|Year : 2013 | Volume
| Issue : 3 | Page : 165-169
Evaluating a new surgical dosage calculation method for esotropia
Siddharth Agrawal1, Vinita Singh1, Sanjiv Kumar Gupta1, Saurabh Agrawal2
1 Department of Ophthalmology, King Georges' Medical University, Lucknow, Uttar Pradesh, India
2 Sukriti Eye Clinic, Lucknow, Uttar Pradesh, India
|Date of Web Publication||30-Nov-2013|
Department of Ophthalmology, King Georges'Medical University, Lucknow, Uttar Pradesh
Source of Support: None, Conflict of Interest: None
| Abstract|| |
Purpose: To evaluate a simplified method for correction of ocular deviation in patients of infantile and acquired basic esotropia.
Materials and Methods: Thirty-six consecutive patients of infantile and acquired basic esotropia were selected for this study. Patients underwent unilateral recession-resection surgery as per the new norm gram. Patients underwent 3.5-7 mm recession of medial rectus (MR) in one eye depending on the pre-operative deviation and patient's age. Together they also underwent 6 or 7 mm resection of the lateral rectus (LR) in the same eye depending on patient's age (6 mm for 3 years and below and 7 mm for older age). In patients 3 years and below, a correction of 6, 7, or 8 PD/mm of recession of MR was expected when the pre-operative deviation was lesser than 30 PD, between 30 and 60 PD, or above 60 PD, respectively. Similarly, these values were 5, 6, and 7 PD/mm of MR recession in patients above 3 years. A ratio between achieved and expected correction was calculated and the calculation was deemed successful for a patient if this ratio fell between 0.9 and 1.1.
Results: The calculation procedure was successful in 33 out of 36 patients (91%). The two-tailed probability on paired Wilcoxon test was 0.187.
Conclusions: This simplified method of surgical dosage calculation using MR recession as basis is predictable in patients of infantile and basic Esotropia. It may serve as a useful tool for minimizing variability of surgical results.
Keywords: Esotropia, recession, resection, surgical dosage
|How to cite this article:|
Agrawal S, Singh V, Gupta SK, Agrawal S. Evaluating a new surgical dosage calculation method for esotropia. Oman J Ophthalmol 2013;6:165-9
| Introduction|| |
The issue of surgical dosage has always generated interest amongst the strabismus surgeons. This interest is mainly because of unpredictability of results attributed to several less-understood factors. Every surgeon has encountered situations where the same surgery was optimum for one patient and grossly under- or over-corrected the deviation in another. Developing and modifying one's own dosage patterns are easier said than done. We have attempted to develop a simple calculation method for surgical dosage in patients of esotropia (ET).
To evaluate a simplified calculation method based on amount of deviation and patient's age for correction of ocular deviation by unilateral recession-resection procedure in patients of infantile and acquired basic esotropia.
| Materials and Methods|| |
Thirty-six consecutive patients of infantile and acquired basic esotropia were selected for this study. Patients underwent unilateral recession-resection surgery as per the new norm gram. A calculation method for surgical dosage was developed by us after analyzing our surgical records of over 30 years and also taking into account the various methods of calculation available. ,,,
This dosage pattern was developed keeping in mind that straightening effect is predominantly because of recession and resection has an accessory role. , Correction achieved is more in children and in patients with larger deviation for the same amount of surgery.  [Table 1] shows details of existing standard guidelines.
