A prospective evaluation of the Heidelberg retina flowmeter in diagnosing ischaemia following branch retinal vein occlusion: a masked, controlled comparison with fluorescein angiography

Waveform analysis of human retinal and choroidal blood flow with laser Doppler holography

Laser Doppler holography was introduced as a full-field imaging technique to measure blood flow in the retina and choroid with an as yet unrivaled temporal resolution. We here investigate separating the different contributions to the power Doppler signal in order to isolate the flow waveforms of vessels in the posterior pole of the human eye. Distinct flow behaviors are found in retinal arteries and veins with seemingly interrelated waveforms. We demonstrate a full field mapping of the local resistivity index, and the possibility to perform unambiguous identification of retinal arteries and veins on the basis of their systolodiastolic variations. Finally we investigate the arterial flow waveforms in the retina and choroid and find synchronous and similar waveforms, although with a lower pulsatility in choroidal arteries. This work demonstrates the potential held by laser Doppler holography to study ocular hemodynamics in healthy and diseased eyes.

Optical coherence tomography angiography: A comprehensive review of current methods and clinical applications

OCT has revolutionized the practice of ophthalmology over the past 10-20 years. Advances in OCT technology have allowed for the creation of novel OCT-based methods. OCT-Angiography (OCTA) is one such method that has rapidly gained clinical acceptance since it was approved by the FDA in late 2016. OCTA images are based on the variable backscattering of light from the vascular and neurosensory tissue in the retina. Since the intensity and phase of backscattered light from retinal tissue varies based on the intrinsic movement of the tissue (e.g. red blood cells are moving, but neurosensory tissue is static), OCTA images are essentially motion-contrast images. This motion-contrast imaging provides reliable, high resolution, and non-invasive images of the retinal vasculature in an efficient manner. In many cases, these images are approaching histology level resolution. This unprecedented resolution coupled with the simple, fast and non-invasive imaging platform have allowed a host of basic and clinical research applications. OCTA demonstrates many important clinical findings including areas of macular telangiectasia, impaired perfusion, microaneurysms, capillary remodeling, some types of intraretinal fluid, and neovascularization among many others. More importantly, OCTA provides depth-resolved information that has never before been available. Correspondingly, OCTA has been used to evaluate a spectrum of retinal vascular diseases including diabetic retinopathy (DR), retinal venous occlusion (RVO), uveitis, retinal arterial occlusion, and age-related macular degeneration among others. In this review, we will discuss the methods used to create OCTA images, the practical applications of OCTA in light of invasive dye-imaging studies (e.g. fluorescein angiography) and review clinical studies demonstrating the utility of OCTA for research and clinical practice.

Basic technique and anatomically imposed limitations of confocal scanning laser Doppler flowmetry at the optic nerve head level

Many studies have suggested an association between blood flow dysregulation and glaucomatous damage to the optic nerve. Confocal scanning laser Doppler flowmetry (CSLDF) is a technique that measures the capillary blood flow of the retina and optic nerve head and provides a two-dimensional map of ocular perfusion in these areas. This review discusses the anatomy of the anterior optic nerve vasculature and the capabilities and limitations of the CSLDF. Methods to minimize error and to acquire more reliable measurements of capillary blood flow are also outlined.

Confocal laser Doppler flowmeter measurements in a controlled flow environment in an isolated perfused eye

The aim of this study was to improve our ability to interpret and validate Heidelberg Retina Flowmeter (HRF) flow images by recording flow measurements from specific regions of the retinal vasculature by taking advantage of the ability to precisely regulate perfusion flow in an isolated eye preparation. The retinal vasculature in 16 isolated perfused pig eyes was perfused with a 50%/50% Krebs/RBC solution at known flow rates ranging from 0 to 300 microl min(-1). At each flow rate, HRF images were obtained at a location approximately two disc diameters from the disc. After HRF image acquisition, the retinal vasculature was perfused with fluorescein isothiocyanate for fluorescence microscopy. Using the standard HRF software and a 10 x 10pixel measurement window, flow rates were measured from a retinal artery, vein, arteriole, venule, and the retinal capillary bed and a capillary-free-zone. The relationship between HRF measured flow and perfusion flow in the different measurement locations was determined. At zero perfusion flow the measured HRF flow was consistently greater than zero ( approximately 170 arbitrary units (AU)), and not significantly different at each measurement location except for the retinal vein, which had a significantly higher HRF flow value ( approximately 230AU). At higher perfusion flow rates the flow signal from the larger vascular elements (arteries and veins) increased rapidly thereafter to reach several thousand AU at a total perfusate flow of 50 microlmin(-1) and increased less rapidly at higher flow rates. In arterioles, the HRF flow was more linear over a broader range of perfusate flow rates but the peak flow signal was an order of magnitude smaller than that from the retinal artery. Both the linearity and magnitude of the flow signal in venules was less than that in arterioles. In capillary areas and in the capillary free zone, the HRF flow showed only a very weak relationship to perfusion flow when compared to the background noise. The choice of location for HRF flow analysis greatly influences the ability of the technique to measure changes in retinal blood flow. The major arteries and veins provide the strongest signal and greatest signal to noise ratio. However, the retinal arterioles produce an HRF signal that is more linear over a wider range of perfusate flow rates.

