Session 4: Monday, October 15, 2012:
Anatomy of the Eye, Associated Eye Conditions
and Functional Implications
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Housekeeping
◦ Any questions from last week’s lecture?
◦ Discuss quizzes
 Anatomy Quiz: Date change to October 18th
 Syndromes Quiz: Date change to November 8th
 Optics Quiz: Date change to November 15th
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Anatomy of the Eye, Associated Eye
Conditions and Functional Implications
◦ Optic Nerve
◦ Visual pathway
Optic Nerve and Visual Pathway
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Composed of nerve fiber layer bundles (retina
ganglion cell axons) that exit the eye through
the optic disc
It is the connection between the eye and the
brain
◦ It carries the impulses formed by the retina for
interpretation by they brain
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Is the second cranial nerve
Is part of the Central Nervous System
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Each optic nerve contains both nasal and
temporal fibers from the same eye
◦ Injury to one optic nerve (before the optic chiasm)
affects only fibers to the eye on that side
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Any condition causing degeneration of the
optic nerve
◦ Degeneration of ganglion cells
◦ Damage to optic nerve
◦ Injury to optic chiasm/tract
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Can be congenital or acquired
◦ Congenital: tumor, inflammation, infection, trauma
 Associated with prematurity, hydrocephalus, hypoxia
◦ Acquired: Trauma, tumor, thyroid eye disease, MS
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Is sometimes inherited (AD fashion)
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Degeneration of ganglion cells
Vision loss is progressive
Vision stabilizes between 20/40 and 20/200
Color vision difficulties
Inherited as Leber Hereditary Optic
Neuropathy
Vision decreases rapidly
Loss of color vision and contrast sensitivity
Affects males more than females
Vision loss occurs in one eye at a time
Visual symptoms occur between 15 and 35 years of
age
◦ In 50% of cases, vision can improve spontaneously
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The ophthalmologist will see:
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Visual field defects
Color vision abnormalities
Contrast sensitivity reduction
Optic disc pallor
Pupil reflex abnormalities
Abnormal blood vessels
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There is no treatment for optic atrophy
◦ Treatment of underlying cause is sometimes
possible
◦ Once atrophy has taken place, vision loss is
irreversible
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Damage to the optic nerve and its fibers
Various causes
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Heredity
Inflammation
Trauma
Brain tumor
Radiation
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Vision loss can be sudden or gradual
Can affect one or both eyes
As damage to the nerve progresses, there will
also be impaired color discrimination and
loss of contrast sensitivity
Can be progressive and can recur
Treatment
◦ No treatment to optic nerve once damage has
occurred
◦ Treatment of underlying causes can cause
stabilization of vision
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A congenital condition in which the optic
nerve is underdeveloped
Is diagnosed by the presence of a small, pale
optic nerve, which is confirmed on MRI
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Cause
◦ Most have no identifiable cause
◦ Associated with
 Maternal diabetes
 Maternal alcohol and drug abuse
 Young maternal age (less than 20 years of age)
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Visual impairment ranges from mild to severe
May affect one or both eyes
May see nystagmus or strabismus
In general, is non-progressive
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Treatment
◦ None
◦ Treat associated eye conditions
 Strabismus
 Amblyopia
 Nystagmus
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ONH, OA and ON are commonly associated
with other visual conditions as well as many
syndromes, i.e., DeMorsier Syndrome (SeptoOptic Dysplasia) or CVI.
Is stable throughout life.
Maybe associated with strabismus or
nystagmus.
Low Visual Acuity (No light perception to
perfect vision)
• May appear to change depending on lighting.
• May benefit from low vision aids such as
magnifiers or monoculars.
• May benefit from assistive technology such as
CCTV, Zoomtext or Magnifier mouse.
• May benefit from large print.
• May benefit from descriptions of pictures
• Many of these students need to learn Braille
and cane skills.
Photophobia:
• When completing a FVA, test the student in
different types and levels of lighting.
• May benefit from wearing sunglass or
brimmed hat.
• May benefit from sitting with back to
windows.
• Avoid objects that produce glare such as
magazines or shiny toys. (Different lighting
and/or positioning can reduce glare).
Poor Contrast sensitivity:
 May appear to change depending on lighting.
 May benefit from clear and high contrasting
materials i.e. black on white.
 May benefit from high contrasting pictures or
verbal descriptions.
 May benefit from high contrasting marking in
environment i.e. edges of stairs.
Visual Field Loss
• Scotomas (blind spots) in the central visual field
can cause difficulty reading and traveling. O&M,
cane travel and scanning techniques are all
important. A LMA may be needed if there are
changes.
