Intérêt du DAT-scan pour le diagnostic étiologique des hallucinations chez la personne âgée : une étude comparative

Hallucinations are a complex tableau in the elderly, and sometime causes are not clear. Severe depression, drug side effects, behavioural and psychological symptoms of dementia, Lewy body disease and psychosis can be responsible, and sometimes overlap. A DAT-scan was used in this study to obtain a precise medical diagnosis in 78 cases of patients aged over 65 with visual hallucinations. The protocol was approved by the regional ethical committee. Seventy-eight elderly patients with visual hallucinations were recruited after their formal acceptation to participate. A medical assessment, including research for depression, drug side effects, parkinsonism and antecedent of falls, was performed. The DAT-scan was conducted in the department of nuclear medicine with a semi-quantitative measurement of the dopamine synthesis in the striatum. Nineteen patients with Parkinson's disease but without cognitive disorders or hallucinations had a DAT-scan in order to assess a tremor, and their results were compared with those of patients with Lewy body disease. Thirty-three patients with Lewy bodies had a reduced dopamine synthesis in the striatum. The DAT-scan does not discriminate between Lewy body disease and Parkinson's disease. The DAT-scan for all of them showed a lower activity than for depressed patients, with or without cognitive disorders, or with psychosis, even if a parkinsonism due to a side effect of antipsychotic drugs was present. In conclusion, in certain difficult cases of patients having hallucinations, the DAT-scan can be helpful in making the diagnosis between Lewy body disease and other psychiatric disorders. The threshold of three for the ratio of striatum/cerebellum is the one that allows the best discrimination between presence and absence of dopaminergic denervation.