Accommodation

In an emmetropic eye, we assume that this eye is an emmetropic eye if an object is at how much meters.

6

The mechanism by which the eye changes refractive power by altering the shape of the lens in order to focus the objects at different distances. It is a very complex function or process that involves the crystalline lens, the zonular fiber and ciliary muscle.

Accommodation

Light rays from distant objects are nearly parallel and don’t need as much refraction to bring them to a focus

Distant Vision

Light rays from close objects diverge and require more refraction for focusing

Close Vision

The radius of the anterior surface of the lens is ____ when unaccommodated

10mm

The radius of the the posterior surface of the lens is ____ when unaccommodated

6mm

The position of an object when its image clearly falls in the retina, with no need of any accommodation, usually considered at 6 meters or at infinity

Far Point

Nearest point clearly seen with maximum accommodation. It varies in terms of age of the patient.

Near Point

The first person to study accommodation. He proved that accommodation results from a change in the optical power of the eye and he demonstrated that the eye obeys the law of optics.

Cristoph Scheiner

According to Cristoph Scheiner, there are four focusing devices. These are:

1. Cornea Curvature

2. Axial Length

3. Lens Axial Position

4. Lens Shape

He claimed that the cornea is not involved in accommodation and he reported that the lens is mainly responsible for accommodation. He performed the double slit experiment

Thomas Young

He claimed and observed that aside from observing images formed by refraction on different surfaces. He claimed the key concept is that smaller radius would cause greater curvature, therefore would lead to higher convexity and when the cornea or the lens is more convex, then the purkinje image is much smaller.

Purkinje-Sanson

Most widely accepted theory of accommodation. Sometimes termed as the Reduced Tension Theory, because the tension is at the zonular fibers.

Capsular Theory

Proposed the Capsular Theory. He said that the lens was elastic and in a normal state or unaccommodated state it is stretched and is flattened by the tension of the suspensory ligaments

Helmholtz

Who supported Helmholtz Theory that created a mechanical model of accommodation based on the theory by Helmholtz that shows that in accommodated state, the elasticity of the choroid in stronger than the lens

Gullstrand

Schachar’s Theory also known as _______ It is the second main theory of accommodation proposed in 1992. The main highlight of the theory is the equatorial, anterior and the posterior zonular fiber.

Equatorial Zonular Tension Theory

It states that the contraction of the ciliary muscle pulls zonules directly and increases the tension of the capsule of the equator of the lens which leads to bulging of poles. The lens bulging during accommodation is caused by compression and not because of passive relaxation.

Increased Tension Theory

Another version of tension theory where it claims that the crystalline lens thickness increases during accommodation and the ciliary muscle contraction causes the increased zonular traction

Modified Tscherning Theory

It claims that the lens, zonules, and the anterior vitreous would form a hammock-like diaphragm or trampoline between the anterior and vitreous chambers of the eye.

Catenary Theory

He described the circular muscle of the ciliary body and theorized that the ciliary muscle contraction pulls the vitreous forward, forcing the lens forward as well, which results in increased refractive power.

Muller

During accommodation, the aqueous humor under pressure is forced into the spaces of fontana. The aqueous humor is found in the anterior and posterior chambers of the eye and the increased curvature of the lens is produced by hydraulic pressure from the aqueous humor and not by relaxation of the ciliary muscle tension on zonules as Helmholtz claimed. He negates the claim of Helmholtz.

Johnson

He used a real time A-scan ultrasonography to show the anterior movement of the vitreous and IOL during accommodation. During the experiment, the first photographic documentation of power increased due to the anterior movement of the IOL. He was able to gain insights into accommodation through cataract surgery.

Thorton

He demonstrated the good distance in near vision could be achieved using movable IOL. The stronger power of the IOL, the smaller amount of movement needed to achieve a +2.50D pseudoaccommodation. He also claimed that the anterior movement of the lens is at least partially responsible for accommodation.

Miller

He said that when viewing distant or far objects, the lens flattens, then the eyeball shortens due to the contraction of the four recti muscles and the relaxation of the two oblique muscles. When you move the object closer or trying to view nearer objects then the lens will thicken like a magnifying glass, then the eyeball lengthens due to the relaxation of four recti muscles and the contraction of two oblique muscles.

Bates

This is due to the tonus of the ciliary muscle and is active in absence of a stimulus. Even if there is no near target, there is tonic accommodation. This resting state of accommodation is not at infinity but rather at an intermediate distance.

Tonic Accommodation

Induced by the awareness of the nearness of the object or target. Independent of the actual dioptric stimulus.

Proximal Accommodation

This is an automatic adjustment response to blur which is made to maintain a clear and sharp retinal image. An automatic reflex when someone moves closer in order for you to view the object clearly.

Reflex Accommodation

Amount of accommodation stimulated or relaxed that is associated with convergence. The link between accommodation and convergence is known as the accommodative convergence and it is expressed clinically as the ACA ratio.

Convergence Accommodation

The link between accommodation and convergence is known as the _________ ———— and it is expressed clinically as the ACA ratio.

Accommodative Convergence