Footnotes

• Contributors KK, DKS, RM, MR, PJW and KF were involved in the study conception, data analysis and drafting of the manuscript: IN, HJK and DC were involved in the study conception, data analysis and reviewing of the manuscript; SN, ST, J-WH, LMdO, TS-H, MS, SS, KK. TA, HK TM, NP, TB and AS were involved in the data acquisition and reviewing of the manuscript : KN and MA were involved in the study conception and reviewing of the manuscript.

• Funding This study was partially supported by a grant-in-aid for scientific research from the Japan Society for the Promotion of Science (KAKENHI 15K19472), the Health and Labour Sciences Research Grant on Intractable Diseases (Neuroimmunological Diseases) from the Ministry of Health, Labour and Welfare of Japan, CAPES/Brasil (CSF-PAJT—88887.091277/2014-00), the Bio and Medical Technology Development Program (M3A9B6069339) through the Ministry of Science, ICT and Future Planning, Republic of Korea, and Marató de TV3 (AS; 20141830).

• Competing interests DKS has received a scholarship from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan, a grant-in-aid for scientific research from the Japan Society for the Promotion of Science (KAKENHI 15K19472), research support from CAPES/Brasil (CSF-PAJT—88887.091277/2014-00) and speaker honoraria from Novartis; IN reports grants from Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan, and from Ministry of Health, Labour and Welfare of Japan, personal fees from Mitsubishi Tanabe Pharma Corporation, Biogen Idec Japan, Novartis Pharmaceuticals Japan, Bayer Yakuhin, Ltd, and grants from LSI Medience Corporation; SN JSPS KAKENHI Grant (Grant-in-Aid for Young Scientists(B)) Number 26860656; RM serves on the scientific advisory board for MedImmune and has received funding for travel and honoraria from Biogen Idec, Merck Serono, Novartis, Sanofi-Genzyme and Teva; PJW reports personal fees from Biogen Idec, personal fees from Euroimmun AG, and others from Guthy-Jackson Charitable Foundation, outside the submitted work; In addition, PJW and the University of Oxford hold a patent for antibody testing against CASPR2/LGI1 with royalties paid by Euroimmun AG, and a patent on testing for antibodies to the GABAAR; HK reports grants from the Ministry of Education, Culture, Sports, Science and Technology of Japan; TM has received speaker honoraria from Bayer Schering Pharma, Biogen Idec Japan, Mitsubishi Tanabe Pharma Corporation, Asahi Kasei Medical Co, and Astellas Pharma Inc, and has received research support from Bayer Schering Pharma, Biogen Idec Japan, Asahi Kasei Kuraray Medical Co, The Chemo-Sero-Therapeutic Research Institute, Teva Pharmaceutical KK, Mitsubishi Tanabe Pharma Corporation, Teijin Pharma and Grants-in-Aid for Scientific Research from the Ministry of Education, Science and Technology, and the Ministry of Health, Labor and Welfare of Japan; NP reports grants from Siriraj Research and Development Fund, and personal fees from Novartis and UCB; AS has received compensation for consulting services and for speaking from Bayer-Schering, Merck-Serono, Biogen-Idec, Sanofi-Aventis, Teva Pharmaceutical Industries Ltd and Novartis; HJK has received research grants from the Ministry of Science, ICT and Future Planning, Republic of Korea, Genzyme, Kael-GemVax and Merck Serono; and has given talks, consulted with and received honoraria from Bayer Schering Pharma, Biogen Idec, Genzyme Corp, Merck Serono, Novartis, Teva-Handok and UCB. He serves on a steering committee for MedImmune and as an editor for Multiple Sclerosis Journal-Experimental, Translational and Clinical; KN reports personal fees from Mitsubishi Tanabe Pharma Corporation, personal fees from Astellas Pharma Inc, personal fees from Chemo-Sero-Therapeutic Research Institute and personal fees from Teijin Pharma; KF serves on scientific advisory boards for Bayer Schering Pharma, Biogen Idec, Mitsubishi Tanabe Pharma Corporation, Novartis Pharma, Chugai Pharmaceutical, Ono Pharmaceutical, Nihon Pharmaceutical, Merck Serono, Alexion Pharmaceuticals, Medimmune and Medical Review; has received funding for travel and speaker honoraria from Bayer Schering Pharma, Biogen Idec, Eisai Inc, Mitsubishi Tanabe Pharma Corporation, Novartis Pharma, Astellas Pharma Inc, Takeda Pharmaceutical Company Limited, Asahi Kasei Medical Co, Daiichi Sankyo and Nihon Pharmaceutical; serves as an editorial board member of Clinical and Experimental Neuroimmunology (2009-present) and is an advisory board member of Sri Lanka Journal of Neurology; has received research support from Bayer Schering Pharma, Biogen Idec Japan, Asahi Kasei Medical, The Chemo-Sero-Therapeutic Research Institute, Teva Pharmaceutical, Mitsubishi Tanabe Pharma, Teijin Pharma, Chugai Pharmaceutical, Ono Pharmaceutical, Nihon Pharmaceutical and Genzyme Japan; and is funded as the secondary investigator (#22229008, 2010–2015) by the Grants-in-Aid for Scientific Research from the Ministry of Education, Science and Technology of Japan and as the secondary investigator by the Grants-in-Aid for Scientific Research from the Ministry of Health, Welfare and Labor of Japan (2010-present).

• Ethics approval Ethics committee Tohoku University.

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