The effect of resection on satellite cell activity in rabbit extraocular muscle

The satellite cell in skeletal muscle: A story of heterogeneity

Skeletal muscle is a highly represented tissue in mammals and is composed of fibers that are extremely adaptable and capable of regeneration. This characteristic of muscle fibers is made possible by a cell type called satellite cells. Adjacent to the fibers, satellite cells are found in a quiescent state and located between the muscle fibers membrane and the basal lamina. These cells are required for the growth and regeneration of skeletal muscle through myogenesis. This process is known to be tightly sequenced from the activation to the differentiation/fusion of myofibers. However, for the past fifteen years, researchers have been interested in examining satellite cell heterogeneity and have identified different subpopulations displaying distinct characteristics based on localization, quiescence state, stemness capacity, cell-cycle progression or gene expression. A small subset of satellite cells appears to represent multipotent long-term self-renewing muscle stem cells (MuSC). All these distinctions led us to the hypothesis that the characteristics of myogenesis might not be linear and therefore may be more permissive based on the evidence that satellite cells are a heterogeneous population. In this review, we discuss the different subpopulations that exist within the satellite cell pool to highlight the heterogeneity and to gain further understanding of the myogenesis progress. Finally, we discuss the long term self-renewing MuSC subpopulation that is capable of dividing asymmetrically and discuss the molecular mechanisms regulating MuSC polarization during health and disease.

The Effects of Orthoptic Therapy on the Surgical Outcome in Children with Intermittent Exotropia: Randomised Controlled Clinical Trial

BACKGROUND

To assess the clinical effectiveness of orthoptic therapy in the postoperative stabilisation and rehabilitation of binocular function in children with intermittent exotropia (IXT) after surgery.

METHODS

This was a prospective, parallel, randomised controlled trial. A total of 136 IXT patients (aged from 7 to 17 years) who had been successfully corrected at 1 month after surgery were enrolled in this study, and 117 patients (58 controls) completed the 12-month follow-up visit. The primary outcome was established as the proportion of patients with suboptimal surgical outcomes, which were defined as: (1) exodeviation ≥10 prism diopters (PD) at distance or near using the simultaneous prism and cover test (SPCT), or (2) constant esotropia ≥6 PD at distance or near using SPCT, or (3) loss of 2 or more octaves of stereopsis from baseline. The secondary outcomes were the exodeviation at distance and near using the prism and alternate cover test (PACT), stereopsis, fusional exotropia control and convergence amplitude.

RESULTS

The cumulative probability of suboptimal surgical outcome by 12 months was 20.5% (14/68) in the orthoptic therapy group and 42.6% (29/68) in the control group. There was a significant difference between these two groups (χ² = 7.402, p = 0.007). Improvements in stereopsis, fusional exotropia control and fusional convergence amplitude were found in the orthoptic therapy group. A smaller exodrift was found in the orthoptic therapy group at near fixation (t = 2.26, p = 0.025).

CONCLUSIONS

Early postoperative orthoptic therapy can effectively improve the surgical outcome as well as stereopsis and fusional amplitude.

Larger Than Larger: Huge Amounts of Recession-Resection Surgery for Third Nerve Palsy

PURPOSE

To describe the characteristics and management outcomes of strabismus surgery in patients with third cranial nerve palsy and identify factors associated with long-term outcomes.

METHODS

A 10-year retrospective study of patients with third cranial nerve palsy who underwent strabismus surgery was performed at Zhongshan Ophthalmic Center, Sun Yat-sen University, China, between August 2010 and January 2021. Clinical characteristics and factors associated with long-term outcomes were evaluated. Success of surgery was defined as esotropia of 15 prism diopters (PD) or less at postoperative day 1 (POD1) or any deviation of 10 PD or less at postoperative month 2 (POM2) or later.

RESULTS

A total of 91 patients were included, with a mean age of 24.02 ± 15.38 years (range: 2 to 64 years). Forty-one patients (45.1%) were diagnosed as having complete palsy. Congenital palsy accounted for 40.7% (n = 37). Mean follow-up time was 2.50 ± 1.78 years (range: 8 months to 8 years). One-step surgery was performed in 78.0% of cases (n = 71), a staging procedure in 7.7% (n = 7), and reoperations in 14.3% (n = 13). Most patients had supra-maximal recession and resection of the horizontal rectus muscle with additional techniques, including horizontal rectus transposition, superior oblique transposition, traction sutures, and fixation of the globe. The mean surgical dosage performed on the horizontal rectus muscle was 18.52 ± 5.69 mm (range: 8 to 35 mm). Success rates at postoperative month 8 (POM8) were 69.0% (49 of 71) in the one-step surgery group, 57.1% (4 of 7) in the staging surgery group, and 46.2% (6 of 13) in the reoperation group. Preoperative horizontal deviation (odds ratio [OR]: 1.07, 95% CI: 1.02 to 1.13, P = .011), horizontal deviation at POD1 (OR: 0.86, 95% CI: 0.76 to 0.97, P = .016), and horizontal deviation at POM2 (OR: 1.53, 95% CI: 1.18 to 1.98, P = .001) were associated with the outcomes at POM8. Patients with partial palsy had a higher success rate than those with complete palsy (76.0% vs 51.2%, P = .014).

CONCLUSIONS

Supra-maximal recession and resection with additional techniques yields satisfying surgical results in patients with third nerve palsy when a larger surgical dose is performed for an initial overcorrection for 15 to 25 PD at POD1. The cases with complete palsy showing a larger deviation were prone to failure. [J Pediatr Ophthalmol Strabismus. 20XX;X(X):XX-XX.].

Longitudinal course of consecutive esotropia in children following surgery for basic-type intermittent exotropia

PURPOSE

To evaluate the longitudinal course of consecutive esotropia following surgery for basic-type intermittent exotropia.

METHODS

Patients who underwent surgery (bilateral lateral rectus muscle recession [BLR] or unilateral lateral rectus muscle recession-medial rectus muscle resection [RR]) for the treatment of intermittent exotropia between 2011 and 2017 with a minimum follow-up period of 2 years were retrospectively reviewed. When esodeviation occurred later in patients with orthotropia or exodeviation at postoperative month 1, it was defined as delayed-onset consecutive esotropia. The number of patients with esodeviation at every follow-up and characteristics of patients were evaluated.

RESULTS

A total of 336 patients (6.2 ± 2.1 years; 236 in the BLR group and 100 in the RR group) were included. After surgery, postoperative esodeviation decreased mostly during the 1st postoperative month in both groups. At postoperative year 2, there were 28 patients (8.3%) with consecutive esotropia: six in the RR group and 22 in the BLR group. Among the 284 patients with orthotropia or exodeviation at postoperative month 1, there were 13 patients with delayed-onset consecutive esotropia; they presented larger preoperative angle of exodeviation, poorer stereopsis, younger at the time of surgery and associated with the types of surgeries for exotropia.

CONCLUSIONS

In patients with consecutive esotropia, the angle of esodeviation decreased and patching/prismatic correction helped achieve the good surgical outcomes. However, delayed-onset consecutive esotropia and persistent esotropia also presented, requiring the reoperation. Therefore, postoperative alignment should be carefully monitored after surgery for intermittent exotropia.

Lateral rectus muscle differentiation potential in paralytic esotropia patients

Purpose and background

Recently, we found that maximal medial rectus recession and lateral rectus resection in patients with complete lateral rectus paralysis resulted in a partial restoration of abduction. In an attempt to understand some of the mechanisms involved with this effect we examined gene expression profiles of lateral recti from these patients, with our focus being directed to genes related to myogenesis.

Materials and methods

Lateral recti resected from patients with complete lateral rectus paralysis and those from concomitant esotropia (controls) were collected. Differences in gene expression profiles between these two groups were examined using microarray analysis and quantitative Reverse-transcription PCR (qRT-PCR).

