Accommodative Range Research Articles

Are optical aberrations during accommodation a significant problem for refractive surgery?

To study the limits to a perfect ideal customized wavefront correction due to the change of aberrations during accommodation. METHODS. We measured the dynamic changes of ocular aberrations during accommodation in normal eyes with a real-time Hartmann-Shack wavefront sensor. Those results were used in computer simulations to predict the benefit of a perfect customized correction. Due to the continuous changes of the aberrations over time, an ideal perfect static correction will not provide stable aberration-free optics. For example, when the eye accommodates to near objects, due to the changing aberrations, the eye will become aberrated again. An alternative correction using the aberration pattern for a slightly accommodated condition could provide a better-correction in a larger accommodative range, although at the cost of non-perfect correction for far vision. Due to the dynamic nature of ocular optics, a static perfect correction, for instance performed in customized refractive surgery, would not remain perfect for every condition occurring during normal accommodation.

Aging of the human lens: changes in lens shape upon accommodation and with accommodative loss.

Accommodation in the human eye occurs through controlled changes in crystalline lens shape, thickness, and refractive surface placement relative to the cornea. The changes in lens curvatures, whether surface or internal, have been characterized as a function of accommodation and subject age by use of quantitative analysis of Scheimpflug slit-lamp photographic images. Radii of curvature of the major lens refractive surfaces--the external and nuclear boundaries--decrease linearly with increasing accommodation in all eyes that are capable of accommodation. The rates at which they change with accommodation are age dependent, decreasing steadily toward zero with increased age. For the curves visible in each lens half, arising from boundaries between adjacent zones of discontinuity, radius of curvature and location are linearly related over the entire accommodative range for a given lens and over the age range for the population. The slope of this relationship changes with both accommodation and age, decreasing linearly in both cases. The relationship between these geometric changes and the loss of accommodative amplitude is discussed.

Dynamic accommodation in rhesus monkeys

The dynamics of Edinger–Westphal (EW) stimulated accommodation were studied in two young rhesus monkeys to understand the relationships between accommodative amplitude and rates of accommodation and disaccommodation. Accommodative responses were recorded with infrared photorefraction at five different amplitudes spanning the full EW stimulated accommodative range available to each eye. Combined exponential and polynomial functions were fit to the accommodation and disaccommodation responses. Derivatives of these functions provided the maximum rates of accommodation and disaccommodation as well as time constants for each amplitude. Maximum rates of EW stimulated accommodation and disaccommodation were found to increase linearly with amplitudes from 0.58 to 17.41 D in the two monkeys. The results suggests that the rate of EW stimulated accommodation is dictated by the amplitude. We conclude that if dynamic accommodative responses are to be compared in monkeys of different ages it is necessary to compare responses for the same accommodative amplitudes in order to draw conclusions about age related changes.

Dynamics of the accommodation response to abrupt changes in target vergence as a function of age

The dynamic accommodation responses to small, abrupt changes in an accommodation stimulus were studied in two experiments. In the first, responses of 19 subjects with ages distributed between 18 and 49 years were measured for step stimuli of ±1.05 D. In the second, responses to small step stimuli (±0.75, ±1.75 D) were recorded for a group of six ‘young’ (mean age 22, range 16–26 years) and six ‘old’ (mean age 42, range 36–48 years) subjects. In both experiments, the low target vergences always lay within the accommodation range of all subjects. Whether the data were analysed in terms of simple reaction and response times, or in terms of the frequency response, no marked systematic changes with age were found in the dynamics of the response in either of the experiments. It is concluded that, for small stimuli within the amplitude of accommodation, the response dynamics over the adult age range studied remain remarkably constant, even though the amplitude progressively reduces with age as complete presbyopia is approached.

