Neuro-ophthalmic Anatomy and Physiology

OPTOMETRY · SEMESTER 2

Neuro-ophthalmic Anatomy and Physiology

Ocular Anatomy and Physiology

START READING NOTES

Neuro-ophthalmic Anatomy and Physiology

CHAPTER 12. NEURO-OPHTHALMIC_ ANATOMY AND PHYSIOLOGY

BY THE END OF THIS CHAPTER YOU WILL HAVE COVERED THE

FOLLOWING ASPECTS OF THE TOPIC:

THE ANATOMY OF VISUAL PATHWAYS

THE PHYSIOLOGY OF VISION

COMMON SYMPTOMS IN NEURO-OPHTHALMOLOGY

OCULAR MOTOR NERVES ANATOMY AND PATHOLOGY ASPECTS

PUPILLARY PATHWAYS AND REFLEXES

ARWONS>

1. THE ANATOMY OF THE VISUAL PATHWAYS.
The route taken by the light-generated impulses the eye is called the visual
pathway.
The visual pathway is effectively a tract within the central nervous system
and it is made up of the:
optic nerves;
optic chiasm;
optic tracts;
lateral geniculate bodies;
optic radiations;
visual cortex.
Disorders affecting different parts of the visual pathway produce
characteristic changes in the field of vision.
The nature of visual field disturbances , therefore, can help in determining
which part of the visual pathway is affected.
84
Temporal
retina
Optic
tract
Lateral
‘eniculate
ody
Optic
radiations
Visual
cortex
Fig. 12.1. Diagramatic representation of the visual pathways.
1.1. The Optic Nerve ( Cranial_Nerve II ).
The optic nerve is made up by the axons of the retinal ganglion cells .
It may be divided anatomically, into four portions:
intraocular (1mm);
intraorbital (25mm);
intracanalicular (9mm);
intracranial (16mm).
85
a. The intraocular portion.
This part includes the optic disc and the portion of the optic nerve that lies
re
The optic disc lies in the nasal retina, medial to the macula.
Also called the optic nerve-head or papilla, it represents the confluence of
approximately 1.2 million ganglion cell axons.
It's pale pink in colour, slightly oval vertically . It has a central cup and a
peripheral rim of neural tissue
The central retinal vessels emerge at the centre of the optic disk, pass over
the rim, and radiate out to supply the retina.
The optic disk derives its blood supply via the short ciliary arteries from the
ophthalmic artery, while the retina is fed by the central retinal artery.
The nerve fibres of the intraocular portion are not myelinated.
They traverse the sclera through the lamina cribrosa .
Just posterior to the sclera, the fibres acquire a myelin coating.
b. The intraorbital portion.
The orbital portion of the nerve extends from the globe to the optic canal.
It is 3-4mm thick, double the diameter of the intraocular portion as a result
of the acquisition of myelin and meningeal sheath.
Posteriorly , the meningeal sheath is continuous with the meningeal sheath
around the brain.
Thus, a rise in the intracranial pressure will be communicated to the
subarachnoid space around the optic nerve, and may result in papilloedema.
The length of the intraorbital portion of the optic nerve is longer than the
anteroposterior dimensions of the orbit.
The extra length of the intraorbital optic nerve allows unimpeded globe
rotation as well as axial shifts within the orbit.
c. The intracanalicular portion.
86
In addition to the optic nerve , the canal contains the ophthalmic artery and
sympathetic fibres of the sympathetic carotid plexus.
In the canal the nerve is firmly anchored (the dura and the periosteum are
fused). As aresult, a small lesion in the optic canal can produce a
compressive optic neuropathy even before it becomes easily visible upon
neuroimagining.
d. The intracranial portion.
The optic nerve exits the posterior opening of the optic canal , enters the
middle cranial fossa and continues posteriorly ascending to join the optic
chiasma.
Above the optic nerve lie the inferior surface of the frontal lobe, the
olfactory tract , and the anterior cerebral and anterior communicating
arteries.
The lateral aspect of the optic nerve if often immediately adjacent to the
internal carotid artery .
Inferiorly and medially , the posterior ethmoid and sphenoid sinuses are
adjacent to the nerve.
The blood supply of the optic nerve is through the pial network of vessels.
Arterial branches feeding the pial network originate from the ophthalmic
artery .
The venous drainage of the optic nerve is chiefly by the central retinal vein
and to a lesser extent via the pial venous system.
