The influence of axial length on the response to strabismus surgery

Revisiting the surgical table: An analysis of surgical dose-response in Asian exotropia

BACKGROUND

Previous research on the factors associated with surgical dose-response in strabismus surgery for exotropia has yielded inconsistent results. This study determined the factors influencing surgical dose-response in exotropia patients who underwent recession and resection (R&R).

METHODS

Exotropia patients who underwent unilateral R&R at the National Taiwan University Hospital between 2006 and 2021 were evaluated. Deviation-angle differences in prism diopters (PD) were measured preoperatively and at 1 month postoperatively. Surgical dose-response (PD/mm) was defined as the difference in deviation angle (in PD) divided by the surgical dose in millimeters. Linear and non-linear regression models were used to evaluate the influence of variables including age, sex, axial length, and preoperative deviation on surgical dose-response.

RESULTS

Overall, 295 patients (162 children; 133 adults) were included. Average surgical dose-response in the pediatric and adult groups was 2.82 ± 0.60 PD/mm and 3.02 ± 0.62 PD/mm, respectively. Male sex was negatively correlated with surgical dose-response in children. The surgical dose-response was larger in adults with longer axial length (>25.64 mm) and patients with larger preoperative deviation (>42.6 PD and >38.7 PD in pediatric and adult groups, respectively). Surgical dose-responses peaked at 35.1 years.

CONCLUSION

Age, axial length, and preoperative deviation have a nonlinear effect on surgical dose-responses in exotropia patients undergoing R&R. Surgical dose-responses were larger in patients in young adulthood, with longer axial length and larger preoperative deviation angle. A table with fitted values for surgical dose-response based on age, axial length, and preoperative deviation was established for clinical reference.

Acquired comitant esotropias - comparison of surgical outcomes of accommodative vs non-accommodative types

Purpose: To compare the motor and sensory outcomes of strabismus surgery and the factors affecting surgical success in acquired acute non-accomodative esotropia (ANAET) and partially accommodative refractive esotropia (pARET). Methods: A retrospective chart review of patients with ANAET and pARET who underwent unilateral or bilateral horizontal rectus muscle surgery between January 2020 and December 2021 was conducted. Patients with postoperative follow-up of at least six weeks were included. Patients with pattern deviation, lateral incomitance, and near-distance disparity were excluded. Motor success was defined as a postoperative deviation within eight prism diopters of orthophoria. Sensory success was defined as presence of binocular single vision (BSV) for both distance and near (Worth four dot test). The effect of factors like age at onset, age at surgery, amblyopia before surgery, duration of squint before treatment, presence or absence and magnitude of vertical deviation, preoperative angle of deviation, and spherical equivalent on the motor and sensory success in each group were analyzed and compared. Results: 38 patients with ANAET and 33 patients with pARET were included. The mean age of onset of esotropia was 8.55 ± 4.65 years and 4.39 ± 2.27 years (p < .001) and the mean age at surgery was 10.62 ± 4.99 years and 7.89 ± 2.84 years (p = .006) in the ANAET and the pARET group respectively. The mean duration of the final follow-up was 38.51 weeks in the ANAET and 48.68 weeks in the pARET group (p = .089). Patients were successfully aligned for both distance and near in 81.5% of patients in the ANAET and 78.9% of patients in the pARET group at the final follow-up (p. 0.775). A BSV for both distance and near at the final follow-up was seen in 81.2% vs 66.6% of patients in the ANAET and the pARET group respectively (p = .25). A good near stereoacuity (<120 arcsecs) was seen in 60.6% and 41.9% of the ANAET and the pARET groups respectively (p = .175). The percentage of patients in the ANAET group who had orthophoria, any esodeviation, and any exodeviation for distance at the final follow-up was 63.1%, 34.2% and, 2.6%. The percentage of patients in the pARET group in similar categories was 36.3%, 42.4% and, 21.2%. None of the demographic and preoperative factors were found to affect the surgical outcomes in the two groups. Conclusions: The motor and sensory outcomes were similar in the two groups. A higher proportion of ANAET patients remained orthophoric during the follow-up. The patients in the pARET group showed a tendency toward exodrift.

Comparison of the results of the modified and classical bi-medial hang-back recession in infantile esotropia

PURPOSE

Bi-medial rectus recession, which can also be performed using a hang-back technique, is one of the surgical treatment options for infantile esotropia (IE). This study has modified the surgical approach, with outcomes compared to the traditional hang-back technique.

METHODS

The bi-medial recession was performed with a modified hang-back technique in 120 IE patients and with a traditional hang-back technique in 88 cases. Surgical outcomes were reviewed and compared retrospectively.

RESULTS

The patients in the two groups were compared in terms of surgery time, inferior oblique weakening surgery, and the presence of refractive error. The differences between pre-operative and postoperative first-month, sixth-month, and first-year degrees were statistically significant (p < 0.001).

