Traumatic Brain Injury can have
devastating effects on the visual system
Vision is our dominant sense
More than just sight is measured in terms of visual acuity; vision is
the process of deriving meaning from what is seen. It is a complex,
learned and developed set of functions that involve a multitude of
skills. Research estimates that eighty to eighty five percent of our
perception, learning, cognition and activities are mediated through
vision.
The ultimate purpose of the visual process is to arrive at an
appropriate motor, and/or cognitive response.
There is an extremely high incidence (greater than 50%) of visual and
visual-cognitive disorders in neurologically impaired patients
(traumatic brain injury, cerebral vascular accidents, multiple sclerosis
etc.) Rosalind Gianutsos, Ph.D.
"Visual-perceptual dysfunction is one of the most common devastating
residual impairments of head injury". Barbara Zoltan, M.A., O.T.R.
"The majority of individuals that recover from a traumatic brain injury
will have binocular function difficulties in the form of strabismus,
phoria, oculomotor dysfunction, convergence and accommodative
abnormalities". William Padula, O.D.
The process of vision can be broken down into three general categories;
1) visual acuity and visual field, 2) visual motor abilities and 3)
visual perception.
VISUAL ACUITY and VISUAL FIELD
Visual Acuity - This refers to clarity of sight. It is commonly measured
using the Snellen chart and noted, for example, as 20/20, 20/50, 20/200
etc. Visual acuity becomes blurred in various refractive conditions, for
example, myopia (nearsighted), hyperopia (far-sighted), astigmatism
(mixed), and presbyopia (age related loss of focusing).
Visual Field - This is the complete central and peripheral range, or
panorama of vision. Various neurological conditions, such as stroke,
cause characteristic losses of the visual field, for example
hemianopsia. The person may, or may not, concurrently demonstrate a
visual neglect which is a perceptual loss of vision and visual motor
integration to the side of the visual field loss.
VISUAL MOTOR ABILITIES
Alignment - This refers to eye posture. If the eyes are straight
and aligned the eye posture is termed phoric. If an eye turns in, out,
up or down compared to the other eye then the eyes are not straight or
aligned and the condition is termed strabismus. Exotropia is a form of
strabismus where an eye turns out, esotropia is where an eye turns in,
hypertropia is where an eye turns up, and hypotropia is where an eye
turns down. These can also occur in combination, such as hyper-
exotropia, or hyper-esotropia.
-
Fixation - The ability to steadily
and accurately gaze at an object of regard. This is most
dysfunctional in nystagmus which is an uncontrollable shaking of the
eyes.
-
Pursuits - The ability to smoothly
and accurately track, or follow, a moving object
-
Saccades - The ability to quickly
and accurately look, or scan, from one object to another
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Accommodation - The ability to
accurately focus on an object of regard, sustain that focusing of
the eyes, and to change focusing when looking at different distances
-
Convergence - The ability to
accurately aim the eyes at an object of regard and to track an
object as it moves towards and away from the person
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Binocularity - The integration of
accommodation and convergence
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Stereopsis - Depth perception
VISUAL PERCEPTION
Visual-Motor Integration - Eye-hand, eye-foot, and eye-body
coordination
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Visual-Auditory Integration - The
ability to relate and associate what is seen and heard
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Visual Memory - The ability to
remember and recall information that is seen
-
Visual Closure - The ability "to
fill in the gaps", or complete a visual picture based on seeing only
some of the parts
-
Spatial Relationships - The
ability to know "where I am" in relation to objects and space around
me and to know where objects are in relation to one another
-
Figure-Ground Discrimination - The
ability to discern form and object from background
THE THREE MOST DEVASTATING AND
INTOLERABLE VISUAL PROBLEMS RESULTING FROM BRAIN INJURY AND STROKE
Although there are many visual problems that arise from brain injury and
stroke, three are more devastating and impairing than the rest. These
are visual field loss, intractable double vision, and visual / balance
disorders.
Visual Field Loss
With a visual field loss the patient is literally blind to half of their
field of vision. This places the person at increased risk of further
injury and harm from bumping into objects, being struck by approaching
objects, and falls.
