Primary and secondary overacting inferior oblique muscles: an ultrastructural study

Differences in Inferior Oblique Muscle Satellite Cell Populations between Primary and Secondary Inferior Oblique Muscle Overaction

INTRODUCTION

The aim of the study was to compare the number of paired box 7 (PAX7)-positive, myogenic differentiation 1 (MyoD1)-positive, and neural cell adhesion molecule-1 (NCAM)-positive satellite cells in human primary versus secondary inferior oblique muscle overaction (IOOA).

METHODS

This prospective observational study enrolled patients who underwent inferior oblique muscle myectomy at the Department of Pediatric Ophthalmology and Strabismus in Tianjin Eye Hospital between January 2020 and November 2020. The muscle specimens were processed. Immunohistochemistry and immunofluorescence were used to quantify inferior oblique muscle fiber diameter and PAX7-positive, NCAM-positive, and MyoD1-positive satellite cells.

RESULTS

Thirty-eight patients with inferior oblique overaction were enrolled: 18 with primary IOOA and 20 with secondary IOOA. The participants were significantly younger in the secondary IOOA group than in the primary IOOA group (2.8 ± 1.2 vs. 9.0 ± 3.2 years, p < 0.001). The muscle fiber diameter between the two groups was not significantly different (13.5 ± 1.4 vs. 13.8 ± 0.7 µm, p = 0.530), but PAX7+ (3.3 ± 2.8 vs. 1.8 ± 0.6, p < 0.001), NCAM+ (3.6 ± 1.5 vs. 1.7 ± 0.2, p < 0.001), and MyoD1+ (4.8 ± 1.9 vs. 2.7 ± 0.5, p < 0.001) cell counts were higher in primary IOOA than in secondary IOOA.

CONCLUSIONS

PAX7pos, MyoD-1pos, and NCAMpos cell counts per laminar nucleus were almost 2-fold higher in the primary IOOA group than in the secondary IOOA group.

Differences in gene expression between the primary and secondary inferior oblique overaction

BACKGROUND

This study sought to define different adaptive changes in the molecular levels of the overacting inferior oblique muscle in primary and secondary inferior oblique overaction.

METHODS

The inferior oblique muscles of patients with congenital superior oblique palsy (SOP) and those of patients with congenital esotropia were collected during surgery. RNA-seq technology was performed to detect the differentially expressed genes (DEGs) between the two groups. A comprehensive analysis of the gene expression profiles was then conducted, including the identification of DEGs, a Gene Ontology (GO) analysis, and a gene set enrichment analysis (GSEA). Finally, a protein-protein interaction (PPI) network was constructed with Search Tool for the Retrieval of Interacting Genes/Proteins (STRING) and Cytoscape software.

RESULTS

We identified 221 DEGs, of which 104 were significantly upregulated and 117 were downregulated in the SOP group. Additionally, several isoforms of the myosin heavy chain (MyHC) gene were found to be significantly and differentially expressed in the SOP group, including 3 upregulated fast-twitch MyHC isoforms (i.e., MYH1, MYH4, and MYH13) and 1 downregulated slow-twitch MyHC isoform (i.e., MYH3). The GO analysis indicated that the upregulated DEGs were mainly enriched in the muscle system process and muscle contraction. The GSEA analysis revealed that the upregulated pathways of ribosome, proteasome, oxidative phosphorylation, fatty acid metabolism, viral myocarditis, and cardiac muscle contraction were enriched.

CONCLUSIONS

Our findings provide insights into the different molecular changes of inferior oblique muscle overaction secondary to SOP and suggest the potential pathological mechanisms of inferior oblique overaction (IOOA) in SOP. The results suggest that upregulated fast-twitch MyHC isoforms and downregulated slow-twitch MyHC isoform in SOP may contribute to the increased force of its inferior oblique muscle.

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.

Histopathological features and satellite cell population characteristics in human inferior oblique muscle biopsies: clinicopathological correlation

PURPOSE

To investigate the correlation between clinical characteristics and histopathological and immunohistochemical features of inferior oblique muscles in patients with primary and secondary inferior oblique overaction.

