Investigation of magnetically driven passage of magnetic nanoparticles through eye tissues for magnetic drug targeting.

This paper elucidates the feasibility of magnetic drug targeting to the eye by using magnetic nanoparticles (MNPs) to which pharmaceutical drugs can be linked. Numerical simulations revealed that a magnetic field gradient of 20 T m-1 seems to be promising for dragging magnetic multicore nanoparticles of about 50 nm into the eye. Thus, a targeting magnet system made of superconducting magnets with a magnetic field gradient at the eye of about 20 T m-1 was simulated. For the proof-of-concept tissue experiments presented here the required magnetic field gradient of 20 T m-1 was realized by a permanent magnet array. MNPs with an optimized multicore structure were selected for this application by evaluating their stability against agglomeration of MNPs with different coatings in water for injections, physiological sodium chloride solution and biological media such as artificial tear fluid. From these investigations, starch turned out to be the most promising coating material because of its stability in saline fluids due to its steric stabilization mechanism. To evaluate the passage of MNPs through the sclera and cornea of the eye tissues of domestic pigs (Sus scrofa domesticus), a three-dimensionally printed setup consisting of two chambers (reservoir and target chamber) separated by the eye tissue was developed. With the permanent magnet array emulating the magnetic field gradient of the superconducting setup, experiments on magnetically driven transport of the MNPs from the reservoir chamber into the target chamber via the tissue were performed. The resulting concentration of MNPs in the target chamber was determined by means of quantitative magnetic particle spectroscopy. It was found that none of the tested particles passed the cornea, but starch-coated particles could pass the sclera at a rate of about 5 ng mm-2 within 24 h. These results open the door for future magnetic drug targeting to the eye.

Modulation of Human Intraocular Pressure Using a Pneumatic System.

Purpose: To technically validate a novel pneumatically based system and method for modulation of intraocular pressure (IOP) and to test its application in the human eye. Special attention was paid to the applicability of the pneumatically driven balloon, which realizes the modulation of the IOP through its contact with the conjunctiva. Methods: A force sensor as key component of a customized measurement setup was used to check the applied pressure through the balloon. The IOP of 10 healthy subjects (4 female, 6 male, aged 28.8 ± 6.64 years) was modulated and increased linearly to at least 40 mmHg. At this point, the pressure inside the balloon was kept constant for 2 minutes, with IOP measurements taken every 40 seconds using a rebound tonometer. Results: The technical setup led to an IOP decrease of 0.71 mmHg within 2 minutes at an operating point of 40 mmHg. For all subjects, the IOP could be increased up to 42.8 ± 3.6 mmHg, whereby a mean pressure decrease of 2.4 mmHg/min was determined, which seems to be caused mainly by physiological processes. Conclusions: With the new pneumatically based setup, a targeted modulation in terms of level and constancy of the IOP can be realized. Translational Relevance: Additional and, compared with the technique according to Löw, a more precise and more constant methodology for the modulation of the IOP, can significantly simplify the determination of retinal vessel pressures for clinical application. It is suitable for practical questions concerning an enhanced retinal venous pressure.

Corrigendum: Pulsed Electrical Stimulation of the Human Eye Enhances Retinal Vessel Reaction to Flickering Light.

[This corrects the article DOI: 10.3389/fnhum.2019.00371.].

Pulsed Electrical Stimulation of the Human Eye Enhances Retinal Vessel Reaction to Flickering Light.

Recent studies indicate therapeutic benefits of electrical stimulation in cases of specific ophthalmic diseases that are associated with dysfunctional ocular microcirculation. This suggests effects of electrical stimulation on vascular functions. In the present study, we investigated the effects of electrical stimulation on retinal vessel reactions using dynamic vessel analysis (DVA). Eighty healthy subjects were randomly assigned to one of three groups receiving electrical stimulation with different current intensities: 400 µA (n = 26); 800 µA (n = 27); 1200 µA (n = 27). The electrode montage for electrical stimulation consisted of a ring-shaped active electrode surrounding one eye and a square return electrode at the occiput. Rectangular, monophasic, positive current pulses were applied at 10 Hz for a duration of 60 s per stimulation period. DVA was used to observe the stimulation-induced reactions of retinal vessel diameters in response to different provocations. In three DVA measurements, three stimulus conditions were investigated: flicker light stimulation (FLS); electrical stimulation (ES); simultaneous electrical and flicker light stimulation (ES+FLS). Retinal vasodilation caused by these stimuli was compared using paired t-test. The subjects receiving electrical stimulation with 800 µA showed significantly increased retinal vasodilation for ES+FLS compared to FLS (p < 0.05). No significant differences in retinal vessel reactions were found between ES+FLS and FLS in the 400 and 1200 µA groups. No retinal vasodilation was observed for ES for all investigated current intensities. The results indicate that positive pulsed electrical stimulation of an adequate intensity enhances the flicker light-induced retinal vasodilation.

