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Modeling ametropic retinal shape changes with an ellipsoid of constant equatorial radius has been found to produce types of peripheral astigmatism which are in good agreement with experimental findings.
PURPOSE: To investigate the pseudophakic accommodation effect in dual and mono optic translation accommodative intraocular lenses (AIOL) using linear matrix methods in the paraxial space. METHODS: Dual (anterior optic of power +32 D linked to a compensatory posterior optic of negative power) and mono lens power was determined in the non-accommodated state using linear geometric optics based on the Gullstrand model eye. The position of the AIOL was calculated from a regression formula. Pseudophakic accommodation was assessed with three systems: (1) forward shift of the mono optic lens, (2) anterior translation of the anterior optic in the dual optic lens system with an unchanged position of the posterior minus lens and (3) symmetrical anterior and posterior translation of the anterior and posterior lens. The Gullstrand model eye was modified by changing the axial length (and proportionally changing the phakic anterior chamber depth) to investigate the accommodative effect in myopic and hyperopic eyes. RESULTS: The dual optic lens system (2) yields a nearly constant accommodation amplitude of 2.4-2.5 D mm(-1) movement over the total range of axial lengths. The mono optic lens (1) provides a higher accommodative effect only in extremely short eyes (high refractive power of the lens), whereas for normal eyes (1.4-1.5 D mm(-1) movement) and for long (myopic) eyes the accommodative effect is much less than the dual optic lens. The dual optic lens system under condition (3) yields less accommodation amplitude compared with the dual optic system under condition (2) over the total range of axial length but provides higher accommodation amplitude compared with the mono optic lens system (1) with axial lengths greater than 22.3 mm (lens power 25.5 D). In the accommodated state, with lens translation of 1 mm, the absolute value of the lateral magnification increases with the refractive power of the mono optic lens (1) and decreases in both dual optic lens systems (under conditions 2 and 3). CONCLUSIONS: A mathematical strategy is presented for calculation of the accommodative effect of mono-optic and dual optic AIOL. The dual optic lens yielded a nearly constant accommodation amplitude of about 2.4-2.5 D mm(-1) translation, whereas the mono optic lens yielded an accommodative response of <2 D mm(-1) translation in long myopic or normal eyes. Only in extremely short eyes is the accommodative amplitude of the mono-optic lens higher than the dual optic lens.
Maps of perceptual image quality were generated for a range of intraocular (IOL) tilts and decentrations, using a perceptually based metric i.e. the square integral (SQRI) metric. Maps were generated for a well-corrected eye, and for hyperopic (-0.1 mm) and myopic (+0.1 mm) image planes (for 2 and 4 mm pupils). These perceptually based results were compared with a standard optical measure (i.e. the equivalent SQRI for given spectacle power added at the cornea). The image quality changes in maps were small, but detectable (exceeding the perceptual just noticeable difference (jnd) or 1 jnd threshold). Clockwise (negative) tilt combined with decenter fell below the 'contrast reserve' modulation transfer functions (MTFs) for 0.25 and 0.5 D defocus, 4 mm pupil. These errors were larger than those found previously, probably because of the shape-factor of the IOL used. Pupil size contributed the largest drop in SQRI value, followed by clockwise tilt combined with decenter. Anticlockwise tilt with decenter produced image quality improvements for the hyperopic eye. Spot diagrams confirmed defocus and the balance of coma-like aberration and astigmatism as causes of the changes in SQRI values. The SQRI maps conveniently summarize imaging, and may be a useful tool in assessing image quality in eye modelling applications.
