The relationship between refractive and biometric changes during Edinger-Westphal stimulated accommodation in rhesus monkeys

Experiments were undertaken to understand the relationship between dynamic accommodative refractive and biometric (lens thickness (LT), anterior chamber depth (ACD) and anterior segment length (ASL=ACD+LT)) changes during Edinger-Westphal stimulated accommodation in rhesus monkeys. Experiments were conducted on three rhesus monkeys (aged 11.5, 4.75 and 4.75 years) which had undergone prior, bilateral, complete iridectomies and implantation of a stimulating electrode in the Edinger-Westphal (EW) nucleus. Accommodative refractive responses were first measured dynamically with video-based infrared photorefraction and then ocular biometric responses were measured dynamically with continuous ultrasound biometry (CUB) during EW stimulation. The same stimulus amplitudes were used for the refractive and biometric measurements to allow them to be compared. Main sequence relationships (ratio of peak velocity to amplitude) were calculated. Dynamic accommodative refractive changes are linearly correlated with the biometric changes and accommodative biometric changes in ACD, ASL and LT show systematic linear correlations with increasing accommodative amplitudes. The relationships are relatively similar for the eyes of the different monkeys. Dynamic analysis showed that main sequence relationships for both biometry and refraction are linear. Although accommodative refractive changes in the eye occur primarily due to changes in lens surface curvature, the refractive changes are well correlated with A-scan measured accommodative biometric changes. Accommodative changes in ACD, LT and ASL are all well correlated over the full extent of the accommodative response.     

Comparisons between pharmacologically and Edinger-Westphal-stimulated accommodation in rhesus monkeys

PURPOSE: Accommodation results in increased lens thickness and lens surface curvatures. Previous studies suggest that lens biometric accommodative changes are different with pharmacological and voluntary accommodation. In this study, refractive and biometric changes during Edinger-Westphal (EW) and pharmacologically stimulated accommodation in rhesus monkeys were compared. METHODS: Accommodation was stimulated by an indwelling permanent electrode in the EW nucleus of the midbrain in one eye each of four rhesus monkeys. Dynamic refractive changes were measured with infrared photorefraction, and lens biometric changes were measured with high-resolution, continuous A-scan ultrasonography for increasing stimulus current amplitudes, including supramaximal current amplitudes. Accommodation was then stimulated pharmacologically and biometry was measured continuously for 30 minutes. RESULTS: During EW-stimulated accommodation, lens surfaces move linearly with refraction, with an increase in lens thickness of 0.06 mm/D, an anterior movement of the anterior lens surface of 0.04 mm/D, and a posterior movement of the posterior lens surface of 0.02 mm/D. Peak velocity of accommodation (diopters per second) and lens thickness (in millimeters per second) increased with supramaximal stimulus currents, but without further increase in amplitude or total lens thickness. After carbachol stimulation, there was initially an anterior movement of the anterior lens surface and a posterior movement of the posterior lens surface; but by 30 minutes, there was an overall anterior shift of the lens. CONCLUSIONS: Ocular biometric changes differ with EW and pharmacological stimulation of accommodation. Pharmacological stimulation results in a greater increase in lens thickness, an overall forward movement of the lens and a greater change in dioptric power.     

Simultaneous measurements of refraction and A-scan biometry during accommodation in humans.

PURPOSE: Accommodation is a dioptric change in power of the crystalline lens resulting from ciliary muscle contraction that leads to an increase in lens surface curvatures and thickness and changes in the position of lens surfaces. Previous studies have used A-scan ultrasound to measure changes in the position of lens surfaces with voluntary accommodation, but have not simultaneously measured the change in refraction. The goal of this study is to simultaneously measure and correlate refractive and biometric changes in the lens during voluntary accommodation in humans. METHODS: Refraction was measured off-axis in the right eye and biometry on-axis in the left eye simultaneously during voluntary accommodation in 22 human subjects between the ages of 21 and 30 years (mean +/- standard deviation: 25.8 +/- 2.3 years). Subjects viewed a distant target and four near targets spanning the full accommodative range available to evaluate refraction and lens surface position at each accommodative state. RESULTS: Maximum objectively measured accommodative amplitude of all subjects was 5.64 +/- 0.21 D (mean +/- standard error of mean). Biometric and refractive changes during accommodation were linearly correlated. The mean +/- standard error of mean decrease in anterior chamber depth was 0.051 +/- 0.008 mm/D, increase in lens thickness was 0.067 +/- 0.008 mm/D, and increase in anterior segment length was 0.017 +/- 0.005 mm/D during accommodation. There was a net anterior movement of the lens center of 0.017 +/- 0.005 mm/D. CONCLUSION: Anterior chamber depth, lens thickness, and anterior segment length change linearly with refraction during accommodation. Per-diopter changes in the lens were greater in the current study compared with previous studies in which only accommodative demand was measured, which overestimates the accommodative response.     

