Differential lateral rectus compartmental contraction during ocular counter-rolling

Finite element model of ocular adduction with unconstrained globe translation

Details of the anatomy and behavior of the structures responsible for human eye movements have been extensively elaborated since the first modern biomechanical models were introduced. Based on these findings, a finite element model of human ocular adduction is developed based on connective anatomy and measured optic nerve (ON) properties, as well as active contractility of bilaminar extraocular muscles (EOMs), but incorporating the novel feature that globe translation is not otherwise constrained so that realistic kinematics can be simulated. Anatomy of the hemisymmetric model is defined by magnetic resonance imaging. The globe is modeled as suspended by anatomically realistic connective tissues, orbital fat, and contiguous ON. The model incorporates a material subroutine that implements active EOM contraction based on fiber twitch characteristics. Starting from the initial condition of 26° adduction, the medial rectus (MR) muscle was commanded to contract as the lateral rectus (LR) relaxed. We alternatively modeled absence or presence of orbital fat. During pursuit-like adduction from 26 to 32°, the globe translated 0.52 mm posteriorly and 0.1 mm medially with orbital fat present, but 1.2 mm posteriorly and 0.1 mm medially without fat. Maximum principal strains in the optic disk and peripapillary reached 0.05-0.06, and von-Mises stress 96 kPa. Tension in the MR orbital layer was ~ 24 g-force after 6° adduction, but only ~ 3 gm-f in the whole LR. This physiologically plausible simulation of EOM activation in an anatomically realistic globe suspensory system demonstrates that orbital connective tissues and fat are integral to the biomechanics of adduction, including loading by the ON.

Investigation of Selective Innervation of Extraocular Muscle Compartments

Purpose

Recent magnetic resonance imaging studies have suggested that extraocular muscles (EOM) are further divided into transverse compartments that behave differentially and often unexpectedly during eye movements. Selective innervation of EOM compartments may explain the observation that certain horizontal recti compartments contribute to specific vertical eye movements and that some cyclovertical EOM compartments do not contribute to vertical vergence. We investigated the discharge characteristics of extraocular motoneurons during these eye movement tasks where EOM compartments behaved differentially for evidence of selective innervation.

Methods

We recorded from all six extraocular motoneuron populations in the abducens, oculomotor, and trochlear nuclei as two non-human primates performed vertical vergence and vertical smooth-pursuit. The relationship between motoneuron firing rate, horizontal and vertical eye parameters of the innervated eye during each task was determined using multiple linear regression.

Results

All 26 medial rectus motoneurons recorded showed no significant modulation during vertical smooth-pursuit and vertical vergence. Twenty-eight of 30 abducens motoneurons showed no significant modulation during vertical vergence, and all 30 cells did not modulate during vertical smooth-pursuit. For the cyclovertical motoneurons, 147 of the 149 cells (44/46 inferior rectus, 27/27 superior oblique, 41/41 superior rectus and 35/35 inferior oblique) modulated significantly during vertical vergence.

Conclusions

Extraocular motoneuron activity during vertical vergence and vertical smooth-pursuit does not support the theory that EOM compartments are selectively innervated. The observed differential behavior of EOM compartments is likely not driven by oculomotor control and could be due to passive change in EOM cross-sectional area.

3D Reconstruction of a Unitary Posterior Eye by Converging Optically Corrected Optical Coherence and Magnetic Resonance Tomography Images via 3D CAD

Purpose

In acquiring images of the posterior eye, magnetic resonance imaging (MRI) provides low spatial resolution of the overall shape of the eye while optical coherence tomography (OCT) offers high spatial resolution of the limited range. Through the merger of the two devices, we attempted to acquire detailed anatomy of the posterior eye.

Methods

Optical and display distortions in OCT images were corrected using the Listing reduced eye model. The 3.0T orbital MRI images were placed on the three-dimensional coordinate system of the computer-aided design (CAD) program. Employing anterior scleral canal opening, visual axis, and scleral curvature as references, original and corrected OCT images were ported into the CAD application. The radii of curvature of the choroid–scleral interfaces (Rc values) of all original and corrected OCT images were compared to the MRI images.

Results

Sixty-five eyes of 33 participants (45.58 ± 19.82 years) with a mean Rc of 12.94 ± 1.24 mm on axial MRI and 13.66 ± 2.81 mm on sagittal MRI were included. The uncorrected horizontal OCT (30.51 ± 9.34 mm) and the uncorrected vertical OCT (34.35 ± 18.09 mm) lengths differed significantly from the MRI Rc values (both P < 0.001). However, the mean Rc values of the corrected horizontal (12.50 ± 1.21 mm) and vertical (13.05 ± 1.98 mm) images did not differ significantly from the Rc values of the corresponding MRI planes (P = 0.065 and P = 0.198, respectively).

Conclusions

Features identifiable only on OCT and features only on MRI were successfully integrated into a unitary posterior eye.

Translational Relevance

Our CAD-based converging method may establish the collective anatomy of the posterior eye and the neural canal, beyond the range of the OCT.

Functional Anatomy of Muscle Mechanisms: Compensating Vertical Heterophoria

PURPOSE

Magnetic resonance imaging (MRI) of extraocular muscle function was used to evaluate the role of newly recognized mechanisms underlying compensation of large heterophoria by vertical fusional vergence (VFV).

DESIGN

Prospective case series.

METHODS

At one academic center, 8 adults with large hyperphoria and supernormal VFV underwent MRI during monocular and binocular fixation of a centered, near target. Contractility of the rectus and superior oblique (SO) extraocular muscles in hypertropic and hypotropic eyes was determined from changes in posterior partial volume (PPV).

