**6. Anatomy of the optic disc (fig 7)**

The optic disc is the area in the posterior pole where the ganglion cell axons converge to exit the eye and travel towards the brain. Its margins are defined by a dense fibrous tissue, the Elsching's ring. The disc area is covered by the neuroretinal rim which contains the retinal ganglion cells axons and the disc cup in the center. The ganglion cells axons leave the eye by piercing the thinned part of the sclera called the lamina cribrosa. The axons are arranged in bundles and exit the eye via the pores of the lamina cribrosa to form the optic nerves. The size and shape of the neuroretinal rim and cup depend on the total size of the optic disc and the number of the axons that travel through it.

Microdisc is a disc with a disc area two standard deviations less than the mean of the normal disc area and macrodisc is a disc with a disc area two standard deviations above the mean. The Blue Mountain Study [32] classified the discs as small (1.1 – 1.3), medium (1.4 – 1.7) and large (1.8 – 2.0) based on the vertical disc diameter measured in mm. The European Glaucoma Society classified the disc based on the disc area as small (<1.6mm2 ), medium (1,6-2.8 mm2

**Figure 7.** Normal optic disc. Note the presence of a small cup, thick neuroretinal rim, absence of peripapillary atrophy, equal distance of the exit of the trunk of the vessel from the superior and inferior sectors of the rim and normal arte‐

have normal disc size. In non-glaucomatous pathologies, optic disc drusen, pseudopapillede‐ ma, nonarteretic anterior ischemic optic neuropathy and tilted disc [33] are associated with small disc sizes while morning glory syndrome and optic disc pits with large discs. Further‐ more larger discs have more axons in absolute number but less axons per disc area [34]. Patients with pseudoexfoliation glaucoma tend to have smaller discs and those with normal tension glaucoma larger discs [35,36]. Glaucoma, however, may occur in conjunction with abnormal

The optic disc is elongated along the vertical axis with the vertical axis being 7-10% longer than the horizontal. The disc shape as expressed by the ratio of minimal to maximal diameter shows less variability between individuals than the disc area. The disc shape is independent from sex, age, right and left eye and body weight and height and does not show interindividual variability for a refractive error less than -8.00D. In POAG patients the disc shape is not associated with the visual field defects. However for in high myopes >-12D it is more elongated.

). Patients with primary open angle (POAG) and pigmentary glaucoma

and large (>2.8 mm2

riolar caliber.

**6.2. Optic disc shape [26]**

disc size as well as with other disc pathologies.

)

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The following morphological features of the optic disc should be taken into account when assessing an optic nerve head [26]:

#### **6.1. Optic disc size**

The size of the optic disc shows great variability between different populations [27]. The range of the mean disc area measured in mm2 for people of different ethnic backgrounds is: africans 1.84-2.50, whites 1.65-2.34, Indians 2.24-2.93, Asians 1.97-2.67 and latinos 1.95-2.56 [28].The size was shown to be independent of the age after the age of 10, the body height, gender and refractive errors between -5.00 and +5.00 D. In contrast the optic disc is smaller in high hypermetropes and larger in high myopes. Optic disc size abruptly increases for myopia above -8.00 diopters (D) and significantly decreases for hyperopia above +4.00 D [29] The optic disc area was found to have great variability between healthy individuals by many researchers [28,30,31]. For this reason the terms <<microdisc>> and <<macrodisc>> have been coined.

**Figure 7.** Normal optic disc. Note the presence of a small cup, thick neuroretinal rim, absence of peripapillary atrophy, equal distance of the exit of the trunk of the vessel from the superior and inferior sectors of the rim and normal arte‐ riolar caliber.

