Visual hallucinations
VH are among the most characteristic neuropsychiatric features of PD, and may even aid in the differentiation of PD from other parkinsonian disorders [49]. VH are associated with both a higher rate of cognitive decline [50] and a higher risk for development of dementia [25, 31]. The association of VH with dementia is probably related to VH being associated with both Lewy body pathology in the temporal lobe, particularly in the amygdala [51], and cholinergic deficits [52]. Patients who develop VH soon after the initiation of dopaminergic treatment are also at higher risk of developing dementia [53].
Rapid eye movement sleep behaviour disorder
RBD is another common and characteristic symptom of PD. Rates of MCI are about six to seven times higher in PD patients with RBD compared with PD patients without RBD [54], and RBD in PD is a marker for earlier onset of PD-D compared with PD patients without RBD [32, 55]. RBD is associated with a lower Braak neurofibrillary tangle stage and much lower neuritic plaque scores [56, 57]. Of note, it may be more difficult to diagnose RBD in PD patients than in those without PD [56, 58, 59].
Olfactory dysfunction
The risk of dementia is higher in PD patients with severe hyposmia than in those who do not exhibit olfactory dysfunction. Furthermore, severe hyposmia is correlated with profound cerebral atrophy which can be observed before the onset of dementia. It has been shown that hyposmia and visuoperceptual impairment should be considered as independent risk factors for future PD-D [31].
Cerebrovascular risk factors
Cerebrovascular disease is common in the elderly and is associated with cognitive decline; it may thus contribute to PD-D as well. There is, however, inconsistent evidence regarding the role of cerebrovascular disease in PD-D. Some studies have shown that cerebrovascular risk factors are not associated with PD-D [33-36], whereas others suggest that chronic cerebral hypoperfusion with large and small vessel disease negatively affects cognition in PD [34, 60, 61].