RESULTS

Five of 8 patients could sustain binocular fusion in the scanner. In those patients, VFV corrected approximately 5-degree misalignment, approximately 5-fold greater than normal VFV. Vertical strabismus was compensated mainly by significant contractility of the lateral more than the medial compartment of the inferior rectus (IR) in both eyes (P < .005). The superior rectus (SR) and inferior oblique muscles had no significant contractile contribution, although the hypotropic SO relaxed significantly. The IR lateral compartment and SR medial compartment significantly co-relaxed when binocular fusion was attained from monocular target fixation (P < .01).

CONCLUSIONS

Although VFV protects patients from small muscle imbalances over the lifespan, even enhanced VFV may be inadequate to avert diplopia. Compensation of hyperphoria by VFV is accomplished mainly by IR muscle relaxation in the hypotropic eye, principally in its selectively innervated lateral compartment, whereas the SO contributes little. Fusion involves compartmentally selective co-relaxation in hypotropic eye vertical rectus muscles. Taken together, these overall findings suggest a physiologic basis to prefer therapeutic surgical weakening of the medial IR in the hypotropic eye.

Comparison of the Superior and Inferior Rectus Muscles in Humans: An Anatomical Study with Notes on Morphology, Anatomical Variations, and Intramuscular Innervation Patterns

A comparison of the superior and inferior rectus muscles was performed to determine whether they have similar structures and innervation attributable to their participation in the same type of, although antagonistic, eye movements. The study was conducted on 70 cadaveric hemiheads, and the anatomical variations in the superior and inferior rectus muscles were assessed. Sihler's whole mount nerve staining technique was used on 20 isolated superior and 20 isolated inferior rectus muscle specimens to visualize the intramuscular distribution of the oculomotor nerve subbranches. In two cases (~2.8%), variant muscular slips were found that connected the superior and inferior rectus muscles. In 80% of cases, muscular branches arising directly from the inferior branch of the oculomotor nerve innervated the inferior rectus muscle, while in 20% of cases, the nerve to the inferior oblique muscle pierced the inferior rectus muscle and provided its innervation. In 15 of 70 specimens (21.4%), a branch to the levator palpebrae superioris muscle pierced the superior rectus muscle. The distance between the specific rectus muscle's insertion and the anterior-most terminations of the nerves' subbranches with reference to the muscle's total length ranged from 26.9% to 47.2% for the inferior rectus and from 34.8% to 46.6% for the superior rectus, respectively. The superior rectus muscle is slightly longer and its insertion is farther from the limbus of the cornea than is the inferior rectus muscle. Both muscles share a common general pattern of intramuscular nerve subbranches' arborization, with characteristic Y-shaped ramifications that form the terminal nerve plexus located near half of the muscles' length. Unexpected anatomical variations of the extraocular muscles may be relevant during orbital imaging or surgical procedures.

Comparison of lateral and medial rectus muscle in human: an anatomical study with particular emphasis on morphology, intramuscular innervation pattern variations and discussion on clinical significance

This paper aims to present various aspects of the anatomy of horizontal (i.e., lateral and medial) rectus muscles. It mainly compares morphology and detailed patterns of intramuscular innervation of those muscles. It is also one of the first reports that uses the Sihler's stain to examine human extraocular muscles. The study was conducted on 80 isolated cadaveric hemi-heads. Sihler technique of nerves staining served to expose the course of intramuscular branches of the oculomotor and abducens nerves. The lateral rectus was longer (48 mm versus 46.5 mm) and more distant from the limbus (6.8 mm versus 5.7 mm) than the medial rectus muscle. Three variants of the abducens nerve primary division were observed in the lateral rectus muscle. In the medial rectus muscle, the motor branch from the oculomotor nerve was more evenly divided into sub-branches. In both examined horizontal rectus muscles, primary muscular branches split into secondary sub-branches, which undergo numerous further divisions. The most numerous terminal sub-branches formed the terminal plexus. The distance between the insertion and the anterior-most end of the terminal plexus referenced to the total length of the muscle ranged from 35.4 to 48.5% for the lateral rectus muscle and from 36.3 to 50.5% for medial rectus muscle. Both horizontal rectus muscles share similar general pattern of distribution of intramuscular nerves, with characteristic root-like arborizations of sub-branches. The terminal nerve plexus was observed near half of both muscles' length. Knowledge of variations and innervation pattern of the extraocular muscles may be relevant during ophthalmology surgeries.

Normal Anatomy and Anomalies of the Rectus Extraocular Muscles in Human: A Review of the Recent Data and Findings

Development of modern surgical techniques is associated with the need for a thorough knowledge of surgical anatomy and, in the case of ophthalmologic surgery, also functional aspects of extraocular muscles. Thus, the leading idea of this review was to summarize the most recent findings regarding the normal anatomy and anomalies of the extraocular rectus muscles (ERMs). Particular attention was paid to the presentation of detailed and structured data on the gross anatomy of the ERMs, including their attachments, anatomical relationships, vascularization, and innervation. This issue of ERMs innervation was presented in detail, considering the research that has recently been carried out on human material using advanced anatomical techniques such as Sihler's technique of the nerves staining. The text was supplemented with a carefully selected graphic material (including anatomical specimens prepared specially for the purpose of this review) and discussion of the clinical cases and practical significance of the presented issues.

