Hemispatial neglect

Hemispatial neglect, also called unilateral neglect, spatial neglect or neglect syndrome is a neurological condition in which, after damage to one hemisphere of the brain, a deficit in attention to the opposite side of space is observed.

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Presentation

It most commonly results from brain injury to the right cerebral hemisphere, causing visual neglect of the left-hand side of space. Although most strikingly affecting 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 and 1 to 6, the other side being distorted or left blank. Neglect patients may also ignore the contralesional side of their body, shaving or adding make-up only to the non-neglected side.

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 other signs of mental illness, it is often labeled as a monothematic delusion.

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. This is particularly evident in a related, but separate, condition known as extinction. A patient with extinction following right-hemisphere damage will successfully report the presence of an object in left space when it is the only object present. However, if the same object is presented simultaneously with an object in right space, the patient will only report the object on the right.

Neglect patients can also have problems with "interior" left information, since cases have been reported in which the patient when asked to describe something he knew before his brain damage describes only its right part.

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. Hence the right hemisphere is able to compensate for the loss of left hemisphere function, but not vice versa.

Varieties of hemispatial neglect

Unilateral neglect is a heterogeneous syndrome with several subtypes, and it’s possible that many distinct disorders have been inaccurately lumped together under a single label. There is growing consensus that no single mechanism accounts for the full range of demonstrated symptoms. And it appears that impairments of several different mechanisms converge to result in neglect—each of which could exist singly without causing it. The complexity of attention alone—just one of several mechanisms that may interact—has generated multiple competing hypothetical explanations of neglect. So it’s not surprising that it’s proven difficult to assign particular presentations of neglect to specific neuroanatomical loci. But despite such limitations, we may loosely describe unilateral neglect with four overlapping variables: type, range, axis, and orientation.

Type

We can broadly divide types of neglect 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 has difficulty directing them into contralesional space. Thus a patient with pre-motor neglect may struggle with grasping an object on the left side even when using the unaffected right arm.

Range

Patients may neglect their own body, the reaching space around them, or the far space beyond their reach. This differentiation is significant because the majority of assessment measures test only for neglect withinin 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 extreme cases of personal neglect, patients may 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 ride 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? 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, amazingly, 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. This also occurs with slanted or mirror-image presentations. A patient looking at a mirror image of a map of the United States may neglect to see California and Oregon despite their inverted placement onto the right side of the map.

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 CVA is a stronger predictor of poor functional recovery after one year than several other variables, including hemiparesis, hemianopia, 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.

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. Throwing a soft ball from the left, playing a game that involves attending to the left, replying to speakers on the left, or manipulating objects on the left may help. Rehabilitation of neglect is often carried out by neuropsychologists and occupational therapists.

Other 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.

See also

• Blindsight
• Brain damage
• Anosognosia
• Delusion
• Monothematic delusion

References

• Hans-Otto Karnath, A. David Milner, and Giuseppe Vallar (2002). The cognitive and neural bases of spatial neglect. Oxford [Oxfordshire]: Oxford University Press. ISBN 0-19-850833-6. 

• Robertson, I.H., & Halligan, P.W. (1999). Spatial neglect: A clinical handbook for diagnosis and treatment. Hove, East Sussex:Erlbaum.

• Heilman, K.M and Valenstein, E. (2003) Clinical Neuropsychology: Fourth Edition