Muscle path length in horizontal strabismus

Comitant strabismus etiology: extraocular muscle integrity and central nervous system involvement-a narrative review

Strabismus is not a condition in itself but the consequence of an underlying problem. Eye misalignment can be caused by disease, injury, and/or abnormalities in any of the structures and processes involved in visual perception and oculomotor control, from the extraocular muscles and their innervations to the oculomotor and visual processing areas in the brain. A small percentage of all strabismus cases are the consequence of well-described genetic syndromes, acquired insult, or disease affecting the extraocular muscles (EOMs) or their innervations. We will refer to them as strabismus of peripheral origin since their etiology lies in the peripheral nervous system. However, in most strabismus cases, that is comitant, non-restrictive, non-paralytic strabismus, the EOMs and their innervations function properly. These cases are not related to specific syndromes and their precise causes remain poorly understood. They are generally believed to be caused by deficits in the central neural pathways involved in visual perception and oculomotor control. Therefore, we will refer to them as central strabismus. The goal of this narrative review is to discuss the possible causes behind this particular type of eye misalignment and to raise awareness among eyecare professionals about the important role the central nervous system plays in strabismus etiology, and the subsequent implications regarding its treatment. A non-systematic search was conducted using PubMed, Medline, Cochrane, and Google Scholar databases with the keywords "origins," "causes," and "etiology" combined with "strabismus." A snowball approach was also used to find relevant references. In the following article, we will first describe EOM integrity in central strabismus; next, we will address numerous reasons that support the idea of central nervous system (CNS) involvement in the origin of the deviation, followed by listing several possible central causes of the ocular misalignment. Finally, we will discuss the implications CNS etiology has on strabismus treatment.

Binocular discrepancy in lateral rectus muscle attachment in intermittent exotropia with eye dominance

PURPOSE

To determine whether there is asymmetry in the lateral rectus (LR) muscle attachment between both eyes in patients with intermittent exotropia (IXT) with a non-dominant eye, but without amblyopia or anisometropia.

METHODS

In total, 109 patients who underwent bilateral lateral rectus recession for IXT were included, 81 with and 28 without eye dominance. The limbus-insertion distance and tendon width of the LR muscle were measured intraoperatively using callipers. The insertion-equator distance (presumed arc of contact), area of contact (arc of contact × tendon width), and torque value (radius of globe × arc of contact) of the LR muscle were calculated based on intraoperative measurements and axial length measured using a partial interferometer. Parameters regarding LR muscle attachment were compared between fellow eyes and between groups.

RESULTS

Mean measurements in all parameters related to LR muscle attachments other than tendon width were not different between the two eyes or between groups. The mean tendon width of the non-dominant eye was 9.2 ± 0.7 mm, narrower than the 9.4 ± 0.5 mm width in either eye of patients without dominance (p = 0.020). However, there was no difference in all parameters in 21 pairs of patients after matching. The proportion of patients who showed binocular discrepancies in attachment measurements beyond that attributable to potential measuring errors did not differ between the two groups.

CONCLUSIONS

Structural parameters related to LR muscle attachments did not differ based on eye dominance, suggesting that the anatomic structure of LR muscle attachments is not responsible for eye dominance in IXT.

Intermediate filaments in the medial rectus muscles in patients with concomitant exotropia

PURPOSE

Distribution of intermediate filament (IF) proteins in normal extraocular muscles (EOMs) showed that the EOMs differ significantly from the other muscles in the body with respect to their IFs composition, including desmin and nestin. The aim of the present study was to investigate the pathological changes in the medial rectus (MR) in patients with concomitant exotropia (XT).

METHODS

Forty-six MR muscle samples from 46 patients with XT were analyzed pathologically and processed for immunohistochemistry with specific antibodies against desmin and nestin.

RESULTS

Although most of MR muscles remained normal structures relatively, they presented high expression of desmin, and in contrast, nestin was absent in a large proportion of the MR muscles.

CONCLUSION

Desmin, which is downregulated in normal EOMs, had high expression in MR muscles of patients with XT. Nestin, which is present in a high proportion of normal EOMs, was downregulated in MR muscles of patients with XT.

Long-term Surgical Outcomes in the Sagging Eye Syndrome

BACKGROUND/AIMS

Strabismus due to sagging eye syndrome (SES) caused by age-related connective tissue involution is now an established cause of diplopia in older people. High suspicion of the condition results in early recognition, often obviating extensive neurological investigations and enabling surgical correction of the strabismus.