Evaluation of the Effects of a Head-mounted Display on Ocular Accommodation

We evaluated a commercially produced head-mounted display (HMD) to determine its short-term effects on human ocular accommodation. Thirteen subjects (seven men and six women, ranging from 13 to 44 years old) were tested for changes in a number of parameters before and after viewing a full-length movie (approximately two hours) on a HMD. As a control, subjects were also tested before and after viewing a movie on a high-quality NTSC color television, and also before and after a one-hour intermission. Accommodation dynamics and range were measured. Data showed wellknown trends due to subject age. Only one statistically significant change was found: a slight increase in the latency of relaxation accommodation after HMD viewing.

Refractive changes induced by electrocautery of the rabbit anterior lens capsule

The pathologic basis for presbyopia is classically attributed to lenticular sclerosis or atrophy of the ciliary muscle, but recent work suggests that thickening and loss of elasticity of the anterior lens capsule play an important role. As no practical means for attenuating or reversing the aging process of lens protein has been identified, alteration of the lens capsule eventually might prove to be a desirable alternative to spectacle correction in presbyopic individuals. This paper describes changes in the refractive properties of the lens resulting from alteration of the anterior lens capsule by application of focal cautery, using both an in vitro, in situ and an in vivo rabbit model. In vitro thermal treatment (electrocautery) of the capsule significantly increased the anterior curvature of the lens by an average of +2.95 diopters. Histologic examination of the treated lenses showed thinning of the capsule in the treated areas, as well as focal vacuolar degeneration in the lens substance beneath the lesions. In vivo thermal treatment of eyes induced a significant shift toward myopia, compared with control eyes. The accommodative range increased post-treatment relative to the controls, but the effect diminished over time, stabilizing near baseline at two to three weeks after treatment. Histologic examination showed localized changes but no signs of diffuse cataract formation. We conclude that the anterior capsule may play a significant role in the refractive power and accommodative changes in the crystalline lens. With further study of short-term and long-term effects and with development of noninvasive laser techniques, thermal treatment of the anterior lens capsule might eventually become a practical method of managing hyperopia and loss of accommodation in patients with advanced presbyopia.

Objective evaluation of visual fatigue in VDU workers.

Prolonged work with visual display units (VDUs) may result in a wide range of subjective complaints of visual fatigue and somatic disorders. Accepted tests for measuring the physiological correlates of eye discomfort are lacking. In this study, we tested accommodation and convergence, which are most often linked experimentally with VDU work. The tests were carried out on 16 VDU and 13 control workers aged 24 to 43 years, before work at the beginning of the week (first examination) and again four days later at the end of the work day (second examination). The results in VDU workers showed that both accommodation and convergence range significantly decreased. In addition, those workers with powerful accommodative or convergence ranges at the initial examination had a higher decrease. We suggest that accommodation and convergence measurements can be used to evaluate visual fatigue objectively in VDU workers.

Refractive state, corneal curvature, accommodative range and ocular anatomy of the Asian elephant ( Elephas maximus)

The resting refractive state of six mature, female, Asian elephants (Elephas maximus) was determined using streak retinoscopy and neutralizing video retinoscopy. The amplitude of accommodation was also measured by neutralizing video retinoscopy of two animals and the corneal curvatures of three animals was measured by photokeratoscopy. The net spherical refraction was found to be +0.23 D. No difference was observed between cyclopleged and non-cyclopleged eyes (data from three animals), nor was there any difference between right and left eyes. Nine of the twelve eyes refracted had > or = 0.5 D astigmatism. The mean corneal power, as measured by photokeratometry was 21.3 D (SD = 1.8 D). There was a tendency towards with-the-rule corneal astigmatism in our sample (mean value: 1.2 D), though it did not reach statistical significance (P = 0.06). Two elephants were examined using neutralizing video photoretinoscopy. They were able to accommodate through 3 D. Three fixed eyes from three different elephants were obtained for gross and microscopic examination. The mean axial length of the eye was 38.75 mm and the lens had an axial diameter of approx. 10 mm. The posterior sclera was thick (8.0-8.5 mm). Histologically, the cornea was comprised of five distinct layers. A thin, meridionally oriented smooth ciliary muscle was identified. Individual muscle fibers were also observed associated with the posterior trabeculae of the uveal meshwork.

