A New Vision of the Mechanism of Crystalline Lens Accommodation and the Hydraulic Component in its Implementation

Background: According to modern concepts, the implementation of the accommodation mechanism in humans involves such eye structures as the iridocyclolenticular complex, the vitreous body and extraocular muscles. However, a key role in the mechanism of accommodation is played by changes in the shape of the crystalline lens.

Purpose: Consider the energy-saving of the mechanism of crystalline lens accommodation and to justify the hydraulic component in its implementation.

Materials and Methods: Various theories of the mechanism of accommodation were analyzed taking into account data from ultrasound biomicroscopy, magnetic resonance imaging and optical coherence tomography. The mechanism of rapid decrease in pressure in the anterior and posterior chambers of the eye during contractions of the meridional portion of the ciliary muscle was considered. Particular emphasis was placed on the participation of mechanosensitive aquaporins in the exchange of intraocular fluid and ensuring pressure in the closed hydrostatic system of the lens, exceeding the level of IOP.

Results: An analysis of various theories of accommodation showed directly opposite views about the participation of the ciliary muscle and ligamentous apparatus in the implementation of the mechanism of the crystalline lens accommodation. In this work, we are the first to consider the crystalline lens as a unique closed hydrostatic system in which the pressure level is established due to ultrafiltration and diffusion of intraocular fluid with the participation of aquaporins. Aquaporins form ion channels in the capsule, cuboidal epithelial cells, the crystalline 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 crystalline lens. The hydrostatic balance between the pressure in the crystalline lens and the anterior and posterior chambers of the eye is ensured by the 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 occurs with contraction of the meridional portion of the ciliary muscle, tension of the scleral spur and 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.