03 the Contour Line

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CHAPTER 3

THE CONTOUR LINE

3.1 The optic disc - Elschnigs Ring

The optic disc is the anterior end of the optic nerve and it is formed by the conver-

gence and grouping of the retinal nerve fibers that go through common holes of the three

ocular tunics.

In an ophthalmological examination it looks like a yellowish pink circle, much

lighter than the fundus color. In a binocular biomicroscopic examination, it looks like a

disc.

The lamina cribrosa coming from the sclera divides the optic disc into: an anterior

part where the nerve fibers have no myelin (amyelinic), and a myelinic one posterior to it.

This makes the anterior part transparent or translucid, which allows for its examination

with the HRT (as it only scans transparent tissue). It is the only part accessible to this

examination. The whole constitutes the optic nerves bulbar segment.

The narrowest part that the fibers go through on their way within the bulbar segmentof the optic nerve is located at the level of Bruchs membrane (choroid vitreous sheet

foramen). This foramen divides the intrabulbar segment of the optic nerve into two trun-

cated cones that are joined by their minor bases (figure 3.1). It is thus divided into two

portions: a) an anterior retinic portion and, b) a posterior chorioscleral portion.

The Retinal Portion (a in figure 3.1) is made up of two protrusions: a nasal one

(larger as it has a higher number of axons) and a temporal one (smaller). Between both

protrusions there is a depression that may have an umbilical, or at times cylindrical,

shape. This depression is a physiological cupping that is not central but eccentric, and

which is located at the temporal half. The pigment epithelium is separated from the most

peripheral fibers of the optic nerve by a conjunctive tissue known as intermedial Kuhnt

tissue. In this part, the retinal central artery trunk crosses the whole retinal portion of the

optic disc and is bifurcated. The arterial vessels are always medial to the venous vessels.The vein trunk, in contrast, is formed by the joining of two branches, one superior, and

the other, inferior, at the level of the lamina cribrosa, i.e. more towards the back. The

vessels are surrounded by glia without interposed retinal fibers. This glia becomes thick

at the center where it surrounds the vessels, and it forms the central support meniscus of

Kuhnt.

The same tissue accompanies the vessels within the optic nerve, where it is called

Elschnigs intercalar tissue. The internal limiting retinal membrane (feet of Mullers fi-

bers) does not exist in the optic disc, instead there is glia. In the periphery, it continues


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with the internal limiting membrane. This is the internal retinal limit of the optic disc,

which, as mentioned before, is greater than the external one (scleral ring). This accountsfor its cone trunk shape.

The chorioscleral portion of the optic disc (figure 3.1) is that portion extending

from the orifice formed by Bruchs membrane, towards the back. It consists a truncated

cone whose smaller base is anterior and larger base posterior. This portion is the optic

nerve canal. The lamina cribrosa, which is at the level of the sclera, divides this chorio-

scleral portion into two parts: an anterior one and a posterior one. The anterior part is

formed by amyelinic fibers and the posterior part by myelinic fibers. The lamina cribrosa

is opaque and conjunctival in its posterior part. It is called scleral lamina cribrosa. The

choroidal lamina cribrosa, which is glial and transparent, is located in front of the scleral

lamina cribrosa, and at the level of the choroid.

It should be stressed that the precribiform amyelinic portion has ectodermal glia (as-troglia), and that the retrocribiform myelinic portion has ectodermal and mesodermal glia

(astrodendroglia, microdendroglia and oligodendroglia )

Elschnigs mesenchymal tissue forms, in its anterior part, the scleral spur (E in fig-

ure 3.1) and as a whole, it constitutes the scleral optic foramen, that when ophthalmo-

scopically visible, is referred to as Elschnigs Scleral Ring. In front of it there is a trans-

parent glial ring [1].

Everything inside the scleral ring is the optic disc, and everything outside it is the

retina. The parapapillary zone is the one surrounding the scleral ring.

Fig. 3.1: R: retina, EP: pigment epithelium, B: Bruch membrane, C: choroid, E: sclera, l:

choroidal lamina cribrosa, 2: scleral lamina cribrosa, p: scleral spur, a: retinal portion, b:

chorioscleral portion. At the bottom right: scleral spur in detail; it forms Elschnigs Ring.

Its anterior projection, whose internal margin corresponds with the external limit of the

optic disc is clearly seen. On the left side, the correlation with a topographic image.


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The internal margin of Elschnigs Ring corresponds anatomically with the external

margin of the optic disc, and therefore, with the external margin of the Neuroretinal rim

[2, 3].

