Anterior segment optical coherence tomography (AS-OCT) is popularly used to obtain images of the anterior segment of the eye and for quantitative analysis of the anterior chamber depth, cornea and lens thicknesses, angle, and bleb structure after glaucoma filtering surgery. Measuring the distance between the limbus and extraocular muscle (EOM) insertion site with AS-OCT has shown good intra-examiner reliability and good agreement with intraoperative measurements.1–9 Moreover, this modality has revealed the characteristic muscle shape changes before and after thyroid ophthalmopathy treatment,10 and the changes in conjunctival-scleral thickness before and after recession or resection/plication on the lateral rectus and medial rectus muscles.11
A previous study measured the distance between the corneal limbus and EOM insertion site in nonstrabismic participants using AS-OCT and observed excellent consistency between examiners,3 with no age-dependent changes in the distance between the corneal limbus and the EOM insertion site in patients older than 4 years.12 AS-OCT analysis of the horizontal muscle insertion site was reliable, and modest rotation of the medial rectus muscle yielded a clear image. However, we noticed that too much abduction resulted in an obscure muscle image and the inability to distinguish between the sclera and the EOM insertion site. A search on PubMed and Google found no comparative study regarding the fixation points during the measurement of EOM insertion sites with AS-OCT. In the current study, we aimed to determine the optimal gaze fixation points for lateral rectus and medial rectus muscle insertions using AS-OCT compared with the intraoperative measurements.
Patients and Methods
This study was performed according to the tenets of the Declaration of Helsinki for research involving human subjects and received approval from the Medical Ethics Committee of Hamamatsu University School of Medicine. All patients and/or guardians of the patients received an explanation of the experimental design and the possible consequences of the study and signed informed consent forms. We enrolled patients who were planning to undergo horizontal strabismus surgery between July 2016 and March 2019 at the hospital of the Hamamatsu University School of Medicine. We excluded patients who had undergone previous EOM surgeries, were younger than 4 years, and had difficulty fixating on targets or were uncooperative with the recordings.
We used a TOMEY SS-1000 CASIA OCT system (Tomey Corporation) for the AS-OCT measurements. This OCT system is based on optical frequency-domain imaging technology. The light source had a center wavelength of 1,310 nm and a high-speed swept-source OCT, enabling greater penetration of the ocular tissue than the 840-nm light source used in conventional OCT for fundus examinations. The A-scan rate of the instrument was 30,000 scans per second and A/B was set to 2,048 with a scanning range of 16 mm in diameter. The instrument had a 10-µm vertical resolution and a 30-µm horizontal resolution. Auto-shot and auto-alignment functions were not used.
We placed 4-mm square stickers for the two gaze fixation targets. Red stickers were placed 2 to 2.4 cm from the objective lens for the inner gaze fixation targets outside of the two illumination windows, and green stickers were placed 3.5 to 3.9 cm from the objective lens for the outer gaze fixation targets outside of the two peripheral fixation targets originally installed on the AS-OCT instrument. The target gaze locations required 20.3 to 23.7 and 33 to 35.8 degrees of ocular rotation with a 5.4-cm distance between the ocular surface and the measurement window (Figure 1).
Fixation targets for the measurement of the angle and extraocular muscle insertion site distance using anterior segment optical coherence tomography (AS-OCT); 4-mm square stickers for two gaze fixation targets were placed. A red sticker was placed 2 to 2.4 cm from the objective lens for the inner gaze fixation target and a green sticker was placed 3.5 to 3.9 cm from the objective lens for the outer gaze fixation target on the AS-OCT instrument. The target gaze locations required 20.3 to 23.7 and 33 to 35.8 degrees of ocular rotation with a 5.4-cm distance between the ocular surface and the measurement window.
During the recording, the head of the patient was stabilized with a head pad, and the non-recorded eye was occluded using either adhesive tape or the patient's own hand. The patient was instructed to look at either of the two gaze fixation targets on the inner or outer gaze for the medial rectus or lateral rectus muscle recordings, respectively (Figure 2). The distance between the muscle insertion site and the angle were manually measured on the AS-OCT monitor by four orthoptists after obtaining an image of the insertion site while the patient's gaze was fixed on the inner and outer targets. By attaching a fixation target, the patient's gaze could be firmly fixed, and OCT imaging was taken after confirming that the EOM was clearly visible. For the subsequent analyses, the measured distance (millimeters) between the angle and the muscle insertion site using AS-OCT, was rounded to two decimal places.
Anterior segment optical coherence tomography (AS-OCT) images of the lateral rectus (LR) and medial rectus (MR) muscles fixing at the outer and inner gaze fixation target (left eye). The LR muscle appears more clearly at the (A) outer than (B) inner gaze fixation target. The MR muscle appears more clearly at the (C) inner than (D) outer gaze fixation target.
