Clinical risk factors and management of consecutive strabismus

Material and Methods: The study was conducted at Pediatric Ophthalmology and Strabismus clinic at our tertiary care institute PGIMS, Rohtak. Patients who developed consecutive tropia after squint surgery for horizontal strabismus from April 2015 to August 2016 were enrolled. Continued esodeviation after surgery for exotropia was termed as consecutive esotropia and post-operative exodeviation present even 2 weeks after surgery for esotropia was termed as consecutive exotropia. The patients were followed up for at least 3 months. Ocular movements, preoperative deviation (PBCT), presence of vertical deviation, DVD, A-V patterns was noted, AC/A ratio, sensory status by Worth Four Dot Test and Titmus test. Immediate post-operative deviation was measured. Follow-up was done at 2 weeks, 6 weeks, 3 months and 6 months. Results: There were a total of 92 patients who underwent squint surgeries for horizontal strabismus. 12 patients developed consecutive squint. Out of 12 patients, 10 patients developed consecutive exotropia and 2 patients developed consecutive esotropia. The mean pre-operative deviation of resolved cases in both the groups was 45±8.16PD for near and 42.5±8.66PD for distance and the mean pre-operative deviation of unresolved cases in both the groups was 39.16±10.68PD for near and 38.33±6.83PD for distance. Discussion: In our study, the incidence of consecutive squint was 13%. Out of which 10.86% had consecutive exotropia and 2.17% had consecutive esotropia. The patients who developed consecutive squint had infantile esotropia, infantile exotropia, partially accommodative esotropia and nonaccommodative acquired esotropia. Amongst consecutive exotropias group, 40% cases resolved while in consecutive esotropia group, 50% resolved. Conclusion: Consecutive squint is a complication of squint surgery which can develop immediately after surgery or years later. Taking care of the errors during measurement of pre-operative deviation improves the success of surgical outcomes. © 2019 Published by Innovative Publication. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by/4.0/)


Introduction
Consecutive esotropia (ET) and exotropia (XT) are the types of manifest squint that develop after optical or surgical treatment for exotropia and esotropia respectively. The incidence of consecutive ET has been reported to be between 6 to 15% of patients operated for intermittent XT. [1][2][3] Consecutive XT is reported to be more common than consecutive ET, seen in 20-27% of patients operated for esotropia. 4,5 This study was planned to find out who are those patients who were at risk for development of consecutive esotropia and consecutive exotropia and to assess whether the risk factors reported in literature were applicable to Indian population coming from rural background or not. This study also tried to find any additional unreported risk or associated factor.

Material and Methods
The study was conducted at Pediatric Ophthalmology and Strabismus clinic at Regional Institute of Ophthalmology, PGIMS The mean pre-operative deviation was 41.5 ± 9.73PD for near and 40 ± 7.45PD for distance in consecutive exotropia group and 42.5 ± 3.53 PD for near and 32.5 ± 3.53PD for distance in consecutive esotropia group. For preoperative deviation, bilateral medial rectus recession was done in 3 (25%) cases, bilateral lateral rectus recession was done in 2 (16.66%) cases and recession with resection was done in 7 (58.33%) cases. 2 (16.66%) patients underwent bilateral superior rectus recession. Regarding treatment of consecutive tropias, alternating patching was advised in 6(50%) patients, dominant eye patching was advised in 5(41.6%) patients. 6(50%) patients were advised to wear minus lenses. 4(33.33%) patients were advised convergence exercises. Amongst consecutive exotropias group, 4 cases resolved while in consecutive esotropia group, 1 case resolved. The mean pre-operative deviation of resolved cases in both the groups was 45 ±8.16PD for near and 42.5±8.66PD for distance and the mean pre-operative deviation of unresolved cases in both the groups was 39.16±10.68PD for near and 38.33±6.83PD for distance.