The increasing sclerosis of the human lens with age and its relevance to accommodation and presbyopia

By means of a fine conical probe and a miniature dynamometer, the resistance to penetration of different lens layers was measured. In clear human lenses the power of resistance of the lens nucleus increases with age, mostly due to the "hardening" of the nucleus. A distinct hardening of the nucleus as opposed to the cortex has been found to occur in lenses as young as 20 years of age. This "firmness" of the lens nucleus, occurring between the ages of 20 and 60 years, coincides with the decrease in accommodation range and the onset of presbyopia.

Refractive state, ocular anatomy, and accommodative range of the sea otter ( Enhydra lutris)

Sea otters are carnivorous, amphibious mammals that are active both above and under water. Accordingly, it might be expected that their eyes are adapted for both aerial and aqueous vision. We examined the anatomy and physiological optics of the sea otter eye with a view towards describing and explaining its amphibious visual characteristics. We employed photokeratoscopy to measure the refractive power of the sea otter cornea, which we found to be 59 D. Using video dynamic photorefraction, we found that sea otters can focus targets clearly both in air and water, relying on accommodation to compensate for the refractive loss of their corneas upon immersion in water. Our anatomical investigations revealed that the anterior epithelium of the cornea is extensively developed, as is the iris musculature, meridional ciliary muscle, and the corneoscleral venous plexus. The first feature is most likely an adaptation to the salinity of the marine environment. We believe the latter features are part of a novel, well-developed lenticular accommodative mechanism.

Intraocular muscles of the Canadian river otter and Canadian beaver and their optical function

The gross morphology and microscopic anatomy of the ciliary and iris musculature of the Canadian river otter (Lutra canadensis) and Canadian beaver (Castor canadensis) were studied to help evaluate whether either species has an exaggerated accommodative mechanism for use underwater. The intraocular muscles of the otter are of a size and organization indicative of an extreme lens-deforming accommodative ability involving both the ciliary body and iris. By contrast, the intraocular muscles of the beaver eye indicate that accommodation is not significant in this animal. Thus, the intraocular musculature of the class Mammalia is not homogeneous, even within the amphibious mammals. Refractive and corneal measurements on two living river otters confirm that this animal is capable of an accommodative range that can neutralize the loss of corneal refractive power when the eye is in water.

The effects of a single intramuscular injection of atropine sulphate on visual performance in man.

The effects of a single intramuscular injection of 2 mg atropine sulphate on visual function were studied in volunteer subjects. The well-known effects of increased heart rate, dryness of the mouth, increased pupil diameter and reduced accommodative range were confirmed. Visual acuity, stereoacuity, red-green colour balance and reaction time to a visual stimulus were unaffected by atropine, while extraocular muscle balance (horizontal heterophoria and cyclophoria) underwent a transient change. There was no significant change in contrast sensitivity measurements to stationary sinusoidal grating patterns of spatial frequencies 1-30 c/deg; however contrast sensitivity to moving grating patterns of spatial frequencies 1-5 c/deg showed a sustained reduction which was still present at 6 h post-injection. It is concluded that atropine adversely affects movement detection but not stationary visual function.

The Basel longitudinal study on aging (1955-1978). Ophthalmo-gerontological research results.

