Comparison of surgically induced astigmatism in patients with horizontal rectus muscle recession

Abstract

AIM

To compare surgically induced astigmatism (SIA) following horizontal rectus muscle recession surgery between suspension recession with both the “hang-back” technique and conventional recession technique.

METHODS

Totally, 48 eyes of 24 patients who had undergone horizontal rectus muscle recession surgery were reviewed retrospectively. The patients were divided into two groups. Twelve patients were operated on by the hang-back technique (Group 1), and 12 by the conventional recession technique (Group 2). SIA was calculated on the 1^st wk, 1^st and in the 3^rd mo after surgery using the SIA calculator.

RESULTS

SIA was statistically higher in the Group 1 all postoperative follow-up. SIA was the highest in the 1^st wk, and decreased gradually in both groups.

CONCLUSION

The suspension recession technique induced much more SIA than the conventional recession technique. This difference also continued in the following visits. Therefore, the refractive power should be checked postoperatively in order to avoid refractive amblyopia. Conventional recession surgery should be the preferred method so as to minimize the postoperative refractive changes in patients with amblyopia.

INTRODUCTION

Refractive changes after strabismus surgery have been reported in the literature^[1]–^[5]. Some of the researchers thought that these refractive changes were transient and non-significant^[6],^[7]. However, some researchers advocate that significant refractive changes occur after strabismus surgery^[8],^[9].

There are different theories about the change in refraction after strabismus surgery. These theories include torsional effects of oblique muscle surgery, changes in the lens curvatures may be affected by changes in the circulation of the ciliary body, scleral wound healing, orbital and eyelid edema, and intraocular pressure changes. These are correlated with corneal power changes associated with due to changes in muscle tension^[10]–^[14].

The effect of strabismus surgery on corneal power has been studied extensively, but there are no studies about the effect of strabismus surgery on surgically induced astigmatism (SIA) in the literature. The current retrospective study was performed to further examine the role of horizontal muscle recession surgery on SIA. We compared SIA following horizontal rectus recession surgery between the suspension recession (hang-back) technique and the conventional recession surgery technique.

SUBJECTS AND METHODS

We retrospectively reviewed the medical records of the patients who had undergone horizontal rectus muscle recession surgery between December 2011 and August 2013. Exclusion criteria for this study included patients with amblyopia, ocular or neurologic diseases, previous history of ocular surgery, coexisting vertical strabismus, extraocular muscle paralysis, nystagmus, corneal opacities, follow-up by duration less than 3mo, poor cooperation to corneal topography. Informed consent was obtained from the patients. The study was approved by Adnan Menderes University's local ethics committee and was performed according to the Declaration of Helsinki.

Totally, 48 eyes of 24 patients were enrolled in this study. The patients in both groups were independent from each other. The patients were divided in the two groups. Horizontal rectus surgery was performed with a suspension recession surgery technique (Group 1; 24 eyes, 12 patients) and was performed with a conventional recession surgery technique (Group 2; 24 eyes, 12 patients). Age, gender, prism cover test, best corrected visual acuity, slit lamp examination, and corneal topography were reviewed and analyzed.

Corneal topography was performed using the Orbscan II (Bausch & Lomb, Rochester, NY, USA) preoperatively, on the 1^st wk, and the 1^st and 3^rd mo postoperatively.

Strabismus surgery was planned according to the amount of deviation, and performed by one surgeon (HÇ). SIA was evaluated with SIA calculator version 3.1, a free software program (http://www.insighteyeclinic.in/SIA_calculator.php).

Data analysis was performed with the SPSS^® (Statistical Package for Social Sciences), version 18.0. The Kolmogorov-Smirnov test was used to assess the normality of numeric variables. For the normally distributed variables, a comparison between the two groups was made by the independent sample t-test, and the results were expressed as a mean standard deviation. For the non-normally distributed variables, the comparison between the two groups was made with the Mann-Whitney U test, and the descriptive statistics were expressed as median (25-75 percentiles). Significance was defined as P<0.05. The Keratometric measurements were evaluated with the SIA calculator version 3.1. In repeatability analysis, any astigmatism smaller than 0.16 diopters (D) was accepted in the “variable interval” which changes according to device and operator. The real-induced changes in vectorial astigmatism greater than 0.16 D were defined as SIA.

RESULTS

Clinical findings of the patients are shown in Table 1. There were 24 eyes of 12 patients (7 males and 5 females) with the mean age of 12.83±5.9y in Group 1 and there were 24 eyes of 12 patients (8 males and 4 females) with the mean age of 12.75±5.7y in Group 2. In our study the mean age and age range of the groups were similar. There were no statistically difference in terms of age, sex, deviation at near and far, amount of recession and operated muscle.

