FP348 : The Role of Anterior Segment Optical Coherence Tomography in Strabismus Surgery: Prospective Study

Dr. Joseph John,J14619, Dr. Elizabeth Joseph, Dr. Meena Chembil Kakkitampara

ABSTRACT

AIM:

To evaluate the accuracy of Anterior Segment Optical Coherence Tomography (AS-OCT) in measuring the distance from the limbus to the insertion of primary and previously operated horizontal rectus muscles.

MATERIALS & METHODS:

Prospective observational study; we did AS-OCT preoperatively and measured the distance from angle of anterior chamber to muscle insertion using the caliper tool provided in the software. We calculated limbus-insertion distance by adding 1mm to the angle-insertion distance. We compared this value with intraoperative measurement obtained with surgical caliper.

RESULTS:

35 muscles were evaluated, including 12 previously operated muscles. 88.6% of the measurements were within the 1 mm difference considered clinically acceptable. Bland-Altman plots showed good agreement between the two methods.

CONCLUSIONS:

AS-OCT can image the horizontal rectus muscle insertions well and provide good reliability and accuracy in measurement of the limbus-insertion distance.

Introduction

The knowledge of the exact limbus-insertion distance of rectus muscles may benefit the calculation of the surgical amount to be performed in strabismus surgery. However, the insertion is invisible in routine ophthalmic examination such as slit lamp biomicroscopy until surgery. Thus, a preoperative technique that could accurately locate the insertion and evaluate the exact limbus-insertion distance may be useful for improving the surgical plan subsequently needed.

A number of techniques have been tried to image the extraocular muscle (EOM) before strabismus surgery. Computed tomography scans and high-resolution magnetic resonance imaging have been used to define the size and location of the EOM. However, these techniques are unable to define the exact EOM insertion and the distances from the limbus.

B scan allows anatomic visualization of the EOM and measurement of limbus-insertion distance, but its accuracy is limited by the relatively low image resolution. Several studies have reported that ultrasound biomicroscopy (UBM) can accurately measure the limbus-insertion distance before strabismus surgery, for both primary surgeries and reoperations. However, due to UBM’s contact feature (placing a cup on the ocular surface), it is difficult to perform on children. Moreover, significant measurement errors are often induced by pressing the eyeball during UBM examination. The newly developed anterior segment optical coherence tomography (AS-OCT; Visante by Carl Zeiss Meditec, Dublin, CA), wavelength 1310 nm, generates a two-dimensional high resolution image from a reflected light beam. It has faster scanning (minimizes motion artifact), low scattering, and high penetration into turbid tissue such as sclera, iris, angle, and opaque corneas. Moreover, AS-OCT evaluation is a noncontact examination, which is particularly suitable for children and postoperative examination1,2.

Aim

To evaluate the accuracy of Anterior Segment Optical Coherence Tomography (AS-OCT) in measuring the distance from the limbus to the insertion of primary and previously operated horizontal rectus muscles.

Materials & methods

Consecutive patients undergoing primary or repeat strabismus surgery on horizontal rectus muscles between September 2015 and April 2016 were included. All AS-OCT measurements were performed by one operator, and all intraoperative measurements were performed by one surgeon. Both examiners were masked to the other’s measurements.

After explaining the procedures used in the study, informed consent was obtained from all participants. The research complied with the tenets of the Declaration of Helsinki. Ethical approval was obtained from the hospital’s research ethics board.

Preoperative AS-OCT Measurements

Before surgery, we did AS-OCT scan (Visante OCT; Carl Zeiss Meditec) of the horizontal muscles with the patients in sitting position. We used Anterior segment biometry protocol to acquire the scans. With the help of a fixation light, we guided the patients to make a 15° temporal gaze for the medial rectus muscle scanning and to make a 30° nasal gaze for the lateral rectus muscle scanning. Such gaze directions allowed the tendinous insertion to fall in the scanning sector of the AS-OCT, so that the posterior face of the insertion becomes visible on AS-OCT images. To obtain a longitudinal scan of the horizontal rectus muscles allowing for a simultaneous visualization from the limbus up to the muscle belly through its inserting tendon, the scanning plane (0°–180°) was oriented parallel to the long axis of the muscle.

