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Biomedical subjects

G L van der Heijde

Publications and source records attributed to G L van der Heijde.

At least 19 recordsLinked to original sources
PubMed2006

Estimating the external force acting on the human eye lens during accommodation by finite element modelling.

Using new geometric information on the shape of the lens that has recently become available, a finite element model has been developed in order to estimate the forces that act on the lens during accommodation for a typical 29-year-old human eye. To investigate the influence of the anterior, posterior and central zonular fibres insertion regions, three models with different configurations were built. All three configurations appeared to be capable of inducing the required accommodative changes in the lens. Based on material properties from the literature, the estimated summed net force for each of the three models was approximately 0.08 N.

Accommodation, Ocular↗
PubMed2001

Histopathologic ocular examination after lensectomy in a Macacus rhesus monkey, an unexpected event of secondary glaucoma.

In a Macacus rhesus monkey an unilateral lensectomy was performed on the 15th day of life to study the emmetropization proces. During a period of 20.5 months refractional state, axial length, corneal dioptric power and intraocular pressure were measured in the aphakic and fellow eye. Results showed that axial elongation of the aphakic eye was larger in comparison to the phakic fellow eye. Histopathologic examination of the aphakic eye revealed glaucomatous changes due to obstruction in the anterior chamber angle.

Animals↗
PubMed2001

The thickness of the aging human lens obtained from corrected Scheimpflug images.

Commonly, measurements of lens thickness are performed using A-scan ultrasonography or slitlamp Scheimpflug photography. Both techniques have their drawbacks in the study of presbyopia: ultrasonography requires the velocity of sound in the lens which may change with age, whereas Scheimpflug photography requires knowing the refractive index of the lens to enable correction of the photographs for the distortion due to the refraction of the cornea and lens. By combining Scheimpflug photography and axial optical eye-length measurements, we were able to individually correct the Scheimpflug images for distortion and calculate the refractive index and thickness of the human lens. Lens thickness of 90 subjects ranging in age between 16 and 65 years was measured, and an average increase of 24 microm/year was found. This value is consistent with ultrasonographic measurements assuming an age-independent velocity of sound in the lens of 1641 m/s. The posterior lens surface recedes from the cornea with age, and this backward movement does not differ significantly from the forward movement of the anterior lens surface.

Adolescent↗
PubMed2001

Involvement of the peripheral nervous system in primary Sjögren's syndrome.

BACKGROUND: Involvement of the peripheral nervous system in patients with primary Sjögren's syndrome (SS) has been reported, but its prevalence in neurologically asymptomatic patients is not well known. OBJECTIVE: To assess clinical and neurophysiological features of the peripheral nervous system in patients with primary SS. PATIENTS AND METHODS: 39 (38 female) consecutive patients with primary SS, aged 20-81 years (mean 50), with a disease duration of 1-30 years (mean 8) were studied. The peripheral nervous system was evaluated by a questionnaire, physical examination, quantified sensory neurological examination, and neurophysiological measurements (nerve conduction studies). To assess autonomic cardiovascular function an orthostatic challenge test, a Valsalva manoeuvre, a forced respiration test, and pupillography were done. RESULTS: Abnormalities as indicated in the questionnaire were found in 8/39 (21%) patients, while an abnormal neurological examination was found in 7/39 (18%) patients. Abnormalities in quantified sensory neurological examination were found in 22/38 (58%) patients. In 9/39 (23%) patients, neurophysiological signs compatible with a sensory polyneuropathy were found. No differences were found in the autonomic test results, disease duration, serological parameters, or erythrocyte sedimentation rate between the patients with primary SS with and those without evidence of peripheral nervous involvement. CONCLUSION: Subclinical abnormalities of the peripheral nervous system may occur in patients with primary SS selected from a department of rheumatology, but clinically relevant involvement of the peripheral nervous system in this patient group is rare.

Adult↗
PubMed2001

[Improvement of vision in nursing home patients through eyeglasses].

Adequate refraction correction may contribute to the quality of life of elderly persons who will be less dependent on care in daily life and will be less prone to fall. In nearly 20% of 102 nursing home residents binocular visual acuity improved with at least one line on the Snellen Chart by adjustment of refractive correction.

Accidental Falls↗
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↗
PubMed1996

Parasympathetic dysfunction in rheumatoid arthritis patients with ocular dryness.