Results

A total of 3056 differentially expressed genes (DEGs) were identified between these two groups. Within the paralytic esotropia group, 2081 genes were up-regulated and 975 down-regulated. The results of RT-PCR revealed that PAX7, MYOG, PITX1, SIX1 and SIX4 showed higher levels of expression, while that of MYOD a lower level of expression within the paralytic esotropia group as compared with that in the control group (p < 0.05).

Conclusion

The decreased expression of MYOD in the paralytic esotropia group suggested that extraocular muscle satellite cell (EOMSCs) differentiation processes were inhibited. Whereas the high expression levels of PAX7, SIX1/4 and MYOG, suggested that the EOMSCs were showing an effective potential for differentiation. The stimulation resulting from muscle surgery may induce EOMSCs to differentiate and thus restore abduction function.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12886-021-01994-4.

Childhood Onset Strabismus: A Neurotrophic Factor Hypothesis

Strabismus is a genetically heterogeneous disorder with complex molecular and neurophysiological causes. Evidence in the literature suggests a strong role for motor innervation in the etiology of strabismus, which connects central neural processes to the peripheral extraocular muscles. Current treatments of strabismus through surgery show that an inherent sensorimotor plasticity in the ocular motor system decreases the effectiveness of treatment, often driving eye alignment back toward its misaligned pre-surgical state by altering extraocular muscle tonus. There is recent interest in capitalizing on existing biological processes in extraocular muscles to overcome these compensatory mechanisms. Neurotrophins are trophic factors that regulate survival and development in neurons and muscle, including extraocular muscles. Local administration of neurotrophins to extraocular muscles partially reversed strabismus in an animal model of strabismus. The hypothesis is that sustained release of neurotrophins gives more time for the ocular motor system to adapt to a slow change in alignment in the desired direction. The effect of neurotrophins on extraocular muscles is complex, as different neurotrophic factors have diverse effects on extraocular muscle contraction profiles, patterns of innervation, and density of extraocular muscle precursor cells. Neurotrophic factors show promise as a therapeutic option for strabismus, which may help to improve treatment outcomes and offset devastating amblyopia and psychosocial effects of disease in strabismus patients.

Morphological Differences in the Inferior Oblique Muscles from Subjects with Over-elevation in Adduction

Purpose

We examined inferior oblique muscles from subjects with over-elevation in adduction for characteristics that might shed light on the potential mechanisms for their abnormal eye position.

Methods

The inferior oblique muscles were obtained at the time of surgery in subjects diagnosed with either primary inferior oblique overaction or Apert syndrome. The muscles were frozen and processed for morphometric analysis of myofiber size, central nucleation, myosin heavy chain (MyHC) isoform expression, nerve density, and numbers of neuromuscular junctions per muscle section.

Results

The inferior oblique muscles from subjects with Apert Syndrome were smaller, and had a much more heterogeneous profile relative to myofiber cross-sectional area compared to controls. Increased central nucleation in the Apert syndrome muscles suggested on-going myofiber regeneration or reinnervation over time. Complex changes were seen in the MyHC isoform patterns that would predict slower and more sustained contractions than in the control muscles. Nerve fiber densities were significantly increased compared to controls for the muscles with primary inferior oblique overaction and Apert syndrome that had no prior surgery. The muscles from Apert syndrome subjects as well as those with primary inferior oblique overaction with no prior surgery had significantly elevated numbers of neuromuscular junctions relative to the whole muscle area.

Conclusions

The muscles from both sets of subjects were significantly different from control muscles in a number of properties examined. These data support the view that despite similar manifestations of eye misalignment, the potential mechanism behind the strabismus in these subjects is significantly different.

Increased myofiber size and reduced satellite cell numbers in medial rectus muscle of patients with intermittent exotropia

PURPOSE

To elucidate the differences in muscle bundle and satellite cells in medial rectus muscle through histological and Immunofluorescence studies of intermittent exotropia patients and normal controls. Materials and Methods: From January 2015 to December 2017, 15 patients who underwent medial rectus resection surgery at Kosin University Gospel Hospital were enrolled. Four medial recti muscles collected from two brain-dead men without strabismus were used as controls and compared with the intermittent exotropia group. Hematoxylin and eosin (HE) staining were performed, and all muscle bundle diameters were measured with the Image J program and compared to the mean value. Immunological staining for MyoHC (Myosin Heavy Chain), PAX7 (Transcription Factor), and PCNA (Proliferating Cell Nuclear Antigen) were performed to analyze the distribution of myocytes and PAX7-positive and PCNA-positive cells. Results: The mean ages of the strabismus and control groups were 17.33 ± 13.05 and 22.0 ± 5.85 years, respectively, and the male to female ratio was 7:2 and 2:0. The average angle of deviation in the exotropia patients was 36.0 ± 16.83 prism diopters. The mean muscle bundle diameter under light microscopy was 60.21 ± 1.48 in the exotropia group and 52.27 ± 0.74 in the control group. The exotropia group showed significantly greater mean muscle bundle diameter (p < .001) and diameter regularity than the control group. The PAX7(+)/muscle bundle number ratio was 0.016 ± 0.014 and 0.056 ± 0.015 in the exotropia group and control group, respectively (p < .001), and the PCNA(+)/muscular bundle number ratio was 0.015 ± 0.017 and 0.182 ± 0.102 (p < .001). Both were significantly higher in the control group compared to that in the exotropia patients. Conclusion: In the exotropia group, mean diameter of medial rectus muscle bundle was significantly larger than that of control group. The ratios of PAX7 and PCNA to muscle bundle number were significantly higher in the control group than intermittent extropia group. We found the negative relationship between activation of satellite cells and muscle bundle diameter, and it might take one step forward to elucidate the pathogenesis of intermittent extropia.

Eye alignment changes caused by sustained GDNF treatment of an extraocular muscle in infant non-human primates

The ability of sustained treatment of a single extraocular muscle with glial cell line-derived neurotrophic factor (GDNF) to produce a strabismus in infant non-human primates was tested. Six infant non-human primates received a pellet containing GDNF, releasing 2 µg/day for 90 days, on one medial rectus muscle. Eye alignment was assessed up to 6 months. Five of the six animals showed a slow decrease in eye misalignment from the significant exotropia present at birth, ending with approximately 10° of exotropia. Controls became orthotropic. Misalignment averaged 8° three months after treatment ended. After sustained GDNF treatment, few changes were seen in mean myofiber cross-sectional areas compared to age-matched naïve controls. Neuromuscular junction number was unaltered in the medial rectus muscles, but were significantly reduced in the untreated lateral recti. Neuromuscular junctions on slow fibers became multiply innervated after this sustained GDNF treatment. Pitx2-positive cells significantly decreased in treated and contralateral medial rectus muscles. Our study suggests that balanced GDNF signaling plays a role in normal development and maintenance of orthotropia. Sustained GDNF treatment of one medial rectus muscle resulted in a measurable misalignment largely maintained 3 months after treatment ended. Structural changes suggest mechanisms for producing an imbalance in muscle function.

Neural Plasticity Following Surgical Correction of Strabismus in Monkeys

Purpose

Although widely practiced, surgical treatment of strabismus has varying levels of success and permanence. In this study we investigated adaptive responses within the brain and the extraocular muscles (EOM) that occur following surgery and therefore determine long-term success of the treatment.

Methods

Single cell responses were collected from cells in the oculomotor and abducens nuclei before and after two monkeys (M1, M2) with exotropia (divergent strabismus) underwent a strabismus correction surgery that involved weakening of the lateral rectus (LR) and strengthening of the medial rectus (MR) muscle of one eye. Eye movement and neuronal data were collected for up to 10 months after surgery during a monocular viewing smooth-pursuit task. These data were fit with a first-order equation and resulting coefficients were used to estimate the population neuronal drive (ND) to each EOM of both eyes.

