Sound Eye Versus Amblyopic Eye Surgery for Correction of Unilateral Sensory Strabismus

Med. J. Cairo Univ., Vol. 81, No. 2, September: 243-247, 2013 www.medicaljournalofcairouniversity.net Sound Eye Versus Amblyopic Eye Surgery for Correction of Unilateral Sensory Strabismus MOHAMED M.K. DIAB, M.D.* and KAMAL A.M. SOLAIMAN, M.D.** The Department of Ophthalmology, Faculty of Medicine, AM Shams* and Zagzig** Universities Abstract Purpose: Based on Herring's law of equal innervations to the yoke muscles, this study aims at comparing the anatomical outcome following sound eye surgery versus amblyopic eye surgery for correction of unilateral sensory strabismus in adult patients. Patients and Methods: A prospective study that included 148 adult patients with unilateral sensory strabismus (50 prism Diopters) and amblyopia. Patients were randomly allocated between two groups: included patients subjected to surgery in the sound eye only, and group B included patients subjected to surgery in amblyopic eye only. Patients were followed-up for at least 6 months to detect the anatomical success rate as well as any residual, consecutive or recurrent strabismus. Results: After a mean follow up period of 7.68±1.93 months in group A, 61 patients (82.4%) had orthotropia, two patients (2.7%) had residual strabismus, 8 patients (10.8%) showed consecutive overcorrection, and three patients (4.1%) had recurrent strabismus. In group B and after a mean followup period of 7.24±1.72 months, 49 patients (66.2%) had orthotropia, 3 patients (4.1%) had residual strabismus, 9 patients (12.2%) had consecutive overcorrection, and lastly thirteen patients (17.6%) had recurrent strabismus. The differences between the results of both groups were statistically significant (p<0.05) only in the patients who achieved orthotropia and those with recurrent strabismus. Conclusion: For unilateral sensory strabismus and amblyopia in adults, sound eye surgery could give a higher success rate with a lower chance for recurrence at six months, as compared to amblyopic eye surgery. Key Words: Sound eye Surgery Unilateral Sensory strabismus. Introduction SENSORY strabismus is a condition of unilateral ocular deviation as a sequel to sensory deprivation, loss of vision or long-standing poor vision in one Correspondence to: Dr. Mohamed M.K. Diab, The Department of Ophthalmology, Faculty of Medicine, Ain Shams eye. It may be caused by anisometropic refractive errors, unilateral aphakia, cataract, retinal detachment or other organic reasons. This unilateral or bilateral asymmetrical visual impairment destroys fusion, and thus disrupts the eyeball array 111. When visual loss occurs in childhood there is a tendency towards esotropia (ET) and when it occurs in adults the tendency is to exotropia (XT) [11. With respect to the causes of sensory strabismus, Sidikaro and von Noorden [2] reported that anisometropic amblyopia was the most common cause, followed by cataracts and corneal opacity. However, Havertape et al., [3] found the most frequent cause to be cataracts, followed by optic nerve disorder and then anisometropic amblyopia. The typical objectives of strabismus surgery are to achieve ocular parallelism in order to re-establish binocularity and stereopsis, and increase the binocular field of vision but in sensory strabismus these objectives may not be achieved. However, considering the negative psychosocial impact in those patients, sensory strabismus surgery should be considered as a reconstructive surgery and not only aesthetic [4]. Surgery on the amblyopic eye is the treatment of choice in almost all literatures, while sound eye surgery is an astonishing and a striking idea as many surgeons avoid operating on the sound eye and also many patients refuse this way of treatment. Sound eye surgery relies on Herring's law of equal innervations to the yolk muscles in both eyes; also called the law of motor correspondence of the eyes [5,6]. When an impulse for the performance of an eye movement is sent out, corresponding yoke muscles of each eye receive equal innervations to contract or relax. During version movements, equal innervation is sent to both eyes, telling them to rotate at the same speed, through the same angle, in the same direction. According to Hering's law, weakening the sound eye muscle will slow down its steps and induces more additional functional 243