Both systems drain into the ophthalmic venous system ( superior and/or
inferior ophthalmic veins ) in the orbit and less commonly directly into the
cavernous sinus.
1.2. The Optic Chiasm.
Fibres from the two optic nerves merge to form the optic chiasm.
The chiasm is located anterior to the hypothalamus and above the sella
turcica.
87
Fig. 12.2. Relation between the optic nerve and chiasm, the sellar
structures
and the third ventricle (III ).
The chiasm overlies the sella turcica with the pituitary gland below, being
The relatively large space between the chiasm and the pituitary explains the
fact that only large tumours of the pituitary gland will compress on the
chiasm and cause visual field defects.
The chiasm forms the floor the third ventricle and carotid arteries bound it
at either side.
The circle of Willis surrounds the chiasm.
88
Anterior cerebral artery
|
Internal
j carotid
/ artery
I
Posterior
communicating
Basilar artery artery
Fig. 12.3. Relationship between the Optic Chiasm and the Circle of Willis.
Fibres from each nasal retina cross in the optic chiasm , whereas fibres from
each temporal retina do not cross.
Slightly more than half of the fibres (65% ) decussate , because the nasal
retina contains more ganglion cells than the temporal retina.
Lesions of the chiasm characteristically involve crossing nasal fibres. As such
, they cause bilateral temporal visual field defects, that respect the vertical
line.
1.3. The Optic Tracts.
The optic tracts begin at the posterior aspect of the optic chiasm , diverge
laterally , and continue posteriorly to terminate primarily in the lateral
geniculate bodies.
A group of fibres leave the optic tract just before the lateral geniculate body
for the pretectal area; the pretectal pathway controls light-mediated
pupillary constriction.
89
Each optic tract contains crossed nasal fibres from the contralateral retina
and uncrossed temporal fibres from the ipsilateral retina.
Optic tract lesions cause incongruous field defects explained by the fact that,
corresponding fibres from the two eyes are not closely aligned.
1.4. The Lateral Geniculate Body.
The lateral geniculate body (nucleus ) is located in the thalamus. _
At this level , the axons of the retinal ganglion ells terminate (synapse).
The cells of the lateral geniculate body are organised in 6 major layers:
e The four superior layers receive input from the parvocellular retinal
ganglion cells;
e The two inferior layers are the terminus for the magnocellular axons.
The bulk of LGB sends its fibres via the optic radiations to the visual cortex.
1.5. The Optic Radiations.
Following the synapse in the lateral geniculate bodies, the axons of
geniculate neurons projecting to the primary visual cortex , become the optic
radiations.
The optic radiations are of major clinical importance as they are frequently
involved in cerebrovascular disturbance or tumours.
The optic radiations contain three main groups of fibres:
e the superior portion containing fibres serving the inferior visual field
(fibres passing through the parietal lobe);
e the inferior portion containing fibres serving the superior visual field
(fan shaped loop – of Meyer- passing through the temporal lobe);
e thecentral portion containing the macular fibres.
As the optic radiations pass posteriorly , corresponding fibers from the right
and left eyes, lie progressively closer together.
For this reason, field defects produced by lesions of the posterior radiations
are more congruous than those involving the anterior radiations.
90
1.6. The Visual Cortex.
The primary visual (striate, calcarine cortex, Brodmann’s area 17) cortex,
lies on the medial aspect of the occipital lobe , above and below the calcarine
Fibres of the optic radiations terminate in the layers of the primary visual
cortex.
The visual fields of both eyes are represented (mapped ) over the calcarine
cortex. —
This mapping of visual fields respects the specific retinotopic distribution.
The peripheral visual fields are represented anteriorly while central macular
vision is represented posteriorly, at the tip of the calcarine cortex.
The representation of the central portion of the visual field is greatly
magnified whereas the representation of peripheral visual fields is relatively
compressed.
This exaggerated representation of central vision correlates with the higher
