Robert W. Flower

Visualization of a developing vasculature

The events involved in vasculogenesis still remain obscure. One difficulty has been the techniques employed to visualize angioblasts, i.e., vascular precursors, during the genesis of blood vessels. The retina provides a unique model for studying these events since it is not completely vascularized in some mammals at birth. Using a previously published magnesium-dependent ATPase technique to visualize the developing retinal vasculature and its precursors, and embedding this tissue in JB-4 methacrylate for serial sectioning, has permitted examination of the retinal vasculogenic processes in dual perspective. The technique has permitted observation of the stages in angioblast differentiation and the apparent importance of glycosaminoglycan-rich cell-free spaces in this process. Perhaps the most important observation is that initial vessel formation occurs by coalescence of angioblasts after differentiation in situ.

Retrolental fibroplasia : Evidence for a role of the prostaglandin cascade in the pathogenesis of oxygen-induced retinopathy in the newborn beagle

Summary: Aspirin administration, at a dosage producing plasma levels within the human therepeutic range, caused marked inhibition of production of both vascular prostacyclin (a vasodilator) and platelet thromboxane (a vasoconstrictor) in beagle puppies. In addition, aspirin-treated, oxygen-exposed puppies developed retinopathy of significantly greater severity than their unmedicated, oxygen-exposed littermates. Direct ophthalmoscopic observations indicated that whereas sustained oxygen breathing produced retinal vasoconstriction in unmedicated puppies, retinal vessels of aspirintreated littermates became more dilated or remained unchanged. It is postulated that retinal vasoconstriction may be a normal physiologic mechanism to protect the immature retina from damaging effects of high blood oxygen levels; i.e., it may be a protective rather than a pathologic process in response to hyperoxia.Many vascular anomalies which characterize the human disease were present in the retinas of the puppies. Several of the most severely affected puppies treated with aspirin even displayed grade III cicatricial retinopathy (falciform retinal fold). Thus, a major criticism of the retrolental fibroplasia animal model has been addressed by producing cicatricial retrolental fibroplasia in puppies, and the confidence with which results from experimental animal studies might be extrapolated to the clinical situation is thereby strengthened.Speculation: It is conceivable that the susceptibility of any eye to oxygen-associated retinopathy at birth depends upon the extent to which the vasoconstriction protective response is functional as well as upon the degree of retinal vascular maturity attained. In utero, the arterial PO2 is low, and blood flow to the immature retina is high. When the PO2 rises at birth, retinal blood flow does not change, although arterial blood pressure rises; this could be explained by the occurrence of some degree of vasoconstriction in response to elevated arterial PO2 or reduced PO2 which establishes the clinically normal retinal vasotonia. In light of this, those cases of “spontaneous” retrolental fibroplasia reported in premature infants never administered oxygen or in full-term infants who were administered oxygen might simply be examples of individuals with inadequate retinal vasotonia for protection of structurally immature vessels from the effects of elevated arterial PO2, and/or excessively high transmural pressure.

INDOCYANINE GREEN FLUORESCENCE ANGIOGRAPHY

Indocyanine green (ICG) fluorescence angiography has been further refined for use in both laboratory and clinical investigations. In the present modification of the Zeiss fundus camera all lenses except the aspherical objective lens have been specially antireflection coated to increase light transmission in the spectral region around 800 nm. A 300 watt indium iodide lamp continuous light source has replaced the conventional xenon flash lamp. This light source produces a retinal irradiance of 265 mw, and therefore restricts retinal exposure time to 11.9 seconds, but that time is more than adequate to record passage of dye through the choroid. Spatial resolution of the fundus on the film has been increased from 11.7 microns to 7.4 microns.

Quantification of indicator dye concentration in ocular blood vessels

The behavior of an indocyanine green (ICG) dye bolus as it passes through the ocular blood vessels provides a calibration standard which permits quantitative dye concentration measurements. Unlike fluorescein, ICG dye can be delivered to the eye in sufficiently high concentrations by intravenous injection that concentration fluorescence quenching takes place in the ocular blood vessels. The relative maximum fluorescence intensity occurring in every blood vessel in which quenching is observed always corresponds to a 0·03 mg/ml dye concentration. Knowing this point and dye concentration in blood as a function of intensity permits routine quantification of dye concentration in blood vessels of the living eye.