The age-dependent decrease in the myogenic response of retinal arterioles as studied with the Retinal Vessel Analyzer

PURPOSE

To study the age-dependent change in myogenic response of retinal arterioles.

METHODS

Fifty-one healthy volunteers with at least ten persons in each of the five age decades (I-V) between 20 and 69 years were subjected to diameter measurement of retinal arterioles using the Retinal Vessel Analyzer (RVA) during rest and during an increase in the systemic blood pressure when lifting hand weights. The transmural pressure in the retinal arterioles during the procedures was estimated from the blood pressure and the intraocular pressure and was compared to the accompanying diameter response.

RESULTS

The retinal arteriolar diameter showed a significant decrease as a function of increasing weight for the two younger age groups below the age of 40 years (P=0.007, group I, and P=0.049, group II), compatible with perfect autoregulation, whereas no such change was observed in persons above this age (P=0.41, 0.053, 0.29 for groups III-V, respectively).

CONCLUSION

Studies on autoregulation in retinal disease should consider the normal age-related decrease in diameter response of retinal arterioles when the blood pressure is changed.

Scanning laser Doppler flowmeter study of retinal blood flow in macular area of healthy volunteers

AIM

To compare the interocular and intraocular differences of capillary perfusion, and the intraocular regional differences of retinal blood flow in the macular area of healthy volunteers.

METHODS

Tissue blood flow in the macula was examined in both eyes of 20 healthy volunteers with the Heidelberg retinal flowmeter. Blood flow measurements were made in a 10 degrees x 2.5 degrees area superior and inferior to the macula. The mean blood flow (MBF) was calculated by an automatic full field perfusion image analyser program. The MBF in the right and left eyes and in the superior and inferior macular areas of the same eye were compared.

RESULTS

The ratios of the MBF in the right eye to the left eye in the macular areas were 1.00, and 1.03, respectively. The ratio of the MBF in the superior macular area to the inferior area was 1.01 for the right eyes and 1.04 for the left eyes.

CONCLUSIONS

Because no significant differences were found in the MBF between the two eyes and between the superior and inferior macular areas in the same eye, interocular (for example, affected eye versus fellow eye) and intraocular (superior versus inferior macular areas) comparisons of MBF can be made to determine if changes in retinal perfusion have occurred.

The capillary blood flow in ischaemic type central retinal vein occlusion: the effect of laser photocoagulation

PURPOSE

To determine the effect of photocoagulation on retinal blood flow (RBF) in eyes with ischaemic type central retinal vein occlusion (CRVO).

PATIENTS AND METHODS

Retinal blood flow was measured in 12 eyes with CRVO, 12 fellow eyes and 12 eyes of 12 age-matched healthy subjects using the Heidelberg retinal flowmeter (HRF). Microvascular blood flow values (volume, flow, velocity) were recorded from the upper temporal retina and macula. Eyes were re-examined 1 month after photocoagulation. We investigated whether there was a difference in RBF measurements before and after photocoagulation treatment.

RESULTS

In eyes with CRVO, mean RBF values (volume, flow and velocity) obtained from the upper temporal retina increased significantly after treatment (paired t-test, p < 0.05). In contrast, mean RBF values from the macula were unaffected by photocoagulation (paired t-test, p > 0.05). Retinal blood flow values from the upper temporal retina obtained from control subjects were significantly higher than the values in eyes with CRVO before and after photocoagulation (unpaired t-test, p < 0.05), but there was no significant difference between control subjects and CRVO patients in RBF values from the macula (unpaired t-test, p > 0.05). Mean RBF values were significantly higher in CRVO patients' fellow-eyes before photocoagulation in the eyes with CRVO (paired t-test, p < 0.05) but were lower than in age-matched healthy control eyes (unpaired t-test, p < 0.05). Macular blood flow did not differ between the eyes with CRVO and fellow eyes (paired t-test, p > 0.05).

CONCLUSION

Laser photocoagulation increased retinal blood flow in eyes with CRVO, but RBF did not reach normal values. Photocoagulation was found to have no effect on RBF in the macular area.