| Hemispatial neglect | |
|---|---|
| Other names | Hemiagnosia, hemineglect, unilateral neglect, spatial neglect, contralateral neglect, unilateral visual inattention, hemi-inattention, neglect syndrome, one-side neglect, or contralateral hemispatialagnosia |
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| Hemispatial neglect is most frequently associated with a lesion of the right parietal lobe (in yellow, at top). | |
| Specialty | Psychiatry, Neurology |
Hemispatial neglect is a neuropsychological condition in which, after damage to one hemisphere of the brain is sustained, a deficit in attention to and awareness of one side of the field of vision is observed. It is defined by the inability of a person to process and perceive stimuli on one side of the body or environment, where that inability is not due to a lack of sensation. Hemispatial neglect is very commonly contralateral to the damaged hemisphere, but instances of ipsilesional neglect (on the same side as the lesion) have been reported.
Presentation
Hemispatial neglect results most commonly from strokes and brain unilateral injury to the right cerebral hemisphere,
with rates in the critical stage of up to 80% causing visual neglect of
the left-hand side of space. Neglect is often produced by massive
strokes in the middle cerebral artery region and is variegated, so that
most sufferers do not exhibit all of the syndrome's traits.
Right-sided spatial neglect is rare because there is redundant
processing of the right space by both the left and right cerebral
hemispheres, whereas in most left-dominant brains the left space is only
processed by the right cerebral hemisphere. Although it most
strikingly affects visual perception ('visual neglect'), neglect in other forms of perception can also be found, either alone or in combination with visual neglect.
For example, a stroke affecting the right parietal lobe of the brain can lead to neglect for the left side of the visual field,
causing a patient with neglect to behave as if the left side of sensory
space is nonexistent (although they can still turn left). In an extreme
case, a patient with neglect might fail to eat the food on the left
half of their plate, even though they complain of being hungry. If
someone with neglect is asked to draw a clock, their drawing might show
only the numbers 12 to 6, or all 12 numbers might be on one half of the
clock face with the other half distorted or blank. Neglect patients may
also ignore the contralesional side of their body; for instance, they
might only shave, or apply make-up to, the non-neglected side. These
patients may frequently collide with objects or structures such as door
frames on the side being neglected.
Neglect may also present as a delusional form, where the patient denies ownership
of a limb or an entire side of the body. Since this delusion often
occurs alone, without the accompaniment of other delusions, it is often
labeled as a monothematic delusion.
Neglect not only affects present sensation but memory and recall
perception as well. A patient suffering from neglect may also, when
asked to recall a memory of a certain object and then draw said object,
draw only half of the object. It is unclear, however, if this is due to a
perceptive deficit of the memory (to the patient having lost pieces of
spatial information of the memory) or whether the information within the
memory is whole and intact but simply being ignored, the same way
portions of a physical object in the patient's presence would be
ignored.
Some forms of neglect may also be very mild—for example, in a condition called extinction
where competition from the ipsilesional stimulus impedes perception of
the contralesional stimulus. These patients, when asked to fixate on the
examiner's nose, can detect fingers being wiggled on the affected side.
If the examiner were to wiggle his or her fingers on both the affected
and unaffected sides of the patient, the patient will report seeing
movement only on the ipsilesional side.
Sequelae
Though
frequently underappreciated, unilateral neglect can have dramatic
consequences. It has more negative effect on functional ability, as
measured by the Barthel ADL index, than age, sex, power, side of stroke, balance, proprioception, cognition, or premorbid ADL status. Its presence within the first 10 days of a stroke is a stronger predictor of poor functional recovery after one year than several other variables, including hemiparesis, hemianopsia, age, visual memory,
verbal memory, or visuoconstructional ability. Neglect is likely among
the reasons that patients with right hemisphere damage are twice as
likely to fall as those with left brain damage. Patients with neglect
rehabilitate longer and make less daily progress than other patients
with similar functional status. And patients with neglect are less
likely to live independently than even patients who have both severe aphasia and right hemiparesis.
Causes
Brain areas in the parietal and frontal lobes are associated with the deployment of attention (internally, or through eye movements, head turns or limb reaches) into contralateral space. Neglect is most closely related to damage to the temporo-parietal junction and posterior parietal cortex.
The lack of attention to the left side of space can manifest in the
visual, auditory, proprioceptive, and olfactory domains. Although
hemispatial neglect often manifests as a sensory deficit (and is
frequently co-morbid with sensory deficit), it is essentially a failure to pay sufficient attention to sensory input.
