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A, Meadows AT, Shields JA.The international classication of retinoblastoma predicts chemoreduction
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15.1 Anatomical Considerations
Nearly 1.2 million retinal ganglion cells (RGCs)
axons exit at the back of the eye through ne fenestrations in the sclera termed lamina cribrosa.
These RGC axons, also called the retinal nerve
bre (RNF), form the anteriormost layer of the
neurosensory retina (NSR). The RGCs are the
thickest at the fovea and 7-cell thick; in the retinal periphery, they are barely one cell thick. Glial
septa separate the RNF layer (RNFL) bundles
and converge at the optic disc to exit the eye as
the optic nerve, the second cranial nerve. The
RGC axons turn 90° over the Bruch’s membrane
and retinal pigment epithelium (RPE) at the border of the scleral canal to exit the eye through the
ne fenestrations in the lamina cribrosa.
Till they exit the eye, the RGC axons are
unmyelinated. The RGC axons are arranged in a
laminar fashion. The most peripheral axons lie
deepest in the RNFL and enter the optic disc in
the most peripheral part. The retinal bres from
the posterior retina enter the more central part of
the optic disc. Nearly 90% of the axons leaving
the eye arise from the macula [1]. Each axon
receives inputs from nearly 100 rod photoreceptors and 4–5 cone photoreceptors. These RGC
axons are also accompanied by glial cells, including astrocytes (provide nutrition), microglia
(phagocytic), and oligodendrocytes seen only
posterior to the lamina cribrosa (myelination).
The optic disc, also known as the optic nerve head
(ONH), marks the beginning of the optic nerve,
which courses through the orbit and nally exits
the orbit through the optic canal to enter the cranial cavity. As the optic nerve exits the sclera, it
gets myelinated by the oligodendrocytes and
becomes ~twice the thickness of the ONH
(~3mm). The pia mater and the arachnoid cover
the optic nerve. The pia mater is a brovascular
covering of the nerve which sends delicate brovascular septa and segregates the axons into fascicles or bundles. It provides blood supply to the
core of the optic nerve. The arachnoid layer is a
loose web-like syncytial bro cellular layer that
covers the optic nerve. The dura mater is the outermost tough brous sheath, also called the optic
nerve sheath, which gets fused with the sclera
anteriorly and, through the optic canal, continues
posteriorly with the dural lining of the brain. The
subarachnoid space around the optic nerve contains cerebrospinal uid (CSF) right up to the
sclera and is continuous with the CSF in the brain.
The intraocular part of the ONHis ~1mm and
has four parts, supercial nerve bre layer, prelaminar, laminar, and retrolaminar. The internal
limiting membrane of the retina continues over
the ONH as Elschnig’s membrane. The centre of
the ONH has a gliotic central meniscus of Kuhnt,
a remnant of the hyaloid artery. The axons in the
ONH are separated from the NSR, RPE, Bruch’s
membrane, choroid, and sclera by an astrocytic
© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023
A. Gupta et al., Ophthalmic Signs in Practice of Medicine,
https://doi.org/10.1007/978-981-99-7923-3_15
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15 Optic Disc Signs—Cupping, Swelling, Inammation, andPallor
ring of connective tissue, the intermediary tissue
of Kuhnt, the border tissue of Jacoby, and the
border tissue of Elschnig, respectively.
In the intraocular course, the macular bres lie
temporally, and the nasal bres lie nasally.
However, the macular bres lie in the optic
nerve’s centre as they course through the orbit.
The optic nerve has a sinuous course in orbit of
~25 to 30mm (for the eye’s movement in orbit).
It runs for about ~10mm in the optic canal and is
rmly anchored to the optic canal. The optic
nerve is surrounded by the pia mater, the arachnoid, and the dura mater extending from behind
the globe into the optic canal.
