Presbyopia

Basics of accommodation physiology

For clear and sharp vision of objects located at various distances from a person, the eye must have the ability to dynamically change its refractive power. This vital function is called accommodation.

The physiological mechanism of accommodation is implemented through the coordinated work of several structures of the eyeball. Key roles in this process are played by the high natural elasticity of the lens itself, as well as the functioning of the parasympathetic nervous system, which ensures the timely activation of the muscular apparatus of the eye when the focal length changes.

Distance-vision mechanism

When fixing gaze on distant objects, parallel rays of light enter the eye. At this moment, the accommodation apparatus is in a specific state of rest, characterized by the following sequence of processes:

1. Muscle state: The ciliary muscle is in a state of complete relaxation.
2. Anatomy of the muscle ring: The diameter of the ciliary body muscle ring reaches its maximum values.
3. Tension of the ligamentous apparatus: The fibers of the suspensory ligament, also known as the zonules of Zinn, are in a highly taut state.
4. Change in lens shape: The powerful tension of the zonules is mechanically transmitted directly to the lens capsule. This leads to its stretching, as a result of which the lens significantly flattens.

The result of these physiological changes is that the refractive power of the eye becomes minimal. Parallel rays of light pass through the flattened lens, and the focus is formed strictly on the surface of the retina, providing clear distance vision.

Near-vision mechanism

When viewing objects located at close range, non-parallel, diverging rays of light enter the eye. To converge and focus them, a significant increase in the refractive power of the eye is required. This process is triggered by the parasympathetic nervous system:

• Active contraction of the ciliary muscle occurs.
• The muscle ring of the ciliary body narrows (its diameter decreases markedly) and shifts anteriorly.
• Due to this displacement, the tension of the zonules (suspensory ligament fibers) significantly weakens, and they relax.
• Freed from the constant stretching tension of the ligaments, the lens, thanks to its high natural elasticity, tends to assume a more convex, spherical shape. Its thickness increases in the process.

As a result of these coordinated actions, the refractive power of the eye increases. Diverging rays of light are converged much more strongly, allowing the image to focus accurately on the retina once again.

Comparative characteristics of accommodation processes

Pathogenesis of presbyopia

Over time, as the body ages, the lens undergoes involutional changes. It gradually hardens and steadily loses its high natural elasticity, which is critically important for normal accommodation.

When a person with age-related lens changes tries to view objects up close, the ciliary muscle contracts and the zonules relax normally. However, due to the loss of elasticity and acquired density, the lens is no longer able to assume a sufficiently convex (spherical) shape.

Its curvature becomes insufficient to properly increase the refractive power of the eye and converge diverging rays of light on the retina. This is how presbyopia—age-related farsightedness—develops. To compensate for the insufficient refractive power of the lens, patients require mandatory optical correction with special reading glasses.

Class and exam questions

What else to study on this topic

• Parasympathetic innervation of the eye and regulation of ciliary muscle function
• Anatomical structure of the suspensory apparatus (zonules of Zinn)
• Biomechanics of the lens capsule during changes in its curvature
• Optical laws of refraction for diverging and parallel light rays
• Principles of selecting spectacle correction for age-related farsightedness

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.