Microdisc is a disc with a disc area two standard deviations less than the mean of the normal disc area and macrodisc is a disc with a disc area two standard deviations above the mean. The Blue Mountain Study [32] classified the discs as small (1.1 – 1.3), medium (1.4 – 1.7) and large (1.8 – 2.0) based on the vertical disc diameter measured in mm. The European Glaucoma Society classified the disc based on the disc area as small (<1.6mm2 ), medium (1,6-2.8 mm2 ) and large (>2.8 mm2 ). Patients with primary open angle (POAG) and pigmentary glaucoma have normal disc size. In non-glaucomatous pathologies, optic disc drusen, pseudopapillede‐ ma, nonarteretic anterior ischemic optic neuropathy and tilted disc [33] are associated with small disc sizes while morning glory syndrome and optic disc pits with large discs. Further‐ more larger discs have more axons in absolute number but less axons per disc area [34]. Patients with pseudoexfoliation glaucoma tend to have smaller discs and those with normal tension glaucoma larger discs [35,36]. Glaucoma, however, may occur in conjunction with abnormal disc size as well as with other disc pathologies.

#### **6.2. Optic disc shape [26]**

glaucoma detection of the GDx is the nerve fiber index and that for Cirrus OCT the inferior RNFL thickness. GDx was also more accurate in detecting glaucoma than the Cirrus OCT. Two recent studies [18,19] showed that the diagnostic accuracy for glaucoma of the HRT II is

The severity of the glaucomatous process also affects the accuracy of glaucoma diagnosis of the various imaging technologies. The more advanced the disease the more accurate the diagno‐ sis of glaucomatous optic neuropathy [20,21]. OCT and SLP performed better than CSLO in discriminating between early glaucomatous eyes with or without visual field defects [22]. In eyes with early glaucoma the most accurate parameter is the inferior RNFL thickness which performs better than the most accurate parameter of the CSLO (vertical cup-to-disc ratio). In glaucoma suspect eyes the most accurate parameter for the OCT is the average RNFL thick‐ ness, for the SLP the nerve fiber indicator and for the CSLO the vertical cup-to-disc ratio. The first two parameters performed better than the vertical cup-to-disc ratio. Leung et al [23] confirmed that SD-OCT performed better than HRT in recognizing patients with glaucoma. RNFL thickness changes performed better than optic nerve head parameters as evaluated with CSLO. The nerve fiber index of the SLP was more accurate in diagnosing glaucoma than the rim volume parameter of the CSLO [24]. SLP was also superior in detecting glaucoma progres‐ sion by analyzingRNFL thickness compared to CSLO analysis ofthe neuroretinalrim area [25].

The optic disc is the area in the posterior pole where the ganglion cell axons converge to exit the eye and travel towards the brain. Its margins are defined by a dense fibrous tissue, the Elsching's ring. The disc area is covered by the neuroretinal rim which contains the retinal ganglion cells axons and the disc cup in the center. The ganglion cells axons leave the eye by piercing the thinned part of the sclera called the lamina cribrosa. The axons are arranged in bundles and exit the eye via the pores of the lamina cribrosa to form the optic nerves. The size and shape of the neuroretinal rim and cup depend on the total size of the optic disc and the

The following morphological features of the optic disc should be taken into account when

The size of the optic disc shows great variability between different populations [27]. The range of the mean disc area measured in mm2 for people of different ethnic backgrounds is: africans 1.84-2.50, whites 1.65-2.34, Indians 2.24-2.93, Asians 1.97-2.67 and latinos 1.95-2.56 [28].The size was shown to be independent of the age after the age of 10, the body height, gender and refractive errors between -5.00 and +5.00 D. In contrast the optic disc is smaller in high hypermetropes and larger in high myopes. Optic disc size abruptly increases for myopia above -8.00 diopters (D) and significantly decreases for hyperopia above +4.00 D [29] The optic disc area was found to have great variability between healthy individuals by many researchers [28,30,31]. For this reason the terms <<microdisc>> and <<macrodisc>> have been coined.

dependent on the disc size which is not the case for OCT and GDx.

