must be bound to large molecules. Fluoresceinlabeled isothiocyanate (FITC)Ðdextran conjugates are molecules of high molecular weight that do not leak from anterior segment vessels. FITCÐ dextrans have been shown to enhance the diagnostic value of the angiograms in retinal vessels of rats, cats, and monkeys [103Ð105] and in episcleral vessels of rabbits [94]. Additionally, lowdose ßuorescein angiogram techniques have been shown to give better quality anterior segment angiograms than does conventional-dose ßuorescein [94]. Approximately 90% of injected ßuorescein is bound to serum albumin and 10% remains unbound [106]. It is known that the unbound ßuorescein leaks from the vessels; [105] because the time for binding to serum albumin is directly proportional to the dose given, by reducing the dose of injected ßuorescein, leakage from the episcleral vessels can be minimized. Intravenous injection of six-tenths of a milliliter of 20% sodium ßuorescein, followed by Þlm photography, provides better dynamic studies in normal and diseased conjunctival and episcleral vessels [94]. But although photographic low-dose anterior segment ßuorescein angiography gives high spatial resolution, the slow recycling rate of most ßash units (one frame per second) restricts the temporal resolution of ßow characteristics and direction. Low-dose anterior segment ßuorescein videoangiography with an image capture rate of 25 frames per second, associated with an image intensiÞer that enhances sensitivity in spite of high luminescence, improves temporal resolution and magniÞcation for ßow dynamic studies in the anterior segment of the human eye [95]. The use of a microcomputer program in conjunction with low-dose anterior segment videoangiography provides complete control of the angiogram, allowing immediate access to any frame, comparison between different frames, and subtraction of any sequence of the study from the remaining ones [107]. Anterior segment ßuorescein videoangiography with a scanning angiographic microscope shows advantages over the photographic and video camera methods through longer depth of focus, larger Þeld of view, lower light levels, coaxial illumination, and real-time traverse of conjunctival/episcleral vasculature

[96]. Anterior segment ßuorescein videoangiography techniques with ßuorescein-labeled dextrans may become the probes of choice for the study of the anterior segment vasculature of the human eye.

3.2.6.2 Normal Anterior Segment Fluorescein Angiography

Anterior segment ßuorescein angiography occurs in three phases: an arterial phase, a capillary phase, and a venous phase [94]. Vessels that Þll early with high ßuorescence and high tortuosity, thick walls, and pulsatile ßow are considered as arteries. Vessels that Þll after arteries, with lower ßuorescence, lower tortuosity, thinner walls, and no pulsatile ßow are considered as veins. Furthermore, arteries never show streaming of blood and branch rarely, whereas veins often show laminar ßow and branch a good deal. However, because veins Þll gradually and diffusely, subsequent to artery Þlling, the moment of their Þrst perfusion is difÞcult to evaluate.

Despite excellent anatomical descriptions [108Ð111] and modern videoangiographic techniques [95, 96], controversy still exists regarding the ßow patterns within the vessels of the anterior segment of the eye. Whereas some studies support the view that the anterior ciliary arterial ßow is from the region of the recti muscles toward the inside of the eye through perforating vessels, that is, centripetal [96, 112Ð116], others suggest that the anterior ciliary arterial ßow is primarily supplied by retrograde ßow from the intraocular medial and lateral long posterior ciliary arteries, that is, centrifugal [81, 88, 93, 95, 117]. Some investigators who favor the centripetal distribution theory believe that other interpretations of the dynamic events result from deÞciencies in photographic and conventional video camera techniques [96]. Resolution of this controversy will require additional studies. Because the main applicability of anterior segment ßuorescein angiography in scleral diseases is to detect areas of vascular closure in the episcleral or conjunctival circulation, the issue of direction of ßow is not critical for patient management.

The different phases of the angiography presented below assume the conventional centripetal

Fig. 3.23 Anterior segment ßuorescein angiogram: arterial phase. Note the extraordinary radiality of the anterior ciliary tributaries, culminating in the formation of loops and anastamoses at the corneoscleral limbus

Fig. 3.24 Anterior segment ßuorescein angiogram: capillary phase. Note the rich abundance of the capillary vascular supply in the episclera. Note also the tiny capillary twigs extending into the far corneal periphery

distribution of the anterior segment circulation of the eye.

Arterial Phase

The Þrst vessels to Þll in an angiography of the bulbar conjunctiva and episclera are the anterior ciliary arteries. These run radially within the episclera toward the limbus, following variable courses (Fig. 3.23). Between 2 and 5 mm posterior to the limbus, the anterior ciliary arteries divide into two branches, which run circumferentially to meet other branches from adjacent anterior ciliary arteries. These anastomoses form the anterior episcleral arterial circle, which broadly resolves into Þve distinct vascular networks: (1) anterior conjunctival, (2) superÞcial episcleral,

(3) deep episcleral, (4) limbal, and (5) iris. Because the anterior episcleral arterial circle is a variable anatomical entity, it may take between 1.5 and 14 s to Þll.

The arteries from the anterior episcleral circle run forward to the limbus, curve backward radially, and divide to form the anterior conjunctival arteriolar plexus. The anterior conjunctival arterioles Þll approximately 1.5 s after the segment of the anterior episcleral segment that supplies them. The anterior conjunctival circulation, supplied by the anterior ciliary arteries, ßuoresces approximately 4 s before the posterior conjunctival circulation, supplied by the peripheral palpebral

Fig. 3.25 Anterior segment ßuorescein angiogram: venous phase. Filling of venous collectors is shown. Note the scattered Òbright spotsÓ residual from the capillary phase. Leakage from capillaries is normal

arch, which is itself formed by terminal vessels derived from the ophthalmic artery. This explains the watershed zone between anterior and posterior conjunctival circulation, which can Þll late.

Branches from the anterior episcleral arterial circle run posteriorly and divide to form the anterior episcleral arteriolar plexus. Neither superÞcial nor deep vascular layers can be detected. These vessels Þll shortly after the anterior episcleral arterial circle.

The limbal vessels often share their origins with the anterior conjunctival vessels. Unlike conjunctival and episcleral vessels, they do not