Although hemispatial neglect has been identified following left hemisphere damage (resulting in the neglect of the right side of space), it is most common after damage to the right hemisphere.
This disparity is thought to reflect the fact that the right hemisphere
of the brain is specialized for spatial perception and memory, whereas
the left hemisphere is specialized for language - there is redundant
processing of the right visual fields by both hemispheres. Hence the
right hemisphere is able to compensate for the loss of left hemisphere
function, but not vice versa.
Neglect is not to be confused with hemianopsia.
Hemianopsia arises from damage to the primary visual pathways cutting
off the input to the cerebral hemispheres from the retinas. Neglect is
damage to the processing areas. The cerebral hemispheres receive the
input, but there is an error in the processing that is not well
understood.
Theories of mechanism
Researchers have argued whether neglect is a disorder of spatial attention or spatial representation.
Spatial attention
Spatial attention is the process where objects in one location are chosen for processing over objects in another location. This would imply that neglect is more intentional. The patient has an affinity to direct attention to the unaffected side. Neglect is caused by a decrease in stimuli in the contralesional side because of a lack of ipsilesional stimulation of the visual cortex and an increased inhibition of the contralesional side.
In this theory neglect is seen as disorder of attention and orientation
caused by disruption of the visual cortex. Patients with this disorder
will direct attention and movements to the ipsilesional side and neglect
stimuli in the contralesional side despite having preserved visual
fields. The result of all of this is an increased sensitivity of visual
performance in the unaffected side. The patient shows an affinity to the ipsilesional side being unable to disengage attention from that side.
Spatial representation
Spatial representation is the way space is represented in the brain.
In this theory it is believed that the underlying cause of neglect is
the inability to form contralateral representations of space.
In this theory neglect patients demonstrate a failure to describe the
contralesional side of a familiar scene, from a given point, from memory.
To support this theory, evidence from Bisiach and Luzzatti's
study of Piazza del Duomo can be considered. For the study, patients
with hemispatial neglect, that were also familiar with the layout of the
Piazza del Duomo square, were observed. The patients were asked to
imagine themselves at various vantage points in the square, without
physically being in the square. They were then asked to describe
different landmarks around the square, such as stores. At each separate
vantage point, patients consistently only described landmarks on the
right side, ignoring the left side of the representation. However, the
results of their multiple descriptions at the different vantage points
showed that they knew information around the entire square, but could
only identify the right side of the represented field at any given
vantage point.When asked to switch vantage points so that the scene that
was on the contralesional side is now on the ipsilesional side the
patient was able to describe with details the scene they had earlier
neglected.
The same patterns can be found with comparing actual visual stimuli to imaging in the brain (Rossetti et al., 2010).
A neglect patient who was very familiar with the map of France was
asked to name French towns on a map of the country, both by a mental
image of the map and by a physical image of the map. The image was then
rotated 180 degrees, both mentally and physically. With the mental
image, the neglect stayed consistent with the image; that is, when the
map was in its original orientation, the patient named towns mostly on
the East side of France, and when they mentally rotated the map they
named towns mostly on the West side of France because the West coast was
now on the right side of the represented field. However, with the
physical copy of the map, the patient's focus was on the East side of
France with either orientation. This leads researchers to believe that
neglect for images in memory may be disassociated from the neglect of
stimuli in extrapersonal space. In this case patients have no loss of memory
making their neglect a disorder of spatial representation which is the
ability to reconstruct spatial frames in which the spatial relationship
of objects, that may be perceived, imagined or remembered, with respect
to the subject and each other are organized to be correctly acted on.
This theory can also be supported by neglect in dreams (Figliozzi et al., 2007).
The study was run on a neglect patient by tracking his eye movements
while he slept, during the REM cycle. Results showed that the majority
of the eye movements were aimed to his right side, indicating that the
images represented in his dreams were also affected by hemispatial
neglect.
Another example would be a left neglect patient failing to
describe left turns while describing a familiar route. This shows that
the failure to describe things in the contralesional side can also
affect verbal items. These findings show that space representation is
more topological than symbolic. Patients show a contralesional loss of space representation with a deviation of spatial reference to the ipsilesional side. In these cases we see a left-right dissimilarity of representation rather than a decline of representational competence.
