Mechanisms of divergent torsional strain in myopia

Abstract

Perpose. To study the mechanism of extralenticular accommodation under conditions of divergent torsional strain (DTS). Material and methods. The study involved 14 students (28 eyes) with mild and moderate myopia, whose average age was 21.7 years. Among them were 4 boys and 10 girls. The average value of refraction, determined by the subjective method, was (–) 2.5 diopters. To conduct the study, optical systems were used to simulate DTS in the downward and outward direction and to simulate DTS in the upward and outward direction. The features of extralenticular accommodation under conditions of divergent torsional strain were comparatively assessed using the method of determining the binocular stability of visual perception (SVP) to hyperopic retinal defocus. Results. Studies of SVP in the far-vision mode under no vergent strain showed that the average total value of visual resolution (VR) was equal to 70.2 %. Under DTS in the downward and outward direction, the average total value of VR was 67.1 %. At the same time, along the curve there was a fall of VR that on average equaled to 9.5 %. Under DTS in the upward and outward direction, the average total value of VR was 76.4 %. Moreover, along the curve there was a fall of VR, that on average equaled to 7.1 %. The presence of the fall zones indirectly indicates a temporary increase in the transverse diameter of the eyeball under the influence of divergent torsional strain. Conclusions. Divergent torsional strain can stimulate combined lenticular and extralenticular accommodation caused by a change in the tone of the ciliary muscle and the pressure of strained oculomotor muscles on the sclera. Thus, divergent torsional strain can change the geometry of the eyeball. Keywords: divergent torsional strain, transverse deformation of the eyeball