Biomedical subjects

Andreas G Boehm

Publications and source records attributed to Andreas G Boehm.

10 recordsLinked to original sources

PubMed2006

Effect of central corneal thickness, corneal curvature, and axial length on applanation tonometry.

OBJECTIVE: To evaluate the effect of central corneal thickness (CCT), corneal curvature, and axial length on applanation tonometry in an in vivo study. METHODS: In a masked, prospective clinical trial, we examined 125 eyes of 125 patients scheduled for cataract surgery. Corneal curvature was measured by means of keratometry and axial length by A-scan ultrasonography. By cannulating the anterior chamber before surgery, intraocular pressure (IOP) was set to 20, 35, and 50 mm Hg in a closed system by means of a water column. After measuring thickness, the IOP was measured with an applanation tonometer. Pearson product moment correlations and multiple linear regression analyses were performed, and significance levels were evaluated by the paired, 2-tailed t test. RESULTS: The difference between measured and real IOP was significantly dependent (P < .001) on CCT. The associations between IOP and corneal curvature or IOP and axial length were not statistically significant (P = .31). The association between IOP reading and CCT is shown in the "Dresdner correction table," which illustrates an approximately 1-mm Hg correction for every 25-microm deviation from a CCT of 550 microm. The correction values were positive as thickness decreased and negative as thickness increased. CONCLUSIONS: Central corneal thickness significantly affects IOP readings obtained by applanation tonometry according to the Goldmann principle. A correction of IOP readings by considering CCT according to the Dresdner correction table might be helpful for determining an accurate IOP value.

Adolescent↗

PubMed2006

A correction formula for the real intraocular pressure after LASIK for the correction of myopic astigmatism.

PURPOSE: To create a correction formula to determine the real intraocular pressure (IOP) after LASIK considering the altered corneal thickness, corneal curvature, and corneal stability. METHODS: This prospective clinical trial comprised 101 eyes of 59 patients (34 women and 25 men) that underwent LASIK with a mean preoperative spherical equivalent refraction of -6.3 +/- 2.17 diopters (D) (-3.0 to -11.5 D). Mean patient age was 32 +/- 9 years. Preoperatively and 6 months postoperatively, IOP (by Goldmann applanation tonometry), keratometry (by topography), and central corneal thickness (CCT) (by ultrasound pachymetry) were evaluated. These parameters were measured in all patients between 8 and 11 o'clock in the morning. RESULTS: Due to the LASIK procedure, IOP was reduced from 16.5 +/- 2.1 mmHg (range: 12 to 22 mmHg) to 12.9 +/- 1.9 mmHg (range: 8 to 16 mmHg). Multiple linear regression analysis of the IOP values before and after LASIK showed a significant correlation between the measured IOP and CCT and keratometry values (R2=0.631; P<.001). After LASIK, the biomechanical bending strength of the cornea is reduced by the cut so that the measured IOP must be additionally corrected by 0.75 mmHg. An equation containing all three changes is given: IOP (real) = IOP (measured) + (540-CCT)/71 + (43-K-value)/2.7 + 0.75 mmHg. CONCLUSIONS: Intraocular pressure measurements after LASIK for the correction of myopia are inaccurate as a consequence of changes in CCT, corneal curvature, and corneal flap stability. After LASIK, the measured IOP should be corrected to avoid false low IOP applanation readings.

Adult↗

PubMed2005

The influence of various substances on the biomechanical behavior of lamina cribrosa and peripapillary sclera.

