Evaluation of the ultrasound biomicroscope in strabismus surgery

Comparison of anterior segment optical coherence tomography and ultrasound biomicroscopy in localizing horizontal rectus muscle insertions

PURPOSE

To evaluate the viability and precision of measuring the distance from the limbus to extraocular muscle insertion using anterior segment optical coherence tomography (AS-OCT) and panoramic ultrasound biomicroscopy (UBM) before and after strabismus surgery.

METHODS

We recruited primary strabismus patients and measured the limbus-insertion distance by AS-OCT and UBM preoperatively, 2 weeks, and 1, 3, and 6 months postoperatively. Values were also measured using callipers intraoperatively before and after the planned procedures. Preoperative AS-OCT and UBM values were compared to intraoperative calliper measurements as the gold standard. Postoperative AS-OCT and UBM values were compared to the new postoperative limbus-insertion distance. The limit of agreement deemed clinically acceptable was defined as 1 mm.

RESULTS

A total of 85 horizontal muscles of 40 patients, including 48 lateral rectus muscles and 37 medial rectus muscles, were analysed. Primary muscles could be successfully detected by AS-OCT (95%) and UBM (100%). At 2 weeks and 1, 3, and 6 months postoperatively, the new rectus muscle attachment site detection rate by AS-OCT was 6%, 32%, 80%, and 89%, respectively, and that by UBM was 24%, 60%, 85%, and 93%, respectively. The Bland-Altman plots revealed better consistency in pairs of AS-OCT, UBM, and calliper measurements of primary muscles than postoperative muscles. For primary muscles, 89% of AS-OCT measurements fell within the permissible range of surgical measurements (1 mm), but this dropped to 67% at 6 months postoperatively (P < 0.001). The accuracy of UBM measurements of primary muscles was 81%, and this decreased to 59% at 6 months postoperatively (P = 0.001).

CONCLUSIONS

AS-OCT and UBM performed well in terms of imaging primary horizontal rectus muscles, but showed decreased accuracy and reproducibility in postoperative muscle measures.

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.

Ultrasound biomicroscopy in ophthalmology

This review presents data on the use of the method of ultrasonic biomicroscopy (UBM) of the anterior segment of the eye in ophthalmological practice in adults and children. Ultrasound biomicroscopy (UBM) is a contact non-invasive method for visualizing structures of the anterior segment of the eye using high-frequency ultrasound in the range from 35 to 100 MHz. Literature data indicate that UBM can be used to visualize almost all structures of the anterior segment, including the cornea, iridocorneal angle, anterior chamber, iris, ciliary body and lens, as well as the peripheral parts of the retina, vasculature and vitreous. There is data on the use of this method in the study of pathogenetic aspects of glaucoma, pseudoecfoliative syndrome, various types of cataracts, post-traumatic injuries of the anterior segment of the eye, meimobium gland dysfunction and other ophthalmopathologies. The use of UBM in children, due to the peculiarities of its implementation, is not widespread, but due to the specificity of the data obtained using it, it is promising. The limited information about the use of UBM in retinopathy of prematurity and the diagnostic capabilities of the method makes its use especially relevant in this severe retinal disease of premature newborns.

A systematic review of ultrasound biomicroscopy use in pediatric ophthalmology

Ultrasound biomicroscopy (UBM) is the only available option for noninvasive, high-resolution imaging of the intricate iridociliary complex, and for anterior segment imaging with corneal haze or opacity. While these unique features render UBM essential for specific types of trauma, congenital anomalies, and anterior segment tumors, UBM imaging has found clinical utility in a broad spectrum of diseases for structural assessments not limited to the anterior intraocular anatomy, but also for eyelid and orbit anatomy. This imaging tool has a very specific niche in the pediatric population where anterior segment disease can be accompanied by corneal opacity or clouding, and anomalies posterior to the iris may be present. Pediatric patients present additional diagnostic challenges. They are often unable to offer detailed histories or fully cooperate with examination, thus amplifying the need for high-resolution imaging. This purpose of this systematic review is to identify and synthesize the body of literature involving use of UBM to describe, evaluate, diagnose, or optimize treatment of pediatric ocular disease. The collated peer-reviewed research details the utility of this imaging modality, clarifies the structures and diseases most relevant for this tool, and describes quantitative and qualitative features of UBM imaging among pediatric subjects. This summary will include information about the specific applications available to enhance clinical care for pediatric eye disease.

