**2. Why optic nerve?**

Optic nerve evaluation is the cornerstone of management of glaucoma. It remains the most crucial step in the early diagnosis of glaucoma and monitoring progressive nerve damage. Stereoscopic changes in the optic nerve head and retinal nerve fiber layer, which are seen clinically, are actually the manifestations of loss

of the ganglion cell layer which cannot be seen using slit lamp biomicroscopy. Moreover, since the structural abnormalities precede the functional changes, it is imperative to have an objective, quantitative, and reproducible imaging technique which is capable of early diagnosis and helps monitoring of the disease.

There are various imaging modalities being used by glaucoma experts today. Confocal scanning laser ophthalmoscopy (HRT; Heidelberg Retina Tomography; Heidelberg Engineering, Heidelberg, Germany), scanning laser polarimetry (GDX; Carl Zeiss Meditec, Dublin, California, USA), and optical coherence tomography (OCT; Carl Zeiss Meditec and others) are among the popular ones. However, subjective optic disc evaluation with stereo optic disc photography still remains the mainstay of every clinical practice.

### **3. Concept of preperimetric glaucoma**

Glaucomatous optic neuropathy is characterized by structural changes in the optic disc in the form of thinning of neuroretinal rim, pallor, and progressive cupping of the optic disc. Since it is a disease which can be treated but not cured, it is crucial for the treating ophthalmologist to catch the disease in its early stages. In glaucoma, structural injury has been documented to precede functional injury in most eyes [3]. One of the reasons observed by many researchers was that it took almost a loss of 40% of the ganglion cells to pick up a defect on the standard automated perimetry. Recently, a change in the diagnostic criteria of glaucoma has been promoted so that glaucoma diagnosis may be made before the old prerequisites functional criteria of standard automatic perimetry visual field (SAP-VF) loss are apparent, namely the "preperimetric glaucoma" (PPG).

To be diagnosed as a case of PPG, a patient needs to have a structural injury to the optic nerve head (ONH) sufficient enough to be classified under glaucomatous optic neuropathy (GON) and ought to be clinically proven. The introduction of newer imaging devices such as confocal scanning laser ophthalmoscopy, scanning laser polarimetry, and optical coherence tomography for measuring structural changes in the optic nerve head and retinal nerve fiber layer seems promising for early detection of glaucoma. Although an effort has been made to diagnose glaucoma in its early stages, there is no evidence that a single measurement is superior to the others and a combination of tests may be needed for detecting early damage in glaucoma.

#### **4. Diagnostic aids in preperimetric glaucoma**

Standard automated perimetry is the gold standard in the diagnosis of glaucoma as it gives us an assessment of the functional loss occurring in glaucoma. Optic disc photography gives a structural assessment of the optic nerve and surrounding nerve fiber layer but can be challenging due to inter-individual variability. Serial 3D imaging on the other hand may seem to be a better way of the subjective diagnosis of glaucomatous optic neuropathy during its early stages [4, 5]. Capturing early loss of retinal nerve fibers both clinically and by means of red-free photos may not be easy and sometimes could be indecisive, particularly in diffuse than in localized retinal nerve fibers loss [6].

### **5. Optical coherence tomography (OCT)**

Optical coherence tomography (OCT) is a non-invasive diagnostic technique that renders an *in vivo* cross-sectional view of the retina, retinal nerve fiber layer, and the

**123**

**Figure 1.**

**Figure 2.**

*Principle of optical coherence tomography.*

*Role of Optical Coherence Tomography in the Evaluation and Management of Glaucoma*

optic nerve head. OCT utilizes a concept known as low coherence interferometry. A broadband width light from a superluminescent diode is projected onto the retina. This is divided into a reference and a sample beam, and further the echo time delays of light reflected from the retina as well as reference mirror at known distances is compared. The light waves that are backscattered from the retina, interfere with the reference beam, and this interference pattern is measured by a photodetector (**Figure 1**). This is

Spectral domain OCT (SD-OCT) also works on similar principles, however, with a much higher data acquisition speed as compared to TD-OCT. This is achieved by the Michelson type interferometer with a stationary reference mirror. Instead of the interference signal being captured by a point detector, after the two returning beams

the basic principle on which the Sratus OCT works.