PURPOSE: Changes in the biomechanical properties of the lamina cribrosa (LC) and of the peripapillary sclera (ppSc) may play a role in the pathogenesis of glaucoma. The purpose of this study was to assess the influence of glyceraldehyde, methylglyoxal, and collagenase A on the mechanical properties of the LC and ppSc. METHODS: Two strips of 1-mm width were cut from each of 80 porcine eyes and 24 pairs of enucleated human eyes. One strip contained the LC and the other the adjacent superior ppSc. One half of the strips was divided into groups and treated with 0.5 M glyceraldehyde, 0.5 M methylglyoxal, and 0.1% collagenase A. The other strips served as the control. The stress strain relation was measured in the stress range of 0.02 to 6.0 MPa by a biomaterial tester. RESULTS: Stress values at 20% strain of the human LC changed from 1.97 +/- 1.48 to 3.40 +/- 1.60 MPa after incubation with glyceraldehyde (P = 0.029), from 2.42 +/- 2.22 to 5.46 +/- 1.91 MPa (P = 0.014) after incubation with methylglyoxal, and from 2.43 +/- 1.3 to 1.35 +/- 0.19 MPa after incubation with collagenase A. The stress values of human ppSc without glyceraldehyde were 3.40 +/- 2.59 and 7.45 +/- 4.46 MPa after incubation with glyceraldehyde (P = 0.047), 4.80 +/- 3.05 MPa without methylglyoxal and 16.10 +/- 5.53 MPa (P = 0.001) after incubation with methylglyoxal, 4.14 +/- 2.56 MPa without collagenase A, and 1.97 +/- 0.55 MPa after incubation with collagenase A. At a 20% strain, Young's moduli of the untreated LC were in the range of E = 11.8 to 15.6 MPa and E = 28.5 to 36.0 MPa of the untreated ppSc. CONCLUSIONS: Glyceraldehyde and methylglyoxal increase the stiffness of the LC and of the ppSc in human and in porcine eyes. These substances induce changes in the extracellular matrix according to the Maillard reaction as it occurs during the ageing process or in case of high blood glucose levels. Collagenase reduces the stiffness of the tissues.

Animals↗

PubMed2005

The effect of age on optic nerve head blood flow.

PURPOSE: To examine whether optic nerve head blood flow changes with aging. METHODS: One randomly chosen eye of each of 103 healthy subjects (age range, 22-76 years) was examined by laser Doppler flowmetry. Relative capillary blood flow, velocity, and volume of the moving red blood cells were measured at the temporal and nasal neuroretinal rim of the optic nerve head. For statistical analysis, linear regression analysis and partial correlations were calculated. RESULTS: Velocity increased with age. The Pearson correlation coefficient (R) between age and velocity was 0.49 temporally and 0.56 nasally. The correlation was significantly different from 0 (both P < 0.0005). Volume decreased with increasing age. The Pearson correlation coefficient between age and volume was -0.47 temporally and -0.40 nasally. The correlation was significantly different from 0 (both P < 0.0005). Flow decreased with increasing age. The Pearson correlation coefficient between age and flow was -0.27 temporally and -0.31 nasally. The correlations were significantly different from 0 (P = 0.006 and P = 0.002, respectively). Partial correlation coefficients between the perfusion parameters and age, after correcting for systolic and diastolic blood pressure, heart rate, gender, and IOP, were similar compared to the Pearson correlation coefficients and remained statistically significant. CONCLUSIONS: The perfusion of the optic nerve head is altered with increasing age. The results suggest that the blood supply is reduced in elderly subjects.

Adult↗

PubMed2002

Visual function and perfusion of the optic nerve head after application of centrally acting calcium-channel blockers.

BACKGROUND: In a previous study it was shown that nimodipine 30 mg twice daily leads to an improvement in the visual field in a subgroup of normal-pressure glaucoma patients. To understand the mechanism of action of nimodipine on the visual system, the aim of this study was to examine the influence of nimodipine on different hemodynamic parameters and contrast sensitivity in healthy subjects. METHODS: Thirty-two healthy subjects (21-49 years old, mean age 28 years, 10 male, 22 female) received either nimodipine 30 mg twice a day or a placebo according to the same dosage regimen in a double-blind cross-over study design. The ocular blood flow was measured by means of the ocular blood flow system, the optic nerve head blood flow with the continuous laser Doppler flowmeter (Riva), and contrast sensitivity using the MCT 8000 Multivision Contrast Tester. Measurements were taken at baseline (1T0), 120 min after initial dose (1T3) and after 3 days (3T3) of therapy with 150 mg nimodipine or placebo in total. RESULTS: Contrast sensitivity improved significantly throughout almost all spatial frequencies in the nimodipine-treated subjects ( P=0.01), whereas there was no change in the placebo group. Ocular blood flow and optic nerve head blood flow increased slightly but not significantly in the nimodipine group (1T0: 706.6 microl/min, 9.33 AU; 3T3: 854.3 microl/min, 9.39 AU) and remained unchanged or were even lower in the placebo group ( P>0.05). CONCLUSION: The results showed a significant increase in contrast sensitivity during treatment with nimodipine in healthy subjects. This increase in visual function, however, was not correlated with an increase in ocular or optic nerve head blood flow. Therefore, another mechanism, e.g., a direct effect on the visual system, might be responsible for the improvement in visual function in healthy volunteers under nimodipine therapy.