Accommodative changes in lens diameter in rhesus monkeys

PURPOSE: Some debate surrounds the accommodative mechanism in primates, particularly whether the lens equatorial diameter increases or decreases during accommodation. This study has been undertaken to measure the relationship between changes in lens diameter and refraction during accommodation in rhesus monkeys. METHODS: Photorefraction was used to measure accommodation, and goniovideography was used to measure accommodative changes in lens diameter in the iridectomized eyes of two rhesus monkeys. Accommodation was stimulated through the full amplitude available to each eye by stimulation of the Edinger-Westphal nucleus of the brain. Dynamic measurement of refractive changes followed by dynamic measurements of changes in lens diameter for the same stimulus current amplitudes allow the relationship between refraction and lens diameter to be determined. RESULTS: Lens diameter decreased relatively linearly during accommodation by 0.055 mm/diopter (D), resulting in an overall decrease in lens diameter of approximately 7% of the unaccommodated lens diameter for approximately 12 D of accommodation. CONCLUSIONS: The rhesus monkey lens diameter decreases systematically with the refractive change during accommodation in accordance with the Helmholtz accommodative mechanism and in contrast to the accommodative mechanism originally proposed by Tscherning.     

The effects of phenylephrine on pupil diameter and accommodation in rhesus monkeys

PURPOSE: Phenylephrine is used to dilate the iris through alpha-adrenergic stimulation of the iris dilator muscle. Sympathetic stimulation of the ciliary muscle is believed to be inhibitory, decreasing accommodative amplitude. Investigations in humans have suggested some loss of functional accommodation after phenylephrine. It is unclear whether this loss is due to direct action of phenylephrine on the ciliary muscle or to secondary optical factors associated with mydriasis. The purpose of this study was to determine whether phenylephrine affects Edinger-Westphal (EW)-stimulated accommodation in rhesus monkeys. METHODS: The time course for maximum mydriasis was determined by videographic pupillography after phenylephrine instillation in 10 normal rhesus monkeys. Static and dynamic EW-stimulated accommodative responses were studied in five iridectomized rhesus monkeys before and after phenylephrine instillation. Accommodative amplitude was measured with a Hartinger coincidence refractometer. Dynamic accommodative responses were measured with infrared photorefraction, and functions were fitted to the data to determine peak velocity versus accommodative response relationships. RESULTS: The maximum dilated pupil diameter of 8.39 +/- 0.23 mm occurred 15 minutes after administration of phenylephrine. In iridectomized monkeys, postphenylephrine accommodative amplitudes were similar to prephenylephrine amplitudes. Dynamic analysis of the accommodative responses showed linear peak velocity versus accommodative amplitude relationships that were not statistically different before and after phenylephrine. CONCLUSIONS: alpha-Adrenergic stimulation causes a strong pupil dilation in noniridectomized monkey eyes but does not affect EW-stimulated accommodative amplitude or dynamics in anesthetized, iridectomized rhesus monkeys.     

Lens diameter and thickness as a function of age and pharmacologically stimulated accommodation in rhesus monkeys

