Dopaminergic Treatment and Episodic Memory in Parkinson’s Disease: A Meta-analysis of the Literature

To date, few studies have focused on the benefits of dopaminergic treatment on episodic memory functions in patients affected by Parkinson’s disease (PD). We conducted a meta-analysis to determine the effects of pharmacological therapy with dopamine in alleviating episodic memory deficits in Parkinson’s patients. A secondary aim was to evaluate the role of dopamine in episodic memory circuits and thus in different memory systems. We conducted a comprehensive literature search in PubMed (1971–2022) to find studies that met specific inclusion criteria. The studies had to provide sufficient data (means and standard deviations) to evaluate performance on neuropsychological tests of episodic memory. A total of k = 36 measures were included in the analysis. A statistically significant difference suggested better performance following dopaminergic therapy assumption (ON condition) than following dopaminergic withdrawal (OFF condition), specifically the estimated pooled effect calculated through a random-effects restricted maximum likelihood model was log ratio of means (RoM) = 0.047 (p = 0.011). The back-transformed RoM, indicating a 4.8% improvement, provides an interpretable measure of the effect size, as it reflects the multiplicative change in performance associated with the ON condition. A meta-regression analysis was also performed to assess the influence of specific memory tasks and relevant covariates/factors on the overall meta-analytic effect: four memory contrasts (verbal/visual, immediate/delayed, recall/recognition, word-list/short-story), age of participants, years of education, severity of illness, duration of illness in years, country of study, proportion of women in the sample, type of medication, counterbalancing. Word list/short story and proportion of women in the sample were the only two statistically significant predictors in the model, both associated with a positive higher pooled effect size. The present study revealed a significant overall difference between the results obtained in the ON and OFF conditions. We also found a significantly greater pharmacological effect in the recall of short stories than word lists, which supports the hypothesis of a beneficial effect of dopamine on the hippocampal circuit rather than on prefrontal cortical areas.