Monday, May 22, 2006

26

Accommodation

Accommodation is the mechanism by which the eye changes its refractive power by altering the shape of its crystalline lens. During accommodation, the ciliary muscle contracts allowing the zonular fibers to relax. This relaxation causes the equatorial edge of the lens to move away from the sclera during accommodation resulting in increased lens convexity (roundness). This increase in roundness primarily occurs on the front surface of the lens.

a. The Amplitude of Accommodation, also known as the accommodative response, is the maximum increase in diopter power obtainable by an eye. The amplitude of accommodation is measured monocularly.

b. The Range of Accommodation denotes the linear distance (expressed in centimeters or meters) over which the accommodative power allows an individual to maintain clear vision. The range lies between the near point of accommodation and the far point of accommodation. This is considered the most useful clinical measurement of accommodation. It helps answer the question as to whether an individual’s accommodative range comfortably encompasses his visual needs.

Clinically, accommodative ranges are measured from the anterior corneal surface (reference position). Optically, for the purist, that reference should be the primary principle plane of the eye, 1.4 mm behind the anterior corneal surface.

c. Resting Level of Accommodation:

In the absence of visual stimuli, the eye assumes an accommodative posture approximately 1D inside the far point, at the so-called "dark focus". This phenomenon helps explain "night" myopia and "empty field" myopia. Activation of the sympathetic nervous system is apparently involved in driving the accommodative state from the resting level to the far point in ordinary seeing.

d. Measuring Accommodation:

Tests of accommodation are performed monocularly.

When measuring the accommodative amplitude, it is assumed that you are testing an emmetropia, or someone who is corrected with spectacles, so that their far point is at infinity.

Target size, target illumination, and speed of target approach will affect the measurement of the amplitude of accommodation. The push up method works well for emmetropes, or fully corrected ametropes.

1. Near point of accommodation “Push Up Test”: For this test, use relatively small letters (0.4M or 0.5M) to help better control accommodation. Slowly move these letters closer to the eye until they become blurry. Measure the distance the letters became blurry. This is the near point of accommodation.

2.Prince Rule: A scaled accommodative ruler is used. Normally it is done with +3.00D sphere over the distance correction. A standard reading card is used and moved slowly towards and away from the individual to locate both the near and far points as in the push up method.

Question: An emmetrope views the reading card through a +3.00 diopter lens. She finds that as the card is moved towards her, the print that was blurred when held at the far end of the prince scale (50 cm) becomes clear at 33 cm (3.00D) and remains clear until it reaches 10 cm (10D). What is her accommodative amplitude?

Answer: Accommodative amplitude then is 7 diopters, 10D - 3D.

3. Spherical Lens Test: Spherical lenses are used in this test. The individual focuses on a stationery target while plus or minus lenses are used to measure the accommodative amplitude. A reading card is put at a convenient distance, say 40 cm, and the individual fixates on threshold size type (0.5M). Plus lenses are added until the print is blurred and then minus spheres are gradually added until the print blurs again. The difference is the accommodative amplitude.

Always test for accommodate relaxation with plus lenses before performing accommodative stimulation with minus lenses. This is because some individuals cannot adequately relax accommodation after exerting a maximum accommodative effort.

Question: How does the lens of the eye allow us to accommodate?

Answer: During the act of accommodation, there is a thickening of the lens and a decrease in its diameter (vertically and horizontally), with at the same time, a protrusion forward of the center and a relative flattening of the periphery, the whole process being accomplished by an axial movement of the lens substance which is evident, particularly in the central regions (Duke-Elder, 1938)

Question: What is the interval of clear vision for an uncorrected 5.00D myope with 10D of accommodative amplitude?

Answer: Far point = 100/5 = 20cm

Near point = 100/(5 + 10) = 6.67cm

Interval of clear vision = 6.67cm to 20cm

Question: What is the interval of clear vision for an uncorrected 2.00D hyperope with 4.00D of accommodative amplitude?

Answer: Far point is 50 cm behind the eye

With +2.00D of accommodation, the far point is at infinity

Near point = 100/(4-2) = 50cm

Interval of clear vision is 50cm to infinity

Question: What is the relative effect of spectacles versus contact lenses on convergence for a myope? For a hyperope?

Answer: When wearing contact lenses, the convergence requirement is the same as that of an emmetrope, because the lenses rotate with the eye and the line of sight remains relatively well directed through the center of the lenses. When an ametrope wearing spectacle lenses centered for his distance pupillary distance fixates a near object, the amount of convergence required is not only a function of his intrapupillary distance and the distance of the object, but will also be a function of the refracting power of the spectacle lenses. As a myope converges to bi-fixate a near object, his line of sight departs from the center of his spectacle lenses and encounters increasing amounts of base in prismatic effect. The spectacle wearing hyperope encounters base out prismatic effect, as he converges to bi-fixate a near object. Thus, to bi-fixate a given object at a distance less than infinity, the bespectacled myope converges less than the emmetrope or the contact lens wearer, while the hyperope wearing spectacles converges more than the emmetrope or the contact lens wearer. Therefore, the myope who discards his spectacle lenses in favor of contact lenses, must converge more to bi-fixate a given near object, while the hyperope will converge less under the same conditions.


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