Dorzolamide influences the autoregulation of major retinal vessels caused by artificial intraocular pressure elevation in patients with POAG: a clinical study.

PURPOSE: The study investigated whether dorzolamide influences the autoregulatory behavior of major retinal arterioles in glaucoma patients via a moderate perfusion pressure reduction. METHODS: The study included one eye each of 12 untreated patients with a primary open-angle glaucoma (POAG) (age 60.8 +/- 8.3, IOP 22.3 +/- 6.5 mmHg). Changes in the diameter of a retinal artery segment before (120 s), during (100 s), and after (380 s) artificial IOP elevation to 38 mmHg for 100 s were recorded continuously by means of a Retinal Vessel Analyzer. The measurement was repeated after 4-week treatment with dorzolamide eye drops t.i.d. RESULTS: Ocular perfusion pressure (mmHg) was reduced by the intraocular pressure (IOP) elevation from 58 (+/- 10) to 41 (+/- 11) in the pretreatment examination and from 60 (+/- 8) to 40 (+/- 8) posttreatment (differences between the examinations n.s.). Before IOP elevation, the arterial diameter was found to be +1.7 +/- 3.5% greater in the posttreated eyes than in the pretreated eyes (p < 0.02). During IOP elevation, the arterial diameter decreased by -1.8% +/- 3.8 in the pretreated eyes, whereas dilatation by +1.4% +/- 2.5 was observed in the posttreated eyes (p = 0.02). At the end of the observation period following IOP elevation, the vessel diameter in the pretreated eyes had increased by +1.8% +/- 4.2, whereas in the posttreated eyes it had decreased by -1.7% +/- 3.0. On average, dorzolamide reduced IOP by -5.6 mmHg (p = 0.001). CONCLUSIONS: The arterial diameter dilatation during IOP elevation in dorzolamide-treated eyes could be an accelerated counter-regulation on the induced elevated IOP and could constitute an additional therapeutic effect.

Age, blood pressure, and vessel diameter as factors influencing the arterial retinal flicker response.

PURPOSE: The present study investigated whether age, blood pressure (BP), and baseline vessel diameter influence the retinal arterial response to flicker light. METHOD: Thirty healthy subjects (mean age, 46.3; range, 22-73 years) and 15 patients with untreated essential arterial hypertension (mean, 50.9; range, 26-69 years) were examined. The diameter of the retinal arterioles was measured by a Retinal Vessel Analyzer (RVA; Imedos, Weimar, Germany). Each examination consisted of a 100-second baseline measurement and five 20-second periods of flicker stimulation, followed by an 80-second observation period. The five stimulation periods were then averaged. The rectangular luminance flicker operated at 12.5 Hz at a wavelength of 530 to 600 nm. The baseline-corrected flicker response (bFR) was defined as the difference between the peak dilatation and subsequent constriction after flicker stimulation minus the fluctuation of the baseline. The BP was measured at 1-minute intervals during the examination. RESULTS: In 26 subjects with normal BP, flicker light induced a bFR of +6.4% +/- 2.7%. The bFR decreased nonsignificantly in healthy subjects with increasing age (y = 8.48-0.048x; r = 0.26). The baseline diameter did not influence the amplitude of the flicker response over a range of 70 to 140 measuring units. The hypertensive patients reacted with a bFR of +2.2% +/- 2.5% (P < 0.001). Four hitherto healthy subjects with elevated BP during the examination were excluded from analysis. CONCLUSIONS: A significant correlation of age and bFR was not found in the small sample examined. Untreated arterial hypertension appeared to be associated with a reduced flicker response. The value of such functional vessel properties in the screening of vasosclerosis and in diagnostics in arterial hypertension should be examined in further studies.

Online human conjunctival vessel diameter analysis. A clinical-methodical study.

