Transactions 29th European Strabismological Association Meeting – de Faber (ed) © 2005 European Strabismological Association, ISBN 04 1537 211 9

MRI measurements of horizontal rectus muscles in esotropia: the role of amblyopia

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F.M. Mutlu, G. Dinçer, H. Durukan, T. Mumcuog˘lu & H.I. Alt nsoy

GATA Department of Ophthalmology, Ankara, TURKEY

ABSTRACT:

Purpose: To evaluate the thickness of horizontal rectus muscles in patients with esotropia by using magnetic resonance imaging (MRI), and to determine the effect of amblyopia on muscle tickness. Methods: Eighteen orthotropic volunteers and 50 patients with non-refractive and non-acom- modative esotropia underwent orbital imaging prospectively. Esotropic patients were divided into 2 groups according to existance of amblyopia: Group I, 26 patients with amblyopia; group II, 24 patients with good visual acuity and alternating fixation. MRI was performed by using a fixation target and occluding each eye separetely. All MRI images were performed with axial T1 weighting by using 2 mm slice tickness. Mean tickness of horizontal rectus muscles of both groups were compared with that of the control group of the same age and sex.

Results: The thickest MR (3.84 0.88 mm) and the thinnest lateral rectus (1.92 0.68 mm) values were with the amblyopic eye of group I. Lateral rectus thickness in the amblyopic eye of group I (1.92 0.68 mm) was thinner than both group II (2.37 0.59 mm) and controls (2.91 0.94 mm) (respectively p 0.029; p 0.000) The difference between the mean tickness of the medial rectus muscles of amblyopic eye of group I (3.84 0.88 mm) and controls (3.16 0.60 mm) was also significant (p 0.001).

Conclusions: There are significant morphometric changes of horizontal rectus muscles in patients with non-refractive and non-acommodative esotropia and amblyopia. Further studies are necessary for different types of strabismus, and the correlation between our findings and surgical outcomes.

1INTRODUCTION

Strabismus is a frequently encountered ophthalmic problem which results in visual, cosmetic and socioeconomic problems. Several mechanisms have been suggested to explain this challenging problem. So far, there is no data to explain whether strabismus occured due to central or peripheral nervous system abnormalities or anomalies of extraocular muscles (EOM) or both. Whatever the mechanism, EOMs should be affected morphologically. The outcomes of strabismus surgery may be corralated with various factors one of which is morphological (Domenici-Lombardo, 1992) and structural (Martinez, 1980) abnormalities of extraocular muscles. Besides, Weakly et al. (1997) reported a correlation between the surgical outcomes of esotropia with and without amblyopia.

The aim of the present study is to evaluate the thickness of horizontal rectus muscles in esotropic patients by using magnetic resonance imaging, and to determine the effect of amblyopia on muscle tickness.

2METHODS

The MRI studies were done on 50 consecutive patients with esotropia (all males with a range of age of 20 to 25 years of age), and 18 sex and age matched healty volunteer subjects with normal eyes,

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visual acuity and ocular motility at the Ophthalmology Department of our institution between January 2001 and April 2002.

All patients with non-refractive and non-acommodative esotropia (onset of esotropia prior to 5 year of age and refractive error less than 3.00 spherical equivalent) included the study in group I and II. Patients of group I had amblyopia but group II patients did not have amblyopia. Those with associated features below were not included the study. The history of previous ocular surgery or inflammation (Domenici-Lombardo, 1992), ocular and systemic pathologies such as diabetes mellitus (Martinez, 1980), thyroid disease, high myopia, orbital or rethinal pathologies (Weakley, 1997) amblyopia other than strabismic, visual acuity less than 0.2 Snellen lines in amblyopic eye (Byrne, 1991), paralytic or restrictive strabismus (Lee, 2001).

All subjects underwent a complete ocular examination, with particular attention to ocular motility. A total of 50 patiens with esotropia were divided into 2 groups. Group I comprised of 26 esotropia patients with amblyopia, and group II comprised of 24 esotropia patiens without amblyopia and with alternating fixation.

