Hemispatial neglect is a failure to attend to, represent, or act upon stimuli located in the contralesional side of space, despite intact primary sensory and motor function. First described by Heidenhain in 1919[@heidenhain1919], neglect is now recognized as a common and disabling feature of corticobasal syndrome (CBS). The characteristic asymmetric cortical involvement in CBS, particularly affecting the right hemisphere's parietal lobe, creates a predilection for left-sided neglect[@karnath2001].
Unlike the primarily subcortical pathology in conditions like progressive supranuclear palsy (PSP), CBS involves the cortical mantle directly, particularly the inferior and superior parietal lobules, leading to more frequent and severe neglect syndromes.
Epidemiology and Prevalence
Prevalence in CBS: 25-40% of clinically diagnosed CBS cases demonstrate clinically significant neglect
Lateralization: Almost exclusively left neglect (right hemisphere lesions) in right-handed patients
Onset timing: Typically emerges 1-3 years after motor symptom onset
Severity: Can range from mild inattention to severe functional disability
Pathological correlation: Strongest with tau-predominant CBD pathology affecting right parietal cortex[@hillis2005]
Neuroanatomical Basis
Brain Regions Involved
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Overview
Hemispatial neglect is a failure to attend to, represent, or act upon stimuli located in the contralesional side of space, despite intact primary sensory and motor function. First described by Heidenhain in 1919[@heidenhain1919], neglect is now recognized as a common and disabling feature of corticobasal syndrome (CBS). The characteristic asymmetric cortical involvement in CBS, particularly affecting the right hemisphere's parietal lobe, creates a predilection for left-sided neglect[@karnath2001].
Unlike the primarily subcortical pathology in conditions like progressive supranuclear palsy (PSP), CBS involves the cortical mantle directly, particularly the inferior and superior parietal lobules, leading to more frequent and severe neglect syndromes.
Epidemiology and Prevalence
Prevalence in CBS: 25-40% of clinically diagnosed CBS cases demonstrate clinically significant neglect
Lateralization: Almost exclusively left neglect (right hemisphere lesions) in right-handed patients
Onset timing: Typically emerges 1-3 years after motor symptom onset
Severity: Can range from mild inattention to severe functional disability
Pathological correlation: Strongest with tau-predominant CBD pathology affecting right parietal cortex[@hillis2005]
Neuroanatomical Basis
Brain Regions Involved
| Region | Function | CBS Involvement | |--------|----------|-----------------| | Inferior Parietal Lobule (IPL) | Spatial attention, reorienting | Primary target | | Superior Parietal Lobule (SPL) | Spatial representation | Variable involvement | | Posterior Parietal Cortex (PPC) | Visuospatial processing | Almost always affected | | Superior Temporal Gyrus | Multimodal integration | Often co-affected | | Frontal Eye Fields (FEF) | Oculomotor control | Involvement common | | Basal Ganglia (Putamen) | Attention shifts | Subcortical component |
Pathophysiological Mechanisms
The dominant theory of neglect involves a disruption of the "attentional gradient" or "spatial priority map" in the right hemisphere. The right hemisphere is thought to maintain vigilance over both hemispaces (bilateral attention), while the left hemisphere attends primarily to the right hemispace. In CBS:
Right parietal damage disrupts the bilateral attention system
Left hemispace becomes underrepresented or ignored
Rightward bias in orienting emerges as the intact left hemisphere dominates
Clinical Features
Core Components
| Feature | Description | Prevalence in CBS | |---------|-------------|-------------------| | Left inattention | Failure to respond to stimuli on left side | 25-40% | | Hemisensory loss | Reduced awareness of left-sided sensation | 30-50% | | Oculomotor neglect | Difficulty initiating leftward saccades | 35-50% | | Motor neglect | Underutilization of left limb despite strength | 20-35% |
Associated Deficits
Extinction: Failure to perceive left stimuli when bilateral stimulation
Visuospatial dysfunction: 60-70% co-occurrence
Constructional dyspraxia: 40-50% difficulty with copying tasks
Dressing apraxia: 30-40% difficulty with dressing, especially left side
Assessment Approaches
Bedside Testing
Line bisection: Patient marks center of 20cm horizontal line
Clock drawing: Patient draws clock face and positions 12, 3, 6, 9
Cancellation tasks: Patient crosses out targets scattered on page