Ординатура / Офтальмология / Английские материалы / Retinal and Vitreoretinal Diseases and Surgery_Boyd, Cortez, Sabates_2010

angiogram, there is a marked leakage of dye from the fronds of the vessels and subsequent filling of the vitreous with fluorescein.

Retinal Vein Occlusions

In central retinal vein occlusion, the red-free photograph shows a combination of dilated and tortuous veins, intraretinal hemorrhages and blurred disc margins. The marked dilatation and tortuosity of the veins can be appreciated while their walls become stained with the fluorescein dye.

The most characteristic appearance of a branch retinal vein occlusion is its limitation of involvement to one side of the horizontal raphe. The site of the venous occlusion often appears as a hyperfluorescent area on angiography. The fluorescein dye often shows an increase in retinal venous circulation time distal to the site of obstruction (Figure 12).

Angiography is especially useful in documenting the extent of the macular edema in selected macular venous occlusions. Late photos are useful in detecting the presence or absence of cystoid spaces.

Central Serous Chorioretinopathy

Central serous retinopathy, also known as serous detachment of the sensory retina, is a spontaneous detachment of the sensory retina. At the beginning there is a small area of fluorescein leakage into a larger blister-like elevation of the sensory retina. Fluorescein dye diffuses throughout the volume of the serous elevation. The early venous phase shows the area of leakage identified in a small point. The late venous phase shows the extension of the fluid adopting a particular configuration (smokestack) into a large serous detachment under the sensory retina. The margins of the lesion are usually fairly well defined showing the extent of the sensory detachment (Figure 13).

Cystoid Macular Edema

This type of macular edema is a recognized complication of cataract extraction. This condition is characterized by radial separation of the nerve fibers and subsequent collection of pockets of fluid. Fluorescein accumulation may not be noticeable by angiography until quite late, requiring photographs at 30 minutes after dye injection. A characteristic “flower-petal” pattern appears on the angiogram during the late arteriovenous phase.

Age-Related Macular Degeneration:

Sub-Retinal Neovascularization

Sub-retinal neovascularization is characterized by the presence of a tuft of vessels under the retinal pigment epithelium. This type of degeneration is associated with drusen, pigmentary and atrophic changes in

Figure 12: Superior Branch Retinal Vein Occlusion. In this case there is some staining of the wall of the

Boyd., M.D.)

Figure 13: Central Serous Chorioretinopathy. Characteristically, the venous phase of the angiogram shows the area of leakage. The late venous phase will demonstrate extravasation of fluorescein under the sensory retina resembling the typical fluorescent balloon-type image. Such pattern occurs because of a small break in the retinal pigment epithelium and Bruch ́s membrane.

(Courtesy of Samuel Boyd., M.D.)

the retinal pigment epithelium, serous and hemorrhagic detachments of the retinal pigment epithelium and sensory retina.

The new sub-retinal vessels fill with fluorescein at an earlier (pre-arterial) phase of the angiogram than do most other retinal vessels. In the presence of subretinal blood, this may appear like a black zone around the neovascular net (Figure 14).

Retinoblastoma

Retinoblastoma is predominantly a tumor of infancy (between 2-6 years old). The whitish color reflex of the tumor through the pupil is very characteristic, especially in its endophytic phase. The tumor may extend from the optic nerve throughout the orbit and/or the brain. Its main characteristics are the intratumoral calcifications and the presence of seeds floating in the vitreous. In cases of endophytic tumors a second circulation is visible. The fluorescein angiogram shows dye leakage into the tumor and staining of the tumor at different stages. During the early phase of the angiogram there is minimal leakage into the tumor evolving to extensive staining of the lesion at later stages of the angiogram.

Recent Developments in Fluorescein

Angiography

Two important improvements have refined the use of fluorescein angiography. First, the ability to digitize images and to use the overlay techniques have helped to assess

Fluorescein Angiography

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Figure 14: Sub-Retinal Neovascularization. SRNV may have angiographically a variety of appearances. Differentiation between the two typical patterns (classic and occult) is important for treatment guidelines. Here you may observe a well-demarcated area of hyperfluorescence with significant leakage in the subretinal space. This picture shows an extrafoveal lesion with a better prognosis for the patient. (Courtesy of Samuel Boyd., M.D.)

how effective laser photocoagulation has been and to minimize the area of destruction of the retina and the choriocapillaris by having better control of imaging.

The second improvement is the capacity for continuous recording of angiography through video techniques. This technology enables us to view and detect specific new aspects of long familiar problems in different retinal disorders.

Optical Coherence Tomography (OCT) is a modern diagnostic imaging technique that enables the visualization “in vivo” of the cross sectional structure of the retina, the vitreo-retina interface and the anterior segment of the eye with higher resolution than any other non invasive imaging technique.

It is based on a complex analysis of the reflections of low coherence radiation from the tissue under examination.

The resolution available with current instrumentation at present varies approximately from 5 to 10 microns, according to the instruments used. These imaging techniques, which can provide cross-sectional images of intraocular structures, give important diagnostic information complementary to conventional fundus photography, fluorescein angiography and indocyanine green angiography.

OCT is rapidly emerging as a basic imaging tool for the diagnosis and consequently for the control of the evolution of macular diseases: diabetic retinopathy, age-related-macular degen-

eration, macular holes, epiretinal membranes and other retinal diseases.

The discomfort of patients is minimized because the acquisition of the images is rapid and this permits us to acquire many images in different cross-sectional planes of the retina, the vitreo-retina interface and also of the anterior segment of the eye.

OCT images contain much information on the retina structure and have an important role in the evaluation of the disease progression and the response to therapy.

The introduction of OCT systems, using Spectral/Fourier domain, has allowed a higher resolution of the retinal images and a faster images acquisition.

