11/13/24, 7\:11 PM Guide | Visual field defects

Visual

Table of contents

Introduction

The visual pathway is the route by which retinal stimuli are transferred to the occipital cortex of the brain. It encompasses the
retina, optic nerve, optic chiasm, optic radiations and the visual centre of the occipital lobe.
Understanding the anatomy of the optic nerve and visual pathways is key to appreciating how various focal lesions of the
brain cause characteristic visual

Visual

When discussing vision, it is important to understand the visual
The visual
point.
The image of an object in the visual
Confrontational assessment of the visual cranial nerve examination. It can be more accurately
assessed using perimetry studies.
Figure 1. Visual
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Visual pathway

The photoreceptors (rods and cones) in the retina are stimulated by photons of light entering the eye. Light-sensitive surface
membrane proteins (e.g. rhodopsin) of these cells are stimulated to propagate second messenger responses which convert
light energy into electrical signals. The photoreceptors synapse with retinal bipolar cells, which in turn transmit these signals
to retinal ganglion cells.
The retinal ganglion cells converge at the optic disc, forming the optic nerve. The optic nerve then exits the eye, travelling
through a defect in the lamina cribrosa of the sclera.
The optic nerve is the second cranial nerve, responsible for transmitting the special sensory information for vision. It may be
considered an extension of the forebrain as it is covered by the meninges of the central nervous system, rather than by
epi/peri/endoneurium like other peripheral nerves.
The optic nerve travels through the bony orbit and enters the middle cranial fossa through the optic canal, a defect in the
lesser wing of the sphenoid.
Once inside the skull, the optic nerve travels along the
cavernous sinus. Left and right optic nerves then converge at the optic chiasm.
The optic chiasm is located directly above the sella turcica of the sphenoid bone. The pituitary gland projects down
immediately behind the chiasm.
At the chiasm,
from the temporal retina remain on their respective sides, as illustrated in
In this way, left-sided post-chiasmal
Figure 2. The visual pathway
The optic tracts extend from the chiasm to the thalamus. Here, a
order sensory neurones at the lateral geniculate nucleus in the thalamus. From here, the sensory nerves radiate dorsally to
the calcarine sulcus of the occipital lobe.
Optic radiations loop either through the parietal lobe or through the temporal lobe (Meyer’s loop).
As illustrated in

retina/upper visual
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Figure 3. The visual pathway [2]
The optic radiations terminate in the calcarine sulcus of the occipital lobe, where the cortical visual centre is situated.
The calcarine sulcus is responsible for retinal image processing. Here, images from both eyes are
image is formed. This image is inverted, as represented in
image so that information is correctly oriented in space.
From the occipital visual centre, signals are sent to the frontal, parietal and temporal lobes to further make sense of the input
information (e.g. reading/facial recognition).

Blood supply

The blood supply of the visual pathway is outlined in table 1.
Table 1. Blood supply of the visual pathway.
Locus of the visual
pathway
Blood supply
Optic nerve Ophthalmic artery
Optic chiasm
Optic tract Branches from\:
Internal carotid
Posterior communicating artery
Anterior cerebral artery
Anterior communicating artery
Middle cerebral artery (MCA)
Anterior choroidal branches of the MCA
Lateral geniculate
nucleus
Thalamogeniculate branches of the posterior cerebral artery (PCA)
Optic radiations MCA
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Mostly by the PCA. The MCA also helps supply the anterior portion, which corresponds to the
Calcarine sulcus
macula.

Visual

As described above, components of the visual pathway carry information corresponding to speci
Destructive lesions of the visual pathway, therefore, result in characteristic visual
As a rule, pre-chiasmal lesions will result in an ipsilateral monocular visual
homonymous visual
Lesions of the chiasm most commonly result in bitemporal hemianopia, as demonstrated in
Figure 4. Visual
At the optic chiasm,
compressing the chiasm, such as pituitary adenomas, therefore cause bitemporal hemianopia.
Lesions to optic radiations result in homonymous contralateral quadrantanopia. Those a
lobe result in a homonymous upper quadrantanopia (“pie in the sky”), while those a
quadrantanopia (“pie on the
Lesions a
Macular vision is spared given the dual blood supply to the anterior portion of the visual centre (PCA and MCA).
Table 2 describes the typical visual
Table 2. Lesions of the visual pathway and their respective visual
Site of
lesion/pressure
Field defect Typical lesions
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Optic nerve Ipsilateral monocular blindness
Optic chiasm (central) Bitemporal hemianopia
Optic chiasm (lateral) Ipsilateral monocular nasal hemianopia
Optic tract Contralateral homonymous hemianopia
Optic radiation Contralateral homonymous quadrantanopia
Occipital cortex
Contralateral homonymous hemianopia with
macular sparing
Optic neuritis
Amaurosis fugax
Optic atrophy
Retrobulbar optic neuropathy
Trauma
Pituitary adenoma
Suprasellar aneurysm
rd
Distension of the 3 ventricle
Internal carotid/posterior communicating
artery atheroma
MCA stroke
Tumours
MCA stroke
Tumour
Trauma
PCA stroke
Trauma

Key points

The visual pathway comprises the retina, optic nerve, optic chiasm, optic radiations, and the visual centre in the occipital
lobe.
Optic nerve lesions tend to cause ipsilateral monocular blindness.
At the optic chiasm,
compressing the chiasm, such as pituitary adenomas, therefore cause bitemporal hemianopia.
Lesions to the optic radiations result in contralateral quadrantanopia. Those a
quadrantinopia ("pie in the sky
"), while those a
on the
Lesions a
macula.

References

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