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

OCT can analyze both the morphology of the optic disc and RNFL (fig 3,4). However automatic recognition of the edges of the optic disc as the end of the RPE layer can give incorrect measurements in patients with peripapillary atrophy. This is especially true for glaucoma patients who tend to have greater peripapillary atrophic areas that progress overtime. In this case what is incorrectly measured as optic disc area is the area of the optic nerve head plus the peripapillary atrophy. Furthermore as the information of the optic nerve head data between the scans are interpolated small defects of the neuroretinal rim may be missed.

#### **3.1. Scanning Laser Polarimetry (SLP) (fig 5,6)**

Scanning laser polarimetry is used in the GDx (GDX; Carl Zeis Meditec, Dublin, CA, USA). It is based on the principle of retardation. The RNFL has linear birefringence due to the parallel orientation of the microtubules in the axons of the RNFL. When polarized light travels through the RNFL the beam parallel to the RNFL slows down compared to the one that travels

optic nerve head the OCT runs six scans across the optic disc in a spoke-like pattern (fig 3). The measurements of the area between the scans are interpolated from the values across the scans. The edge of the optic nerve head is automatically defined as the end of the retinal pigment epithelium (RPE)/choriocapillaris layer. A straight line is taken from one edge of the RPE to the other and a reference plane is set 150 µm above this line. Neuroretinal rim is defined

**Figure 2.** The beam from super-lumiscent diode laser source is split as it travels through the beam splitter (BS). One beam goes to the reference mirror (mirror) and the second beam in the tissue to be examined. The two beams are reflected back and they interfere as they enter the interfereometer (spectrometer in the figure). The mirror moves back and forth in order to create constructive interference at different depths (represented by different colors in the sample) of the examined tissue (z axis). The beam also travels across x axis in order to capture a slice of the sample.

OCT can analyze both the morphology of the optic disc and RNFL (fig 3,4). However automatic recognition of the edges of the optic disc as the end of the RPE layer can give incorrect measurements in patients with peripapillary atrophy. This is especially true for glaucoma patients who tend to have greater peripapillary atrophic areas that progress overtime. In this case what is incorrectly measured as optic disc area is the area of the optic nerve head plus the peripapillary atrophy. Furthermore as the information of the optic nerve head data between

Scanning laser polarimetry is used in the GDx (GDX; Carl Zeis Meditec, Dublin, CA, USA). It is based on the principle of retardation. The RNFL has linear birefringence due to the parallel orientation of the microtubules in the axons of the RNFL. When polarized light travels through the RNFL the beam parallel to the RNFL slows down compared to the one that travels

the scans are interpolated small defects of the neuroretinal rim may be missed.

as the area above the reference plane and cup the area below it.

298 Glaucoma - Basic and Clinical Aspects

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

**3.1. Scanning Laser Polarimetry (SLP) (fig 5,6)**

**Figure 3.** Optic nerve head analysis of a normal optic disc. The disc margins are identified by the OCT but the examiner can accurately identify the true disc border by manually moving the blue squares. The parameters measured are shown in the figure

**Figure 4.** RNFL analysis with OCT. The numbers refer to RNFL thickness in μm. The green shaded area represents the normal RNFL thickness in the normative database of the OPKO spectral-domain OCT/SLO (Opko/OTI, Ophthalmic Technologies Inc, Toronto, Canada). Ninety five percent of the age-matched subjects with normal RNFL thickness will be included in the green area. On the other hand <5% of the subjects with normal RNFL thickness will fall in the yel‐ low shaded area and <1% of the normal subjects will be in the red shaded area. In this patient the blue contour line of their RNFL thickness has the characteristic double hump appearance and falls in the green area. The RNFL thickness is normal for the age of this patient. The double hump pattern of the RNFL is due to the increased thickness of the fiber layer in the superotemporal and inferotemporal sector. The RNFL thickness is measured around a 3.46 mm diameter circle centered on the optic disc.

(128×128 pixels) field centered on the optic disc. The warmer the colors the thicker the RNFL. Below the thickness map is the deviation map which represents the deviation of the RNFL thickness from the normal age matched value. At the bot‐ tom is the TSNIT (Temporal – Superior – Nasal – Inferior – Temporal) map which shows the RNFL thickness along the cal‐ culation ring. The latter is a ring 0.4 mm wide centered around the optic disc with the outer diameter being 3.2 mm and the inner 2.4 mm. The shaded areas (green for the right eye and purple for the left) represent the 95% of the normal val‐ ues for this age group. The TSNIT contour line has a double hump appearance as for the OCT. The TSNIT parameters are the RNFL thickness along the calculation ring for the average, superior and inferior sector RNFL thickness. The TSNIT standard deviation is the modulation from peak to trough values of the double hump pattern. Because in glaucoma the superior and inferior sectors become thinned the difference between the peks and troughs decreases the TSNIT stand‐ ard deviation value decreases as well. The intereye symmetry measures the symmetry between the eyes (values be‐ tween -1 and 1). Normal eyes show good symmetry but the glaucomatous eyes tend to be asymmetrical as glaucoma can affect one eye more than the other. The Nerve Fiber Indicator is a global value based on the entire RNFL thickness map.