In the prospective Basel longitudinal study on aging (1955-1978) 123 men (age at entry from 6-61 years) were investigated in 2 year (average) intervals. Complete case histories are available on 67 subjects over the entire period (19.6 +/- 0.85 years). Part I of the study was to confirm the hypothesis of Bernstein and of Steinhaus, according to which life expectancy can be estimated from the speed of development of presbyopia. Parallel to the measurement of accommodation range (after preliminary determination of refraction and visual acuity), the development of height, body weight, vital capacity, expiratory volume, chest circumference, abdominal circumference, blood pressure, ECG and pulse wave velocity were measured. Invasive investigations were not undertaken. Only when hypertension was combined with obesity was the diminution of accommodation range striking (Fig. 10c; however there were only 3 subjects in this risk group). Taking everything into consideration there was a concomitance between decrease of accommodation range and changes of medical parameters (Table 6). Intercurrent illness did not influence the accommodation range. Longitudinal measurements and cross-sectional comparisons (data averaged to the same point as of the same age) were carried out. The results did not always coincide. We could not confirm the hypothesis of Bernstein and of Steinhaus. In Part II the results of the objective measurements are given. Apparently growth of the skull does not stop entirely. The increase of interpupillary distance can be complete at 17 years of age, but also can continue to the 30th year. The palpebral fissure increases an average of 3 mm more horizontally between the 6th and the 20th year of life. The corneal diameter remains constant in all age classes, that is, the growth of the cornea should be complete before the 6th year of life. Early arcus senilis changes are found already in the 20-year-old. The increase in the course of time of arcus senilis is obvious, it is most pronounced in obese hypertensives. Still there are persons who at 65 years of age show no arcus senilis. In the 20th year degenerative deposits in the conjunctiva begin. They increase in number and above the 60th year all subjects showed degenerative deposits. Aqueous veins are less visible in younger subjects. With increasing age--as a result of degenerative thinning of the conjunctiva--they are recognizable in ever greater numbers. The depth of the anterior chamber attains its greatest extent between the 20th and the 30th year.(ABSTRACT TRUNCATED AT 400 WORDS)

Corneal accommodation in chick and pigeon.

We have investigated the role of changes in corneal radius of curvature in effecting accommodation in the bird's eye. It was found that in natural accommodation (measured by IR photoretinoscopy) changes of corneal radius of curvature (measured by IR photokeratometry) play an important role in both the chick and the pigeon. In the adult pigeon the cornea is indeed responsible for the largest part of natural accommodation (up to approx. 9 D). In this animal the corneal diameter (as seen from the optical axis of the eye) decreases in accommodation which can be taken to explain the change of corneal radius of curvature. In the chicken, corneal accommodation is combined with other mechanisms (total accommodative range 15-17 D, corneal accommodation about 8 D). The chicken's cornea is aspherical within the pupil area leading to large measurement variation in photokeratometry if the Purkinje images are not symmetrical to the pupillary axis.

Visual accommodation and virtual image displays: target detection and recognition.

Twelve subjects performed a complex task, detecting and recognizing small targets presented at infinity, while simultaneously monitoring a virtual image display (VID). The VID was presented at one of four optical distances (2.0, 0.5, 0.0, and-0.5 D). Optical distance was found to affect detection and recognition performance, mainly at the extreme value of 2.0 D. Interactions between optical distance and grouping of subjects according to measures of accommodation indicated that the three other optical distances affect performance differentially. The subjects' resting position of accommodation (RPA) and a combined measure of accommodation range and RPA were significantly correlated with performance (0.58). A narrowing of the functional visual field (complete misses of peripheral targets) was found at the 2.0-D optical distance for all subjects, and at the other optical distances for the subjects with an inferior accommodative mechanism (i.e., a near RPA and/or a narrow accommodation range).

Accommodative range in amblyopic monkeys ( Macaca nemestrina)

Three naturally strabismic and two chronic bilaterally atropinized monkeys were tested for spatial contrast sensitivity and range of accommodation. All eyes that showed deficits in contrast sensitivity also showed deficits in accommodation. The strabismic monkeys all showed interocular differences in their CSFs and in their accommodative ranges. The atropine monkeys had no interocular differences on either measure. However, one had depressed contrast sensitivity relative to normal and also showed a reduced accommodative range. A statistically significant correlation was found between the high frequency cutoffs of the CSFs and accommodative ranges. These deficits in accommodative range that accompany contrast sensitivity losses in the monkey are similar to the deficits in accommodative range that accompany amblyopia in humans.