CONCLUSIONS

This modified novel technique aims to avoid unwanted muscle movement in the horizontal and vertical axes and a gap in the middle of the recessed muscle, as seen in the traditional hang-back technique. Further, the modified technique resulted in less over- and under-correction as well as alphabetic pattern deviation.

Augmented-Dose Unilateral Recession-Resection Procedure in Acute Acquired Comitant Esotropia

PURPOSE

To compare the surgical amount of unilateral medial rectus recession and lateral rectus resection (RR) in patients with acute acquired comitant esotropia (AACE) versus common forms of esotropia and to provide dose-response reference for surgical planning in AACE.

DESIGN

Retrospective study.

PARTICIPANTS

Consecutive patients who underwent unilateral RR for AACE or common forms of esotropia correction from January 2018 to January 2022. Only patients who achieved motor and sensory success with a minimum follow-up of 3 months were analyzed.

METHODS

Group differences in the amount of medial rectus (MR) recession and lateral rectus (LR) resection were analyzed using multivariate regression models. Surgical dose responses in AACE were analyzed using multivariable regression models. Regression models were performed with and without adjustment for clinical confounders. Piecewise analysis was used to detect segmented results.

MAIN OUTCOME MEASURES

Group difference in surgical amount and dose responses of unilateral RR in AACE.

RESULTS

Fifty-four patients with AACE and 98 patients with common forms of esotropia were included. To correct comparable deviations, surgical amount performed was significant larger in AACE patients, with the adjusted group differences of 0.49 mm (95% confidence interval [CI], 0.34-0.65 mm; P < 0.001) in MR recession and 1.68 mm (95% CI, 1.25-2.11 mm; P < 0.001) in LR resection in patients with deviation < 40 prism diopters (Δ) and with a group difference of 1.22 mm (95% CI, 0.76-1.68 mm; P < 0.001) in LR resection in those with deviation ≥ 40 Δ. In AACE patients, in the adjusted model taking into account the intercept, dose responses of MR recession and LR resection in patients with deviation < 30 Δ were 5.11 Δ/mm (95% CI, 0.98-9.23 Δ/mm; P = 0.02) and 2.51 Δ/mm (95% CI, 0.57-4.45 Δ/mm; P = 0.02), respectively, and for those with deviation ≥ 30 Δ, the dose response of additional LR resection was 5.48 Δ/mm (95% CI, 4.56-6.40 Δ/mm; P < 0.001) to correct remaining deviation beyond 30 Δ.

CONCLUSIONS

These findings provide quantitative evidence that augmented-dose unilateral RR should be performed in AACE for favorable surgical outcomes. The new surgical dose calculation proposed provides reference for surgical planning.

FINANCIAL DISCLOSURE(S)

The author(s) have no proprietary or commercial interest in any materials discussed in this article.

Outcomes of a simplified, fixed surgical dosage calculation for uncomplicated, horizontal, concomitant strabismus in adults

Purpose

The aim of this study was to evaluate the outcomes of a simplified, fixed surgical dosage calculation for uncomplicated, horizontal, concomitant strabismus in adults.

Methods

Outcomes analysis of a fixed-dose calculation method for uncomplicated, horizontal, concomitant strabismus in adults (≥18 years) wherein 1-mm recession/resection equals 2 PD for lateral rectus and 3 PD for medial rectus. This was a retrospective case series.

Results

The mean age of the patients was 28.50 ± 8.43 years, the mean amount of preoperative deviation was 35.16 ± 9.97 PD, the mean expected correction was 36.26 ± 9.49 PD, and the mean correction achieved was 35.92 ± 10.74 PD. There were 22 monocular exotropias, six monocular esotropias, eight alternate divergent squints, and two alternate convergent squints. There were 20 cases of sensory strabismus (54.28%). There was no statistically significant difference between the expected correction and correction achieved (P = 0.519), meaning that our fixed-dose calculation method was effective. Thirty-five out of 38 patients had successful outcomes as per our criteria of less than 10-PD deviations from straight in primary position. Therefore, the success rate achieved by this procedure was 92.10%. There was no statistically significant correlation between age and the amount of deviation (P = 0.611).

Conclusion

Our case series had a high postoperative success rate in terms of motor alignment. It is hoped that this simplified, fixed-dosage calculation method would help the numerous novice strabismus surgeons, make their starting steps easier and give them the confidence to do strabismus surgeries. They can modify the dosages later as per their own experiences.

The Effect of Axial Length on Extraocular Muscle Leverage

PURPOSE

Magnetic resonance imaging was used to determine the effect of axial length (AL) on globe rotational axis and horizontal extraocular muscle leverage during horizontal duction.

DESIGN

Prospective observational case series.