Horizontal Extraocular Muscle Insertion Site in Relation to Axial Length Using Swept-Source Anterior Segment OCT

Purpose

The purpose of this study was to correlate between the axial length of the globe and the insertion site of horizontal extraocular muscles using swept-source anterior segment optical coherence tomography (SS-ASOCT), with posing an equation to calculate the muscle insertion site from the axial length.

Methods

The study design was observational and cross-sectional. It was performed on 157 eyes of 157 healthy subjects. The distance of the medial rectus (MR) and the lateral rectus (LR) insertion sites from the limbus were measured using SS-ASOCT. The insertion sites’ distances were correlated to the axial length (hypermetropes < 22.5 mm, myopes > 24.5). Correlation between numerical variables was done by Pearson’s correlation coefficient and confirmed by linear regression analysis and scatter diagrams.

Results

The mean MR insertion site was 5.47 ± 0.19 mm in hypermetropes versus 5.68 ± 0.23 mm in myopes, whereas the mean LR insertion site was 6.81± 0.23 mm in hyperopes versus 7.08 ± 0.16 mm in myopes. The axial length showed a moderate positive, but significant, correlation to the insertional position for the medial and lateral rectus muscles (MR: r=0.417, p<0.001; LR: r=0.410, p<0.001).

Conclusion

Comparing the horizontal extraocular muscle insertion site to axial length using SS-ASOCT showed a significant positive correlation. The model equation for MR insertion: MR (mm) = 4.522 + 0.045 (AXL in mm) with an R = 0.437, R²= 0.191, F=12.071, P<0.001. The model equation for LR insertion: LR (mm) = 5.72 + 0.048 (AXL in mm) with an R = 0.438, R²= 0.192, F=12.116, P<0.001.

Accuracy of anterior segment optical coherence tomography for pre-operative localization of insertions of extraocular recti muscles

Introduction:

To study the accuracy of anterior segment optical coherence tomography (AS-OCT) function of Cirrus HD-OCT 500 (Carl Zeiss Meditec, Dublin, CA) in calculating the insertion of rectus muscles from the limbus in patients undergoing primary squint surgery.

Methodology:

A prospective, double-masked, observational study was conducted on 128 muscles of 46 patients with strabismus who were planned for strabismus surgery between January 2019 and December 2019. Insertion distance from limbus was measured using AS-OCT function of Cirrus HD-OCT 500 preoperatively. Intraoperative measurements were taken using Castroviejo caliper. A Bland-Altman analysis was performed to determine the agreement between the readings.

Results:

A total of 128 muscles were successfully imaged and evaluated, including 13 superior recti (10.15%), 23 inferior recti (17.96%), 35 medial recti (27.34%) and 57 lateral recti (44.53%). 124 muscles (96.9%) were within 1mm difference, which was considered clinically acceptable. Bland-Altman plots showed the level of agreement between the two methods was good.

Conclusion:

Anterior segment scan function of Cirrus HD-OCT 500 is an accurate method to detect the insertion of extraocular recti muscles in patients of primary strabismus surgery.

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.

Accuracy of the Ultrasound Biomicroscopy Bag/Balloon Technique in Locating Horizontal Extraocular Muscle Insertions Before and After Strabismus Surgery

PURPOSE

To investigate the accuracy of high frequency ultrasound biomicroscopy using the bag/balloon technique to locate recti muscle insertions before and after strabismus surgery.

METHODS

This was a prospective masked study. The distance from the limbus to horizontal recti muscle insertions was measured by caliper intraoperatively and ultrasound biomicroscopy preoperatively and during follow-up. Accuracy was defined as the difference between ultrasound biomicroscopy and caliper measurements.

RESULTS

Thirty-nine muscles (19 medial rectus and 20 lateral rectus) of 22 patients were included. The mean age of the patients was 34.7 ± 15.5 years (range: 18 to 78 years). Follow-up measurements were available for 25 muscles: 12 muscles were recessed and 13 were resected/advanced. Twelve of the included muscles underwent prior surgery. There was a strong correlation between preoperative mean ultrasound biomicroscopy and caliper measurements (P < .001, r_p = 0.872) without significant difference between the accuracy of identification of medial rectus and lateral rectus muscles (P = .116 and .377, respectively). On average, postoperative ultrasound biomicroscopy measurements of recessed muscles were 1.15 mm greater than caliper measurements at the end of surgery (P = .003), whereas the mean difference in the resected muscles was not statistically significant (-0.07 mm, P > .999). Overall, in 16 of 25 muscles (64%), the differences between the postoperative measurements and the end of surgery measurements were less than 1 mm. There was no correlation between the accuracy and the time of ultrasound biomicroscopy after surgery (P = .516).