Uncertainty exists regarding accommodative and age changes in lens diameter and thickness in humans and monkeys. In this study, unaccommodated and accommodated refraction, lens diameter, and lens thickness were measured in rhesus monkeys across a range of ages. Iridectomized eyes were studied in 33 anesthetized monkeys aged 4-23 years. Refraction was measured using a Hartinger coincidence refractometer and lens thickness was measured with A-scan ultrasound. Lens diameters were measured with image analysis from slit-lamp images captured via a video camera while a saline filled, plano perfusion lens was placed on the cornea. Accommodation was pharmacologically stimulated with 2% pilocarpine via the perfusion lens in 21 of the monkeys and lens diameters were measured until a stable minimum was achieved. Refraction and lens thickness were measured again after the eye was accommodated. Unaccommodated lens thickness increased linearly with age by 0.029 mm/year while unaccommodated lens diameter showed no systematic change with age. Accommodative amplitude decreased by 0.462 D/year in response to pilocarpine. The accommodative increase in lens thickness decreased with age by 0.022 mm/year. The accommodative decrease in lens diameter declined linearly with age by 0.021 mm/year. Rhesus monkeys undergo the expected presbyopic changes including increasing lens thickness and a decreasing ability of the lens to undergo changes in thickness and diameter with accommodation, however without an age-related change in unaccommodated lens diameter. As in humans, the age-related decrease in accommodative amplitude in rhesus monkeys cannot be attributed to an age-related increase in lens diameter.     

Edinger--Westphal stimulated accommodative dynamics in anesthetized, middle-aged rhesus monkeys

The relationships between peak velocity and amplitude of Edinger-Westphal (EW) stimulated accommodation and disaccommodation were investigated in anesthetized, middle-aged rhesus monkeys. Accommodative responses were recorded at 30Hz with infrared photorefraction. Peak velocity of accommodation and disaccommodation increased linearly with stimulus amplitude. Peak velocities of accommodation continued to increase with stimulus amplitudes greater than required to produce the maximum response. The peak velocity of disaccommodation did not further increase with supramaximal stimulus amplitudes beyond that achieved with maximal stimulus amplitudes. Although maximum accommodative response amplitude is reduced in older rhesus monkeys, within the methodological constraints of this study, older monkeys appear to achieve accommodative and disaccommodative peak velocities similar to adolescent monkeys for the same response amplitudes.     

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.     

Linear relationship of refractive and biometric lenticular changes during accommodation in emmetropic and myopic eyes

AIM: Aim of this study was to investigate the relationship between refractive changes in the eye and biometric changes of the human crystalline lens during accommodation. Furthermore, differences in these relationships between young, healthy emmetropic and myopic subjects were analyzed. METHODS: Mean relative change in anterior chamber depth (ACD), lens thickness (LT), anterior segment length (ASL = ACD + LT) and in objective refraction were simultaneously assessed during near-point-induced accommodation in 10 emmetropic and 10 myopic subjects. Via a beam splitter, measurements were performed simultaneously using partial coherence interferometry (PCI) and infrared (IR) photorefraction. RESULTS: On average, for each diopter of accommodation LT increased by 0.063 mm in emmetropic and by 0.072 mm in myopic eyes, and ACD decreased by 0.047 mm and 0.057 mm, respectively. Mean ASL, indicating the position of the posterior lens pole, increased by 0.009 mm in emmetropic and by 0.013 mm in myopic eyes. The correlation between refractive and biometric changes was found to be essentially linear in both subgroups. Differences in ACD between emmetropic and myopic eyes were statistically significant at an accommodative stimulus of -1 D (p<0.04) and -2 D (p<0.02). CONCLUSION: The biometric and refractive changes of the human lens are highly correlated and linear in function in both emmetropic and myopic eyes.     

Ocular measurements throughout the adult life span of rhesus monkeys

PURPOSE: To examine the relationship of ocular components to refraction throughout the adult life span of the rhesus monkey (Macaca mulatta). METHODS: Cycloplegic retinoscopy, A-scan ultrasonography, slit lamp examination, indirect ophthalmoscopy, and keratometry were performed in a cross-sectional study of 111 monkeys, aged 5 to 31 years. Lens thickness and anterior and vitreous chamber depths were measured from the echograms. The intercorrelations of these variables were analyzed, as well as their association with age and sex. RESULTS: In monkeys aged 5 to 15 years, the mean refractive value of +1.5 D with an SD of 1.7 D was maintained near the previously established developmental asymptote of +2 D. In monkeys older than 15 years, there was greater interindividual variation (SD = 4.5 D), including extreme myopia and hyperopia. The cornea became steeper with age. The axial length of the eyes increased up to 12 years of age and began to shorten after 20 years. Changes also occurred in the other individual components that constitute eye length. These age-related changes were decreased vitreous chamber depth, decreased anterior chamber depth, and increased lens thickness. In general, males had longer eyes than females. The eyes of old monkeys were more likely to exhibit cataract and drusen, but age-related changes in focal atrophy of the retinal pigment epithelium did not achieve statistical significance. CONCLUSIONS: The components of the monkey eye change with age in a pattern similar to that reported in humans. Age-related changes in individual ocular components that could be detrimental to refraction appear to be compensated for by changes in other components.     