The intracranial part of the optic nerve, also
termed the cisternal segment, is covered with
only the pia mater. As the optic nerves emerge
from the optic foramen, they extend posteriorly,
Fig. 15.1 Inferior
aspect of the human
brain showing some of
the components of the
visual pathway. Image
courtesy of Prof Daisy
Sahni (Ex-Professor)
and Prof Anjali
Aggarwal (Head),
Department of Anatomy,
Post Graduate Institute
of Medical Education
and Research,
Chandigarh, India
rise by about 45°, and converge to form the optic
chiasm at the base of the brain (Fig.15.1). The
optic chiasm lies in a suprasellar cistern, ~10mm
above the pituitary gland in the sella turcica. The
anterior part of thethird ventricle lies just above
the optic chiasm.
In the chiasm, the bres from the nasal half of
the retina from both eyes decussate (cross in ‘X’)
to the opposite side, while the temporal bres
remain uncrossed. Therefore, the optic tracts that
diverge from the optic chiasm carry bres for the
opposite half of the eld of vision, i.e. the right
optic tract projects to the left hemield and vice
versa for the left optic tract. Nearly 53% of the
retinal bres cross over to the opposite side.
In the chiasm, the crossing nasal axons and the
uncrossed temporal axons from the inferior retina
cross anteriorly on the ventral aspect of the optic
15.2 Blood Supply oftheVisual Pathways
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chiasm, while the superior axons cross on the
chiasma’s dorsal aspect. These axons become
vulnerable to an enlarging pituitary tumour producing initially a bitemporal quadrantanopia that
progresses to a bitemporal hemianopia. The
crossing axons from the nasal macula cross in the
posterior part of the chiasma. Nearly 85–90% of
axons proceed to the lateral geniculate nucleus
(LGN) located on the posteroinferior aspect of
the thalamus; the rest go to the pretectal nucleus.
The lower nasal crossing axons from the left eye
and uncrossed lower temporal axons from the
right eye synapse in the lateral third of the right
LGN.The lateral third of the right LGN projects
to the left-sided upper temporal quadrant of the
homonymous eld of vision. The upper nasal
crossing axons from the left eye and the uncrossed
upper temporal axons from the right eye end up
in the medial third of the right LGN.These project to the left-sided lower temporal quadrant of
the homonymous eld of vision. The crossing
axons from the nasal retina synapse in the LGN
in layers 1, 4, and 6 and the uncrossed temporal
bres synapse in layers 2, 3, and 5. The macular
axons synapse in LGN in its upper and posterior
portions, and the post-synaptic axons extend into
the optic radiation in the central third. The superior and inferior axons extend posteriorly in the
optic radiation’s superior and inferior third,
respectively. The inferior axons form Meyer’s
loop. In the visual cortex, the axons from the
inferior retina synapse in the inferior bank of the
calcarine ssure and the axons from the superior
retina synapse in the superior bank of the calcarine ssure. The macular bres synapse in the
caudal visual cortex, where they are most vulnerable to the trauma sustained during a fall on the
occiput. [2].
15.2 Blood Supply oftheVisual
Pathways
15.2.1 Blood Supply oftheOptic
Chiasm
The optic chiasm gets its blood supply from the
anterior cerebral arteries, anterior communicat-
ing arteries on its dorsal and anterior aspect and
the basilar artery, posterior communicating arteries, and the posterior cerebral arteries from
below. This network of arteries is called the arterial circle of Willis. The internal carotid arteries
ank the chiasm, and the cavernous sinus lies
below and lateral.
LGN gets its blood supply from the anterior
and posterior choroidal arteries. The optic tracts
do so from the anterior choroidal and middle
cerebral arteries. The visual cortex in the calcarine sulcus gets its blood supply from the posterior cerebral artery.
