Biomedical subjects

A P Beers

Publications and source records attributed to A P Beers.

8 recordsLinked to original sources

PubMed1998

[Aging of the crystalline lens and presbyopia].

The crystalline lens changes both optically and mechanically with age: it becomes less transparent and more difficult to deform. Using a new ultrasound technique, Continuous Ultrasonographic Biometry (CUB), we were able to demonstrate that presbyopia is caused by increased stiffness of the lens due to changes in lens fibre membranes and cytoskeleton. These results are important for further research on the ageing processes that lead to cataract and presbyopia, and the treatment of these conditions.

Adolescent↗

PubMed1996

Microfluctuations of steady-state accommodation measured with ultrasonography.

Over the past 30 years investigators have noted fluctuations in accommodation when the eye views a stationary target. These microfluctuations have been studied mainly with the use of infrared optometers. Two dominant components have been found: a low frequency component (LFC; 0.05-0.5 Hz) and a high frequency component (HFC; 1.0-2.2 Hz). The LFC probably has a neurological origin. The HFC is supposed to be related to the arterial pulse. The aim of this study was to investigate accommodative fluctuations and gain more information about their origin by using a non-optical method. We used continuous ultrasonographic biometry to measure changes in anterior chamber (AC) depth, lens thickness and vitreous length during steady-state accommodation. Simultaneously, the electrocardiogram (ECG) was recorded. Changes in axial length were calculated offline by taking the sum of the changes in the three intraocular distances. Results show that fluctuations with an LFC were present in all three intraocular distances but not in the axial length, proving that only the lens was involved. In contrast with the findings of investigators using optical methods, no HFC was present in fluctuations of AC depth and lens thickness. However, a small HFC was found in registrations of the vitreous and axial length, which appeared to correspond with the heart rate.

Accommodation, Ocular↗

PubMed1996

Age-related changes in the accommodation mechanism.

Presbyopia is caused by changes in the visco-elastic properties of the lens or the choroid or both. Far-to-near (FN) accommodation dynamics are dominated by the properties of the lens, whereas near-to-far (NF) accommodation dynamics depend on the properties of the lens as well as the choroid. The aim of this study was to gain more insight into the cause of presbyopia by analyzing how FN and NF accommodation dynamics change as a function of age. Changes in axial lens thickness during accommodation were measured in vivo using continuous ultrasonographic biometry. The measurements were analyzed with the use of a biomechanical model of the mechanism of accommodation. Fitting the model to the responses yields time constants of FN and NF accommodation. The time constants FN and NF increase with age at an approximately equal rate: 7 ms/year (+/- 1.8 SEM) and 6 ms/year (+/- 1.6 SEM), respectively. This result supports a lenticular cause of presbyopia. In addition, estimations of the lens damping coefficients at different ages are given for the first time: the damping coefficient of the lens increases 20-fold between 15 and 55 years of age.

Accommodation, Ocular↗

PubMed1994

In vivo determination of the biomechanical properties of the component elements of the accommodation mechanism.

A method of obtaining in vivo information about the component elements of the accommodation mechanism is described. Changes in axial lens thickness during accommodation are measured continuously by ultrasonographic biometry. The measurements are fitted with a dynamic biomechanical model of the mechanism of accommodation. This gives a time constant that depends both on the direction of accommodation and on the properties of the lens, zonules and choroid. Two ratios can be calculated that give information about the visco-elastic properties of the lens, zonules and choroid. This information is especially useful to evaluate the changes that lead to presbyopia.

Accommodation, Ocular↗

PubMed1994

Presbyopia and velocity of sound in the lens.

The elastic properties of lens matter change with age and this contributes to presbyopia. The changes in elasticity of lens matter could be the result of a change in water and soluble proteins (1) or a change in lens fiber cytoskeleton and membranes (2). If it is caused by (1) then the velocity of sound in the lens should change with age. If it is caused by (2) the velocity of sound in the lens will not change. Using the technique of continuous ultrasonographic biometry, the velocity of sound in clear lenses was measured in vivo in a group of 24 healthy subjects aged 15 to 45 years with a visual acuity of 6/6 or better. In this group maximum accommodative amplitude decreased with age. It was found that, despite the occurrence of presbyopia, the velocity of sound did not change with age. Our results support the hypothesis that age-related changes in lens fiber cytoskeleton and membranes are responsible for the change in elastic properties of lens matter and thus contribute to presbyopia.

Accommodation, Ocular↗

PubMed1992

Visual evoked potential estimation of visual activity with a Laplacian derivation.

Visual acuity was estimated with visual evoked potentials in 13 healthy subjects. A Laplacian derivation of 5 electrodes was used to improve the signal-to-noise ratio and to enhance striate cortical activity selectively. In 6 subjects, the Laplacian derivation gave a more reliable estimation of visual acuity than did a single midline derivation. In the other 7 subjects, the quality of both estimations was comparable.

Electrodes↗

PubMed1992

The application of Laplacian analysis in the recording of half-field pattern-onset evoked potentials.

The Laplacian operator in electroencephalographic measurements consists of a mathematical combination of the responses from a number of electrodes (e.g., five in a crosswise montage). It enhances activity from sources lying underneath the area covered, relative to activity from outside this area. Thus, by appropriate positioning, the contributions of extrastriate and striate sources can be recorded selectively. To quantify the contribution of each hemisphere to half-field onset evoked potentials, the responses in two Laplacian operators, one over each hemisphere, were analyzed and compared to monopolar derivations and a bipolar derivation between the two hemispheres. Both the Laplacian and bipolar analyses were helpful in interpretation of the responses.

Electrodes↗

PubMedDate not supplied

Analysis of accommodation function with ultrasonography.

The aim of this study was to present a new method that uses ultrasonography to analyse accommodation function, and to do a preliminary investigation of its possible use in clinical practice. Using the method of continuous ultrasonographic biometry, changes in lens thickness were measured during accommodation. From these measurements response latency and duration were determined. Normal values for latency and duration were obtained by measuring 20 healthy subjects of different ages. Measurements were also performed on three patients with different accommodation disorders: diabetes, Adie's syndrome and third nerve palsy. Normal response latency is 394 ms (+/-46 SD) and independent of are. Normal response duration increases with age from an average 306 ms at 15 years-of-age to an average 954 ms at 55 years-of-age. Normal latency as well as duration appear to have a large interindividual variability. The diabetic patient had a delayed latency but a normal response duration. The patient with Adie's syndrome had a delayed latency and prolonged duration. The patient with third nerve palsy had a normal latency and duration. We conclude that ultrasonographically determine latency and duration give additional information on accommodation function that is more complete and objective than maximum accommodative amplitude alone. The results in our patients suggest that, in selected cases, this information may aid in the diagnosis and management of patient's complaints.

Accommodation, Ocular↗