METHODS

At a single academic center, 36 orthophoric adults with a wide range of ALs underwent high-resolution axial orbital magnetic resonance imaging in target-controlled adduction and abduction. ALs were measured in planes containing maximum globe cross-sections. Area centroids were calculated to determine globe centers. Rotational axes in orbital coordinates were calculated from displacements of lens centers and globe-optic nerve attachments. Lever arms were calculated as distances between published extraocular muscle insertions and rotational axes.

RESULTS

ALs averaged 26.3 ± 0.3 mm (standard error [range 21.5-33.4 mm]). Rotational axes from adduction to abduction averaged 1.1 ± 0.2 mm medial and 1.1 ± 0.2 mm anterior to the globe's geometric center in adduction. Linear regression demonstrated no significant correlation between AL and rotational axis horizontal (R² = 0.06) or anteroposterior (R² = 0.07) position. Medial rectus (MR) lever arms averaged 12.0 ± 0.2 mm and lateral rectus (LR) lever arms averaged 12.8 ± 0.2 mm. Both MR (R² = 0.24, P < .001) and LR (R² = 0.32, P < .001) lever arms significantly increased by about 0.3 mm per 1.0-mm of increased AL, with a corresponding reduction in predicted per-millimeter effect of surgical repositioning of their insertions.

CONCLUSIONS

Regardless of AL, the globe rotates about a point nasal and anterior to its geometric center, giving the LR more leverage than the MR. This eccentricity may diminish the effect of tendon repositioning in moderate to highly myopic patients, with reductions in per-mill imeter dose/response predicted with longer AL.

Consideration of eyeball length and prismatic side-effects of spectacle lenses in strabismus surgery-a randomised, double-blind interventional study

PURPOSE

Potential sources of error in dosage planning in strabismus surgery are (a) prismatic side-effects of spectacle lenses when measuring the preoperative angle with the alternating prism cover test and (b) a potential influence of eye ball axial length on dose response. As both errors take effect in opposite directions, many strabismus surgeons set aside their consideration. This study investigates whether considering both factors for dosage planning yields better operative results.

METHODS

In this prospective, randomised, double-blind, interventional pilot study, we included patients scheduled for purely horizontal strabismus surgery and determined each patient's surgical dose (total amount of recession/plication) either with (study group) or without (control) consideration of the two factors. The deviation of the resulting angle from the target angle 3 months postoperatively was the primary endpoint.

RESULTS

One hundred one patients were included, 51 of which in the intervention group and 50 in the control group. The primary endpoint showed a median deviation from the target of 3.0° in the intervention group and 4.8° in the control group. We observed a group difference of 1.8° in favour of the intervention group (p = 0.053). Subgroup analysis showed a difference between groups of 2.2° for esotropic patients and of 5.1° for patients with hyperopia > + 2 D.

CONCLUSION

Taking prismatic side-effects of spectacle lenses and eye ball length into account when calculating strabismus surgery doses showed a trend towards more accurate results. Esotropic patients and patients with hyperopia > + 2 D seemed to benefit most.

TRIAL REGISTRATION

International Clinical Trials Registry Platform: DRKS00011121.

Orbital Imaging in Strabismus

Normal orbital anatomy plays a foundational role in stabilizing binocular eye movements. Abnormal orbital anatomy, contrariwise, destabilizes binocular eye alignment by introducing eccentric and unbalanced EOM forces. These abnormalities can be categorized into five broad etiologies: (1) orbital structural disorders; (2) globe size disorders; (3) degenerative disorders; (4) innervational disorders; and (5) trauma. Orbital imaging provides important diagnostic information on EOM path and innervational status, but only if performed properly. The three critical elements are (1) maximize the field of view by focusing on the orbit of interest; (2) control gaze, ideally imaging in primary position; and (3) image perpendicular (direct coronals) and parallel (axial for horizontal, sagittal for vertical) to the EOM(s) of interest. Images should be analyzed systematically by comparing EOM size and location between orbits and with established normative values. The single most critical image is the most anterior direct coronal plane that contains both globe and clearly defined EOM cross sections. EOM positional abnormalities in this plane establish the diagnosis for the first three categories of orbital abnormalities. Innervational abnormalities are best evaluated in the mid-orbit; asymmetry in mid-orbital EOM size and shape defines or confirms innervational disorders like complete or partial cranial nerve palsies.

Spectral domain optical coherence tomography to assess the insertion of extraocular rectus muscles

PURPOSE

To determine the distance of the horizontal rectus muscle insertion to the limbus using spectral domain optical coherence tomography (SD-OCT) and to evaluate whether results are correlated with sex, age, or axial length.

METHODS

The right eyes of healthy, white subjects were imaged with SD-OCT. Subjects' sex and age were recorded, and axial length was measured using an optical biometer. The distance from the horizontal rectus insertion to the limbus was measured. The intraclass correlation coefficient (ICC) was used to assess the reproducibility of the measurements with a subset of images. A multivariate model was adjusted to analyze whether sex, age, and axial length was correlated with insertion distance.