acuity and greater density of photoreceptors and ganglion cells in the
macula compared with the peripheral retina.
Field defects, caused by lesions of the calcarine cortex are congruous.
Above and below the primary visual cortex ( area 17), are located the
secondary visual association areas (18 and 19); the connections,
arrangement and functions of these areas, are currently the subject of
intensive investigations.
2. THE PHYSIOLOGY OF VISION
The ability to detect, recognise, and discriminate objects in space, is
fundamental to survival, and it is achieved mainly through visual acuity.
The visual process is initiated by the detection of a light signal by
photoreceptor cells in the outer retina.
These cells convert light energy to an electrochemical stimulus.
The conversion of light energy to an electrochemical response, is called
phototransduction.
These signals started in the photoreceptor cells, are transmitted to the
bipolar cells , then to the ganglion cells.
91
The information from the ganglion cells is further transmitted in the axons
of these cells ( via the optic nerves, chiasm, and optic tracts) to the lateral
geniculate bodies.
At this level there is synaptic contact , and signals are further transmitted
via the optic radiations to the visual cortex , where the information is
analysed and the sensation of vision takes place.
Input is also received by the visual cortex from many other areas,
particularly those controlling general motor function and eye movement ,
cerebellar and special sense, memory and many other functions.
3. COMMON _SYMPTOMS_IN_NEURO-OPHTHALMOLOGY.
3.1. Decreased visual acuity .
Assessment of visual acuity (Va ), is the most common measure of central
visual function.
Unilateral visual loss always indicates a lesion anterior to the chiasm,
whereas bilateral loss may reflect bilateral optic nerve disease or a chiasmal
or retrochiasmal process.
Sudden onset usually indicates an ischaemic event. Gradual progression over
months is typical of toxic lesions.
Associated symptoms should be sought out , as they often suggest aetiology.
3.2. Colour vision disturbance.
Testing of colour vision complements assessment of visual acuity.
Decreased colour vision (colour desaturation )is characteristic of optic nerve
disease, particularly demyelinating optic neuritis.
3.3. Visual field defects.
Visual field testing is the first consideration in patients complaining of visual
loss but demonstrating normal visual acuity.
Visual field testing helps in the localisation of the lesion and quantifies the
defect.
Scotoma is an area of depressed visual function surrounded by normal visual
function.
There is a physiological scotoma ( blind spot ) in the temporal visual field,
representing the optic disc.
Enlargement of the blind spot occurs in papilloedema.
92
Visual field defects characteristic of optic nerve pathology, are the central
and centrocaecal scotomas.
Visual field defects due to chiasmal and retrochiasmal lesions, typically
respect the vertical midline.
Hemianopia means that one half of visual field is involved ,either nasal or
temporal.
Bitemporal hemianopia which obeys the vertical line, is the classical visual
field abnormality associated with chiasmal disease.
It may complete or incomplete.
Lesions posterior to the chiasm cause homonymous (temporal side of one
eye and the nasal side of the other one) field defects.
More posterior lesions result in more congruous ( similar) defects, while
anterior lesions tend to produce dissimilar, incongruous ,defects.
3.4. Diplopia.
Diplopia means double vision.
It is important to determine whether diplopia is monocular or binocular.
Monocular diplopia is usually caused by disorders of the ocular refractive
media ,i.e. corneal irregularities, cataract.
Binocular diplopia represents a disturbance of ocular motility.
4. OCULAR MOTOR NERVES ANATOMY AND PATHOLOGY ASPECTS.
4.1. The Oculomotor Nerve (Cranial Nerve III ).
The third nerve originates withinithe midbrain , thejnuclearcomplexibeing
situated at the level of the superior colliculi.
It is composed of paired and unpaired subnuclei with projections to the
individual extraocular muscles; the Edinger- Westphal nucleus gives
parasympathetic input to the ipsilateral pupil and ciliary muscle.
After leaving the midbrain follows a subarachnoid course.