METHODS

Inferior oblique muscle specimens of patients who underwent inferior oblique-weakening procedures for primary or secondary inferior oblique overaction were recruited. Subjects were mainly divided into two groups, each of which was further divided into two subgroups: group 1 included patients with primary inferior oblique overaction (subgroups, infantile esotropia vs acquired V-pattern esotropia), and group 2 included patients with secondary inferior oblique overaction (subgroups, congenital vs acquired trochlear nerve palsy). Inferior oblique overaction was graded between 0-4. Histopathologic changes, such as angular fibers, endo- and perimysial fibrosis, and vacuolization were categorized from mild to severe. Immunohistochemical markers Pax7, NCAM, and MyoD1 were used to detect satellite cells, a unique stem cell population in muscles presumably responsible from myofiber regeneration and maintenance, and their activity. Results were reported as stained cells per cross-section ratio.

RESULTS

A total of 51 patients were included: 36 in group 1 and 15 in group 2. Satellite cell distribution and activity was significantly higher in group 1 (P < 0.001). The angular fiber count and the degree of perimysial fibrosis was higher in the secondary group (P < 0.001 and P = 0.01, resp.). There was no correlation between clinical amount of inferior oblique muscle overaction and immunohistochemical markers.

CONCLUSIONS

The differences in immunohistochemical parameters supported with histopathological changes between different strabismus etiologies imply that satellite cell population behavior varies among strabismus types.

Relationship of inferior oblique overaction to macular and subfoveal choroidal thickness

PURPOSE

To evaluate the macular and subfoveal choroidal thickness of eyes with inferior oblique muscle overaction (IOOA) using enhanced depth imaging spectral domain optical coherence tomography (EDI SD-OCT).

METHODS

The measurements of macular and subfoveal choroidal thickness obtained by EDI SD-OCT of patients with IOOA (24 patients) were compared with those of age- and sex-matched controls (25 subjects).

RESULTS

There were no morphological abnormalities of the macula in patients with IOOA or in control subjects. There were no statistically significant differences in macular and subfoveal choroidal thickness between the eyes with IOOA and the eyes of the control subjects (P > 0.05). When the patients with IOOA were assigned to two distinct groups according to the degree of IOOA, the macular thickness did not differ between groups (P = 0.66), whereas subfoveal choroidal thickness measures were significantly lower in eyes with severe IOOA compared to eyes of the controls (P = 0.01).

CONCLUSIONS

IOOA has no effect on the morphology and the thickness of the macula. Severe IOOA seems to be related to thinning of subfoveal choroid due to possible external mechanical effect.

Histopathological and electron microscopic study for different grades of inferior oblique muscle overaction

Purpose

To correlate between the clinical degree of inferior oblique muscle (IO) overaction and the histopathological changes of the muscle.

Settings

Departments of Ophthalmology and Pathology, Faculty of Medicine, Ain Shams University, Cairo, Egypt.

Patients and methods

Biopsies from 12 IO muscles were taken during strabismus surgery for cases of IO muscle overaction. One biopsy from a normal IO was taken as a control. All samples were examined under light microscopy and transmission electron microscopy.

Results

In light microscopy, all cases showed histopathological changes, in the form of degenerated and regenerating muscle fibers, increased fibrofatty infiltration, and mild variability of muscle fiber size. Hypertrophied nerve bundles also appeared in biopsies from patients with grade II and grade III IO overaction. Endomysial and perimysial fibrosis, mononuclear inflammatory infiltrates, and focal fatty infiltration were seen in biopsies from cases of grade III IO overaction. In electron microscopy, ultrastructural examination revealed an increased number of mitochondria associated with some degree of mitochondrial pleomorphism. Hypercontracted fibers, vacuoles, and fat droplets were also noticed.

Conclusion

IO overaction is always accompanied by histopathological changes that differ in severity according to the clinical grading of the overaction. Changes in nerve fibers can also occur in severe cases.

Differences in gene expression between strabismic and normal human extraocular muscles

PURPOSE

Strabismic extraocular muscles (EOMs) differ from normal EOMs in structural and functional properties, but the gene expression profile of these two types of EOM has not been examined. Differences in gene expression may inform about causes and effects of the strabismic condition in humans.