*Principle of Fourier domain optical coherence tomography.*

*DOI: http://dx.doi.org/10.5772/intechopen.84202*

*Role of Optical Coherence Tomography in the Evaluation and Management of Glaucoma DOI: http://dx.doi.org/10.5772/intechopen.84202*

**Figure 1.**

*A Practical Guide to Clinical Application of OCT in Ophthalmology*

mainstay of every clinical practice.

**3. Concept of preperimetric glaucoma**

apparent, namely the "preperimetric glaucoma" (PPG).

**4. Diagnostic aids in preperimetric glaucoma**

**5. Optical coherence tomography (OCT)**

of the ganglion cell layer which cannot be seen using slit lamp biomicroscopy. Moreover, since the structural abnormalities precede the functional changes, it is imperative to have an objective, quantitative, and reproducible imaging technique

There are various imaging modalities being used by glaucoma experts today. Confocal scanning laser ophthalmoscopy (HRT; Heidelberg Retina Tomography; Heidelberg Engineering, Heidelberg, Germany), scanning laser polarimetry (GDX; Carl Zeiss Meditec, Dublin, California, USA), and optical coherence tomography (OCT; Carl Zeiss Meditec and others) are among the popular ones. However, subjective optic disc evaluation with stereo optic disc photography still remains the

Glaucomatous optic neuropathy is characterized by structural changes in the optic disc in the form of thinning of neuroretinal rim, pallor, and progressive cupping of the optic disc. Since it is a disease which can be treated but not cured, it is crucial for the treating ophthalmologist to catch the disease in its early stages. In glaucoma, structural injury has been documented to precede functional injury in most eyes [3]. One of the reasons observed by many researchers was that it took almost a loss of 40% of the ganglion cells to pick up a defect on the standard automated perimetry. Recently, a change in the diagnostic criteria of glaucoma has been promoted so that glaucoma diagnosis may be made before the old prerequisites functional criteria of standard automatic perimetry visual field (SAP-VF) loss are

To be diagnosed as a case of PPG, a patient needs to have a structural injury to the optic nerve head (ONH) sufficient enough to be classified under glaucomatous optic neuropathy (GON) and ought to be clinically proven. The introduction of newer imaging devices such as confocal scanning laser ophthalmoscopy, scanning laser polarimetry, and optical coherence tomography for measuring structural changes in the optic nerve head and retinal nerve fiber layer seems promising for early detection of glaucoma. Although an effort has been made to diagnose glaucoma in its early stages, there is no evidence that a single measurement is superior to the others and a

Standard automated perimetry is the gold standard in the diagnosis of glaucoma as it gives us an assessment of the functional loss occurring in glaucoma. Optic disc photography gives a structural assessment of the optic nerve and surrounding nerve fiber layer but can be challenging due to inter-individual variability. Serial 3D imaging on the other hand may seem to be a better way of the subjective diagnosis of glaucomatous optic neuropathy during its early stages [4, 5]. Capturing early loss of retinal nerve fibers both clinically and by means of red-free photos may not be easy and sometimes could be indecisive, particularly in diffuse than in localized retinal nerve fibers loss [6].

Optical coherence tomography (OCT) is a non-invasive diagnostic technique that renders an *in vivo* cross-sectional view of the retina, retinal nerve fiber layer, and the

combination of tests may be needed for detecting early damage in glaucoma.

which is capable of early diagnosis and helps monitoring of the disease.

**122**

*Principle of optical coherence tomography.*

#### **Figure 2.**

*Principle of Fourier domain optical coherence tomography.*

optic nerve head. OCT utilizes a concept known as low coherence interferometry. A broadband width light from a superluminescent diode is projected onto the retina. This is divided into a reference and a sample beam, and further the echo time delays of light reflected from the retina as well as reference mirror at known distances is compared. The light waves that are backscattered from the retina, interfere with the reference beam, and this interference pattern is measured by a photodetector (**Figure 1**). This is the basic principle on which the Sratus OCT works.

Spectral domain OCT (SD-OCT) also works on similar principles, however, with a much higher data acquisition speed as compared to TD-OCT. This is achieved by the Michelson type interferometer with a stationary reference mirror. Instead of the interference signal being captured by a point detector, after the two returning beams recombine and form the interference pattern at the beam splitter, the interference pattern is split by a grating into its frequency components, all of these components are simultaneously detected by a charge-coupled device (CCD) (**Figure 2**). SD-OCT is also known as Fourier domain OCT (FD-OCT) because the distances are encoded in the Fourier transform of the frequencies of light reflected.
