06 Correlation Between Pathological Anatomy

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CHAPTER 6 CORRELATION BETWEEN PATHOLOGICAL ANATOMY AND OPTIC NERVE COMPUTERIZED TOMOGRAPHY: NEW DEFINITION OF THE HISTOLOGIC NOMENCLATURE Prof. Dr. Jorge Oscar Zrate

6.1 Introduction With the new terminology for the study of the optic nerve head, through the noninvasive tomographic method, histopathologists need to use a language shared with such methodology in order to determine the normal range and to interpret eventual pathologic processes. The conventional section of the eyeball through a horizontal section covering all the main structures of the eyeball in an anteroposterior direction (cornea, anterior chamber, iris, lens, vitreous, retina, macula and optic disc, choroid, sclera and optic nerve) may, in certain circumstances, not adjust to said correlation. In order to better understand this aspect, it should be kept in mind that in this histologic section only thickness can be obtained. Therefore, a modification of the type of histologic section used was necessary, so as to later match it with the HRT. The section of the eyeball in a vertical equatorial plane, dividing it in an anterior hemicasque and a posterior one, allows perpendicular sections of the anteroposterior axis of the optic nerve head to be performed on it. These sections are similar to those obtained with tomography and can be correlated to them. The thickness of each histologic section can be measured, and volumes can be obtained as they are added together. Perimeters and areas can be measured by these sections and greater accuracy is achieved for the detection of damaged bundles and fibers in certain sectors by quadrants and octants, when there are localized defects. Furthermore, greater accuracy is achieved in the detection of damaged bundles and fibers, in certain areas, in relation to quadrants and octants when there are localized defects. Both sections (the conventional one, horizontal in an anteroposterior direction, and the equatorial vertical section, dividing the eyeball into hemicasques) may be used in combination. If the posterior casque is divided into two by a horizontal plane that crosses the optic disc center, conventional sections can be performed in one half, while in the other, sections can be made perpendicular to the anteroposterior axis of the optic nerve.

66 In brief, nomenclature and histologic examination techniques must be renewed to better understand the correlation between histology and HRT. 6.2 Material and methods Six eyeballs were studied performing perpendicular sections of the optic nerve, three of them were enucleated from fetuses (34-36-38 weeks) and the other three belonged to adults with intraocular tumors. The method used to perform these sections is shown in figure 6.1, where between each histologic section, represented by a horizontal line there is a distance of 50 microns. Sections of the eyeballs were performed successively from the vitreoretinal interface towards the back. In the posterior casque, the histologic sections were performed from the front towards the back until reaching the plane corresponding to the vitreoretinal interface at the optic disc area. This plane is shown by image number 11 of the computerized tomography. The planes were stained with hematoxillineosin and argentic impregnation techniques. From said plane on, about 30 semiseried sections were performed towards the back.

Fig. 6.1 Figure 6.1 shows the following measures as examples: perimeters and areas of large and small optic nerve fiber bundles, optic disc perimeter and area, cup perimeter and area, and neuroretinal rim perimeter and area. The sections were measured with an image analyzer. The image analyzer has a microscope where the specimen to be studied is placed. A TV camera transmitting the selected image to a monitor connected to the computer, is at the third tube. The image is digitized by means of a computer program (Pro-Image). The boundary of the region to be measured is drawn with the mouse on the image on the screen. The measures are obtained in microns (µm). Figure 6.2 belongs to the images obtained with the image analyzer, where the areas of the different successive histologic sections to be measured can be observed with their perimeters marked out.

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Fig. 6.2: Digitized images in successive sections. On the left there are 16 semiseried sections of the optic nerve. The image on the top right-hand shows part of the neuroretinal rim and the optic nerve. On the bottom left-hand there is a section of the optic nerve. At the center of both images, the hyaloid duct and the optic nerve vessels can be seen. The perimeter and surface to be measured are shown as they were drawn with the mouse. Furthermore, conventional sections were performed through the optic disc region in 40 fetuses and 40 adults, where only the thickness was measured and the volumes were obtained by multiplying area by thickness. An optic disc region was then studied by octants (figure 6.3a). This technique was carried out only once due to its operational difficulty. Figure 6.3b shows the sections performed in order to study tissue and thickness. We believe that an easier method to study each octant in the future would be the one described by figure 6.3c.

Fig. 6.3a

Fig. 6.3b

Fig. 6.3c

68 6.3 Definitions and parameter correlation

Fig. 6.4a

Fig. 6.4b

Optic disc (figure 6.4): The elevation produced by a large number of axons converging on the optic nerve head; it constitutes a surface of which perimeter and area can be measured. It can be steep, moderately steep or cupped, depending on the quantity and persistence of the axons, glia, connective tissue, central vessels ramifications and optic nerve head size. This elevation under normal conditions is greater in the nasal and superior parts, smaller in the inferior part, and less evident in the temporal side (figure 6.4a: fetal optic disc; figure 6.4b: adult optic disc).