• Scotomas or Blind spots in the visual field may
make reading or recognizing people and places
difficult.
• Students with blind spots may exhibit eccentric
viewing or appear to be looking to the side
instead of at you.
• “wavy vision” or “blurry spots”
Colour Vision:
 Colour vision can be affected.
 Label pictures that are colour dependent (i.e.
maps or diagrams) with symbols or tactiles.
 Use high contrasting colours.
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The optic nerves leave the back of each orbit
and further recede where they converge to
meet at the optic chiasm
Optic chiasm
◦ All nasal retinal fibers from both eyes cross over to
the opposite side
 Some inferior nasal fibers loop briefly up into the
opposite optic nerve before moving into the optic tract
(von Wilebrandt’s knee)
◦ Is located near the pituitary
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After leaving the chiasm, nasal fibers from
the OPPOSITE eye and temporal fibers from
the SAME eye join to form the optic tracts
◦ Right optic tract
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Temporal fibers from right eye
Nasal fibers from the left eye
Come from the right half of each eye’s retina
Contain information from the left half of the visual
field
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These paired halves of each visual field are
homonymous
◦ Meaning they contain information from the same
side of each eye
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Each optic tract continues traveling backward
around the outside of the midbrain portion of
the brain stem to end in the lateral geniculate
body (LGN)
◦ Nerve fibers from the ganglion cells synapse here
◦ This gives rise to the neurons that travel together
as the optic radiations
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Optic Radiations
◦ When the nerve fibers leave the LGN, they fan out
◦ Some wing forward and laterally around the lateral
ventricle into the temporal lobe
◦ Others pass superiorly through the parietal lobe
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Nerve fibers eventually terminate around the
calcarine fissure in the occipital lobe
The area responsible for vision at the end of
this area is Brodmann area 17 or visual cortex
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A large portion of the posterior tip of the
occipital cortex contains macular fibers
Each area in the in the retina is represented in
a corresponding area of the visual cortex
◦ Left visual cortex perceives objects in the right
visual field of each eye
◦ Superior fibers responsible for the inferior visual
field terminate along the upper lip of the calcarine
fissure
◦ Inferior fibers responsible for the superior visual
field terminate along the lower lip
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After visual information is sent to the visual
cortex, it must be sent on for further
processing
Some areas of the brain are specialized for
processing certain aspects of vision
◦ Dorsal stream
 The “where” pathway
 Detection of motion, spatial orientation, localization
 Parietal lobe
◦ Ventral stream
 The “what” pathway
 Recognition of objects (color and form)
 Temporal lobe
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The brain is much more complicated!
Messages are then sent on to other parts of
the brain that control:
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Eye movements
Pupil reactions
Reactions to visual stimuli
And so much more…
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Characteristic patterns of visual field defects
are associated with abnormalities in different
portions of the sensory visual pathway
Three anatomic sections:
◦ Prechiasmal – affecting visual field of involved eye
◦ Chiasmal – affecting the temporal half of each eye’s
field
◦ Postchiasmal – defects affect either both right
halves or both left halves of each eye’s field
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The more posterior the lesion, the more
congruous the visual field defect!!!
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Causes of visual field defects are numerous
and include:
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Glaucoma
Vascular disease (stroke)
Tumours
Retinal disease (RP)
Hereditary disease
Optic neuritis
Toxins
Drugs
Trauma
Post surgery
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Hemianopsia is a loss of vision affecting half
of the visual field
See Figure 6.1 in text!
Hemianopsia occurs more frequently in stoke
and traumatic brain injuries
Some improvement in visual fields may be
possible in hemianopsia
◦ Is dependent on the cause of the loss and extent of
the damage
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There is no specific treatment for VF loss
◦ Rehabilitation is helpful!
Visual Field Loss:
 Learning reading strategies that can help with
field loss, such as, where to focus or using
outlines to focus attention.
 Orientation and Mobility, white cane.
 Learning to scan the environment, head
turns.
 Preferential seating in class or during
presentations.
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A non-progressive condition resulting in
damage to the brain before, during or shortly
after birth
Can cause:
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Developmental delay
Seizures
Damage to motor system (movement/posture)
Visual impairment
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Visual Impairment
◦ Multiple injuries throughout the visual system due
to inadequate oxygen
◦ Can lead to:
 Optic nerve damage (optic atrophy)
 Retinal disease (ROP)
 CVI
◦ Can sometimes have poor control of eye
movements
Anatomy Quiz to be completed by October 18th!