Suna Tian

MRI measurements of normal extraocular muscles and other orbital structures.

Abstract Background: Studies of extraocular muscle (EOM) by magnetic resonance imaging (MRI) need to be extended to normal subjects of different ages to obtain data on the muscle thickness, cross-sectional area, and the volume of EOM and other orbital tissues. Methods: Forty-two orbits of 21 normal subjects in three age groups with an age range of 19–70 years were examined with surface-coil MRI. The transverse and sagittal images were used to measure the thickness of the four rectus muscles during fixation in different gaze positions. The coronal images with eyes in the primary position were used to calculate the cross-sectional areas. The volumes of all six EOM, orbital fatty tissue, the optic nerve and the eyeball were measured in the coronal plane and in either the transverse or the sagittal plane. Results: The horizontal muscles were thinner than vertical muscles. Muscle volume was larger in SR (superior rectus) than in IR (inferior rectus), larger in SO (superior oblique) than in IO (inferior oblique), and the same in LR (lateral rectus) as in MR (medial rectus). No significant differences were found in the values of the cross-sectional area in any image plane between the three age groups. There were no significant differences in muscle thickness and size and fatty tissue volume between age groups. The muscle thickness was linearly correlated to the angle of the eye deviation for all four rectus muscles, both in the ”on” and ”off” directions of the muscles. Conclusions: The study provides quantitative data, in normal subjects of different ages, on the thickness and size of EOM and the volume of other orbital tissues by MRI, to serve as a basis for further studies on the morphological changes of EOM in various orbital diseases.

Effects of eye muscle proprioceptive activation on eye position in normal and exotropic subjects.

Abstract• Background: Activation of muscle spindles by vibration of eye muscles is known to induce illusory movements of fixated targets, but the effects on eye position have not been studied, either in normal subjects or in patients with exotropia. • Methods: Eye position was recorded from the covered, non-dominant eye with an infrared system in 11 subjects with normal eyes and binocular vision and in 10 patients with exotropia and abnormal binocular function. Activation of eye muscle spindles was done by vibration at 70 Hz of the inferior and lateral rectus muscles of the dominant eye, fixating a light-emitting diode in subdued light. • Results: Vibratory activation of proprioceptors in the inferior rectus muscle induced an eye movement mainly directed upward in both normal and exotropic subjects. The magnitude of the movement was on average 2.7 deg in normals and 2.4 deg in exotropes. Lateral rectus vibration induced a movement that was mainly temporally directed (abduction) of an average 2.1 deg in normal subjects, but a nasally directed (adduction) movement of 4.2 deg in exotropic subjects. In normal subjects the eye movement is of the same direction as the earlier reported visual illusory movements induced by the same type of proprioceptive activation, but in exotropic subjects the movements is in the opposite direction. • Conclusions: Proprioceptive activation of eye muscles affects eye position, and the results also indicate that signals from eye muscles are processed differently in normals and strabismics, probably depending on the level of binocular function.

Isometric force measured in human horizontal eye muscles attached to or detached from the globe

BackgroundHuman eye muscle tension has been measured directly only in detached condition. The purpose of this study is to compare force development in single, horizontal human eye muscle during saccadic eye movements, first when the muscle was still attached and later when it was detached from the globe.MethodsEleven horizontal muscles of eight patients were examined during surgery under topical anesthesia for concomitant strabismus. None of the muscles examined had been operated before. Isometric muscle tension was recorded with a strain gauge system, to which the muscle tendon was attached by a silk suture. The subjects made saccadic eye movements with the non-recorded eye by fixating light-emitting diodes in the center and at 10, 20, and 30° horizontally to each side. Continuous and stepwise saccades were produced. In the tension signals, peak tension (Fp), steady tension (Fs), and the ratio Fp/Fs were measured. Statistical analysis was done with multivariate analysis of variance.ResultsThe values of Fp, Fs, and Fp/Fs at different amplitudes of the saccadic eye movements were compared in the attached and the detached muscle. There were no consistent statistical differences between the values obtained in the two conditions.ConclusionsThe muscle force development, measured at the tendon, was the same in muscles attached to the globe and in muscles free from the globe. Thus isometric muscle tension can be adequately recorded in muscles still attached to the globe, which increases the possibilities for studying contractile properties of various eye muscles during ophthalmic surgery procedures performed under topical anesthesia.

