Measurement of extraocular horizontal muscle insertion distance via anterior segment optical coherence tomography of healthy children and comparison with healthy adults

Conformation of Horizontal Extraocular Muscle Insertions in Acute Acquired Concomitant Esotropia Induced by Excessive Digital Device Usage

Purpose

Acute acquired concomitant esotropia induced by excessive digital device usage, especially smartphones (SAACE), has been increasing over the past few years. Convergence spasm induced by excessive near work has been suggested as a mechanism. Anatomical differences could also potentially contribute to SAACE onset. The present study investigated the conformation of horizontal recti between SAACE patients and normal subjects.

Patients and Methods

In 15 SAACE patients (SAACE group), the distances between the limbus and insertion of the horizontal recti (LI distance) and the widths of horizontal recti on the insertion (insertion width) were measured. The control group consisted of 30 patients who underwent retinal detachment surgery. Differences in LI distances and insertion widths were compared between SAACE and control groups.

Results

While there were no differences between the two groups for LI distances and insertion widths of lateral recti, there were significantly shorter LI distances for the medial recti in the SAACE group (P<0.05). Moreover, the SAACE group tended to exhibit larger insertion widths of the medial recti. Medial/lateral ratio of LI distances were significantly lower and insertion widths were significantly higher in the SAACE compared to the control group (P<0.05).

Conclusion

Based on the observations of more anterior insertion and larger muscle widths, this suggests there are stronger forces of medial recti in SAACE. In addition to excessive accommodation followed by increases in medial recti tonus, the results also suggest that an anatomical imbalance between lateral and medial recti contributes to esotropia onset following excessive near work.

Efforts to improve the surgical outcome for exotropia and investigation of the insertion locations of the extraocular muscles

In our previous retrospective study, we found that using the strabismus surgery dosages established by western strabismus mentors tends to result in undercorrection of Taiwanese exotropia (XT) patients compared with those in western populations. We also discovered that the location of extraocular muscle (EOM) insertion could vary by ethnicity. In this study, using a generalized estimation equation model, we compared the XT surgery outcome between augmented and original strabismus surgery dosages in Taiwanese patients. We also conducted an observational study to investigate the horizontal EOM insertion location in a Taiwanese population and compared the data with Dr. Apt L.'s study. For Taiwanese XT patients, augmented surgical dosages resulted in significantly better outcome at 6 months and 1 year postoperatively compared with original surgical dosages (p = 0.003 and p < 0.001, respectively). The distance from the lateral recuts muscle (LR) insertion location to the limbus was significantly shorter in Taiwanese than in white Americans (6.5 vs. 6.9 mm, respectively, p = 0.0001). Furthermore, the medial rectus muscle and LR insertion locations differed significantly between males and females (p < 0.001 and p = 0.023, respectively). The patients' sex did not affect the surgery outcome. Augmented surgery doses modified from western strabismus mentors produce better surgery outcome for Taiwanese XT patients. Surgeons may require country-specific guidelines for strabismus surgery dosage. We also demonstrated a simple method for young ophthalmologists to establish their own normograms to improve their surgical success rate. Our study confirms that LR insertion locations differ between Taiwanese and White Americans.

Shape analysis of rectus extraocular muscles with age and axial length using anterior segment optical coherence tomography

Purpose

This study aimed to evaluate the shape of the extraocular muscles (EOMs) in normal subjects using the en-face images of anterior segment optical coherence tomography (AS-OCT). The EOM insertion and the direction of the muscle fibers were investigated.

Subjects and methods

A total of 97 healthy normal subjects (194 eyes) at Okayama University Hospital (age, 47.1±21.5 years; range, 8–79 years) participated in the study. A series of 256 tomographic images of the rectus EOMs were captured using the C-scan function of the AS-OCT (CASIA2, TOMEY Co., Japan), and the images were converted to en-face images in multi-TIFF format. The anterior chamber angle to EOM insertion distance (AID) and the angle of the muscle fibers from the insertion site (angle of muscles) were measured from the images. The correlations of AID and angle of muscles with age and axial length were investigated and evaluated.