15.2.2 Blood Supply oftheOptic
Nerve
The ophthalmic artery is the rst branch that
arises from the intradural part of the internal
carotid artery, and the diameter varies from
2.16mm to 2.25mm at its origin. It courses anteriorly through the oor of the optic canal
ensheathed with the optic nerve and, just before
entering the orbital apex, penetrates the dura
mater again to exit the optic foramen from the
lateral aspect of the optic nerve [3]. The intracranial part of the optic nerve is supplied by the
superior hypophyseal arteries, 1–4in number and
0.1–0.5mm in size. These arise from the internal
carotid artery and the posterior communicating
artery. The intracanalicular part is supplied by the
subpial and the intraneural branches of the superior hypophyseal arteries, and some may come
from the intracranial (one, <0.1mm) or the intraorbital (2–5, 0.1–0.4mm) part of the ophthalmic
artery. Because of the rigid canal with a small
diameter, the vessels are vulnerable to rupture by
trauma and cause compression of the optic nerve
by swelling or haemorrhage in the subdural space
[4]. The intraorbital part of the optic nerve is supplied by branches from the short and long posterior ciliary arteries which enter the dura, cross the
dural space, and form a subpial and intraneural
plexus [4]. The central retina artery enters the
dural sheath about 12mm behind the globe, runs
in the subarachnoid space, and then runs an intraneural course to enter the eye.
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15 Optic Disc Signs—Cupping, Swelling, Inammation, andPallor
15.2.3 Blood Supply oftheOptic
Nerve Head
The supercial nerve bre layer gets blood supply through the capillary network drawn from the
retinal arterial supply (Fig.15.2). The prelaminar
part of the optic disc gets its blood supply from
the peripapillary choroid, which in turn gets its
supply from 2–3 posterior ciliary arteries, varying from 1 to 5. There is no supply from the choriocapillaris or the central retina artery in the
prelaminar part. The laminar part of the ONH
gets supplied from the arterial circle of Zinn and
Haller (ZHAC) and, when present (~50%), is
formed by branches of the posterior ciliary arteries. Thus, individuals’ arterial supply to the laminar ONH is highly variable [5]. The ZHAC is
located in the posterior sclera surrounding the
ONH at the junction of the dura mater with the
sclera [6]. The ZHAC has strict distribution in the
superior and inferior parts of the ONH.This part
of theONH has a rich capillary plexus. The lami-
nar part of the ONH is packed with 1.2 million
bres in a very narrow and conned space. It is
highly vulnerable to the pressure differential
between the intraocular and cerebrospinal uid
pressure in the optic nerve sheath. Notably, the
posterior ciliary arteries are strictly end-arterial
under autonomic control, making the ONH’s
laminar part highly vulnerable to developing
ischaemic events due to a fall in perfusion pressure. The fundus uorescein angiography often
reveals a watershed zone between the blood supply from the lateral and medial ciliary arteries
[5]. The retrolaminar part gets supply from the
centripetal pial branches, some branches from the
central retinal artery or the rich plexus formed by
the recurrent branches from the circle of ZinnHaller (Fig.15.2). Longitudinal vessels have also
been demonstrated to extend from the retrolaminar optic nerve to the anterior surface of the
ONH. These are seen to anastomose with the
transverse system drawn from the ciliary circulation [7].
Fig. 15.2 Highly schematic representation of the blood
supply of the optic nerve head. (CRA =Centra Retinal
Artery; ILM = Internal limiting membrane;
PCA = Posterior ciliary artery; RPE = Retinal pigment
epithelium). Note that the laminar optic nerve head
receives blood supply from the branches of the posterior
ciliary arteries. In contrast, the prelaminar optic nerve
head gets supply from the peripapillary choroid. The
supercial optic nerve head gets its supply from the capillaries drawn from the retinal artery branches. The central
retinal artery does not supply blood to the retrolaminar
optic nerve, which is served by the pial branches of the
posterior ciliary arteries. Graphic by Kritika Thakur
15.3 Evaluation oftheOptic Disc
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15.3 Evaluation oftheOptic Disc
The anterior end of the optic nerve is visible in
the eye on ophthalmoscopy. It is called the ONH
or the optic disc (OD). It is a pink, vertically oval
disc-like structure. The ONH/OD size measured
on histopathological examination in eye bank
eyes is 1.88mm vertical and 1.77mm horizontal, irrespective of the axial length of the eyeballs. It is larger than the diameter measured
with imaging techniques. The vertical diameter
in blacks is signicantly greater than that in
whites. Women have a narrower horizontal
diameter than men [8].