Intramuscular innervation of the lateral rectus muscle evaluated using sihler's staining technique: Potential application to strabismus surgery

The latest research suggests that the abducens nerve may be divided into subbranches that reach functionally distinct zones of the lateral rectus muscle. The goal of the study was to examine this muscle's innervation, including the detailed distribution of the intramuscular subbranches of the abducens nerve. Twenty-five lateral rectus muscle specimens were harvested (with the orbital segment of the abducens nerve), fixed in 10% formalin solution, and stained with Sihler's whole mount nerve staining technique. Subbranches running to the lateral rectus divided into two main groups: superior and inferior. Both groups of subbranches are distributed in a fan-shaped manner, show a characteristic "tree-like" branching pattern and form terminal plexus near the proximal half of the lateral rectus muscle. However, some smaller subbranches run as far as the muscle's insertion, and recurrent subbranches also reach its origin. With respect to their course to the muscle's origin or insertion, the smallest subbranches running within the muscle may be associated with innervation of the tendon. In the majority of cases (88%), superior and inferior subbranches of the abducens nerve overlapped in the central one-third of the lateral rectus muscle's width so that any clearly distinct anatomical segments of the muscle could be observed based on Sihler's technique. In the remaining 12% of specimens, superior and inferior groups of subbranches innervated two distinct compartments of the lateral rectus muscle with no overlapping. Dense, fan-shaped distribution of abducens nerve intramuscular subbranches can be observed within the lateral rectus muscle. Clin. Anat. 33:585-591, 2020. © 2019 Wiley Periodicals, Inc.

Vertical vergence in nonhuman primates depends on horizontal gaze position

The goal of this study was to compare vertical fusion capability at different orbital eye positions in normal nonhuman primates and attempt to use this information to isolate the extraocular muscles (EOMs) that mediate vertical vergence. Scleral search coils were used to record movements of both eyes as two normal nonhuman primates (M1, M2) performed a vertical vergence task at different horizontal eye positions. In a control experiment, M1 was also tested at different angles of horizontal vergence. To elicit vertical vergence, a 50° x 50° stimulus comprising a central fixation cross and random dots elsewhere was presented separately to each eye under dichoptic viewing conditions. Vertical disparity was introduced by slowly displacing the stimulus for one eye vertically. Vertical fusion amplitude (maximum disparity that the monkey was able to fuse) and vertical vergence (maximum difference in vertical position of the two eyes) were measured. Vertical fusion capability differed at different orbital eye positions. Monkey M1 had significantly smaller vertical fusion capabilities when the right eye (RE) was abducted than left eye (LE) while M2 had significantly smaller vertical fusion capabilities when the RE was adducted and LE abducted. M1 also showed greater vertical fusion capability for near gaze. M1 data suggested that the vertical recti mediated vertical vergence in the RE and the oblique muscles in the LE while M2 data suggested that the oblique muscles mediated vertical vergence in the RE and the vertical recti in the LE. The variable results within the same animal and across animals suggest that EOM involvement during vertical fusional vergence is idiosyncratic and likely a weighted combination of multiple cyclovertical muscles.

Non-commutative, nonlinear, and non-analytic aspects of the ocular motor plant

The ocular motor plant, consisting of the globe, extraocular muscles (EOMs), and connective tissue suspension, constitutes an intricate and non-linear actuator of eye movements. The pulley system of the rectus EOMs constitutes a non-linear inner gimbal actuated by the orbital layers of these EOMs that renders the sequence of ocular rotations effectively commutative to the central controller, and can be rotated by the outer gimbal driven by the oblique EOMs. Optic nerve (ON) length is insufficient to permit large angle adduction without tethering by the ON and sheath, creating at and beyond this threshold a large additional load on the medial rectus muscle. Finite element modeling suggests that adduction may eventually cause repetitive strain injury to the ON and glaucomatous optic nerve damage.

Functional anatomy of extraocular muscles during human vergence compensation of horizontal heterophoria

We employed magnetic resonance imaging to quantify human extraocular muscle (EOM) contractility during intermittent convergent and divergent strabismus with each eye viewing monocularly at 20 cm compared with centered target fusion. Contractility, indicated by posterior partial volume change, was analyzed in transverse rectus and in medial and lateral superior oblique (SO) muscle compartments. In five subjects with intermittent esotropia, abduction of the deviated eye to monocular target fixation was associated with significant whole lateral rectus (LR) contraction, but with medial rectus (MR) relaxation that was significantly greater in the superior than inferior compartment. Esotropic eye abduction to binocular fusion was associated with similar relaxation in the two MR compartments, but with greater contraction in the LR's superior than inferior compartment. The whole diverging eye SO muscle relaxed. In three subjects with intermittent exotropia, converging eye fusional adduction was associated with significant whole LR relaxation and with MR contraction attributable to significantly greater contraction in the superior than inferior compartment. In adduction of the exotropic eye to monocular target fixation but not fusional adduction, the whole SO exhibited significant relaxation. Rectus pulley positions were not significantly altered by fusion of either form of intermittent strabismus. Globe rotational axis was eccentric in intermittent strabismus, rolling the eye so that rectus EOM lever arms facilitated vergence. These results confirm, and extend to fusion of intermittent horizontal strabismus, differential compartmental function in horizontal rectus EOMs and suggest a novel role for the SO in compensation of both intermittent esotropia and exotropia. NEW & NOTEWORTHY Disjunctive eye movements normally permit binocular fixation in near visual space but also compensate for mechanical imbalances in binocular alignment developing over the life span. Magnetic resonance imaging of the extraocular muscles demonstrates important differential function in muscle compartments during compensation of large-angle intermittent convergent and divergent strabismus in humans. Eye translation during rotation also enhances vergence compensation of intermittent strabismus.

Intramuscular Nerve Distribution in the Medial Rectus Muscle and Its Clinical Implications

PURPOSE

The intramuscular nerve distribution in the extraocular muscles may be crucial for understanding their physiological and pathological responses. This study aimed to determine the oculomotor nerve distribution in the medial rectus muscle (MR) using Sihler's staining.

