Tag Archives: emmetropy

Refraction Fluctuations in the Eye

The following article is presented to you by Dmitry Romashchenko
Disclaimer: The following text may content specific terms, requiring more in deep knowledge in the field.
  • Refraction

As it was said previously, the mechanisms for the myopia onset are currently not completely understood. That makes every difference in static or dynamic behavior of emmetropic and myopic eye of particular interest. Refraction, or optical power of the eye, (compared to its length) is the main criterion by which the judgment about ammetropia (myopia or hypermetropia (far-sightedness)) is made. Refraction is the reciprocal (1/value) of the distance to the plane on which the eye is focused. For the relaxed (not accommodating) emmetropic (healthy) eye refraction is 0D. That means that infinitely far objects (1/infinity =  0) will be in focus on the retina. Relaxed myopic eyes have refraction 0. The signs are showing the position of the plane in focus (negative – forward, in front of the eye, positive – backwards, behind the eye) and the number is showing the amount of image blur: the bigger is the absolute refraction value the more the image is unfocused.

  • Refraction fluctuations

When talking about the ‘refraction’ of the eye the ‘mean refraction value’ is meant as the value is not completely constant over time (see the fig.1). These changes can clearly be seen on the figure 1.

Fluctuations_Dmitri
Fig. 1 Dynamics of the eye refraction

This microfluctuations in the eye optical power can be categorized in 2 groups by their frequency: lower (below 1 Hz) and higher (above 1 Hz) frequency domains. The second group fluctuations are lower than those of the first one (2). The low-frequency group is believed to be responsible for ‘physiological control’ of the eye refractive state (2). In other words, when looking at any object, the eye ‘checks’ if the refractive power of the eye is still optimal for viewing the particular object. It is represented by slow fluctuations of the optical power from the mean value.
Comparing myopic and emmetropic eyes showed that myopic patients have larger refraction fluctuations for far and near targets than emmetropic ones (1). This is one of the clues to the theory idea that in myopic eyes the whole accommodation mechanism (change of the optical power depending on the distance to the object) is working differently than in emmetropic eyes. Since the loop ‘accommodation mechanics + neurological control’ is not fully understood as well, this difference can play a major role in myopia onset processes by itself or be a significant part of it. On the other hand, this observed changes can be not the cause but the result of the myopia development in the eye. In both cases it gives a better understanding of the myopic and emmetropic eyes dynamics, creates new and answers previously arisen questions on the matter.


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References.

1) Seidel; N.C. Strang; L.S. Gray; E.A. H. Mallen. The Influence of Target Vergence Upon the Magnitude of the Accommodative Microfluctuations in Emmetropia, Early–Onset Myopia and Late–Onset Myopia, 2005

2) Ronald B. Rabbets, Edward E. A. Mallen, 2007. Clinical Visual Optics, 4th edn., p. 125 – 149. References to the chapter:

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