Controversial Issues of the Mechanism of Crystalline Lens Accommodation and the Rationale the Hydraulic Component in its Implementation

Abstract

Purpose: To consider controversial issues of the mechanism of crystalline lens accommodation and to justify the hydraulic component in its implementation. Materials and Methods: Theories of the mechanism of accommodation and its assessment according to ultrasound biomicroscopy, magnetic resonance imaging and optical coherence tomography were analyzed. For the first time, the features of accommodative activation of intraocular fluid exchange in the closed hydrostatic system of the lens with the participation of mechanosensitive aquaporins were considered. When substantiating the hydraulic component in the mechanism of the crystalline lens accommodation, special emphasis was placed on the rapid decrease in pressure in the anterior and posterior chambers of the eye during contraction of the meridional portion of the ciliary muscle. Results: Analysis of various theories of accommodation has shown that mechanism of the crystalline lens its implementation continues to be discussed to this day. For the first time, the lens was considered as a unique closed hydrostatic system in which the pressure level is established through ultrafiltration and diffusion of intraocular fluid with the participation of aquaporins. Aquaporins form ion channels in the capsule, cuboidal epithelial cells, lens fibers and are mechanosensitive receptor proteins. The opening and closing of ion channels regulates the potassium-sodium pump, directed transport and exchange of intraocular fluid in the lens. The hydrostatic balance between the pressure in the crystalline lens and the anterior and posterior chambers of the eye is ensured by the crystalline lens capsule. The capsular bag of the crystalline lens can be considered as a curved diaphragm that separates two hydrostatic systems with different levels of pressure. Due to the hydrostatic buffering effect, the IOP level does not affect the crystalline lens, but it responds to a rapid decrease. This decrease in pressure in the anterior and posterior chambers is realized through the tension of the scleral spur by the meridional portion of the ciliary muscle and the activation of the valve mechanism of the scleral sinus. The greater the decrease in pressure, the more convex the crystalline lens takes on and increases its refraction. Conclusion: The presence of a hydraulic component in the mechanism of crystalline lens accommodation allows us to understand how the contraction of the small ciliary muscle can change the shape and refractive power of the large crystalline lens.