A prospective open label 2-8 year extension of the randomised controlled ICON trial on the long-term efficacy and safety of occipital nerve stimulation in medically intractable chronic cluster headache.

"Declaration of interests WM reports honoraria from Novartis, Teva, AbbVie, Lundbeck and Lilly, and consultancy and lecture fees from Lilly; RF reports consultancy and lecture fees from Novartis, Lundbeck, AbbVie, Lilly and TEVA, and independent support from the Dutch Brain Foundation, Leiden University Fund and Innovation Fund Dutch Healthcare Providers; FH reports consultancy and lecture fees form ABBOTT, Saluda, Grunenthal and Pfizer; RB, EZ, LW, JH, IC and MF report no relevant conflict of interest. No funding was received for the ICON study group."

"This study was funded by the 10.13039/501100003246Netherlands Organisation for Scientific Research, the Dutch Ministry of Health, the NutsOhra Foundation from the Dutch Health Insurance Companies, and Medtronic. The funders had no role in the study design, collection, analysis and interpretation of data; in the writing of the report; and in the decision to submit the paper for publication."