Five surgeons using surgical microscopes performed the horizontal EOM surgeries. Briefly, a fornix- or limbal-based conjunctival incision was performed to reveal the horizontal muscle, which was grasped with a muscle hook. The eye was rotated slightly in the medial direction to identify the lateral rectus muscle insertion site or laterally to identify the medial rectus muscle insertion site until both the corneal limbus and the insertion sites were clearly visible. Then, the eye was maintained in the position that enabled the clear visualization of both the corneal limbus and the insertion site while obtaining the measurements. The distance between the corneal limbus and the anterior edge of the muscle hook was measured using surgical calipers at the vertical center of the muscle insertion before muscle removal from the sclera twice. The surgical calipers were used for measurements at a 1-mm interval with a minimum of 0.5 mm.
The AS-OCT–measured distance between the angle of the recess and the corneal limbus was increased by 1 mm to better reflect the actual distance between the corneal limbus and the muscle insertion site, following previous reports.3,6,7,12 Intraoperative and AS-OCT measurements based on the two fixation targets were statistically analyzed using Microsoft Excel 2013 for Windows 8 software (Microsoft Corporation). A one-way analysis of variance, multiple comparisons incorporating Bonferroni correction, and Bland-Altman plots were used to evaluate the accuracy of the AS-OCT measurements. A P value of less than .05 was considered statistically significant.
We recorded 82 muscles (46 lateral rectus muscles and 36 medial rectus muscles) from 44 patients (22 males and 22 females) aged 5 to 83 years (median age: 16 years).
The average intraoperative distance between the limbus and lateral rectus muscle insertion site was 6.3 ± 0.7 mm. The average AS-OCT–measured distance was 5.8 ± 0.7 mm for the inner gaze fixation target (P = .0017) and 6.4 ± 0.5 mm for the outer gaze fixation target (P = 1.00). Comparison of the AS-OCT and intraoperative measurements revealed a less than 1 mm difference for 34 patients (73.9%) who fixated their gaze at the inner target and 41 patients (89.1%) who fixated their gaze at the outer target. The intraoperative measurement for the lateral rectus muscle insertion site fit better at the outer gaze fixation target than at the inner gaze fixation target (Table 1).
Comparison of AS-OCT and Intraoperative Measurements
The average intraoperative distance between the limbus and medial rectus muscle insertion site was 4.9 ± 0.6 mm. The average AS-OCT–measured distance was 4.9 ± 0.6 mm for the inner gaze fixation target (P = 1.000) and 5.7 ± 0.9 mm for the outer gaze fixation target (P = .0003). Comparison of the AS-OCT and intraoperative measurements revealed a less than 1 mm difference for 29 patients (80.6%) who fixated their gaze at the inner target and 21 patients (58.3%) who fixated their gaze at the outer target. The intraoperative measurement for the medial rectus muscle fit better at the inner gaze fixation target than the outer gaze fixation target (Table 1).
The Bland-Altman plots for the lateral rectus muscle showed better consistency with the outer gaze fixation target than with the inner gaze fixation target; the opposite was observed for the medial rectus muscle (Figure 3).
Bland-Altman plots showing the differences in the limbus and extraocular muscle insertion site distance for the (A) lateral rectus (LR) and (B) medial rectus (MR) muscles between intraoperative and anterior segment optical coherence tomography (AS-OCT) measurements at inner and outer gaze fixation. The difference between the intraoperative and AS-OCT measurements for the LR muscle is larger at inner gaze fixation (mean: 0.59; 95% CI: −0.86 to 2.04) than at outer gaze fixation (mean: −0.02; 95% CI: −1.25 to 1.21). The AS-OCT measurement at inner gaze fixation is smaller than the actual intraoperative measurement. The difference between the intraoperative and AS-OCT measurements for the MR muscle is smaller at inner gaze fixation (mean: −0.02; 95% CI: −1.53 to 1.51) than at outer gaze fixation (mean: −0.83; 95% CI: −2.81 to 1.15). The AS-OCT measurement at inner gaze fixation is closer to zero. SD = standard deviation
The TOMEY SS-1000 CASIA OCT system is equipped with peripheral fixation targets suitable for visualizing angles but not for EOM insertions. We placed fixation targets near the observational illumination windows (inner fixation targets) and peripheral fixation targets (outer fixation targets). These points were calculated as 20 and 35 degrees, respectively. In a previous study, Liu et al6 instructed patients to fixate their gaze medially at 30 degrees and laterally at 15 degrees during lateral rectus and medial rectus muscle recordings, respectively. Park et al1 measured their patients' fixed gaze at 15, 40, 50, and 60 degrees.