Results

Surgery resulted in a ∼70% reduction in strabismus angle in both animals that reverted toward presurgical misalignment by approximately 6 months after treatment. In the first month after surgery, the ND to the treated MR reduced in one animal and ND to the LR increased in the other animal, both indicating active neural plasticity that reduced the effectiveness of the treatment. Adaptive changes in ND to the untreated eye were also identified.

Conclusions

Active neural and muscle plasticity corresponding to both the treated and the untreated eye determines longitudinal success following surgical correction of strabismus. Outcome of surgical treatment could be improved by identifying ways to enhance “positive” adaptation and limit “negative” adaptation.

Gene expression profile of extraocular muscles following resection strabismus surgery

This paper aims to identify key biological processes triggered by resection surgery in the extraocular muscles (EOMs) of a rabbit model of strabismus surgery by studying changes in gene expression. Resection surgery was performed in the superior rectus of 16 rabbits and a group of non-operated rabbits served as control. Muscle samples were collected from groups of four animals 1, 2, 4 and 6 weeks after surgery and processed for RNA-sequencing and immunohistochemistry. We identified a total of 164; 136; 64 and 12 differentially expressed genes 1, 2, 4 and 6 weeks after surgery. Gene Ontology enrichment analysis revealed that differentially expressed genes were involved in biological pathways related to metabolism, response to stimulus mainly related with regulation of immune response, cell cycle and extracellular matrix. A complementary pathway analysis and network analysis performed with Ingenuity Pathway Analysis tool corroborated and completed these findings. Collagen I, fibronectin and versican, evaluated by immunofluorescence, showed that changes at the gene expression level resulted in variation at the protein level. Tenascin-C staining in resected muscles demonstrated the formation of new tendon and myotendinous junctions. These data provide new insights about the biological response of the EOMs to resection surgery and may form the basis for future strategies to improve the outcome of strabismus surgery.

Histological changes underlying bupivacaine's effect on extra ocular muscle

To determine the changes in the cross-sectional area (CSA) of myofibers and their subtype distribution based on the myosin isoform expression after bupivacaine (BUP) injection in the EOM of rabbits and help the understanding of strabismus correction after BUP injection in the clinical practice. A total of 32 rabbits received 0.3 mL of 1.5% BUP in the superior rectus muscle (SR) of the right eye (OD) and were sacrificed at days 7, 28, 60, and 92. Additional eight untouched rabbits were included as controls. Hematoxylin and eosin staining was performed, and ImageJ software was used to measure CSA. Immunohistochemical analysis was performed to analyze the proportion of myofibers positive for myosin types 1 (slow), 2 (fast) and embryonic. Myofiber area measurement decreased 7 days after BUP injection [SR, 1271 ± 412 μm² (control) to 909 ± 255 μm² (day 7)] after BUP injection, followed by an increasing trend after 28 days and normalization after 92 days [SR; 1062 ± 363 μm² (day 28), 1492 ± 404 μm² (day 60), 1317 ± 334 μm² (day 92)]. The proportion of slow myosin-positive fibers increased in the 60-day group (88.5% ± 16.2%). There was no statistically significant difference in fast myosin-positive fibers. The inferior rectus of both eyes showed an increase in CSA. No increase of endomysial fibrous tissue was observed after 60 and 92 days of BUP injection. Bupivacaine, when injected into the SR of rabbits, initially decreases the fiber area followed by a transient increasing trend and normalization. There is a transient increase in the proportion of slow myosin-positive fibers in the injected muscle. Muscle adaptation in untreated EOM was found with increased CSA. These findings help clarify the clinical effects of BUP in extraocular muscle.

Extraocular Muscle Repair and Regeneration

PURPOSE OF REVIEW

The goal of this review is to summarize the unique regenerative milieu within mature mammalian extraocular muscles (EOMs). This will aid in understanding disease propensity for and sparing of EOMs in skeletal muscle diseases as well as the recalcitrance of the EOM to injury.

RECENT FINDINGS

The EOMs continually remodel throughout life and contain an extremely enriched number of myogenic precursor cells that differ in number and functional characteristics from those in limb skeletal muscle. The EOMs also contain a large population of Pitx2-positive myogenic precursor cells that provide the EOMs with many of their unusual biological characteristics, such as myofiber remodeling and skeletal muscle disease sparing. This environment provides for rapid and efficient remodeling and regeneration after various types of injury. In addition, the EOMs show a remarkable ability to respond to perturbations of single muscles with coordinated changes in the other EOMs that move in the same plane.

SUMMARY

These data will inform Ophthalmologists as they work toward developing new treatments for eye movement disorders, new approaches for repair after nerve or direct EOMs injury, as well as suggest potential explanations for the unusual disease propensity and disease sparing characteristics of human EOM.

Longitudinal Evaluation of Eye Misalignment and Eye Movements Following Surgical Correction of Strabismus in Monkeys

Purpose

Strabismus correction surgery is well documented in both the literature and practice with varying levels of success and permanence. Our goal was to characterize longitudinal changes in eye alignment and eye movements following strabismus correction surgery in a monkey model for developmental strabismus.

Methods

We studied two juvenile rhesus monkeys with exotropia previously induced via an optical prism-rearing paradigm in infancy. Eye misalignment was corrected via a resection–recession surgery of the horizontal rectus muscles of one eye. Binocular search coils were used to collect eye movement data during smooth-pursuit, saccades, and fixation tasks before surgical treatment, immediately after surgery, and through 6 months after treatment.

Results

Both animals showed an immediate ∼70% reduction in misalignment as a consequence of surgery that regressed to a 20%–40% improvement by 6 months after treatment. Significant changes were observed in saccade and smooth-pursuit gain of the nonviewing eye after surgery, which also reverted to presurgical values by 6 months. A temporary improvement in fixation stability of the nonviewing eye was observed after surgery; naso-temporal (N/T) asymmetry of monocular smooth-pursuit remained unchanged.

Conclusions

Surgical realignment is followed by plastic changes that often lead to reversal of surgery effects. Immediate improvement in misalignment and changes in eye movement gains are likely a result of contractility changes at the level of the extraocular muscle, whereas longer-term effects are likely a combination of neural and muscle adaptation.

Improvement of Eye Alignment in Adult Strabismic Monkeys by Sustained IGF-1 Treatment

Purpose

The goal of this study was to determine if continuous application of insulin-like growth factor-1 (IGF-1) could improve eye alignment of adult strabismic nonhuman primates and to assess possible mechanisms of effect.

Methods

A continuous release pellet of IGF-1 was placed on one medial rectus muscle in two adult nonhuman primates (M1, M2) rendered exotropic by the alternating monocular occlusion method during the first months of life. Eye alignment and eye movements were recorded for 3 months, after which M1 was euthanized, and the lateral and medial rectus muscles were removed for morphometric analysis of fiber size, nerve, and neuromuscular density.

Results

Monkey 1 showed a 40% reduction in strabismus angle, a reduction of exotropia of approximately 11° to 14° after 3 months. Monkey 2 showed a 15% improvement, with a reduction of its exotropia by approximately 3°. The treated medial rectus muscle of M1 showed increased mean myofiber cross-sectional areas. Increases in myofiber size also were seen in the contralateral medial rectus and lateral rectus muscles. Similarly, nerve density increased in the contralateral medial rectus and yoked lateral rectus.

Conclusions

This study demonstrates that in adult nonhuman primates with a sensory-induced exotropia in infancy, continuous IGF-1 treatment improves eye alignment, resulting in muscle fiber enlargement and altered innervational density that includes the untreated muscles. This supports the view that there is sufficient plasticity in the adult ocular motor system to allow continuous IGF-1 treatment over months to produce improvement in eye alignment in early-onset strabismus.