CONCLUSIONS

The ultrasound biomicroscopy bag/balloon technique is an accurate and reliable method of locating recti muscle insertions before and after strabismus surgery. Ultrasound biomicroscopy may assist strabismologists in planning repeated operations. [J Pediatr Ophthalmol Strabismus. 2020;57(1):12-20.].

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.

Magnetic Resonance Imaging of the Globe-Tendon Interface for Extraocular Muscles: Is There an "Arc of Contact"?

PURPOSE

To determine if the "arc of contact" is an accurate approximation of the globe-tendon interface for the biomechanical modeling of extraocular muscle (EOM) force transfer onto the globe.

METHODS

At a single academic institution, 18 normal and 14 strabismic subjects were prospectively recruited for surface-coil enhanced magnetic resonance imaging at 312- or 390-μm resolution in axial planes for horizontal EOMs (23 subjects, 26 orbits) and sagittal planes for vertical EOMs (13 subjects, 22 orbits) during large ipsiversive ductions. The measured angle at insertion and the predicted angle assuming an "arc of contact" were compared using paired t tests.

RESULTS

For normal EOMs, the measured angle at insertion was significantly greater than predicted assuming an "arc of contact" for the medial rectus (MR) (5.0 ± 4.8 degrees vs 0.0 ± 0.0 degrees, P = .03), lateral rectus (LR) (4.9 ± 3.0 degrees vs 0.0 ± 0.0 degrees, P = .02), inferior rectus (7.4 ± 4.8 degrees vs 1.2 ± 2.6 degrees, P = .00003), and superior rectus (0.6 ± 1.1 degrees vs 0.0 ± 0.0 degrees, P = .04). In strabismic subjects, the measured angle was significantly greater for the MR in abducens palsy (9.9 ± 4.3 degrees vs 0.5 ± 0.7 degrees, P = .0007) and after MR resection (9.0 ± 6.9 degrees vs 1.2 ± 2.4 degrees, P = .02), but not after LR recession (2.9 vs 0.0 degrees). Single subjects had comparable angles after MR recession, but markedly different angles after MR and LR posterior fixation.

CONCLUSIONS

Contrary to the "arc of contact" biomechanical model, normal and postsurgical EOMs are significantly non-tangent to the globe at their scleral insertions. The "arc of contact" should be replaced in biomechanical modeling by the experimentally measured angles at tendon insertions. NOTE: Publication of this article is sponsored by the American Ophthalmological Society.

Horizontal Extraocular Muscle and Scleral Anatomy in Children: A Swept-Source Anterior Segment Optical Coherence Tomography Study

PURPOSE

To evaluate the efficacy of anterior segment swept-source optical coherence tomography (SS-OCT) for examining horizontal extraocular muscle thickness, distance from the corneal limbus to the insertion of the horizontal extraocular muscle (limbus to insertion distance), and scleral thickness in Korean pediatric strabismus patients.

METHODS

This study included pediatric strabismus patients between 5 and 10 years of age. Children with any ocular disease other than strabismus or a history of ocular surgery were excluded. SS-OCT was used to measure horizontal extraocular muscle thickness, limbus to insertion distance, and scleral thickness. Eyes were classified into subgroups by sex, spherical equivalent of the refractive error (measured with cycloplegic refraction), and fixating/deviating eye.

RESULTS

One patient initially included in this study was excluded due to poor cooperation. Of the remaining 35 eyes of 20 patients, 19 eyes (54.3%) were from male patients and 16 eyes (45.7%) were from female patients. The mean patient age was 7.86 ± 1.38 years. Lateral scleral thickness was greater in male eyes than in female eyes (p = 0.048). No other differences were noted between male and female children. Additionally, there were no statistically significant differences between fixating and deviating eyes or among spherical equivalent groups for any parameter examined.

CONCLUSIONS

Anterior segment SS-OCT can successfully and comfortably measure horizontal extraocular muscle and scleral anatomy in children. These measurements may be helpful for treatment and follow-up of pediatric strabismus patients.

The role of imaging in strabismus

PURPOSE OF REVIEW

To review the most recent literature regarding the clinical experience of imaging modalities in strabismus.

