Methods, Indications, Examples
Present and Future
In the year 1969, F. Lescure presented the first results concerning fluorescein angiography of the ocular fundus in animals. Since this date, angiography has been taught at the National veterinary school in Toulouse (France).
Fluorescein angiography is a dynamic method to explore the fundus vasculature and tissues.
The fluorescent dye used is sodium fluorescein injected intra venously. The induction is obtained by a first excitation, the white beam of light of the angiograph machine passing through a blue filter (400-500 A). Actually, the filters of the angiographs are interferencials, very selective, and with a narrow wavelength. The ocular structures impregnated by fluorescein are illuminated by a blue radiation which makes them luminescent. The yellow-green light so induced is observed through a barrier filter which will allow only the rays emitted by the fluorescein to be examined (near 5250 A)
The angiographic apparatus we use has a flash generator, with a rapid regeneration (1 or 2 frames per second), a variable power, from 10 W/sec to 300 W/sec., a wide angle lens (45 ° up to 60 °), two cameras, one for color retinography and the other one, motorized, for the black and white angiography. The apparatus is equiped with a timer, and two interferencial filters of excitation and barrier. The black and white film is a 400 ASA emulsion, which is processed in 800 ASA.
For the retinography, a 50 ASA film is available, the animal fundus being more reflective than the human one.
Performing An Angiography
Half an hour before the test, the pupils of the animal will be dilated using tropicamide drops. General anaesthesia is useful to obtain a complete motionless of the globe.
First, a color picture is done, and a second one with a green filter (red free).
The intravenous injection of fluorescein is done just after having changed the color camera by the black and white one. Each second, we take a picture during 15 seconds, then more slowly when we reach the late venous phases. During all the time, it is necessary to maintain the cornea wet, the lids being maintained open.
The film is then processed; the pictures on paper are selected and the interpretation is possible.
There are four stages:
The choroidal phase
The arterial phase
The arterio venous phase-capillar phase-
The venous phase
During this time, it is interesting to make pictures of later phases. Also, we consider the optic nerve head aspects.
The choroidal phase is visible only in the tapetal area; in the non tapetal area, the retinal pigment epithelium (RPE) containing melanin pigments does not allow the visualisation of the underlying choroid. The staining of the choroid can be progressive, the chorio capillaris filling by sectors, so the diffusion of the dye is not immediate.
The retinal phases are
The arterial phase, one or two seconds after the choroid, the arteries of the non tapetal area are first visible. Within 2 or 3 seconds, all the arteries are impregnated, from the papilla until the peripheral retina. The diameter of the vessels is superior to what we can observe by ophthalmoscopy. This phenomenon is due to the staining of the peripheric plasmatic column associated to that of the vascular endothelium.
The arterio venous phase (capillar phase) presents a laminar aspect due, at this stage of the test, to the simultaneous presence of non fluorescent cells in the center and fluorescent plasma in periphery. During this time, the capillars are impregnated; their study is easier in the non tapetal area where they are more visible (over the black fundus of the RPE).
The venous phase occurs then, all the vessels are totally fluorescent. The capillars become empty, and, finally, completely black.
The late phases: after 40 seconds, the non-tapetal area becomes more and more dark, the fluorescence disappears progressively. After 15 minutes, only the papillar fluorescence is persistent.
The papilla is dark in the early phases, and then the arteries of the Zinn circle are fluorescent. At the capillary phase, a diffuse fluorescence is noted, the center remaining black.
It is due to the lack of fluorescence, in a place where it is normally present at a given time.
The filling defect could be even a delay or an absence of filling. The cause is usually a vasculopathy, an arterial obliteration, a capillar or a venous obliteration.. In case of choroidal ischaemia, the angiographic aspect is a triangular zone which remains black, in the early phases.
The masking effect is the interposition of a screen, more or less complete, according to the level where it is situated.. When the screen (hæmorhage for instance) is pre retinal, the masking is present all along the angiogram. When the screen is intra retinal, one can observe the superficiel retinal vesels.
Three situations are encountered:
The window effect
Leakage of the dye
Window effect. The choroidal windows are visible in the tapetal fundus, all along the angiogram. When the R.P.E. is alterated, the window effect can be observed in the non tapetal area, since the choroidal phase.
Leakage. It is due to defects in the permeability of the vascular compartment. When leakage occurs in a liquid formation, it is named «pooling effect», for instance in oedema, serous cavity. When leakage occurs in a tissue, it is called «staining effect», for instance in cases of retinal epitheliopathy. (C.P.R.A., P.R.A.).
Abnormal vascular structures: neovessels. This vascular proliferation is encountered in chronic uveitis, and in the margin of ischæmic areas, for instance in the Triangular Amalric syndrome. The leakage of the dye is constant.
It is often useful to perform an angiography in two groups of cases:
1. When the ophthalmoscopy indicates lesions:
a. Of vessels (narrowing, granular aspect, hæmorhages)
b. Of tissues: partial or total hyperreflectivity of the tapetal area, pigment migrations
2. When there is a history of lowering or loss of vision, even without lesions visibles in the eye fundus.
We shall show and discuss some examples of angiograms, one normal and others pathologic:
Serous detachment of the R.P.E.
Amalric triangular syndrome
It is a particular situation that, in human medicine, all the retinal lesions are explored by an angiographic test. In veterinary ophthalmology, for reasons unknown by myself, the use of angiography is very restricted. I don't think that the cost of the angiographic machine could be the cause, neither the relative difficulty of the interpretation. As we can see, some diseases cannot be diagnosed without angiography, some others need this test for a sure differential diagnosis, and in some others cases, angiography allows to follow correctly the evolution of the disease.
I hope this lecture will give to someone in the audience the stimulation to perform this so gratifying ophthalmological test.