Combined recession and resection surgery in the management of convergence excess esotropia with different levels of AC/A ratio

PURPOSE

To study prospectively the effect of extirpating the proprioceptive impulse at the myotendinous junction combined with recession of the medial rectus muscles in patients with convergence excess esotropia.

METHODS

A total of 21 patients with different sizes of AC/A ratios (high, 8; normal, 12; low, 1) underwent a surgical procedure consisting of combining resection of 2.5 mm of the insertional end of the medial rectus muscles with recession from the original insertion, based on the patient's angle of esotropia at 1/3 m while wearing full cycloplegic refraction, with an additional recession of 1 mm for each rectus muscle based on current surgical tables. A satisfactory outcome was defined as orthotropia or esotropia of <10^Δ at near and distance fixations with available correction and reduction of the distance--near disparity to <10^Δ.

RESULTS

All patients, regardless of the size of AC/A ratio and the amount of near-distance disparity, had satisfactory alignments at near and distance fixations, with residual near-distance disparity of <10^Δ. Consecutive distance exotropia did not develop even when there was preoperative distance orthotropia. Outcome measures remained stable for a mean of 4.3 years. None of the 8 patients with high AC/A ratios required bifocal wear or overcorrection prescriptions to maintain alignment postoperatively.

CONCLUSIONS

This technique of combined resection and recession of the medial rectus muscle shows promise in the treatment of convergence excess esotropia. The main advantage is improvement in distance alignment while selectively reducing the near angle in patients with different levels of AC/A ratios.

Strabismus and the Oculomotor System: Insights from Macaque Models

Disrupting binocular vision in infancy leads to strabismus and oftentimes to a variety of associated visual sensory deficits and oculomotor abnormalities. Investigation of this disorder has been aided by the development of various animal models, each of which has advantages and disadvantages. In comparison to studies of binocular visual responses in cortical structures, investigations of neural oculomotor structures that mediate the misalignment and abnormalities of eye movements have been more recent, and these studies have shown that different brain areas are intimately involved in driving several aspects of the strabismic condition, including horizontal misalignment, dissociated deviations, A and V patterns of strabismus, disconjugate eye movements, nystagmus, and fixation switch. The responses of cells in visual and oculomotor areas that potentially drive the sensory deficits and also eye alignment and eye movement abnormalities follow a general theme of disrupted calibration, lower sensitivity, and poorer specificity compared with the normally developed visual oculomotor system.

Prolongation of Relaxation Time in Extraocular Muscles With Brain Derived Neurotrophic Factor in Adult Rabbit

PURPOSE

We tested the hypothesis that short-term treatment with brain derived neurotrophic factor (BDNF) would alter the contractile characteristics of rabbit extraocular muscle (EOM).

METHODS

One week after injections of BDNF in adult rabbit superior rectus muscles, twitch properties were determined in treated and control muscles in vitro. Muscles were also examined for changes in mean cross-sectional areas, neuromuscular junction size, and percent of myofibers expressing specific myosin heavy chain isoforms, and sarcoendoplasmic reticulum calcium ATPases (SERCA) 1 and 2.

RESULTS

Brain derived neurotrophic factor-treated muscles had prolonged relaxation times compared with control muscles. Time to 50% relaxation, time to 100% relaxation, and maximum rate of relaxation were increased by 24%, 27%, and 25%, respectively. No significant differences were seen in time to peak force, twitch force, or maximum rate of contraction. Brain derived neurotrophic factor treatment significantly increased mean cross-sectional areas of slow twitch and tonic myofibers, with increased areas ranging from 54% to 146%. Brain derived neurotrophic factor also resulted in an increased percentage of slow twitch myofibers in the orbital layers, ranging from 54% to 77%, and slow-tonic myofibers, ranging from 44% to 62%. No significant changes were seen SERCA1 or 2 expression or in neuromuscular junction size.

CONCLUSIONS

Short-term treatment with BDNF significantly prolonged the duration and rate of relaxation time and increased expression of both slow-twitch and slow-tonic myosin-expressing myofibers without changes in neuromuscular junctions or SERCA expression. The changes induced by BDNF treatment might have potential therapeutic value in dampening/reducing uncontrolled eye oscillations in nystagmus.

Surgical planning and innervation in pontine gaze palsy with ipsilateral esotropia

PURPOSE

To discuss surgical intervention strategies among patients with horizontal gaze palsy with concurrent esotropia.

METHODS

Five consecutive patients with dorsal pontine lesions are presented. Each patient had horizontal gaze palsy with symptomatic diplopia as a consequence of esotropia in primary gaze and an anomalous head turn to attain single binocular vision.

RESULTS

Clinical findings in the first 2 patients led us to presume there was complete loss of rectus muscle function from rectus muscle palsy. Based on this assumption, medial rectus recessions with simultaneous partial vertical muscle transposition (VRT) on the ipsilateral eye of the gaze palsy and recession-resection surgery on the contralateral eye were performed, resulting in significant motility limitation. Sequential recession-resection surgery without simultaneous VRT on the 3rd patient created an unexpected motility improvement to the side of gaze palsy, an observation differentiating rectus muscle palsy from paresis. Recession combined with VRT approach in the esotropic eye was abandoned on subsequent patients. Simultaneous recession-resection surgery without VRT in the next 2 patients resulted in alleviation of head postures, resolution of esotropia, and also substantial motility improvements to the ipsilateral hemifield of gaze palsy without limitations in adduction and vertical deviations.

CONCLUSIONS

Ocular misalignment and abnormal head posture as a result of conjugate gaze palsy can be successfully treated by basic recession-resection surgery, with the advantage of increasing versions to the ipsilateral side of the gaze palsy. Improved motility after surgery presumably represents paresis, not "paralysis," with residual innervation in rectus muscles.

Abnormally Small Neuromuscular Junctions in the Extraocular Muscles From Subjects With Idiopathic Nystagmus and Nystagmus Associated With Albinism

Purpose

Infantile nystagmus syndrome (INS) is often associated with abnormalities of axonal outgrowth and connectivity. To determine if this manifests in extraocular muscle innervation, specimens from children with idiopathic INS or INS and albinism were examined and compared to normal age-matched control extraocular muscles.

Methods

Extraocular muscles removed during normal surgery on children with idiopathic INS or INS and albinism were immunostained for neuromuscular junctions, myofiber type, the immature form of the acetylcholine receptor, and brain-derived neurotrophic factor (BDNF) and compared to age-matched controls.

Results

Muscles from both the idiopathic INS and INS and albinism groups had neuromuscular junctions that were 35% to 71% smaller based on myofiber area and myofiber perimeter than found in age-matched controls, and this was seen on both fast and slow myosin heavy chain isoform–expressing myofibers (all P < 0.015). Muscles from subjects with INS and albinism showed a 7-fold increase in neuromuscular junction numbers on fast myofibers expressing the immature gamma subunit of the acetylcholine receptor. The extraocular muscles from both INS subgroups showed a significant increase in the number and size of slow myofibers compared to age-matched controls. Brain-derived neurotrophic factor was expressed in control muscle but was virtually absent in the INS muscles.

Conclusions

These studies suggest that, relative to the final common pathway, INS is not the same between different patient etiologies. It should be possible to modulate these final common pathway abnormalities, via exogenous application of appropriate drugs, with the hope that this type of treatment may reduce the involuntary oscillatory movements in these children.