RECENT FINDINGS

MRI of extraocular muscles (EOMs) has elucidated the roles of variation in compartmental contraction of EOMs and further evidence of EOM pulley heterotopy in various strabismus patterns, which may contribute to the clinical and surgical management of patients as this mode of imaging becomes more readily available. Ultrasound biomicroscopy (UBM) and anterior-segment optical coherence tomography (AS-OCT) may play a role in determining the location of EOMs which could also aid in devising a preoperative surgical plan, especially in those who have had prior strabismus surgery.

SUMMARY

Although MRI, UBM, and AS-OCT show promise in the formulation of a treatment plan in complex strabismus, more research regarding the role and limitations of these imaging modalities is required before they become the mainstay of strabismus evaluation.

Postoperative change in lateral rectus muscle insertion measured by anterior segment optical coherence tomography

AimsThe aims of this study were to investigate the longitudinal change in lateral rectus (LR) muscle insertion after recession surgery, and to evaluate a relationship between insertion distance and postoperative amount of deviation.MethodsWe recruited 31 patients who underwent primary LR recession surgery with normal anterior segment structures. An AS-OCT scan of the LR muscle was performed at every visit. Data on sex, age, degree of deviation (prism diopter), and spur-LR insertion distance using AS-OCT were collected at preoperatively and postoperative months 1, 3, and 6. Spur-LR insertion was defined as the shortest distance between the insertion of the LR muscle and the scleral spur was measured using the caliper function in the AS-OCT software.ResultsThe mean distance of spur-LR insertion measured with AS-OCT was 5.5±0.7 mm preoperatively, and 11.1±0.9 mm at 1 month, 11.5±0.8 mm at 3 months, and 11.0±0.7 mm at 6 months postoperatively. There were significant differences in spur-LR insertion between postoperative 1 and 3 months, and between 3 and 6 months (generalized estimating equation, all P<0.001). The overall spur-LR insertion measurement has a significantly negative correlation with postoperative amount of deviation (Pearson's correlation, P=0.035, r=-0.218).ConclusionsAS-OCT is a useful instrument for understanding postoperative changes in EOM after tissue swelling subsides.

Accuracy of Optical Coherence Tomography Measurements of Rectus Muscle Insertions in Adult Patients Undergoing Strabismus Surgery

PURPOSE

To assess the accuracy of anterior segment optical coherence tomography (AS-OCT) in measuring the distance of extraocular muscle (EOM) insertion to the limbus to improve preoperative assessment of adult patients undergoing strabismus surgery.

DESIGN

Reliability analysis.

METHODS

setting: An institutional practice.

PATIENT POPULATION

Seventy-four adult patients scheduled for strabismus surgery on rectus muscles.

OBSERVATION PROCEDURE

The distance between the EOM insertion and the limbus was measured preoperatively with AS-OCT. The value was compared with the intraoperative measurement obtained with calipers. Additional measurements included the limbus-anterior chamber angle distance with AS-OCT and the axial length with IOLMaster.

MAIN OUTCOME MEASURE

Agreement between preoperative AS-OCT and intraoperative measurements. A difference of ≤1 mm was "clinically acceptable."

RESULTS

A total of 144 muscles were analyzed. Thirty-one of 33 reoperated muscles were successfully imaged. AS-OCT measurements were within 1 mm of intraoperative measurements in 77% of all muscles. The accuracy was higher for muscles with no prior surgery (83%), as compared with reoperated muscles (58%). Although the accuracy decreased when comparing reoperations to primary surgeries for the medial (79% to 63%; P = .09; 95% confidence interval [CI], -1.38 to 0.11) and the lateral rectus (81% to 49%; P = .11; 95% CI, -2.06 to 0.22), the difference was not significant. No correlation between limbus-anterior chamber angle distance and axial length was established.

CONCLUSIONS

AS-OCT is valuable in identifying EOM insertions in primary strabismus surgeries, but the accuracy decreases in reoperations.

Agreement between intraoperative measurements and optical coherence tomography of the limbus-insertion distance of the extraocular muscles

OBJECTIVE

To assess the agreement between intraoperative measurements of the limbus-insertion distance of the extraocular muscles with those measured by spectral domain optical coherence tomography.

METHODS

An analysis was made of a total of 67 muscles of 21 patients with strabismus. The limbus-insertion distance of the horizontal rectus muscles were measured using pre-operative SD-OCT and intra-operatively in 2 ways: 1) direct, after a conjunctival dissection in patients who underwent surgery, or 2) transconjunctival in patients who were treated with botulinum toxin, or in those who were not going to be operated. The intraclass correlation coefficient and Bland-Altman plots were calculated to determine the concordance between the 2 methods.