RESULTS

A total of 187 right eyes were included. Mean participant age was 43.7 ± 22.1 years (range, 6-85). Of the 187 patients, 129 were female; 25 were children. Mean axial length was 23.9 ± 1.6 mm (range, 20.4-29.3). Mean rectus insertion distance to the limbus was 6.47 ± 0.52 mm (range, 5.2-7.6 mm) for the lateral rectus muscle and 5.22 ± 0.51 mm (range, 4.1-6.1 mm) for the medial rectus muscle. ICC was >0.87 for intra- and interobserver reproducibility. The insertion-limbus distance was correlated with sex, being greater in males (P = 0.040 for the lateral rectus muscle; P = 0.036 for medial rectus muscle). There was no correlation between this distance and axial length or age (P > 0.156).

CONCLUSIONS

In this study cohort, the insertion distance of the horizontal rectus muscles to the limbus on SD-OCT was greater in males than females; however, age and axial length were not correlated with insertion distance.

An external pilot study to test the feasibility of a randomised controlled trial comparing eye muscle surgery against active monitoring for childhood intermittent exotropia [X(T)]

INTRODUCTION

The evidence base for the treatment of strabismus (squint) is poor. Our main aim is to improve this evidence base for the treatment of a common type of childhood squint {intermittent exotropia, [X(T)]}. We conducted an external pilot study in order to inform the design and conduct of a future full randomised controlled trial (RCT).

METHODS

Children of between 6 months and 16 years with a recent diagnosis of X(T) were eligible for recruitment. Participants were recruited from secondary care at the ophthalmology departments at four UK NHS foundation trusts. Participants were randomised to either active monitoring or surgery. This report describes the findings of the Pilot Rehearsal Trial and Qualitative Study, and assesses the success against the objectives proposed.

RECRUITMENT AND RETENTION

The experience gained during the Pilot Rehearsal Trial demonstrates the ability to recruit and retain sites that are willing to randomise children to both trial arms, and for parents to agree to randomisation of their children to such a study. One child declined the group allocation. A total of 231 children were screened (expected 240), of whom 138 (60%) were eligible (expected 228: 95%) and 49 (35% of eligible) children were recruited (expected 144: 63% of eligible). Strategies that improved recruitment over the course of the trial are discussed, together with the reasons why fewer children were eligible for recruitment than initially anticipated. Attrition was low. Outcome data were obtained for 47 of 49 randomised children.

TRIAL PROCESSES AND DATA COLLECTION

The Trial Management processes proved effective. There were high levels of completion on all of the data collection forms. However, the feedback from the treatment orthoptists revealed that some modifications should be made to the length and frequency of the health service assessment and travel assessment questionnaires, thus reducing the burden on participants in the main trial. Modifications to the wording of the questions also need to be made.

MONITORING OF BIAS

Children who recruited to the trial were older and had more severe strabismus than those children eligible but declining participation. Strategies to account for this in a full trial are proposed.

REASONS FOR PARTICIPATION OR DECLINING STUDY

These were identified using qualitative interviews. The principal reasons for declining entry into the study were strong preferences for and against surgical treatment.

HARMS

There were no serious unexpected adverse events. Two children had overcorrection of their X(T) with reduction in binocular vision following surgery, which is in line with previous studies. No children in the active monitoring arm developed a constant strabismus although two showed some reduction in control.

CONCLUSIONS

The SamExo study has demonstrated that it is possible to recruit and retain participants to a randomised trial of surgery compared with active monitoring for X(T). For longer-term full RCTs, in order to maximise the generalisability of future studies, consideration needs to be given to planning more time and clinic appointments to assess eligibility and to allow consideration of participation; the greater use of research nurses for recruitment; and accommodating the strong preferences of some parents both for and against surgical intervention.

TRIAL REGISTRATION

Current Controlled Trials ISRCTN44114892.

FUNDING

This project was funded by the NIHR Health Technology Assessment programme and will be published in full in Health Technology Assessment; Vol. 19, No. 39. See the NIHR Journals Library website for further project information.

Hyperopic refractive errors as a prognostic factor in intermittent exotropia surgery

Purpose: To evaluate and compare surgical outcomes with respect to refractive errors in strabismus surgery for the treatment of intermittent exotropia (IXT).

METHODS

The medical records of patients with IXT who were treated by one surgeon from January 2005 and June 2011 were reviewed. Three hundred and thirty-three IXT patients were included and divided into three groups according to preoperative refractive error: IXT with hyperopia (group I), IXT with emmetropia (group II), and IXT with myopia (group III). The surgical outcomes with respect to sensory and motor criteria were compared among the three groups.