METHODS

This retrospective study reviewed surgical results in 93 patients (40 males) of average age 68 ± 12 years, who had small-angle strabismus due to SES, during the 20-year period 1994-2014.

RESULTS

In central gaze, mean distance pre-operative esotropia was 4.2 ± 7.5Δ (mean, SD), while mean hypertropia was 4.7 ± 5.9Δ. Surgeries comprised medial rectus (MR) recession; lateral rectus (LR) resection; plication, imbrication, and superior transposition of the LR to the superior rectus (SR) for esotropia; and graded vertical rectus tenotomy (GVRT) or vertical muscle recession for hypertropia. Mean post-operative immediate and long-term deviation after 316 ± 265 months average follow-up was 0.2 ± 1.2Δ and 1.1 ± 2.7Δ esotropia (both distance), respectively, and 0.00Δ and 1.1 ± 2.7Δ, respectively, for hypertropia. Strabismus recurred in 19 cases.

CONCLUSIONS

Progressive connective tissue involution in SES may occasionally result in symptomatic recurrences of the small-angle strabismus in about 20% of patients, irrespective of surgical procedures performed, possibly because of progressive involutional changes. This risk should be disclosed pre-operatively.

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Rectus Extraocular Muscle Size and Pulley Location in Concomitant and Pattern Exotropia

PURPOSE

To determine whether rectus extraocular muscle (EOM) sizes and pulley locations contribute to exotropia, we used magnetic resonance imaging (MRI) to measure these factors in normal control participants and in patients with concomitant and pattern exotropia.

DESIGN

Prospective case-control study.

PARTICIPANTS

Nine patients with concomitant exotropia, 6 patients with pattern exotropia, and 21 orthotropic normal control participants.

METHODS

High-resolution surface-coil MRI scans were obtained in contiguous, quasicoronal planes. Rectus pulley locations were determined in oculocentric coordinates for central gaze, supraduction, and infraduction. Cross sections in 4 contiguous image planes were summed and multiplied by the 2-mm slice thickness to obtain horizontal rectus posterior partial volumes (PPVs).

MAIN OUTCOME MEASURES

Rectus pulley locations and horizontal rectus PPVs.

RESULTS

Rectus pulleys were located differently in patients with A-pattern, versus V- and Y-pattern, exotropia. The lateral rectus (LR) pulleys were displaced significantly superiorly, the medial rectus (MR) pulleys were displaced inferiorly, and the inferior rectus pulleys were displaced laterally in A-pattern exotropia. However, the array of all rectus pulleys was excyclorotated in V- and Y-pattern exotropia. The PPV of the medial rectus muscle was statistically subnormal by approximately 29% in concomitant, but not pattern, exotropia (P < 0.05). The ratio of the PPV of the LR relative to the MR muscles in concomitant exotropia was significantly greater than in control participants and those with pattern exotropia (P < 0.05).

CONCLUSIONS

Abnormalities of EOMs and pulleys contribute differently in pattern versus concomitant exotropia. Abnormal rectus pulley locations derange EOM pulling directions that contribute to pattern exotropia, but in concomitant exotropia, pulley locations are normal, and relatively small medial rectus size reduces relative adducting force.

Contractile Force of Human Extraocular Muscle: A Theoretical Analysis

Aim. The length-contractile force relationships of six human extraocular muscles (EOMs) in primary innervations should be determined during eye movement modeling and surgery of clinical EOMs. This study aims to investigate these relationships. Method. The proposal is based on the assumption that six EOMs have similar constitutive relationships, with the eye suspended in the primary position. The constitutive relationships of EOMs are obtained by optimizing from previous experimental data and the theory of mechanical equilibrium using traditional model. Further, simulate the existing experiment of resistance force, and then compare the simulated results with the existing experimental results. Finally, the mechanical constitutive relationships of EOMs are obtained. Results. The results show that the simulated resistance forces from the other four EOMs except for the horizontal recti well agree with previous experimental results. Conclusion. The mechanical constitutive relationships of six EOMs in primary innervations are obtained, and the rationality of the constitutive relationships is verified. Whereafter, the active stress-strain relationships of the six EOMs in the primary innervations are obtained. The research results can improve the eye movement model to predict the surgical amounts of EOMs before EOM surgery more precisely.