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At least 19 recordsLinked to original sources
PubMed2005

[Changes of eye refraction, corneal power and lens power during growth in emmetropia, myopia and hyperopia].

PURPOSE: The aim of this study was to evaluate changes of eye refraction, corneal power and lens power during growth in emmetropia, myopia and hyperopia. MATERIAL AND METHODS: We examined 183 children (363 eyes) aged 4 to 19 with emmetropia, myopia and hyperopia. All measurements were performed after cycloplegia with 1% tropicamidum. Total refraction and corneal power was examined with autokeratorefractometer. Then we used ultrasound biometer Ocuscan (Alcon, USA), to measure axial length of the eye. Lens power was calculated with use of SRK II formula. RESULTS AND CONCLUSIONS: Mean refractive error in whole group in the age of 4 was +2,86D and was gradually decreasing to reach OD in the age of 14. Between 4th and 14th years old, myopia increases slowly and then acceleration of this process was observed. In hyperopic eyes between 4th and 16th years old, refractive error decreases gradually and then stabilization was noted. Mean corneal power between 4th and 19th years old, decreased in emmetropia and myopia by 1.24D and 2.19D respectively, and increased by 0.38D in children with hyperopia. This changes took place before 10th years old. Mean lens power between 4th and 19th years old, decreased in emmetropia by 2.01 D, in myopia by 1.43D and in hyperopia by 1.78D. This changes took place before 12th years old.

Adolescent↗
PubMed1978

Biometric analysis of intraocular lens power required to produce emmetropia: results of 450 implants.

We analyzed 450 consecutive cases of intraocular lens implantation (omitting only two inadvertent implantations in patients with high myopia) to determine the dioptric lens power in each case required to produce emmetropia. The mean was 18.3 D with a standard deviation of 2.6 D. The data did not conform to a normal or Gaussian distribution because of an abnormally high number of cases that required greater than 25 D to achieve emmetropia and clustering between 17 to 19 D. The deviation from a normal distribution may be explained by our considering patients with high hyperopia and high myopia (which are for the most part excluded from intraocular lens data) to be separate populations. The data are valuable to the intraocular lens surgeons, in that they demonstrate the range and frequency of intraocular lens powers needed to achieve emmetropia in a large population sample.

Aphakia↗
PubMed1999

The development and maintenance of emmetropia.

The human eye is programmed to achieve emmetropia in youth and to maintain emmetropia with advancing years. This is despite the changes in all eye dimensions during the period of growth and the continuing growth of the lens throughout life. The process of emmetropisation in the child's eye is indicated by a shift from the Gaussian distribution of refractive errors around a hypermetropic mean value at birth to the non-Gaussian leptokurtosis around an emmetropic mean value in the adult. Emmetropisation is the result of both passive and active processes. The passive process is that of proportional enlargement of the eye in the child. The proportional enlargement of the eye reduces the power of the dioptric system in proportion to the increasing axial length. The power of the cornea is reduced by lengthening of the radius of curvature. The power of the lens is reduced by lengthening radii of curvature and the effectivity of the lens is reduced by deepening of the anterior chamber. Ametropia results when these changes are not proportional. The active mechanism involves the feedback of image focus information from the retina and consequent adjustment of the axial length. Defective image formation interferes with this feedback and ametropia then results. Heredity determines the tendency to certain globe proportions and environment plays a part in influencing the action of active emmetropisation. The maintenance of emmetropia in the adult in spite of continuing lens growth with increasing lens thickness and increasing lens curvature, which is known as the lens paradox, is due to the refractive index changes balancing the effect of the increased curvature. These changes may be due to the differences between nucleus and cortex or to gradient changes within the cortex.

Aging↗
PubMed1982

Intraocular lens power calculation for emmetropia: a clinical study.

A series of 50 eyes received an intraocular lens (IOL) of power calculated for emmetropia from data of axial length, corneal curvature, and postoperative anterior chamber depth by R. D. Binkhorst's formulae. The postoperative refraction results were compared with those of 100 control eyes which received +19 D standard power IOLs without calculation. The calculated group had postoperative refractions which were closer to emmetropia, and the difference was os statistical significance, with 92% within the +/- 1D range and 98% within the +/- 2 D range from emmetropia. The calculated predictions of postoperative refraction were of a useful level of accuracy. Consideration of the sources of error indicates that there is no justification for the use of IOLs in power steps of less than 1 D. The calculation of IOL power allows the surgeon to control the postoperative refraction and avoid unwanted ametropia.

Aged↗
PubMedDate not supplied

Older age as risk factor for deviation from emmetropia in pseudophakia.