Table 1. Clinical findings of the patients.

Parameters	Suspension group	Conventional group
No. of patients	12	12
Age	12.83±5.9	12.75±5.7
Sex (M/F)	7/5	8/4
Deviation at near (PD)	48.33±8.69	49.17±9.74
Deviation at far (PD)	47.22±8.82	48.21±8.73
MR recession (eye)	16	18
LR recession (eye)	8	6
Amount of recession (mm)	6.21±0.49	6.13±0.51

PD: Prism diopters; MR: Medial rectus; LR: Lateral rectus

x±s

Sixteen eyes underwent medial rectus (MR) recession and 8 eyes underwent lateral rectus (LR) recession (Group 1). Eighteen eyes were underwent MR recession and 6 eyes underwent LR recession (Group 2). Preoperatively the prism cover test revealed 48.33±8.69 PD and 49.17±9.74 PD at near deviation, 47.22±8.82 PD and 48.21±8.73 PD at far deviation in Groups 1 and 2, respectively. The mean preoperative keratometric values in steep axis (KS) were 44.28 (41.55-45.78) D and 44.21 (43.6-45.00) D in Groups 1 and 2, respectively. The mean preoperative keratometric values in flat axis (KF) were 42.58 (39.38-44.28) and 43.15 (42.43-43.80) in Groups 1 and 2, respectively. The mean preoperative keratometric differences (KD) were 1.79 (1.03-2.63) D in Group 1 and 1.81 (1.10-2.40) D in Group 2. The astigmatisms were within the rule. There was no statistical difference in terms of preoperative keratometric values between the groups.

Keratometric values are seen in Table 2 preoperatively and in the 1^st wk, 1^st mo and 3^rd mo postoperatively. KD was higher in Group 1 in all postoperative periods, and was seen to be increasing in 1^st mo and then decreased in both groups (Figure 1). Although there was no difference statistically in terms of keratometric values between the groups, SIA values were statistically higher in Group 1. After the 1^st wk, they were found 0.87 (0.66-1.12) and 0.64 (0.45-0.74) D in Group 1 and 2, respectively (P=0.01). In the 1^st mo, it was found to be 0.64 (0.40-0.83) D in Group 1 and 0.41 (0.20-0.54) D in Group 2 (P=0.004). At the end of the follow-up period of 3mo, SIA was found 0.44 (0.29-0.54) D and 0.25 (0.12-0.29) D in Groups 1 and 2, respectively (P=0.005) (Figure 2). We analyzed SIA for MR recession patients and values are shown in Figure 3. SIA values were found to be high in suspension group for all follow ups and this difference was statistically significant (Figure 3).

Table 2. Keratometric values of the patients.

Parameters	Suspension group	Convensional group
Preop. KS	44.28 (41.55-45.78)	44.21 (43.60-45.00)
Preop. KF	42.58 (39.38-44.28)	43.15 (42.43-43.80)
Preop. KD	1.79 (1.03-2.63)	1.81 (1.10-2.40)
Postp. 1st wk KS	43.91 (42.15-45.75)	44.3 (43.43-45.13)
Postp. 1st wk KF	41.86 (39.93-43.70)	43.14 (42.35-43.80)
Postp. 1st wk KD	1.99 (1.15-2.90)	1.89 (1.23-2.63)
Postp. 1th mo KS	43.63 (40.93-45.98)	44.23 (43.10-45.08)
Postp. 1th mo KF	41.62 (38.45-43.58)	43.12 (42.5-43.90)
Postp. 1th mo KD	2.02 (1.03-2.88)	1.87 (1.20-2.60)
Postp. 3rd mo KS	44.11 (42.13-45.65)	44.31 (43.20-45.33)
Postp. 3rd mo KF	42.16 (39.83-43.90)	43.27 (42.70-44.05)
Postp. 3rd mo KD	1.95 (1.10-2.85)	1.78 (1.13-2.50)

KS: Steep keratometric values; KF: Flat keratometric values; KD: Keratometric difference values; The descriptive statistics were expressed as median (25-75 percentiles). There was no difference in preoperative and postoperative keratometric values between the two groups.

(D)

Figure 1. Preoperative and postoperative KD.

KD: Keratometric difference. KD was higher in suspension group all postoperative period. But this difference wasn't statistically significant.

Figure 2. Amount of SIA.

SIA: Surgically induced astigmatism. SIA was higher in suspension group all postoperative follow-ups and this difference was statistically significant.

Figure 3. Amount of SIA for medial rectus recession.