METHOD

Thirty-seven MRs from 23 cadavers were investigated. The MR including the oculomotor nerve was finely dissected from its origin to its insertion point into the sclera. The total length of the muscle-belly, tendon length and maximum width of the muscle were measured. We evaluated the pattern of distribution and the length of the intramuscular nerve distribution by gross observation after performing Sihler's staining, which is a method for visualizing the distribution of nerve fibers without alteration of the nerve.

RESULTS

The total length of the muscle-belly, tendon length, and muscle width were 37.6 ± 4.6 mm, 4.4 ± 1.9 mm, and 10 ± 1.8 mm, respectively. The oculomotor nerve enters the MR at a mean of two-fifths along the muscle (24 ± 2.0 mm posterior to the insertion point) and then typically divides into a few branches (mean of 2.1). The intramuscular nerve distribution showed a Y-shaped ramification, forming the terminal nerve plexus, and its course typically finished at around 17 ± 1.5 mm posterior to the muscle insertion point by gross observation. The nerve plexus in the upper part generally coursed more distally than the lower part.

CONCLUSION

This new information regarding the nerve distribution pattern of MR will be helpful for understanding MR function and the diverse pathophysiology of strabismus.

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.

Functional anatomy of human extraocular muscles during fusional divergence

We employed magnetic resonance imaging to quantify human extraocular muscle contractility during centered target fusion and fusional divergence repeated with each eye viewing monocularly at 20 cm through 8Δ and at 400 cm through 4Δ base in prism. Contractility, indicated by posterior partial volume (PPV) change, was analyzed in transverse rectus and in medial and lateral superior oblique (SO) muscle compartments and by cross-sectional area change in the inferior oblique (IO). At 20 cm, 3.1 ± 0.5° (SE) diverging eye abduction in 10 subjects was associated with 4.2 ± 1.5% whole lateral rectus (LR) PPV increase ( P < 0.05) and 1.7 ± 1.1% overall medial rectus (MR) PPV decrease attributable to 3.1 ± 1.8% reduction in the superior compartment ( P < 0.025), without change in its inferior compartment or in muscles of the aligned eye. At 400 cm, 2.2 ± 0.5° diverging eye abduction in nine subjects was associated with 6.1 ± 1.3% whole LR PPV increase ( P < 10^-5) but no change in MR, with compartmentally similar relaxation in the LR and MR of the aligned eye. Unlike convergence, there were no IO or SO contractile changes for divergence to either target nor any change in rectus pulley positions. Results confirm and extend to proximal divergence the unique role of the superior MR compartment, yet no MR role for far divergence. Corelaxation of aligned eye LR and MR combined with failure of MR relaxation during divergence is consistent with the limited behavioral range of divergence. NEW & NOTEWORTHY Magnetic resonance imaging shows that the lateral rectus muscle must overcome continued contraction by its opponent the medial rectus when humans diverge their visual axes to achieve single, binocular vision. While the upper but not lower compartment of the medial rectus assists by relaxing for near targets, it does not do so when targets are far away. This behavior violates Sherrington's law of reciprocal action of antagonists and conventional assumptions about the ocular motor system.

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.

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.

Extraocular Muscle Compartments in Superior Oblique Palsy

Purpose

To investigate changes in volumes of extraocular muscle (EOM) compartments in unilateral superior oblique (SO) palsy using magnetic resonance imaging (MRI).

Methods

High-resolution, surface-coil MRI was obtained in 19 patients with unilateral SO palsy and 19 age-matched orthotropic control subjects. Rectus EOMs and the SO were divided into two anatomic compartments for volume analysis in patients with unilateral SO palsy, allowing comparison of total compartmental volumes versus controls. Medial and lateral compartmental volumes of the SO muscle were compared in patients with isotropic (round shape) versus anisotropic (elongated shape) SO atrophy.

Results

The medial and lateral compartments of the ipsilesional SO muscles were equally atrophic in isotropic SO palsy, whereas the lateral compartment was significantly smaller than the medial in anisotropic SO palsy (P = 0.01). In contrast to the SO, there were no differential compartmental volume changes in rectus EOMs; however, there was significant total muscle hypertrophy in the ipsilesional inferior rectus (IR) and lateral rectus (LR) muscles and contralesional superior rectus (SR) muscles. Medial rectus (MR) volume was normal both ipsi- and contralesionally.

Conclusions

A subset of patients with SO palsy exhibit selective atrophy of the lateral, predominantly vertically acting SO compartment. Superior oblique atrophy is associated with whole-muscle volume changes in the ipsilesional IR, ipsilesional LR, and contralesional SR; however, SO muscle atrophy is not associated with compartmentally selective volume changes in the rectus EOMs. Selective compartmental SO pathology may provide an anatomic mechanism that explains some of the variability in clinical presentations of SO palsy.

Optic Nerve Sheath as a Novel Mechanical Load on the Globe in Ocular Duction

Purpose

The optic nerve (ON) sheath's role in limiting duction has been previously unappreciated. This study employed magnetic resonance imaging (MRI) to demonstrate this constraint on adduction.

Methods

High-resolution, surface coil axial MRI was obtained in 11 normal adults, 14 subjects with esotropia (ET) having normal axial length (AL) < 25.8 mm, 13 myopic subjects with ET and mean AL 29.3 ± 3.3 (SD) mm, and 7 subjects with exotropia (XT). Gaze angles and ON lengths were measured for scans employing eccentric lateral fixation in which an ON became completely straightened.