244 Sound Eye Versus Amblyopic Eye Surgery for Correction symmetrical amount of nerve impulses to it and its yoke muscle in the other amblyopic eye, and so increases the innervations to the weak yoke muscle in the sensory squinted eye doing both mechanical and functional effects, resulting in a more effect than weakening that muscle in the squinted eye. In other words weakening the sound eye will slow down its steps and increase impulses to the squinting amblyopic eye to arrive together to orthotropia, decreasing angle of deviation and increasing the size of the field of binocular single vision. According to this idea we can treat sensory strabismus by creating an artificial iatrogenic weakening or strengthening of the yoke muscle in the sound eye [6,7]. Although severe amblyopia is considered to be a poor prognosis factor to obtain good motor and sensory results in the short and long-term [8], there are very few published studies on the results of sensory strabismus surgery [9]. The purpose of this study is to compare the anatomical outcome following sound eye surgery versus amblyopic eye surgery for correction of unilateral sensory strabismus in adults. Subjects and Methods This prospective interventional comparative study was conducted on 148 eyes of 148 patients with unilateral sensory horizontal strabismus and amblyopia, who attended the unit of pediatric ophthalmology and strabismus at Magrabi eye hospital in Aseer region, KSA from July 2010 to May 2013. Informed consent was obtained from all patients prior to enrollment in the study, and after explanation of the procedure to be done and the possible outcomes, as well as the need for repeating several examinations at each scheduled follow-up visit. Patients were equally and randomly distributed between two groups according to their order of presentation: included patients subjected to surgery in the sound eye only and group B included patients subjected to surgery in the amblyopic eye only. All surgeries were performed by the first author, using the same maneuver for muscle recession and resection. The inclusion criteria were adult patients with horizontal sensory strabismus <50 prism Diopters to be treated only by two muscle one eye surgery (recession and resection, unilateral amblyopia, and free ocular motility with at least 6 months follow-up after surgery. Degree of amblyopia was defined according to Dadeya [10] classification (Table 1). Table (1): Degree of amblyopia according to Dadeya [10] classification. Degree of amblyopia Verbal None Equal visual acuity (eg. 20/20, 20/30 both eyes) Mild Moderate Sever One to two lines acuity difference 3-5 lines acuity difference More than five lines acuity differences Exclusion criteria: Included patients who had previous squint surgery in one or both eyes, vertical strabismus, dissociated vertical deviation, intermittent exotropia and eyes with conventional scleral buckle surgery, cases of sensory strabismus with angle of deviation more than 50 prism diopters as it may in need to three muscle surgery. All patients had full preoperative ophthalmic examination especially for visual acuity, errors of refraction, cover-uncover and alternate cover tests, ocular motility, estimation of angle of strabismus for distance and near, with and without glasses by using the Krimsky test and cover-uncover test utilizing prisms. All patients in both groups had unilateral recession and/or resection surgery. Hangback technique was used for muscle recession. Surgical measurement and parameters were determined according to Wright's surgical formula 1111, Table (2). Table (2): Wright's [11] surgical formula. Esotropia MR LR Exotropia LR MR A recession resection A resection recession 15 3.0mm 4.0mm 15 4.0mm 3.0mm 20 3.5mm 5.0mm 20 5.0mm 4.0mm 25 4.0mm 6.0mm 25 6.0mm 4.5mm 30 4.5mm 7.0mm 30 6.5mm 5.0mm 35 5.0mm 8.0mm 35 7.0mm 5.5mm 40 5.5mm 9.0mm 40 7.5mm 6.0mm 45 5.5mm 9.5mm 45 8.0mm 6.5mm 50 6.0mm 10.0mm 50 9.0mm 7.0mm Postoperatively, all patients were examined at day 1, 1 week, 1 month and then monthly till the end of the follow-up period which extended for at least 6 months. At each postoperative visit, all eyes had a complete ophthalmic examination with stress on the surgical outcome by evaluating orthotropia or any residual, consecutive or recurrent strabismus using the same preoperative tests. Anatomical success was considered when eye deviation in the last postoperative visit was within 10 prisms Diopter of orthotropia.

