Accommodation is the ability of the eye to change the refractive power of the crystalline lens which enables the eyes to focus at various distances. The accommodative process involves contraction of the ciliary muscle, which releases the tension on the zonular fibers, allowing the elastic lens capsule to increase its curvature, especially that of the front surface. Accommodation is measured in diopters (D), which is the reciprocal of the fixation distance. If the fixation distance is 1.00 m, the accommodation is said to be 1.00 D; if it is 0.50 m or 0.33 m, the accommodation is 2.00 D or 3.00 D respectively.
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| Accommodation |
The furthest distance at which an object can be seen clearly is called the Far Point (punctum remotum). In order to see such an object the eye is in a state of rest, the ciliary muscle is relaxed, and the refractivity is at minimum. When maximum accommodation is in force, the nearest point which the eye can see clearly is called the Near Point (punctum proximum). The difference between the refractivity of the eye in the two conditions – when at rest with minimal refraction, and when fully accommodated with maximal refraction – is called the Amplitude of Accommodation.
In order to accurately perform visually guided daily tasks, it is necessary for the accommodative system to be dynamic, fast, and precise to ensure a well-focused image on the retina. The amplitude of accommodation is age dependent, it is least around the age of 65 years and is very strong below the age of 20 years. Aphakes and pseudophakes have no accommodation. During refraction active accommodation can result in over correction in myopia or undercorrection hypermetropia. Accommodation is the enemy of the good refraction. Accommodation clears vision in hyperopia and blurs vision in myopia. Controlling accommodation is very critical to the success of the results of the clinical refraction. There are two ways of controlling accommodation:
1. Fogging
2. Cycloplegic Refraction
Fogging
Fogging implies controlling accommodation by using plus lens. In case you follow the fogging method of refraction, start with higher plus than the results of objective method of refraction in case of hypermetropia and with low plus in case of low myopia and no plus lens is needed in case of high myopia. There are
rules to follow to reduce the fogging-put new plus lens first then remove old plus lens from the trial frame and while removing minus lenses, remove old minus lens from the trial frame first, then put new. In case of astigmatic eye under fog, both the principal meridian will not focus at the same distance. The difference between the two principal meridian power is the cylinder, which is designed in minus form.
Cycloplegic Refraction
Cycloplegic refraction is the procedure used to determine patient’s true refractive error by temporarily paralyzing the muscles that aid in focusing, using dilating drops. Cyclopentolate 1% is used to paralyze the muscles. Doses 1 drop each eye 6 times at 15 minutes interval on the day of refraction. Refraction
is done after 45 minutes of last drop. The younger patient needs stronger drops and /or more application is needed. Patients with light colored iris need less strength or fewer applications. Patients who have never been dilated, or who are new to the clinic, should be examined for narrow angles prior to dilation.
Another thing to take care is that the patient can be allergic to any of the cycloplegics, which needs to be enquired.
Cycloplegic refraction is indicated in the following cases:
1. When esophoria is present or latent hypermetropia is suspected
2. A young patient with hypermetropic eyes
3. Children below the age of 8 years.
Only distance correction can be done after cycloplegia. Near vision is not examined as accommodation is being suspended. Near vision must be checked during manifest refraction prior to cycloplegia. Same visual acuity may not be achieved with cycloplegic refraction as dilated eyes lacks the pinhole effect
of small pupil. Therefore, cycloplegic refraction should be conducted after completing all other eye tests. Cycloplegic refraction indicates the magnitude of refractive error and noncycloplegic refraction indicates the acceptability. Usually it gives a little high plus correction. Prescription of lens power has to be backed with clinical decision. About 0.75D should be subtracted from the net finding, if we know that complete cycloplegia has been achieved before the refraction.
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