Persistence of eye movement following disinsertion of extraocular muscle

Three-muscle surgery for large-angle esotropia in chronic sixth nerve palsy: comparison of two approaches

AIMS

To report the effect of two three-muscle surgeries, inferior rectus belly transposition plus augmented superior rectus transposition plus medial rectus recession (ISM) and modified vertical rectus belly transposition plus medial rectus recession (VM), in the management of large-angle esotropia in Chinese patients with chronic sixth nerve palsy.

METHODS

Twenty-eight consecutive patients with large-angle esotropia ≥50Δ were prospectively enrolled and underwent either ISM or VM. Main outcomes included preoperative and postoperative deviation in primary position, abduction limitation and complications. Follow-up was at least 6 months.

RESULTS

Of the included patients, 13 underwent ISM and 15 underwent VM. Preoperatively, ISM group displayed larger esotropia and more severe abduction limitation. 27 patients completed the follow-up. The postoperative horizontal deviation and abduction limitation were similar in both groups. At the last follow-up, ISM group demonstrated greater improvement of abduction limitation than VM group in both grading (group difference -2.1, p<0.001) and quantitation (group difference 2.6 mm, p=0.001). However, eight (30%) patients revealed an induced adduction limitation ≤-1. Of the 22 patients with unilateral palsy, more esotropia of 14.8Δ was corrected in ISM group, compared with VM group (p=0.003). Three patients (14%) developed vertical diplopia and three (14%) developed torsional diplopia. Unexpectedly, keratitis was observed in 4 of 27 (15%) patients, all with concurrent fifth and/or seventh nerve palsy. Three patients aggravated to corneal ulceration.

CONCLUSIONS

Two three-muscle surgeries, ISM and VM were both effective for large-angle esotropia in Chinese patients with chronic sixth nerve palsy. However, attention should be paid to potential complications.

Medial rectus disinsertion for management of chronic complete sixth nerve palsy

PURPOSE

To report the outcomes of the medial rectus (MR) disinsertion procedure for the management of refractory esotropia (ET) with or without Abnormal head posture (AHP) in chronic complete sixth nerve palsy.

METHODS

This is a retrospective case series of patients with sixth nerve palsy who suffered from residual ET and diplopia following the conventional strabismus surgeries and underwent MR disinsertion procedure between April 2017 and February 2020. This procedure was offered to the patients who declined to use prism and did not wish to perform surgery on the fellow eye. The demographic and clinical data, including sex, age, visual acuity, pre and postoperative angle of strabismus, duction limitations, results of forced duction and force generation tests, details of prior strabismus surgeries, orbital CT scan findings, and follow up duration were collected from the medical records.

RESULTS

Six patients were enrolled in this study. Mean age was 35.0  ±  14.0 years, and mean follow-up was 15.3  ±  5.9 months. The ET at the Primary position (PP) was 35.0  ±  18.4 prism dioptre (PD) before MR disinsertion, which decreased to 14.2  ±  17.4 PD after MR disinsertion procedure. Four cases needed additional complementary surgeries to improve residual ET in PP. No case developed overcorrection. Abduction deficiency was -5.0  ±  1.3 before MR disinsertion, which improved to - 2.8  ±  0.5 units at last follow-up. The mean of induced adduction deficiency was - 2.9  ±  0.4 at last follow-up.

CONCLUSIONS

MR disinsertion can be considered in patients with chronic complete sixth nerve palsy and refractory diplopia when the conventional methods have failed.

Intraoperative Findings in Consecutive Exotropia with and without Adduction Deficit

PURPOSE

Consecutive exotropia may be associated with limited adduction, which has been reported to be caused by 1 or more anatomic abnormalities of rectus muscles or their insertions. We studied the relative frequency of grades of adduction deficit and the relative frequency of abnormal anatomic findings.

DESIGN

Retrospective cohort study.

PARTICIPANTS

Patients undergoing surgery for consecutive exotropia.

METHODS

Preoperative duction deficits were graded on a -5 (severe limitation) to 0 (normal) scale. Operative reports were reviewed to classify intraoperative factors: (1) medial rectus muscle attachment type (normal, abnormal [slipped or stretched scar], attached to pulley, behind pulley, or mixed [a tenuous normal attachment, but with muscle fibers also attached to the pulley or behind the pulley]), (2) medial rectus muscle distal fiber location (millimeters from original insertion), and (3) lateral rectus muscle tightness (normal, mild restriction, moderate restriction).

