Erratum to: COPPADIS-2015 (COhort of Patients with PArkinson's DIsease in Spain, 2015), a global -clinical evaluations, serum biomarkers, genetic studies and neuroimaging- prospective, multicenter, non-interventional, long-term study on Parkinson's disease progression [BMC Neurol. 16, (2015), 26] DOI: 10.1186/s12883-016-0548-9