BACKGROUND: The present study investigates the possible application of a commercially available on-line measuring device of retinal vessels for conjunctival vessel assessment. METHODS: Repeated measurements in one randomly chosen eye were performed in 11 healthy volunteers (mean age 42.9 +/- 10 years). Measurements of one conjunctival vessel were obtained first without a stimulus followed by measurements after the application of one drop of a topical vasoconstrictor. The examinations were performed by Retinal Vessel Analyzer (RVA, IMEDOS/Germany). This system determines automatically on-line the vessel diameter along a chosen vessel segment. RESULTS: Measurements in the native state without eye drop application showed an intraclass correlation coefficient of 0.97 and a mean variation coefficient of 1.8%. After application of the topical vasoconstrictor a short acting vasodilatation was observed with a magnitude of +10.9% +/- 14.9 (p < 0.001), followed by an increasing vasoconstriction (after 4 min -12.0% +/- 7.6; p = 0.004). One volunteer had no measurable conjunctival vessels in the baseline measurements and was therefore excluded from the study. DISCUSSION: The suggested technique allows the measurement of changes in conjunctival vessel diameter with high precision. The method represents a non invasive technique for the assessment of effects on conjunctival vessels caused by topical or systemic drugs.

Repeatability of autofluorescence lifetime imaging at the human fundus in healthy volunteers.

PURPOSE: We aim to evaluate the repeatability of a new fluorescence lifetime imaging (FLIM) technique which measures time-resolved autofluorescence to assess metabolism of the retina. MATERIALS AND METHODS: We performed FLIM with two spectral channels (channel 1: 490-560 nm and channel 2: 560-700 nm) on 10 healthy volunteers, with 10 replicates per volunteer. From the 30° fundus FLIM images, we selected three regions: the fovea, the optic disc and the papillo-macular bundle. For each channel in these regions, we determined an average multi-exponential approximation with three components, and the six resulting parameters, α1-α3 (amplitudes) and τ1-τ3 (fluorescence lifetimes), were analyzed in terms of the coefficient of variation (CV). RESULTS: Repeatability was highest in the papillo-macular bundle, followed by the fovea and the optic disc. Repeatability was higher in channel 1 (mean CV of 7.9%) than in channel 2 (mean CV of 17.7%). The average CV for the diagnostically most relevant channel 1 and the most relevant parameters was as follows: τ1 (5.5%) and τ2 (4.7%) in the papillo-macular bundle, and τ1 (6.8%) and τ2 (6.9%) in the fovea. CONCLUSIONS: We demonstrated repeatability of FLIM measurement results within acceptable ranges of variation. Based on the detailed coefficients of variation, we derived recommendations for parameter ranges suitable for diagnostic applications.

Microstructural alterations of retinal arterial blood column along the vessel axis in systemic hypertension.

Purpose. Image analysis by the retinal vessel analyzer (RVA) observes retinal vessels in their dynamic state online noninvasively along a chosen vessel segment. It has been found that high-frequency diameter changes in the retinal artery blood column along the vessel increase significantly in anamnestically healthy volunteers with increasing age and in patients with glaucoma during vascular dilation. This study was undertaken to investigate whether longitudinal sections of the retinal artery blood column are altered in systemic hypertension. Methods. Retinal arteries of 15 untreated patients with essential arterial hypertension (age, 50.9 +/- 11.9 years) and of 15 age-matched anamnestically healthy volunteers were examined by RVA. After baseline assessment, a monochromatic luminance flicker (530-600 nm; 12.5 Hz; 20 s) was applied to evoke retinal vasodilation. Differences in amplitude and frequency of spatial artery blood column diameter change along segments (longitudinal arterial profiles) of 1 mm in length were measured and analyzed using Fourier transformation. Results. In the control group, average reduced power spectra (ARPS) of longitudinal arterial profiles did not differ when arteries changed from constriction to dilation. In the systemic hypertension group, ARPS during constriction, baseline, and restoration were identical and differed from ARPS during dilation (P < 0.05). Longitudinal arterial profiles in both groups showed significant dissimilitude at baseline and restoration (P < 0.05). Conclusions. The retinal artery blood column demonstrates microstructural alterations in systemic hypertension and is less irregular along the vessel axis during vessel dilation. These microstructural changes may be an indication of alterations in vessel wall rigidity, vascular endothelial function, and smooth muscle cells in this disease, leading to impaired perfusion and regulation.

Fixed combination of bimatoprost and timolol in patients with primary open-angle glaucoma or ocular hypertension with inadequate IOP adjustment.