In the cross-section image, the end of the cleft between the muscle and sclera was defined as the insertion site.

Figure 1. AS-OCT image of a muscle for primary surgery. The lateral rectus muscle of the right eye is imaged. Inset shows the position of the eye and the orientation of the scan.

However, the corneoscleral limbus is difficult to determine in AS-OCT images. The iris root (the anterior chamber angle) lies approximately 1.0 mm posterior to the limbus in the horizontal meridian3. With the help of the caliper function in the AS-OCT software, we placed the calipers on the perpendicular projection of iris root on the corneoscleral surface and on the muscle insertion site to measure the angle-insertion distance. The actual value of limbus-insertion distance was adjusted by adding an additional 1.0 mm to the angle-insertion distance. Five consecutive readings were taken, and the mean value was taken for analysis.

Figure 2. Measurement of angle-insertion distance of the muscle shown in Figure 1 using the caliper function in the AS-OCT software. The distance from the insertion site of the lateral rectus to the anterior chamber angle as measured with the caliper is 5.44 mm. 1mm is added to calculate limbus -insertion distance. In this case, limbus- insertion distance for right lateral rectus is 6.44mm.
Intraoperative Direct Measurements
During surgery, the muscle was exposed with a squint hook after a conjunctival incision. After dissection of a muscle from the sclera, the distance from the limbus to the insertion of the muscle was measured with Castroviejo calipers at the midpoint of the insertion. Three consecutive readings were taken and then the surgery was completed as planned. The mean was calculated with the three measured values.

Statistical Methods

In the present study, the intraoperative surgical calipers measurement was considered the gold standard because of its direct view and measurement. Bland-Altman plots was used in the statistical analysis to compare the AS-OCT measurements with the intraoperative surgical caliper measurements of limbus-insertion distance. In addition, a clinically acceptable difference of up to 1.0 mm between the two measurements was defined as a good agreement criterion for this study. The statistical analysis was done using MedCalc version 12.5.

Results

Twenty eight patients (18 females) were enrolled in the study. Thirty five out of 38 (92.1%) muscles could be clearly seen in AS-OCT. Three previously operated muscles could not be properly imaged in AS-OCT; in these eyes the lateral rectus was found to be 14mm from limbus in two eyes and 16 mm from limbus in one eye during surgery.

So 35 muscles of 25 patients (17 females; mean age 17.9 +/- 9.4 years, range 8–38 years) were evaluated, including 12 previously operated muscles. In the primarily operated eyes, limbus-insertion distance obtained with AS-OCT for medial rectus muscle was 5.80+/0.24mm (range 5.61- 6.44mm) and that for lateral rectus was 7.23+/-0.16mm (range 6.95-7.51mm). The Bland-Altman plots showed that a total of 16 of 17 (94%) and 17 of 18 (94%) AS-OCT-caliper differences, were found in the Medial rectus group(Figure 3) and Lateral rectus group(Figure 4) respectively, to be within 1.96 standard deviation of the mean difference. Bland-Altman plots for various subgroups are shown in figures 5-8.
Figure 3 Bland- Altman plot 1 Figure 4 Bland- Altman plot 2 Figure 5 Bland-Altman plot3 Figure 8 Bland-Altman plot6 Overall 88.6% of the measurements were within the 1 mm difference that was considered as a criterion for good agreement. In the 23 eyes undergoing surgery for the first time, 100% of the measurements met this criterion. In the reoperated eyes, 3 out of 6 (50%) in the medial rectus group and 5 out of 6 (83%) in the lateral rectus group met this criterion.

Discussion

The EOM insertion site is generally treated as a reference point in the calculation of the amount of strabismus surgery. However, the insertion site varies considerably in some strabismus patients, which may result in inaccuracies in determining the site to which the muscle is to be recessed. Because the prior strabismus surgery may alter the insertion points of the muscles and the surgical undercorrection and overcorrection of horizontal tropia are common problems for strabismus surgery, a careful preoperative evaluation is mandatory for the management of such conditions. However, the information regarding prior surgery is sometimes unavailable. The AS-OCT may help to design a surgical plan for reoperation by localization of the displaced EOM insertion.