In infantile and acquired basic esotropes we have been using the dosage calculation method as per guidelines given by Stallard  in early 70s, i.e. per-mm medial rectus (MR) recession gives a correction of 3-4 Prism Dioptres (PD)/mm lateral rectus (LR) resection gives a correction of 2-3 PD plus 25% advantage when both muscles are done in the same sitting. Though variables that affect the outcome are discussed, no clear figures are available in the literature as to how much to account for amount and duration of deviation. In patients <3 years with >60 PD deviations, we calculate using higher figures, i.e., 5 PD for MR recession and 4 PD for LR resection. If the deviation is so small that minimum surgery on both muscles is likely to over-correct, then we do recession of one muscle. Thus for 30 PD essential ET correction in less than 3 years age we would plan 3-mm-MR recession + 4-mm-LR resection which would give a correction of 12 PD + 12 PD + 6 PD (25% advantage) =30PD, in more than 3 years age, 4.0 mm-MR recession + 6 mm-LR-resection which would give a correction of 12 PD + 12 PD + 6 PD (25% advantage) =30PD. Similarly, for 70PD esotropia in <3 years age, 4.5 mm-MR recession + 8mm-LR-resection which would give a correction of 22.5PD + 32 PD = 54.5 PD +13.5 PD (25% advantage) =68 PD, and in >3 years age we would plan 6mm-MR recession + 8-mm-LR resection which would give a correction of 24 PD + 24 PD = 48 + 12 PD (25% advantage) =60 PD along with recession of MR of the other eye.
An overview of our surgical results is illustrated in [Table 2]. We achieved surgical alignment within ± 10 PD in 69-80% of our patients. Keeping in mind that the straightening effect is predominantly because of recession and resection has an accessory role , we studied the mean MR recession in above group of patients and found statistically significant lower value in patients with under-correction (4.2 mm ± 0.8) versus those with over-correction (5.9 mm ± 0.6). In the patients who achieved alignment within ± 10PD the mean MR recession was 5.6 ± 0.7 mm. To avoid the skew effect of the difference in the number of patients in the various groups we compared the values obtained after making the denominator (resection) constant. In view of these observations and other reports in the literature we decided to revise our dosage calculation method on the basis of MR recession alone, and test its accuracy.
|Table 2: An overview of our results in patients operated for infantile and basic esotropia, over the last 30 years|
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In our revised calculation correction per mm of MR recession is calculated according to [Table 3]. The surgical dosage is decided according to the pre-operative ocular deviation.
|Table 3: The expected correction per mm of recession of medial rectus muscle. The expected correction per mm of surgery will be maximum in children below 3 years of age with ET of greater than 60 PD|
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As can been seen from [Table 3], a maximum of 56PD of correction can be expected in children up to 3 years and 49PD in patients above 3 years by 7 mm recession. Although some authors recommend up to 8mm recession of MR we prefer to keep the limit at 7 mm due to the possibility of placing the new insertion beyond the functional equator and thereby limiting adduction and convergence. 
Along with different amounts of MR recession thus calculated, the patients undergo LR resection according to age. A 6mm resection of LR is carried out in children aged 3 years and below and a 7 mm resection is done above this age.
This method was tested prospectively in 36 consecutive patients with infantile and acquired basic type  of ET undergoing unilateral recession-resection procedure at Department of Ophthalmology, King Georges'Medical University, Lucknow, India between June 2008 and January 2011. Patients with incomitance or coexisting vertical deviation were eliminated.
Some authors prefer bilateral MR recession for infantile ET while others believe that unilateral recession-resection is equally effective. ,,, Unilateral surgery is our procedure of choice in these patients (see discussion).
Patients fulfilling the selection criteria underwent a detailed orthoptic work-up. Angle measurements were done by Prism Bar and Cover Test (PBCT). PBCT was performed fixing each eye (FEE) for distance and near fixation in all nine gazes. Largest deviation in primary gaze was considered for calculations.
In patients where the pre-operative deviation was larger than manageable by single recession-resection procedure, possibility of second-stage surgery in other eye was explained to the patient. We performed the same unilateral recession-resection procedure in these patients to evaluate the accuracy of our calculation method. Patients unwilling for staged procedure were excluded. The surgeries were carried out by standard limbal incision technique and measurements on the globe were made from the muscle insertion with a pre-measured piece of suture. , Axial Length (AL) measurements were performed prior to surgery by A-scan using the contact technique. AL measurements are useful to have an idea of the functional equator and safe recession.  In patients where the calculated recession of MR exceeded the safe recession by over 1.5 mm we modified our surgical technique by using hang-back sutures on MR. ,, We were also able to correlate our calculation failures with extremes of AL although this was not a part of initial study protocol.
