[Hallucinations and dementia. Prevalence, clinical presentation and pathophysiology].

: Hallucinations are a common feature of certain degenerative diseases with a risk of dementia such as Alzheimer's disease, Lewy body dementia, and Parkinson's disease. Obtaining valid epidemiological data is nevertheless quite difficult because of methodological problems. As a rule, hallucinations are more prevalent in Lewy body disease than Parkinson's disease or Alzheimer's disease. The prevalence in parkinsonian dementia is about the same as in Lewy body disease. Complex visual hallucinations predominate, auditory or tactile hallucinations are more exceptional. Minor forms (illusions, sensation of presence) are also observed. Recurrence is common, mainly in the evening or at night. Patients with advanced mental impairment generally take the hallucinations for reality. The hallucinations can be associated with psychological and behavioral disorders such as delusionnal idea or identification disorders. It is important to search for other causes of hallucinations such as drugs, ocular disorders, or depression, but many of these disorders are common comorbidities in elderly patients with degenerative disease. There is no unique model fitting all the hypothesized pathogenic mechanisms. Complex visual hallucinations most likely arise from abnormal activation of the extra-striat temporal associative regions, but only hypothetical mechanisms have been proposed. Genetic studies and functional imaging have not provided convincing evidence. Current focus is placed on an imbalance between deficient cholinergic transmission and preserved or augmented monoaminergic transmission at the cortical level, but other neurotransmission systems could be involved. The dream dysregulation mechanism proposed in Parkinson's disease cannot be generalized. The link between cognitive disorders and hallucination is also poorly understood: hallucinations are associated with more severe cognitive impairments or more rapid cognitive deline in Parkinson's disease and Alzheimer's disease, but the association with specific cognitive disorders remains to be fully explored.