RESULTS

The surgical success rates according to motor criteria and sensory and motor criteria combined were higher in groups I (29 patients) and III (124 patients) than in group II (180 patients) at postoperative 3 and 6 months and at the last follow-up. Stereopsis was significantly better in groups II and III than in group I preoperatively (P=0.002 by one-way analysis of variance test); however, the difference was not significant postoperatively. Twenty patients in group I (69.0%) were prescribed undercorrected hyperopic spectacles postoperatively, while only 22 patients in group III (17.7%) were prescribed spectacles with more myopic power than their refractive errors.

CONCLUSION

In the surgical treatment of IXT, hyperopia was not an indicator of poor prognosis. Taking into consideration the age effect, follow-up period after IXT surgery, and stereopsis improvement, hyperopic refractive error is rather a good prognostic factor.

Preoperative factors predicting the surgical response of bilateral lateral rectus recession surgery in patients with infantile exotropia

PURPOSE

To determine the preoperative factors affecting the efficacy in bilateral lateral rectus recession surgery for infantile exotropia.

METHODS

This was a retrospective study of 50 patients with infantile exotropia who had bilateral lateral rectus recession surgery correction with more than 1 year of follow-up. Preoperative parameters influencing the response of the surgery were obtained and evaluated using multiple regression analysis including age at onset, age at surgery, interval between onset and surgery, preoperative deviation, refractive error, anisometropia, amount of surgery performed, presence of A- or V-pattern, and preoperative phase of exotropia. The response to surgery was defined as the change in deviation divided by the amount of the surgery.

RESULTS

Thirty-two (64%) and 24 (48%) patients had successful outcome at 6 weeks and 1 year postoperatively, respectively. The mean response to surgery was 1.79 ± 0.63 prism diopters (PD)/mm recession at 6 weeks and 1.58 ± 0.75 PD/mm recession at 1 year postoperatively. Exotropic drift was 5.12 ± 7.27 PD at 6 weeks and 8.02 ± 6.81 PD at 1 year postoperatively. Pre-operative deviation was correlated with the response of surgery at 6 weeks (β = 0.342, P = .020) and 1 year (β = 0.391, P = .007) postoperatively.

CONCLUSIONS

Surgical response decreases over time due to the exotropic drift. Larger preoperative distance deviation was associated with larger surgical response. Both the exotropic drift and the preoperative deviation should be considered in applying the surgical dose in infantile exotropia.

Posterior inflection of weakened lateral rectus path: connective tissue factors reduce response to lateral rectus recession

PURPOSE

To determine why lateral rectus (LR) muscle recession has a variable effect on binocular alignment using magnetic resonance imaging (MRI).

DESIGN

Prospective, observational, interventional case series.

METHODS

Posterior LR muscle path lengths from the orbital apex to first globe contact were determined by axial plane, surface coil MRI in eight patients with unilateral LR muscle palsy and in four patients before and after bilateral LR muscle recession.

RESULTS

Posterior paths of paretic LR muscles were 2.2 to 6.0 mm longer (mean, 3.4 mm; P = .0002) than normal contralateral paths. Each paretic LR muscle was sharply inflected laterally at a point in the anterior orbit corresponding to the histologic location of the LR muscle pulley sleeve. Every recessed LR muscle was 0.8 to 4.4 mm (mean, 2.4 mm; P = .0008) longer after surgery than before surgery, with less temporal deflection.

CONCLUSIONS

The LR muscle pulley suspension contributes to LR muscle tension, tightening the muscle belly by stretching it temporally when LR muscle tone is reduced. The increase in LR muscle path length resulting from temporal inflection offsets the effect of recession by up to 4 mm. Connective tissue action explains some response variability after LR muscle recession.

Role of the equator in the early overcorrection of intermittent exotropia

PURPOSE

To determine the cause of consecutive esotropia on the basis of the relationship between the location of the equator and a new insertional site of the recessed lateral rectus muscle in cases of intermittent exotropia.

METHODS

Ninety-two patients with intermittent exotropia, all of whom underwent recessions of both lateral rectus muscles (5 to 8 mm), were included. The distances from the limbus to the equator (LE distance) and from the equator to the new insertion of the recessed lateral rectus muscle (EIN distance) were calculated using the following formula: (LE = axial length x pi/4 - corneal diameter/2). Overcorrection was defined as esophoria in excess of 5 prism diopters, and all patients were followed up for at least 3 months after surgery.

RESULTS

In all patients, the mean LE distance was 12.7 mm in the right eye and 12.6 mm in the left eye. The mean EIN distance in the successfully corrected patients was -0.82 mm in the right eye and -0.95 mm in the left eye. A negative distance value indicates a new insertion anterior to the equator. Overcorrection was detected in 15 patients (16.3%), whose mean EIN distances were -0.93 mm in the right eye and -0.78 mm in the left eye. No significant differences were determined to exist between the corrected and overcorrected patients with regard to the affected eye (P > .05).

