Dementia and visual function

Neurodegenerative diseases with dementia include Alzheimer's disease (AD), dementia with Lewy body (DLB), Parkinson's disease (PD), frontotemporal dementia (FTD), Machado-Joseph disease (MJD), progressive supranuclear palsy (PSP), and corticobasal degeneration (CBD). These dementias are often associated with visual symptoms. Characteristic occurrences of visual dysfunctions are recognized as visual attention impairment in AD, vivid hallucination in DLB and PD, narrowed visual fields secondary to supranuclear downward gaze palsy in PSP, and hemispatial neglect in CBD. Dementia due to non-degenerative diseases of the brain is also often associated with visual symptoms, for example in head injury, neurosyphilis, stroke, brain tumor, and temporal lobe epilepsy. Visual hallucination is also seen in excessive alcohol drinking, and in pellagra. Visual symptoms are often an initial sign of progressive dementia in Creutzfeldt-Jakob disease. Visual dysfunctions are classified into three categories; impaired sensory visual function, impaired perceptive visual function, and impaired cognitive/emotional visual function. Non-invasive measurements of electroencephalograms, and visual evoked potentials (VEPs) are useful for evaluating attentional or cognitive functions in patients with dementia. VEPs are classified into three categories; sensory VEPs, perceptive VEPs, and cognitive/emotional VEPs. Sensory VEPs are related to non-attentional visual function. Sensory VEPs are a/b waves of eletroretinograms (30-60ms) originated from photoreceptor cells and retinal ganglion cells, and Cl wave (50-90ms) originated from V1 cortex. Perceptive VEPs are P1/N1 waves (80-190ms) originated from extrastriate and V1 cortex, and N2 wave (240-320ms) originated from V4 and V8 cortex. P1/N1 waves are related to visuospatial attention and depth perception, while N2 wave is related to non-spatial attention and feature-specific channels. Cognitive/emotional VEPs are related to attentional resources, and are miss match negativity and P3 wave of event-related potentials (300-450ms) originated from bitemporal cortex. Neuroimaging tests are useful for evaluating cerebral lesions which are responsible for visual dysfunctions in patients with dementia, and consist of structural (MRI) and functional (SPECT) imaging studies. MRI shows brain atrophy at the hippocampus in AD, and at the frontotemporal lobes in FTD, while brain atrophy is usually not apparent in DLB and PD. SPECT shows reduced cerebral blood flow (CBF) initially at the posterior cingulated gyrus, and at the precuneus, and later at the temporoparietal association cortex. CBF is often reduced at the visual cortex or the visual association cortex in DLB, and at the frontotemporal lobes in FTD. In conclusion, it is important for clinicians to evaluate visual dysfunctions resulting from impaired visual information processing of the brain in patients with dementia.