**6. Anatomy of the optic disc (fig 7)**

302 Glaucoma - Basic and Clinical Aspects

number of the axons that travel through it.

assessing an optic nerve head [26]:

**6.1. Optic disc size**

The optic disc is elongated along the vertical axis with the vertical axis being 7-10% longer than the horizontal. The disc shape as expressed by the ratio of minimal to maximal diameter shows less variability between individuals than the disc area. The disc shape is independent from sex, age, right and left eye and body weight and height and does not show interindividual variability for a refractive error less than -8.00D. In POAG patients the disc shape is not associated with the visual field defects. However for in high myopes >-12D it is more elongated. Elongated optic discs were associated with increased corneal astigmatism. Overall disc shape bears little value in the diagnosis of glaucoma.

#### **6.3. Neuroretinal rim shape and cup-to-disc ratio (C/D ratio)**

It represents the quotient of the vertical cup diameter to the vertical overall disc diameter. In normal eyes the cup is horizontally elongated with the horizontal diameter being 8% longer than the vertical one. On the other hand the disc is vertically oval shaped. As a consequence the neuroretinal rim is thicker at superior and inferior poles. The mnemonic ISNT rule dictates that the neuroretinal rim is thicker in the inferior pole of the disc followed by the superior, the nasal and finally the temporal which is the thinnest. The C/D ratio also shows interindividual variability being higher in large discs and lower in smaller discs. Clinicians should bear in mind the opposite configuration of the cup and optic disc when assessing the disc for glau‐ comatous damage. They should also take into account that a high C/D ratio is not necessarily pathognomonic for glaucoma as it can occur in large diameter discs (fig 8,9). Conversely early glaucomatous damage can be overlooked in small discs with small cups.

**Figure 9.** Retinal nerve fiber analysis (RNFL) with optical coherence tomography (OCT) of the same eye as in fig 8. The blue contour line represents the thickness of the RNFL of this patient and falls in the falls area. It is normal for this

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In the early to moderate glaucoma the axons in the superotemporal and inferotemporal areas of the disc are affected usually first and this leads to an increase of the C/D vertical diameter faster than the horizontal causing an increased vertical C/D ratio with violation of the ISNT rule.

**Figure 10.** Typical RNFL loss in the superior and inferior RNFL sectors in a glaucoma patient. The RNFL contour line is flattened and crosses the red shaded abnormal area. Less than 1% of the normal subjects will have a similar RNFL

thickness in the affected sectors as this patient has.

patient's age

**Figure 8.** Large optic disc with a large cup. Neuroretinal rim shape respects **ISNT** rule (rim sectors wider to thinner (**I**nferior-**S**uperior-**N**asal-**T**emporal), there is no peripapillary atrophy and no optic disc haemorrhages are detected. The main vessels emerge with a dual trunk. The exit of each trunk lies at equal distance from the superior and inferior rim

Elongated optic discs were associated with increased corneal astigmatism. Overall disc shape

It represents the quotient of the vertical cup diameter to the vertical overall disc diameter. In normal eyes the cup is horizontally elongated with the horizontal diameter being 8% longer than the vertical one. On the other hand the disc is vertically oval shaped. As a consequence the neuroretinal rim is thicker at superior and inferior poles. The mnemonic ISNT rule dictates that the neuroretinal rim is thicker in the inferior pole of the disc followed by the superior, the nasal and finally the temporal which is the thinnest. The C/D ratio also shows interindividual variability being higher in large discs and lower in smaller discs. Clinicians should bear in mind the opposite configuration of the cup and optic disc when assessing the disc for glau‐ comatous damage. They should also take into account that a high C/D ratio is not necessarily pathognomonic for glaucoma as it can occur in large diameter discs (fig 8,9). Conversely early

**Figure 8.** Large optic disc with a large cup. Neuroretinal rim shape respects **ISNT** rule (rim sectors wider to thinner (**I**nferior-**S**uperior-**N**asal-**T**emporal), there is no peripapillary atrophy and no optic disc haemorrhages are detected. The main vessels emerge with a dual trunk. The exit of each trunk lies at equal distance from the superior and inferior

rim

bears little value in the diagnosis of glaucoma.

304 Glaucoma - Basic and Clinical Aspects

**6.3. Neuroretinal rim shape and cup-to-disc ratio (C/D ratio)**

glaucomatous damage can be overlooked in small discs with small cups.