Within the subarachnoid space it passes betweefiithelposterionicerebral
aiteryrandithelsuperioncerebellarartery, parallel with the posterior
communicating artery.
93
A posterior communicating artery aneurysm , which typically arises at the
junction of th
artery) is the most common cause of spontaneous, acute , complete third
nerve palsy with pupil involvement.
Blood vessels on pia mater supply surface
of the nerve including pupillary fibres
~ { damaged by compressive lesions )
rn
Vasa vasorum supply part
of nerve but not pupillary fibres
{ damaged by medical lesions )
Pupillary fibres lie
dorsal and peripheral
Fig. 12. 4. Location of the pupillary fibres within cranial nerve III.
Because pupillary fibres are located peripherally in the third nerve, the pupil
is involved in essentially all cases of aneurysm-induced third nerve palsy.
Acute elevation of intracranial pressure (result of mass lesion or
haemorrhage), may also cause third nerve palsy by stretching or
compressing the nerve ; pupillary involvement is again an early sign ,
because of the superficial location of the pupillary fibres.
Within the cavernous sinus, the third nerve runs in the lateral wall.
Because Oflitsiclose|proximity to/other cranialinerves , cranial nerve III
lesions in the cavernous sinus are usually associated with involvement of
other nerves.
94
In thévanterior Cavernous'sinus the nerve divides into superior and inferior
branches which enter the orbit through thé\superior orbital fissure),
Within the orbit the superior division innervates the levator palpebrae
superioris and superior rectus muscles.
The inferior division supplies the medial rectus, the inferior rectus and the
inferior oblique.
The inferior branch of the third nerve within the orbit also contains
parasympathetic fibres from the Edinger- Westphal subnucleus , which
innervate the sphincter/pupillaevand the ciliary muscle:
Damage to cranial nerve III may result in :
e eye deviated outward;
inability to look upward, downward and inward;
diplopia;
ptosis;
dilated, unreactive pupil;
lack of accommodation.
4.2. The Trochlear Nerve ( Cranial Nerve IV ).
The fourth cranial nerve originates within the midbrain , the nucleus being
located just caudal to the oculomotor complex , at the level of the inferior
collicullus.
The axons from the nucleus decussate completely in the anterior brain stem,
then emerge on its dorsal surface, on the lower border of the pons, above
the medulla.
These properties are probably responsible for its frequent involvement in
head trauma.
In the subarachnoid space, passes betwen the posterior cerebral artery and
superior cerebellar artery . Within the subarachnoid space it is easily
damaged during neurosurgical procedures that involve the tentorium.
Just below the tentorium pierces the dura into the lateral wall of the
cavernous sinus.
Enters the orbit through the superior orbital fissure to innervate the superior
oblique muscle.
Damage to the trochlear nerve may result in :
eye deviated upward;
inability to look downward;
diplopia;
head tilt.
95
4.3. The Abducens Nerve (Cranial Nerve VI ).
The sixth nerve nucleus is located in the pons, beneath the floor of the
fourth ventricle.
The fasciculus of cranial nerve VII, the facial nerve, loops over the top of the
sixth nerve nucleus , forming the genu of cranial nerve VII.
Leaves the midbrain at the pontomedullary junction; here it may be involved
in cerebellopontine angle tumours such as acoustic neuromas and
meningeomas.
In the subarachnoid space ascends the petrous bone then angles sharply
forwards over the tip of the petrous bone, passes through Dorello’s canal
(under the petroclinoid ligament ) to enter the cavernous sinus.
The sixth nerve may be stretched over the petrous tip as a result of raised
intracranial pressure causing a non-specific abducens palsy . In this case the
sixth nerve palsy may be bilateral.
Infectious or inflammatory processes originating in the middle ear or
mastoid may affect the sixth nerve secondary to petrositis (Gradenigo
syndrome).
In the cavernous sinus the nerve runs through the middle, in close relation
to the carotid artery.
Whereas the third and fourth nerve are protected within the lateral wall of
the sinus,
the sixth nerve running freely in the middle , is more prone to damage than
the others.

