc. Table 1 shows the comparison of stereoacuity between the cases and the controls. Although there was improvement in the stereoacuity after the surgery, there was significant difference in distance stereo acuity between cases and controls even at 6 months postoperatively (p=0.017).

Figure 1

Graph showing the trend in improvement in distance stereoacuity in cases after the surgery and its comparison with the controls. Note that although there is a significant improvement in the distance stereoacuity in X (T) after the surgery, the difference between the cases and control group was significant.

Table 1

Comparison of distance and near stereoacuity in cases and control

Five out of 30 patients did not show any improvement after surgery out of which 2 patients already had normal stereoacuity preoperatively, thus only 3 out of 30 patients did not show any improvement in distance stereoacuity on FD2 with 15/30 (50%) patients achieving normal stereoacuity postoperatively. Table 2 shows comparison of various parameters between those improved to normal level as compared with cases those who gained subnormal stereoacuity. There was significant difference in the preoperative near and distance stereoacuity between the two groups and no difference was found in age and amount of pre and postoperative deviation.

Table 2

Characteristics of cases those who improved to normal level on Frisby–Davis Distance stereotest versus those who did not gain normal distance stereoacuity

Near Stereoacuity

The median near stereo acuity improved from preoperative value of 240 s of arc to 120 s of arc at 1 week postoperatively (p=0.003). It improved further to 90 s of arc at 1 month postoperatively, though the change was not significant and remained stable at 90 s of arc until 6 months of follow-up (figure 2). The median near stereo acuity in the controls was 60 s of arc. The difference between the control and the cases was significant, even at 6 months of follow-up (p=0.001). Only 20% (6/30) of patients showed normal stereoacuity preoperatively, which increased to 46.6% (14/30) postoperatively. Table 3 compares the characteristics of those patients who gained normal near stereoacuity versus those who did not gain normal stereoacuity. Similar to the results for the distance stereoacuity, there was a significant difference in the preoperative near and distance stereoacuity in those patients who achieved normal near stereoacuity versus those who had subnormal stereoacuity, while age and amount of pre- and postoperative deviation were not found to have any affect. There was a moderate correlation between near and distance stereoacuity preoperatively (Spearman coefficient=0.5, p=0.03) and postoperatively (Spearman coefficient=0.5, p=0.05) in X (T) cases.

Figure 2

Graph showing trend in improvement of near stereoacuity in cases after the surgery and its comparison with controls. Note that although there is a significant improvement in the near stereoacuity in X (T) after the surgery, the difference between the cases and control group was significant.

Table 3

Characteristics of cases those who improved to normal level on The Netherlands Organization versus those who did not gain normal near stereoacuity

Surgical correction

All the patients included in the study were of the basic type. The mean preoperative exodeviation for distance fixation was 33.9±9.8 (median 30) PD, which significantly improved to the mean of 5.8±4.2 (median 4) PD at 1 week and 6.4±4.9 (median 6) PD at 6 months postoperatively. None of the patients had postoperative esotropia at 1 week. Five out of 30 patients (16%) had residual exotropia of more than 8 PD at 6 months of follow-up. We compared the characteristics of patients with successful surgical outcome and patients with unsuccessful surgical outcome (table 4). There was no significant difference between the two subgroups in any of the parameters compared.

Table 4

Comparison of various parameters between surgically successful cases and surgically failed cases

Fusional vergence evaluation

There was a poor correlation between fusional vergences and stereo acuity preoperatively (Spearman coefficient=0.3, p=0.05) and postoperatively (Spearman coefficient=0.3, p=0.06) in X (T) cases.

Discussion

Distance stereoacuity has been recommended by some studies as an objective method for assessing the progression of the disease in cases of intermittent exotropia.11 12 Few studies in the literature have assessed the pre- and postoperative change in distance stereoacuity.12 14 15 FD2 is a real depth test and has been validated for measuring distance stereopsis in normal as well as strabismic patients.16–18 In the present study, we prospectively evaluated the sensory outcome in terms of improvement in distance stereoacuity on FD2 in cases of intermittent exotropia and assessed factors affecting achievement of normal stereoacuity after surgery.