Figure 3.2 shows a histological section of a normal optic disc, where the scleral spur

and the fibers that cross the scleral canal can be clearly seen. Notice the shape of the two

cones joined at the vertex.Elschnigs scleral ring can be most frequently seen in the temporal area of the optic

disc, and at times it can be seen in the nasal area as well. However, it is more difficult to

see it in the superior or inferior quadrant since there is a greater number of fibers there

(figure 3.3). The image on the left shows that the superior and inferior quadrants are

darker because it is a topographic image presenting the more elevated structures with

darker colors, and the more depressed structures with lighter colors.

It is more difficult to see Elschnigs ring in normal optic discs than in pathological

ones [4, 5], because retinal fibers are located on the anterior face of Elschnigs ring, thus

hindering observation. Elschnigs ring can be seen almost entirely in severely damaged

optic discs because all the retinal fibers have disappeared.

In figure 3.3, in the image on the right, both the internal and external margins of the

ring can be clearly seen in the temporal margin. The ring cannot be seen so clearly in thenasal rim due to the greater number of fibers crossing its anterior face and because of

vessel disposition. At the poles the ring is completely covered by the great number of

retinal fibers coming into the optic nerve.

Figure 3.4 illustrates Elschnigs ring at 360 degrees. This optic disc belongs to a late

congenital glaucoma. This is made evident by the fiber loss in every sector, allowing the

ring to be seen clearly.

Fig. 3.2


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From the center of the papilla towards periphery the following structures can be

observed:

- external margin of the cup = internal margin of the neuroretinal rim

- external margin of the neuroretinal rim = external margin of the optic disc = internalmargin of Elschnigs ring

- external margin of the Elschnigs ring

- parapapillary area

In figure 3.5 we can see the same optic disc, where these limits are marked in an im-

age similar to the one observed in the eye fundus, which is the summation image (on the

right) and in a topographical image (on the left). It must be noted that the limits are accu-

rate in both images and therefore, they are both taken into account when the contour line

Fig. 3.3

Fig. 3.4


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is drawn. There is a graph at the bottom of this figure that shows the disposition adopted

by Elschnigs ring with regard to HRT sections. As it is not parallel to the retina, it cannot

be completely seen in any of the 32 planes. If the planes are displayed one after the other

at high speed, the different parts of Elschnigs ring combine together in such a way that

the image of the whole is produced. This can be achieved with the Movie menu [5].

3.2 Contour line drawing

What makes Elschnigs scleral ring so important is that its internal margin signals the

external limit of the optic disc. It has been proven that this ring does not vary in structure

nor dimensions throughout glaucoma evolution, because, as it belongs to the sclera it is

not an elastic tissue [6].

If we measure the optic disc taking this structure as a limit, we will always be meas-

uring the same thing since the frame of reference does not vary. This does not happen in

the optic discs of children with congenital glaucoma [7].

The contour line is a tool for optic disc examination that serves as an aid to deter-

mine the external limit of the optic disc, because it is a line drawn at the internal margin

of Elschnigs ring. This is done with a mouse handled by the observer, who must care-

fully follow the edge of the internal margins of the ring. Most of the quantitative pa-

rameters provided by stereometric analysis depend exclusively on this drawing. Because

of this, if the contour line is not drawn properly the parameters are not valid for optic discanalysis.

Once the 32 planes have been processed (S, A, T), the Processing menu must be

selected, then the Analyze Topography submenu must be chosen. Two images will ap-

pear on the screen: The summation or extended focus image (on the right) which is the

most similar to what is usually seen in the eye fundus; and the topographical image (on

the left), which presents the anterior structures in dark and the posterior ones with light

colors [5].

Fig. 3.5


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The next step is to choose the Contour Line menu, within which two submenus

must be chosen. The first one is the Draw option, which allows for contour line drawing

with the mouse; the second one is the Display Images or Image Selection option which

allows you to choose, from the image on the right, any of the 32 images obtained in the

series, in order to make the drawing easier [5]. As seen in figure 3.3, the complete scleral

ring path cannot be seen in the summation image, only a part of it can be seen. The ob-

server must then draw the contour line with the mouse, on the visible parts, and he must

go on drawing it on subsequent planes until completed, with no discontinuity. Figures

3.6a and b show the ring segments appearing in the images of the tomography. They also

illustrate three different individual planes.

The contour line can be drawn in either image (topographical or summation) since

the computer automatically and simultaneously draws the line on the other image.