Vertical saccadic velocity and force development in superior oblique palsy

Vertical saccadic movements recorded with electrooculography (EOG) and force development measured by means of a contact lens-strain gauge technique were investigated in 13 patients with a clinical diagnosis of unilateral superior oblique palsy (SOP) and 10 normal subjects. Upward and downward movements from the horizontal level to 5, 10 and 20 deg of deviation with monocular fixation were made in the 20 deg abduction and adduction fields of gaze. Peak velocity (Vp) and the ratio of downward over upward movement (VpD/U) of the paretic eye (PE) and the sound eye (SE) of the patients were compared with the values of the covered, non-fixating eye (CE) and the fixating eye (FE) of the normals respectively. Peak tension (Fp), steady-state tension (Fs) and the ratios of downward over upward movement (Fp D/U, Fs D/U) of PE were compared with the values of CE. Comparisons were done (a) between normals and the whole group of 13 patients, (b) between normals and two subgroups of five patients with acquired and five with congenital SOP, (c) and between the subgroups. Mean Vp for the whole group of SOP showed reduced values from normals in many directions of gaze. However, mean Fp and ratios of Vp D/U and Fp D/U were not different in SOP and normals, and such measures could not be used to differentiate SOP from normal eye muscles in the sample of patients studied. Clear differences were seen in the comparisons between normals and the subtypes of SOP. Vp was significantly reduced in PE for the downward movements in the adduction field of gaze, more marked in acquired than in congenital SOP. Slowing of upward saccades was also found in the congenital group. This may be due to adaptational changes in concomitization of congenital paretic strabismus. Fp for downward saccade, and Vp D/U and Fp D/U were lower for PE in the acquired than in congenital SOP and normals in the lower adduction field of gaze, where the superior oblique has its main action. A linear relation was found between Vp and Fp of downward saccades for the congenital group in the lower adduction of gaze, but the slope was steeper than in normals. However, in acquired SOP the relationship was non-linear, which indicates a force production different from that of a normal muscle. Vertical saccadic velocity and force development measured at 20 deg adduction could reveal weakness of the superior oblique muscle and the imbalance of agonist-antagonist actions in the paretic eye.(ABSTRACT TRUNCATED AT 400 WORDS)

Eye muscle force development and saccadic velocity in thyroid-associated ophthalmopathy

BackgroundOcular motility disturbances are very common in patients with thyroid-associated ophthalmopathy (TAO). The force duction test has demonstrated that limitations of eye movements may be caused by mechanical restrictions in the muscles. A quantitative evaluation of isometric force production and ocular motility in all directions of gaze could further elucidate the mechanism of the movement disorder.MethodsIn eight patients with TAO and eight controls of the same age range, horizontal and vertical saccadic movements of both eyes were recorded by means of electro-oculography (EOG). The corresponding force development in muscles of the more affected eye in patients and the non-dominant eye in normals was measured, using a suction contact lens/strain gauge technique.ResultsSteady-state tension in the patients was increased in all directions of gaze both horizontally and vertically, most markedly for vertical fixations in the upper field of gaze. However, peak tension was increased only for the downward movements in the upper field of gaze and for the horizontal movements in the abduction field of gaze. Vertical saccadic velocity was slower than normal in downward movements, and horizontal saccadic velocity was reduced in the abduction field of gaze.ConclusionThe increased active eye muscle tension in different directions of gaze could represent an adaptational mechanism of the ocular motor system to overcome the eye movement restriction in TAO.

Succinylcholine activation of human horizontal eye muscles.

Acta Ophthalmol. 2010: 88: 872–876

Force development and velocity of saccadic eye movements in exotropia

The authors have measured saccadic peak velocity with elec-trooculography (EOG) and force development with a suction contact lens/strain gauge technique in 11 patients with intermittent exotropia (IXT) and ten patients with constant exotropia (CXT). Saccadic movements with the dominant eye fixating were elicited horizontally to targets positioned at 5, 10 and 20 deg in the adduction and abduction fields of gaze, and vertically to targets at 10 deg in the 20 deg adduction and abduction fields of gaze. Peak velocity (Vp) was measured in both eyes and peak force (Fp) and steady-state force (Fs) in the deviated eye. Fp of abduction was higher in CXT patients than in normals, while Fp in adduction was higher in IXT than in normals and CXT. Vp did not show clear differences between patients and normals. However, the ratios of Vp(abd/add) and Fp(abd/add) of the deviated eye were significantly higher in CXT and lower in IXT than in normals. This is probably related to fusion induced muscle activity in IXT and the...

Isometric force development in human horizontal eye muscles and pulleys during saccadic eye movements.

Purpose:  The connective tissue elements forming the check ligaments and portals of the human eye muscles have recently been ascribed with a pulley function. Active positioning of the pulleys over orbital layer contraction during eye movements has been suggested. Other studies have instead demonstrated fibrous tissue connections between all parts of the muscle and the pulleys. We aimed to compare the isometric force developed at the muscle tendon and at the pulleys of the horizontal eye muscles, and to investigate which eye muscle structures might exert force on the pulleys.