Discussion
This study was planned to study the incidence of consecutive esotropia and exotropia and to report the risk factors associated with occurrence of consecutive squint at our hospital. The mean age of the patients at the time of the study was 8.33 years. Mean age at the onset of primary squint was 18.5 months and mean age at the time of surgery was 7.5 years. Our observation was similar to a study done by Ganesh et al 5 on 85 patients with consecutive exotropia, where the mean age at the onset of eotropia was 1.85 years while at the time of surgery it was 6 years. In a study by Kim et al, 6 mean age at the onset of exotropia was 2.91 years and mean age at the time of surgery was 4.8 years. The timing of surgery for primary squint has been a matter of debate. Pratt-Johnson et al reported that patients who got operated before 4 years of age, consecutive esotropia developed more frequently in them, although success rate was higher. 7 Keech and Stewart divided patients operated for intermittent exotropia into two groups (overcorrection with esodeviation ≥3 PD vs no overcorrection) at the last follow-up (at a 2 minimum of 6 months), and found that the overcorrection group had a two-fold higher mean age at surgery. 8 Dunlop observed that age at the time of surgery did not influence the extent of overcorrection. 9 Jampolsky recommended that operation at an early age should be avoided as performing surgery after the age of 7 years had many advantages including an ability to observe the progress of exotropia, precise diagnostic determination, accurate management of the angle of deviation, ability to train patients to conduct pre-and post-operative visual function exercises, a low risk for amblyopia, and suppression even if overcorrection had in fact occurred. 10 In our study, the incidence of consecutive squint was 13%. Out of which 10.86% had consecutive exotropia and 2.17% had consecutive esotropia. In literature, the incidence of consecutive exotropia has been reported to be 4-27%. 11,12 The incidence of consecutive esotropia after surgery for intermittent exotropia ranges from 6-15% depending upon the length of follow-up and measures to diagnose the esotropia. [1][2][3] In the present study, the patients who developed consecutive squint had infantile esotropia, infantile exotropia, partially accommodative esotropia and non-accommodative acquired esotropia mainly as primary diagnosis. More than half of the patients had pre-existing amblyopia. In a study by Paduca et al, 67.6% patients who developed consecutive exotropia had amblyopia as an associated factor. 13 Many studies have considered amblyopia as an important contributing factor in the development of consecutive exotropia. 14- 16 Kim et al reported that amblyopia was a statistically significant risk factor for consecutive esotropia in their study. 6 So, it is very important that we should treat amblyopia fully in children before taking them for surgery and the treatment should continue even after the surgery for residual amblyopia as good final visual acuity and fully developed binocular function help in post-operative stability of correction.
In the present study, most of the patients had one or the other refractive error like hyperopia ≤3D, hyperopia >3 D, myopia and astigmatism. In a study by Ganesh et al, 25% of patients with high hypermetropia developed consecutive exotropia. 5 Patients with hyperopia have been reported to develop consecutive exotropia even without surgery. 17,18 In our study, other associated factors were dissociative vertical deviation, vertical strabismus, pattern and oblique dysfunction. Jang et al studied factors predisposing to consecutive esotropia after surgery to correct intermittent exotropia. Significant correlations were found with high myopia, amblyopia, preoperative angle of deviation of 25-40 PD at distance, deviation at distance -deviation at near > 10 PD, lateral incomitance, tenacious proximal convergence fusion type, unilateral lateral rectus muscle recession and medial rectus muscle resection. 19 Some studies report medial rectus limitation 20 and the presence of an exodeviation in the upgaze and downgaze positions 21 as risk factors for consecutive exotropia after surgery for undercorrected esotropia. One should be careful while performing surgeries on such patients and should aim for undercorrection of their squint. Edelman et al 22 and Raab and Parks 23 found that high AC/A ratio increased the incidence of consecutive esotropia in their studies. Beneish and Flanders reported that good pre-operative stereopsis resulted in good surgical results. 24 In our study, the mean pre-operative deviation in consecutive exotropia group was 41.5D for near and 40PD for distance, while in consecutive esotropia group, it was 42.5PD for near and 32.5PD for distance. Abbasoglu et al found that pre-operative deviation was the only factor influencing response to surgery. The response was higher for larger preoperative deviations. 25 Gordon and Bachar found that response was determined by the magnitude of the larger pre-operative deviation and besides this average corrected visual acuity, the degree of anisometropia and the average spherical equivalent also influenced the surgical response. 26 Graf et al reported that pre -operative deviation did not influence the response to surgery in exotropic patients. 27 This lack of consensus may be due to variability in pre-operative deviation in exotropic patients. Error in measuring pre-operative deviation is also an important factor which plays a role in final outcome of surgery. It is advisable to treat overcorrection by non-surgical means upto a year. Deviation less than 20PD usually resolves with conservative methods. The success rates of surgical outcomes are 10-100%. 24,28 In our study, the only non-surgical measures were taken, which were alternating patching, dominant eye patching and convergence exercises. Amongst consecutive exotropias group, 40% cases resolved while in consecutive esotropia group, 50% cases resolved.

Conclusion
Consecutive squint is a complication of squint surgery which can develop immediately after surgery or years later. A little amount of post-operative overcorrection may resolve spontaneously, but if it doesn't resolve it can cause amblyopia in children and bothersome diplopia in adults. Younger age at surgery, amblyopia, divergence excess exotropia are reported as risk factors for consecutive esotropia, and DVD and amblyopia are proven risk factors for consecutive exotropia in many studies. Taking care of these risk factors and avoiding errors during measurement of pre-operative deviation improve the success and predictability of surgical outcomes.

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Conflict of interest
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