**1. Introduction**

Glaucoma is an optic neuropathy with its hallmark being a characteristic loss of the ganglion cell axons which in turn leads to an excavation of the optic disc. Although optic disc cupping occurs in many other ocular diseases [1] the assessment of the optic nerve head with either optic disc photography or the newer modalities remains of utmost importance in the diagnosis and follow up of the glaucomatous process. The digital stereophotographs allow storage of optic disc photos for future comparison and offer qualitative assessment of the optic nerve head. The new imaging modalities can quantitatively and objectively analyze various param‐ eters of the optic nerve head and the retinal nerve fiber layer in order to discriminate between glaucomatous and nonglaucomatous optic discs. They can also compare scans of the same patient overtime and detect any changes. As glaucoma is a progressive optic neuropathy patient's assessment overtime is of paramount importance in order to tract changes and monitor the progression of the disease.

#### **1.1. New modalities for the imaging and analysis of the optic disc and retinal nerve fiber layer (RNFL)**

#### *1.1.1. Red-free photography of the optic disc and RNFL*

Photography is not a new imaging technique [2,3]. However newer photographic methods allow stereographic assessment of the optic nerve head and more detailed visualization of the RNFL. Retinal nerve fiber layer is better visualized when the refractive media are clear and in pigmented fundi. Its defects can be broadly classified as localized and diffuse and the former are easier to identify.Red free photography of the RNFL is as accurate in distinguishing glaucomatous from nonglaucomatous patients as optical coherence tomography (OCT),

© 2013 Kozobolis et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

scanning laser polarimetry (SLP) and confocal scanning laser ophthalmoscope (SCLO) [4,5]. Stereophotographs of the optic discs was proven to be as efficacious in detecting glaucoma as the objective analysis the optic nerve head with the new modalities [6,7].

reflected from that specific plane (as determined by the position of the pinhole in front of the laser device) enters the

Recognizing a Glaucomatous Optic Disc http://dx.doi.org/10.5772/55157 297

The HRT can analyze both the RNFL thickness and optic nerve head. A fundamental part of the analysis is the identification of the boundaries of the optic disc. The operator can draw a line along the edge of the optic disc. As the retinal vessels and peripapillary atrophy can make the exact identification of the boundaries difficult the examiner can use the 3-dimensional

The RNFL thickness is measured at the edge of the optic nerve head for 360° and follows a double hump appearance as the RNFL is thicker in the superotemporal and inferotemporal sectors. The optic disc parameters analyzed are: the disc area, cup area, rim area, cup volume, rim volume, linear cup/disc ratio, mean cup depth. Mean cup depth, maximum cup depth, cup shape measure, height variation contour, mean RNFL thickness and RNFL cross-sectional area. The Moorfields regression analysis provides an overall assessment of the field of view

The advantages of the new version of CSLO (HRT 3) is the large normative database which includes subjects European, African and Indian ancestry and can analyze both optic nerve head and RNF. Its limitation is that some optic nerve head measurements rely on a reference plane based on a hand drown contour line around the disc margins. The Glaucoma Probability Score does not need a reference plane. HRT measurements can be influenced by intraocular

Optical coherence tomography uses the principle of interferometry to construct high resolution cross-sectional images of the retina. An 800 nm laser light is split into two beams before entering the eye. The imaging beam consists of short pulses of light (the duration of each pulse is defined as the coherence length). One beam enters the eye and is reflected from the retina and the second beam is reflected from a reference mirror that moves back and forth along the Z axis. When the two reflected light beams constructively interfere they create a signal read by the interferometer. The time delay of the back scattered light from each layer of the retina differentiates the depth location of each layer (time-domain OCT). As a consequence in time domain OCT the instrument needs to perform two scans: a transverse scan across the eye (x axis) and a depth scan (z axis).The upgrade of time-domain OCT is the spectral-domain or Fourier-domain OCT (SD OCT/FD OCT). The SD OCT instead of the mechanical movement of the reference mirror analyzes with the aid of a mathematical equation (Fourier transform: FS(z) ∝ FT{AS(K} ) multiple wavelengths reflected from the retina. SD OCT obtains retina scans much faster (as the movement of the reference mirror along the z axis is omitted and only the scanning of the beam along the x axis is used) and with a better resolution (5-6 µm axial resolution, 10-15 µm transverse resolution) than the time-domain OCT. For the analysis of the

photodetector. The focal plane can be changed by moving the pinhole of the laser device.

and classifies it as "normal", "borderline" and "outside normal limits".

image of the optic nerve head in order to draw the line.

**2. Strengths and limitations [8]**

**2.1. Optical Coherence Tomography (OCT, fig 2)**

pressure fluctuations [9].

The new imaging modalities on the optic disc and RNFL include the confocal scanning laser ophthalmoscopy (CSLO), optical coherence tomography (OCT) and scanning laser polarime‐ try (SLP). The first two technologies can analyze both the optic nerve head and RNFL while SLP analyzes the thickness of the RNFL only.

#### *1.1.2. Confocal Scanning Laser Ophthalmoscopy (CSLO, fig 1)*

The CSLO technology is used by the Heidelberg Retinal Tomograph (HRT, Heidelberg Engineering, Heidelberg, Germany). It is based on the principle of two conjugated pinholes. Laser light (670nm) enters through one pinhole and focuses on a plane of the retina or the optic disc. The reflected light passes through the confocal pupil and allows reflected light only from that specific plane to enter the photodetector. The focused laser light scans across the optic nerve head (ONH) and RNFL along the x and y axes at planes of different depth acquiring a series of images. This series is reconstructed to produce a three dimensional image. Each series consists of 16 images per mm and for a 4 mm depth scan 64 images are captured. A fundamental part of the SCLO technology is the reference plane. It is defined as a plane parallel to the retina and lies 50 µm below the temporal part of the scleral ring of Elsching. In ONH analysis structures above the reference plane are read as neuroretinal ring and structures below are read as disc cup. SCLO has a transverse resolution of 10 µm and an axial resolution of 300 µm. The field of view of the image is 15°×15°.

**Figure 1.** Light from the laser device passes through a pinhole sitting in front of it and focuses on a certain plane in the retina. The reflected light from the retina enters a confocal pinhole sitting in front of the photodetector. Only light

reflected from that specific plane (as determined by the position of the pinhole in front of the laser device) enters the photodetector. The focal plane can be changed by moving the pinhole of the laser device.

The HRT can analyze both the RNFL thickness and optic nerve head. A fundamental part of the analysis is the identification of the boundaries of the optic disc. The operator can draw a line along the edge of the optic disc. As the retinal vessels and peripapillary atrophy can make the exact identification of the boundaries difficult the examiner can use the 3-dimensional image of the optic nerve head in order to draw the line.

The RNFL thickness is measured at the edge of the optic nerve head for 360° and follows a double hump appearance as the RNFL is thicker in the superotemporal and inferotemporal sectors. The optic disc parameters analyzed are: the disc area, cup area, rim area, cup volume, rim volume, linear cup/disc ratio, mean cup depth. Mean cup depth, maximum cup depth, cup shape measure, height variation contour, mean RNFL thickness and RNFL cross-sectional area. The Moorfields regression analysis provides an overall assessment of the field of view and classifies it as "normal", "borderline" and "outside normal limits".