Pituitary Chiasma

Sphenoid Cavernous Internal

sinus sinus carotid
artery
Fig. 12.5. Location of the cranial nerves in the cavernous sinus.
The sixth nerve enters the orbit through the superior orbital fissure to
supply the lateral rectus muscle.
Damage to the abducens may result in :
e eye deviated inward;
e inability to look outward;
e diplopia.
97
5. PUPILLARY PATHWAYS AND REFLEXES.
5.1. The pupil is the aperture in the middle of the iris, which allows the light
rays to enter the eye.
The size of the pupil regulates the amount of light entering the eye, and it is
controlled by two iris muscles:
e the sphincter muscle —constricts the pupil ( miosis ) ,via
parasympathetic innervation; miosis occurs in illuminated conditions.
e the dilator muscle – dilates the pupil (mydriasis ), via sympathetic
innervation; mydriasis occurs in dark conditions.
The pupil may vary from 1-8mm in diameter and there may be a slight
degree of asymmetry ( anisocoria ) between right and left eyes in normal
individuals.
5.2. The Parasympathetic Pupillary Pathway (The Light Reflex Pathway ).
The light reflex, which consists of pupillary constriction in response to
stimulation by light, has an:
e afferent limb/tract
e efferent limb/tract.
The afferent response commences in photoreceptors, is transmitted to
retinal ganglion cells , enters the optic nerve, decussates at the chiasm ,
traverses the optic tract and terminates in pretectal nucleus ( bypassing the
lateral geniculate body).
Fibres synapse in the pretectal nuclei;these nuclei then project to both
ipsilateral and contra-lateral Edinger- Westphal nuclei in the oculomotor
complex (this explains why a unilateral light stimulus evokes a bilateral and
symmetrical pupillary constriction).
Efferent parasympathetic pupillary fibres from the Edinger- Westphal nuclei,
exit the midbrain with cranial nerve I// (oculomotor).
Initially the pupillary fibres are located superficially , where they are
vulnerable to compression (i.e. aneurysm).
98
As the third nerve courses forward, the pupillary fibres join the inferior
division then the inferior oblique branch , to synapse finally in the ciliary
ganglion.
From the ganglion , the short ciliary nerves distribute the postganglionic

fibres to the iris sphincter muscle and the ciliary muscle.

Edinger Westphal

nucleus
Pretecto-oculomotor
tract
Posterior
commissure
Pretectal nucleus
Fig. 12.6. Diagram representing the pathway of the pupillary light
reflex.
99
5.3. The Sympathetic Pupillary Pathway.
The afferent sympathetic pupillary pathway is not well known.
The efferent sympathetic fibres arise in the posterior hypothalamus ( first
order neuron ) ; descend the brain stem to terminate in the ciliospinal centre
of Budge, between C8 and T2, ( second order neuron).
A synapse occurs at this level, then the axons ascend .
During this long course the axons are closely related to the subclavian artery
and to the apical pleura, where it may be damaged by an apical bronchial
carcinoma (Pancoast tumour ) or during neck surgery.
These axons synapse in the superior cervical ganglion( third order neuron) ;
the postganglionic fibres then ascend and proceed along the carotid artery
and subsequently the ophthalmic artery and its branches.
The sympathetic fibres reach the ciliary body and the dilator of the iris via
the nasociliary nerve and the long ciliary nerves.
100
Pa a
I
a
pe mn Maller’s muscle
€ |
nerve
Long ciliary
3rd order nerve
neuron
2nd order
neuron
Subslavian artery
Ciliogpinal
center of
Budge
Fig. 12.7. Diagram representing the ocular sympathetic pathway.
5.4. The Direct Pupillary Light Reflex.
When light is shine directly into an eye, the pupil of the eye constricts .
Responsible for the direct light reflex is, the parasympathetic pathway
(afferent & efferent).
5.5. The Consensual Pupillary Light Reflex.
When one eye is stimulated by light, the fellow eye simultaneously and
equally constricts.
101
This is explained by the crossing of nerve fibres ( decussations ) at the level
of the chiasm and pretectal area.
5.6. The Relative Afferent Pupillary Defect (RAPD ).
This is an abnormal pupillary reaction.
The reaction is best detected by the so-called “ swinging/alternating light
test”.
Normally, when swinging the light from one eye to another, the pupils
constrict.
When the light is brought from the normal eye to the abnormal eye, the
pupil of the abnormal eye will dilate instead of constricting.
This paradoxical reaction, also called Marcus Gunn reaction, occurs because
the dilatation of the pupil ( by withdrawing the light from the normal eye ),
outweighs the constriction produced by stimulating the abnormal eye.
The stimulus from the normal eye is transmitted, while the stimulus from the
abnormal eye is not ( due to optic nerve damage mostly).
5.7. The Near Reflex (response).
The near reflex occurs when attention is changed from distance to near.
It consists of:
e increased accommodation;
e convergence of the visual axis;
e constriction of the pupils.
There is no clinical condition in which the light reflex is present but the near
response is absent, therefore if the pupillary response is intact there is no

need to test its reaction to near.