This is very useful because sometimes much information can be found in the topographi-

Fig. 3.6a

Fig. 3.6b


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cal image. It must be stressed that the contour line tracing should be drawn exactly on the

internal rim of Elschnigs ring, not a pixel inside or outside this structure. Furthermore,the line drawing in the first examination of the patient is very important since this very

same contour line may later be exported to the subsequent tomographies for a follow-up,

thus eliminating intraobserver variation.

As we have mentioned before, this contour line may be drawn on one of the series

images and then be exported to the mean. Also, it can be drawn directly on the mean

without the aid of the 32 planes. Once it is drawn, the option Accept of the Contour

Line menu, can be chosen in order to accept the new contour line. If the line were not

complete, the computer would indicate so with a white circle on the place where it is

discontinued. If it has been drawn over one of the original series, it should be exported to

the mean with the Export function in the Contour Line menu.

The correlation between the projection of the scleral rings internal rim and the con-

Fig. 3.7

Fig. 3.8


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tour line drawing can be observed in figure 3.7.

Figure 3.8 shows how the computer analyzes everything inside the contour line. Theright side of the optic disc was not analyzed in order to show where the contour line was

drawn.

3.3 Understanding the contour line

Once the contour line is traced and accepted, the computer analyzes any structure in-

side of it. This includes a list of quantitative parameters, the stereometric analysis, the

color surfaces analysis, the parameters studied by quadrants and octants, and various

other different displays of results.

Much of this information is explained in chapters 4 and 5. From now on we will de-

vote ourselves to the explanation of those topics closely related to the contour line.

Once the line is accepted, the computer automatically locates the Reference Plane,which is at 50 m beneath the retinal surface. It is important to make it clear that it is not

located 50 m beneath the total optic disc surface, but rather beneath a sector of the tem-

poral quadrant.

Based on a study by Quigley and Addicks [8], and Airaksinen and Tuulonen [9], it

was determined that the sector that least varies in thickness throughout glaucomatous

optic disc changes is the temporal sector, more specifically, the angle between -4 and -10

degrees at the center of the papillomacular bundle. This sector becomes narrower only in

the final stages of the disease.

In a 44 year-old female patient with glaucoma due to goniodysgenesis, with an optic

disc in the terminal stage and a severely damaged visual field (stage 3) the retinal thick-

ness in every quadrant and octant was compared with normal values. Thickness decreased

by 75% in the nasal quadrant, 50% in the superior temporal octant, 80% in the inferior

temporal octant and 6% in the temporal quadrant. This shows that the loss of retinal

thickness in the papillomacular bundle sector does not occur until the optic disc is se-

verely damaged. This 6% loss might be due to the fact that the temporal quadrant covers

90 degrees and the sector where the plane is located includes only 6 degrees (see chapter

9).

We can then say that the Reference Plane (software version 2.01) is located 50 m be-

neath the retinal surface in the temporal sector, within the -4 to -10 degrees angle (papil-

lomacular bundle) (figure 3.9) [5,8,9].

As the contour line goes through the superior and inferior poles, where there is agreater number of fibers than in the rest of the optic disc, its height in both of these sec-

tors in relation to the reference plane increases, whereas it decreases in the nasal and

temporal sectors.

If we take the contour line and extend it linearly so as to cover the 360 degrees from

0 to 360, we can observe that its course has the appearance of a double camel hump or

peak. The first hump represents the height increase of the contour line in relation to the

reference plane as it goes through the superior pole of the optic disc. The second hump

represents the inferior pole.


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In 1993, J. Caprioli published a paper where the double hump image is obtained by a

different method [10]. Caprioli then showed how the humps height decreases in glauco-

matous patients when compared to a control group (figure 3.10).

Figure 3.11 shows the spatial relationship between the contour line and the refer-

ence plane and also shows the location of the humps on the contour line in a three-

dimensional graph. Figure 3.12 is the same image in a two-dimensional graph referred to

as height variation along the contour line.

The height variation along the contour line graph represents one of the programs

most important piece of information. Together with the stereometric parameters it helps

understand the optic disc condition, and unlike the previous ones, it helps distinguish a

generalized defect from a localized one. Also, in this latter case, it allows to locate the

defect within the 360 degrees. It also helps correlate the fiber defects in the visual field,

Fig. 3.9

Fig. 3.10


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in the gray scale as well as in the scale of values. Figure 3.12 shows the contour line dia-

gram exactly as it appears on the screen.

Three different color lines can be seen in this figure: The green line is the contour

line, the red line represents the reference plane, and the white line, the mean height of

the contour line. This last line is often located between the green line and the red line

[5].

