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.

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:

- Adams C W, Johnson, C A 1991 Steady-state and dynamic response properties of the Mandelbaum effect. Vision Research 31:752-760
- Anonymous 1980 Near and intermediate additions in trifocals. Optician 179(25):18, 20, 22
- Atchison D A 1995 Accommodation and presbyopia. Ophthalmic and Physiological Optics 15:255-272
- Ball G V 1951 Twilight myopia. International Optical Congress 1951. British Optical Association, London, p 92-103
- Bannon R E 1946 A study of astigmatism at the near point with special reference to astigmatic accommodation. American Journal of Optometry 23:53-75
- Bergman S 1957 Research into the amplitudes of accommodation of Afrikaans group. British Journal of Physiological Optics 14:59-64
- Borish I M 1970 Clinical refraction, 3rd edn. Professional Press, Chicago, p 103-109
- Brown N A P 1973 The change in shape and internal form of the lens of the eye on accommodation. Experimental Eye Research 15:441-459
- Brown N A P 1974 The shape of the lens equator. Experimental Eye Research 19:571-576
- Brown N A P 1986 How the lens accommodates. Optician 191(5045):15-16, 18
- Bullimore M A, Gilmartin B, Hogan R E 1986 Objective and subjective measurement of tonic accommodation. Ophthalmic and Physiological Optics 6:57-62
- Burns D 1995 Blur due to pupil area when using progressive addition lenses. Ophthalmic and Physiological Optics 15:273-279
- Burns D, Obstfeld H, Saunders J 1993 Prescribing for presbyopes who use VDUs. Ophthalmic and Physiological Optics 13:409-414
- Bussin H 1990 Reading additions the easy way. Optician 200(2282):12-13
- Campbell F W 1954 The minimum quantity of light required to elicit the accommodation reflex in man. Journal of Physiology 123:357-366
- Campbell F W, Robson J G 1959 High-speed infra-red optometer. Journal of the Optical Society of America 49:268-272
- Campbell F W, Westheimer G, Robson J G 1958 Significance of fluctuations of accommodation. Journal of the Optical Society of America 48:669
- Charman W N 1989 The path to presbyopia: straight or crooked? Ophthalmic and Physiological Optics 9:424-430
- Charman W N 1996 Night myopia and driving. Ophthalmic and Physiological Optics 16:474-485
- Charman W N, Tucker j 1978 Accommodation as a function of object form. American Journal of Optometry 55:84-92
- Ciuffreda K j, Hokoda S C 1985 Effect of instruction and higher level control on the accommodative response spatial frequency profile. Ophthalmic and Physiological Optics 5:221-223
- Coates W R 1955 Amplitude of accommodation in South Africa. British Journal of Physiological Optics 12:76-81, 86
- Cornsweet T N, Crane H D 1973 Training the visual accommodation system. Vision Research 13:713—715
- Donders F C 1864 Accommodation and refraction of the eye. The New Sydenham Society, London
- Duane A 1922 Studies in monocular and binocular accommodation with their clinical applications. American Journal of Ophthalmology 5:865-877
- Dubbelman M, van der Heijde G L, Weeber H A 2005 Change in shape of the aging human crystalline lens with accommodation Vision Research 45:117-132
- Dul M, Ciuffreda K J, Fisher S K 1988 Accommodative accuracy to harmonically related complex grating patterns and their components. Ophthalmic and Physiological Optics 8:146-152
- Edwards M H, Law F, Lee C M, Leung K M, Lui W O 1993 Clinical norms for amplitude of accommodation in Chinese. Ophthalmic and Physiological Optics 13:199-204 (and matters arising, 431)
- Fairmaid J A 1959 The constancy of corneal curvature. British Journal of Physiological Optics 16:2-23
- Fisher R F 1971 The elastic constants of the human lens. Journal of Physiology 212:147-180
- Fitch R C 1971 Procedural effects on the manifest human amplitude of accommodation. American Journal of Optometry 48:918-926
- Fletcher R J 1951/2 Astigmatic accommodation. British Journal of Physiological Optics 8:73-94,129-160,193-224; 9:8-32
- Francis J L, Rabbetts R B, Stone J 1979 Depressed accommodation in young people. Ophthalmic Optician 19:803 804, 807-808, 811
- Giles G H 1960 Principles and practice of refraction. Hammond, Hammond & Co., London
- Gilmartin B 1986 A review of the role of the sympathetic innervation of the ciliary muscle in ocular accommodation. Ophthalmic and Physiological Optics 6:23-37
- Gilmartin B 1989 Personal communication
- Gilmartin B 1995 The aetiology of presbyopia: a summary of the role of lenticular and extralenticular structures. Ophthalmic and Physiological Optics 15:431-437
- Hamasaki D, Ong J, Marg E 1956 The amplitude of accommodation in presbyopia. American Journal of Optometry 33:3-14
- Harris W F 2000 Step-along vergence procedures in stigmatic and astigmatic systems. Ophthalmic and Physiological Optics 20:487-493
- Heath G G 1956 The influence of visual acuity on the accommodative responses of the eye. American Journal of Optometry 33:513-524
- van. der Heijde G L, Beers A P A, Dubbelman M 1996 Microfluctuations of steady-state accommodation measured with ultrasonography. Ophthalmic and Physiological Optics I 6:216-221
- Hennessy R T 1975 Instrument myopia. Journal of the Optical Society of America 65:1114— 1120
- Heron G, Smith A C, Winn B 1981 The influence of method on the stability of dark focus position of accommodation. Ophthalmic and Physiological Optics 1:79-90
- Hofstetter W H 1944 A comparison of Duane's and Donders' tables of the amplitude of accommodation. American Journal of raid Optometry 21:345-363
- Hofstetter H W 1965 A longitudinal study of amplitude changes of in presbyopia. American Journal of Optometry 42:3-8
- Hofstetter H W 1968 Further data on presbyopia in different ethnic groups. American journal of Optometry 45:522-527
- Hokoda S C, Ciuffreda K J 1982 Measurement of accommodative amplitude in a mbiyopia. Ophthalmic and Physiological Optics :a. 2:205-212
- Howland H C, Dobson V, Sayles N 1987 Accommodation in infants as measured by photorefraction. Vision Research 27:2141-2152
- Johnson C A 1976 Effects of luminance and stimulus distance on accommodation and visual resolution. Journal of the Optical Society of America 66:138-142
- Jaschinski-Kruza W, Toenies U 1988 Effect of a mental arithmetic task on dark-focus of accommodation. Ophthalmic and in Physiological Optics 8:432-437
- Knoll H A 1952 A brief history of `nocturnal myopia' and related phenomena. American Journal of Optometry 29:69-81
- Koomen M, Scolnik R, Tousey R 1951 A study of night myopia. journal of the Optical Society of America 41:80-90
- Kragha I KO K 1986 Amplitude of accommodation: population and methodological differences. Ophthalmic and Physiological t. Optics 6:75-80
- Kragha I K O K, Hofstetter H 1986 Bifocal aids and environmental temperature. American Journal of Optometry 63:372-376
- Kruger P B, Mathews S, Aggarwala K R, Sanchez N 1993 Chromatic aberration and ocular focus: Fincham revisited. Vision Research 33:1391-1411
- Leat S 1996 Reduced accommodation in children with cerebral palsy. Ophthalmic and Physiological Optics 16:385-390
- Leibowitz H W, Owens D A 1975 Anomalous myopias and myopias: the intermediate dark focus of accommodation. Science 189:646-648
- Leibowitz H W, Owens D A 1978 New evidence for the intermediate position of relaxed accommodation. Documenta Ophthalmologica 46:133 147