SIA: Surgically induced astigmatism. SIA was higher in suspension group all postoperative follow-ups and this difference was statistically significant. P values were 0.004, 0.001 and 0.002 for 1^st wk, 1^st mo and 3^rd mo, respectively.

DISCUSSION

In this study, we evaluated SIA following the horizontal rectus muscle recession in a 3mo follow-up period. We found statistically significant difference in the postoperative SIA values between the conventional recession technique and the suspension recession technique.

Astigmatism can be measured by keratometer. However, the SIA needs to be calculated. We used the “SIA Calculator Version 3.1” to evaluate SIA.

Astigmatic changes after recession horizontal rectus muscles or horizontal rectus muscle recession-resection procedures were previously reported^[10]–^[14]. There are different theories about the change in astigmatism after strabismus surgery. The change of extraocular muscle tension on corneal power is seen as the most important factor^[2].

There are many publications that describe the hang-back suspension technique. Suspension technique has some advantages when compared to conventional technique. These include shorter surgery duration, less risk of scleral perforation, better exposure of scleral sutures^[15].

Mohan and Sharma^[16] found that conventional bilateral lateral rectus recessions success rate were more than hang-back recessions in patients with true divergence excess intermittent exotropia. On the other hand, many studies report that the hang-back technique is as effective as the conventional rectus muscle recession technique^[17]–^[19].

Although many studies have evaluated the efficacy between the two techniques, there are few studies that have been carried out on the impact of astigmatism. Betts and Olitsky^[20] selected one eye to randomly underwent strabismus surgery with suspension technique, and the other eye underwent strabismus surgery using the conventional technique. They measured the keratometric values in the operating room immediately prior to and immediately after the operation, and found that the suspension recession technique induced less corneal astigmatism^[20]. Unlike our study, they only studied corneal curvature changes in the early perioperative period in the operating room; they did not follow up on the patients after the long postoperative period in terms of astigmatism. In the perioperative period, there are many reasons that may affect astigmatism such as inflammation, kemosis, tissue edema, irritating sutures, and patients' discomfort. It should be measured in the late postoperative period also. On the other hand the method which was used in their prospective study is a valid approach to compare the results based on the same tissue response and ocular conditions.

Rajavi et al^[21] evaluated refractive error changes following horizontal muscle recessions, and found statistically significant differences in some aspects; however, these differences did not seem to be important clinically. They did not recommend early cycloplegic refraction after the horizontal muscle recessions in order to discover any surgically induced refractive changes. They recommended that a cycloplegic refraction is required in all cases 3mo after the operation.

Chun et al's^[22] study evaluated changes in the induced astigmatism by lateral rectus recession. They found that, the larger recessions had more induced astigmatism in the 1^st wk postoperatively. However, there was no statistically significant difference in the induced astigmatism between the two groups. In our study the horizontal rectus muscles recession range was very small and therefore we think that the small amount of recession didn't influence the amount of astigmatism and other parameters which Chun et al^[22] mentioned in their study.

According to international practice, even by performing the same amount of recession on MR and LR muscles, the results may not be the same. We analyzed also SIA values separately for MR and LR recession in the same group. In the MR recession group, SIA values were found to be high in suspension group for all follow-ups and this difference was statistically significant (Figure 3). Unfortunately, we couldn't evaluate these parameters for LR recession cases; because of small number of patients.

The adjustable suture technique is partially a similar technique to the suspension recession technique. Eustis et al^[23] found more tissue response to surgery in patients with adjustable sutures. After strabismus surgery using the adjustable suture technique, some non-serious complications and temporary problems can occur, such as conjunctival dehiscence, suture granuloma or cyst, conjunctival edema and/or hyperemia^[24]–^[26].

In suspension recession technique, remodeling may take longer due to delayed scleral wound healing, and other tissue responses to the suture between the conjunctiva and sclera. In addition, the long suture material in the subconjunctival space (not buried in the sclera) may cause increased reactions when compared to other techniques. In the conventional technique, there is no suture between the new insertion point and the cornea in the subconjunctival space.

There are some limitations in our study; the groups are paired for age and age ranges are similar. Although age ranges could be more homogenous in each group itself. There are three patients around 6 years old in each group and there are two patients around 19 years old in each group. SIA may be influenced by age at operation. Further prospective studies with more homogenous group in terms of age are needed.

In conclusion, our study showed that the suspension recession technique caused much more SIA than the conventional horizontal rectus recession technique. In addition, this difference continues during the three month follow-up. Because these refractive differences may increase the astigmatism, this may affect amblyopic patients. Thus, the refractive power should be checked postoperatively, and conventional recession surgery should be the preferred technique in patients with amblyopia.