RESULTS

The mean age was 45.9 ±20.9 years (range 16 to 85), with 52% being women. The percentage of identification by direct intraoperative measurement was 95.6% (22/23), by transconjunctival intraoperative measurement 90.9% (40/44), and by OCT 85% (57/67), with 22 muscles finally being analysed for the agreement study between direct intraoperative measurement and OCT measurements, and 35 muscles for the agreement between transconjuctival intraoperative measurement and OCT. The intraclass correlation coefficient showed good agreement with OCT and direct intraoperative measurements (0.931; 95% confidence interval (95% CI): 0.839-0.972; P<.001), and with transconjunctival intraoperative measurements (0.889; 95% CI: 0.790-0.942; P<.001).

CONCLUSIONS

The SD-OCT is an effective technique to measure the distance from the insertion of the horizontal rectus muscles to the limbus, with a high agreement with intraoperative measurements being demonstrated.

Anterior Segment Optical Coherence Tomography of the Horizontal and Vertical Extraocular Muscles With Measurement of the Insertion to Limbus Distance

PURPOSE

To assess the possibility of determining the insertion distance from the limbus of horizontal and vertical extraocular rectus muscles with anterior segment optical coherence tomography (AS-OCT).

METHODS

The right eyes of 46 patients underwent AS-OCT. The horizontal and vertical extraocular rectus muscle insertion distances from the limbus were measured in a masked fashion by two pediatric ophthalmologists.

RESULTS

Forty-two lateral rectus, 43 medial rectus, 35 inferior rectus, and 40 superior rectus muscles of the right eyes of 46 patients were included. Insertion to limbus measurements (mean ± SD) were as follows: lateral rectus = mean 6.8 ± 0.7 mm, range = 4.8 to 8.4 mm; medial rectus = mean 5.7 ± 0.8 mm, range = 4.3 to 7.8 mm; inferior rectus = mean 6.0 ± 0.6 mm, range = 4.8 to 7.0 mm; superior rectus = mean 6.8 ± 0.6 mm, range = 5.5 to 8.1 mm. The intraobserver and interobserver correlation coefficients for the insertion to limbus measurements of all four rectus muscles exceeded 0.75 (excellent correlation).

CONCLUSIONS

The study showed that AS-OCT is capable of imaging all four of the rectus muscle insertions and measuring the insertion to limbus distance, and is the second AS-OCT study to image the superior and inferior rectus muscle insertions. The insertion to limbus measurements between examiners and on repeat measurements were consistent and reproducible. The ability to accurately image extraocular rectus muscle insertions may have future implications for the preoperative procedure planning in patients who have had previous surgery. [J Pediatr Ophthalmol Strabismus. 2016;53(3):141-145.].

Evaluation of vertical rectus muscles using ultrasound biomicroscopy

PURPOSE

To evaluate the accuracy of ultrasound biomicroscopy (UBM) in measuring the distance (in mm) from limbus to the insertion of vertical rectus muscles (superior rectus and inferior rectus compared with the "gold standard" surgical caliper at the time of surgery.

METHODS

Prospective, masked, observational study of 31 vertical rectus muscle insertions in which we compared the measurements from the limbus as measured by 50 MHz UBM, either preoperatively or at the time of anesthesia, with that measured by surgical caliper intraoperatively. Measurements (UBM and surgical) were evaluated by two different observers and analyzed using the Bland-Altman method. All UBM measurements were done by the same author. The intraclass correlation coefficient (ICC) and Pearson coefficient with 95% confidence intervals were used to quantify the degree of agreement between the two methods.

RESULTS

Thirty-one vertical muscles were evaluated (13 superior rectus and 18 inferior rectus, of which 7 muscles were reoperations). The average for UBM measurements was 6.63 mm and for surgical caliper was 7.09 mm. The measurements for the two methods were all within +/-2 standard deviations of the mean. Only three measurements showed differences more than 1 mm. The ICC was 0.78 and Pearson coefficient was 0.85, indicating a "very good" correlation between the two methods. The longest distance from the limbus that could be accurately measured with the UBM was 12 mm. In one case a pseudotendon was differentiated from the true insertion of a previously recessed superior rectus muscle.

CONCLUSIONS

The UBM and surgical measurements showed "very good" correlation when allowing for a margin of error of +/-1.0 mm between the two modalities, indicating that the UBM is a good predictor of the position of the vertical rectus muscles.

Ultrasound biomicroscopy in strabismus reoperations

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.