15.3.1 Clinical Assessment
oftheOptic Disc Size
The optic disc size can be easily estimated using
the middle viewing aperture on a direct ophthalmoscope which projects a 1.5mm diameter (5°)
of the light spot on the retina and about covers the
entire extent of the ONH.The optic disc size can
also be measured using high + biconvex lenses,
60D, 78D, or 90D, during the biomicroscopic
examination of the optic disc. A narrow slit-lamp
beam is projected to measure the horizontal or
vertical diameter; the beam height is shortened
until it covers the optic disc edge to edge. A correction must be applied to the beam length (beam
height scale shown on the slit-lamp) 1.0X, 1.1X,
and 1.3X, respectively, for the 60, 78, and 90 D
lenses. The same technique can measure the optic
disc cup (ODC) height and width.
15.3.2 Optic Disc Cup (ODC)
andGlaucoma
A normal-sized ONH shows a small round or
slight vertical oval depression centred a little to
the temporal side called the physiological cup
(Fig. 15.3a). The central retinal artery and the
central retinal vein usually emerge from the nasal
side of the cup. The area between the margins of
the cup and the scleral rim is packed with exiting
RGC neurons called the neuroretinal rim (NRR).
The cup is covered with a remnant gliotic membrane of Kuhnt. There are wide variations in the
ONH size depending on the size of the scleral
canal. Irrespective of the size of the ONH, almost
1.2 million axons get packed; thus, a small ONH
crowded with axons may not show any physiological cup. Such eyes are at risk of developing
non-arteritic anterior ischaemic optic neuropathy
(ni-AION).
On the other hand, a large-sized ONH carries
the same number of axons and is still left with a
large cup (Fig. 15.3b). The space between the
lamina cribrosa and the surface of the ONH is
lled with prelaminar tissue. The cup is usually
assessed for its vertical and horizontal diameter.
The cup diameter is compared to the diameter of
the ONH and is expressed as a fraction termed
cup-to-disc ratio, the C/D ratio, or CDR.Enlarging
optic disc cup (ODC) is a critical sign of the progression of open-angle glaucoma (OAG)
(Fig. 15.3 c, d). OAG is now believed to be an
optic neuropathy resulting in progressive, irreversible RGC axons loss (Fig.15.4). The signicant risk factors for OAG are higher than normal
intraocular pressure (IOP), myopia, age, and race.
Nearly 1/3rd of the patients with OAG may show
normal or lower than-normal IOP (Fig.15.4). The
loss of RGC axons is reected in their attrition
from the ONH, with a consequent increase in the
size of the ODC.A uniform enlargement of the
ODC may be mistaken for a large physiological
cup unless a smaller cup has been imaged previously in the same eye. The median CDR is 0.3.
Usually, the CDR<0.7 is normal if the optic disc
size is average. CDR≥0.8 is highly suspicious of
OAG, irrespective of the disc size. Asymmetric
enlargement of the ODC, especially when seen at
the ONH’s upper or lower pole as a focal notch, is
almost always acquired and pathognomonic of
OAG (Fig.15.4). ONH smaller than average often
has no ODC.Thus, ODC of any size in such eyes
highly suggests OAG. Notably, the ODC in the
two eyes are symmetrical, and variation of C/D
Raito ≥1 between the two eyes should raise suspicion of glaucoma.
The loss of the RGC axons is reected in
visual eld defects called scotomas (Fig.15.4c).