Sparing of the extraocular muscles in mdx mice with absent or reduced utrophin expression: A life span analysis

Sparing of the extraocular muscles in muscular dystrophy is controversial. To address the potential role of utrophin in this sparing, mdx:utrophin(+/-) and mdx:utrophin(-/-) mice were examined for changes in myofiber size, central nucleation, and Pax7-positive and MyoD-positive cell density at intervals over their life span. Known to be spared in the mdx mouse, and contrary to previous reports, the extraocular muscles from both the mdx:utrophin(+/-) and mdx:utrophin(-/-) mice were also morphologically spared. In the mdx:utrophin(+/)(-) mice, which have a normal life span compared to the mdx:utrophin(-/-) mice, the myofibers were larger at 3 and 12 months than the wild type age-matched eye muscles. While there was a significant increase in central nucleation in the extraocular muscles from all mdx:utrophin(+/)(-) mice, the levels were still very low compared to age-matched limb skeletal muscles. Pax7- and MyoD-positive myogenic precursor cell populations were retained and were similar to age-matched wild type controls. These results support the hypothesis that utrophin is not involved in extraocular muscle sparing in these genotypes. In addition, it appears that these muscles retain the myogenic precursors that would allow them to maintain their regenerative capacity and normal morphology over a lifetime even in these more severe models of muscular dystrophy.

A muscle stem cell for every muscle: variability of satellite cell biology among different muscle groups

The human body contains approximately 640 individual skeletal muscles. Despite the fact that all of these muscles are composed of striated muscle tissue, the biology of these muscles and their associated muscle stem cell populations are quite diverse. Skeletal muscles are affected differentially by various muscular dystrophies (MDs), such that certain genetic mutations specifically alter muscle function in only a subset of muscles. Additionally, defective muscle stem cells have been implicated in the pathology of some MDs. The biology of muscle stem cells varies depending on the muscles with which they are associated. Here we review the biology of skeletal muscle stem cell populations of eight different muscle groups. Understanding the biological variation of skeletal muscles and their resident stem cells could provide valuable insight into mechanisms underlying the susceptibility of certain muscles to myopathic disease.

Adaptability of the Immature Ocular Motor Control System: Unilateral IGF-1 Medial Rectus Treatment

PURPOSE

Unilateral treatment with sustained release IGF-1 to one medial rectus muscle in infant monkeys was performed to test the hypothesis that strabismus would develop as a result of changes in extraocular muscles during the critical period of development of binocularity.

METHODS

Sustained release IGF-1 pellets were implanted unilaterally on one medial rectus muscle in normal infant monkeys during the first 2 weeks of life. Eye position was monitored using standard photographic methods. After 3 months of treatment, myofiber and neuromuscular size, myosin composition, and innervation density were quantified in all rectus muscles and compared to those in age-matched controls.

RESULTS

Sustained unilateral IGF-1 treatments resulted in strabismus for all treated subjects; 3 of the 4 subjects had a clinically significant strabismus of more than 10°. Both the treated medial rectus and the untreated ipsilateral antagonist lateral rectus muscles had significantly larger myofibers. No adaptation in myofiber size occurred in the contralateral functionally yoked lateral rectus or in myosin composition, neuromuscular junction size, or nerve density.

CONCLUSIONS

Sustained unilateral IGF-1 treatment to extraocular muscles during the sensitive period of development of orthotropic eye alignment and binocularity was sufficient to disturb ocular motor development, resulting in strabismus in infant monkeys. This could be due to altering fusion of gaze during the early sensitive period. Serial measurements of eye alignment suggested the IGF-1-treated infants received insufficient coordinated binocular experience, preventing the establishment of normal eye alignment. Our results uniquely suggest that abnormal signaling by the extraocular muscles may be a cause of strabismus.

Adaptation of slow myofibers: the effect of sustained BDNF treatment of extraocular muscles in infant nonhuman primates

PURPOSE

We evaluated promising new treatment options for strabismus. Neurotrophic factors have emerged as a potential treatment for oculomotor disorders because of diverse roles in signaling to muscles and motor neurons. Unilateral treatment with sustained release brain-derived neurotrophic factor (BDNF) to a single lateral rectus muscle in infant monkeys was performed to test the hypothesis that strabismus would develop in correlation with extraocular muscle (EOM) changes during the critical period for development of binocularity.

METHODS

The lateral rectus muscles of one eye in two infant macaques were treated with sustained delivery of BDNF for 3 months. Eye alignment was assessed using standard photographic methods. Muscle specimens were analyzed to examine the effects of BDNF on the density, morphology, and size of neuromuscular junctions, as well as myofiber size. Counts were compared to age-matched controls.

RESULTS

No change in eye alignment occurred with BDNF treatment. Compared to control muscle, neuromuscular junctions on myofibers expressing slow myosins had a larger area. Myofibers expressing slow myosin had larger diameters, and the percentage of myofibers expressing slow myosins increased in the proximal end of the muscle. Expression of BDNF was examined in control EOM, and observed to have strongest immunoreactivity outside the endplate zone.

CONCLUSIONS

We hypothesize that the oculomotor system adapted to sustained BDNF treatment to preserve normal alignment. Our results suggest that BDNF treatment preferentially altered myofibers expressing slow myosins. This implicates BDNF signaling as influencing the slow twitch properties of EOM.

Abnormal activity of neurons in abducens nucleus of strabismic monkeys

PURPOSE

Infantile strabismus is characterized by persistent misalignment of the eyes. Mounting evidence suggests that the disorder is associated with abnormalities at the neural level, but few details are known. This study investigated the signals carried by abducens neurons in monkeys with experimentally induced strabismus. We wanted to know whether the firing rates of individual neurons are exclusively related to the position and velocity of one eye and whether the overall level of activity of the abducens nucleus was in the normal range.

METHODS

We recorded 58 neurons in right and left abducens nuclei while strabismic monkeys (one esotrope and one exotrope) performed a saccade task. We analyzed the firing rates associated with static horizontal eye position and saccades by fitting the data with a dynamic equation that included position and velocity terms for each eye. Results were compared to previously published data in normal monkeys.

RESULTS

For both strabismic monkeys the overall tonic activity was 50 to 100 spikes/s lower, for every suprathreshold eye position, than what has previously been reported for normal monkeys. This was mostly the result of lower baseline activity; the slopes of rate-position curves were similar to those in previous reports in normal monkeys. The saccade velocity sensitivities were similar to those of normal monkeys, 0.35 for the esotrope and 0.40 for the exotrope. For most neurons the firing rate was more closely related to the position and velocity of the ipsilateral eye.

CONCLUSIONS

These data suggest that strabismus can be associated with reduced neural activity in the abducens nucleus.

Dystrophic changes in extraocular muscles after gamma irradiation in mdx:utrophin(+/-) mice

Extraocular muscles (EOM) have a strikingly different disease profile than limb skeletal muscles. It has long been known that they are spared in Duchenne (DMD) and other forms of muscular dystrophy. Despite many studies, the cause for this sparing is not understood. We have proposed that differences in myogenic precursor cell properties in EOM maintain normal morphology over the lifetime of individuals with DMD due to either greater proliferative potential or greater resistance to injury. This hypothesis was tested by exposing wild type and mdx:utrophin(+/-) (het) mouse EOM and limb skeletal muscles to 18 Gy gamma irradiation, a dose known to inhibit satellite cell proliferation in limb muscles. As expected, over time het limb skeletal muscles displayed reduced central nucleation mirrored by a reduction in Pax7-positive cells, demonstrating a significant loss in regenerative potential. In contrast, in the first month post-irradiation in the het EOM, myofiber cross-sectional areas first decreased, then increased, but ultimately returned to normal compared to non-irradiated het EOM. Central nucleation significantly increased in the first post-irradiation month, resembling the dystrophic limb phenotype. This correlated with decreased EECD34 stem cells and a concomitant increase and subsequent return to normalcy of both Pax7 and Pitx2-positive cell density. By two months, normal het EOM morphology returned. It appears that irradiation disrupts the normal method of EOM remodeling, which react paradoxically to produce increased numbers of myogenic precursor cells. This suggests that the EOM contain myogenic precursor cell types resistant to 18 Gy gamma irradiation, allowing return to normal morphology 2 months post-irradiation. This supports our hypothesis that ongoing proliferation of specialized regenerative populations in the het EOM actively maintains normal EOM morphology in DMD. Ongoing studies are working to define the differences in the myogenic precursor cells in EOM as well as the cellular milieu in which they reside.