OBJECTIVE: To assess the efficacy and tolerability of a fixed combination of bimatoprost and timolol (BTFC) in a large patient sample in a clinical setting. METHODS: In this multicenter, observational, noncontrolled, open-label study, patients (n = 1862) with primary open-angle glaucoma or ocular hypertension were treated with BTFC. Assessments were made at baseline, six weeks, and three months. RESULTS: Prior to starting BTFC, 92.3% of patients were taking other ocular hypotensive medications. In the overall group at three months, mean intraocular pressure was reduced from baseline (21.7 ± 4.5 mmHg and 21.8 ± 4.9 mmHg for the right and left eye, respectively) to 16.1 ± 3.0 mmHg for each eye (P < 0.0001). The majority of patients (92%) reported no adverse events. The most commonly reported adverse events (in >1% of patients) were eye irritation, and ocular and conjunctival hyperemia. Adherence to treatment was generally better than (35.4%) or the same as (57.5%) with prior therapy. BTFC tolerability was rated as excellent or good by 92.3% of physicians and 85.8% of patients. CONCLUSIONS: In a large group of patients with primary open-angle glaucoma or ocular hypertension, treatment with BTFC was associated with consistent reductions in IOP, improved adherence to treatment, and good tolerability.

Use of the retinal vessel analyzer in ocular blood flow research.

The present article describes a standard instrument for the continuous online determination of retinal vessel diameters, the commercially available retinal vessel analyzer. This report is intended to provide informed guidelines for measuring ocular blood flow with this system. The report describes the principles underlying the method and the instruments currently available, and discusses clinical protocol and the specific parameters measured by the system. Unresolved questions and the possible limitations of the technique are also discussed.

Reproducibility of the retinal vascular response to flicker light in Asians.

PURPOSE: Dilation of retinal vessels in response to diffuse luminance flicker may reflect endothelial function. Although this has previously been shown to be reproducible in whites, there have been no similar data in Asians. We assess the reproducibility of repeated measurements of this response in Asians. MATERIAL AND METHODS: Healthy Asians (n = 33) with normal vision and no history of glaucoma, age-related macular degeneration, cataract, or retinal arterial/venous occlusion participated in this study. Repeated measures from the same subjects were taken 30-60 min apart using the Dynamic Vessel Analyser (DVA, IMEDOS, Jena, Germany). Modification was made to the shape of the light source for Asian participants. Correlations of the first and second measures were assessed using Pearson correlation (R(2)), and agreement between the two measures was shown using Bland-Altman plots. RESULTS: After modification to the shape of the light source, almost perfect correlation was found between the 1st and 2nd measurements of baseline arteriolar (R(2) = 0.95) and venular diameters (R(2) = 0.98) of arteriolar maximum dilation (R(2) = 0.85). Substantially high correlation between the 1st and 2nd measurements of venular maximum dilation was found (R(2) = 0.80). CONCLUSIONS: Measurements of the dilation response of retinal vessels to diffuse luminance flicker an Asian sample using the DVA show high reproducibility for repeated measures over a short period of time. Such measurements may allow non-invasive quantification of endothelial function to study its association with systemic and ocular diseases.

Functional in vivo assessment of retinal artery microirregularities in glaucoma.

PURPOSE: We investigated whether retinal branch arteries in healthy subjects, and non-treated and treated primary open-angle glaucoma (POAG) patients show irregular local patterns during dynamic reaction to acute increases of different magnitudes in intraocular pressure (IOP). METHODS: Nine POAG patients and nine age-matched normal volunteers were examined with the retinal vessel analyser (RVA) using a suprasystolic IOP increase (Study 1). Fourteen POAG patients and 13 age-matched controls were examined using a moderate IOP increase for 100 seconds (Study 2). Longitudinal arterial profiles were obtained for the chosen time intervals. The high-frequency waviness (HFW) of these profiles was analysed quantitatively. RESULTS: No significant changes in HFW were found in controls in different phases of the arterial reaction. Significant increases in HFW from baseline to dilation (Study 1, P < 0.03) and from dilation to constriction (Study 2, P < 0.05) were found in POAG patients. High-frequency waviness was higher in POAG patients than in controls during dilation (P < 0.05) in both studies. CONCLUSIONS: Our results indicate a local vessel wall difference in glaucoma patients compared with age-matched controls. Increasing HFW might worsen hydraulic resistance of the vessel segment to blood flow. Significant increase of arterial microirregularities in the POAG retina during vascular dilation might be an indication for vascular endothelial alterations in glaucoma, leading to impaired perfusion in response to IOP increase.

Autoregulative behavior of retinal arteries and veins during changes of perfusion pressure: a clinical study.