OAG is an asymptomatic disorder and is a sig-
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15 Optic Disc Signs—Cupping, Swelling, Inammation, andPallor
Fig. 15.3 ONH evaluation and glaucoma: A normalsized ONH shows a small round or slight vertical oval
depression centred a little to the temporal (T) side called
the physiological cup (a). The central retinal artery and
the central retinal vein usually emerge (black arrows)
from the cup’s nasal (N) side. The area between the margins of the cup and the scleral rim is packed with exiting
RGC neurons called the neuroretinal rim (blue arrows). A
nicant cause of blindness. ODC evaluation
during a screening visit is the most critical sign
in suspecting and preventing blindness from
OAG. A visual eld examination conrms the
loss of RGC axons (Fig.15.4c). Since IOP is a
signicant risk factor for OAG, the only known
intervention reducing the risk of further damage
is lowering IOP using local therapy or ltration
surgical procedures. While a higher- than- normal
IOP may provide a clue to OAG, many such
patients may not show glaucomatous ODC or
visual eld changes and are labelled ocular
large-sized ONH carries the same number of axons and is
still left with a large cup (b). An enlarged optic disc cup
(ODC) is a critical sign of the progression of open-angle
glaucoma (OAG) (c, d). Images courtesy of Prof SS
Pandav, Professor and Head, Advanced Eye Centre, Post
Graduate Institute of Medical Education and Research,
Chandigarh, India
hypertension. Given the uncertainty of the IOP
as a diagnostic sign and the logistic and time
constraints of doing visual eld testing in a
screening program, evaluation of the optic disc
for glaucomatous cupping either on the ophthalmoscopy or remote evaluation from the optic
disc imaging remains the most favoured
technique.
Using enhanced depth imaging with spectral
domain optical coherence tomography (EDIOCT), the depth of the physiological cup can be
measured and the volume can be calculated.
15.3 Evaluation oftheOptic Disc
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a
d
b
c
e
Fig. 15.4 A 77- year-old man was referred as a case of
open-angle glaucoma. He had a history of sudden blurring
of vision. His visual acuity was 6/12in the right eye and
6/6 in the left eye. The intraocular pressures were
12mmHg in both eyes. The superior half of the neuroretinal rim of the right optic disc showed pallor (a). The left
optic disc appeared normal (b). A 24.2 visual eld in the
right eye showed an inferior altitudinal defect correspond-
15.3.3 Challenges ofEvaluation
oftheOptic Disc Cupping
Typically, the optic disc is slightly vertically oval,
and the margins are nely blurred as the bundles
of RGC axons cross over the edge. The vertical
diameter is the maximum diameter, and the horizontal diameter is the minimum, with a difference
of about 7%. In a normal population study, the
horizontal diameter was 1.76 ± 0.31mm (0.91–
2.61 mm), and the vertical 1.92 ± 0.29 mm. A
shallow cup, if present, is slightly horizontally
oval. The mean horizontal cup diameter was 0.83
± 0.58mm (0.00–2.08mm), and the mean verti-
ing to the rim pallor(c). SD-OCT showed thinning of the
superior peripapillary RNFL in the right eye (d). There
was signicant thinning of the macular ganglion cells in
the superior half of the right eye and some thinning of the
lower nasal sector in the left eye (e). Images courtesy of
Prof SS Pandav, Professor and Head, Advanced Eye
Centre, Post Graduate Institute of Medical Education and
Research, Chandigarh, India
cal diameter was 0.77 ± 0.55mm (0.00–2.13mm)
[9]. This appearance is due to the usual direct
entry of the optic nerve into the eye. However, the
nerve may enter the eye obliquely, producing the
appearance of either a vertical or horizontally
tilted disc. These are more often seen in eyes with
myopia. The vertical tilt most often results in the
temporally tilted disc and shows parapapillary
atrophy temporally to the optic disc. In a temporally tilted disc, the temporal NRR is thinned out,
and the horizontal boundary of the cup may be
difcult to dene.