Changes in crystalline lens radii of curvature and lens tilt and decentration during dynamic accommodation in rhesus monkeys

Dynamic changes in crystalline lens radii of curvature and lens tilt and decentration were measured during centrally stimulated accommodation in four iridectomized eyes of two adolescent rhesus monkeys. Phakometry measurements were performed dynamically using a custom-built, video-based, Purkinje-image instrument. Lens anterior and posterior radii were calculated from reflections of paired light sources from the ocular surfaces (Purkinje images PI, PIII, and PIV). Lens tilt and decentration were calculated assuming linearity between Purkinje image positions, eye rotation, lens tilt, and decentration. Because the monkey eyes were iridectomized, Purkinje images were referred to the mid-point of the double first Purkinje image (PI). Mean unaccommodated values of anterior and posterior lens radii of curvature were 11.11 +/- 1.58 mm and -6.64 +/- 0.62 mm, respectively, and these decreased relatively linearly with accommodation in all eyes, at a rate of 0.48 +/- 0.14 mm/D and 0.17 +/- 0.03 mm/D for anterior and posterior lens surfaces, respectively. Tilt and decentration did not change significantly with accommodation except for tilt around the horizontal axis, which changed at a rate of 0.147 +/- 0.25 deg/D. These results are important to fully characterize accommodation in rhesus monkeys.     

Accommodative lens refilling in rhesus monkeys

PURPOSE: Accommodation can be restored to presbyopic human eyes by refilling the capsular bag with a soft polymer. This study was conducted to test whether accommodation, measurable as changes in optical refraction, can be restored with a newly developed refilling polymer in a rhesus monkey model. A specific intra- and postoperative treatment protocol was used to minimize postoperative inflammation and to delay capsular opacification. METHODS: Nine adolescent rhesus monkeys underwent refilling of the lens capsular bag with a polymer. In the first four monkeys (group A) the surgical procedure was followed by two weekly subconjunctival injections of corticosteroids. In a second group of five monkeys (group B) a treatment intended to delay the development of capsular opacification was applied during the surgery, and, in the postoperative period, eye drops and two subconjunctival injections of corticosteroids were applied. Accommodation was stimulated with carbachol iontophoresis or pilocarpine and was measured with a Hartinger refractometer at regular times during a follow-up period of 37 weeks in five monkeys. In one monkey, lens thickness changes were measured with A-scan ultrasound. RESULTS: In group A, refraction measurement was possible in one monkey. In the three other animals in group A, postoperative inflammation and capsular opacification prevented refraction measurements. In group B, the maximum accommodative amplitude of the surgically treated eyes was 6.3 D. In three monkeys the accommodative amplitude decreased to almost 0 D after 37 weeks. In the two other monkeys, the accommodative amplitude remained stable at +/-4 D during the follow-up period. In group B, capsular opacification developed in the postoperative period, but refraction measurements could still be performed during the whole follow-up period of 37 weeks. CONCLUSIONS: A certain level of accommodation can be restored after lens refilling in adolescent rhesus monkeys. During the follow-up period refraction measurements were possible in all five monkeys that underwent the treatment designed to prevent inflammation and capsular opacification.     

视力不良儿童调节幅度与眼生物参数的相关性

目的 探讨8～10岁视力不良儿童调节幅度与眼生物参数的相关性.方法 横断面研究.2010年7月对北京市密云区三所小学三、四年级300名视力不良学生进行了调查.所有学生均接受睫状肌麻痹后电脑验光与检影验光、调节幅度测定以及生物参数测量.根据屈光度范围,将受试者分为A组(+0.75 D≤SE≤-0.75 D)、B组(-0.75 D＜SE＜-1.75 D)和C组(SE≥-1.75 D),分别进行调节幅度与屈光度、眼生物参数的Pearson相关和线性回归分析.结果 A组调节幅度仅与前房深度具有相关性(r=-0.30,P=0.05).B组调节幅度与屈光度、眼轴长度及前房深度均具有显著相关性,有统计学意义(r=0.40、-0.41和-0.40,P均＜0.05),而与晶状体厚度、角膜厚度以及角膜曲率无相关性.C组中调节幅度与各生物参数均无相关性.结论 在一定的屈光度范围内(-0.75 D＜SE＜-1.75 D),调节幅度的大小与屈光度、眼轴长度及前房深度具有显著相关性.提示在近视初发时,可以通过视觉训练来改变调节功能的异常,间接、部分地控制近视的发展.     

