| Purpose : The path of the rectus extraocular muscle (EOM) are constrained by pulleys coupled to the orbital walls. The purpose of this study is to investigate , using magnetic resonance imaging (MRI) , the effect of the pulley on the path of EOM in different gazing position and to investigate the anatomic changes in some kinds of incomitant strabismus.Methods: control group had 8 volunteers , 8 eyes . Strabismus group had 2 cases with A pattern exotropia , 1 case with Brown's syndrome and 17 cases with superior oblique muscle palsy including 13 cases with congenital superior oblique (SO) palsy and 4 cases with acquired SO palsy , in which 4 cases were bilateral and 13 cases were unilateral . Multiple contiguous coronal MRI scans spanning the anterior-to-posterior extent of the orbit during primary gaze , upgaze , downgaze , adduction and abduction were analyzed in control group and strabismus group .The horizontal and vertical distances of the EOM's centroid relative to the orbital center were measured . The maximal cross-section area and the volume of the superior obliques were measured and calculated . In A pattern cases , the angles between SO reflect tendons and optic axes were measured in the axial MRI plane .Results : Data from 8 orbits of 8 normal subjects established the normal paths of the EOM . The paths of the EOM posterior to the global equator were stable relative to the orbit in the different gaze positions . The average maximal cross-section area of SO was 19.78 1.93mm 2 and the average volume of SO was 233.23 14.96 mm3, while they were respectively 15.74 6.31mm2 and 188.39 63.65 mm3 in the SO palsy group (respectively ,16.91 5.73 mm2 and 200.30 60.49mm3 inpatients with congenital palsy , 14.16+ 10.40rnm 2 and 137.81+ 57.23mm3 in patients with secondary palsy ). There were significant differences in maximal cross-section area and the volume of SO between the normal group and SO palsy group (P<0.05) but no significant differens between congenital and secondary SO palsy . There was no significant co-relation between SO volume and aging in the congenital palsy group . In the SO palsy group of which SO volume was more than 85% of the controlled group , the vertical position of MR , relative to the orbital center, was averagely shifted superiorly 1.14mm compared with the normal group ( significance<0.05 ) , while LR had no change , in the coronal MRI plane which contained the globe-optic nerve junction . In the SO palsy group of which SO volume was less than 85% of the controlled group , there were no significant change in the positions of medial rectus and lateral rectus . In two cases with A pattern exotropia , we found that one case had the volume of SO less than that of normal group and MR shifting inferiorly compared with the normal average in the coronal MRI plane which contained the globe-optic nerve junction , and the other case had the volume of SO more than that of normal group and LR shifting superiorly . The angles between the SO tendon and optic axial in two cases were smaller than the normal average . In a case with Brown's syndrome , we found the reflect tendon of SO was thicker than that of sound eye and the cross-section area of it was nearly equal to that of the SO belly .Conclusion : In this study , MRI showed the orbital layer of EOMs . The EOMs' bellys remained stable relative to the orbit , which confirmed the existence of EOM's pulley . There may be various congenital abnormality in anatomy in some kinds of incomitant strabismus. Both of the tendon and muscle belly of SO may be abnormal in SO palsy , while in the cases with the normal ornearly normal volume of SO bellies , the displacement of EOM may cause the clinical signs of SO underaction and V pattern . In A pattern , SO primary overaction was not unique factor. The heterotopic pulleys of EOM and the abnormal angle with SO tendon and optic axial may also cause A pattern . In a word , applied to strabismic ophthalmology , MRI can display explicitly the anatomic abnormality in the orbits to instruct...
|
PDF Full Text Request