Analysis of human crystalline lens curvature as a function of accommodative state and age

Slit-lamp photographs from four human subjects, aged 11, 19, 29, and 45 were reanalyzed using computer-based digitization and curve-fitting methods in order to obtain more complete information on internal lens curvature changes during accommodation. All discernible curves ( N = 742) could be fit to parabolas with χ 2 less than or equal to 0.001 irrespective of lens age, accommodative state, or curve location within the lens. For each lens, the coefficients of the parabolas, when displayed in graphic form, exhibit a linear relationship between location within the lens and the coefficient of the χ 2 term. The slope of this line remains unchanged over accommodation for a given lens, but is shifted in position. The slope changes as a function of age. The age 45 lens exhibits these characteristics to a limited extent only, the differences possibly related to the development of presbyopia. The further a given curve is located from the lens surface, the smaller the region of its arc that can be considered approximately circular. A roughly hourglass figure is generated by these circular bounds; the waist of the hourglass decreases with increasing accommodation, since changes in radius of curvature with accommodation are more pronounced internally. Calculations of arc lengths as a function of increasing accommodation indicate that these lengths change very little over the entire accommodative range.

Accommodative Esotropia Long Range Follow-up

Thirty-nine adult patients treated for typical (refractive) accommodative esotropia in childhood continue to have problems because they have not outgrown their hypermetropia, and the majority have not developed stable binocular vision. Their hypermetropia became maximal (median 5.7 diopters) by age 6, decreased in adolescence, and then stabilized (median 4 diopters). Thirty-eight of the 39 adults wear correcting lenses full-time. Nearly all depend on relaxed accommodation to maintain alignment when they remove their glasses. Ten patients, all of whom received treatment before a constant esotropia developed are essentially orthophoric with glasses and have normal binocular vision. The remainder have small-angle deviations with glasses, 14 with varying degrees of amblyopia and peripheral fusion and 15 with anomalous correspondence and suppression. As adults, only one patient with normal binocular function has required surgery whereas 13 of the patients lacking normal fusion have had surgery for increasing esotropia, postoperative exotropia, or consecutive exotropia.

Convergence accommodation

Steady-state accommodation responses were measured in both eyes as a function of vergence angle and direction of lateral gaze. The measurements were made with a binocular laser optometer. Small speckle patterns were used as fusional stimuli in an otherwise dark field. These patterns have the advantage of providing no blur stimulus to accommodation. Convergence accommodation for vergence angles ranging from 0 to 25 deg was measured for lateral-gaze angles of +32, -32, and 0 deg. The average accommodation of the two eyes was linearly related to vergence angle over the observer's accommodation range but was independent of the angle of lateral gaze. The mean convergence accommodation/convergence ratio for three subjects, in diopters per meter-angle, was 0.9. Our measurements of convergence accommodation using laser-speckle targets are in good agreement with previous studies that used small pupils. Accommodation responses for binocular viewing of letters of a Snellen chart were also measured. When luminance was reduced, night myopia was observed. No similar effect was found for convergence accommodation. Accommodation to a dim target corresponded closely to the convergence accommodation.

A model for accommodation in the young human eye: The effects of lens elastic anisotropy on the mechanism

An explanation of the mechanism of visual accommodation depends heavily on understanding the mechanical properties of the lens, as well as the way in which its shape is altered in small accommodative changes. An initial attempt to relate these properties to a mechanism has already been performed (Koretz and Handelman, 1982) for the young (age 11 yr) human lens, using certain simplifying assumptions (spherical curvature on the anterior lens surface and elastic isotropy). However, since it has been shown that the lens behaves as an anisotropic body, the previous treatment has been extended to include the variation of lens elastic properties in the polar and radial directions. With this modified representation, it is found that only one combination of elastic constants is consistent with the generally accepted qualitative theory of accommodation and with clinical data on the accommodative range of the emmetropic age 11 human eye. For this unique solution of the equations, however, the general mechanism already suggested by us, which includes support by the vitreous and alteration of the magnitude and angle of application of zonular force with accommodation, remains little changed.