Fig 6.5a

Fig. 6.5b

Optic nerve head or intraocular portion (figure 6.5): It is made up of the choroidal and retinal portions of the optic nerve, including the tissues between the optic disc surface and the lamina cribrosa (microphotograph of figure 6.5a). Its lateral limits are the internal margin of Elschnig’s Ring (as shown in the diagram in figure 6.5b). It is possible to determine the volume of the optic nerve head, which is made up of the greatest part of the intraocular portion of the optic nerve, as well as its perimeter and the areas of sections performed at different heights of its course.

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Fig. 6.6 Optic disc (figure 6.6): It is the only group of structures included within the contour line analyzed by the tomograph. The internal margin of the scleral Elschnig’s Ring is considered to be the lateral or peripheral limit, and the first tomographic section with incidence on the retina is considered its anterior limit.

Fig. 6.7 Optic nerve head or intraocular portion (figure 6.7): The limits of the optic nerve head are: the internal margin of Elschnig’s Ring as the lateral or peripheral limit, the first section with incidence on the retina as its anterior limit, and the lamina cribrosa as the posterior limit. The sections obtained with the tomograph that follow plane number 11 are distributed in this segment.

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Fig. 6.8a

Fig. 6.8b

Neuroretinal rim (figure 6.8): The entrance of the axons surrounded by glia, to the optic nerve head area. Its external limit is the internal margin of Elschnig’s Ring, that separates it from the retina proper, and its internal limit is the cup. It is a mass of tissue (axons - glial tissue) indicated in figure 6.8a with an arrow. Its perimeter, area and total volume can be measured. Likewise, relative volumes of predetermined conventional limits are also measured as in the case of the reference plane used by the HRT. Figure 6.8b shows a perpendicular section at the level of the neuroretinal rim.

Fig. 6.9a

Fig 6.9b

Cup (figure 6.9): Figure 6.9a shows the optic disc cupping and figure 6.9b a diagram of the cup with the hyaloid duct. The cup is located in front of the lamina cribrosa. It is continuous with the hyaloid duct towards the posterior part and it is surrounded by the neuroretinal rim. Part of the cup (which may be eccentric), in normal conditions, is caused by the reabsorption of the glia accompanying the central vessels during development. The cup depends on the diameter of the optic nerve canal and its volume is inversely proportional to the number of nerve fibers. The cup perimeter, area, total volume and relative volumes can be measured.

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Fig. 6.10 Neuroretinal rim (figure 6.10): The structures above the standard reference plane (software version 2.01) and within the projection of the contour line drawing (internal margin of Elschnig’s Ring) are referred to as the neuroretinal rim. Its volume and surface can be measured and it is divided into a flat neuroretinal rim (in green) and tilted neuroretinal rim (in blue). The limit between both parts is determined by the plane projected from the peripapillary retinal surface known as Curved Surface, whose area is located at the same level as the peripapillary retina.

Fig. 6.11 Cup (figure 6.11): It is defined as the space not occupied by nerve fibers (free space) below the reference plane and surrounded neuroretinal rim fibers. Its volume is calculated through an estimated volume of the structures below the reference plane from

72 which the volume outside the cup is subtracted. Its surface, as well as its volume are shown in red.

Fig. 6.12a

Fig. 6.12b

Contour line (figure 6.12): It is an imaginary line. Its histologic representation is obtained from the drawing of a perpendicular line from the internal border of the scleral spur to the retinal surface (figure 6.12a). Morphologically it constitutes the limit between the actual internal limiting membrane of the retina and the astroglial membranous layer that covers the optic disc (figure 6.12b). In both figures, the glial membranous layer is thicker and hyperchromic. It constitutes the optic disc perimeter.

Fig. 6.13a

Fig. 6.13b

Fig. 6.13c

Cup shape measure (figure 6.13): In normal conditions, the axons of the retinal nerve fiber layer enter the optic canal with a slight slope that shifts them from a horizontal direction to a vertical one, which morphologically determines a curve that depends on the volume occupied by these fibers (figure 6.13a: negative cup shape measure). When this volume decreases, the transition between the horizontal direction and the vertical one of this entrance may form a right angle (figure 6.13b: cup shape measure = 0), or an acute angle (figure 6.13c: positive cup shape measure). Therefore the shapes that fibers take upon penetrating the canal determine the histologic representation of the cup shape measure.

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Fig. 6.14 Contour line (figure 6.14): This line is drawn by the observer in order to determine the exterior limit of the optic disc. It should be drawn at the internal margin of Elschnig’s Ring. Since it is not laid out parallel to the retina and, as it is covered by different numbers of fibers in its different parts, its margins must be looked for in the different planes. Once the optic disc has been delimited with the contour line, the tomograph measures all the structures found within its course.

Fig. 6.15 Cup shape measure (figure 6.15): The cup shape measure is defined as the skewness of the distribution frequency of the depths within the contour line. When the optic disc is normal and shallow with slight slopes, the cup shape measure has a negative value. When the slope starts to become steeper, the value gets closer to zero, and finally, when there is a great cupping of the disc in association with the presence of bayonetshaped vessels, the value of the cup shape measure is positive.

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