**Figure 9.** Retinal nerve fiber analysis (RNFL) with optical coherence tomography (OCT) of the same eye as in fig 8. The blue contour line represents the thickness of the RNFL of this patient and falls in the falls area. It is normal for this patient's age

In the early to moderate glaucoma the axons in the superotemporal and inferotemporal areas of the disc are affected usually first and this leads to an increase of the C/D vertical diameter faster than the horizontal causing an increased vertical C/D ratio with violation of the ISNT rule.

**Figure 10.** Typical RNFL loss in the superior and inferior RNFL sectors in a glaucoma patient. The RNFL contour line is flattened and crosses the red shaded abnormal area. Less than 1% of the normal subjects will have a similar RNFL thickness in the affected sectors as this patient has.

A large C/D ratio has been shown to be a risk factor for glaucoma progression [37], although Cioffi et al argued that it merely represents undetected damage [38].

red <0.5%. The TSNIT map of the right eye has lost the double hump appearance, there is high inter-eye asymmetry as

Recognizing a Glaucomatous Optic Disc http://dx.doi.org/10.5772/55157 307

Research has shown that the area on the optic disc most susceptible to glaucomatous damage is the area that is the furthest away from the main vessel trunk (fig 12) [43]. The exit of the main vessel trunk is usually displaced superonasally which makes the inferotem‐ poral quadrant more susceptible to the glaucomatous damage. The disc arterioles follow the contour of the neuroretinal rim. As the rim recedes in the glaucomatous process the arterioles tend to become displaced towards the periphery of the optic disc. If the rim becomes extremely thin the vessels may be pushed to the far periphery of the disc just next to the Elsching's ring and then they sharply angle on the retinal surface giving rise to the bayoneting sign (fig 13). The presence of a temporal cilioretinal artery has a protective role

**Figure 12.** Right optic disc: The main vessel trunk emerges in the superotemporal disc quadrant. The neuroretinal rim is thinnest in inferior and nasal quadrants. The distance of the main vessel trunk to the inferior rim is longer compared to the distance to the superior rim. The glaucomatous damage is greatest in the inferior rim. There is no arteriolar

Optic disc haemorrhages are an independent risk factor for glaucoma and ocular hypertensive patients with disc haemorrhages are six times more likely to develop glaucoma than those patients without haemorrhages [45]. The frequency of disc haemorrhages in glaucoma eyes ranged from 9-20% [46,47]. Their frequency is not statistically different in glaucoma eyes with high or normal IOP [46]. Their prevalence in non-glaucomatous eyes ranges from 0.2% - 1.03%

the left eye has not been affected by glaucoma and the NFI is high in the right eye.

**6.5. Point of exit of the large vessel trunk on the optic disc**

against the glaucomatous process [44]

**6.6. Optic disc haemorrhages (fig 14)**

narrowing

[48-51]

#### **6.4. Retinal Nerve Fiber Layer (RNFL)**

The retinal nerve fiber layer is made up of the nonmyelinated axons of the retinal ganglion cells. They are more visible in the inferotemporal and superotemporal areas of the fundus and least visible in the horizontal nasal and temporal sectors. The visibility of the RNFL corresponds to the configuration of the neuroretinal rim which is thicker in the superior and inferior poles of the disc [26] giving a double hump configuration in the OCT RNFL analysis (fig 4). Defects in RNFLprecede opticdisc cupping in the corresponding sectors [39] as well as visualfielddefects with standard automated perimetry [40]. The most common sectors affected in glaucoma are the inferotemporal followed by the superotemporal [41]. This pattern of RNFL loss leads to the disappearance ofthedoublehumpconfigurationoftheRNFL(fig 10,11). Nerve fiberdefects are encountered in other optic nerve diseases such as optic disc drusen, toxoplasmic retinochoroi‐ dal scars, diabetic retinopathy and optic neuritis secondary to multiple sclerosis [26,42].

