Purpose: To evaluate the accuracy of ultrasound biomicroscopy (UBM) for measuring the insertional distance from the limbus of horizontal extraocular muscles (EOM) that had previous surgery.
Methods: This prospective, masked, observational study compared the distance of the horizontal EOM insertion from the limbus as measured by 50-MHz UBM, either preoperatively or at the time of the anesthesia, with that measured by surgical caliper intraoperatively.
Results: Forty-three horizontal rectus muscles in 23 subjects were measured using the UBM. Of these 43 muscles, 41 (22 medial rectus and 19 lateral rectus muscles) were successfully imaged by UBM and also received intraoperative surgical measurements. Two medial rectus muscles of one patient were inserted too far from the limbus (beyond 12 mm) to be detected by UBM. The mean measured distances of the horizontal muscle insertions from the limbus were 9.0 mm with UBM and 9.3 mm at surgery (P = 0.0001, showing no significant difference). For 33 (80.5%) of the muscles, the 2 methods agreed within 1 mm (P < 0.0001). There was no difference in accuracy for lateral rectus compared with medial rectus muscles or when comparing the muscles imaged by UBM under topical versus general anesthesia.
Conclusion: The 50-MHz UBM can accurately measure the horizontal EOM insertion distances from the limbus for muscles in patients that had previous surgery. The limit of detection was 12 mm from the limbus for the MR and 14 mm for the LR. Depending on the patient's age and level of cooperation, the UBM study can be done under either general anesthesia or topical anesthesia.