PURPOSE: To investigate the autoregulative response of large retinal vessels to artificial reduction of perfusion pressure. METHODS: The diameters of a venous and an arterial segment (each approx.1.5 mm in length) in one eye of each of 13 healthy volunteers (age 54.5+/-18 years) were measured continuously using the Retinal Vessel Analyzer (Imedos, Weimar, Germany). The intraocular pressure (IOP; mean before examination 13.7+/-2.9 mmHg) was increased by 21.2+/-3.5 mmHg by means of a suction cup in order to produce a temporary reduction of the retinal perfusion pressure. The RVA measurements were taken for 2 min without artificial intervention (baseline), for 100 s during IOP elevation, and for up to 10 min after removal of the suction cup. RESULTS: A significant response of arterial and venous diameters to the provocation was found ( P<0.02, ANOVA). The arterial and venous responses were opposite: Whereas the artificially elevated IOP increased the arterial diameter by +1.9+/-4.5%, the venous vessel diameter decreased by -2.6+/-3.5% ( P<0.02, Mann-Whitney U-test). After normalization of the IOP the arterial diameter fell slightly below the baseline value, while the veins underwent temporary dilation by +5.9+/-3.3% ( P<0.001). The mean systemic blood pressure did not change significantly during the investigation. CONCLUSION: Retinal arteries and veins of healthy volunteers exhibited opposite autoregulative behavior in response to perfusion pressure changes. This is believed to be due to the different regulative functions of arteries and veins.

[Reproducibility of measurements with the retinal vessel analyzer under optimal conditions]. / Reproduzierbarkeit der Messungen mit dem Retinal Vessel Analyzer unter Optimalbedingungen.

BACKGROUND: [corrected] To evaluate the reproducibility of measurements with the Retinal Vessel Analyzer (RVA) in healthy subjects and to describe which measuring conditions should be guaranteed to obtain optimal results. METHODS: The diameter of retinal arteries and veins of 20 healthy subjects (M : F = 11 : 9; mean age: 33 +/- 12 years) were measured with the RVA at baseline, after 2 hours, and after 2 weeks. RESULTS: No statistically significant differences in the diameter of retinal arteries and veins between the single measurements were present. Short-term variability of arterial diameter was 1 %, and the intraclass correlation coefficient kappa was 0.96. Long-term variability was 1.8 %, kappa was 0.98. In retinal veins, a short-term variability of 1 % was calculated, with a kappa of 0.97. Long-term variability was 1.5 %, with a kappa of 0.98. CONCLUSION: Due to the high reproducibility of its measurements, the RVA appears to be a useful device for both analysis and follow-up of retinal vessel diameters.

Retinal vessel reaction in response to chromatic flickering light.

BACKGROUND: Flickering light stimulation of the retina is known to increase retinal vessel diameter in animals and humans. The aim of the study was to quantify the response of retinal vessel diameter to red-green and blue-green flickering light. METHODS: In 11 normal healthy volunteers (mean age: 25.2+/-6.8 years) retinal arterial and venous diameters were examined by Retinal Vessel Analyzer (IMEDOS Ltd., Weimar, Germany) before, during and after red-green and blue-green flicker stimulation with a frequency of 12 Hz and duration of 10 and 30 s. RESULTS: For red-green flicker at 10 s there was a 2.4+/-1.4% arterial diameter increase at 9.1+/-3.3 s with a return to baseline after 30 s and a 2.4+/-1.1% venous diameter increase at 12.1+/-2.6 s with a return to baseline after 30 s. For red-green flicker at 30 s there was a 3.2+/-1.5% arterial diameter increase at 26.9+/-12.6 s with a return to baseline after 40 s and a 4.9+/-1.8% venous diameter increase at 31.4+/-7.6 s with a return to baseline after 40 s. For blue-green flicker at 10 s there was a 2.0+/-0.7% arterial diameter increase at 10.6+/-5.3 s with a return to baseline after 30 s and a 2.3+/-1.1% venous diameter increase at 12.0+/-5.5 s with a return to baseline after 30 s. For blue-green flicker at 30 s there was a 2.6+/-1.3% arterial diameter increase at 20.7+/-8.0 s with a return to baseline after 40 s and a 3.4+/-2.2% venous diameter increase at 28.8+/-10.5 s with a return to baseline after 40 s. CONCLUSIONS: Retinal vessel diameter dilation is a reproducible response to the applied flicker stimuli. This finding supports the existence of neurovascular coupling in the human retina. Flicker stimulation in either red-green or blue-green might be a useful stimulus for examination of retinal vessel behavior to regulatory demands.