Diagnosis
In
order to assess not only the type but also the severity of neglect,
doctors employ a variety of tests, most of which are carried out at the
patient's bedside. Perhaps one of the most-used and quickest is the line
bisection. In this test, a line a few inches long is drawn on a piece
of paper and the patient is then asked to dissect the line at the
midpoint. Patients exhibiting, for example, left-sided neglect will
exhibit a rightward deviation of the line's true midpoint.
Another widely used test is the line cancellation test. Here a patient
is presented with a piece of paper that has various lines scattered
across it and is asked to mark each of the lines. Patients who exhibit
left-sided neglect will completely ignore all lines on the left side of
the paper.
Visual neglect can also be assessed by having the patient draw a copy
of a picture with which they are presented. If the patient is asked to
draw a complex picture they may neglect the entire contralesional side
of the picture. If asked to draw an individual object, the patient will
not draw the contralesional side of that object.
A patient may also be asked to read a page out of a book. The patient
will be unable to orient their eyes to the left margin and will begin
reading the page from the center. Presenting a single word to a patient
will result in the patient either reading only the ipsilesional part of
the word or replacing the part they cannot see with a logical
substitute. For example, if they are presented with the word "peanut",
they may read "nut" or say "walnut".
Varieties
Neglect
is a heterogenous disorder that manifests itself radically differently
in different patients. No single mechanism can account for these
different manifestations. A vast array of impaired mechanisms are found in neglect. These mechanisms alone would not cause neglect.
The complexity of attention alone—just one of several mechanisms that
may interact—has generated multiple competing hypothetical explanations
of neglect. So it is not surprising that it has proven difficult to
assign particular presentations of neglect to specific neuroanatomical
loci. Despite such limitations, we may loosely describe unilateral
neglect with four overlapping variables: type, range, axis, and
orientation.
Type
Types of
hemispatial neglect are broadly divided into disorders of input and
disorders of output. The neglect of input, or "inattention", includes
ignoring contralesional sights, sounds, smells, or tactile stimuli.
Surprisingly, this inattention can even apply to imagined stimuli. In
what's termed "representational neglect", patients may ignore the left
side of memories, dreams, and hallucinations.
Output neglect includes motor and pre-motor deficits. A patient with motor neglect does not use a contralesional limb despite the neuromuscular ability to do so. One with pre-motor neglect, or directional hypokinesia,
can move unaffected limbs ably in ipsilateral space but have difficulty
directing them into contralesional space. Thus a patient with pre-motor
neglect may struggle to grasp an object on the left side even when
using the unaffected right arm.
Range
Hemispatial
neglect can have a wide range in terms of what the patient neglects.
The first range of neglect, commonly referred to as "egocentric"
neglect, is found in patients who neglect their own body or personal
space. These patients tend to neglect the opposite side of their lesion, based on the midline of the body, head, or retina.
For example, in a gap detection test, subjects with egocentric
hemispatial neglect on the right side often make errors on the far right
side of the page, as they are neglecting the space in their right visual field.
The next range of neglect is "allocentric" neglect, where
individuals neglect either their peri-personal or extrapersonal space.
Peri-personal space refers to the space within the patient's normal
reach, whereas extrapersonal space refers to the objects/environment
beyond the body's current contact or reaching ability.
Patients with allocentric neglect tend to neglect the contralesional
side of individual items, regardless of where they appear with respect
to the viewer.
For example, In the same gap detection test mentioned above, subjects
with allocentric hemispatial neglect on the right side will make errors
on all areas of the page, specifically neglecting the right side of each
individual item.
This differentiation is significant because the majority of
assessment measures test only for neglect within the reaching, or
peri-personal, range. But a patient who passes a standard
paper-and-pencil test of neglect may nonetheless ignore a left arm or
not notice distant objects on the left side of the room.
In cases of somatoparaphrenia,
which may be caused by personal neglect, patients deny ownership of
contralesional limbs. Sacks (1985) described a patient who fell out of
bed after pushing out what he perceived to be the severed leg of a
cadaver that the staff had hidden under his blanket. Patients may say
things like, "I don't know whose hand that is, but they'd better get my
ring off!" or, "This is a fake arm someone put on me. I sent my daughter
to find my real one."
Axis
Most tests
for neglect look for rightward or leftward errors. But patients may also
neglect stimuli on one side of a horizontal or radial axis. For
example, when asked to circle all the stars on a printed page, they may
locate targets on both the left and right sides of the page while
ignoring those across the top or bottom.
