Eight randomised controlled trials (RCTs; n=121) and one quasi-RCT (n=20) were included, all of double-blind crossover design.
The authors evaluated the studies as being of relatively good quality. However, all were small, only four had adequate allocation concealment and none described blinding of the outcome assessment. They were clinically heterogeneous with a variety of outcomes. It is unclear whether the washout periods between treatments were adequate.
Subjective measures of sleep quality and quality of life: 5 RCTs (n=61) reported statistically significant improvements in subjective measures of sleep quality compared with placebo (p≤0.05 where reported). The results for these outcomes in the other 3 RCTs and the quasi-RCT (n=80) were inconsistent.
PSG: a meta-analysis of data on eight PSG measures of sleep quality (5 RCTs, n=77) reported a statistically significant benefit associated with levodopa, compared with placebo, for periodic movements in sleep (PLMS) index (WMD –26.84, 95% CI: -36.41, -17.27, p<0.00001; 4 RCTs, n=64) and sleep latency (WMD –4.91, 95% CI: -9.69, -0.12, p=0.04; 4 RCTs, n=54). There was statistically significant heterogeneity for the PLMS outcome when using a fixed-effect model (p=0.006; I2=76%). No statistically significant difference between the groups was reported for another six PMS measures .
Adverse events: a meta-analysis of data on adverse events found a statistically significant benefit for the placebo group for the outcomes of augmentation (RR 9.09, 95% CI: 1.05, 30.51, p=0.04; 3 RCTs, n=48) and gastrointestinal symptoms (RR 2.78, 95% CI: 1.01, 7.67, p=0.05; 3 RCTs, n=58). No statistically significant difference between the groups was reported for the outcomes of headache, dry mouth and dizziness.
Other results were reported in the review.