Footnotes

• Contributors All authors have participated sufficiently in the work to fulfill the criteria for authorship. All authors certify that they have participated sufficiently in the work to take responsibility for the content including participation in the concept, design, analysis and writing and revision of the manuscript. Study concept and design: MG and MdC. Acquisition and analysis of data, critical revision of the manuscript and reporting of the work described in the article: all authors. Statistical analysis and drafting of the manuscript: GM-M, VAC and MdC. Obtained funding: MdC.

• Funding This study is included in the ONWebDUALS project (JPND- PS/0001/2013), an EU Joint Programme—Neurodegenerative Disease Research (JPND)—Fundação para a Ciência e Tecnologia initiative. VAC was independently funded by a PhD fellowship (PD/BD/105852/2014) from Fundação para a Ciência e Tecnologia, Portugal.

• Patient consent Obtained.

• Ethics approval Comissão de Ética do Centro Académico de Medicina de Lisboa.

• Provenance and peer review Not commissioned; externally peer reviewed.

• Data sharing statement Additional data are provided as supplementary documents.

• Correction notice Since this letter was first published online Peter M Andersen has been added to the author list.