On the right side of the graph there is a scale of absolute height values (axis z) and

on the left, a scale ofvalues relative to the mean height of the contour. The latter gives a

negative value to anything located above the mean height of the contour line, and a posi-

tive value to anything below it. In other words, it considers the mean height of the con-

tour line as the mean of the retinal surface in the optic disc contour. This is why we say

that everything before the retinal surface has a negative value and everything behind it

has a positive one.

Fig. 3.11

Fig. 3.12


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As mentioned before, the line is extended in the direction from 0 to 360 degrees, that

is to say that the line goes through the four quadrants in the following order: superior

temporal quadrant (STQ), superior nasal quadrant (SNQ), inferior nasal quadrant

(INQ) and inferior temporal quadrant (ITQ).

On the right of the graph, the word tilted appears on the screen. It indicates that the

computer has corrected the tomography in such a way that even if there were a certaininclination in the laser incidence angle, the line curve corresponds to a perfectly perpen-

dicular beam incidence on the retina. The word relative indicates that coordinates along

the z axis are relative to the average height of the peripapillary retinal surface.

Hrefindicates the location of the reference plane along the z axis.

Fig. 3.13 and 3.14 illustrate the same graphs but belonging to a normal optic disc

and a pathological one.

Fig. 3.13 shows the two humps in the contour line graph. In this case the superior

one is smaller than the inferior one. It is important to note that the green line (contour

Fig. 3.13

Fig. 3.14


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line) seems normal and is far away from the line representing the reference plane. The

white line, which indicates the mean height of the contour line, is also far away from thereference plane, which indicates that the mean retinal thickness below the contour line is

well maintained.

Fig. 3.14 shows a pathological optic disc. The contour line profile is different from

that of a normal optic disc. As in most cases, one of the first changes is the decrease, then

the loss, of one or both humps. In this case, no localized depression can be observed

throughout the contour line profile, which means that there are no localized defects or

bundles of damaged fibers. However, a general loss in height of the contour line can be

seen, which might be related to an increase in the mean defect of the visual field.

It is also important to note that the white line (mean height of the contour line) is

closer to the reference plane than in the above mentioned case (compare both yellow

segments). The height of both humps has diminished. During the next examination of thesame patient it will be very important to check that the contour line does not show any

localized depression. Should this happen, it would be logical to eventually find a sco-

tomatous defect in the visual field. Turning back to figure 3.14, this homogenous de-

crease of the mean height of the contour line should be related to a damaged neuroretinal

rim with a concentric cupping that does not show any notch or localized thinning in all its

course. If this were the case, the line would show a significantly worse depression or

decrease in height which would be easy to locate in a quadrant or a segment.

It should also be stressed that sometimes these localized depressions of the contour

line may be due to the fact that though the examiner has drawn the contour line correctly,

he has drawn that sector inside the actual optic disc limit, thus letting the contour line fall

inside the cup. Similarly, the absence of both humps with a lack of localized depressions

may be due to the fact that the examiner has drawn the contour line outside the internalmargin of the scleral ring (i.e. outside the external optic disc limit).

In light of all of this, we reaffirm the importance of a correct contour line drawing

by experienced examiners who are well-acquainted with optic disc anatomy.

3.4 Basics and Fundamentals

When the 1.10 software version was used, significant differences were found re-

garding the parameters resulting from the contour line drawing because it placed the ref-

erence plane at 320 m below the contour line mean height.

The differences were smaller in normals and became larger according to the severity

of the optic disc glaucomatous damage.

We must also bear in mind that every reference plane is relative and parallel to theretinal surface. If we select a point in the temporal optic disc area it may or may not cor-

respond with the scleral level in the nasal zone according to the thickness of the nerve

fiber layer.

The problem with this reference level lay with the fact that it was chosen on the ba-

sis of reflectivity images, then it was written down and used for measurements.

The next step was to select a point on the temporal contour line for the measure-

ments. This new reference level was called papillomacular. With this new level or refer-

ence plane, good results were obtained from normal optic discs to advanced glaucomas,


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where the differences between both groups are statistically significant (Airaksinen and

Tuulonen).In version 1.11, a modification was introduced in the papillomacular reference level.

Instead of choosing a point in the temporal area of the optic disc, an area between -4 and

-10 degrees is now chosen. Consequently, the papillomacular bundle is taken from below

the zero degree of the horizontal line, since the macula is anatomically slightly below it.