OBJECTIVE: To determine whether abnormalities in the function of the autonomic nervous system are associated with oral and ocular dryness in rheumatoid arthritis. METHODS: Pupillography was done using an infrared light reflection method (IRIS) to measure both parasympathetic function (constriction latency and the latency of maximum constriction velocity (MCV)) and sympathetic function (dilatation latency) in rheumatoid arthritis patients with and without ocular dryness. The Schirmer and Saxon tests were used to measure the tear and saliva production respectively. RESULTS: The Schirmer and Saxon test results in rheumatoid arthritis patients with ocular dryness were reduced (P < 0.05) compared with rheumatoid arthritis patients without ocular dryness and healthy controls. Constriction latency and MCV latency were prolonged in rheumatoid arthritis patients with ocular dryness compared to the other two groups (P < 0.05). A negative correlation was found between the degree of ocular dryness and both constriction latency and MCV latency. No correlation was found between the results of pupillography and saliva production. CONCLUSIONS: Parasympathetic dysfunction may play a role in ocular dryness in patients with rheumatoid arthritis.

Adult↗
PubMed1989

The correction of myopia by lens implantation into phakic eyes.

We corrected myopia in 62 phakic eyes by fixing a concave intraocular lens to the midperiphery of the iris in the anterior chamber. No inflammation, glaucoma, corneal edema, or intraocular lens induced-cataract has occurred. The lens did not touch the corneal endothelium. The achieved correction was within 10% in 47 of the 62 eyes (76%) and within 20% of the expected correction in all eyes.

Adult↗
PubMed1989

Effect of timolol on the amplitude and dynamics of accommodation.

There is still debate about whether the ciliary muscle is innervated by the sympathetic nervous system. We investigated the amplitude and the dynamics of accommodation under influence of the non-selective beta-blocker timolol. For this purpose the variations in thickness of the human lens during step-like changes in accommodation were measured with high-resolution A-scan echography. Results showed that the dynamics of accommodation, expressed in the time constants of the response, were affected as well as the amplitude.

Accommodation, Ocular↗
PubMed1989

Accommodation used to determine ultrasound velocity in the human lens.

Intraocular distances can be measured by echography. In this determination the velocity of sound in the various media is assumed to be constant and equal to the velocity found by in vitro measurements. However, interindividual variability causes an inaccurate determination of these distances. We present a method to determine the velocity of ultrasound in the lens "in vivo." For this purpose continuous registrations of the transit times for the anterior chamber, lens, and vitreous were made during accommodation. Regression analysis of the variation of transit times provides the velocity of ultrasound in the lens with an accuracy of about +/- 3 m/s.

Accommodation, Ocular↗
PubMed1988

[Intraocular lens for the correction of myopia of the phakic eye].

A biconcave intraocular lens for the correction of myopia is described and preliminary results are presented. An iris claw-type lens is used as a carrier for the biconcave lens. This lens is fixed in front of the natural lens in the mid-periphery of the iris. It maintains a sufficient distance from both the natural lens and the corneal endothelium. The surgical technique is described in detail and the indications and contraindications are discussed. The authors' results so far are encouraging.

Follow-Up Studies↗
PubMed1988

[Optical consequences of implantation of a negative intraocular lens in myopic patients].

The implantation of a concave intraocular lens (IOL) (Worst-Fechner Biconcave Lens) is a new method of correcting myopia. This paper describes how to compute the effective power of an IOL on the basis of corneal curvature and anterior chamber depth, i.e., which lens will neutralize a specific myopia. The authors also discuss the size of the retinal image of an eye corrected by an intraocular lens as compared to an eye corrected by a spectacle glass.

Humans↗
PubMed1987

Are eye movement evoked potentials different from pattern reversal evoked potentials?

The averaged lambda wave elicited by saccadic eye movements across a checkerboard pattern has been reported to differ from checkerboard reversal evoked responses, even when the electroencephalographic responses were corrected for the artefact caused by the movement of the dipole moment of the eye itself. Because of these differences it was suggested that the recording of the lambda wave might provide extra information in pathological circumstances. We performed experiments in which the parameters of the pattern (high contrast checkerboard pattern, 20' checks, large field 72 x 72 degrees) shift across the retina were carefully adjusted. For instance, eye movements were made across an integer and odd number of checks in order to mimic the pattern reversal. Furthermore, the timing of the pattern movement in the pattern reversal condition was adjusted so as to simulate the saccadic eye movement. The results seem to suggest that the reported dissimilarities between pattern reversal and eye movement evoked responses can be accounted for by the small differences of the retinal shift in the two conditions.

Evoked Potentials↗