Mohamed M.K. Diab & Kamal A.M. Solaiman 245 Statistical analyses were done and quantitative data were reported as Mean±Standard deviation using SPSS version 15 (SPSS Inc., Chicago, IL). Paired t-test was used to compare the mean angle of deviation before and after surgical correction within each group. Student's t-test was used to compare the mean preoperative and the mean postoperative angles of deviation between the two groups. p-value of 0.05 or less was considered significant. Results The study included 148 patients who were randomly allocated between group A (74 patients) and group B (74 patients) according their order of presentation. included 74 patients; 48 males and 26 females, with a mean age of 22.28±3.76 years. Esotropia was found in 33 patients while exotropia was found in 41 patients. The mean angle of deviation before surgery was 42.1±9.4 prism Diopters. included 74 patients; 42 males and 32 females, with a mean age of 21.76±4.27 years. Esotropia was found in 38 patients and exotropia in 36 patients. The mean angle of deviation before surgery was 40.6±8.6 prism Diopters. No statistically significant differences were found between both groups as regards the preoperative data. The causes of vision loss and amblyopia in both groups are showed in Table (3). The mean refractive errors (spherical equivalent) in patients are shown in Table (4), with no significant differences between both groups. Table (3): Causes of amblyopia in group A and group B. Causes of vision loss in the amblyopic eye Anisometropia 28 26 Corneal opacity 19 23 Rupture globe or previous ocular trauma 15 13 Pediatric cataract 10 8 Optic disc and macula disorders 2 4 Table (4): The mean refractive error (spherical equivalent) in patients of both groups. Refractive error (Spherical equivalent) 2.0 to +2.50D 49 49 Hyperopia 2.50D 8 8 Myopia 2.0D 17 17 *D = Diopter. Consecutive overcorrection occurred in 8 patients (10.8%) in group A and 9 patients (12.2%) in group B, and the difference between the two groups was not statistically significant (p>0.05), Table (5). Most cases of overcorrection in group A occurred between the earlier cases of the study; 6 cases within 1st 11 cases. Undercorrection was reported in 2 patients (2.7%) in group A and in 3 patients (4.1%) in group B (p>0.05), Table (5). Table (5): Anatomical outcome 6 months after surgery in group A and group B. Anatomical Outcome p-value Within 10 A of orthotropia 61 (82.4%) 49 (66.2%) <0.05 Overcorrection >10 A 8 (10.8%) 9 (12.2%) >0.05 Undercorrection >10 A 2 (2.7%) 3 (4.1%) >0.05 Recurrence 3 (4.1%) 13 (17.6%) < 0.05 Recurrence of strabismus was statistically significant (p<0.05) more in group B (17.6%) than in group A (4.1%). Recurrence was more likely to occur in patients with deep amblyopia. In group A, recurrence occurred in one case with moderate amblyopia and in 4 cases with deep amblyopia while in group B, recurrence occurred in 2 cases with mild amblyopia, 5 cases with moderate amblyopia and in 9 cases with sever amblyopia (Table 6). Table (6): Amblyopia and Recurrence rates for all grades. Degree of amblyopia Mild Moderate Deep (sever) Preop- Postoperative Preoperative Recurrence erative 19 31 24 1 2 17 29 28 Postoperative Recurrence Incomitance complication with some sort of abduction or adduction limitation in the operated eye occurred in four patients (5.6%) in group A and in two patients (2.8%) in group B (p>0.05). In group A, 3 patients with sensory esotropia had -1 degree restricted adduction and one patient with sensory exotropia had -1 degree limited abduction. In group B, two patients with sensory exotropia had -1 degree limited abduction. Postoperative diplopia occurred in 8 patients (10.8%) in group A and in 6 patients (8.1%) in group B (p>0.05). Narrowing of the palpebral fissure, enophthalmos or orbital cellulites was not reported in any case in the two groups. Discussion For surgical correction of amblyopic strabismus in adults, most surgeons prefer to operate on the amblyopic eye only when the angle of strabismus could be corrected by operating on one or two muscles. However, surgery on muscles of the sound eye only in such cases could be an astonishing alternative. This relies on Herring's law of equal simultaneous innervations for synergistic muscles in both eyes [5]. 1 3 9