Recognizing a Glaucomatous Optic Disc http://dx.doi.org/10.5772/55157 301

SLP can only measure data from RNFL. Areas of peripapillary atrophy give false information about the RNFL. A minority of the eyes examined show atypical retardation patterns (APRs) which are overcome by the GDx-ECC machines [11]. Atypical patterns are those which do not follow the normal histological distribution of the RNFL with the supero- and inferotemporal sectors being the thickest. APRs give falsely high RNFL measurements [12]. Newer SLP models are not compatible with the older ones. On the other hand RNFL analysis with SLP does not

Badala et al [4] compared the efficacy of stereoptic disc assessment and that of all three imaging modalities (OCT, GDx, HRT 3) in diagnosing glaucoma. The sensitivity at 95% specificity of the best performing parameter of each modality is: for the OCT (average RNFL thickness) 89%, for the GDx VCC (nerve fiber indicator) 78% and for the HRT 3 [Frederick S. Mikelberg (FSM) discriminant function] 70%. Optic disc stereophtographs are as accurate in detecting glaucoma

Retinal nerve fiber analysis with all the above modalities exhibit a characteristic double hump because the RNFL in thicker in the superotemporal and inferotemporal sectors compared to

All of the above imaging modalities have been employed in the diagnosis and follow up of patients with various stages of glaucomatous optic neuropathy. Studies have shown that there is a discrepancy between the measurements of the optic disc parameters taken with OCT and HRT in glaucomatous eyes [13]. HRT II had higher values for disc and rim area while RTVue-100 OCT had higher values for cup area, cup-to-disc area ratio, and vertical and horizontal cup-to-disc ratio. Leite et al [14] compared three FD-OCT machines and reported that their performance in detecting glaucoma is similar. FD-OCT out-performed SD-OCT in detecting progression of the glaucomatous process [15] but they were comparable in detecting glaucomatous damage [16]. Lee et al [17] found that the best performing parameter for

The values range from 1-100 (1-30: normal, 31-50: borderline, >51: abnormal).

**4. Strengths and limitations [7]**

require a reference plane.

**5. Sensitivity and specificity**

as the other imaging modalities.

the nasal and temporal ones.

**Figure 5.** Polarized light that travels parallel to the RNFL slows down. This retardation is proportional to the thickness of the RNFL

perpendicular to the fiber layer. This difference in the speed between the two beams is called retardation and is proportional to the RNFL thickness. The scanning laser beam used is 785nm.

Because the corneal also exhibits birefringence the GDx has a variable corneal compensator (VCC) in order to subtract the retardation from the cornea and the only retardation measured is that derived from the RNFL. The newer GDx machines have an enhanced corneal compen‐ sator that offers better reproducibility of the measurements and is more accurate in the diagnosis of glaucoma [10]. The transverse resolution of the GDx is 45 µm.

**Figure 6.** Printout of the RNFL analysis with the GDx VCC. The colored images at the top of the printout are the fundus photos. Below them is the thickness map. It is a color coded representation of the thickness of the RNFL within a 20° × 20°

(128×128 pixels) field centered on the optic disc. The warmer the colors the thicker the RNFL. Below the thickness map is the deviation map which represents the deviation of the RNFL thickness from the normal age matched value. At the bot‐ tom is the TSNIT (Temporal – Superior – Nasal – Inferior – Temporal) map which shows the RNFL thickness along the cal‐ culation ring. The latter is a ring 0.4 mm wide centered around the optic disc with the outer diameter being 3.2 mm and the inner 2.4 mm. The shaded areas (green for the right eye and purple for the left) represent the 95% of the normal val‐ ues for this age group. The TSNIT contour line has a double hump appearance as for the OCT. The TSNIT parameters are the RNFL thickness along the calculation ring for the average, superior and inferior sector RNFL thickness. The TSNIT standard deviation is the modulation from peak to trough values of the double hump pattern. Because in glaucoma the superior and inferior sectors become thinned the difference between the peks and troughs decreases the TSNIT stand‐ ard deviation value decreases as well. The intereye symmetry measures the symmetry between the eyes (values be‐ tween -1 and 1). Normal eyes show good symmetry but the glaucomatous eyes tend to be asymmetrical as glaucoma can affect one eye more than the other. The Nerve Fiber Indicator is a global value based on the entire RNFL thickness map. The values range from 1-100 (1-30: normal, 31-50: borderline, >51: abnormal).
