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Biomedical subjects

P Mierdel

Publications and source records attributed to P Mierdel.

At least 19 recordsLinked to original sources
PubMed2001

Increased higher-order optical aberrations after laser refractive surgery: a problem of subclinical decentration.

PURPOSE: To study the clinical and theoretical effects of subclinical decentrations on the optical performance of the eye after photorefractive laser surgery. SETTING: Department of Ophthalmology, University of Dresden, Dresden, Germany. METHODS: Ocular aberrations were determined before and 1 month after uneventful photorefractive keratectomy (PRK) with the Multiscan laser (Schwind) in 10 eyes of 8 patients. The corrections ranged from -2.5 to -6.0 diopters, and ablation zones of 6.0 mm and larger were used. The measured wavefront errors were compared to numerical simulations using the individually determined decentrations and currently used ablation profiles. RESULTS: The PRK-induced aberrations were significantly greater than the preoperative aberrations. The numerically calculated increase in the higher-order optical aberrations correlated with the clinical results, demonstrating a major increase in coma- and spherical-like aberrations. Subclinical decentration (less than 1.0 mm) was found to be a major factor in increased coma-like and spherical-like aberrations after corneal laser surgery. CONCLUSION: To minimize higher-order optical errors, special efforts to center the ablation zone are necessary; for example, by eye-tracking systems that consider the visual axis.

Astigmatism↗
PubMed2001

Ocular optical aberrometer for clinical use.

Higher-order optical errors of the human eye are often responsible for reduced visual acuity in spite of an optimal spherical or cylindrical refraction. These optical aberrations are of natural origin or can result from operations in the eye that involve optical structures. The ocular aberrometer presented is based on Tscherning's aberroscope. A collimated laser beam (532 nm, 10 mW) illuminates a mask with a regular matrix of holes which forms a bundle of thin parallel rays of 0.3 mm diameter. These rays are focused by a lens in front of the eye so that their intraocular focus point is located a certain distance in front of the retina, generating a corresponding pattern of light spots on it. According to the existing ocular optical errors, this spot pattern is more or less distorted in comparison to the mask matrix. For a 6 mm pupil diameter 68 retinal spots are plottable for assessment of the optical aberrations. The retinal spot pattern is imaged onto the sensor of a low-light charge coupled device video camera by indirect ophthalmoscopy. Deviations of all spots from their ideal regular positions are measured by means of a PC, and from these values the intraocular wave front aberration is computed in the form of the sum of Zernike polynomials up to sixth order.

Eye↗
PubMed2000

Ocular optical aberrations after photorefractive keratectomy for myopia and myopic astigmatism.

OBJECTIVES: To study the effects of photorefractive keratectomy on ocular optical aberrations and to establish correlations with glare vision and low-contrast vision. METHODS: Preoperative ocular aberroscopy of 15 eyes undergoing photorefractive keratectomy was compared with aberroscopy at 3 months postoperatively by means of a newly developed automated aberroscope of the Tscherning type. The correlation of the wavefront errors with best spectacle-corrected visual acuity, low-contrast visual acuity, and visual acuity under glare conditions was analyzed. RESULTS: In any individual treated, the total wavefront error increased. On average, the total wavefront error increased by a factor of 17.65; this increase was highly statistically significant (P = .001). Also, the correlation with best-corrected visual acuity, low-contrast visual acuity, and glare visual acuity was statistically significant (P = .02, P = .001, and P = .03, respectively). The increase in ocular aberrations was significantly related with the virtual pupil size. CONCLUSIONS: Photorefractive keratectomy increases the ocular aberrations, impairing the visual performance of the eyes treated. In detail, scotopic visual measures such as low-contrast visual acuity and glare visual acuity suffer most from the myopia correction. Aberroscopy-guided photorefractive keratectomy may avoid such effects.

Adolescent↗
PubMed1999

Effects of photorefractive keratectomy and cataract surgery on ocular optical errors of higher order.