Results

In all groups, ON straightening occurred only in the adducting, not abducting, eye. Adduction at ON straightening was 26.0 ± 8.8° in normal subjects, not significantly different from XT at 22.2 ± 11.8°. However, there was significant increase in comparable adduction in ET to 36.3 ± 9.3°, and in myopic ET to 33.6 ± 10.7° (P < 0.04). Optic nerve length at straightening was 27.6 ± 2.7 mm in normals, not significantly different from 28.2 ± 2.8 mm in ET and 27.8 ± 2.7 mm in XT. In myopic ET, ON length at straightening was significantly reduced to 24.0 ± 2.9 mm (P < 0.002) and was associated with globe retraction in adduction, suggesting ON tethering.

Conclusions

Large adduction may exhaust length redundancy in the normally sinuous ON and sheath, so that additional adduction must stretch the sheath and retract or deform the globe. These mechanical effects are most significant in ET with axial myopia, but may also exert traction on the posterior sclera absent strabismus or myopia. Tethering by the ON sheath in adduction is an important, novel mechanical load on the globe.

Functional morphometry demonstrates extraocular muscle compartmental contraction during vertical gaze changes

Anatomical studies demonstrate selective compartmental innervation of most human extraocular muscles (EOMs), suggesting the potential for differential compartmental control. This was supported by magnetic resonance imaging (MRI) demonstrating differential lateral rectus (LR) compartmental contraction during ocular counterrolling, differential medial rectus (MR) compartmental contraction during asymmetric convergence, and differential LR, inferior rectus (IR), and superior oblique (SO) compartmental contraction during vertical vergence. To ascertain possible differential compartmental EOM contraction during vertical ductions, surface coil MRI was performed over a range of target-controlled vertical gaze positions in 25 orbits of 13 normal volunteers. Cross-sectional areas and partial volumes of EOMs were analyzed in contiguous, quasi-coronal 2-mm image planes spanning origins to globe equator to determine morphometric features correlating best with contractility. Confirming and extending prior findings for horizontal EOMs during horizontal ductions, the percent change in posterior partial volume (PPV) of vertical EOMs from 8 to 14 mm posterior to the globe correlated best with vertical duction. EOMs were then divided into equal transverse compartments to evaluate the effect of vertical gaze on changes in PPV. Differential contractile changes were detected in the two compartments of the same EOM during infraduction for the IR medial vs. lateral (+4.4%, P = 0.03), LR inferior vs. superior (+4.0%, P = 0.0002), MR superior vs. inferior (-6.0%, P = 0.001), and SO lateral vs. medial (+9.7%, P = 0.007) compartments, with no differential contractile changes in the superior rectus. These findings suggest that differential compartmental activity occurs during normal vertical ductions. Thus all EOMs may contribute to cyclovertical actions.

Compartmental Innervation of the Superior Oblique Muscle in Mammals

PURPOSE

Intramuscular innervation of mammalian horizontal rectus extraocular muscles (EOMs) is compartmental. We sought evidence of similar compartmental innervation of the superior oblique (SO) muscle.

METHODS

Three fresh bovine orbits and one human orbit were dissected to trace continuity of SO muscle and tendon fibers to the scleral insertions. Whole orbits were also obtained from four humans (two adults, a 17-month-old child, and a 33-week stillborn fetus), two rhesus monkeys, one rabbit, and one cow. Orbits were formalin fixed, embedded whole in paraffin, serially sectioned in the coronal plane at 10-μm thickness, and stained with Masson trichrome. Extraocular muscle fibers and branches of the trochlear nerve (CN4) were traced in serial sections and reconstructed in three dimensions.

RESULTS

In the human, the lateral SO belly is in continuity with tendon fibers inserting more posteriorly on the sclera for infraducting mechanical advantage, while the medial belly is continuous with anteriorly inserting fibers having mechanical advantage for incycloduction. Fibers in the monkey superior SO insert more posteriorly on the sclera to favor infraduction, while the inferior portion inserts more anteriorly to favor incycloduction. In all species, CN4 bifurcates prior to penetrating the SO belly. Each branch innervates a nonoverlapping compartment of EOM fibers, consisting of medial and lateral compartments in humans and monkeys, and superior and inferior compartments in cows and rabbits.

CONCLUSIONS

The SO muscle of humans and other mammals is compartmentally innervated in a manner that could permit separate CN4 branches to selectively influence vertical versus torsional action.

Graded vertical rectus tenotomy for small-angle cyclovertical strabismus in sagging eye syndrome

BACKGROUND/AIMS

Graded vertical rectus tenotomy (GVRT) is postulated as effective for small-angle vertical heterotropia. We aimed to determine the dosing recommendations for GVRT in sagging eye syndrome (SES).

METHODS

This was a retrospective, observational study of surgical outcomes for GVRT from 2009 to 2014 in a single surgeon's academic practice. There were 37 (20 women) patients of average age 68±10 (SD) years with comitant or incomitant hypertropia ≤10Δ caused by SES. The main outcome measure was the dose-effect of GVRT required to correct intraoperative hypertropia.

RESULTS

Preoperative average central gaze hypertropia measured 4.7±2.2Δ. Three patients underwent repeat GVRT for residual or consecutive hypertropia, one undergoing it twice. All surgeries were analysed, increasing the total operations to 41. The inferior rectus tendon in the hypotropic eye was operated in 32 eyes, and the superior rectus tendon in the hypertropic eye in 9 eyes. Mean tenotomy was 68±19% of tendon width. Hypertropia was always eliminated intraoperatively by progressive GVRT. Mean hypertropia was 1.1±1.6Δ at average 93 days postoperatively. Linear regression demonstrated that 3-6Δ hypertropia correction requires 30%-90% graded tenotomy (R(2)=0.32, p<0.0001), but with substantial individual variability. Undercorrection necessitated reoperation in 10% of cases.

