Other Approaches

Core Messages

¥Beside the classical techniques to assess ocular blood ßow, several methods exists which aim to measure speciÞc components of ocular blood ßow. In this chapter, the blue Þeld entoptic technique, the laser specle technique and methods that assess the pulsatile ocular blood ßow will be covered.

9.1Blue Field Entoptic Technique

The blue Þeld entoptic technique is a semiquantitative, subjective method that uses the optical blue Þeld entoptic phenomenon to estimate retinal capillary blood ßow in retinal perifoveal vessels [27]. Basically, entoptic phenomena are deÞned as visual effects, observed under certain illumination conditions, whose source lies within the eye itself. The blue Þeld effect is the most well known among these entoptic phenomena.

G. Garhšfer, M.D. (*)

Department of Clinical Pharmacology,

Medical University of Vienna, Waehringer Guertel 18-20, Vienna A-1090, Austria

e-mail: gerhard.garhoefer@meduniwien.ac.at

L. Schmetterer, Ph.D.

Department of Clinical Pharmacology, Center of Medical Physics and Biomedical Engineering, Medical University of Vienna,

Waehringer Guertel 18-20, Vienna A-1090, Austria e-mail: leopold.schmetterer@meduniwien.ac.at

The blue Þeld entoptic phenomenon can easily be seen when looking into a blue light with a narrow optical spectrum at a wavelength of approximately 430 nm. In daily life, the blue Þeld effect can be produced when looking into the blue sky on a bright, sunny day. Under these illumination conditions, many tiny corpuscles that move in a ßowing manner with synchronous acceleration corresponding to the cardiac rhythm can be observed around an area of the center of the fovea. The particles often show a tiny dark tail, seem to appear suddenly, and follow Þxed, often curving paths before disappearing.

Given that entoptic images are generated within the observerÕs own eye, it has been hypothesized that the observed effects may carry information about the anatomical and physiological properties of the observerÕs eye. In particular, it was suggested that the speed and the density of the moving particles observed could reßect quantitative information about blood ßow in the retina. However, given that entoptic phenomena are produced within the subjectÕs eye, the subject cannot share the speciÞc view of the phenomenon with others. Thus, the quantiÞcation of the blue Þeld effect remained impossible for a long time. Furthermore, the cellular origin of the entoptically produced bright particles remained a matter of controversy for a long time. Although it has been hypothesized since the early interpretations of the blue Þeld effect by Helmholtz and others that the blue Þeld effect is caused by circulating leukocytes, the assumptions concerning the source of the entoptic phenomenon were mainly

Leukocyte count (x109/l)

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Fig. 9.1 Effects of granulocyte colony stimulating factor (G-CSF) or placebo on retinal hemodnymic parameters in healthy subjects. The time course of leukocyte counts, white blood cell velocity (WBCV), white blood cell density (WBCD), mean retinal red blood cell velocity (Vel), retinal

venous diameter (Diameter) and retinal blood ßow through a major retinal vein (Flow) after administration of G-CSF (solid up triangles) or placebo (open down triangles) is shown. Data are presented as means ± SD (n=15 per group). Asterisks indicate signiÞcant effects of G-CSF versus placebo

based on physiological and optical considerations without experimental evidence.

In particular, the movement patterns of the observed particles share similarities to histologically identiÞable capillary loops. Furthermore, the observation that the corpuscles are not visible in the innermost of the fovea, which may represent the foveal avascular zone, strengthens the hypothesis that moving particles in the blood stream cause the entopic effect. Evidence that leukocytes cause the blue Þeld effect is even nowadays mainly derived from indirect evidence. Bauermann observed that in patients with leukemia, the number of particles observed under blue Þeld conditions is paralleled by the leukocyte count in the peripheral blood [2]. Furthermore, it has been shown in animal experiments that the blue Þeld entoptic phenomenon can be reproduced in microvascular preparations [35]. More precisely, the authors used a video microscopic setup with lighting conditions similar to those under which the entopic phenomenon is visualized within the human eye. Under these conditions, the cellular ßow within small blood vessels in a wing of a hibernating bat and a rat cremaster muscle was studied. In both preparations, effects of bright moving particles, which could microscopically be identiÞed as leukocytes, were observed [35].