MAIN OUTCOME MEASURES

Relationship of grade of adduction deficit to each intraoperative factor.

RESULTS

Of 143 eyes, 124 (87%) had an adduction deficit. Eyes with abnormal (n = 23), pulley (n = 9), behind pulley (n = 8), or mixed (n = 7) attachments had worse adduction deficits than normal attachments (n = 96; P < 0.02). There was a significant correlation between distal medial rectus muscle fiber location (0-19.5 mm recessed) and grade of adduction deficit (P < 0.0001). Eyes with mild or moderate lateral rectus muscle tightness on forced duction testing (n = 48/143 eyes) had worse adduction deficits than eyes without tightness (P < 0.001). Nevertheless, despite overall correlation, there was considerable individual variability. For example, for -1 and -2 adduction deficits, medial rectus muscle attachment could be at the pulley, behind the pulley, or include the pulley (19/87 eyes [22%]), and the lateral rectus muscle was tight in 36 of 87 eyes (41%).

CONCLUSIONS

Adduction deficits are common in patients with consecutive exotropia. Overall, more severe preoperative adduction deficits are associated with medial rectus muscle insertion abnormalities and abnormal forced ductions, but frequently there are exceptions. Severe medial rectus muscle insertion abnormalities, including lost muscles, may be found despite mild preoperative adduction deficits.

Effect of Limited Tenon Capsule and Intermuscular Membranes Dissection on the Outcome of Surgery in Patients with Horizontal Strabismus

PURPOSE

The aim of the study was to evaluate the effect of limited dissection of Tenon capsule on the outcome of strabismus surgery.

METHODS

Patients between the ages of 2 and 50 years with pure horizontal strabismus were enrolled in a prospective study. Patients were divided into two groups: case and control as a non-randomized study. The method of operation was similar in both groups except for the amount of sheath Tenon dissection. In the control group the intermuscular connections and Tenon capsule were cut as much as possible. In the study group, Tenon capsule, 3-4 mm posterior to the location of the sutures over the muscle (recessed or resected), were cut and intermuscular connections remained intact.

RESULTS

The study enrolled 54 patients with 27 patients in each group. In both groups, after operation, regardless of the type of surgery, the angle of strabismus was reduced (P< 0.05). For patients who had undergone bimedial recession, the angle of strabismus was corrected to 2.6 ± 0.4 prism diopters per mm (PD/mm) of muscle recession in the case group, and 2.2 ± 0.9 in the control group (P=0.2). For patients who had undergone monocular recession and resection (R&R) for esotropia, the angle of strabismus was corrected 3.4 ± 0.3 PD/mm of muscle recession or resection in the case group, and 3.2 ± 0.2 in the control group (P=0.05). For patients who had undergone bilateral recession, the angle of strabismus was corrected 2.3 ± 0.2 PD/mm of muscle recession in the case group, and 2.2 ± 0.2 in the control group (P=0.03). For patients who had undergone R&R for exotropia, the angle of strabismus was corrected 3.1 ± 0.5 PD/mm of muscle recession or resection in the case group, and 2.7 ± 0.3 in the control group (P=0.02).

CONCLUSION

Connective tissue ensheathments, whether disturbed or removed, do not ultimately affect the success of the strabismus surgery.

Viscoelastic characterization of extraocular Z-myotomy

PURPOSE

Z-myotomy is an extraocular muscle (EOM) weakening procedure in which two incisions are made from longitudinally-separated, opposite EOM margins for treatment of strabismus. We examined the in vitro biomechanics of Z-myotomy using tensile loading.

METHODS

Fresh bovine rectus EOMs were reduced to 20 × 10 × 2-mm dimensions, and clamped in a microtensile load cell under physiological conditions. Extraocular muscles were elongated until failure following scissors incisions made from opposite sides, spaced 8 mm apart and each encompassing 0%, 40%, 50%, 60%, or 80% EOM width. Initial strain to 30% elongation was imposed at 100 mm/s, after which elongation was maintained for greater than 100 seconds during force recording at maintained deformation. Stress relaxation tests with nonincised specimens having widths ranging from 1 to 9 mm were conducted for viscoelastic characterization of corresponding equivalence to 20% to 80% Z-myotomy. Data were modeled using the Wiechert viscoelastic formulation.

RESULTS

There was progressively reduced EOM failure force to an asymptotic minimum at 60% or greater Z-myotomy. Each Z-myotomy specimen could be matched for equivalent failure force to a non-Z-myotomy specimen with a different width. Both tensile and stress relaxation data could be modeled accurately using the Wiechert viscoelastic formulation.