CONCLUSION

GVRT precisely corrects hypertropia of up to 10Δ, but because of variable effect, it should be performed with intraoperative monitoring under topical anaesthesia.

Superior oblique extraocular muscle shape in superior oblique palsy

PURPOSE

To investigate the superior oblique (SO) extraocular muscle cross section in normal controls and in SO palsy using high-resolution magnetic resonance imaging (MRI).

DESIGN

Prospective observational study.

METHODS

At a single academic medical center, high-resolution MRI was obtained at 312 μm in-plane resolution using surface coils in multiple, contiguous, quasi-coronal planes perpendicular to the orbital axis in 12 controls and 62 subjects with SO palsy. Previous strabismus surgery was excluded. Imaging was repeated in central gaze and infraduction. In each image plane along the SO, its cross section was outlined to compute cross-sectional area and the major and minor axes of the best-fitting ellipse. Main outcome measures were SO morphology and ocular motility.

RESULTS

The major and minor axes, cross-sectional area distributions, and volume of the SO belly were subnormal in orbits with SO palsy at most anteroposterior locations (P = .001), but discriminant analysis showed that palsied SO cross sections segregated distinctly into round and elongate shapes representing isotropic vs anisotropic atrophy, respectively. The major axis was relatively preserved in anisotropic atrophy (P = .0146). Cases with isotropic atrophy exhibited greater hypertropia in infraversion than central gaze, as well as greater excyclotorsion, than cases with anisotropic atrophy (P < .05 for all).

CONCLUSIONS

Characteristic differences in shape of the palsied SO belly correlate with different clinical features, and may reflect both the degree of differential pathology in the medial vs lateral neuromuscular SO compartments and the basis for diversity in patterns of resulting hypertropia.

Compartmentalization of extraocular muscle function

Ocular motor diversity exceeds capabilities of only six extraocular muscles (EOMs), but this deficiency is overcome by the plethora of fibers within individual EOMs surpassing requirements of homogeneous actuators. This paper reviews emerging evidence that regions of individual EOMs can be differentially innervated to exert independent oculorotary torques, broadening the oculomotor repertoire, and potentially explaining diverse strabismus pathophysiology. Parallel structure characterizes EOM and tendon fibers, with little transverse coupling of experimentally imposed or actively generated tension. This arrangement enables arbitrary groupings of tendon and muscle fibers to act relatively independently. Coordinated force generation among EOM fibers occurs only upon potentially mutable coordination of innervational commands, whose central basis is suggested by preliminary findings of apparent compartmental segregation of abducens motor neuron pools. Humans, monkeys, and other mammals demonstrate separate, nonoverlapping intramuscular nerve arborizations in the superior vs inferior compartments of the medial rectus (MR) and lateral rectus (LR) EOMs that could apply force at the superior vs inferior portions of scleral insertions, and in the medial vs lateral compartments of the superior oblique that act at the equatorial vs posterior scleral insertions that might preferentially implement incycloduction vs infraduction. Magnetic resonance imaging of the MR during several physiological ocular motor behaviors indicates differential compartmental function. Differential compartmental pathology can influence clinical strabismus. Partial abducens palsy commonly affects the superior LR compartment more than the inferior, inducing vertical strabismus that might erroneously be attributed to cyclovertical EOM pathology. Surgery may selectively manipulate EOM compartments.

Magnetic resonance imaging demonstrates compartmental muscle mechanisms of human vertical fusional vergence

Vertical fusional vergence (VFV) normally compensates for slight vertical heterophorias. We employed magnetic resonance imaging to clarify extraocular muscle contributions to VFV induced by monocular two-prism diopter (1.15°) base-up prism in 14 normal adults. Fusion during prism viewing requires monocular infraduction. Scans were repeated without prism, and with prism shifted contralaterally. Contractility indicated by morphometric indexes was separately analyzed in medial and lateral vertical rectus and superior oblique (SO) putative compartments, and superior and inferior horizontal rectus extraocular muscle putative compartments, but in the whole inferior oblique (IO). Images confirmed appropriate VFV that was implemented by the inferior rectus (IR) medial compartment contracting ipsilateral and relaxing contralateral to prism. There was no significant contractility in the IR lateral compartment. The superior but not inferior lateral rectus (LR) compartment contracted significantly in the prism viewing eye, but not contralateral to prism. The IO contracted ipsilateral but not contralateral to the prism. In the infraducting eye, the SO medial compartment relaxed significantly, while the lateral compartment was unchanged; contralateral to prism, the SO lateral compartment contracted, while the medial compartment was unchanged. There was no contractility in the superior or medial rectus muscles in either eye. There was no globe retraction. We conclude that the vertical component of VFV is primarily implemented by IR medial compartment contraction. Since appropriate vertical rotation is not directly implemented, or is opposed, by associated differential LR and SO compartmental activity, and IO contraction, these actions probably implement a torsional component of VFV.

Independent active contraction of extraocular muscle compartments

PURPOSE

Intramuscular innervation of horizontal rectus extraocular muscle (EOMs) is segregated into superior and inferior (transverse) compartments, whereas all EOMs are also divided into global (GL) and orbital (OL) layers with scleral and pulley insertions, respectively. Mechanical independence between both types of compartments has been demonstrated during passive tensile loading. We examined coupling between EOM compartments during active, ex vivo contraction.

METHODS

Fresh bovine EOMs were removed, and one compartment of each was coated with hydrophobic petrolatum. Contraction of the uncoated compartment was induced by immersion in a solution of 50 mM CaCl2 at 38°C labeled with sodium fluorescein dye, whereas tensions in both compartments were monitored by strain gauges. Control experiments omitted petrolatum so that the entire EOM contracted. After physiological experiments, EOMs were sectioned transversely to demonstrate specificity of CaCl2 permeation by yellow fluorescence dye excited by blue light.

