Do frontal dysfunctions play a role in visual hallucinations in Alzheimer's disease as in Parkinson's disease? A comparative study

Recent studies have demonstrated that nondemented patients with Parkinson's disease with visual hallucinations had lower scores on frontal-executive tasks than parkinsonian patients without hallucinations, most likely due to defective cholinergic circuitry. The aim of the present study is to investigate whether development of visual hallucinations in patients with Alzheimer's disease may also be related to more severe frontal dysfunctions. In the present study, 36 patients were included who were affected by probable Alzheimer's disease (18 with visual hallucinations and 18 without) and 38 patients affected by idiopathic Parkinson's disease (19 with visual hallucinations and 19 without). Patients completed a neuropsychological test battery and a short questionnaire to collect information about hallucination types and features. Multivariate analysis showed that patients with Alzheimer's disease scored significantly lower than patients with Parkinson's disease and that patients with hallucinations scored significantly lower than patients without hallucinations. Within both the Alzheimer's disease group and the Parkinson's disease group, patients with visual hallucinations scored significantly lower than patients without visual hallucinations, particularly on tests evaluating frontal-executive functions. These results demonstrate that patients with visual hallucinations show a significant impairment on tests tapping frontal-executive functions in Alzheimer's disease, as previously demonstrated (and verified here) in Parkinson's disease. On this basis it seems likely that analogous cognitive mechanisms underlie development of visual hallucinations in both degenerative diseases. Moreover, we may speculate that a defective circuitry of the prefrontal cortex is crucial for the genesis of hallucinations.