CONCLUSIONS

Overcorrection of intermittent exotropia did not appear to be related to the location of new insertions of the lateral rectus muscle of up to 8 mm of recession. This may indicate the redistributing of relatively more innervational inputs to the medial rectus muscle after the completion of lateral rectus recession in consecutive esotropia.

The clinical course of recurrent exotropia after reoperation for exodeviation

PURPOSE

To determine the clinical course of recurrent exotropia after a secondary operation for exotropia.

METHODS

The surgical results in 58 patients who had undergone reoperation for recurrent exotropia (reoperation group) were retrospectively investigated and compared with those of 100 patients who had undergone primary strabismus surgery only (primary operation group) using survival analysis.

RESULTS

In the reoperation group, recurrence occurred in 19 of the 58 patients (33%). Survival analysis revealed that the recurrence rates in the reoperation group were significantly lower than those in the primary operation group at the same follow-up period after the corresponding strabismus surgery (p=0.018). The distant esodeviation at the postoperative 1st week after reoperation was the only significant factor associated with the recurrence after reoperation (p=0.01).

CONCLUSIONS

Exotropia did recur after a secondary operation, although the recurrence rate was lower than that after a primary operation only.

Bimedial hang-back recession— outcomes and surgical response

AIM

To report outcomes and identify factors affecting surgical response for constant esotropia using 'hang-back' bimedial rectus recession.

STUDY TYPE

Retrospective case series analysis.

METHODS

Patients managed by a single surgeon over a 4-year study period were categorized into esotropia types: infantile, partially accommodative, nonaccommodative and secondary esotropia. Postoperative alignment was compared between types, and regression modelling used to examine factors predicting surgical response.

RESULTS

In all, 95% (18/19) of children with partially accommodative esotropia achieved postoperative deviation <15 prism dioptres from orthotropia, compared to 56% (15/27) of children with infantile esotropia, 69% (11/16) of children with non-accommodative esotropia and all (2/2) of those with secondary esotropia. Surgical response (Delta/mm recession performed) increased with the magnitude of both preoperative deviation (P<0.001) and anisometropia (P<0.001); the effect of deviation on surgical response was reduced by amblyopia (P=0.02). Age at surgery was statistically associated (P=0.002) but had negligible clinical effect on response.

CONCLUSIONS

Surgical response to hang-back recession may be partially predicted by preoperative factors.

Re-recession of the medial rectus muscles in patients with recurrent esotropia

PURPOSE

Retrospective evaluation of changes in ocular motility after surgical re-recession of the medial rectus (MR) muscles as treatment of recurrent esotropia (ET).

METHODS

We describe 115 patients (age, 11 months-77 years; median, 11.1 years; 83 children and 32 adults) with an average amount of non-accommodative ET before surgery of 18.7 PD (SD = 8.8 PD). Preoperative alignment, amount of re-recession, distance from insertion to the limbus, and postoperative alignment and versions were collected.

RESULTS

In most cases, MR muscles were re-recessed to a fixed distance of 12 mm from the limbus, with unilateral re-recessions in cases with relatively small ET (typically < 20 PD) and bilateral re-recessions in cases with larger amounts of ET (typically > 20 PD). No clear relation was found between the amount of re-recession and the change in alignment in prism diopters. The success rate (esotropia [ET] < or = 10 PD or exotropia [XT] < or = 8 PD) 4 weeks to 8 months after surgery was 85%, with 4 patients still showing ET and 13 patients showing XT. Incidence of XT was higher for bilateral than for unilateral re- recessions. Significant underaction of the MR muscles was noted in 7% of the patients. None of the undercorrected patients and only 1 of the overcorrected patients were adults. Among adults, incidence of MR underaction was 4%. Long-term follow-up (8-120 months; median, 25 months) data from 59 patients indicated that good stability in alignment can be expected.

CONCLUSION

The results support the notion that MR re-recession to 12 mm from the limbus successfully corrects recurrent ET up to 35 PD and that it is particularly effective in adults.

Instrument-induced measurement errors during strabismus surgery

PURPOSE

The purpose of this study was to determine the clinical setting where errors in measurements of muscle position during strabismus surgery made by the Scott curved ruler or by calipers become important and to characterize the magnitude of those errors.

METHODS

Geometric analysis was used to determine the measurement error between true arc lengths of 3.0 to 20.0 mm versus Scott curved ruler measurements and caliper measurements for axial lengths ranging from 18 to 30 mm.

RESULTS

For measurements less than 9.0 mm, neither the Scott curved ruler nor calipers had any clinically important measurement error for any axial length. For axial lengths substantially smaller than 21 mm or larger than 24 mm, the Scott curved ruler, although more accurate than calipers, caused clinically important measurement errors with arc length measurements as small as 12 mm in very small eyes and 14 mm in large eyes. For axial lengths of 30 mm or more, both calipers and the Scott curved ruler had similar accuracy for measuring long arc lengths.

