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1.4 Refractive Myopia

Besides mere static geometrical anomalies of the lens system of the eye, there are two more dynamic refractive effects:

1.4.1 Night Myopia and Tonic Accommodation

Night myopia is when the eye adjusts in the dark or already in dim light, or due to a lack of image contrast to near focus, even if all the objects are far off. It is also called dark focus of accommodation, or tonic accommodation, or resting state of accommodation¹⁷. Consequently, a measurement of the refraction (i.e. the determination of appropriate optical glasses to obtain good vision on these distant objects) gives the result that the eye is myopic.
Night myopia can reach values of up to about - 4.0 D²⁸, more typical around – 1.0 D (D stands for diopters, the measure for the degree of myopia; people with myopia of - 1.0 D can see still clearly at a distance of 1 m, at – 4.0 D the distance is 0.25 m. For more details about the definition of diopters see section 1.11). At presentations where projectors with poor brightness are used, this effect might be important as well. Some authors recommend some additional optical correction for night use only, e.g. for night driving⁶.
The reason for this myopia is some basic residual accommodation²⁹, as stated by Leibowitz et al.³⁰: "the focus of the eye tends to return passively to an individually characteristic intermediate resting position or dark-focus whenever the stimulus to accommodation is degraded or when the quality of the image is independent of focus."
Night myopia decreases with age³¹ and therefore it can be of special importance for young drivers at night. In one experiment with people aged 16 to 25 years, 38% had night myopia of - 0.75 D or more, and 4% had – 2.50 D or more³².
As mentioned above, the reduced distance vision at low light is caused mainly by residual accommodation, but some other effects have an impact as well:

• The optical effect of the larger pupil decreases the depth of focus – like a large aperture of a camera
• At low light levels there is a change of the biochemistry of imaging on the retina: in bright light the receptors in the retina are cones with higher image acuity, and imaging of colors, whereas in low light the receptors in the retina are rods with reduced image acuity, and imaging of black and white only. The transition between both states, i.e. to get maximum sensitivity when moving from the bright to the dark, doesn't happen immediately, but takes some minutes33. This is a problem e.g. when driving a car in sunlight and entering a tunnel.
• The low light imaging by rods in the retina can be further reduced by a lack of vitamin A and zinc.

There is conflicting evidence regarding a connection between tonic accommodation and myopia. Some studies associate higher tonic accommodation with myopia, while others indicate that lower tonic accommodation is associated with myopia¹⁷.

After vacations (summer or winter), the dark focus was found to be lower and the progression of myopia to be suspended³⁴.

1.4.2 Pseudomyopia

If there is a transient spasm or excessive stress in the ciliary muscle during excessive near work, the zonular fibers are relaxed (and the ciliary muscle is not relaxed) even at distant-focus, giving the impression of a myopic eye. It is said that this pseudomyopia frequently precedes axial myopia, and it is most frequently found with young people⁶.

Another mechanism for pseudomyopia has been postulated: It was stated7 [reference was missing in the sample] that as an automatic reaction to mental stress the axes of the two eyes are set parallel and the focus is set to "distant", and the pupils are opened wide. In a hunter-gatherer lifestyle, this has a survival advantage in that it may lead to the early detection of danger. However, if a child working at near focus - like during a test - will subject the eyes to competing forces:

• Between near focus and distant focus (ciliary muscle), and
• Between inward adjustment of the axes of the eyes (convergence for near work) and parallel adjustment.

This can lead to a spasm of the involved muscles and further to pseudomyopia (and maybe to permanent myopia).

Besides these effects of the ciliary muscle, transient myopia, which is caused by a hysteresis (i.e. a longer time necessary for readjustment) of accommodation, can be based on:

• A transient ocular elongation caused by accommodation (see section 3.6.5).
• A hysteresis of the shape of the lens as well (see sections 1.3.3 and 3.6.4).

1.4.3 Other Types of Myopia

There are some other types of myopia:

• Keratoconus is when the shape of the cornea is not uniform but more pointed; this can add additional refractive power of up to –20 D.
• With increasing age the refractive index of the cornea can be changed, to result in very moderate myopia.
• Beyond 40 to 50 years presbyopia appears, i.e. the lens loses some (and later all) of its flexibility due to structural changes. Fewer authors attribute presbyopia to a weakening of the ciliary muscle by age³⁵. Generally, this results in problems with near work and accommodation. If, however, the lens shape is "frozen" in a slightly accommodated state, problems with distant focusing may appear, which corresponds to myopia.
• Diabetes can change the refractive index of the lens, leading to myopia if the blood sugar is elevated, and to hyperopia (farsightedness) if the blood sugar is low³⁶ (see section 3.16.1).
• Antibiotics like sulfonamides, tetracyclines and corticosteroids can induce myopia³⁷.
• Amblyopia is when the visual acuity is reduced without visible pathologic defects. In most cases one eye only is affected. The affected eye is often highly myopic. This is named anisometropic amblyopia (see section 3.18).

1.5 Axial Myopia

Axial myopia occurs if the length of the eyeball is more than the average length of about 24 mm³⁸. In this case the ratio of the length of the eye (anteroposterior dimension) to the height/width of the eye (transverse dimension) is larger than 1.0. Roughly 1 mm in length corresponds to - 3.0 D. The increase in the length of the eye is said to happen only at daytime³⁹. There are several forms of axial myopia:

• Simple myopia (sometimes called school myopia), which normally starts at age 10 – 12, stays normally under - 6 D and remains quite stable after the age of 20 years. No structural defects of the eye can be diagnosed in this case.
• Benign progressive myopia up to 12 D, which is often stabilized at an age of 30 years. Most likely structural / biochemical defects of the eye can be diagnosed.
• Malign myopia, which does not stop progressing at all. Up to – 30 D can be reached, with serious consequences, which may lead to blindness. Structural / biochemical defects of the eye can be diagnosed.
• Pathological myopia, if there are already pathological changes in the eye (see section 1.7), independent from the refractive error.

1.6 "What Type of Myopia Do I Have?"

The main and most worrying question is, whether it is a simple (not dangerous) myopia, or a myopia that can lead to a permanent damage of the vision (see section 1.7). This question can be answered only be an optometrist or an ophthalmologist, who will check the background of the eye for some signs of already appearing damage.

Note:
You may regret doing nothing if myopia appears at a young age, or if it is increasing. Even if there is no visible damage yet, follow the recommendations in section 4.8.

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