METHODS

EOM samples were obtained during corrective surgery from patients with horizontal strabismus and from deceased organ donors with normal EOMs. Microarrays and quantitative PCR identified significantly up- and down-regulated genes in EOM samples. Analysis was performed on probe sets with more than 3-fold differential expression between normal and strabismic samples, with an adjusted P value of ≤ 0.05.

RESULTS

Microarray analysis showed that 604 genes in these samples had significantly different expression. Expression predominantly was upregulated in genes involved in extracellular matrix structure, and down-regulated in genes related to contractility. Expression of genes associated with signaling, calcium handling, mitochondria function and biogenesis, and energy homeostasis also was significantly different between normal and strabismic EOM. Skeletal muscle PCR array identified 22 (25%) of 87 muscle-specific genes that were significantly down-regulated in strabismic EOMs; none was significantly upregulated.

CONCLUSIONS

Differences in gene expression between strabismic and normal human EOMs point to a relevant contribution of the peripheral oculomotor system to the strabismic condition. Decreases in expression of contractility genes and increases of extracellular matrix-associated genes indicate imbalances in EOM structure. We conclude that gene regulation of proteins fundamental to contractile mechanics and extracellular matrix structure is involved in pathogenesis and/or consequences of strabismus, suggesting potential novel therapeutic targets.

The double-bellied inferior oblique muscle: clinical correlates

PURPOSE

We previously reported an 8% incidence of double-bellied inferior oblique (IO) muscles at the surgical capture site (10-12 mm from insertion) in cadaveric specimens. This companion study sought to determine how often this anomaly is encountered at surgery for clinically overacting IO muscles and whether clinical findings or surgical outcomes in cases with double-bellied muscles differ from those with single-bellied muscles.

METHODS

For 7 years we collected preoperative, intraoperative, and postoperative data on all patients for whom one surgeon performed primary IO weakening operations for overactions. We compared eyes with double-bellied IO muscles to those with single-bellied muscles on 4 variables--gradings of preoperative IO and superior oblique (SO) actions, presence of fundus excyclotropia, differences between horizontal deviations in upgaze and downgaze, and presence and sizes of primary position hypertropias--to determine whether one or more of them could predict the presence of a double-bellied muscle. Finally, we assessed postoperative IO actions to determine whether the presence of a double-bellied muscle influenced the effectiveness of IO weakening surgery in reducing overaction.

RESULTS

Among 162 patients (247 eyes) who underwent this surgery, 77 (77 eyes) had unilateral surgery and 85 (170 eyes) bilateral. Twenty-seven (10.9%) of the 247 muscles had double bellies. Among all variables compared, only the incidence of fundus excyclotropia differed significantly between groups, occurring more often in eyes with double-bellied IO muscles (48% vs 27%; P =.041). The efficacy of weakening surgery in reducing overactions was similar in both groups.

CONCLUSION

The finding that eyes with double-bellied IO muscles showed a higher incidence of fundus excyclotropia suggests that the presence of a second belly may alter the physiologic action of the IO muscle.

Histological appearances of the levator palpebrae superioris muscle in the Marcus Gunn phenomenon

The levator palpebrae superioris muscles from 12 cases of Marcus Gunn phenomenon were examined histologically by means of histochemistry and morphometry. The appearances were of a neurogenic atrophy with pathological changes occurring in both the affected side and the clinically normal side, suggesting that the causative lesion was located in the central nervous system.

Central trochlear palsy

Historically, the trochlear (IV) nerve has been "neglected" by neurologists and ophthalmologists. However, the reported incidence of trochlear palsy in two large series has more than doubled in the past two decades, indicating increasing awareness of this nerve. Trauma is the most common cause of trochlear palsy, as the trochlear nerve is anatomically more vulnerable to trauma than the other ocular motor nerves. Trochlear palsy can also be caused by vascular and inflammatory diseases, congenital factors, toxic substances and tumors. Diplopia secondary to vertical and horizontal deviation is the most common presentation. The trochlear nerve has a relatively high recovery rate after the underlying cause of injury has been corrected. In this article, the anatomy and physiology of the trochlear nerve are described, and the various etiologies, methods of diagnosis and differential diagnosis of trochlear palsy are reviewed.