**Figure 11.** Thinning of the superior sector of the RNFL of the right eye due to glaucoma. Note the probabolity of each soperpixel (each superpixel includes 4 pixels) of the deviation to be normal. The purple pixel represent a 5% probabili‐ ty that the RNFL thickness in that superpixel is normal, the blue color represents a <2% probability, yellow <1% and

red <0.5%. The TSNIT map of the right eye has lost the double hump appearance, there is high inter-eye asymmetry as the left eye has not been affected by glaucoma and the NFI is high in the right eye.

#### **6.5. Point of exit of the large vessel trunk on the optic disc**

A large C/D ratio has been shown to be a risk factor for glaucoma progression [37], although

The retinal nerve fiber layer is made up of the nonmyelinated axons of the retinal ganglion cells. They are more visible in the inferotemporal and superotemporal areas of the fundus and least visible in the horizontal nasal and temporal sectors. The visibility of the RNFL corresponds to the configuration of the neuroretinal rim which is thicker in the superior and inferior poles of the disc [26] giving a double hump configuration in the OCT RNFL analysis (fig 4). Defects in RNFLprecede opticdisc cupping in the corresponding sectors [39] as well as visualfielddefects with standard automated perimetry [40]. The most common sectors affected in glaucoma are the inferotemporal followed by the superotemporal [41]. This pattern of RNFL loss leads to the disappearance ofthedoublehumpconfigurationoftheRNFL(fig 10,11). Nerve fiberdefects are encountered in other optic nerve diseases such as optic disc drusen, toxoplasmic retinochoroi‐ dal scars, diabetic retinopathy and optic neuritis secondary to multiple sclerosis [26,42].

**Figure 11.** Thinning of the superior sector of the RNFL of the right eye due to glaucoma. Note the probabolity of each soperpixel (each superpixel includes 4 pixels) of the deviation to be normal. The purple pixel represent a 5% probabili‐ ty that the RNFL thickness in that superpixel is normal, the blue color represents a <2% probability, yellow <1% and

Cioffi et al argued that it merely represents undetected damage [38].

**6.4. Retinal Nerve Fiber Layer (RNFL)**

306 Glaucoma - Basic and Clinical Aspects

Research has shown that the area on the optic disc most susceptible to glaucomatous damage is the area that is the furthest away from the main vessel trunk (fig 12) [43]. The exit of the main vessel trunk is usually displaced superonasally which makes the inferotem‐ poral quadrant more susceptible to the glaucomatous damage. The disc arterioles follow the contour of the neuroretinal rim. As the rim recedes in the glaucomatous process the arterioles tend to become displaced towards the periphery of the optic disc. If the rim becomes extremely thin the vessels may be pushed to the far periphery of the disc just next to the Elsching's ring and then they sharply angle on the retinal surface giving rise to the bayoneting sign (fig 13). The presence of a temporal cilioretinal artery has a protective role against the glaucomatous process [44]

**Figure 12.** Right optic disc: The main vessel trunk emerges in the superotemporal disc quadrant. The neuroretinal rim is thinnest in inferior and nasal quadrants. The distance of the main vessel trunk to the inferior rim is longer compared to the distance to the superior rim. The glaucomatous damage is greatest in the inferior rim. There is no arteriolar narrowing

#### **6.6. Optic disc haemorrhages (fig 14)**

Optic disc haemorrhages are an independent risk factor for glaucoma and ocular hypertensive patients with disc haemorrhages are six times more likely to develop glaucoma than those patients without haemorrhages [45]. The frequency of disc haemorrhages in glaucoma eyes ranged from 9-20% [46,47]. Their frequency is not statistically different in glaucoma eyes with high or normal IOP [46]. Their prevalence in non-glaucomatous eyes ranges from 0.2% - 1.03% [48-51]

**6.7. Peripapillary Chorioretinal Atrophy (PPCA, fig 13)**

in glaucomatous eyes (1.21 ±1.92 mm 2

larger in the eyes with disc haemorrhage.

rather the cause of the optic nerve damage [26]

**7. Glaucomatous versus non-glaucomatous damage**

**6.8. Retinal arterioles diameter**

damage [62].