CONCLUSIONS

The parallel fiber structure results in low shear force transfer across EOM width, explaining the biomechanics of Z-myotomy. Z-myotomy progressively reduces force transmission to an asymptotic minimum for less than 60% surgical dose, with no further reduction for greater amounts of surgery. Equivalence to EOM specimens having regular cross-sections permits viscoelastic biomechanical characterization of Z-myotomy specimens with irregular cross-sections.

Rectus muscle flap tear as an independent cause of restricted motility

Most published cases of rectus muscle flap tear have been associated with orbital trauma of various degrees of severity. When they accompany an orbital fracture, however, it is difficult to determine whether the flap tear is merely an incidental additional finding or a major contributing cause of the resulting restriction. How to treat the flap itself remains an open question. We report a 24-year-old man with an inferior rectus muscle flap tear caused by direct laceration of the muscle. The major finding was a "reverse leash" vertical restriction. Discarding the flap instead of reattaching it did not prevent a successful result. Our case supports the proposition that rectus muscle flap tear can be a restriction-producing entity.

Traumatic superior oblique tendon rupture

Traumatic rupture of an extraocular muscle is rare, and most commonly affects the horizontal and vertical rectus muscles. We report a case of a 45-year-old man injured by a metal hook, with rupture of the superior oblique tendon. The resulting deficit was less severe than expected, raising the question of whether a few superior oblique fibers remained intact.

The viscoelastic properties of passive eye muscle in primates. I: static forces and step responses

The viscoelastic properties of passive eye muscles are prime determinants of the deficits observed following eye muscle paralysis, the root cause of several types of strabismus. Our limited knowledge about such properties is hindering the ability of eye plant models to assist in formulating a patient's diagnosis and prognosis. To investigate these properties we conducted an extensive in vivo study of the mechanics of passive eye muscles in deeply anesthetized and paralyzed monkeys. We describe here the static length-tension relationship and the transient forces elicited by small step-like elongations. We found that the static force increases nonlinearly with length, as previously shown. As expected, an elongation step induces a fast rise in force, followed by a prolonged decay. The time course of the decay is however considerably more complex than previously thought, indicating the presence of several relaxation processes, with time constants ranging from 1 ms to at least 40 s. The mechanical properties of passive eye muscles are thus similar to those of many other biological passive tissues. Eye plant models, which for lack of data had to rely on (erroneous) assumptions, will have to be updated to incorporate these properties.

"Heavy Eye" syndrome in the absence of high myopia: A connective tissue degeneration in elderly strabismic patients

PURPOSE

In axial high myopes with "heavy eye" syndrome, orbital MRI can be use to demonstrate degeneration of the lateral rectus-superior rectus (LR-SR) band, with the result that the lateral rectus muscle slips inferiorly and causes esotropia and hypotropia. We investigated whether this degeneration might also cause strabismus in nonmyopic elderly patients.

METHODS

Three elderly patients with strabismus, 3 strabismic high myopes, and 12 orthotropic elderly subjects underwent ophthalmic examinations and orbital MRI. The lateral rectus muscle position was determined relative to globe center from quasicoronal images and correlated with LR-SR band structure. MRI scans were compared with histology of 4 cadaveric orbits ranging in age from 17 months to 93 years.

RESULTS

Two strabismic patients exhibited hypotropia; one exhibited esotropia. Mean axial length was 24.1 +/- 0.8 mm (mean +/- SD), compared with 31.6 +/- 1.4 mm for myopes. The lateral rectus muscle position of elderly strabismic subjects averaged 4.6 +/- 1.7 mm inferior to globe center, which was significantly lower than that of orthotropic elderly subjects (2.1 +/- 1.9 mm; p = 0.01) and similar to that of high myopes (5.1 +/- 3.2 mm). On MRI scanning, 100% of strabismic elderly orbits, 67% of strabismic myopic orbits, and 12.5% of control elderly orbits showed LR-SR band thinning, discontinuity, or displacement. LR-SR band degeneration was present histologically only in older cadavers.

CONCLUSIONS

Age-related LR-SR band degeneration permits the lateral rectus muscle to slip inferiorly in elderly nonmyopes, a mechanism of strabismus similar to myopic "heavy eye" syndrome. Imaging may assist in diagnosing this mechanical cause of age-related strabismus.

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.