Postulating a role for connective tissue elements in inferior oblique muscle overaction (an American Ophthalmological Society thesis)

PURPOSE

To compare the localization and density of collagens I, IV, VI, and elastin, the major protein components of connective tissue, in the inferior oblique muscle of patients with overelevation in adduction and in controls and to characterize changes that develop following surgery. Biomechanical studies suggest that the connective tissue matrix plays a critical role in extraocular muscle function, determining tensile strength and force transmission during contraction.

METHODS

Prospective laboratory-based case-control study of inferior oblique muscle specimens from 31 subjects: 16 with primary inferior oblique overaction, 6 with craniofacial dysostosis, and 9 normal controls. Collagen I, IV, VI, and elastin were localized and quantified using immunohistochemical staining. Densities were compared using analysis of variance and post hoc comparisons.

RESULTS

In primary inferior oblique overaction, all connective tissue components in unoperated specimens were elevated compared to controls (P<.0001). Previously operated muscles showed normal levels of collagens IV and VI (P>.27) but increased collagen I. In unoperated craniofacial dysostosis specimens, only elastin was elevated (P=.03), whereas density of collagens IV and VI was lower in previously operated vs unoperated specimens (P=.015).

CONCLUSIONS

Elevated collagen and elastin levels in the cohort with primary inferior oblique overaction are consistent with the clinical finding of muscle stiffness. Contrarily, normal connective tissue densities in craniofacial dysostosis support the hypothesis that overelevation in this group reflects anomalous muscle vectors rather than tissue changes. Surgical intervention was associated with changes in the connective tissue matrix in both cohorts. These results have ramifications for treating patients with overelevation in adduction.

Bupivacaine injection remodels extraocular muscles and corrects comitant strabismus

PURPOSE

To evaluate the clinical effectiveness and anatomic changes resulting from bupivacaine injection into extraocular muscles to treat comitant horizontal strabismus.

DESIGN

Prospective, observational clinical series.

PARTICIPANTS

Thirty-one comitant horizontal strabismus patients.

METHODS

Nineteen patients with esotropia received bupivacaine injections in the lateral rectus muscle, and 12 patients with exotropia received bupivacaine injections in the medial rectus. Sixteen of these, with large strabismus angles, also received botulinum type A toxin injections in the antagonist muscle at the same treatment session. A second treatment was given to 13 patients who had residual strabismus after the first treatment.

MAIN OUTCOME MEASURES

Clinical alignment measures and muscle volume, maximum cross-sectional area, and shape derived from magnetic resonance imaging, with follow-up examinations for up to 3 years.

RESULTS

At an average of 15.3 months after the final treatment, original misalignment was reduced by 10.5 prism diopters (Δ; 6.0°) with residual deviations of 10Δ or less in 53% of patients. A single treatment with bupivacaine alone reduced misalignment at 11.3 months by 4.7Δ (2.7°) with residual deviations of 10Δ or less in 50% of patients. Alignment corrections were remarkably stable over follow-ups for as long as 3 years. Six months after bupivacaine injection, muscle volume had increased by 6.6%, and maximum cross-sectional area had increased by 8.5%, gradually relaxing toward pretreatment values thereafter. Computer modeling with Orbit 1.8 (Eidactics, San Francisco, CA) suggested that changes in agonist and antagonist muscle lengths were responsible for the enduring changes in eye alignment.

CONCLUSIONS

Bupivacaine injection alone or together with botulinum toxin injection in the antagonist muscle improves eye alignment in comitant horizontal strabismus by inducing changes in rectus muscle structure and length.

A novel GFP reporter mouse reveals Mustn1 expression in adult regenerating skeletal muscle, activated satellite cells and differentiating myoblasts

AIM

Mustn1 has been implicated in myofusion as well as skeletal muscle growth and repair; however, the exact role and spatio-temporal expression of Mustn1 have yet to be fully defined.

METHODS

Transgenic mice were generated with a 1512-bp sequence of the Mustn1 promoter directing the expression of GFP (Mustn1(PRO) -GFP). These mice were used to investigate the spatio-temporal expression of Mustn1(PRO) -GFP during skeletal muscle development and adult skeletal muscle repair, as well as various phases of the satellite cell lifespan (i.e. quiescence, activation, proliferation, differentiation).

RESULTS

Mustn1(PRO) -GFP expression was observed within somites at embryonic day 12 and developing skeletal muscles at embryonic day 15 and 18. While uninjured adult tibialis anterior muscle displayed no detectable Mustn1(PRO) -GFP expression, cardiotoxin injury robustly elevated Mustn1(PRO) -GFP expression at 3 days post-injury with decreasing levels observed at 5 days and minimal, focal expression seen at 10 days. The expression of Mustn1(PRO) -GFP at 3 days post-injury consistently overlaid with MyoD although the strongest expression of Mustn1(PRO) -GFP was noted in newly formed myotubes that were expressing minimal levels of MyoD. By 5 days post-injury, Mustn1(PRO) -GFP overlaid in all myotubes expressing myogenin although cells were present expressing Mustn1(PRO) -GFP alone. The expression patterns of Mustn1(PRO) -GFP in regenerating muscle preceded the expression of desmin throughout the regenerative time course consistent with Mustn1 being upstream of this myogenic protein. Further, quiescent satellite cells located on freshly isolated, single myofibers rarely expressed Mustn1(PRO) -GFP, but within 24 h of isolation, all activated satellite cells expressed Mustn1(PRO) -GFP. Expression of Mustn1(PRO) -GFP in primary myoblasts diminished with prolonged time in proliferation media. However, in response to serum withdrawal, the expression of Mustn1(PRO) -GFP increased during myofusion (day 2) followed by declining expression thereafter.

CONCLUSION

Mustn1(PRO) -GFP is expressed in activated satellite cells and myoblasts but continued time in proliferation media diminished Mustn1(PRO) -GFP expression. However, myoblasts exposed to serum withdrawal increased Mustn1(PRO) -GFP expression consistent with its demonstrated role in myofusion. The in vivo expression pattern of Mustn1 observed in regenerating and developing skeletal muscle is consistent with its presence in satellite cells and its critical role in myofusion.

Effects of sequential injections of hepatocyte growth factor and insulin-like growth factor-I on adult rabbit extraocular muscle

PURPOSE

To determine whether hepatocyte growth factor (HGF) and insulin-like growth factor-I (IGF-I) have synergistic effects in promoting extraocular muscle fiber growth and force generation.

METHODS

A superior rectus muscle of adult rabbits was treated with either a single injection of HGF or sequential injections of HGF followed 1 week later by IGF-I. One week after HGF alone and 1 week after the IGF-I injection, the superior rectus muscles from treated and control orbits were examined for alterations in force generation as well as changes in myofiber size.

RESULTS

Injection of HGF alone did not result in changes to muscle force, specific tension, or myofiber cross-sectional area; however, it did result in a significant increase in numbers of satellite cells. Sequential injection of HGF and IGF-I resulted in significantly increased force, specific tension, and myofiber cross-sectional areas as well as increased numbers of satellite cells.

CONCLUSIONS

Preinjection with HGF augments the treatment effect of IGF-I. This synergistic effect is likely a result of HGF-induced activation of satellite cells and should allow a reduction in IGF-I dosing required to produce a given increase in extraocular muscle force generation.

Effects of the sustained release of IGF-1 on extraocular muscle of the infant non-human primate: adaptations at the effector organ level

PURPOSE

The authors have demonstrated that prolonged exposure of adult rabbit extraocular muscle (EOM) to insulin-like growth factor-1 (IGF-1) results in significantly increased cross-sectional area and muscle force generation lasting over 3 months. Here the authors assess the effects on EOM of sustained IGF-1 treatment on normal binocular infant Macaca mulatta.