On the other hand, the horizontal tilt results in
an inferior tilted disc with parapapillary atrophy
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15 Optic Disc Signs—Cupping, Swelling, Inammation, andPallor
below the disc. Since these eyes also have an axial
elongation, the IOP exerts maximum pressure on
the axons passing through the disc’s upper and
lower poles, resulting in the loss of superior and
inferior arcuate retinal nerve bres. The horizontal tilt leads to the loss of the inferior retinal arcuate bres [10]. Moreover, the optic disc tilt eyes
show signicant corneal astigmatism, the steeper
axis aligned with the tilt axis [11]. There is frequently a parapapillary chorioretinal atrophy
(PPCA) beyond the scleral border tissue of
Elschnig that surrounds the optic disc. Any PPCA
is seen in 59%, zone α in 58% and zone β in 13%.
The zone β, a greyish-white area, is caused by
RPE falling short of Bruch’s membrane opening,
exposing the underlying choroidal vessels and
sclera. In zone β, there is a complete loss of choriocapillaris, RPE, and overlying photoreceptors.
Large choroidal vessels are seen overlying the
sclera. It is seen as a crescent- shaped area next to
the optic disc. It can be measured in histopathology as the distance between the RPE and the
Bruch’s membrane ending. Beyond the zone β,
RPE hypo and hyper pigmentary are labelled
Zone α. It has no signicance. The presence of
zone β is three times more likely to be present in
patients with POAG [12]. PPCA is most frequent
temporal to the optic disc and is infrequent in the
nasal sector. In patients with early POAG, zone α
(23%) and β are seen (42%) [13]. Zone γ is the
bare peripapillary scleral ange in axial myopia
without overlying choriocapillaris, BM, or RPE
and is seen in the juxtapapillary area. The zone
within the zone γ, which lacks blood vessels
larger than 50μm, is labelled zone 𝛿. Zone 𝛿 is
seen in very high axial myopia [14].
15.3.4 OCT Evaluation
oftheGlaucomatous Optic
Neuropathy(GON)
In recent years, remarkable progress has been
made in evaluating the optic disc, RNFL, and
macular parameters in the early detection and
progression of glaucoma damage. All three measurements complement each other, and the sensitivity to detect changes varies with the specicity
required. Several OCT machines and different
algorithms are currently used to evaluate these
parameters. The data are not transferrable
between the machines [15].
The earliest measured cRNFL thickness at a
xed 3.5 mm circle centred on the optic disc
remains the gold standard in diagnosing glaucoma [15].
In contrast to a rough estimation of the optic
disc size (within the white scleral ring), automatic algorithms on OCT consistently identify
and measure the Bruch’s membrane opening
(BMO) which currently denes the size of the
optic disc. It measures the vectors between 180
points in Cirrus and 48in Spectralis. On clinical
examination, BMO is not visible. The termination of Bruch’s membrane denes the edge of the
disc. The ILM denes the delineation of the
NRR.Termination of the internal limiting membrane provides the edge of the cup, and the size
of the opening gives the diameter of the ODC
[16]. Clinical estimation of the CDR on stereoscopic fundus pictures consistently underestimates the CDR. Although the clinical
examination may provide a quick clue for suspecting OAG, the SD-OCT provides a more
objective measurement of glaucoma damage
progression [16].
15.3.5 Bruch’s Membrane Opening—
Minimum Rim Width inGON
On Spectralis OCT (Heidelberg Engineering,
Inc., Heidelberg, Germany), 24 radial line
B-scans (48 points on the BMO) can automatically measure the distance between the BMO and
the ILM to give the rim width in sectors. The
minimum rim width (MRW) parameter in the
lower temporal sector is highly sensitive in
detecting early glaucoma in younger people compared to the older, as this sector shows age-related
MRW.
However, the cRNFL thickness has the highest sensitivity in detecting perimetric glaucoma
[17, 18]. Guided progression analysis of the
cRNFL provides the highest accuracy in detecting glaucoma progression [15].
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