A location 50 m below the retinal surface, which is the reference level used in

software versions 1.11 and 2.01, is taken within these 7 degrees. These 50 m come from

Dr. Pickli's studies of the primates' eyes where he discovered that the thickness of the

nerve fiber layer in the papillomacular area is 50 m [11], and from the studies of Quig-

ley and Addicks [8].

We should keep in mind that studies on the location of the reference plane are still in

process. When the optic disc is severely damaged, the reference plane may be locatedeven lower than 50 m due to a decrease in retinal thickness (even in the temporal sector

in terminal optic discs). As a consequence of this, the results may show a greater volume

of the neuroretinal rim and a lower cup volume in a longitudinal study.

Extensive research is being conducted at present to achieve an automatic location of

the reference plane, but based on the scleral reflex finding, which should be quantifiable

[12].

3.5 Papillomacular bundle persists normal in the course of optic nerve head deterio-

ration in glaucoma evolution. A case report.

In figure 3.15 the right eye of a patient is shown, presenting a damaged optic nerve

head, which is in phase 4 of our classification (see chapter 10). As the stereometric analy-

sis shows, the rim area is decreased while the cup area is increased. The contour line dia-

gram shows a small retinal thickness remaining above the reference plane. There is no

presence of localized nerve fiber bundle defects.

The yellow arrows on the right side indicate the remaining neuroretinal rim, and the

arrows on the picture on the left show the same structure in green and blue colors.

Figure 3.16 shows the same optic nerve head in a 20 degrees field examination,

where the fundus presents a diffuse loss of the RNFL, but just one nerve fiber bundle -

the papillomacular bundle - is still normal. Its reflectivity is superior in relationship to the

rest of the retina, and the values for the contour profile are bigger.

On the upper left picture, the topographic image shows that there are no localized

defects. Only one bundle is darker because it is more anterior than the rest. On the right

side, the papillomacular bundle appears more clearly than the rest.On the bottom left, the three-dimensional structure shows the normal bundle in the

temporal quadrant, and on the right side, the reflectivity image shows more clearly this

bundle.

All these features indicate the fact that the papillomacular bundle remains intact up

to the last stage of glaucoma evolution and it helps to hold the reference plane level along

this area. The visual field of this patient is in stage 3 with the 15 central degrees con-

served.


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On the left eye of this patient we can observe the same phenomenon as in the right

eye, and the visual field is almost the same (figures 3.17 and 3.18).We conclude that the present location of the new standard reference plane is good

enough to measure the parameters of the optic nerve head along glaucoma evolution.

Nevertheless, in some cases, it is very useful to control the Height Reference values in

order to evaluate if the papillomacular bundle is still preserved.

Fig. 3.15

Fig. 3.16


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Fig. 3.17

Fig. 3.18


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Bibliography

1. Sampaolesi R: Glaucoma. Second edition, 1994, p. 299-301. Editorial Panameri-

cana.

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3. Jonas JB, Airaksinen PJ, Robert Y: Definitionsentwurf der intra- und parapapil-

lren Parameter fr die Biomorphometrie des Nervus Opticus. Klin Mbl Augen-

heilkd 1988;192-621.

4. Sampaolesi JR, Sampaolesi R: Lecture: Study of optic nerve head normality. In:

Curso y Simposio Argentino de Glaucoma, July 1995, Buenos Aires, Republica Ar-gentina.

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2.01. Heidelberg Engineering GmbH, Heidelberg, Germany, January 1997.

6. Jonas JB: Biomorphometrie des Nervus Optikus. Stuttgart, Enke-Verlag, 1990.

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International Meeting on Scanning Laser Ophthalmoscopy, Tomography and Mi-

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8. Quigley HA, Addiks EM: Quantitative studies of retinal nerve fiber layer in glau-

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9. Airaksinen PJ, Burk ROW, Vihanninjoki K, Toulonen A, Alanco HI: Neuro-

retinal rim volume measurements with the Heidelberg Retina Tomograph. Glaucoma

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10. Boeglin RJ, Caprioli J: Actual clinical evaluation of the optic nerve in glaucoma.

In: Clinicas Oftalmologicas de Norteamerica, Actualizaciones en Glaucoma. Editor

Stamper RL, Editorial Inter-Medica, Buenos Aires, Argentina 1993, pp. 67-88.

11. Toulonen A: Factors determining the location of the standard reference plane. Im-

aging of the ONH and retina with the Heidelberg Retina Tomograph and Flowmeter,

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12. Burk ROW: Analisis de la capa de fibras nerviosas. Imaging of the ONH and Ret-ina with the Heidelberg Retina Tomograph and Flowmeter, Inaugural European

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18th, 1995.


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