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1.3 Accommodation

Accommodation is the adjustment of the refractive power of the lens of the eye to achieve an exact image of the object on the retina.

Myopia (or shortsightedness, or nearsightedness) is a condition in which distant objects are not displayed sharply on the retina by the optical system of the eye, because the rays converge already before they hit the retina.

Figure 2
The accommodation effort of myopes and non-myopes

Figure 2 shows effective accommodation, when a proper focusing on the retina is achieved. E.g., in typical axial myopia even with relaxed accommodation proper focus of distant objects cannot be achieved. The emmetropic eye can adjust for all distances by an appropriate accommodation effort. Low accommodation amplitude results in an exact vision of a narrow range of distances only.







The states of myopia and emmetropia are defined according to the handling of parallel rays of light (i.e. far distance):

Emmetropia is most often defined as a state, where parallel rays of light can be focused properly on the retina. For myopia this focus point lies in front of the retina.

Obviously, emmetropia can be achieved not by one specific ocular model, but by a range of ocular configurations, which may all lead to a good vision at distant objects.
Emmetropia and hyperopia (farsightedness) blur into each other:

• Proper distant focus can be achieved without accommodation as well as with some residual accommodation.
• Near focus (e.g. for reading) is largely dependent on the person's ability to accommodate (i.e. the individual amplitude of accommodation), which decreases with age (see section 1.8).

The reasons for myopia can be (see Figure 3):

• The refractive power of the lens system is too high: refractive myopia. Parameters of the lens system are the curvature of the cornea, the curvature of the lens at the front as well at the back, and the refractive indices of the anterior chamber, the lens and the vitreous body.
• The distance between the lens system and the retina is too large: axial myopia.

The critical reason is the second one, as it can lead to the dangerous progressive myopia by excessive stretching of the sclera. In the next sections the basics of accommodation, i.e. the adjustment of the eye to different distances of the objects, and some specific sub-categories of myopia will be discussed. Literature-references to ophthalmology in general and to literature about myopia are given at the end of this book in chapter 6 ^{6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17}.
Figure 3 The focusing of the myopic eye

1.3.2 Theory of Accommodation

The general view of ophthalmic science, based on Helmholtz, is:

• Due to its own original shape the lens would get into the shape of a round ball without a force on it. This shape corresponds to near focus.
• The pulling action of the zonular fibers, which connect the lens with the ring-shaped ciliary muscle, can flatten the lens. This shape corresponds to focus on a distant object.

Consequence:

• If the ciliary muscle is relaxed, there is a pulling tension on the lens via the zonular fibers, and therefore the focus is set to "distant object".
• If the ciliary muscle is contracted, the diameter of this ring-shaped muscle is decreased, and there is no more pulling action on the lens via the zonular fibers and therefore the focus is set to "near object".

A modified, and in a way contrary theory, the Schachar theory "suggests that the ciliary muscle contracts during accommodation, placing more tension on the equatorial zonules while relaxing the anterior and posterior zonules. (In the Helmholtz theory, all the zonules relax). This causes an increase in the equatorial diameter of the lens, decreasing the peripheral volume while increasing the central volume. As the central volume increases, so does the power of the lens."^{18, 19, 20, 21}. This means, as Burd stated²², "that increasing the zonular tension increases rather than decreases the power of the lens."

Both theories agree, however, that it is the ciliary muscle, which controls accommodation - in contrast to the theory described in section 1.3.3.

The idea that accommodation is a more complex function than the Helmholtz model suggests is supported by the results of Gao L et al.: "The ocular accommodation has a great influence on refractive components in children. It is not only the process by which the refractive power of the lens is increased. Furthermore, the lens itself moved forward relatively... AD [anterior chamber depth] increased while both LT [lens thickness] and VL [vitreous chamber depth] decreased significantly after cycloplegia regardless of their refractive state. However, AL increased for hyperopic eyes and decreased for myopic eyes after cycloplegia."²³

Additional information about the impact of accommodation on the ocular shape can be found in section 3.6.5.

1.3.3 A Controversial Hypothesis

Based on experiments, which he conducted on his own eyes, McCollim published the following very different model^{24, 25}:

• "Compression of the globe by the extraocular muscles [which move the axes of the eyeballs] can cause the lens to accommodate" ... "accommodation can be actuated without the intervention of the ciliary muscle".
• "... a single factor, external pressure on the globe, produces two separate effects, in opposite directions: anteriorly it accommodates the lens [by forcing the vitreous against the lens], and posteriorly it elongates the globe."

The fact that it was found by measurements that the eye is elongating during near accommodation^{26, 27} supports aspects of this thesis. Because of these experimental results, it was concluded by McCollim²⁴ that "with repeated periods of prolonged accommodation the lens would never have sufficient time to return completely to the unaccommodated state", i.e. there is a substantial time lag for the reshaping of the lens.

Conclusion: While the reshaping is taking place the refraction will diagnose myopia. After the reshaping the refraction will not find myopia.

Note:
This time lag for the reshaping of the lens was generally found in myopic eyes (see section 3.2.1). The model of a longer lasting time lag of the shape change of the lens could explain a progression of myopia: The fitted glasses would become inadequate, and could induce a further increase of myopia (see section 3.2.1.3). In fact, the progression would depend on how long the eye takes to return to shape. This effect of a time lag for readjustment is called hysteresis.

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