246 Sound Eye Versus Amblyopic Eye Surgery for Correction The main objective of strabismus surgery is to achieve parallelism of eyes in order to establish or improve binocularity. However, in sensory strabismus the long-term maintenance of postsurgical alignment and the attainment of normal binocular vision are expected to be poor resulting in high recurrence rate of strabismus due to disruption of fusion and the low chance of regaining stable binocular vision [10]. According to Herring's law, surgery on the sound eye muscle(s) will affect the yolk muscle(s) in the deviating amblyopic eye in such a way that decreases the angle of deviation and maintains the postsurgical alignment. Edelman [12] reported that in unilateral sensory esotropia, recession of the medial rectus muscle in the sound eye will lead to its weakness. This will stimulate bilateral extra impulses to it and its yoke muscle in the amblyopic eye according to Herring's law, leading to an increase in the drive to the lateral rectus muscle of the amblyopic eye resulting in stronger abduction in the amblyopic eye and thus decreases the chance for recurrence. In this study and after 6 months of surgery, the success rate in the group of sound eye surgery (80.2%) is significantly higher than the success rate (66.2%) of amblyopic eye surgery. Also, the recurrence rate is significantly less in the group of sound eye surgery (4.1%) than in the group of amblyopic eye surgery (17.6%). Chun et al., [13] studied the effect of the unilateral recession/resection procedure and reported that the effects of nondominant eye surgery are mainly mechanical, whereas dominant eye surgery could achieve both mechanical and functional effects. On the other hand, Jeoung and associates [14] were unable to confirm the superiority of dominant eye surgery over amblyopic eye surgery. Incomitance complication with some sort of abduction or adduction limitation in the operated eyes occurred in four eyes (5.6%) in group A and in two eyes (2.8%) in group B. This matches with the results reported by Havertape et al., [3] in their study on patients with unilateral strabismus surgery. The surgical amount required for correction of the deviation angle is very difficult to predict in patients with sensory strabismus and lost fusion. In this study, surgical measurements were determined according to Wright's surgical formula 1111, Table (2). However, early in the study, a high incidence of postoperative overcorrection was reported in group A (6 overcorrection within the first 11 cases). So, recession and resection were performed by one mm less in the group of sound eye surgery to avoid overcorrection, and the result was successful (only 2 overcorrected cases within the remaining 63 cases). In group B, the 9 overcorrected cases occurred all over the whole study. This matches with the results reported by Kim et al., [15] but our cases were primary treatment on virgin muscles while their study was performed as a second session on previously operated eyes. Adjustable sutures were not used in this study because no significant difference had been detected by many studies between the adjustable and the non adjustable sutures in the treatment of sensory strabismus [16,17]. Some studies have studied the dose-effect relation between the millimeters of reoperation and the reduction of the deviation angle in the previously operated eye. However, these studies were performed on small patient numbers, and the dose-effect relationship was evaluated for the recession or advancement of only one rectus muscle and included only recurrent cases. Therefore, the results of these studies cannot be compared with the results of this study in which we used one or two muscle surgery and also our cases were primary sensory and not recurrent cases [9,18]. According to Dickmann et al., [19] Surgical treatment of strabismus in adults with long-standing childhood-onset constant strabismus can result in patients feeling that their quality of life has improved but this is not comparable with our study as our postoperative parameters depend on anatomical not psychological outcome according to degree of prism diopter improvement in deviation and not on patients feeling and their quality of life. Fraine [20] reported a success rate of 90.5% with amblyopic eye surgery in sensory strabismus, which is higher than the 66.2% reported in group B of our study. This might be because the success in our study was considered as 10 A within orthotropia while in their study the success was considered as 15 A within orthotropia. No significant difference was found between the two groups as regards postoperative diplopia. However, the postoperative diplopia reported in both groups of this study is slightly more than the postoperative diplopia reported in anther literatures [21]. This could be due to variation in the age of patients as well as the depth of amblyopia. Dawson et al., [22] have reported that the botulin toxin is a option to surgery for sensory strabismus with poor results only obtained 8% of good results with a single injection of Botulinum toxin whereas 20% of their cases required maintenance treatment with repeated botulinum toxin injections, and 43% required surgery after botulinum treatment but this results incomparable with our results as we not use this line of treatment.