**Summary box**

Peripapillary atrophy consists of an outer alpha zone with irregular hyper- hypopigmentation and an inner beta zone with visible large choroidal vessels and sclera. Alpha zone is present in most normal eyes but beta zone is more common in glaucoma eyes and tends to enlarge in eyes with progressing normal tension glaucoma [52]. They both tend to increase in size with advancing glaucoma damage [53]. The frequency of the beta zone varies between 59.5% and 69% in glaucoma patients and 17.4% and 24% in healthy subjects [53,54]. Beta zone is also larger

is conflicting evidence as to whether the PPCA corresponds to areas of neuroretinal rim thinning. Uchida et al [55] reported that PPCA progression correlated to progressive disc damage and visual field defects. On the other hand See et al [56] showed that neuroretinal area decrease did not correlate with PPCA progression. The extent of PPCA positively correlated with the presence of optic disc haemorrhage in glaucoma eyes [57]. In this study beta zone was

The diameter of retinal arterioles is decreased in both glaucomatous and non-glaucomatous optic nerve damage (fig 14). It merely represents the limited needs of the retina for oxygen

Optic disc cupping is not pathognomonic for the glaucomatous optic neuropathy only [58]. Other diseases such as arteritic ischemic optic neuropathy (AION), optic neuritis, optic disc pit, colobomas, tilted disc, traumatic optic neuropathy, methanol toxicity, compressive lesions of the anterior visual pathways [59], disc drusen, long standing papilledema [26]. However nonglaucomatous disc damage produces optic disc rim pallor while glaucomatous damage produces focal or diffuse obliteration of the neuroretinal rim [60]. Glaucoma damage tends to produce deeper cups than the nonglaucomatous type [61]. In this study open angle glaucoma eyes had larger and deeper cups and smaller neuretinal rims compared to eyes with nonar‐ teritic and arteritic AION. Contrary to glaucoma PPCA does not increase in nonglaucomatous

There is great variability among healthy subjects and people from different races in the morphology of the optic disc

which makes the diagnosis of glaucoma very complicated. The clinician should take into consideration various aspects

of the anatomy of the optic nerve head and the RNFL before deciding whether a patient has glaucoma or not

) compared to healthy ones (0.32 ± 0.99 mm2

) [54]. There

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**Figure 13.** Large PPCA in advanced glaucoma with small alpha zone temporally (arrow) and a large beta zone (arrow‐ head). Bayoneting of the arterioles (white arrowhead)

**Figure 14.** Optic disc haemorrhage in advanced glaucoma. Note the presence of focal arteriolar narrowing (arrow).

### **6.7. Peripapillary Chorioretinal Atrophy (PPCA, fig 13)**

Peripapillary atrophy consists of an outer alpha zone with irregular hyper- hypopigmentation and an inner beta zone with visible large choroidal vessels and sclera. Alpha zone is present in most normal eyes but beta zone is more common in glaucoma eyes and tends to enlarge in eyes with progressing normal tension glaucoma [52]. They both tend to increase in size with advancing glaucoma damage [53]. The frequency of the beta zone varies between 59.5% and 69% in glaucoma patients and 17.4% and 24% in healthy subjects [53,54]. Beta zone is also larger in glaucomatous eyes (1.21 ±1.92 mm 2 ) compared to healthy ones (0.32 ± 0.99 mm2 ) [54]. There is conflicting evidence as to whether the PPCA corresponds to areas of neuroretinal rim thinning. Uchida et al [55] reported that PPCA progression correlated to progressive disc damage and visual field defects. On the other hand See et al [56] showed that neuroretinal area decrease did not correlate with PPCA progression. The extent of PPCA positively correlated with the presence of optic disc haemorrhage in glaucoma eyes [57]. In this study beta zone was larger in the eyes with disc haemorrhage.

#### **6.8. Retinal arterioles diameter**

The diameter of retinal arterioles is decreased in both glaucomatous and non-glaucomatous optic nerve damage (fig 14). It merely represents the limited needs of the retina for oxygen rather the cause of the optic nerve damage [26]