Changes of the accommodative amplitude and the anterior chamber depth after implantation of an accommodative intraocular lens

Background

Modern cataract surgery is interested in recovery of the accommodative power. This investigation aimed at determining pseudophakic accommodation in subjects implanted with the accommodative Human Optics 1 CU intraocular lens after drug-induced ciliary muscle stimulation by measuring the objective refraction and the changes in anterior chamber depth in comparison with a PMMA intraocular lens with rigid haptics.

Methods

The studied sample involved 30 eyes of 30 patients undergoing cataract surgery due to age-related cataract. Patients were between 50 and 77 years of age (67.71±8.0). No randomization was performed. The 1 CU accommodative intraocular lens and the PMMA intraocular lens were implanted in 15 eyes of patients with an expected visual acuity of at least 0.7. Objective refraction under pilocarpine-stimulated ciliary muscle contraction was determined with a Hartinger coincidence refractometer. The anterior chamber depth was measured with Jäger’s Haag–Streit slit-lamp attachment. The accommodative amplitude and the anterior chamber flattening were calculated from the measured values.

Results

Twelve weeks after surgery the average accommodative amplitude in eyes with a 1 CU intraocular lens calculated from the refractive change under drug-induced stimulation was 0.48±0.36 D (with a maximum of 1.25 D). The measured change of anterior chamber depth under drug-induced stimulation was 0.3±0.32 mm (at a maximum of 0.9 mm). In the reference group with PMMA lenses, the mean accommodative amplitude derived from the refractive changes under drug-induced stimulation was 0.34±0.27 D (at a maximum of 0.85 D). The measured change in anterior chamber depth under drug-induced stimulation was 0.18±0.09 mm (at a maximum of 0.31 mm). No statistically significant differences were found between the two groups of lenses concerning change in anterior chamber depth and accommodative amplitude.

Conclusions

This investigations indicate a mean anterior 1 CU shift of only 0.32 mm and a maximum of 0.9 mm. The accommodative amplitudes measured with the Hartinger coincidence refractometer (mean value 0.47 D) correspond to these values. Similar conclusions may be drawn from existing investigative results of the reference group, which are on the same order of magnitude as those of the 1 CU group. Objective accommodation measurements are needed to evaluate commercially available accommodative intraocular lenses in a scientifically satisfactory manner. Objectively measurable parameters include changes of the anterior chamber depth as well as refraction, as determined for instance by coincidence refractometry and streak retinoscopy. Future studies should also consider the IOL properties, astigmatism, and pupillary diameter. This is the only way to identify pseudoaccommodation and a decisive factor for further development of accommodative artificial lenses.     

Effects of pharmacologically manipulated amplitude and starting point on edinger-westphal-stimulated accommodative dynamics in rhesus monkeys

PURPOSE: The aim of this study was to determine whether pharmacologically manipulated resting refraction, amplitude, and starting point affect accommodative and disaccommodative dynamics in anesthetized adolescent rhesus monkeys. METHODS: Pilocarpine and atropine were applied topically to manipulate resting refraction, accommodative amplitude, starting point, and end point in two monkeys with permanent electrodes in the Edinger-Westphal nucleus. Accommodation was centrally stimulated with submaximal and maximal current amplitudes. Dynamic accommodative responses were measured with infrared photorefraction before and during the course of action of the drugs. Accommodative and disaccommodative dynamics were analyzed in terms of peak velocity as a function of amplitude, starting point, and end point. RESULTS: Pilocarpine caused a myopic shift in resting refraction of 11.62 +/- 1.17 D. Centrally stimulated accommodative amplitude was 10.08 +/- 1.15 D before pilocarpine and 0.68 +/- 0.29 D after pilocarpine. Changes were found in accommodative dynamics as a function of starting point and in disaccommodative dynamics as a function of amplitude and end point. Accommodative amplitude was 11.25 +/- 0.18 D before atropine administration and 0.52 +/- 0.11 D after atropine administration. Accommodative dynamics as a function of amplitude were not substantially altered during the course of pilocarpine-induced accommodation or atropine-induced cycloplegia. CONCLUSIONS: Accommodative response amplitude is reduced with pilocarpine by shifting the eye to a more myopic state and with atropine by cycloplegia. Pharmacologic manipulations showed that accommodative and disaccommodative dynamics in anesthetized monkeys depend on amplitude, starting point, and end point of the response and on the contributions of neural and receptor activity.     

