Accommodation of Eye: Definition, Mechanism, Anomalies

Today’s topic is all about the accommodation of the eye, its definition, mechanism, and several anomalies of accommodation of the eye. So, stay connected.

accommodation-of-eye-definition-theories-anomalies

Definition of Accommodation of Eye

Accommodation of the human eye is the process of adjusting the lens to focus on objects at various distances, thereby allowing clear vision. The process can be compared to zooming in or out with a camera lens.

The lens changes shape to help bring images into focus on the retina. In young people, this adjustment happens quickly and automatically when looking at objects at different distances.

The accommodation of the eye depends on intraocular muscles or ciliary muscles inside your eye that control the shape of your lens. When you look at something far away, these muscles relax and allow the lens to flatten out; when you look at something close up, they contract and thicken the lens.

During the process of accommodation of the eye, the curvature of the crystalline lens is altered so that it becomes more convex in order to focus on near objects (less than 6 meters) or more concave in order to focus on distant objects (greater than 6 meters).

The process of accommodation involves the action of three intraocular muscles: the ciliary muscle, the sphincter pupillae, and the dilator pupillae.

The lens is mostly made up of water and elastic proteins and changes shape when it is needed to focus on objects at different distances.

When looking at something far away, such as stars, the ciliary muscle relaxes allowing the lens to become flatter; when viewing something up close, such as a book or computer screen, the ciliary muscle contracts forcing the lens to round out in order for an individual’s vision to properly focus.

The young human eye can change focus from distance (infinity) to as near as 6.5 cm from the eye in 350 milliseconds.

Accommodation depends on reflexes initiated by focusing on an object. In particular, when light rays from a near object enter the eye and fall on the retina, they stimulate two reflexes:

The pupillary light reflex causes the iris muscles to contract and makes the pupil smaller (the light reflex constricts the pupil). As a result, less light enters through the pupil and reaches the retina. This reflex has two effects: it protects the retina from bright light and it helps reduce spherical aberration by reducing the amount of peripheral light reaching it.

The accommodation reflex is mediated by sympathetic stimulation of ciliary muscles that change the shape of the lens for near vision (accommodation increases optical power). This reflex has one effect; it focuses incoming parallel rays from a near object onto a single point on the retina.

Eye Structures Responsible for Accommodation of Eye

There are three main structures in your eyes that are involved in the accommodation of the eye. These include the ciliary muscles, the lens, and the pupil. Check out this explanation of these structures below.

Ciliary Muscle

The ciliary muscle is a ring-shaped smooth muscle that can be found in the center of the eyes. It serves the purpose of holding the lens in place and adjusting its shape so it can maintain visual clarity during accommodation.

Lens

The lens is a transparent, biconvex structure inside the eye that is shaped like the exterior of a circle. The lens is held in place by the suspensory ligament and has a diameter of 10 mm and a height of 4 mm in adults.

These measurements vary regardless of whether or not the accommodative function is used and/or if the lens structure has changed due to aging.

Pupil

The pupil is a round opening at the center of each eye. It’s usually black in color and shrinks when light enters to make sure the light only reaches the retina instead of going to other parts of the eyeball. It also limits the rays of light entering the eye in order to prevent damage to the retina.

Near Triad or Accommodation Reflex

When humans see something really close to them, they also have the tendency to converge their eyes. This makes small pupils. However, constricting your pupils is not part of the eye lens accommodation.

The accommodation reflex is the reaction by the eye to allow for clear vision. The three movements (accommodation, convergence, and miosis) are controlled by the Edinger-Westphal nucleus and are called the near triad.

Even though convergence is important in order to avoid double vision, the exact function of pupillary constriction remains unclear. The depth of field might increase by reducing the size of your pupils. This can reduce the need for a lot of accommodation, and focus the image onto the retina with ease.

Convergence

When we look at something, the two images we see should overlap. The brain compares the difference and interprets it as vision. For that, the two eyes should converge and maintain sharp vision in the retina.

Pupil Constriction

The pupils constrict to increase the depth of focus.

Increase in Eye Lens Power

The muscle at the base of the eye’s lens contracts to change its shape and bring objects that are close up into focus.

Eye accommodation weakens as we get older

Yes, we’re all aging and it’s causing changes in our lenses.

With increasing age, the eye’s lens becomes less flexible and is unable to change focus. This condition is referred to as presbyopia.

People who read a lot during their life may start to notice the effects of presbyopia as they get older. For those over 40, reading up close becomes difficult as it becomes difficult to see a text such as what’s on a book or phone.

Presbyopia is not exclusive to reading but can happen to anyone over 40. Without getting an eye exam, you may find that you need glasses in order to read small fonts.

Aging can lead to a change in the lens of the eye where it thickens and becomes less elastic. As a result, the eye is unable to change its shape any longer. This means that as you get older it becomes more difficult to see objects up close as your eyesight worsens.

You have plenty of options for treating presbyopia – including progressive glasses, bifocal glasses, reading glasses, and even contact lenses.

If you’re also interested in having cataract surgery, an accommodating intraocular lens is one of the options for how to treat presbyopia.