RESULTS

In control experiments without petrolatum, both transverse and GL and OL compartments contracted similarly. Selective compartmental omission of petrolatum caused markedly independent compartmental contraction whether measured at the GL or the OL insertions or for transverse compartments at the scleral insertion. Although some CaCl2 spread occurred, mean (±SD) tension in the coated compartments averaged only 10.5 ± 3.3% and 6.0 ± 1.5% in GL/OL and transverse compartments, respectively relative to uncoated compartments. Fluorescein penetration confirmed selective CaCl2 permeation.

CONCLUSIONS

These data confirm passive tensile findings of mechanical independence of EOM compartments and extend results to active contraction. EOMs behave actively as if composed of mechanically independent parallel fiber bundles having different insertional targets, consistent with the active pulley and transverse compartmental hypotheses.

Sensitivity of the three-step test in diagnosis of superior oblique palsy

PURPOSE

Although the Parks-Bielschowsky three-step test is the cornerstone of cyclovertical strabismus diagnosis, it has not been validated against an external benchmark. We evaluated the test's sensitivity in clinical diagnosis of superior oblique palsy in patients with unequivocal magnetic resonance imaging (MRI) evidence of superior oblique atrophy.

METHODS

A total of 73 strabismic patients were selected from a prospective MRI study because they exhibited superior oblique atrophy indicative of superior oblique denervation and thus confirmatory of superior oblique palsy. Of these, 50 patients who had no confounding factors were included for detailed study. Ocular motility data were evaluated to determine sensitivity of single and combined clinical findings in diagnosis of superior oblique palsy.

RESULTS

Maximum mean ipsilesional superior oblique cross section was reduced to 9.6 ± 0.6 mm(2) (mean ± standard error) in superior oblique palsy, representing 52% of the 18.5 ± 0.6 mm(2) contralesional superior oblique maximum cross section and 52% of the 18.4 ± 0.4 mm(2) control maximum superior oblique cross section (P < 0.001). Of the 50 patients, 35 (70%) with superior oblique atrophy fulfilled the entire three-step test. In 14 (28%) patients two steps were fulfilled; in 1 patient (2%), only one step. Affected superior oblique cross section was similar in orbits that fulfilled the three-step test (9.8 ± 0.9 mm(2)) and those that did not (9.1 ± 0.7 mm(2); P = 0.58).

CONCLUSIONS

The complete three-step test fails to detect 30% of cases of superior oblique atrophy. Often only two of three steps are positive in superior oblique palsy.

How does the structure of extraocular muscles and their nerves affect their function?

The sensory and motor control of human extraocular muscles (EOMs) have been subjected to considerable speculation in ophthalmic literature, often related to infranuclear structures such as the unique complement of muscle fibres and their associated sensory organs. The intrafusal fibres do not resemble their somatic counterparts and their peculiar morphology has raised questions about their proprioceptive capacity. No Golgi tendon organs have so far been observed and the myotendinous nerve endings, previously assumed to convey sensory information, have recently been argued to merely represent constituents of the efferent innervation serving the multiply innervated muscles fibres. These observations raise questions about the overall capacity to monitor the activity created by the generous efferent nerve supply observed in these muscles. Furthermore, the argued independent activity of muscular layers and compartments suggest that the required feedback must be highly structured and more specific than previously assumed. Yet, uncertainty about the source of such information remains. The purpose of this paper is to provide a short review of neuromuscular properties of human extraocular muscles. Their functional implications and the most reputable sources of proprioception will also be discussed. The promoted views are based on pertinent literature and previous research undertaken by the authors.

Hypertropia in unilateral isolated abducens palsy

PURPOSE

To evaluate the incidence and features of hypertropia in abducens nerve palsy.

METHODS

The records of consecutive patients with unilateral, isolated, previously unoperated abducens nerve palsy were reviewed for binocular alignment on cover testing, Krimsky measurement, or Hess screen testing. Patients with associated cranial nerve palsy (including bilateral abducens palsies), orbital disease, myasthenia gravis, Horner syndrome, hemiplegia, cerebellar signs, arteritis, or previous strabismus surgery were excluded. Control subjects underwent complete examination to confirm normality.

RESULTS

A total of 79 patients were included (40 males; mean age 49.2 years). Hypertropia in lateral or central gazes was present in 15 of 79 cases (19%) on alternate cover or Krimsky testing, in 32 of 56 cases (57%) on Hess screen testing, and absent in all 30 normal controls. Of cases with hypertropia, the mean of the greatest hypertropia in lateral or central gaze on was 5.0(Δ) ± 2.3(Δ) (standard deviation; range, 1(Δ)-8(Δ)) routine clinical examination, and 5.8(Δ) ± 4.2(Δ) (range, 2(Δ)-24(Δ)) on Hess screen testing. Of 39 cases with partial abducens palsy evaluated by Hess screen testing, the ipsilesional eye was hypertropic in 24 (61%) and hypotropic in 15 cases (39%).

CONCLUSIONS

Small-angle hypertropia is common in isolated, unilateral abducens and does not imply existence of multiple cranial neuropathies or skew deviation.