Evolution of indocyanine green dye choroidal angiography

During the past three years, indocyanine green (ICG) fluorescence angiography has enjoyed an increasing popularity, especially among clinicians who treat age-related macular degeneration. Although the basic methodology for performing ICG angiography was developed and demonstrated in the early 1970s, nearly 20 years passed before the technology became commercialized for widespread clinical use. The developmental evolution of ICG choroidal angiography and its future clinical uses are discussed.

Clinical Application of Indocyanine Green Angiography

FLOWER, elsewhere in this monograph, has reviewed the fluorescence characteristics of indocyanine green (ICG) and the filter combinations and film best suited for ICG angiography. KOGURE and co-workers (1970) first studied infrared absorption angiography utilizing intra-arterial injections of ICG in monkeys. Flower & Hochheimer (1973, 1974, 1976) further perfected ICG angiography so that intravenous injections could be utilized. This development permitted the practical clinical testing of the dye which is noted for its low incidence of side effects and toxicity. Flower & Hochheimer (1976) have continued their studies and made available the ICG fluorescein angiography camera for the examination of a series of patients with pigmented fundus tumors, retinal vascular and macular disorders. The present report summarizes our experience in approximately 150 consecutive patients with these conditions.

Perinatal ocular physiology and ROP in the experimental animal model

In order to overcome the scarcity of premature human ocular tissues and the enormous obstacles to direct examination of immature human ocular vasculatures, a number of animal models have been employed by investigators in order to study various aspects of ROP. A variety of factors may influence selection of the particular model used, but ultimately it is the faithfulness with which the model mimics human ROP that is most important. The validity of the models has been and remains a controversial subject, but evidence appears strong in favor of the beagle puppy model for studying physiology of the ocular vasculatures during perinatal development.Human ROP pathology usually is defined in terms of static morphological state, physiological dysfunction being considerably more difficult to assess. Most of the animal models fall short of mimicking the pathological lesions found in human eyes, especially those associated with severe, or end-stage ROP, yet they do fairly well in terms of mimicking the retinal vascular physiological changes associated with onset of the disease. Unfortunately, where the physiological aspects of ROP are concerned, focus is primarily on the effects of hyperoxia; other physiological factors as well as the potential role of the choroid are essentially ignored. This paper discusses the potential of physiological changes which occur during the perinatal period to play a role in ROP pathogenesis.

High Speed Human Choroidal Angiography Using Indocyanine Green Dye and a Continuous Light Source

Fluorescein angiography has not proved to be an ideal dye for study of the human choroidal circulation. There are two main reasons for this: First, its unbound molecules readily ‘leak’ from the fenestrated choriocapillaris; and second, its absorption and fluorescence spectra lie in a region where the ocular tissues do not efficiently transmit light energy — this is particularly true of the macular area. On the other hand, indocyanine green (ICG) dye does not tend to leak from the choroidal vessels and has its absorption and fluorescence spectra in the near-infrared region where light energy is transmitted fairly efficiently by the ocular tissues. An attempt has been made therefore, to develop a clinically feasible and relatively simple method of routinely visualizing the human choroidal circulation using ICG.

Electroretinographic Changes and Choroidal Defects in a Case of Central Retinal Artery Occlusion

A 62-year-old white woman with hypertensive diabetes had central retinal artery occlusion of her right eye. Serial electroretinographic (ERG) studies demonstrated reversible changes in the scotopic components but permanent reduction of the photopic b-wave. Serial simultaneous fluorescein and indocyanine green angiographic studies demonstrated a perfusion defect in the choroid involving part of the macula which subsided after reopening the retinal circulation. Differential hypoxic susceptibility of the ERG photopic and scotopic b-waves was demonstrated, and changes in the patients retinal tissue oxygenation were postulated on the basis of retinal and choroidal angiographic studies performed during and after central retinal artery occlusion to account for the observed changes in serial ERGS.

Retinopathy of Prematurity and the Role of Oxygen

The retinopathy of prematurity (retrolental fibroplasia—RLF) has a unique story in the annals of twentieth century medicine. First described by Terry (1) in the early 1940s, RLF became within a decade the largest single cause of child blindness in the United States and, indeed, a greater cause of child blindness than all other conditions combined. The early and mid-1950s saw the incrimination of oxygen as the principal cause of the disease at that time (2–5). Following the identification of oxygen in the etiology of RLF, a drastic curtailment in oxygen usage was instituted in nurseries throughout the world. A dramatic decrease in incidence and severity of this condition then rapidly followed.