Accommodation is the ability of the eye to dynamically adjust the refraction of light and, for this reason, to see objects clearly and sharply at any distance.
What is accommodation?
Accommodation is the ability of the eye to dynamically adjust the refraction of light and, for this reason, to see objects at any distance clearly and sharply. The process of accommodation is made possible by the flexible lens of the eye, which can change its shape by muscle power. As a person ages, the ability to accommodate deteriorates. Effects can be, for example, presbyopia, i.e. presbyopia of old age. Vision and imaging of objects on the retinal plane occurs under an optical near and far point. The near point is the shortest distance, and the far point is the farthest to the eye. The change between the two settings of the eye is accommodation, or near adjustment. More precisely, the change from distance adjustment to near adjustment is near accommodation, and the reverse process is distance accommodation.
Function and task
The elastic lens of the eye, which is changeable by the intraocular pressure, is responsible for the proper adjustment of seeing near or far objects. It is basically a fluid-filled sphere that is flexible like a balloon but has the appearance of a flattened lens. This is due to the vitreous humor, a gel-like and transparent fluid in the eye. The vitreous body presses against the lens of the eye, which thus takes on its shape. Accommodation is a reflex that can be influenced by the will and thus varies the refractive power. This happens through the change of the lens, more precisely through the ciliary muscle, which is located at the front inner side of the eye. This is ring-shaped and has zonular fibers that hold the lens of the eye. As soon as the ciliary muscle tenses, it simultaneously shrinks. The result is that the intraocular pressure decreases and the lens of the eye enlarges or becomes more spherical. In this way, it is now possible to recognize things that are close by. On the other hand, if the lens of the eye is flat, because the lens has formed into an elliptical shape and the elastic fibers have retracted, the person recognizes objects that are far away and are now in focus on the retina through accommodation. The closer the object is in front of the eye, the more muscle power is needed to lower the intraocular pressure. There is also a rearrangement of microstructures, which in turn cause the fibers and lens to change shape. The mechanisms by the ciliary muscle are called external accommodation, and the changes caused by the rearrangement are called internal accommodation. The entire process of accommodation begins in the primary visual corex. Fibers move to the “area pretectalis” and run to the Edinger-Westphal nucleus. This produces bilateral responses of the eyes, even when the person is blind. Now the ciliary muscle comes into play. Its fibers run in two different directions, through the Brück’s muscle and the Müller’s muscle. The latter is innervated as soon as near accommodation occurs and also performs a small active movement during distance adjustment of the eye to establish visual balance. Under these conditions, a relaxation tonus is created, which is located between the near and far points. Forces acting alongside this emanate from the elasticity elements of the ciliary muscle and lead to myopia. This resting position is present when the visual field is empty of stimuli, such as during night vision.
Diseases and conditions
As soon as an object is viewed up close, convergence of the eyes occurs, accompanied by miosis, which is a constriction of the pupil. If the interaction of all factors is disturbed, strabismus may occur, for example. Various disorders of accommodation lead to defective vision. One is when the ability to accommodate is lost, which is always the case over time due to aging. In this case, the minimum visual range shifts more and more into the distance. The cause is the hardening of the eye lens, which loses its elasticity in the process. Medicine calls this age-related long-sightedness presbyopia. The ability to see near is lost due to age and, moreover, cannot be prevented, since this is not a disease but an age-related and normal process of loss of function. A disease could be caused, for example, by a paralysis of accommodation.This is called cycloplegia in ophthalmology and is accompanied by loss of function of the “Musculus ciliaris”. Causes can be damage to the parasympathetic nerve fibers or active induction by anesthesia with pharmacological agents. For example, when a diagnostic examination is performed on the eye. During the period of paralysis, sharp vision is then not possible. Another disorder is hypoaccommodation, i.e. a restricted range of accommodation, which is understood to mean the maximum possible change in refractive power in the eye. This also decreases with age, although hypoaccommodation occurs rather rarely: if it does, it is usually already in childhood. In the case of hypoaccommodation, the accommodation does not correspond to the necessary innervation impulse and the near point is shifted into the distance. This is usually accompanied by various complaints, such as fluctuating visual acuity of near objects, reading difficulties and the like. In cases of particularly extreme myopia, an accommodation spasm or spasm may occur. The accommodation effort to detect distant objects does not match the impulse. The result is blurred vision and, in worse cases, headaches. The remedy is glasses adjusted to the shifted vision and, if necessary, antispasmodic medication. However, an accommodation spasm can also lead to temporary myopia. This is called pseudomyopia but has nothing in common with myopia.