Adult↗

PubMed2002

Comparison of two grading methods to evaluate focal narrowing of retinal arterioles in glaucoma.

BACKGROUND: Focal arteriolar narrowing has been reported to be more common in glaucoma eyes compared to normal eyes. The current study was performed to compare two methods for standardized assessment of focal arteriolar narrowing. MATERIAL AND METHODS: Stereoscopic optic disc photographs (from one randomly selected eye per subject) of 48 normal subjects, 20 ocular hypertensive patients (OHT), and 29 glaucoma patients, were reviewed independently in a masked fashion by two graders. Focal arteriolar narrowing within one disc diameter from the rim edge was evaluated based on two different grading methods: (1) narrowing present if the arteriole was wider distal to the narrowing, and (2) narrowing present if the arteriole was wider both distal and proximal to the narrowing. RESULTS: Focal arteriolar narrowing was observed with grading method 1 in 37.5% (18/48), 35.0% (7/20), and 65.5% (19/29) of normals, OHT, and glaucoma patients, respectively. It was observed with grading method 2 in 18.8% (9/48), 0% (0/22), and 48.3% (14/29) of normals, OHT, and glaucoma patients, respectively. With both grading methods, focal arteriolar narrowing was significantly more frequent in glaucoma versus normal eyes (chi-square test: grading method 1 P=0.03, and grading method 2 P=0.0001). The number of eyes with focal arteriolar narrowing was significantly more frequent with method 1 than with method 2 across all photographs and all subgroups (chi-square test: P=0.0001). The overall agreement between the two graders was kappa 0.77+/-0.06 for grading method 1, and 0.43+/-0.11 for grading method 2. CONCLUSIONS: The prevalence of focal arteriolar narrowing is highly dependent upon the grading method. A uniform grading method of focal arterial narrowing is needed to achieve comparable and reproducible results among studies.

Aged↗

PubMed2002

The structure-function relationship in eyes with glaucomatous visual field loss that crosses the horizontal meridian.

OBJECTIVE: To evaluate the relationship between visual field loss and glaucomatous optic discs in eyes in which field loss spreads across the horizontal meridian. SUBJECTS AND METHODS: Ninety-six patients with glaucoma (9 advanced, 60 moderate, and 27 early) with 2 successive abnormal fields were included. Standard achromatic automated perimetry defects were identified with a nerve fiber bundle map to identify abnormal sectors. Crossover was present if the superior and inferior sectors at the horizontal meridian (nasal, central, or temporal) were both abnormal. Optic disc damage was assessed by masked grading of simultaneous stereophotographs. RESULTS: Only 30% (29) of glaucomatous eyes showed crossover, and only 2 of those eyes had early loss. The most frequent pattern of visual field loss (41% of eyes) was single hemifield damage with defects in contiguous sectors. Regardless of the pattern or severity of visual loss, most eyes (66 [69%] of 96) had both superior and inferior optic disc damage. CONCLUSIONS: Early glaucomatous visual field loss rarely crosses the horizontal meridian, but defects in both hemifields at the horizontal meridian are more common in more advanced field loss. In 26 (90%) of 29 eyes with crossover, it could be explained by changes at the optic nerve head.

Adult↗

PubMed2002

Comparison of two transducers for color Doppler imaging of the retrobulbar vessels.