Results

AID and angle of muscles were significantly correlated with age or axial length in some EOMs. The AIDs of medial rectus (MR) (P = 0.000) and superior rectus (SR) (P = 0.005) shortened with age. The AIDs of MR (P = 0.001) and inferior rectus (IR) (P = 0.035) elongated with axial length, whereas lateral rectus (LR) (P = 0.013) shortened. The angles of MR (P = 0.001) and LR (P = 0.000) were found to have a more downward direction toward the posterior in older subjects.

Conclusion

En-face images can be created by AS-OCT, and the shape of the EOMs in normal subjects using these image measurements was available. With the ability to assess the EOMs, AID and angle of muscles are expected give useful information for treating and diagnosing strabismus-related diseases.

Extraocular muscle insertion shift after disinsertion during strabismus surgery

PURPOSE

To quantify the amount of insertion shift after disinsertion of the rectus muscles and identify factors that may influence the shift.

METHODS

Patients who underwent rectus muscle surgery between November 2018 and April 2019 were included. During surgery, the limbal-insertion distance (LID) distance was measured in millimeters with calipers from the limbus to the center of the insertion at the anterior border of the rectus muscle prior to and after disinsertion. The primary outcome was the shift of the rectus muscle insertion after disinsertion. This was calculated by subtracting the LID after disinsertion from the LID before disinsertion. The secondary outcome was the identification of preoperative and intraoperative factors that influenced insertion shift. Randomization was performed to select one rectus muscle per patient. Patients with a history of prior scleral buckle procedure, glaucoma drainage device, orbital wall fracture and strabismus surgery in the same rectus muscle were excluded.

RESULTS

110 patients were included. The median (Q₁, Q₃) LID before disinsertion was shortest for the medial rectus muscle [5.0 (4.5, 6.0)], followed by lateral rectus muscle [6.0 (6.0, 7.0)] and inferior rectus muscle [6.0 (5.5, 7.0)]. The overall median (Q₁, Q₃) insertion shift was 1.0 (0.8, 1.0) mm (p < .001), which did not significantly differ between each rectus muscle subgroup (p = .158). Factors that influenced the amount of shift were moderate to severe restriction on forced duction testing (FDT) (B = 0.320, SE = 0.105, p = .003) and longer LID before disinsertion (B = 0.172, SE = 0.036, p < .001).

CONCLUSIONS

We found a significant anterior insertion shift after disinsertion of rectus muscles. Moderate to severe restriction on FDT and longer LID before disinsertion can result in larger insertion shifts.

Pre-, Intra-, and Post-Operative Evaluation of Extraocular Muscle Insertions Using Optical Coherence Tomography: A Comparison of Four Devices

OCT (optical coherence tomography) is widely used in ophthalmology and pediatric ophthalmology, but limited research has been done on the use of OCT in strabismus. This study investigates the use of different OCT machines to image rectus muscle insertions pre-, intra-, and post-operatively in pediatric strabismus patients. The OCT machines used in the study were a Bioptigen (Leica Microsystems Inc., Buffalo Grove, IL, USA), Spectralis HRA+OCT with Anterior Segment Module (Heidelberg Engineering, Heidelberg, Germany), Visante (Carl Zeiss, Oberkochen, Germany), and Zeiss Rescan 700 (Carl Zeiss, Oberkochen, Germany). Measurements from the machines were compared with the caliper distance measured during the strabismus surgery before disinsertion or after reattachment. The OCT machines had moderate (Bioptigen: 0.62) to good intraclass correlation coefficients (Rescan: 0.83, Spectralis: 0.85, Visante: 0.88) with intra-operative measurements. To our knowledge, this is the first study to use an operating microscope with integrated intra-operative OCT to image rectus muscle insertions. OCT is a useful tool in strabismus surgical patients in the pre-, intra-, and post-operative settings, particularly in patients who have had previous surgery, when the muscle insertion is unknown. The ability to accurately image rectus muscle insertions has significant implications for the management of strabismus patients.