All subjects were examined by 1.5 Tesla, superconducting MRI scanner (Vision Plus, Siemens, Erlangen, Germany) using a circulary polarized head coil. After apropriate localization and fixation of the head with standard fixation devices, scout images were obtained in three orhogonal planes. Contiguous axial T1 weighted (583/15 ms, TR/TE, one exitation) spin-echo images parallel to the optic nerves seen on saggital scout images were obtained by using 2 mm slice thickness without any intersection gap, 220 220 mm field of view (FOV) and 256 256 matrix size. Each eye was imaged separetely, while the other was occluded. The imaging eye was fixed by using a 1.5-cm- diameter, black fixation target located in head coil 17 cm away from anterior eye plain. After adjusment of appropriate window settings, horizontal diameters of medial and lateral rectus muscles were measured directly with a calipper on magnified images with the same magnification factor and then converted to true dimentions in millimeters by using standard reference scale of the system. Results are reported as the mean standard deviation (SD). Differences between the groups were tested for significance by Kruskall Wallis, Bonferroni adjusted Mann-Whitney U test, t-Test and Chi-Square test. Differences were considered significant at p 0.05. Statistical analysis was performed with Statistical Package for the Social Sciences for Windows (SPSS Inc, version 10.0, Chicago, Il, USA).

3RESULTS

All subjects were males. The mean age of control group was 21.3 1.4 years (range 21 to 23 years), and 21.8 1.6 years (range 20 to 24 years) for patients with esotropia (p 0.887). There was no difference between group I and II according to age (p 0.780), gender, angle of deviation (p 0.191) and laterality (p 0.893). Mean values and ranges for thickness of medial and lateral rectus of all groups are given in Table 1.

Table 1. Muscle thickness of fixating and deviating eyes within groups.

SD: Standard deviation; MR: Medial rectus; LR: Lateral rectus; DE: Deviating eye; FE: Fixating eye.

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The differences between thickness of medial and lateral rectus of group I and controls were significant (p 0.001; p 0.000, respectively). So, MR muscle is thicker and LR muscle is thinner in deviating eye of patients with esotropia and amblyopia with respect to controls. In contrast, there was no difference between the thickness of MR and LR of controls and fixating eye of group I patients (p 0.158; p 0.106, respectively). LR thickness of deviating and fixating eyes of group II were significantly thinner with respect to controls (p 0.03; p 0.02). LR muscle thickness of deviating (amblyopic) eyes of group I is significantly thinner than group II (p 0.029). The differences of thickness of MR and LR of fixating eyes of both group I and II were insignificant (p 0.810; p 0.609, respectively).

4CONCLUSION

The pathogenesis of strabismus is unclear. Some believe that mechanical factors related with anomalies of the EOMs or their tendinous insertion cause strabismus while others support the hypothesis that strabismus is mainly due to central or pheriferal innervational factors. Strabismus can be with good visual acuity or amblyopia. Consistent with Weakley’s study [ ] we have seen that surgical results of esotropic patients with amblyopia is poor with respect to the patients without amblyopia (Weakley, 1997). Additionally, we have seen that thickness of medial or lateral recti may vary significantly in some patients with esotropia. Although various reports have been published, there is no report that evaluates the correlation between thickness of EOM and amblyopia in patients with esotropia. The aim of our study was to evaluate the thickness of horizontal rectus muscles in esotropic patients by using magnetic resonance imaging (MRI), and to determine the correlation of amblyopia and horizontal rectus muscle thickness.

Morphological and structural properties of EOMs have been investigated using different techniques such as histopathology, ecography (Demer, 1994a), computer tomography (CT) (Lee, 2001) and magnetic resonance imaging (MRI) (Bloom, 1993). These techniques could provide useful information for clinical diagnosis, classification, and treatment of EOM disorders. Although non-invasive, standardized echography has disadvantage of significant variabilities in echographic measurements. CT has been used in evaluation of EOM. However, the exposure to X-ray radiation is a significant disadvantage of the CT. Recently, MRI has been widely used in clinical diagnosis of different ocular motor disorders. It is possible to demostrate morphological and morphometric changes of EOMs with MRI. Muscle thickness, a morphometric analysis, can give useful information about the changes of EOM in different types of strabismus. Measuring the thickness of an EOM by MRI is the one of the methods to quantitatively evaluate the size of EOMs in a practically, easy-to-use and noninvasively.