PURPOSE: To compare blood flow velocity measurements obtained with two different linear probes (7.5 MHz and 12 MHz). MATERIALS AND METHODS: CDI measurements were performed in 31 eyes of 19 individuals with a 12 MHz and a 7.5 MHz probe. Measurements were taken in the ophthalmic artery (OA), central retinal artery (CRA), temporal short posterior ciliary arteries (TSPCA), and nasal short posterior ciliary arteries (NSPCA). With each probe, three measurements of each vessel were obtained, and end-diastolic velocity (EDV), peak systolic velocity (PSV), and resistance index (RI) were calculated. RESULTS: For EDV, the variation among replicates with the 7.5 MHz probe was between 13.99% and 19.51% and with the 12 MHz probe between 15.40% and 23.17%. For PSV, the variation among replicates was between 5.16% and 9.98% and between 6.58% and 10.71%, respectively with each probe. For RI, the variation among the replicates was between 3.10% and 8.97% and between 3.93% and 10.16%, respectively with each probe. The variation between probes was between 15.02% and 23.07% for EDV, between 10.72% and 13.24% for PSV, and between 4.27% and 6.05% for RI. The difference between estimated means was only significant for EDV in the TSPCA (P = 0.019), PSV in the TSPCA (P = 0.0002), as well as NSPCA (P = 0.003) and RI in the CRA (P < 0.001) and NSPCA (P < 0.044). CONCLUSION: The observed variation among three measurements is similar with both the 7.5 MHz and the 12 MHz probes. However, the 7.5 MHz probe tends to measure slightly higher values for some parameters.

Adult↗

PubMed2002

Imaging of the optic disc and retinal nerve fiber layer: the effects of age, optic disc area, refractive error, and gender.

We cross-sectionally examined the relationship between age, optic disc area, refraction, and gender and optic disc topography and retinal nerve fiber layer (RNFL) measurements, using optical imaging techniques. One eye from each of 155 Caucasian subjects (age range 23.0-80.8 y) without ocular pathology was included. Measurements were obtained by using the Heidelberg Retina Tomography (HRT), the GDx Nerve Fiber Analyzer, and the Optical Coherence Tomograph (OCT). The effects of age were small (R2 < 17%) and were limited to specific HRT, GDx, and OCT parameters. Disc area was significantly associated with most HRT parameters and isolated GDx and OCT parameters. Refraction and gender were not significantly associated with any optic disc or RNFL parameters. Although effects of age on the optic disc and RNFL are small, they should be considered in monitoring ocular disease. Optic disc area should be considered when cross-sectionally evaluating disc topography and, to a lesser extent, RNFL thickness.

Adult↗

PubMed2002

Using machine learning classifiers to identify glaucomatous change earlier in standard visual fields.

PURPOSE: To compare the ability of several machine learning classifiers to predict development of abnormal fields at follow-up in ocular hypertensive (OHT) eyes that had normal visual fields in baseline examination. METHODS: The visual fields of 114 eyes of 114 patients with OHT with four or more visual field tests with standard automated perimetry over three or more years and for whom stereophotographs were available were assessed. The mean (+/-SD) number of visual field tests was 7.89 +/- 3.04. The mean number of years covered (+/-SD) was 5.92 +/- 2.34 (range, 2.81-11.77). Fields were classified as normal or abnormal based on Statpac-like methods (Humphrey Instruments, Dublin, CA) and by several machine learning classifiers. The machine learning classifiers were two types of support vector machine (SVM), a mixture of Gaussian (MoG) classifier, a constrained MoG, and a mixture of generalized Gaussian (MGG). Specificity was set to 96% for all classifiers, using data from 94 normal eyes evaluated longitudinally. Specificity cutoffs required confirmation of abnormality. RESULTS: Thirty-two percent (36/114) of the eyes converted to abnormal fields during follow-up based on the Statpac-like methods. All 36 were identified by at least one machine classifier. In nearly all cases, the machine learning classifiers predicted the confirmed abnormality, on average, 3.92 +/- 0.55 years earlier than traditional Statpac-like methods. CONCLUSIONS: Machine learning classifiers can learn complex patterns and trends in data and adapt to create a decision surface without the constraints imposed by statistical classifiers. This adaptation allowed the machine learning classifiers to identify abnormality in visual field converts much earlier than the traditional methods.

Algorithms↗