Accommodation dynamics as a function of age

Alterations in Lens Free Water Distribution Are Associated with Shape Deformation in Accommodation

Objective

To investigate whether a redistribution of water within the crystalline lens is associated with the shape deformation that occurs during accommodation.

Design

Observational, cross sectional study.

Subjects

Eleven young adults without presbyopia (aged 18-39 years) and 9 middle-aged adults with presbyopia (aged 40-55 years).

Methods

Magnetic resonance imaging (MRI) scans of the lens were acquired on a 3 Tesla clinical MRI scanner, without and with the presentation of a 3 Diopter accommodative stimulus. The MRIs were postprocessed using established methods to extract the geometric dimensions and spatial maps of water distribution of the lens.

Main Outcome Measures

Accommodative changes in the full 3-dimensional description of lens shape, the lens total-water distribution profile, and the lens free-water distribution profile.

Results

Viewing of an accommodative stimulus by young subjects elicited an elastic shape deformation of the lens consistent with accommodation that was associated with an elevated, smoother free-water distribution, primarily in the anterior region of the lens. In contrast, viewing of an accommodative stimulus by presbyopic subjects produced an atypical shape deformation of the lens that was instead associated with a lowered free-water distribution, primarily in the anterior region of the lens. No discernible changes to the lens total-water distribution were observed in response to the accommodative stimulus in either subject cohort.

Conclusions

The present study suggests that protein-mediated alterations in the free-water distribution of the anterior region of the lens influence the shape deformation in accommodation, presenting pharmacological modulation of free-water distribution as an attractive novel approach for treating presbyopia.

Financial Disclosures

The authors have no proprietary or commercial interest in any materials discussed in this article.

Predictability of sinusoidally moving stimuli does not improve the accuracy of the accommodative response

Previous research work suggests that predictable target motion such as sinusoidal movement can be anticipated by the visual system, thereby improving the accommodative response. The validity of predictable motion for studying human dynamic accommodation is sometimes put into question. The aim of this work was to assess the effect of anticipation along with learning (and motivation, etc.) and fatigue (and boredom, loss of attention, etc.) on dynamic accommodation experiments from a practical perspective. Specifically, changes in amplitude and temporal phase lag were estimated within and between trials as 9 adult observers were instructed to focus on a stimulus that oscillated sinusoidally towards and away from the eye at specific temporal frequencies. On average, amplitude decreased whereas phase increased within trials. No evidence of anticipation or learning was observed either within or between trials. Fatigue consistently dominated anticipation and learning within the course of each trial. Even if the eye is equipped by a prediction operator as it is often assumed, fatigue confounds the results from dynamic accommodation experiments more than anticipation or learning.

Validation of Novel Metrics from the Accommodative Dynamic Profile

Objective and subjective methods of assessing time taken for accommodative change (ToAC) include accommodative dynamics (AD) and accommodative facility (AF). This study investigates the validity of novel metrics derived from the AD-profile and explores their relationship with AF. AD were assessed using a modified open-field autorefractor in 43 healthy adults. Non-linear regression curves were fitted to the data to derive: latency-of-accommodation (nLoA) and -disaccomodation (nLoD), Time-for-accommodation (ToA) and -disaccommodation (ToD), and objective-ToAC (oToAC). Latencies were also calculated through visual inspection of the AD data as in previous studies (pLoA and pLoD). AF was used to assess subjective-ToAC. Statistical analysis explored the relationships between the AD-metrics and AF. Subjects were assessed on three visits to examine intra- and inter-observer repeatability. nLoA and nLoD were greater than pLoA (p = 0.001) and pLoD (p = 0.004) respectively. nLoA and nLoD also demonstrated greater intra- and inter-observer repeatability than pLoA and pLoD. AF demonstrated a moderate, inverse correlation with ToA (p = 0.02), ToD (p = 0.007), and oToAC (p = 0.007). ToD was the single best accommodative predictor of AF (p = 0.011). The novel method for deriving latency was more repeatable, but not interchangeable with the techniques used in previous studies. ToD was the most repeatable metric with the greatest association with AF.

Dynamic imaging of accommodation by swept-source anterior segment optical coherence tomography

PURPOSE

To study the accommodation process in normal eyes using a commercially available clinical system based on swept-source anterior segment optical coherence tomography (AS-OCT).

SETTING

Ophthalmology Department, University of Parma, Italy.

DESIGN

Evaluation of diagnostic technology.

METHODS

Right eyes were analyzed using swept-source AS-OCT (Casia SS-1000). The optical vergence of the internal coaxial fixation target was adjusted during imaging to obtain monocular accommodation stimuli with different amplitudes (0, 3.0, 6.0, and 9.0 diopters [D]). Overlapping of real and conjugate OCT images enabled imaging of all the anterior segment optical surfaces in a single frame. Central corneal thickness (CCT), anterior chamber depth (ACD), and lens thickness were extracted from the OCT scans acquired at different static accommodation stimulus amplitudes. The crystalline lens was analyzed dynamically during accommodation and disaccommodation by acquiring sequential OCT images of the anterior segment at a rate of 8 frames per second. The lens thickness was extracted from the temporal sequence of OCT images and plotted as a function of time.

RESULTS

The study analyzed 14 eyes of 14 subjects aged 18 to 46 years. During accommodation, the decrease in the ACD was statistically significant (P < .05), as were the increase in the lens thickness (P < .001) and the slight movement forward of the lens central point (P < .01). The CCT and anterior chamber width measurements did not change statistically significantly during accommodation. The lens thickness at 0 D was positively correlated with age (P < .01).

CONCLUSION

High-resolution real-time imaging and biometry of the accommodating anterior segment can be effectively performed using a commercially available swept-source AS-OCT clinical device.

