To analyze the levels of neuropeptides in CSF of patients with Parkinson’s disease (PD) compared to healthy individuals.

We systematically searched MEDLINE (PubMed), EMBASE, ScienceDirect, and Scopus using specific search strategies comprising terms related to PD, SLI, and SPLI from the date of database inception to March 2024. Following PRISMA guidelines, our screening and extraction included studies investigating neuropeptide concentrations in Parkinson's patients' cerebrospinal fluid (CSF). Utilizing the 'meta' R package, particularly the 'metacont' tool, we analyzed mean concentration levels and standard differences via the random effects model. Data pooling utilized inverse variance weighting, with I² and tau² assessing heterogeneity. The primary outcome was the mean difference in CSF neuropeptide levels between Parkinson's patients and controls, considering reported units separately.

Substance P-like-immunoreactivity showed a significant mean difference of -0.77 fmol/mL  (95% CI: -1.48 to -0.06, I² = 0%) in 4 studies (106 PD, 60 controls). Somatostatin like-immunoreactivity showed a mean difference of -5.86 fmol/mL (95% CI: -11.95 to 0.22, I² = 75%) in 5 studies (87 PD, 76 controls), a difference of 8.30 pg/mL  (95% CI: -44.23 to 60.84, I² = 92%) in 2 studies (26 PD, 37 controls), and a difference of -9.19 pmol/L (95% CI: -16.77 to -1.61, I² = 52%) in 2 studies (44 PD, 49 controls).