PURPOSE: To find risk factors for deviation from emmetropia after cataract surgery in clinical practice. METHODS: We evaluated the refractive outcome in 106 patients who had underone phacoemulsification and in-the-bag IOL placement 115 +/- 10 days after surgery. Postoperative optical correction and refractive error (diopters of spherical equivalent--ED) were related to age and sex, pre-operative axial length and keratometric diopter power, and operative incision technique. RESULTS: Emmetropia was achieved in 15% of cases; 65% of eyes needed a myopic correction, averaging = 0.46 +/- 0.91 ED. The refractive error was 0.74 +/- 0.61 ED (< or = 1 ED in 77% of cases, < or = 2 ED in 97%). Both optical correction and refractive error were correlated to older age at the time of surgery (p=0.002 and p=0.001, respectively). Astigmatism appeared greater in clear-cornea incision than in limbar incision cases (p=0.05). CONCLUSIONS: The higher refractive error in patients aged over 73 years suggests that age may be a risk factor for deviation from emmetropia after cataract surgery.

Adult↗
PubMed1985

Visual system emmetropia.

Based on the fundamental task of the visual system, which is to receive information from the surroundings and to transmit it to the central nervous system, the visual system is described as being emmetropic if the amount of information received through the visual pathways is maximum when the eyes are unaided. A mathematical model of the dependence of the amount of information received through the visual channels on the eye's refractive power and range of clear vision has been worked out. The model, by showing good correlation with common experience, confirms the hypothesis that the amount of information is a coefficient of an eye's approximation to emmetropia and that the criterion of emmetropia of an eye is a particular case of the criterion of emmetropia of the visual system.

Humans↗
PubMed1998

Role of the cornea in emmetropia and myopia.

The purpose of this paper is to review the available evidence concerning the role of the cornea in achieving and maintaining emmetropia, as well as its role in the development and progression of myopia. The role of the cornea in emmetropization is most evident at the two extremes of life. During infancy, when the axial length increases rapidly, both the cornea and the lens become flatter, moving the refractive state of the eye toward emmetropia; during the later years of life, when the axial length remains constant or even shortens, the cornea appears to steepen somewhat. When myopia develops, in some cases the cornea may undergo a "paradoxical" steepening, which accelerates, rather than retards, the development of myopia. The effect of the cornea on the development and progression of myopia is evident when the ratio of axial length to corneal radius--the AL/CR ratio--is considered. Recent studies suggest that a high AL/CR ratio, which has a value of approximately 3.0 in the emmetropic eye, is a risk factor for youth-onset myopia. Several studies have shown that myopic eyes have greater mean corneal powers than emmetropic eyes; this may be mostly due to greater corneal powers being a risk factor for youth-onset myopia.

Accommodation, Ocular↗
PubMed2005

[Changes of axial dimensions of the eye during growth in emmetropia, myopia and hyperopia].

PURPOSE: The aim ot this study was to evaluate changes ot axial dimensions ot the eye during growth in emmetropia, myopia and hyperopia. MATERIAL AND METHODS: We examined 183 children (363 eyes) aged 4 to 19 with emmetropia, myopia and hyperopia. All measurements were performed after cycloplegia with 1% tropicamidum (Polfa Warszawa). Total and corneal refraction was examined with autokeratorefractometer (Nikon NRK-8000). Then we used ultrasound biometer Ocuscan (Alcon, USA), to measure axial length of the eye, axial length of the vitreous cavity, axial dimension of the lens and axial depth of the anterior chamber. RESULTS AND CONCLUSIONS: 1. Growth of the axial length of the emmetropic eyes is finished at the age of 12, in hyperopic eyes in the age of 11 and in myopic eyes growth is proportional until the age of 14 and then significantly accelerates. 2. Growth of the axial length is mainly caused by increasing axial length of vitreous cavity. A little role in human eye growth is also played by increasing depth of the anterior chamber. 3. Between 4 and 19 years old, mean cycloplegic axial dimension of the lens is slightly decreasing in emmetropic and hyperopic eyes, whereas in myopic eyes is constant.

Adolescent↗
PubMedDate not supplied

[Eyeball shape in children with emmetropia and myopia].

In order to determine the eyeball shape, the authors have carried out ultrasonic biometry of its three major parameters, the anteroposterior axis (APA), horizontal diameter (HD), and vertical diameter (VD), and estimated the ratios of these values (APA/HD and APA/VD) in children with emmetropia (234 eyes) and those with slight and medium-grave myopia (660 eyes), aged 7 to 14. The findings evidence a compressed ellipsoidal shape of the eyeball, presenting as a vertical oval, in all subjects with emmetropic refraction, whatever their age. In myopia the eyeball shape transforms, and all the eyeball sizes are increased, but the APA size is growing more rapidly than the rest sizes, and the eyeball acquires the ball shape with a trend to an elongated ellipsoidal shape. The mean APA length in 7-14-year-old children with emmetropia was up to 23 +/- 0.15 mm, whereas in those with the ball shape of the eyeball it was distended.

Adolescent↗
PubMed1997

Integration of a sensory component into the accommodation model reveals differences between emmetropia and late-onset myopia.