FINANCIAL DISCLOSURE

No author has a financial or proprietary interest in any material or method mentioned.

Effects of age on dynamic accommodation

Visual accommodation plays a critical role in one's visual perception and activities of daily living. Age-related accommodation loss poses an increased risk to older adults' safety and independence. Although extensive effort has been made towards understanding the effect of age on steady-state accommodation, dynamic aspects of accommodation is still unknown. A study was therefore conducted to investigate age-related dynamic accommodative characteristics utilising a modified autorefractor. Ten individuals from each of three age groups (i.e. younger group: 20 to 29 years old; middle-aged group: 40 to 49 years old; older group: 60 to 69 years old) were recruited and their dynamic accommodation responses were examined. The laboratory experiment was designed to assess dynamic accommodation associated with an abrupt change from a constant far target (400 cm, 50 cd/m(2)) to a near target (70 cm, 100 cd/m(2) or 20 cd/m(2)), which aimed to simulate car dashboard reading behaviour while driving. The results of the study indicated that age and target intensity both had a significant impact on dynamic accommodation. These effects were attributed to both the age-related physiological limitation of the eye as well as to central neural processing delay. A method of measuring dynamic accommodation and the implications of the study are discussed. STATEMENT OF RELEVANCE: The results of the study indicate that age and target intensity both have a significant impact on dynamic accommodation. These effects are attributed to age-related physiological limitation of the eye as well as central neural processing delay and to decreased sensitivity of the cone photoreceptors. To enhance the visual performance of the ageing population involving dynamic accommodation, target distance and target light intensity should be carefully evaluated to facilitate effective viewing.

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.

Age related changes in accommodative dynamics in humans

Age related changes in the dynamics of accommodation (far to near focus) and disaccommodation (near to far focus) are reported in this study. Dynamic responses to step stimulus demands from 1D to 6D, in 1D steps, were recorded with a PowerRefractor in 66 subjects in the age range 14-45 years. The accommodative and disaccommodative responses were fit with exponential functions to calculate response amplitude, time constant and peak velocity. The latency of accommodation did not change and the latency of disaccommodation increased with age. For accommodation, time constant increased and peak velocity decreased with age. For disaccommodation, no change in time constant or peak velocity was found with age. The form of the peak velocity vs response amplitude relationship (main sequence) of accommodation changed with age. The differences in the dynamics of accommodation and disaccommodation with age are discussed with reference to the age related changes in the eye leading to presbyopia.

Dynamic mechanical properties of human lenses

The purpose of this study was to determine the shear compliance of human crystalline lenses as a function of age and frequency. Dynamic mechanical analysis was performed on 39 human lenses, ranging in age from 18 to 90 years, within the frequency range of 0.001-30 Hz. The lenses were stored at -70 degrees C before being measured. The influence of freezing on the mechanical properties was determined using pairs of porcine lenses, with one lens measured directly after enucleation and the other after freezing. The measurement method had a repeatability standard deviation of 4 and 6% for the storage and loss compliance, respectively. The reproducibility standard deviation was 31 and 33% for the storage and loss compliance respectively. On average, freezing increased the storage compliance by 8% and increased the loss compliance by 32%, both depending slightly on age and frequency. The human lenses exhibited a distinct viscoelastic behavior. The storage and loss compliance depended strongly on age and decreased a factor 1000 over a lifetime. Dynamic mechanical analysis has proven to be a successful technique for characterizing the mechanical properties of the human crystalline lens. The shear compliance decreases exponentially with age.

A pulse-step model of accommodation dynamics in the aging eye

We have developed a dynamic model of accommodation that combines independent phasic-velocity and tonic-position neural signals to control position, velocity and acceleration properties of accommodative step responses. Phasic and tonic signals were obtained from neural integration of a fixed-height acceleration-pulse and variable-height velocity-step respectively to control independent acceleration and velocity properties of the step response. Duration and amplitude of the acceleration-pulse are increased with age to compensate for age-related increases of visco-elastic properties of the lens to maintain youthful velocity. The model illustrates a neural control strategy that is similar to the classical neural control model of step changes by the saccadic and vergence systems.

Accommodation as a function of age and the linearity of the response dynamics

The changes with age in the accommodation responses to dynamic stimuli can reveal useful information on the factors underlying presbyopia development. Analysis of the monocular accommodation responses of 19 normal observers (ages 18-49 years) to stimuli whose vergence varied sinusoidally with time at different temporal frequencies (peak-to-peak stimulus 1.33-2.38 D, at 0.05-1.00 Hz) showed that at all ages both the gain and phase of the response were essentially linear functions of the temporal frequency. Extrapolation of least-squares, regression line fits to the gain data for each subject gave the gain at zero frequency, G0, and the cut-off frequency, fc, at which the gain fell to zero. G0 reduced with age but fc remained essentially constant at about 1.7 Hz, up to at least the age of about 40. The magnitude of the response to step stimuli covering the same stimulus range was well correlated with the value of G0. The linear changes in phase lag with temporal frequency corresponded to simple time delays td. The time lag varied from close to zero for the youngest subjects to about 0.5 s for the subjects in their late forties. There was substantial variation between the responses of subjects of similar age: those subjects with high values of G0 also tended to have low values of td, both effects probably being due to the superior ability of some individuals to predict the sinusoidal changes in the accommodation stimulus. Comparison of theoretical step responses, derived by applying linear theory to the parameters obtained from the results for the sinusoidal stimuli, with the actual responses to unpredictable steps for the same subjects supports the view that prediction effects and other possible factors make linear theory inapplicable to this type of data. The results are discussed in the context of current ideas on the development of presbyopia: it is suggested that the constancy of fc with age is the result of the ciliary body maintaining its efficiency, whereas the fall in G0 and increase in td result from increases in lens rigidity.