A two fold approach is used to treat visual field loss. Visual
rehabilitation activities are prescribed by the doctor and administered
by the therapist to teach scanning of the hemianopic field loss. This is
a difficult task. It is the act of seeing something that brings our
visual attention and scanning to bear. However, these patients do not
see to the field they are being trained to scan and attend. Therapy is
aimed at teaching that and several approaches have been developed to
assist in this, but remediation still requires a lot of effort and
patience.
Special visual field awareness prism lenses are used in treating visual
field loss. As the patient scans into the prism the optics are shifted
so as to perceptually gain about 15 to 20 degrees of visual field
recognition. Since diplopia is perceived when scanning into the prism,
fixation in the prism must be brief. These are used as spotting devices
only to determine if there is an object in the periphery that deserves
further visual attention. When such an object is spotted, the patient
turns their head to view it in detail with their intact central vision.
Double Vision (Diplopia)
Double vision (diplopia) is a serious and intolerable condition that can
be caused by strabismus, ophthalmoplegia, gaze palsy, and decompensated
binocular skills in patients with brain injury, stroke and other
neurologically compromising conditions. Prisms, lenses and / or vision
therapy can oftentimes help the patient achieve fusion (alignment of the
eyes) and alleviate the diplopia. If and when these means are not
employed, the patient may adapt by suppressing the vision of one eye to
eliminate the diplopia. If lenses, prisms, and / or therapy are not
successful and the patient does not suppress, intractable diplopia
ensues.
In this population of patients, patching has frequently been used to
eliminate the diplopia. Although patching is effective in eliminating
diplopia it causes the patient to become monocular. Monocular as opposed
to binocular vision will affect the individual primarily in two ways;
absence of stereopsis and reduction of the peripheral field of vision.
These limitations will directly cause problems in eye hand coordination,
depth judgments, orientation, balance, mobility, and activities of daily
living such as playing sports, driving, climbing stairs, crossing the
street, threading a needle etc.
A new method of treating diplopia that does not have these limitations
has been successfully evaluated. It is called the "spot patch" (invented
and named by this author) and is a method to eliminate intractable
diplopia without compromising peripheral vision. It is a small, usually
round or oval, patch made of dermacil tape, 3-M blurring film (or
another such translucent tape). It is placed on the inside of the lenses
of glasses and directly in the line of sight contributing to the
diplopia. The diameter is generally about one centimeter, but will vary
on the individual angular subtense required for the particular
strabismus, or gaze palsy.
Visual Balance Disorders
Visual balance disorders can be caused by a Visual Midline Shift
Syndrome (VMSS), oculo-motor dysfunction in fixations, nystagmus, and
disruptions of central and peripheral visual processing. A full
description of these disorders is beyond the scope of this paper. The
treatment will depend on the visual diagnosis and etiology. Lenses,
prisms and visual rehabilitation activities are used in the remediation
of these disorders.
Overview of the Pathophysiology of
Vision problems following Brain Injury or Stroke
Vision is frequently disrupted following head injury, stroke and other
neurologically compromising conditions (e.g. multiple sclerosis,
cerebral vasculitis, aneurysm, hypoxia etc). The anatomy and physiology
of the visual system, the vascular network of the brain, and the
dynamics of head trauma all contribute to the incidence of ocular trauma
and visual dysfunction.
Injury to the visual system can be diffuse and / or focal and can
localize to any, or a combination of the ocular structures, cortical
areas, mid brain, or nerve nuclei. Brain injuries affecting vision
typically occur via axonal shearing, hemorrhage, infarct, inflammation,
and / or compression.
The third cranial nerve and third nerve nuclei seem to be particularly
vulnerable to injury following trauma and stroke. These innervate eight
of the twelve extra-ocular muscles (the medial, inferior and superior
recti, and inferior oblique muscles), the ciliary (focusing) muscle
inside of the eye, the levator (eyelid) muscle, and the pupillary
sphincter muscle. Consequently, injury to this area causes some classic
signs and symptoms that will manifest as:
-
Exotropia
-
Exophoria
-
Convergence Insufficiency
-
Convergence Infacility
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Accommodative Insufficiency
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Accommodative Infacility
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Ptosis
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Fixed and Dilated Pupil
Limited Motility (Abduction,
Sursumduction, Infraduction) of the affected eye
The
sixth cranial nerve and nuclei are also prone to injury in TBI and
stroke. These control two of the twelve extra-ocular muscles (the
lateral recti muscles) which are responsible for abduction of the eye.