The baseline median near and distance stereo acuity in X (T) cases was significantly worse than that in the control group. The deterioration in near stereopsis appears to be more than that in the distance stereopsis possibly because of the difference in the basic principle of the two tests used for measuring them. FD2 is a real depth test and allows the eyes to fuse for distance, even when the control is poor,19 while TNO measures stereopsis by dissociating the two eyes. Moreover, the minimum threshold for measuring stereopsis with TNO is 40 s of arc, while that for FD2 is 10 s of arc, and therefore the median value for TNO is greater. Nonetheless, the level of near stereopsis in our study is poor and is similar to that in a previous study in similar settings.14 This could be due to the fact that the patients in our setup present late, only when they become symptomatic and the deviation becomes constant. Postoperatively, there was a significant improvement in both distance and near stereoacuity, as has been documented by previous studies, though differences in stereoacuity levels between the controls and cases persisted even at 6 months postoperatively.12 14 20 A trend analysis showed that postoperatively the improvement in near stereoacuity stabilises at 1 week, while distance stereoacuity continues to show improvement until 3 months. What normally is more resistant to loss logically would recover first and vice versa, and thus near stereopsis, which is maintained until the later stage, is earlier to recover than distance stereoacuity. A moderate correlation was found between distance and near stereoacuity, both preoperatively and postoperatively, similar to a previous report.14

Fifty per cent (15/30) and 46% (14/30) of patients achieved normal distance and near stereoacuity respectively after surgery. We tried to analyse the factors that could result in successful sensory outcomes in cases of intermittent exotropia and compared the patients who improved to the normal level with those who showed a subnormal improvement on FD2. There was a statistically significant difference in the preoperative distance and near stereoacuity between the above two groups (table 2). A similar analysis for near stereoacuity showed comparable results (table 3). This implies that the greater the loss in stereoacuity, the more difficult it is to restore it to normal levels. Therefore, it may be important to intervene before a significant loss of stereoacuity occurs or central suppression develops, which has been reported to prevent restoration of normal stereoacuity, even after successful surgical alignment.20 Unfortunately, we were unable to obtain a cut-off value on either FD2 or TNO, which can allow us to set a measure of when to intervene. No differences were found in age, preoperative deviation and postoperative deviation between the groups, suggesting that the preoperative amount of deviation may not influence the postoperative sensory outcome in terms of stereopsis; and neither does the successful postoperative alignment (surgical outcome) guarantee full recovery of stereoacuity.

We also tried to find out if the preoperative stereopsis status can influence the postoperative motor outcomes. In this study, we did not find any significant difference in preoperative distance and near stereoacuity between the patients with successful as compared with failed outcomes. This is similar to a report by Yildirim et al,20 where they have stated that preoperative central fusion or preoperative near and distance stereopsis was not predictive of surgical outcome in motor terms. The dissociation between the preoperative sensory and motor status in a case of intermittent exotropia is further validated by the fact that no correlation was found between the fusional vergence and stereoacuity in the present study or previously.14

The stereoacuity in X (T) is shown to be variable, and one must be sceptical in interpreting any minor change in stereoacuity.21 22 One of the limitations of this study is that we did not consider the variability in stereoacuity, and hence its influence on the result cannot be stipulated.

Studies have suggested the presence of coexisting monofixation in X (T), especially those which are diagnosed at infancy.23–25 These patients continue to have monofixation postoperatively and have a poorer outcome compared with the patients with pure X (T). Although all of our patients were older than 6 years of age, we do not rule the possibility of its presence and influence on the postoperative subnormal stereoacuity in the patients with good alignment.

Conclusion

The loss of stereoacuity in intermittent exotropia is reversible, but the extent of recovery depends upon the preoperative sensory status of the patient. A good preoperative stereoacuity does not necessarily ensure successful surgical outcome; nor does the successful alignment ensure restoration of normal stereoacuity. The sensory and motor outcomes in cases of intermittent exotropia appear to be exclusive of each other.

References

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Footnotes

  • Competing interests None.

  • Ethics approval Ethics approval was provided by the Institutional review board, All India Institute of Medical Sciences, New Delhi, India.

  • Provenance and peer review Not commissioned; externally peer reviewed.

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