Optical coherence tomography (OCT) is a non-invasive retinal imaging technology that can provide high-resolution cross-sectional images of the peripapillary retinal nerve fiber layer (RNFL) and measure its thickness. A reduction of the RNFL thickness has been detected in several neurodegenerative diseases, such as multiple sclerosis, CADASIL and Alzheimer's disease. Different studies have reported RNFL changes also in Parkinson's disease (PD),a common neurodegenerative disease characterized by motor dysfunctions, originally described by James Parkinson in 1817. PD is characterized by selective dopaminergic neuronal cells loss, which may correlate with RNFL thinning. Previous studies on this subject, however, reported contradicting results. Some investigations reported reductions of the RNFL thickness while others did not. In the present study, in order to determine whether RNFL thickness is reduced in PD patients, we performed a meta-analysis and systematically evaluated RNFL thickness measurements with OCT in a series of PD patients and in the healthy control groups.

Inclusion Criteria:

1. case-control studies;
2. patients with PD were compared with healthy controls;
3. all subjects underwent RNFL thickness measurement by OCT;
4. studies should provide the data of peripapillary RNFL thickness;
5. sample size ≥10 in each group.

Exclusion Criteria:

1. authors did not make RNFL measurements;
2. study without healthy control group;
3. the outcome values can not be used for meta-analysis;
4. duplicated articles.

PD patients and healthy subjects were included and underwent OCT examinations