In the past, fluorescein angiography (FA) andindocyaninegreenangiography(ICGA)have allowed visualization of the retinal vessels; today OCT allows the visualization of the structure of the retina, the retinal pigment epithelium and the choriocapillary inner spaces and highlights

the vitreoretinal interspace. Moreover, OCT can quantify thickness of the retina, the amount of subretinal fluid and the pigment epithelium. For all these aspects OCT is today a very important tool for the assessment of the macular diseases, in the choice of the treatment and in the follow-up of the evolution of chorioretinal diseases.

on near-infrared interferometry, it is not affected by axial length, refraction or by the degree of nuclear sclerosis; however large posterior subcapsular or cortical cataracts, as well as a poor compliance of the patient, do impair the ability to perform OCT. This technology is capable of reproducible measurement of retinal thickness in normal eyes(1,2,3).

What is OCT?

OCT uses near infrared, low coherence light to achieve a resolution of approximately 5-10 microns, depending on the instrument used. Similar to an ultrasound which uses sound waves, a CT scan which uses X-rays, and an MRI which uses electron spin resonance, OCT uses light to obtain a cross-sectional image. It uses a non-contact transpupillary approach to obtain a tomograph of the retina which is displayed in real time through a computer. The scan length for each tomogram may be between 2.83 and 12 mm. Quantitative measurement of retinal thickness is possible because of the well-defined boundaries of optical reflectivity at the inner and outer margins of the neurosensory retina. Quantification of juxtapapillary Retinal Nerve Fiber Layer (RNFL) thickness in glaucomatous eyes is also available. Circular scans around the optic nerve, with a circle diameter of either 2.25 or 3.37 mm, without overlapping the disc itself, can be performed. These measurements are obtained by means of a computer algorithm that searches for the characteristic changes. A transverse sequence of optical ranging measurements is used to construct a false color tomographic image of tissue microstructure which appears incredibly similar to a histologic section. Spectral OCT today can function as a type of “Optical Biopsy” in an even more precise way. Since OCT is based

Interpretation of OCT Maps

OCT images can be presented as either cross sectional images or as topographic maps. Crosssectional or B-mode imaging is accomplished by acquiring a sequence of 100 interferometric A-scans across a section of retina. To facilitate interpretation a false color scheme is added in which bright colors such as red and white correspond to highly reflective areas and darker colors such as blue and black correspond to areas of lower reflectivity. Topographic maps obtained by OCT are displayed by a false-color scheme to facilitate interpretation. For crosssectional images, bright colors correspond to areas of high reflectivity while darker colors correspond to areas of low reflectivity. For topographic maps, bright colors are assigned to areas with increased retinal thickening and darker colors are assigned to areas with less retinal thickness.

Retinal thickness is converted to a false color value for each of the 600 points measured within 3,000 microns from the center. Interpolation of polar coordinates is performed to estimate thickness in the wedge-shaped areas between each cross-sectional scan. To further facilitate interpretation, the macula is divided into 9 ETDRS regions with a central circle of 500 um radius. Two outer circles with radii of 1,500 um and 3,000 um complete the display.

In Figure 1a-b we show the normal retina, for comparison with abnormal cases. Figure 2a-b shows alterations in age-related macular degeneration with choroidal neovascularization.

occlusion with cystoid macular edema.

Figure 5a-b: Central Venous Occlusion with Cystoid Macular Edema. a) Fluorescein Angiography shows the presence of numerous retinal hemorrhages in the macular area, secondary to the venous occlusion. b) In these Optical Coherence Tomography (OCT) we can observe the presence of numerous intraretinal areas of high reflectivity, characteristic of hemorrhage and thick retinal layer tissue.

a

b

Current Clinical Application

Imaging of Anterior Segment

Structures

This technology is also very helpful for the anterior segment surgeon, either the refractive or the cataract surgeon. The strongest reflected signals arise from epithelial surface of the cornea and the highly scattering sclera and the iris. Other clearly identifiable structure is anterior capsule of lens. Structures in the angle region like trabecular meshwork and canal of Schlemm are not clearly visualized in the tomogram since the incident and backscattered light is highly attenuated after traversing the overlying scleral tissue.

Glaucoma

Due to the OCT scan, the user can visualize the angle in multiple cross-sections of the anterior chamber. Because the OCT uses infrared light, the pupil does not constrict, providing a more natural view of the angle without changing its anatomy. A measuring tool can then be used to calculate a definitive angle depth in degrees. Now, patients at risk for angle-closure glaucoma may be monitored more closely as the crystalline lens matures.

Evaluation of RNFL in Glaucoma

Figure 6: Optic nerve head scan on OCT showing a normal nerve head.

conjunction with regular clinical examinations with IOP measurements and periodic visual field testing, retinal tomography offers accurate assessment of the retinal nerve fiber layer integrity. Quantification of the peripapillary retinal nerve fiber layer (RNFL) thickness can provide clinicians with objective information about the optic nerve in different pathologic conditions. Several imaging techniques can be used to obtain such a measurement; most recently, optical coherence tomography (OCT) has demonstrated several merits. This technology has been used extensively to quantify RNFL thickness in atrophic diseases such as glaucoma, Leber hereditary optic neuropathy, traumatic optic neuropathy, and band atrophy.

Refractive Surgery Application

Optical coherence tomography is one of the most reliable, reproducible and accurate methods of monitoring changes in the optic nerve and retinal nerve fiber layer (RNFL), which is imperative for diagnosis and management of early glaucoma (Figure 6). When used in

When applied to refractive evaluation the anterior segment OCT maps corneal thickness in 25 spots across the cornea and has great repeatability. It can also create a differential map to compare past readings and detect subtle changes involved in corneal-thinning