Theories of Mechanism of Eye Accommodation

Helmholtz Theory (1855)

There are multiple theories of accommodation, but the most widely held one is Hermann Von Helmholtz’s. In this theory, when viewing a far object, the circularly arranged ciliary muscles relax and allow the lens to be pulled by the zonules and suspensory ligaments. It causes the lens to flatten its surface.

The tension on the sclera is caused by the outer pressure of fluids (vitreous and aqueous humors) against the eye.

When looking at something up close, the ciliary muscles contract resisting outward tension on the sclera. As a consequence, the zonules get slackened, and the lens is then able to get thicker and more convex.

Schachar Theory (2006)

Schachar’s theory suggests that increased tension on the lens occurs when it is in focus and as the ciliary muscle contracts, those zonular fibers at the equator go into a state of increased tension. As a result, the lens becomes more steeply curved at its center, increasing in thickness and flattening near its edges.

As the tension in the zonular fibers on the equator increases when you accommodate, other zonular fibers will relax in response to this increased tension.

The zonular fibers in the lens have different functions. The anterior and posterior zonular fibers provide passive support for the lens, but the equatorial zonular fibers determine how much a lens bends light (refractive power).

Catenary theory of accommodation (1970)

D. Jackson Coleman theorized that, with the presence of the lens, zonule, and anterior vitreous membranes in the eye’s anterior cavity, they formed a type of diaphragm to separate the anterior chamber from the vitreous chamber.

Ciliary muscle contraction is what helps to produce a pressure difference between the vitreous and aqueous compartments, which both help to shape the lens.

In this lens shape, a steep radius of curvature is in the center with a slight flattening of the periphery. You might recognize it as being shaped like a catenary.

The anterior capsule and zonule form a trampoline-shaped surface. The degree of curvature is largely dependent on the circular diameter of the ciliary body (Müller’s muscle).

The ciliary body shapes the lens like the pylons of a suspension bridge but doesn’t need to support an equatorial traction force to flatten it.

Anomalies of Accommodation of Eye

There are many forms of accommodation dysfunctions. You can broadly classify them into two groups:

Decreased accommodation

Any of these conditions are responsible for decreased accommodation of the eye.

  • Physiological (presbyopia)
  • Pharmacological (cycloplegia)
  • Pathological

Increased accommodation

Increased accommodation of the eye is categorized as:

  • Excessive accommodation
  • Spasm of accommodation

Presbyopia

Gradual blurry vision for near due to age-related physiological changes in the human crystalline lens is known as presbyopia. The insufficiency of accommodation occurs due to increased hardness and decreased elasticity of the human eye lens, and increased ciliary muscle weakness.

Accommodative Insufficiency

Accommodative insufficiency is a lens condition in which the amplitude of accommodation of the eye does not meet the normal accommodation limit for the age.

There are several factors responsible for the accommodative insufficiency such as ciliary weakness, premature lens sclerosis, among others.

Insufficiency of accommodation of the eye lens is categorized as ill-sustained accommodation, paralysis of accommodation, and unequal accommodation.

Accommodative Infacility

Also known as accommodative inertia, the infacility of accommodation is the lens’s inability to change accommodation from one point of fixation to another point.

You might have faced the problem of maintaining clear focus for far or near when you change your point of fixation, or your eyes might have taken several seconds to achieve clear focus while looking at different distances. This eye lens condition is known as accommodative infacility.

Spasm of Accommodation

Pseudomyopia is a result of a spasm of accommodation. Also known as ciliary spasm, accommodative spasm is a condition of abnormally excessive accommodation of the eye lens that is beyond the voluntary control of the person.

Accommodative Excess

Accommodative excess happens when the eye lens is not able to relax accommodation readily. If you force your eyes to use for accommodation for certain near tasks than normally required then accommodative excess happens.

Accommodation of the Eye PPT


Sources

  • Baumeister, M.; Kohnen, T. (June 2008). “Akkommodation und Presbyopie: Teil 1: Physiologie der Akkommodation und Entwicklung der Presbyopie”. Der Ophthalmologe (in German). 105 (6): 597–610. doi:10.1007/s00347-008-1761-8
  • Atchison, David A. (1995). “Accommodation and presbyopia”. Ophthal. Physiol. Opt15 (4): 255–212. 
  • The Mechanism of Accommodation and Presbyopia. The Hague: Kugler Publications. ISBN 978-90-6299-233-1.
  • Khurana, AK (September 2008). “Asthenopia, anomalies of accommodation and convergence”. Theory and practice of optics and refraction (2nd ed.). Elsevier. pp. 100–107. ISBN 978-81-312-1132-8.
  • Duke, Elder’s (1969). “Anomalies of accommodation”. The practice of refraction (8th ed.). London: Churchill. ISBN 0-7000-1410-1.
  • William J., Benjamin (2006). “Accommodation, the Pupil, and Presbyopia”. Borish’s clinical refraction (2nd ed.). St. Louis Mo.: Butterworth Heinemann/Elsevier. p. 112. ISBN 978-0-7506-7524-6.

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