CONCLUSIONS

Both the Scott curved ruler and calipers are accurate in measuring arc lengths 9.0 mm or less. For longer arc length measurements, accuracy becomes dependent on axial length. The Scott curved ruler, although substantially more accurate than calipers for most common axial lengths, can introduce clinically important measurement errors when measuring arc lengths as small as 12 mm. Axial length should be considered when measuring muscle position during strabismus surgery.

Clinical characteristics and long-term postoperative results of infantile esotropia and myopia

PURPOSE

To determine the short-term (6 month) and long-term (4 year) surgical results of infantile esotropia in a control group of 100 patients, as compared to a myopia (-2.0 to -5.0 diopters [D]) group of 13 patients and to a high-myopia (greater than -8.0 D) group of 14 patients.

METHOD

A retrospective chart review with selection according to inclusion criteria.

RESULTS

All 14 high-myopic patients (100%) achieved short-term unacceptable result of large-angle (greater than 20 prism diopters) deviation as compared to 29 (29%) and four (31%) of the patients in the control and myopia groups, respectively (P < 0.0001). Eighty (80%) of the patients in the control group and 10 (77%) in the myopia group achieved long-term favorable postoperative results of subnormal binocular vision, microtropia, or small-angle (less than 20 prism diopters) deviation, with no statistical differences between the two groups. In contrast, 10 (71%) of the patients in the high-myopia group achieved a final unacceptable result of large-angle deviation, as compared to 20 (20%) and three (23%) in the control and myopia groups, respectively (P < 0.0001). Persistent preoperative amblyopia was associated with a higher percentage of unfavorable surgical results and was more frequent in the high-myopia group (50%) as compared to the control group (14%) (P = 0.0004).

CONCLUSIONS

Careful attention must be directed preoperatively toward reversing amblyopia and correcting refractive errors because persistent amblyopia at the time of surgery was associated with a higher percentage of unacceptable postoperative large-angle deviation. The conventional amount of recession or resection of muscles should be increased in high-myopic cases with infantile esotropia, because less effect is produced per millimeter of recession/resection initially and, in the long-term, the constant stimulus to accommodative convergence apparently causes recurrent esotropia.

Axial length estimation in strabismic patients

BACKGROUND

Previous studies have indicated that axial length determination is important in strabismic patients for defining the limit for a safe maximum recession of the medial rectus. Also, the response to strabismus surgery may be, in part, a function of axial length. We previously published a formula for predicting axial length based on age and refractive error; however, its accuracy has not been tested in a patient population that is different from the one used to generate the formula. The purpose of this study is to test a formula for estimating axial length, given age and refractive error, in a population that is different from that from which it was generated.

METHOD

We measured axial length using A-scan ultrasonography in 163 consecutive patients undergoing strabismus surgery. Twenty-nine patients were younger than 18 months of age; 134 patients were between 18 months and 10 years of age. We compared the measured axial length determination with the axial length value estimated by a formula generated from our previous published series.

RESULTS

For patients younger than 18 months of age, the equation estimated axial length within 0.5 mm in 41.4% of patients, within 1.0 mm in 79.3% of patients, and within 1.5 mm in 93.1% of patients. For patients between 18 months and 18 years of age, the formula estimated axial length within 0.5 mm in 37.3% of patients, within 1.0 mm in 73.1% of patients, and within 1.5 mm in 87.3% of patients.

CONCLUSIONS

The formula may be useful for the strabismus surgeon in estimating axial length when A-scan ultrasonography is not available in an operating room setting, particularly in congenital esotropes who require larger recessions in small eyes. If, however, A-scan ultrasonography is available, it is preferable to using the formula. The formula is not sufficiently accurate for use for calculating intraocular lens power.

Factors influencing the successful outcome and response in strabismus surgery

In this retrospective study based on 140 esotropic and 51 exotropic patients, the factors influencing successful outcome and response to strabismus surgery were investigated. Thirteen independent variables were chosen. The pre-operative deviation was found to be the only discriminant factor for early and late successful surgical outcomes in esotropic patients. For exotropic patients the visual acuity of the left eye was the discriminant factor for early successful surgical outcome. In esotropic patients the response to surgery increased with increasing amounts of pre-operative deviation. It was lower for patients with older age of onset and larger amounts of medial rectus recession. For exotropic patients the response to surgery was higher for larger pre-operative deviations. Eliminating possible sources of error when determining the pre-operative deviation will improve the predictability of the response to surgery and surgical outcome.

How far can a medial rectus safely be recessed?