Injury to the communication pathways between sixth nerve nuclei and
their paired third nerve nuclei can also occur. This is termed an
internuclear ophthalmoplegia (INO). Sixth nerve problems will manifest
as:
-
Esotropia
-
Esophoria
-
Divergence Insufficiency
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Divergence Infacility
-
Limited Abduction of the Affected
Eye
Internuclear Ophthalmoplegia will
manifest as:
-
Exotropia
-
Limited Adduction of the Affected
Eye
-
Paralysis of Gaze to the Affected
Side
The
fourth cranial nerve is less frequently injured in TBI and stroke. It is
more frequently injured by direct trauma. The fourth nerve and nucleus
control two of the twelve extra-ocular muscles, the superior obliques.
These muscles are responsible for inferior gaze of the eye when it is
adducted. Damage to this area will manifest as:
-
Hypertropia
-
Limited Down Gaze of the Affected
Eye when Adducted
The
optic nerve (second cranial nerve) and its radiations and pathway back
to the occipital and associated corticies can be injured in TBI and
stroke. The location of the injury will determine the nature of the
deficit. Visual field loss is the manifestation of these injuries.
If the optic nerve is damaged between the eye itself and the optic
chiasm, there will be a monocular loss of sight and a pupillary defect.
This is seen in direct trauma and multiple sclerosis.
When the damage is at the chiasm there will be a classic bi-temporal
visual field loss. This is found frequently in pituitary disorders.
If the injury occurs in the optic radiations coursing back from the
chiasm to the lateral geniculate nucleus (LGN), there will be an
incongruous (unequal) bilateral visual field loss. Although this can be
a hemianopsia (half visual field loss) or quadrantanopsia (quarter field
loss), it will more frequently show as a quadrantanopsia.
When the injury is posterior to the LGN and ranging back to the
occipital cortex there will be a homonymous bilateral visual field loss.
The visual field loss will show more frequently as a hemianopsia, but
can also be a quadrantanopsia.
Visual fibers feed forward to other areas of the brain. One in
particular is the parietal cortex which is frequently referred to as the
association cortex. It is called this because it integrates information
from the various senses in an attempt to derive meaning from the "whole
picture". When there is damage to this area, one will frequently find a
visual neglect.
Visual neglect is a perceptual loss of vision. The person is unaware of
their sight, or lack of sight to the affected side. If the nerve
pathways and occipital cortex are spared and only the parietal cortex is
affected there will a sparing of vision (to a good degree), but the
person will "ignore" and not attend to vision on the affected side. If
the pathways and / or occipital cortex are injured as well as the
parietal cortex, then there will be an actual visual field loss in
addition to the person having a neglect, and / or denial of loss of
vision.
Injury to the frontal cortex can affect vision. The frontal eye fields
are in the frontal cortex and so eye movement disorders can occur from
injury here. This can also cause a perceptual deficit in visual memory.
Parietal and occipital-parietal injuries can cause visual perceptual
deficits. These include visual-motor integration, figure-ground
discrimination, spatial relations, topographic orientation and form
perception and constancy.
Vision integrates with and affects balance. Approximately twenty percent
of the visual fibers are involved in this process. Following brain
injury and stroke problems with nystagmus, ocular motor control and
strabismus will disrupt balance and can cause dizziness and vertigo.
Visual Midline Shift Syndrome {(VMSS) Padula}, is a condition that
affects balance, posture, orientation and mobility can occur following
TBI and stroke In VMSS the persons' visual perception of the world will
appear compressed in one portion and expanded in another. Their
perception of the world will therefore appear slanted, or tipped, and
walls may appear bowed and distorted. Balance is disrupted when attempts
are made to orient to that perception of the visual world. For example,
imagine a patient who has suffered a left cerebral vascular accident (CVA)
with right hemiparesis and right hemianopsia. This patient will be
observed to weight bear left. A shift of the visual perception of their
midline to the non-affected side will often be found. A mismatch results
between the visual perception of self, spatial relations and the
physical and kinesthetic feedback received. Since vision is the dominant
sense attempts are to orient to the visual perception. As a result, VMSS
will cause, and / or, exacerbate problems with balance, orientation and
mobility. VMSS can also be found in patients showing extensor and flexor
postures. In these cases the VMSS will be shifted either up, or down.
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