The medial rectus muscle insertion site was unclear when the patient's gaze was fixated more laterally, and measurement of the distance between the insertion site and corneal limbus was unreliable and inaccurate.
The differences observed for the lateral rectus and medial rectus muscles in the current study may be related to the difference in the distance between the limbus and their respective insertions sites (6.4 and 4.9 mm at their vertical centers, respectively) intraoperatively. When the eye is excessively rotated, the EOM is overly stretched and the AS-OCT image becomes unclear. When the eye is insufficiently rotated, the infrared light perpendicularly enters the muscle and the light is too randomly scattered to reveal a clear image.
Factors that can influence the intraoperative measurement of the distance between the corneal limbus and the muscle insertion site include a surgical caliper interval as large as 1 mm, the obscured localization of the corneal limbus even under microscopic observation, a curved or oblique line for muscle insertion, or variation in the power used to pull the hooked muscle. Any of these factors could lead to a discrepancy between intraoperative and AS-OCT measurements. Although AS-OCT is helpful in determining the location of EOM insertions on previously operated eyes, the measurements after EOM recession should be carefully considered. Notably, Rossetto et al8 reported that although 81% of AS-OCT and intraoperative measurements for the lateral rectus muscle were within 1 mm of agreement for the first surgery, the agreement rate decreased to 49% for the second surgery. For the medial rectus muscle, the agreement rate was 79% for the first surgery and 63% for the second surgery.
Although we realize that the measurements of AS-OCT on the reoperation cases are less accurate than the first surgery as mentioned by Rossetto et al,8 preoperative estimation of the muscle insertion site is important for reoperations, especially when previous surgical reports are not known, or for viewing unexpected muscle rotations after previous surgery to look for slipped muscle/stretched scar. If EOM is not found with AS-OCT in the suggested eye position, it is considered that the muscle was previously recessed or had slipped posteriorly and thus the muscle may not be found intraoperatively. In that situation, a surgeon should prepare an alternative surgical strategy.
Medial fixation at outer gaze for the lateral rectus muscle and lateral fixation at inner gaze for the medial rectus muscle are accurate techniques for assessing the distance between the limbus and EOM insertion site using AS-OCT compared with the intraoperative measurements. The fixation position changes the distance measured between the corneal limbus and muscle insertion site when using AS-OCT. This result indicates that it is necessary to consider the fixation point depending on the position of the EOM insertion site during surgery and AS-OCT may be used to for this purpose.
- Park KA, Lee JY, Oh SY. Reproducibility of horizontal extra-ocular muscle insertion distance in anterior segment optical coherence tomography and the effect of head position. J AAPOS. 2014;18(1):15–20. doi:10.1016/j.jaapos.2013.11.005 [CrossRef]
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- Haseoka T, Arai S, Washiyama M, et al. Evaluation of horizontal extraocular muscle insertion site using anterior segment optical coherence tomography. Article in Japanese. Folia Japonica de Ophthalmologica Clinica. 2017;10:129–133.
- De-Pablo-Gómez-de-Liaño L, Fernández-Vigo JI, Ventura-Abreu N, et al. Spectral domain optical coherence tomography to assess the insertion of extraocular rectus muscles. J AAPOS. 2016;20(3):201–205. doi:10.1016/j.jaapos.2016.02.011 [CrossRef]
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Comparison of AS-OCT and Intraoperative Measurements
|Group||Intraoperative Measurements (mm)||OCT Measurements at Inner (mm)||OCT Measurements at Outer (mm)|
|LR muscle (n = 46)|
| Mean ± SD (range), mm||6.3 ± 0.7 (4.0 to 7.5)||5.8 ± 0.7 (4.1 to 6.8)||6.4 ± 0.5 (4.8 to 7.7)|
| (Intraoperative measurements) minus (OCT measurements)||NA||0.5 ± 0.7 (−0.8 to 2.6)||−0.1 ± 0.6 (−1.3 to 2.0)|
| (Intraoperative measurements) minus (OCT measurements) % of muscles falling within a 1-mm difference||NA||73.9%||89.1%|
|MR muscle (n = 36)|
| Mean ± SD (range), mm||4.9 ± 0.6 (4.0 to 6.0)||4.9 ± 0.6 (3.4 to 6.2)||5.7 ± 0.9 (4.3 to 8.3)|
| (Intraoperative measurements) minus (OCT measurements)||NA||0.0 ± 0.8 (−1.9 to 1.8)||−0.8 ± 1.0 (−3.3 to 1.0)|
| (Intraoperative measurements) minus (OCT measurements) % of muscles falling within a 1-mm difference||NA||80.6%||58.3%|