In a recent study, researchers asked patients with left neglect to project their midline with a neon bulb
and found that they tended to point it straight ahead but position it
rightward of their true midline. This shift may account for the success
of therapeutic prism
glasses, which shift left visual space toward the right. By shifting
visual input, they seem to correct the mind's sense of midline. The
result is not only the amelioration of visual neglect, but also of
tactile, motor, and even representational neglect.
Orientation
An
important question in studies of neglect has been: "left of what?" That
is to say, what frame of reference does a subject adopt when neglecting
the left half of his or her visual, auditory, or tactile field? The
answer has proven complex. It turns out that subjects may neglect
objects to the left of their own midline (egocentric neglect) or may
instead see all the objects in a room but neglect the left half of each
individual object (allocentric neglect).
These two broad categories may be further subdivided. Patients
with egocentric neglect may ignore the stimuli leftward of their trunks,
their heads, or their retinae.
Those with allocentric neglect may neglect the true left of a presented
object, or may first correct in their mind's eye a slanted or inverted
object and then neglect the side then interpreted as being on the left.
So, for example, if patients are presented with an upside-down
photograph of a face, they may mentally flip the object right side up
and then neglect the left side of the adjusted image. In another
example, if patients are presented with a barbell, patients will more
significantly neglect the left side of the barbell, as expected with
right temporal lobe lesion. If the barbell is rotated such that the left
side is now on the right side, patients will more significantly neglect
the left side of the object, even though it is now on the right side of
space.
This also occurs with slanted or mirror-image presentations. A patient
looking at a mirror image of a map of the World may neglect to see the
Western Hemisphere despite their inverted placement onto the right side
of the map.
Various neuropsychological research studies have considered the
role of frame of reference in hemispatial neglect, offering new evidence
to support both allocentric and egocentric neglect. To begin, one study
conducted by Dongyun Li, Hans-Otto Karnath, and Christopher Rorden
examined whether allocentric neglect varies with egocentric position.
This experimental design consisted of testing eleven right hemispheric
stroke patients. Five of these patients showed spatial neglect on their
contralesional side, while the remaining six patients showed no spatial
neglect.
During the study, the patients were presented with two arrays of seven
triangles. The first array ran from southwest to northeast (SW-NE) and
the second array ran from southeast to northwest (SE-NW). In a portion
of the experimental trials, the middle triangle in the array contained a
gap along one side. Participants were tested on their ability to
perceive the presence of this gap, and were instructed to press one
response button if the gap was present and a second response button if
the gap was absent.
To test the neglect frame of reference, the two different arrays
were carefully situated so that gap in the triangle fell on opposite
sides of the allocentric field. In the SW-NE array, the gap in the
triangle fell on the allocentric right of the object-centered axis along
which the triangle pointed. In the SE-NW configuration, the gap in the
triangle fell on the allocentric left of the object-centered axis.
Furthermore, varying the position of the arrays with respect to the
participant's trunk midline was used to test egocentric neglect. The
arrays were therefore presented at 0° (i.e. in line with the
participant's trunk midline), at −40° left, and at +40° right.
Ultimately, varying the position of the array within the testing visual
field allowed for the simultaneous measurement of egocentric neglect
and allocentric neglect. The results of this experimental design showed
that the spatial neglect patients performed more poorly for the
allocentric left side of the triangle, as well as for objects presented
on the egocentric left side of the body.
Furthermore, the poor accuracy for detecting features of the object on
the left side of the object's axis was more severe when the objects were
presented on the contralesional side of the body. Thus, these findings
illustrate that both allocentric and egocentric biases are present
simultaneously, and that egocentric information can influence the
severity of allocentric neglect.
A second study, conducted by Moscovitch and Behrmann,
investigated the reference frame of neglect with respect to the
somatosensory system. Eleven patients with parietal lobe lesions and
subsequent hemispatial neglect were analyzed during this experiment.
A double simultaneous stimulation procedure was utilized, during which
the patients were touched lightly and simultaneously on the left and
right side of the wrist of one hand. The patients were tested both with
their palms facing down and with their palms facing up.
This experimental condition allowed the scientists to determine whether
neglect in the somatosensory system occurs with respect to the sensory
receptor surface (egocentric) or with respect to a higher-order spatial
frame of reference (allocentric). The results of this experiment showed
the hemispatial neglect patients neglected somatosensory stimuli on the
contralesional side of space, regardless of hand orientation.