METHODS

Sustained-release IGF-1 pellets were implanted bilaterally in each medial rectus (MR) muscle of two normal infant non-human primates. Eye position was examined using corneal light reflex testing. After 3 months, morphometric analyses of myofiber cross-sectional area and innervation density in treated MR muscles were compared with an age-matched control and with antagonist lateral rectus (LR) muscles.

RESULTS

After 3 months, the slow-release pellets remained at the implantation site in all four MR muscles treated. The treated MR showed pronounced increases in cross-sectional area and nerve density, mirrored in the untreated antagonist LR.

CONCLUSIONS

Three months of bilateral sustained IGF-1 release in infant non-human primate MR resulted in increased muscle size and innervation density, mirrored in the untreated antagonist LR. It appears that bilateral MR treatment resulted in slow adaptation of both treated MR and contralateral LR muscles over time such that functional homeostasis and near-normal alignment were maintained. Further work is needed to determine what signaling mechanisms maintain proportional innervation when EOMs are forced to adapt to an externally applied perturbation.

Responses of medial rectus motoneurons in monkeys with strabismus

PURPOSE

Monkeys reared under conditions of alternating monocular occlusion during their first few months of life show large horizontal strabismus, "A" patterns, and dissociated vertical deviation. "A" patterns manifest as an inappropriate horizontal component in the deviated eye during vertical eye movements (cross-axis movement). The objective of this study was to investigate response properties of medial rectus motoneurons (MRMNs) in relation to strabismus properties.

METHODS

Burst-tonic activity of 21 MRMNs in the oculomotor nucleus were recorded from two monkeys with exotropia as they performed horizontal and vertical smooth pursuit (0.2 Hz, ±10°) under monocular viewing conditions. Neuronal responses and horizontal component of eye movements were used to identify regression coefficients in a first-order model for each tracking condition.

RESULTS

Comparison of position, velocity, and constant parameter coefficients, estimated from horizontal tracking data with either eye viewing, showed no significant differences (P > 0.07), indicating that neuronal activity could account for the horizontal misalignment. Comparison of the position, velocity, and constant parameter coefficients estimated from horizontal tracking and the cross-axis condition showed no significant differences (P > 0.07), suggesting that motoneuron activity could account for most of the inappropriate horizontal cross-axis movement observed in the covered eye during vertical smooth pursuit.

CONCLUSIONS

These data suggest that, in animals with sensory-induced strabismus, central innervation to extraocular muscles is responsible for setting the state of strabismus. Mechanical factors such as muscle length adaptation (for horizontal misalignment) and pulley heterotopy or static torsion (for "A" patterns) likely do not play a major role in determining properties in a sensory-induced strabismus.

Incomitant strabismus: does extraocular muscle form denote function?

The paradigm that an underacting extraocular muscle (EOM) is always atrophic or hypoplastic, and an overacting EOM should always be enlarged, leads to inconsistencies with clinical observations. It is inconsistent with the findings of normal extraocular muscle diameters in patients with apparent superior oblique muscle palsy, "overacting" inferior oblique muscles, and the superior rectus muscle overaction / contracture syndrome, among other clinical entities. These inconsistencies can be reconciled if one accepts the possibility that EOM contractile activity may reflect a change in neural input to an anatomically normal muscle, and / or that muscle contractile activity may be altered by a shift in fiber type and distribution within a normal-sized muscle. This remodeling may occur as a result of vergence adaptation or any change in neural stimulus to the muscle. There is substantial evidence to suggest that both these theoretical possibilities may likely occur.

A continuum of myofibers in adult rabbit extraocular muscle: force, shortening velocity, and patterns of myosin heavy chain colocalization

Extraocular muscle (EOM) myofibers do not fit the traditional fiber typing classifications normally used in noncranial skeletal muscle, in part, due to the complexity of their individual myofibers. With single skinned myofibers isolated from rectus muscles of normal adult rabbits, force and shortening velocity were determined for 220 fibers. Each fiber was examined for myosin heavy chain (MyHC) isoform composition by densitometric analysis of electrophoresis gels. Rectus muscle serial sections were examined for coexpression of eight MyHC isoforms. A continuum was seen in single myofiber shortening velocities as well as force generation, both in absolute force (g) and specific tension (kN/m(2)). Shortening velocity correlated with MyHCIIB, IIA, and I content, the more abundant MyHC isoforms expressed within individual myofibers. Importantly, single fibers with similar or identical shortening velocities expressed significantly different ratios of MyHC isoforms. The vast majority of myofibers in both the orbital and global layers expressed more than one MyHC isoform, with up to six isoforms in single fiber segments. MyHC expression varied significantly and unpredictably along the length of single myofibers. Thus EOM myofibers represent a continuum in their histological and physiological characteristics. This continuum would facilitate fine motor control of eye position, speed, and direction of movement in all positions of gaze and with all types of eye movements-from slow vergence movements to fast saccades. To fully understand how the brain controls eye position and movements, it is critical that this significant EOM myofiber heterogeneity be integrated into hypotheses of oculomotor control.

Sustained release of bone morphogenetic protein-4 in adult rabbit extraocular muscle results in decreased force and muscle size: potential for strabismus treatment

Purpose. To assess the effect of a sustained-release preparation of bone morphogenetic protein-4 (BMP-4) on EOM force generation and muscle size. Methods. Sustained-release pellets, releasing 500 nanograms/day of BMP-4 for a maximum of 3 months, were implanted beneath the superior rectus muscle (SR) belly in anesthetized adult rabbits. The contralateral side received a placebo pellet as a control. After 1, 3, and 6 months, SRs were removed, and force generation at twitch and tetanic frequencies as well as fatigue resistance were determined in vitro. Myofiber size, myosin heavy chain isoform expression, and satellite cell density were assessed histologically. Results. SR force generation was significantly decreased by BMP-4 compared with the contralateral controls. Force generation was decreased by 25-30% by 1 month, 31-50% by 3 months, and at 6 months, after 3 BMP-4-free months, force was still decreased by 20-31%. No change in fatigue was seen. Significant decreases in muscle size were seen, greatest at 3 months. At all time points Pax7- and MyoD-positive satellite cell densities were significantly decreased. Conclusions. The decreased force generation and muscle size caused by sustained release of BMP-4 suggests that myogenic signaling factors may provide a more biological method of decreasing muscle strength in vivo than exogenously administered toxins. Treating antagonist-agonist pairs of EOM with titratable, naturally occurring myogenic signaling and growth factors may provide safe, efficacious, nonsurgical treatment options for patients with strabismus.

Serial histologic and immunohistochemical changes in anterior digastric myocytes in response to distraction osteogenesis

PURPOSE

To document histologic and immunohistochemical changes in the anterior digastric muscle during distraction osteogenesis (DO).

MATERIALS AND METHODS

Nineteen Yucatan minipigs with mixed dentition were used for these experiments. Group A (n = 16) underwent unilateral mandibular distraction at a rate of 1 mm/day (no latency) for 12 days. Animals were killed at mid-DO (n = 5), end-DO (n = 5), mid-fixation (n = 4), and end-fixation (n = 2). Group B (n = 2) underwent acute 12-mm advancement, and group C (n = 1) dissection and osteotomy. Animals from groups B and C were killed at the end-DO time point. Digastric muscles from treatment and contralateral sides of all animals were harvested and embedded in paraffin. Specimens were stained with hematoxylin/eosin or immunohistochemically for proliferating cell nuclear antigen (PCNA; total cell proliferation), paired Box-7 gene protein (Pax7; satellite cells), or myogenic differentiation 1 protein (MyoD; differentiating myoblasts). Descriptive and bivariate statistics were computed to compare groups (P ≤ .05 statistically significant).