In a study by Liu et al4, 37 muscles of the 16 patients were studied including 18 medial rectus and 19 lateral rectus muscles.The Bland-Altman plots showed that a total of 17 of 18 (94%) and 18 of 19 (95%) AS-OCT-caliperdifferences, were found in the medial rectus and lateral rectus group respectively, to be within the 95% confidence interval of the mean difference. In addition, 92% of the measured values (34 of 37) were within the predefined level (1.0 mm) of agreement between the two methods. In another study by Ngo et al5, a total of 65 muscles were evaluated, including 9 muscles undergoing reoperation. Of these, 62 muscles were successfully imaged. In all reoperated eyes, the AS-OCT measurements were within 1 mm of the intraoperative measurements. Overall, 89.7% of the measurements were within the 1 mm difference considered clinically acceptable. Our study also showed a good agreement between the two methods. 88.6% of the measurements in our study were within predefined level (1.0 mm) of agreement between the two methods.Ngo et al5also reported that they were able to image a previously recessed muscle as far posterior from the limbus as 13.5 mm. In our study, we could locate a medial rectus muscle recessed 11.01 mm from the limbus.

AS-OCT evaluation is a noncontact examination, which is particularly suitable for children. Ngo et al5 reported that they could acquire images in children as young as 4 years of age. In our study, the youngest age was 8 years.

Figure 9 showing agreement between the two methods of measurement

The angle –insertion distance measured with AS-OCT is 10.01mm in this case. Hence the limbus– insertion distance calculated is 11.01mm. B. Intraoperative measurement of limbus –insertion distance with Castroviejo calipers. C. Caliper reading of 11mm.

The present study confirmed that the AS-OCT could provide visualization of the insertion site of horizontal rectus muscles in vivo, and the results supported a reasonable level of concordance between the AS-OCT and intraoperative measurements of limbus-insertion distance. This makes the AS-OCT an ideal instrument for measuring limbus-insertion distance of extraocular muscles in vivo.

This study has some limitations. In the calculation of limbus-insertion distance, we chose 1.0 mm as the limbus-angle distance. However, because there is variation in the distance between anterior chamber angle and the limbus, the method we applied here may cause a systematic measurement error. The slanting extraocular muscle insertions due to congenital abnormality or previous surgery cannot be detected by AS-OCT. It should be noted that the statistical analysis of the present study might be disputed due to the relatively imprecise measurement of the surgical calipers. In the measurement of limbus-insertion distance, the AS-OCT can be resolved to the nearest 0.01 mm whereas the caliper can only be resolved to the nearest 1 mm.

Conclusion

AS-OCT can image the horizontal rectus muscle insertions well and provide good reliability and accuracy in measurement of the limbus-insertion distance.

References

  1. Huang D, Li Y, Radhakrishnan S. Optical coherence tomography of the anterior segment of the eye. Ophthalmol Clin North Am. 2004;17:1– 6.
  2. Ramos JLB, Li Y, Huang D. Clinical and research applications of anterior segment optical coherence tomography – a review. Clin Exp Ophthalmol. 2009;37:81–89.
  3. Park DJJ, Karesh JW. Chapter 1. Topographic anatomy of the eye: an overview. In: Tasman W, Jaeger EA, eds. Duane’s Foundations of Clinical Ophthalmology. Vol. 1. CD-ROM ed. Hagerstown, MD: Lippincott Williams & Wilkins; 2011.
  4. Liu X, Wang F, Xiao Y, Ye X, Hou L. Measurement of the limbus -insertion distance in adult strabismus patients with anterior segment optical coherence tomography. Invest Ophthalmol Vis Sci 2011;52:8370-73.
  5. Ngo CS, Smith D, Kraft SP. The accuracy of anterior segment optical coherence tomography (AS-OCT) in localizing extraocular rectus muscles insertions. J AAPOS 2015;19:233-236.

 

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