These findings suggest that, within the somatosensory system, stimuli
are neglected with respect to the allocentric, spatial frame of
reference, in addition to an egocentric, sensory frame of reference.
Ultimately, the discoveries made by these experiments indicate that
hemispatial neglect occurs with respect to multiple, simultaneously
derived frames of reference, which dictate the nature and extent of
neglect within the visual, auditory, and tactile fields.
Treatment
Treatment
consists of finding ways to bring the patient's attention toward the
left, usually done incrementally, by going just a few degrees past
midline, and progressing from there. Rehabilitation of neglect is often carried out by neuropsychologists, occupational therapist,
speech-language pathologists, neurologic music therapists, physical therapists, optometrists, and orthoptists.
Forms of treatment that have been tested with variable reports of success include prismatic adaptation,
where a prism lens is worn to pull the vision of the patient towards
the left, constrained movement therapy where the "good" limb is
constrained in a sling to encourage use of the contralesional limb.
Eye-patching has similarly been used, placing a patch over the "good"
eye. Pharmaceutical treatments have mostly focused on dopaminergic
therapies such as bromocriptine, levodopa, and amphetamines, though these tests have had mixed results, helping in some cases and accentuating hemispatial neglect in others. Caloric vestibular stimulation (CVS) has been shown to bring about a brief remission in some cases. however this technique has been known to elicit unpleasant side-effects such as nystagmus, vertigo and vomiting.
A study done by Schindler and colleagues examined the use of neck muscle
vibration on the contralesional posterior neck muscles to induce
diversion of gaze from the subjective straight ahead. Subjects received
15 consecutive treatment sessions and were evaluated on different
aspects of the neglect disorder including perception of midline, and
scanning deficits. The study found that there is evidence that neck
muscle stimulation may work, especially if combined with visual scanning
techniques. The improvement was evident 2 months after the completion
of treatment.
Other areas of emerging treatment options include the use of prisms, visual scanning training, mental imagery training, video feedback training, trunk rotation, galvanic vestibular stimulation (GVS), transcranial magnetic stimulation (TMS) and transcranial direct-current stimulation (tDCS). Of these emerging treatment options, the most studied intervention is prism adaptation
and there is evidence of relatively long-term functional gains from
comparatively short-term usage. However, all of these treatment
interventions (particularly the stimulation techniques) are relatively
new and randomised, controlled trial evidence is still limited. Further
research is mandatory in this field of research in order to provide
more support in evidence-based practice.
In a review article by Pierce & Buxbaum (2002), they
concluded that the evidence for Hemispheric Activation Approaches, which
focuses on moving the limb on the side of the neglect, has conflicting
evidence in the literature.
The authors note that a possible limitation in this approach is the
requirement for the patients to actively move the neglected limb, which
may not be possible for many patients. Constraint-Induced Therapy (CIT),
appears to be an effective, long-term treatment for improving neglect
in various studies. However, the use of CIT is limited to patients who
have active control of wrist and hand extension. Prism Glasses,
Hemispatial Glasses, and Eye-Patching have all appear to be effective in
improving performance on neglect tests. Caloric Stimulation treatment
appears to be effective in improving neglect; however, the effects are
generally short-term. The review also suggests that Optokinetic
Stimulation is effective in improving position sense, motor skills, body
orientation, and perceptual neglect on a short-term basis. As with
Caloric Stimulation treatment, long-term studies will be necessary to
show its effectiveness. A few Trunk Rotation Therapy studies suggest its
effectiveness in improving performance on neglect tests as well as the
Functional Independence Measure (FIM). Some less studied treatment
possibilities include treatments that target Dorsal Stream of visual
processing, Mental Imagery Training, and Neck Vibration Therapy.
Trunk rotation therapies aimed at improving postural disorders and
balance deficits in patients with unilateral neglect, have demonstrated
optimistic results in regaining voluntary trunk control when using
specific postural rehabilitative devices. One such device is the Bon
Saint Côme apparatus, which uses spatial exploratory tasks in
combination with auditory and visual feedback mechanisms to develop
trunk control. The Bon Saint Côme device has been shown to be effective
with hemiplegic subjects due to the combination of trunk stability
exercises, along with the cognitive requirements needed to perform the
postural tasks.