PURPOSE: To develop formulae for the refractive state ratios, axial length growth and the age of onset (A*) of myopia. Calculated results are compared with measurements. METHODS: Using an image equation, the axial length in the ametropic state (L) is related to the length in the emmetropic state (L*) and refractive error (D) by a rate of change M (D mm(-1)). Three refractive state ratios are defined: C1 = L/r1, C2 = L/L* and C3 = L/H, where r1 and H are the anterior surface radius of the cornea and the transverse dimension of the eye, respectively. The age of onset A* is calculated by the rate of change of refractive error (M) and the axial growth rates N and N* in the ametropic and emmetropic states, respectively. RESULTS: The three ratios C1, C2 and C3 are increasing function of the myopia power, for example, C1 = 3.4, 3.1, 2.9 for D = -8, 0, +4 dioptres, respectively. The calculated C1* (for the emmetropic state) varies between 3.08 and 3.14, depending on corneal shape, and C3 = 0.985, 1.0, 1.04 for D = +3, 0, -3 dioptres: these values are consistent with measured data. For a typical system with effective focal length F = 22.25 mm, L* = 24.2 mm and L = 23.4, 24.9, 25.9 mm for D = +2, -2, -5 dioptres, respectively. The calculated rates of change of refractive error M = 2.3-2.9 D mm(-1) for F = 22-23 mm are also consistent with measured values 2.4-2.7 D mm(-1). The age of onset A* is calculated to be proportional to 1/(M dN), where dN = N-N* is the axial growth rate difference between the ametropic and emmetropic states and may be used as a better predictor for myopia onset than the conventional ratio L/r1. The A* is given by the crossing of L and L* curves, in which myopia onset occurs earlier for larger M dN. CONCLUSION: The theory provides formulae to calculate various refractive state ratios, which are consistent with measurements. By defining two rate functions, M and N, the onset of myopia can be predicted.
The changes in the patterns of refraction (skiagrams) over the central +/-35 degrees of the horizontal field of 3 originally near-emmetropic eyes of 2 subjects were determined over a time interval of 26 years. The subjects were aged 32 and 40 years at the time of the first measurements. The central refractions shifted in the expected hyperopic direction, while the radial and tangential image fields in the periphery became more myopic. These longitudinal results agree with recent transverse studies, provided that allowance is made for the change in central refraction: the reported loss with age in peripheral visual performance does not seem to be attributable to markedly increased peripheral astigmatism.
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PURPOSE: To conduct an experimental investigation of the effect of accommodative relaxation using far point shift stimuli for the reduction of asthenopia. METHODS: Twenty-two female students accommodated to a far point shift stimuli during a 2-min period immediately after a 15-min sustained task on a three-dimensional display. Before and after the trial, their accommodative step response and symptoms were assessed. The far point shift stimuli in the optical system, which were presented on a refractometer, were created by moving the target scenery images from far to near, lineally centred about the far point position of each eye. During 2 min of fixating on the far point shift stimuli, changes in refraction were recorded in the same eye. RESULTS: While looking at the far point shift stimuli, 10 of 22 subjects had changes in refraction that showed a hypermetropic shift, and the other 12 subjects had changes in refraction that showed a myopic shift. The time taken for the accommodative step response from far to near post-trial in the myopic shift group was markedly prolonged, and the accommodative lag at the far target in the optometer was significantly increased. In the myopic shift group, the symptoms of "eye fatigue", "eye pain", "eye heaviness", and "eye dryness" also increased after the trial. In the hyperopic shift group, however, only the symptom of "eye dryness" increased, with no reduction of accommodation function. CONCLUSIONS: We suggest that accommodative relaxation by accommodative far point shift stimuli is effective in the reduction of asthenopia.
An analysis of visual defects among 310 children referred from a vision screening of 2 178 7-year-old children revealed a 50% frequency of significant eye defects among the referrals (7% of screened children). Of the screened children, one group (1 530 children) had previous visual screening three years earlier. The other group (648 children) had no previous vision screening until the age of seven. A comparison between the two groups showed that the risk of finding a new significant eye disorder in a school entrant was more than 6 times greater for a child who was not examined in his preschool years, and the risk of finding an amblyopic child was more than 10 times greater. The results do indicate the need for continuation of the present vision screening program of pre-school children.
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In a previous ophthalmic study of ex-prematures around the age of 10 years it was accidentally found that cupping of the optic disc was significantly more pronounced in children of a low birth weight (less than 2000 g) than in full-term controls (Fledelius 1976). This was true for cup size (cup/disc diameter ratio) as well as depth (as indicated by a visible cribriform plate). The present analyses make up an extension of the above study, with additional calculations based on some of its data. Within the ex-prematures (n = 268) the cup size did not seem to be related to birth weight, sex, or visual acuity. Eyes with myopia of prematurity were not especially 'loaded' with large cups. For the whole sample (including the 187 full-term control there was a weak association between cup size and refraction (and axial eye length). Larger cups occurred relatively more often in myopic eyes. Possible mechanisms behind early changes in disc are discussed (astroglial hypothesis--distension of disc--loss of retinal nerve fibers). It is felt that this new observation--large disc cupping as a possible low birth weight sequel--has to be substantiated by further clinical evidence, as statistical type 1 error (mass significance) cannot be ruled out.