Standard post-operative follow-ups were carried out on day 1, week 2 and week 12.  PBCT findings at 12 weeks were considered and largest deviation in primary position was taken for calculations [Table 4]. The amount of correction was measured by before-after difference in maximum angle by PBCT in primary gaze.
|Table 4: Master chart showing the patient data, surgery performed, expected and achieved correction, achieved/expected ratio, and fi nal outcome of calculation|
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A ratio between achieved and expected correction of 0.9-1.1 was considered as a successful outcome. As we were also dealing with patients where a second-stage surgery was anticipated the classical satisfactory outcome criteria of orthotropia or asymptomatic heterophoria post-operatively was not applicable. 
As we had already on basis of previous publications accepted a relationship between the variables (age and deviation) included in calculations and surgical dosage, we would be dealing with two groups of paired samples (expected and achieved corrections). ,, We intended to use paired "Student's t-test" or "Wilcoxon" test depending on presence or absence of normality (variables being parametric) in the samples.
| Results|| |
A total of 36 consecutive patients meeting the inclusion criteria were operated according to our norm gram. Seven patients were in the age group of 3 years and below. Eleven patients had infantile ET and 25 had acquired basic type of ET.
The calculation procedure was successful in 33 out of 36 patients (91%). The three failures (Number 8, 18 and 31 of [Table 3]) had an AL outside the normal range. 
On analysis on the two groups (expected and achieved corrections) the D'Agostino-Pearson test for normal distribution was 0.836 and 0.010. Hence, the achieved correction group was non-parametric. Thus we used the Wilcoxon test for paired samples for analysis. The two-tailed probability on this test was 0.187 implying that the two groups were similar and our method of prediction was successful.
| Discussion|| |
Several authors have tried to develop guidelines to dose the amount of correction achieved per mm of surgery on the extra-ocular muscles. ,,, Few publications talk about placing the muscle insertion with relation to certain points on the circumference like the equator, while others give limited importance to AL and equator calculations. , It is believed by and large that recession plays a major part in restoration of ocular alignment and resection has an accessory role. ,, It is also agreed that the correction is larger in children and greater deviations.  We have incorporated these variables in this calculation method.
We prefer doing unilateral surgery in patients with infantile and acquired basic esotropia not only because it has been shown to be as effective as bi-medial recession but also because we have one untouched MR, should a patient require a second surgery later. , It is well-documented that infants with esotropia require 1.9-2.6 operations to achieve stable alignment with some fusion. ,,, Moreover, we have found it easier to convince the parents for a single-eye surgery.
It has also been suggested that the correction achieved in different hands is variable and each surgeon should try to dose his procedures after regular audit of results.  The mere presence of so many calculation methods is evidence of variability of results. We believe that such guidelines for calculation will be helpful for beginners and occasional squint surgeons who are yet to develop their own norm gram.
Average AL in strabismus patients has been estimated as 21.98 ± 1.59 mm (range, 18.75-25.37 mm).  The three (8.3%) failures of our calculation method had ALs outside this range. Of these two eyes were hypermetropic with AL of 18 and 17.5 and one was myopic with AL of 27.4 mm. We achieved over-correction in hyper-metropic eyes and under-correction in the myopic eye. This may be explained by the relative position of the functional equator in eyes with AL outside the normal limits. , One would probably need to reduce the dosage in smaller eyes and increase it in longer ones. 
This simplified method of surgical dosage calculation using MR recession as basis is predictable in patients of infantile and basic ET with AL within normal range.
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[Table 1], [Table 2], [Table 3], [Table 4]