PURPOSE: To evaluate the differences in accommodative function between subjects with emmetropia and those with late-onset myopia (LOM). METHODS: This study suggests a modified model of static accommodation, in which an accommodative sensory gain as a linear operator is added to simulate the sensory part of the system. Results derived from the model show that the sensory part not only affects the slope of the accommodative response function but also increases the system's effective threshold (ET) to the blur signal. This method expands the utility of using the control model to evaluate accommodation behavior. Thirteen emmetropic and 10 LOM subjects participated in this study. The subject's accommodative responses to one-, two-, three-, and four-diopter stimuli were measured by the Canon R-1 optometer, and the differences in dark focus, the slope of the accommodative response function, and the ET were compared between the emmetropic and the LOM subjects. RESULTS: The results show that although the dark-focus values and the slopes of the accommodative response function are not significantly different in emmetropia and LOM, the ETs are significantly different. CONCLUSIONS: The higher ET found among subjects with LOM suggests that either the blur (or the error) signal is degraded significantly in the sensory part of the system, the dead space as an internal threshold of the system is high, or both factors are important. On the basis of further analysis of the data, we speculate that the sensory system in LOM subjects was less sensitive to blur than that of the emmetropic subjects.

Accommodation, Ocular↗
PubMed1984

Secondary IOL power calculations: a comparison of regression formula and refraction method in accurate prediction of emmetropia.

A retrospective review of 51 secondary intraocular lens (IOL) implantations was made to determine the relative accuracy of the SRK regression formula and the refraction method described by Holladay in preoperative prediction of emmetropic IOL power. The regression formula accurately predicted emmetropia within 1 diopter (D) in 63% of patients; the refraction method accurately predicted within 1 D in only 26% of cases. The emmetropic power predicted by the two formulas differed by at least 1 D in 76% of patients. Precise measurement of preoperative variables is imperative to ensure accurate prediction of emmetropic IOL power in the patient considered for secondary implantation.

Aphakia↗
PubMed1996

Achieving emmetropia in extremely short eyes with two piggyback posterior chamber intraocular lenses.

PURPOSE: To examine the refractive results and limitations of current intraocular lens power formulas when implanting two posterior chamber lenses in-the-bag to achieve emmetropia in extremely short eyes. METHODS: Preoperative measurements (corneal diameter, axial length, keratometry, anterior chamber depth, and lens thickness) and postoperative measurements (refraction, corneal vertex to iris depth, and iris to front anterior lens surface) were taken in six eyes from three patients, with axial lengths ranging from 15.09 to 19.95 mm. These data were used to calculate the prediction error for three current third-generation formulas (Holladay, Hoffer Q, SRK/T) and two older formulas (SRK2 and SRK1). RESULTS: None of the formulas accurately predicted the refractions using the optimized lens constants for normal eyes. The third-generation formulas were not different (P > or = 0.602) and averaged 5 diopters (D) of absolute error (Hoffer Q = 4.64 +/- 1.57 D; Holladay = 5.07 +/- 1.28 D; SRK/T = 5.12 +/- 1.43 D). The older formulas were significantly worse (P = 0.0006), with average mean absolute errors of 10.93 +/- 5.09 D for the SRK2 and 13.33 +/- 5.09 D for the SRK1. When the formulas were optimized for these six eyes, the mean absolute errors were Holladay = 1.33 +/- 1.25 D; SRK/T = 2.10 +/- 1.31 D; Hoffer Q = 4.54 +/- 2.00 D; SRK2 = 4.71 +/- 1.94 D; and SRK1 = 4.71 +/- 1.94 D. The Holladay and SRK/T formulas were statistically better (P = 0.0068) than the Hoffer Q and the two older formulas. CONCLUSION: Current third-generation formulas are better than older formulas for extremely short eyes, but still are not acceptable for the desired clinical accuracy. Newer formulas that will use additional anterior segment measurements (corneal diameter, anterior chamber depth, and lens thickness) will be required for improved accuracy, because the anterior segment often is not proportional to the axial length.

Anterior Chamber↗
PubMed1989

[The biomechanical properties of the crystalline lens capsule in emmetropia and myopia].

Morphometric and electronmicroscopic studies of the capsule of a clear lens depending on the degree of myopia and age as compared with the capsule of a clear lens in emmetropia and myopia have shown that with age the anterior capsule of a clear lens becomes thicker, while the posterior capsule becomes thinner and increases in length. This is connected with the weakening of accommodation in the presence of different curvature of the anterior and posterior surfaces of the lens on the background of the lowering of the metabolic level in elderly persons. In myopia, with the development of age changes the thickness of the lens capsule remains almost unchanged. The degree of myopia doesn't influence the thickness and the length of the anterior capsule of the lens, while with the rise of myopia the thickness of the posterior capsules increases. No dystrophic changes in the structure of such a capsule in high myopia were revealed. Hypertrophy of the posterior capsule speaks about the increase of its mechanic strength in a clear lens and is connected with the weakness of accommodation. Such a capsule can serve as a support for IOL implants in patients with myopic refraction.

Adult↗
PubMed1981

The eyes of young chickens grow toward emmetropia.

The distribution of refractive errors was followed in chicks from hatching to 8 weeks of age. A dramatic progressive decrease in the variability of refractions was observed over this period. In addition, there appeared to be a parallel decline in hyperopia, even when the artifactual hyperopia of retinoscopy was taken into account. These results are evidence for a postnatal development regulatory mechanisms, most likely dependent on vision, which directs growth of the eye toward emmetropia.

Animals↗