青少年近视在不同阅读距离调节状态下眼前段结构的变化

目的分析和探讨不同程度青少年近视在不同阅读距离诱导的调节状态下眼前段结构的变化。方法收集我院屈光门诊14～18岁间青少年近视95例（95眼），正视眼34例（34眼）作对照，其中男66例，女63例。所有被试者行主觉验光、调节幅度测定，眼前节OCT测量在不同阅读距离诱导的调节状态下（OD、3D、6D）的前房深度、晶状体厚度及瞳孔直径，采用SPSS11．5forWindows行统计学分析。结果①在调节放松情况下，随着近视度数增加，暗瞳直径明显减小（P〈0．05），各组间晶状体厚度差异无显著性，前房深度差异有显著性（P〈0．05）。（D随着阅读距离的减小，调节运用的增加，各组前房深度变浅值、晶状体厚度变厚值、暗瞳直径变小值均显著增加（P〈0．01）。③随着近视度数增高，诱发3D调节时前房深度变浅值显著增加（P〈0．01），晶状体厚度变厚值明显增加（P〈0．05），暗瞳直径变小值差异无显著性。④随着近视度数增高，诱发6D调节时前房深度变浅值显著增加（P〈0．01），晶状体厚度变厚值明显增加（P〈0．05），暗瞳直径变小值显著增加（P〈0．01）。结论在青少年中，随近视程度加深，阅读距离减小，运用调节量的增加可引起眼前房显著变浅，晶状体明显变厚：暗瞳直径缩小值在诱发6D调节时随近视度数增高而增大。     

Slit-lamp studies of the rhesus monkey eye: II. Changes in crystalline lens shape, thickness and position during accommodation and aging

Changes in crystalline lens shape and axial thickness, anterior chamber depth and anterior cornea-posterior lens distance during accommodation induced by corneal iontophoresis of carbachol or electrical stimulation of the Edinger-Westphal nucleus were studied in 25 living, surgically aniridic rhesus monkey eyes, aged 1-25 years. Intraocular distances and anterior and posterior lens surface curvatures were evaluated from slit-lamp Scheimpflug photographs; distances were also determined by A-scan ultrasonography. With increasing accommodation, both lens surfaces become more sharply curved, the lens thickens and the anterior chamber shallows, while the posterior lens surface remains fixed relative to the cornea. Within statistical limits, the respective curvature and distance changes are the same for a given dioptric accommodation induced by either stimulation technique. The respective intraocular distance-accommodation relationships are identical whether derived from photographic or ultrasonographic measurements. Temporal and contralateral reproducibility of all measurements is excellent. Each parameter-accommodation relationship is strikingly linear in all eyes, although above 20 D the slopes of the lens surface curvature-accommodation relationships may have decreased. The curvature change per D of accommodation averages approximately 20% more for the posterior than for the anterior lens surface. There is relatively little interindividual variation in the slope of each relationship despite the significant interindividual differences in age and accommodative amplitude, indicating that the relationships are independent of age. However, when extrapolated back to the non-accommodated resting state, the data indicate that the lens thickens, both its surfaces become more sharply curved, and the anterior chamber shallows with age in adult greater than 5 years, while opposite trends are seen in younger animals.     