Evidence from an interventional study in healthy humans conÞrmed these results. In this experiment, granulocyte colony-stimulating factor was infused intravenously in a randomized, placebo-controlled double-masked study in healthy young volunteers in order to increase the leukocyte count in the peripheral blood [9]. Leukocyte movement was then assessed by the blue Þeld technique. The authors of the study found a strong correlation between granulocyte colony-stimulating factor-induced changes in leukocyte count in the peripheral blood and leukocyte density as assessed with the blue Þeld entoptic technique (Fig. 9.1). This was observed in the absence of effects on retinal vessel diameters or red blood cell velocities as assessed with bidirectional laser Doppler velocimetry. This is a clear indication that the blue Þled phenomenon reßects leukocyte movement in the perifoveal retinal vasculature [9].

Fig. 9.2 Origin of the blue Þeld entoptic phenomenon (for details see text, courtesy of Martial Geiser)

Today, there is general agreement that the blue Þeld entoptic phenomenon is produced by the different absorption properties of red and white blood cells when the retina is illuminated with blue light. Passing white blood cells do not absorb the short wavelength light, whereas the red blood cells do. Accordingly, white blood cells transversing retinal capillaries that are in front of the photoreceptors are perceived as a ßying corpuscle (Fig. 9.2). Hence, the technique is capable of gaining insight into white blood cell movement in retinal perifoveal capillaries. Whether this is proportional to retinal red blood cell movement or retinal blood ßow under all clinical conditions remains, however, doubtful.

Several methodological approaches have been proposed to quantify leukocyte movement based on the blue Þeld entoptic effect. Given that the blue Þeld phenomenon is produced by the leukocytes moving in the vessel of the observer, it cannot be made visible by others. Thus, all approaches that have been introduced for the quantiÞcation of the blue Þeld phenomenon are strictly subjective in their nature. In an early approach, Riva and Loebl used the blue Þeld phenomenon to investigate autoregulation of the retinal circulation.

For this purpose, subjects were asked to compare the white blood cell velocity observed in one eye with that seen in the other eye. Then the subjects were asked to raise their intraocular pressure (IOP) by pressing a tonometer probe against

the sclera until a reduction in speed of the moving particles was observed [26]. The IOP at which the white blood cell speed started to decrease was deÞned by the authors as the point where the autoregulation of retinal blood ßow was not sufÞcient any more to maintain normal blood ßow.

Later, Riva and Petrig developed a more reÞned technique that has been made commercially available (Blue Field Simulator, Oculix Sarl, Arbaz, Switzerland, Fig. 9.3). This approach uses a computer system to simulate a Þeld with corpuscles, similar to the Þeld that is observed by the subjects under blue Þeld conditions to extract quantitative data. For this purpose, the eye is illuminated with light of a center wavelength of 430 nm. By the means of a connected computer system and a video monitor, a simulated particle Þeld is shown to the subject under study. Then, either the simulated Þeld by the computer system or the subjectsÕ own perception of the blue Þeld phenomenon is alternately shown. The subject is asked to adapt the computer image by adjusting speed and number of the moving particles till the computer image reßects his own perception. By comparing and adjusting the white blood cell (WBC) density and the WBC velocity of the particles in the observed simulated particle Þeld with their own perception, an estimate of perimacular WBC ßux can be obtained. WBC ßux is calculated as

WBC flux = WBC density ´ WBC velocity

These outcome parameters characterize WBC dynamics in perimacular retinal capillaries in arbitrary units.

One of the most important advantages of the blue Þeld technique is that, in contrast to other techniques available, the blue Þeld technique is largely independent from opaque media. Thus, it has been hypothesized that the blue Þeld technique may be used to predict postoperative macular function in patients undergoing future cataract operation [36]. Several experiments indicate that when the blue Þeld phenomenon is observed by patients, retinal function is intact to a large degree [1, 36]. Thus, the blue Þeld technique was proposed to be applicable for the decision whether surgical intervention may be successful for the patients in terms of the postoperative visual outcome, even if opaque ocular media do not allow funduscopy. It has, however, also been reported that the blue Þeld often fails to detect vision loss caused by macular holes and moderate macular dysfunction [21]. Because of these limitations, the blue Þeld entoptic technique has not become generally accepted as a clinical routine test, although it might provide useful information about retinal function in some patient subgroups.

The blue Þeld entoptic system was one of the Þrst methods that allowed for semiquantitative, noninvasive estimation of retinal blood ßow blood ßow in humans. Thus, the system has been widely used for the investigation of retinal blood