We considered only cases with the same sex and the same age group. All pathologic eyes which are of potential for changes in EOMs were excluded from the study. Although there is no study that reports the effect of deviation angle to the muscle thickness, we considered the cases with 20 to 45 prism diopter deviation, and the difference between mean of the deviation angle of group I and II was insignificant. Although cross-sectional area of EOMs by using coronal sections may give useful information about the thickness of EOMs, we prefered to use data obtained on axial sections to calculate the thickness for EOMs. It is reported that visualization of an EOM is optimal if the plane of the section is parallel to the course of the muscle on CT scans. The size and shape of recti muscles are depended on the direction of the patient’s gaze, with contracted muscles having a greater cross-sectional area (Demer, 1994b). For this reason each eye of patients and controls were occluded seperately, and subjects were asked to maintain their fixation to fixation target during the scans to prevent asymmetric extraocular muscle contraction. Tian et al. also used similar monoocular fixation in their study (Tian, 2000).

Scheiber et al. reported that a fixation target located 10 to 30 cm far from the anterior eye plain can result in acommodation and convergence reflex in binocular viewing, and the measurements of EOM thickness can be affected (Scheiber, 1997). For this reason, we patched one eye of each patient, and all patients used the same fixation target for each eye seperately to take the measurements at the same position.

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Although there was no difference between the thickness of horizontal EOMs of deviating and fixating eyes of esotropic patients with and without amblyopia, the LR muscle thickness of deviating eye of amblyopic patients and LR muscle thickness of both eyes of non-amblyopic patients were thinner with respect to controls. Additionally, MR muscle of deviating eye of amblyopic patients (group II) is thicker with respect to controls. LR muscle of deviating eye of esotropic patients with amblyopia (group I) is thinner with respect to esotropic patients without amblyopia (group II). There is no difference between MR thickness of esotropic patients with or without amblyopia. LR muscle thickness of all esotropic patients, especially patients with amblyopia, is thinner with respect to controls.

Although there are significant morphometric changes of medial and lateral recti of esotropic patients, it should be speculative to conclude whether these morphometric changes are cause or result of strabismus. However, changes of EOM thickness are significant in patients with amblyopia with respect to patients with alternating fixation in concommitant strabismus. This finding maybe explain poor surgical outcome on patients with strabismus and amblyopia. Further studies are necessary for deviation angle and morphometric changes, and surgical success in patients with concommitant strabismus and amblyopia.

REFERENCES

Bloom J. N. et al. 1993. A magnetic resonance imaging study of horizontal rectus muscle palsies. J Pedatr Ophthalmol Strabismus 30: 296–300.

Demer J. L. et al. 1994a. Comparison of standardized echography with magnetic resonance imaging to measure extraocular muscle size. Am J Ophthalmol. 118: 351–361.

Demer J. L. et al. 1994b. Quantitative magnetic resonance morphometry of extraocular muscles: a new diagnostic tool in paralytic strabismus. J Pediatr Ophthalmol Strabismus 31: 177–88.

Domenici-Lombardo L. et al. 1992. Extraocular muscles in congenital strabismus: muscle fiber and nerve ending ultrastructure according to different regions. Ophthalmologica 205: 29–39.

Lee J. S. et al. 2001. Normative measurement of Korean orbital structures revealed by computerized Tomography. Acta Ophthalmol. 79: 197–200.

Martinez A. J. et al. 1980. Structural features of extraocular muscles of children with strabismus. Arch Ophthalmol. 98: 533–9.

Scheiber C. et al. 1997. Technique for MRI of ocular motility. J Comput Assist Tomogr. 21: 442–6.

Tian S. et al. 2000. MRI measurements of normal extraocularmuscles and other orbital structures. Graefes Arch Clin Exp Ophthalmol. 238: 393–404.

Weakley D. R. & Holland D. R. 1997. Effect of ongoing treatment of amblyopia on surgical outcome in esotropia. J Pediatr Ophthalmol. Strab. 34: 275–278.

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