BACKGROUND: This pilot study was carried out to assess the effects of photorefractive keratectomy (PRK) for myopia and myopic astigmatism and cataract surgery on the ocular optical aberrations of higher degrees. METHODS: The optical aberrations were measured in 12 patients before and after PRK and in 10 patients after cataract surgery with a video aberroscope for clinical use (based on Tscherning's aberroscope) designed by the authors. To characterize the optical performance of the eye the deviation of the wavefront of a foveal image point from its ideal (spherical) shape (wavefront aberration) was determined. The wavefront aberration is represented mathematically in Zernike polynomials. The first 14 Zernike coefficients K(i) were determined and compared with data from normal eyes with full visual acuity. RESULTS: Most Zernike coefficients were considerably greater after PRK than before surgery. These changes differed significantly from the variability of repeated individual measurements. In particular, coefficients corresponding to astigmatism, spherical aberration or coma were highly significantly increased (P<0.001). After cataract surgery, the averaged Zernike coefficients exhibited no significant differences from normal values, except the coefficient K(5) (astigmatism at 0 degrees or 90 degrees ). However, coefficients showed a significant high variability, especially the coefficients for spherical aberration or astigmatism. CONCLUSION: Both PRK and cataract surgery are operations which may considerably increase the ocular optical errors of higher order. These aberrations are not predictable and can affect the visual acuity despite optimal sphero-cylindrical correction, in particular under mesopic conditions.

Adult↗
PubMed1997

[Measuring device for determining monochromatic aberration of the human eye].

BACKGROUND: After refractive or cataract surgery, ocular optical errors can occur that are not correctable with spherical or astigmatic lenses and are probably responsible for the fact that in many cases the best possible (retinal) acuity is not achieved in spite of an optimum refraction. Assessment of these aberrations in the clinical routine is an important first step towards documentation and correction of these errors with modern photorefractive methods. PRINCIPLE OF MEASURING: Ocular optical errors are assessed from the viewpoint of the wave property of light as an aberration of the real wavefront of a central retinal image point from the ideal spherical from (wavefront aberration). The measurement is based on the principle of the Tscherning aberroscope. RESULTS: A test setup is presented consisting of a laser system, a CCD fundus camera and a PC. A bundle of parallel equidistant rays is refracted by means of a lens in front of the eye producing an equivalent pattern of light spots on the retina. This pattern is more or less distorted according to the ocular aberrations. The deviations of all spots from their ideal (equidistant) positions are measured by means of the PC and from these values the wavefront aberration is computed in the form of Zernike and Taylor polynomials. The first results of emmetropic eyes are presented. CONCLUSIONS: The method allows sufficiently accurate assessment of the ocular wavefront aberration and might be on principle suitable for clinical use, provided that some technical improvements are installed.

Adult↗
PubMed1997

[Laser-induced pressure waves in the eye. Propagation characteristics].

BACKGROUND: The characteristics of shock waves during photoablation were investigated for an IR and a UV laser. These stress waves may be harmful to ocular structures. MATERIAL AND METHODS: The amplitude of shock waves was measured by a needle-shaped hydrophone in enucleated porcine eyes during excimer laser (193 nm, 23 ns, diameter of ablation 1.5-7.5 mm) and Er:YAG laser photoablation (2.94 microns, 200 microseconds, 1.2 mJ/cm2, diameter of ablation 4 mm). RESULTS: With the excimer laser at ablation zones larger than 4.5 mm, a pressure focus occurs at a distance of 4-6 mm behind the cornea. The pressure amplitudes are smaller than 80 bar for a fluence of 180 mJ/cm2 and decrease steadily to values below 10 bar towards the retinal level. Higher fluences produce higher pressure values; in the range of 60 to 220 mJ/cm2 the relation is linear. For the Er:YAG laser, pressure amplitudes are smaller than 0.5 bar. CONCLUSIONS: Mechanical damage of the retina is unlikely during excimer-or Er:YAG-laser ablation. The existence of a pressure focus may result in mechanical damages of the posterior lens or anterior vitreous at large ablation diameters. During Er:YAG laser ablation, shock waves could not be detected with our measurements. Theoretical estimations yield values of less than 700 mbar at a fluence of 1.2 J/cm2. The pressure load of the endothelium is independent of diameter but dependent on fluence.

Animals↗
PubMed1992

Cortical potentials to pattern reversal and luminance onset under ramp stimulation conditions in glaucoma.

Stimulus quantities with a ramp-like temporal course evoke cortical potentials that are delayed and attenuated in comparison with responses evoked by a step-like course. This delay probably depends on temporal transfer characteristics of the activated precortical visual system. In a pilot study, luminance onset and pattern-reversal stimulation using this technique (light-emitting diode stimulator; transition time, 50 ms) were applied to 18 glaucomatous eyes with early visual field damage. The visual evoked cortical potential latency shifts between ramp and step mode were compared with visual field indices (mean defect, corrected loss variance). The pattern-reversal stimulation in ramp mode yielded a significant difference between the glaucoma group and normal subjects (p less than 0.004). The difference between groups with pattern-reversal stimulation in step mode was not significant (p = 0.15). From the determined visual field indices, only the mean defect showed a significant correlation to the measured visual evoked cortical potential latency shift, with both luminance-onset and pattern-reversal stimulation. In glaucoma with early malfunction, the precortical transmission and processing of lower and middle temporal frequencies, which are predominant in ramp-like temporal stimulus courses, are obviously more affected than the transmission of higher frequencies. The ramp stimulation technique for visual evoked cortical potentials might be of interest as a tool to detect early and specific functional defects in glaucoma.