Previous studies have suggested that the location of the equator should be important in determining the site of a "safe maximum recession" of a rectus muscle, and that the location of the equator should be a function of axial length. Exactly where in relationship to the equator a muscle can be safely recessed has never been scientifically determined. Over a 4-year period, we measured axial length on all patients we operated on for strabismus. Using a previously derived formula, we were able to calculate the limbus-to-equator distance, given axial length. Based on our analysis of 28 patients in whom we recessed one or both medial recti posterior to the equator, we believe that recessions of the medial recti up to 1.5 mm posterior to the equator should not produce postoperative medial rectus underaction associated with an overcorrection, but recessions that are further than 1.5 mm posterior to the equator may do so. Recessions to a point greater than 11 mm from the limbus do not appear to be associated with late progressive overcorrection provided that the site of recession is not greater than 1.5 mm posterior to the equator. Using our previously determined formula for estimating the location of the equator, given axial length, we have generated easy-to-use reference tables for determining the location of the equator in terms of millimeters posterior to the limbus. Also, based on axial length data from 180 strabismus patients, we have generated an algorithm for predicting axial length, given age, and refractive error, which may be useful to the strabismus surgeon in predicting the location of the equator when A-scan ultrasonography is not available.

19th annual Frank Costenbader Lecture--the origins of congenital esotropia

Congenital esotropia develops in the first 4 months of life in an infant who lacks the inborn mechanism for motor fusion. It manifests as an esotropia which is not eliminated by correction of hyperopia and occurs in an otherwise neurologically normal infant. The earliest practical time for surgery is 4 months of age. The eye is anatomically suited for surgery at this age and also, this is the earliest age that congenital esotropia can be diagnosed with confidence. The best attainable result of treatment of congenital esotropia is subnormal binocular vision. This result is more likely to be attained if infants are aligned by 18 months of age. Satisfactory alignment is produced in 80% to 85% of infants in one procedure with an appropriate bimedial rectus recession. An array of motor defects including DVD, latent nystagmus, oblique dysfunction, and A- and V-pattern appear at varying times after successful alignment. These associated findings are commonly found with, but are not unique to, congenital esotropia. The onset and clinical picture of congenital esotropia is satisfactorily explained by a theory first suggested by Worth that the strabismus is caused by an inborn defect in the motor fusion mechanism and aggravated by esotropital factors as suggested by Chavasse. In contrast to congenital esotropia, all other strabismus can be thought of as occurring on a secondary basis in a person with the inborn capacity for motor fusion, but who failed to maintain it because of conatal insurmountable strabismus (congenital third nerve palsy), who lost it because of acquired (postnatal) strabismus, who uses a strategy such as head posture alteration to retain fusion under favorable circumstances (Duane syndrome), who has intermittent strabismus with part-time suppression (X(T)), or who is maintaining alignment with nonsurgical means (refractive esotropia). For the future, I believe that advances in the management of congenital esotropia will depend on a better understanding of etiology leading to design and use of innovative nonsurgical techniques to discourage convergence and stimulate bifoveal fusion.

Effect of severing the check ligaments and intermuscular membranes on medial rectus recessions in infantile esotropia

BACKGROUND

The effect of severing the check ligaments and intermuscular membranes on the dose-response curve for medial rectus recessions in patients with infantile esotropia has not been previously studied by randomized assignment techniques using contemporary control and experimental groups and masked precautions.

METHODS

A prospective study of 101 patients was performed over an 8-year period. Patients were randomly assigned to either augmented or nonaugmented groups. In the former group, the medial check ligaments and intermuscular membranes were severed; in the latter group, no surgery was performed on the medial check ligaments or intermuscular membranes. Patients were postoperatively evaluated by an observer masked to group assignment.

RESULTS

No difference in response to surgery was observed between the two groups at the time of evaluation, which took place, on average, 6 to 7 months after surgery.

CONCLUSION

Severing the check ligaments and intermuscular membranes on patients with infantile esotropia undergoing medial rectus recessions does not appear to affect the dose-response curve.

Variation in axial length and anatomical landmarks in strabismic patients

The authors calculated axial length measurements in 185 consecutive patients undergoing strabismus surgery and found a mean measurement of 21.98 +/- 1.59 mm (range, 18.75-25.37 mm). Although significant correlation between axial length, refractive error, and age was found, wide variation was present, which indicates that age and refractive error could not accurately predict axial length. Based on a formula derived from a geometric model to determine the equator-limbus distance, given the axial length, the authors found that the equator had a mean distance from the limbus of 11.56 +/- 1.75 mm (range, 9.10-13.76 mm). Based on the variability found at surgery for the insertion-limbus distance, the number of millimeters of recession of the medial rectus from the insertion that would have been necessary to place it at the equator ranged between 3.5 and 8.5 mm in this series, and for the lateral rectus, 3.5 mm to 7.0 mm. The number of millimeters necessary to recess the lateral rectus to its point of tangency with the globe ranged between 9.5 and 14.4 mm.