Edinger-Westphal and pharmacologically stimulated accommodative refractive changes and lens and ciliary process movements in rhesus monkeys

During accommodation, the refractive changes occur when the ciliary muscle contracts, releasing resting zonular tension and allowing the lens capsule to mold the lens into an accommodated form. This results in centripetal movement of the ciliary processes and lens edge. The goal of this study was to understand the relationship between accommodative refractive changes, ciliary process movements and lens edge movements during Edinger-Westphal (EW) and pharmacologically stimulated accommodation in adolescent rhesus monkeys. Experiments were performed on one eye each of three rhesus monkeys with permanent indwelling electrodes in the EW nucleus of the midbrain. EW stimulated accommodative refractive changes were measured with infrared photorefraction, and ciliary process and lens edge movements were measured with slit-lamp goniovideography on the temporal aspect of the eye. Images were recorded on the nasal aspect for one eye during EW stimulation. Image analysis was performed off-line at 30 Hz to determine refractive changes and ciliary body and lens edge movements during EW stimulated accommodation and after carbachol iontophoresis to determine drug induced accommodative movements. Maximum EW stimulated accommodation was 7.36+/-0.49 D and pharmacologically stimulated accommodation was 14.44+/-1.21 D. During EW stimulated accommodation, the ciliary processes and lens edge moved centripetally linearly by 0.030+/-0.001 mm/D and 0.027+/-0.001 mm/D, with a total movement of 0.219+/-0.034 mm and 0.189+/-0.023 mm, respectively. There was no significant nasal/temporal difference in ciliary process or lens edge movements. 30-40 min after pharmacologically stimulated accommodation, the ciliary processes moved centripetally a total of 0.411+/-0.048 mm, or 0.030+/-0.005 mm/D, and the lens edge moved centripetally 0.258+/-0.014 mm, or 0.019+/-0.003 mm/D. The peaks and valleys of the ciliary processes moved by similar amounts during both supramaximal EW and pharmacologically stimulated accommodation. In conclusion, this study shows, for the first time, that the ciliary processes and lens edge move centripetally, linearly with refraction during EW stimulated accommodation. During pharmacological stimulation, the ciliary processes move to a greater extent than the lens edge, confirming that in adolescent monkeys, lens movement limits the accommodative optical change in the eye.     

Accommodation and presbyopia in the human eye--aging of the anterior segment

Ocular biometric parameters and accommodative amplitude were measured by various techniques in 100 normal emmetropic human subjects age 18-70 yr. Anterior chamber depth decreased and lens thickness increased linearly over the entire age group. Accommodative amplitude declined linearly until a stable nadir was reached at about age 50 yr. The respective slopes and intercepts of the age-dependent decline in anterior chamber depth were essentially the same for measurements made independently by optical pachmetry, A-scan ultrasonography, and slit-lamp Scheimpflug photography. The age-dependent increase in lens thickness differed in slope and intercept for measurements made by photography and ultrasonography if the generally accepted lenticular sound velocity was assumed for all subjects. However, if putative lenticular sound velocity was adjusted for age, the relationships given by the two techniques were essentially identical. Total anterior segment length (defined as the distance between the anterior corneal and posterior lens surfaces), vitreous cavity length (distance between the posterior lens and anterior retinal surfaces), and total globe length were all independent of age. This constellation of findings indicates that the human lens grows throughout adult life while the globe does not, that thickening of the lens completely accounts for shallowing of the anterior chamber with age, but that the posterior surface of the lens remains fixed in position relative to the cornea and retina.     

Variability of the ocular component measurements in children using A-scan ultrasonography.

Knowledge of the precision with which the various ocular components can be measured with available techniques is vital to our ability to track changes in the anatomy of the eye in relation to the development of refractive error. Corneal touch A-scan ultrasound measurements were obtained by either an instrument-mounted or by a hand-held method on 469 children aged 6 to 11 years at their enrollment into the Correction of Myopia Evaluation Trial. Variability of measurements was calculated for overall axial length, anterior chamber depth, lens thickness, and vitreous chamber depth. The mean variability of overall axial length was 0.062 +/- 0.043 mm and 0.061 +/- 0.056 mm for the right and left eyes, respectively. Mean variability of the ocular component measures was similar. The mean variability was not influenced by the age, gender, level of myopia, or residual accommodation of the subjects. Statistically significant differences between the instrument-mounted and the hand-held method and among certain ethnic groups were found, but the differences were not of a magnitude to be clinically significant. A-scan ultrasonography is sensitive to changes in the axial length and vitreous chamber depth equivalent to 0.25 D and is therefore a useful technique to assess changes in these ocular components in children. The precision of lens thickness is poorer than the equivalent of 1.0 D, and, therefore, A-scan may not be sufficiently precise to be useful in studies of active accommodation or lens growth.