Chronic Disease↗
PubMed1992

The pattern ERG in glaucoma: effect of pattern reversal time.

Pattern reversals with a ramp-like temporal course evoke transient pattern electroretinograms (PERG) that are delayed and attenuated in comparison with responses evoked by a step-like course (abrupt pattern reversals). This delay depends on the reversal time and probably represents a measure for temporal characteristics of the activated retinal structures. A pattern reversal stimulator with adjustable reversal periods (matrix of 8 x 14 rectangular red light emitting diodes, element size 2 degrees x 1 degree, stimulus area 16 degrees x 14 degrees) was used to record transient PERGs from 20 glaucomatous eyes with early or moderate visual field damages (measured with computer perimetry). The q-wave (P1) amplitude difference between the glaucoma group and normal subjects with step-like reversal stimulation was not significant (P = 0.23). On the other hand, a highly significant separation (P < 0.0001) between both groups was possible using the P1 latency at ramp-like stimulation or the response latency shift between step and ramp-like stimulation (Pattern reversal time, 30 ms).

Chronic Disease↗
PubMed1990

[Measuring contrast sensitivity using visual acuity tests in retinal and optic nerve diseases].

The luminance contrast needed to discern various test types was measured with monochromatic and achromatic light to detect discrete functional deficiencies of the retina and optic nerve in cases of normal visual acuity. Landolt rings corresponding to visual acuity levels from 0.04 to 1.0 were used as test types. A significant increase in the necessary minimum contrast was detectable with blue test light on large Landolt rings in patients with diabetic retinopathy, ocular hypertension and glaucoma and with green or yellow test light on medium-sized and small Landolt rings in patients with central serous chorioidopathy and optic atrophy. The additional contrast needed to reach the maximum visual acuity amounts to 14-100% compared with normal visual acuity, depending on the color of the test light and the diagnosis. The amount of contrast needed is greatest in retinal diseases, and it is therefore possible to a certain extent to distinguish these from diseases of the optic nerve.

Humans↗
PubMed1989

A model for the determination of VECP latencies.

The time between stimulus event and onset of the visually evoked cortical potential (VECP) reflects the dynamic transmission characteristics of the precortical part of the visual system. A simple mathematical model describing this sub-system with regard to the triggering of luminance VECP is presented. The four functional parameters of the model can be calculated from the measured latency versus stimulus intensity curve of luminance VECP and can provide a quantitative evaluation of the functional state of the precortical visual system for clinical use. Some experiments and first clinical results show the applicability of the model.

Evoked Potentials, Visual↗
PubMed1983

Quantitative evaluation of pallor-disc ratio and colour of the optic disc by a photographic method.

A photographic technique is described, which is based on simultaneous photography of the papilla with green and red filters by means of a stereoscopic fundus camera. Using black and white film, the negatives are photometrically measured and the pallor-disc ratio and colour of the papilla are calculated from the densitometric tracings. The latter parameter is quantitatively expressed by a numerical index. The method is demonstrated in a practical example.

Adult↗
PubMed1980

[The functional examination of the visual system by means of delayed triggered VECP (author's transl)].

The VECP represents a brain reaction evoked by fairly abrupt visually perceived changes of one or several environmental features. If the stimulus consists of a finite temporal luminance rise, then the VECP is triggered later as compared to an abruptly appearing stimulus. This delay is an expression of the dynamic input properties of a VECP trigger mechanism. This system evaluates sudden changes of luminance slope and the temporal luminance integral in the first 50 ms after commencement of the stimulus. It may be assumed that deficiencies of the processing of visual information are manifested especially and first of all in a change of the dynamic trigger conditions of VECP. This is shown for colour stimulation (under isolation of the green and red colour mechanism) in different deficiencies of the visual system.

Cerebral Cortex↗
PubMed1978

Objective detection of reversible deficiencies in glaucoma by means of BECP.

The dynamic properties of the VECP trigger mechanism are used as a functional test for the glaucomatous eye. A light-emitting diode is used as stimulator. The test was performed under the condition of the functional isolation of the red colour mechanism (by means of selective colour adaptation technique). In any case, a delayed rise of stimulus luminance causes a delay of the VECP. This delay increases in patients with glaucoma. After a successful glaucoma operation the VECP delay diminishes again.

Color Perception Tests↗