Mohamed M.K. Diab & Kornai A.M. Solaiman 247 In conclusion, sound eye surgery should be kept in mind while making the surgical plane for unilateral sensory strabismus in adults, as it could give a higher success rate with a lower chance for recurrence at 6 months when compared to sole surgery on the amblyopic eye. Sound eye surgery in such cases using the standard formulae may result in overcorrection due to increase impulses to the operated muscle in the sound eye and its yoke muscle in the other amblyopic eye. So, reduction of the surgical measurement one millimeter in every muscle recession or resection is recommended to avoid postoperative overcorrection. References 1- VON NOORDEN G.K. and CAMPOS E.C.: Binocular vision and ocular motility: Theory and management. 6th ed. St. Louis: Mosby, pp. 345-347, 2002. 2- SIDIKARO Y. and VON NOORDEN G.K.: Observations in sensory heterotropia. J. Pediatr Ophthalmol. Strabismus, 19: 12-19, 1982. 3- HAVERTAPE S.A., CRUZ O.A. and CHU F.C.: Sensory strabismus: Eso or exo? J. Pediatr Ophthalmol. Strabismus, 38: 327-330, 2001. 4- YOON K.C., YOU I.C. and PARK Y.G.: Clinical analysis of sensory strabismus. J. Korean Ophthalmol. Soc., 43: 2483-2488, 2002. 5- KING W.M.: Binocular coordination of eye movements- Hering's Law of equal innervation or uniocular control? Eur. J. Neurosci. Jun., 33 (11): 2139-46, 2011. 6- A. TERRY BAHILL, KENNETH. J. GIFFREDA and LAWRENCE STARK: Dynamic and Static Violations of Hering's Law of equal enervations. American Journal of Optmetry & Physiological Optics, Volume 53, No. 12: 786-796, 1976. 7- VON NOORDEN G.K. and HELVESTON E.M.: Strabismus. A decision making approach. St. Louis: CV Mosby, 178-179, 1994. 8- GROFFMAN S.: Psychological aspects of strabismus and amblyopia-a review of the literature. J. Am. Optom. Assoc., 49: 995-999, 1978. 9- KRAFT S.P.: Adult strabismus surgery: More than just cosmetic. Can. J. Ophthalmol., 43: 9-12, 2008. 10- SUBHASH DADEYA and KAMLESH: Is it mandatory to treat amblyopia prior to surgery in esotropia? Acta. Ophthalmologica Scandinavia Vol., 79: 28-30, 2001. 11-KENNETH W.: Wright, Color Atlas of Strabismus Surgery Strategies and Techniques Third Edition Springer Science+Business Media, LLC Appendix 1 Surgical Numbers 219-220, 2007. 12- PAULA M. EDELMAN: Functional Benefits of adult strabismus surgery American orthoptic Journal, Vol. 60: 43-47, 2010. 13- BO YOUNG CHUN, SOON JEA KWON, SUN HWA CHAE and JUNG YOON KWON.: Reduction of deviation angle during occlusion therapy: In patially accommodative esotropia with moderate amblyopia. Korean J. Ophthalmol., Vol. 21, No. 3, 159-161, 2007. 14- JINWOOK JEOUNG, MIN JEOUNG LEE and JEONG- MIN HWANG.: Bilateral lateral rectus Recession versus Unilateral Recess-Resect procedure for exotropia with a dominant eye American Journal of Ophthalmology April, 683-688, 2006. 15- MOON JUNG MM, SANG IN KHWARG, SEONG- JOON KIM and BONG LEEN CHANG: Results of Reoperation on the Deviated Eye in Patients with Sensory heterotropia Korean Journal of Ophthalmology, 22 (1): 32-36, 2008. 16- YONG CHUL PARK, BO YOUNG CHUN and JUNG YOON KWON: Comparison of the Stability of Postoperative Alignment in Sensory Exotropia: Adjustable Versus Non-Adjustable Surgery Korean J. Ophthalmol., December, 23 (4): 277-280, 2009. 17- BAKER J.D.: The value of adult Strabismus correction to the patient. J. AAPOS, 6: 136-40, 2002. 18- LANGMANN A., LINDNER S., KOCH M., et al.: Doseeffect relation in revision surgery for consecutive divergens strabismus in adults. Ophthalmologe., 102: 869-72, 2005. 19- ANNA DICKMANN, STEFANIA ALIBERTI, MAMA TERESA REBECCHI and IRENE APRILE: Improved sensory status and quality-of-life measures in adult patients after strabismus surgery Journal of AAPOS Volume 17, Issue 1, Pages 25-28, February, 2013. 20- LISA FRAINE: Strabismic Amblyopia: When to Treat the Amblyopia, When to Operate American Orthoptic Journal Volume, 57: 30-34, 2007. 21- ANN L.: Webber Amblyopia treatment: An evidencebased approach to maximising treatment outcome Clinical and Experimental Optometry, 90, 4: 250-257, 2007. 22- DAWSON E.L., SAINANI A. and LEE J P Does botulinum toxin have a role in the treatment of sensory strabismus? Strabismus, 13: 71-73, 2005.