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1.11 The Refraction

To determine if myopia exists, people have to see an optometrist or an ophthalmologist. It is generally recommended, especially for children,^{7, 12} that any remaining accommodation (i.e. stress of the ciliary muscle) is eliminated as much as possible before doing the test. Cycloplegic agents can achieve this, i.e. by the application of eye drops, which relax the ciliary muscle. The residual accommodation of the ciliary muscle, which exists if no cycloplegic agent is applied results in overcorrection of myopia⁵⁸. If this relaxing of the ciliary muscle is not done, pseudomyopia may still exist. In reality, however, hardly any practitioner appears to use these cycloplegic agents – maybe because it takes too much time until the agents work (about 30 minutes).

A comparison between non-cycloplegic and cycloplegic autorefraction showed that noncycloplegic measurements generally result in too high minus diopters. Zhao et al. reported⁵⁸: "Noncycloplegic autorefraction was found to be highly inaccurate in school-age children and, thus, not suitable for studies of refractive error or for prescription of glasses in this population." In detail, they found that the error is largest for hyperopes and smaller for myopes, but still -0.41(+/- 0.46D) for myopia of -2.00 D or more.

Therefore, by not using these drops, often more glasses can be sold, and herewith a cycle of a permanent increase in myopia can be initiated, if no cycloplegic agent is used.

A comparison between autorefraction and "normal" prescription showed a tendency of overcorrection by autorefraction⁵⁹.

Notes:
- According to other results and corresponding theories there is also a substantial accommodation hysteresis besides the one caused by the ciliary muscle and other ocular muscles, i.e. a hysteresis (i.e. a time lag for getting back to the original shape) of the ocular shape (see section 3.6.5) and even of the lens (see section 3.6.4). Cycloplegic agents cannot eliminate all these hysteresis (pseudomyopia) effects.

- The omission of cycloplegic agents can contribute to the progression of myopia because the prescribed glasses can be too strong, which causes a further progression of myopia (see section 3.2 and 3.3).

- According to personal experience cycloplegia is rarely done (this attitude might be very different in different countries).

- Another potential source of errors is a chart for reading, which is badly illuminated: In this case, hidden night myopia may lead to an over-correction (see section 1.4.1).

- Psychological stress can have an immediate impact on refraction (section 3.13). Stress could occur in school, or during an eye examination. If glasses are prescribed as a result, they in turn can initiate myopia that didn’t previously exist. The Bates method (section 3.2.2.1) focuses on this issue.

- As it is shown in the following sections, this over-correction can induce a further increase of "real" axial myopia.

- The fitting of glasses should not be based on autorefraction only; it should be double-checked by a conventional refraction.

In 95% of the cases, the results of the refraction can be reproduced within 0.5 D - assuming there is no effect of pseudomyopia or hysteresis of accommodation at all. These hysteresis effects can make refraction difficult, and can finally contribute to a permanent "lens-induced" progression of myopia (see section 3.3).

In different countries and by different authors different units are used to describe the amount of myopia and loss of visual acuity:

• Diopter, abbreviated D: The refractive power of a lens (to correct the myopia) can be expressed by the reciprocal value of the focal distance: -1 D corresponds with a focal distance of 1 m. People with myopia of -1.0 D can see still clearly at a distance of 1 m (at -2.0 D the distance is 0.5 m). The refractive power of the prescribed glasses is given in D with a minus in front.

• Snellen chart: This standard is more than 100 years old. 20/20 means that you can see at a distance of 20 feet what a "normal" person can see from a distance of 20 feet. 20/40 means that you can see at a distance of 20 feet what a "normal" person can see from a distance of 40 feet, and so on. Standard distance for myopia tests is 5 to 6 m. 20/40 is required in most States of the US to pass the Driver's License Test.

• Decimal quotients: In some countries the basic principle of the Snellen chart is used - the quotient of the distance where the patient can read to the distance where "normal" people can read. The result is given, however, in decimal units. A vision of 1.0 corresponds therefore to the visual acuity of a "normal" person. Some people, especially kids can have better resolutions up to a vision of about 2.0. Sometimes this decimal number is given in %, i.e. a visus of 1.0 corresponds to 100%.

• Visual acuity or visual efficiency: Sometimes another conversion from the Snellen chart numbers to percent is used. The table below contains the corresponding information.

• Angle-resolution: The visual acuity of the "normal" person (i.e. vision 20/20 or 1.0) corresponds with an angle-resolution of rays, which are incident on the eye, of 1 minute.

Besides plain spherical myopia, often astigmatism is diagnosed: This is the contribution of nonspherical irregularities of the eye, which can be compensated by added cylindrical lenscomponents.

The power of these cylindrical lenses is given in D, too (same definition as above). The prescription will give additionally the circular direction of the astigmatism in degrees. Combined spherical and cylindrical glasses are called toric.

The "best spherical fit" for toric glasses (i.e. for eyes with astigmatism) can be roughly estimated as: Spherical D + ˝ · astigmatism D

Visual acuity can be described by several parameters, of which the D figure or its equivalent is one. Other factors include astigmatism. A very rough cross-reference is shown in Table 1^{60, 61}:


Table 1 Some different methods to measure visual acuity

Sometimes the numbers for the diopters D are given in a different representation without the decimal point, e.g. - 1.00 as 100, - 6.50 as 650 etc.

If you want to keep track on your myopia by yourself you can use Snellen Charts, which can be downloaded from the Internet and printed⁶², or which can be displayed and used directly on your computer monitor⁶³.
Very frequent visits to an optometrist for measurement of refraction bears the risk of a subsequent overcorrection, if the optometrist is not very well informed about myopia prevention and all the recent research results.

The result of a study in the USA was that, as Donahue stated⁶⁴ "... a significant percentage of [preschool] children are probably prescribed glasses unnecessarily." In this study, specialized pediatric ophthalmologists were most careful in prescribing glasses.

Note:
The first unnecessary glasses might be the first step towards myopia (see section 3.2.2)

The regulations about who is authorized to examine the eye and to prescribe glasses or contact lenses are different for the various countries (e.g. optometrists, ophthalmologists⁶⁵ ...). Definitely, the selection of the right professional is extremely important, but it is hard to give any selection criteria. A discussion with the respective professional about some of the issues described in this book may be helpful to assess whether the professional is familiar with the current research results.

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