Incomitant Strabismus Does Extraocular Muscle Form Denote Function?
Department of Ophthalmology and Visual Sciences, University of Wisconsin-Madison, 53705, USA. Archives of ophthalmology
(Impact Factor: 4.4).
12/2010; 128(12):1604-9. DOI: 10.1001/archophthalmol.2010.301
The paradigm that an "underacting" extraocular muscle is always atrophic or hypoplastic and that an overacting extraocular muscle should always be enlarged leads to inconsistencies with clinical observations. These include findings of "overacting" inferior oblique muscles, superior rectus muscle overaction or contracture syndrome, and normal extraocular muscle diameters in patients with apparent superior oblique muscle palsy, among other clinical entities. These inconsistencies can be reconciled if one accepts the possibility that extraocular muscle contractile activity may reflect a change in neural input to an anatomically normal muscle or that muscle contractile activity may be altered by shifts in fiber type and distribution within a normal-sized muscle. This remodeling may result from vergence adaptation or from any change in neural stimulus to the muscle. There is substantial evidence to suggest that both of these theoretical possibilities may likely occur.
Available from: Zhipeng Gao
- "At present, no ideal experimental data can be used to determine this complicated constitutive relationship between active force and the stretch of an EOM because of the anatomical difficulty and ethical requirements, although a similar relationship of the skeletal muscle has been determined (Gordon et al., 1966) and has been widely used in the modern finite element modeling of muscles (Böl and Reese, 2008; Ehret et al., 2011). However, the fiber composition of an EOM is different from that of a skeletal muscle (Kushner, 2010); their biomechanical behaviors also differ (Quaia et al., 2009). To use the corresponding research results on the active force of a skeletal muscle in describing the active behavior of an EOM is inappropriate . "
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ABSTRACT: This study proposes a mathematical model to estimate the initial tension forces of the extraocular muscles (EOMs). These forces are responsible for the mechanical equilibrium of the eye suspended in primary position. The passive contributions were obtained using the corresponding Cauchy stress-stretch relationships based on the previous clinical experimental data; whereas the active contributions were obtained using an optimum method with weakening the effect of innervation. The initial tension forces of the EOMs were estimated to be 48.8±14.2mN for the lateral rectus, 89.2±31.6mN for the medial rectus, 50.6±17.6mN for the superior rectus, 46.2±13.4mN for the inferior rectus, 15.6±8.3mN for the superior oblique, and 17.1±12.1mN for the inferior oblique.
Available from: nature.com
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ABSTRACT: Idiopathic superior oblique muscle palsy presents, as quantitative phenotypes, vertical deviation and cyclodeviation in eye alignment on clinical testing, and superior oblique muscle hypoplasia on imaging. We determined ARIX and PHOX2B polymorphisms as genotypes, and analyzed phenotype-phenotype and genotype-phenotype correlations in 37 patients with idiopathic superior oblique muscle palsy. Vertical deviations were measured at upright position of the head and head tilt for 30° to either side, and angles of objective excyclodeviations were determined by image analysis on fundus photographs. Cross-sectional areas of the superior oblique muscle near the eye globe-optic nerve junction were measured by image analysis on coronal sections of magnetic resonance imaging to calculate the paretic-side/normal-side ratios. Among the phenotypes, the increase in vertical deviations elicited by head tilt to the paretic side, the decrease in vertical deviations elicited by head tilt to the normal side and the difference of angles of objective excyclodeviations between the paretic side and normal side were significantly correlated inversely with the paretic-side/normal-side ratios of the cross-sectional areas of the muscle (r=-0.43 with P=0.0084, r=-0.34 with P=0.038, and r=-0.43 with P=0.009, respectively, n=37, Pearson's correlation test). Fifteen patients with ARIX and/or PHOX2B polymorphisms had significantly greater paretic-side/normal-side ratios of the muscle compared with 20 patients without the polymorphisms (P=0.017, n=35, Mann-Whitney U-test). The patients with ARIX and/or PHOX2B polymorphisms had less hypoplastic superior oblique muscles.
Available from: Amy L Altick
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ABSTRACT: 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.
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.
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.
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.
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