RESULTS

All animals survived the operation and observation period; there were no device failures. Two animals (1 at mid-DO, 1 at mid-fixation) were eliminated from the study because of postoperative infection. There was minimal digastric inflammation, fibrosis, and muscle fiber size variability during active DO. Immunohistochemical analysis showed statistically significant increases in PCNA (cellular proliferation), Pax7 (satellite cells), and MyoD (differentiating myoblasts) positive nuclei in digastrics at mid-DO and end-DO.

CONCLUSIONS

Results of this study indicate that there are minimal pathologic changes but significant increases in PCNA, Pax7, and MyoD positive nuclei during active distraction. This supports the hypothesis that the digastric muscle response to DO consists of proliferation and hypertrophy.

Effects of recession versus tenotomy surgery without recession in adult rabbit extraocular muscle

PURPOSE

Surgical recession of an extraocular muscle (EOM) posterior to its original insertion is a common form of strabismus surgery, weakening the rotational force exerted by the muscle on the globe and improving eye alignment. The purpose of this study was to assess myosin heavy chain (MyHC) isoform expression and satellite cell activity as defined by Pax7 expression in recessed EOMs of adult rabbits compared with that in muscles tenotomized but not recessed and with that in normal control muscles.

METHODS

The scleral insertion of the superior rectus muscle was detached and sutured either 7 mm posterior to its original insertion site (recession surgery) or at the same site (tenotomy). One day before euthanization, the rabbits received bromodeoxyuridine (BrdU) injections. After 7 and 14 days, selected EOMs from both orbits were examined for changes in fast, slow, neonatal, and developmental MyHC isoform expression, Pax7 expression, and BrdU incorporation.

RESULTS

Recession and tenotomy surgery resulted in similar changes in the surgical EOMs. These included a decreased proportion of fast MyHC myofibers, an increased proportion of slow MyHC myofibers, and increased BrdU-positive satellite cells. Similar changes were seen in the non-operated contralateral superior rectus muscles. The ipsilateral inferior rectus showed reciprocal changes to the surgical superior rectus muscles.

CONCLUSIONS

The EOMs are extremely adaptive to changes induced by recession and tenotomy surgery, responding with modulations in fiber remodeling and myosin expression. These adaptive responses could be manipulated to improve surgical success rates.

Changes in muscle fiber size and in the composition of myosin heavy chain isoforms of rabbit extraocular rectus muscle following recession surgery

PURPOSE

To assess the changes in the size of muscle fibers and the composition of myosin heavy chain (MyHC) isoforms in the global layer (GL) and the orbital layer (OL) of rabbit rectus extraocular muscle (EOM) after recession.

METHODS

The right superior rectus muscles of two rabbits were harvested at 3 days or 1, 2, or 4 weeks after recession (eight rabbits in total). At each time point, one muscle was used for measuring the cross-sectional area of the muscle fibers and the other for identifying the composition of MyHC. The right superior rectus muscles of three additional naïve rabbits were used as controls.

RESULTS

The mean cross-sectional area of the OL fibers did not change significantly. However, that of the GL fibers significantly decreased at 3 days (P<0.001) and 1 week (P=0.024) postoperatively, and increased thereafter to reach the control levels at 2 and 4 weeks postoperatively. Three days after surgery, the total MyHC content and the proportion of type IIb MyHC (MyHCIIb) plus EOM-specific MyHC (MyHCeom) decreased and remained at its lower level for 4 weeks.

CONCLUSIONS

Transient atrophy and regeneration were observed only in the GL, and the changes in the MyHCIIb plus MyHCeom appeared to be related to these changes.

Activated satellite cells in medial rectus muscles of patients with strabismus

PURPOSE

The goal of this study was to determine whether the medial rectus muscles of patients with a history of medial rectus underaction or overaction show alterations in the process of satellite cell activation when compared with normal age-matched control muscles.

METHODS

Medial rectus muscles were obtained with consent from adult patients undergoing surgical resection due to medial rectus underaction or overaction and were prepared for histologic examination by fixation and paraffin embedding. Control muscles were obtained from cornea donor eyes of adults who had no history of strabismus or neuromuscular disease. Cross sections were obtained and stained immunohistochemically for the presence of activated satellite cells, as identified by MyoD immunoreactivity, and the presence of the total satellite cell population, as identified by Pax7 immunoreactivity. The percentages of MyoD- and Pax7-positive satellite cells per 100 myofibers in cross section were calculated.

RESULTS

As predicted from results in the literature, MyoD-positive satellite cells, indicative of activation, were present in both the control and resected muscles. In the underacting medial rectus muscles, the percentages of MyoD- and Pax7-positive satellite cells, based on the number of myofibers, were approximately twofold higher than the percentages in the control muscles. In the overacting medial rectus muscles, the percentage of MyoD-positive satellite cells was twofold less than in the control muscles, whereas the percentage of Pax7-positive satellite cells significantly increased compared with that in the control specimens.

CONCLUSIONS

The presence of an increased number of activated satellite cells in the resected underacting medial rectus muscles and the decreased numbers of activated satellite cells in the overacting muscles was unexpected. The upregulation in the number of MyoD-positive satellite cells in underacting muscles suggests that there is potential for successful upregulation of size in these muscles, as the cellular machinery for muscle repair and regeneration, the satellite cells, is retained and active in patients with medial rectus underaction. The decreased number of activated satellite cells in overacting MR muscle suggests that factors as yet unknown in these overacting muscles are able to affect the number of satellite cells and/or their responsiveness compared with normal age-matched control muscles. These hypotheses are currently being tested.

Bupivacaine injection of eye muscles to treat strabismus

BACKGROUND

Bupivacaine injected into animal muscles induces a cycle of myotoxicity, degeneration, regeneration and hypertrophy of muscle fibres, without adverse effects on other tissues. This induced hypertrophy can be harnessed to treat strabismus.

METHODS

Bupivacaine, 4.5 ml of a 0.75% solution, was injected into the right lateral rectus (RLR) muscle of a patient who had diplopia and who showed 14-prism-dioptres oesotropia.

RESULTS

RLR paresis persisted for 7 days. Then, the RLR regained its abducting ability, and progressive improvement of alignment to 4-prism-dioptres oesophoria occurred over the next 33 days, with the elimination of diplopia. Alignment remained the same at 54 days after injection. Magnetic resonance imaging showed a focal increase in the size of the injected RLR of 58% in the posterior area, with reduced change in anterior portions of the RLR.

CONCLUSION

Injection of bupivacaine to induce hypertrophy of the injected muscle and thus alter eye alignment was effective in our patient. This approach can be a useful addition to the treatment of strabismus.

Increased extraocular muscle strength with direct injection of insulin-like growth factor-I

PURPOSE

Previous work has demonstrated the effectiveness of insulin-like growth factor (IGF)-II in increasing force generation in extraocular muscle (EOM). Studies in the literature have suggested that IGF-I would be even more effective than IGF-II. This study was performed to assess the effects on muscle mass and force generation of IGF-I injection in adult rabbit superior rectus muscle.

METHODS

Adult rabbits received a single injection of IGF-I at one of several doses into one superior rectus muscle. One week after treatment, the rabbits were euthanatized, and the superior rectus muscle from each orbit was removed. Force generation was measured using an in vitro apparatus, and injected muscles were compared with the contralateral control. A second group of animals were injected similarly, and the muscles were examined at 1 week for changes in cross-sectional area of individual myofibers.

RESULTS

EOMs demonstrate significant numbers of cells expressing the IGF receptor. After the EOMs were injected with IGF-I, there were significant increases both in muscle force generation and cross-sectional area at all doses tested in this study. Doses of 10 and 25 microg IGF-I were most effective.

CONCLUSIONS

Direct muscular injection of IGF-I effectively increases EOM force generation without the potential biomechanical hazards of surgery such as permanently altered muscle length or insertional position on the globe.