Magnetic resonance imaging of differential compartmental function of horizontal rectus extraocular muscles during conjugate and converged ocular adduction

Activity in horizontal rectus extraocular muscles (EOMs) was investigated by magnetic resonance imaging (MRI) of humans during asymmetric convergence to a monocularly aligned target at 15-cm distance or monocular fixation of afocal targets placed over a wide range of conjugate abduction through adduction. Cross sections and posterior partial volumes (PPVs) of EOMs were determined from quasi-coronal image planes and were separately analyzed in the inferior vs. superior compartments, defined by lines bisecting their maximum vertical dimensions. Both inferior and superior compartments of medial (MR) and lateral (LR) rectus exhibited contractile changes in PPV and maximum cross section for both asymmetric convergence and a comparable range of conjugate adduction. Both LR compartments, and the inferior MR compartment, exhibited similar decreases in contractility correlating with relaxation during both convergence and conjugate adduction. In contrast, the superior MR compartment exhibited roughly three times the contractility in conjugate adduction as in similar-magnitude convergence. In the aligned eye that did not move during convergence, summed contractility in all compartments of MR and LR exhibited corelaxation consistent with published EOM force measurements in this paradigm (Miller JM, Bockisch CJ, Pavlovski DS. J Neurophysiol 87: 2421-2433, 2002; Miller JM, Davison RC, Gamlin PD. J Neurophysiol 105: 2863-2873, 2011). The superior MR compartment also exhibited significantly greater contractility than the other compartments over the maximum achievable horizontal globe rotation from abduction to adduction. These findings suggest that the superior MR compartment is controlled differentially from the inferior compartment and suggest that its activity is reduced during convergence as a component of generally altered extraocular mechanics.

Reproducibility of horizontal extraocular muscle insertion distance in anterior segment optical coherence tomography and the effect of head position

PURPOSE

To investigate the reproducibility of horizontal extraocular muscle insertion distance measurements in anterior segment optical coherence tomography (AS-OCT) and to evaluate the effect of eye position on the measurement.

METHODS

The right eyes of 30 healthy young subjects underwent AS-OCT. Varying eye positions were used and the muscle insertion distance was measured by two independent examiners who each measured the insertion distance twice. The measurement was performed for the lateral rectus and medial rectus muscles with the eye rotated 40°, 50°, and 60° to the midline of the instrument. Reproducibility was evaluated with the intraclass correlation coefficient (ICC) and Bland-Altman plot.

RESULTS

The lateral rectus insertion distance was smallest with 50° rotation and the medial rectus insertion distance did not show a consistent pattern in regards to gaze position. The differences in insertion distance between different eye positions were not statistically significant for both muscles. The inter- and intraexaminer ICC reproducibility values were excellent for both lateral and medial rectus insertion distance measurements.

CONCLUSIONS

Inter- and intraexaminer reproducibility were excellent for lateral and medial rectus muscle insertion distance measurements using AS-OCT. The measurements tended to be smallest with the 50° position in lateral rectus measurement; however, medial rectus measurements were variable.

Lateral rectus superior compartment palsy

PURPOSE

To employ magnetic resonance imaging (MRI) to seek evidence of compartmental lateral rectus atrophy consistent with a lesion involving selective denervation of only 1 of the 2 neuromuscular compartments of the lateral rectus.

DESIGN

Prospective observational case-control series.

METHODS

At a single institution, surface coil coronal MRI was obtained at 312 μm resolution in quasi-coronal planes 2 mm thick throughout the orbit in 20 normal volunteers and 18 patients with unilateral lateral rectus palsy fixated monocularly on a target placed in central gaze. Maximum cross sections and posterior volumes of the superior and inferior lateral rectus compartments were computed and correlated with clinical findings.

RESULTS

Twelve patients with lateral rectus palsy demonstrated symmetric, highly significant 40% reductions in maximum cross sections and 50% reductions in posterior volumes from normal for both compartments (P < 10(-6) for all comparisons). Six patients with lateral rectus palsy had similar significant but asymmetric reductions in those measures only for the superior compartment of the affected lateral rectus (P < 10(-4) for all comparisons), with insignificant 20%-30% reductions for the inferior compartment (P > 0.2 for all comparisons).

CONCLUSIONS

A subset of patients with clinical lateral rectus palsy may have palsy limited to the superior compartment. Paralytic esotropia may be caused by lateral rectus superior compartment palsy despite an intact lateral rectus inferior compartment. This finding is consistent with evidence supporting independent innervation of the 2 lateral rectus neuromuscular compartments.

Independent passive mechanical behavior of bovine extraocular muscle compartments

PURPOSE

Intramuscular innervation of horizontal rectus extraocular muscles (EOMs) is segregated into superior and inferior (transverse) compartments, while all EOMs are also divided into global (GL) and orbital (OL) layers with scleral and pulley insertions, respectively. We sought evidence of potential independent action by examining passive mechanical coupling between EOM compartments.

METHODS

Putative compartments of each of the six whole bovine anatomical EOMs were separately clamped to a physiologically controlled, dual channel microtensile load cell (5-mN force resolution) driven by independent, high-speed, linear motors having 20-nm position resolution. One channel at a time was extended or retracted by 3 to 5 mm, with the other channel stationary. Fiducials distributed on the EOM global surface enabled optical tracking of local deformation. Loading rates of 5 to 100 mm/sec were applied to explore speeds from slow vergence to saccades. Control loadings employed transversely loaded EOM and isotropic latex.

RESULTS

All eom bellies and tendons exhibited substantial compartmental independence when loaded in the physiologic direction, both between OL and GL, and for arbitrary transverse parsings of EOM width ranging from 60%: 40% to 80%:20%. Intercompartmental force coupling in the physiologic direction was less than or equal to 10% in all six EOMS even for saccadic loading rates. Coupling was much higher for nonphysiologic transverse EOM loading and isotropic latex. Optical tracking demonstrated independent strain distribution between EOM compartments.

CONCLUSIONS

Substantial mechanical independence exists among physiologically loaded fiber bundles in bovine EOMs and tendons, providing biomechanical support for the proposal that differential compartmental function in horizontal rectus EOMs contributes to novel torsional and vertical actions.