The first phase of the study will consist of implanting electrodes and applying continuous stimulation until the patients stabilise clinically. This period is anticipated between 6 and 9 months. The next phase will consist of the crossover study. Responders will be randomised to either of 2 groups for a period of 3 months: a stimulation-on group and a stimulation-off group. Patients will then be crossed over to the other group for a further 3 months. Fortnightly visits will be made during the entire study to evaluate treatment efficacy and patient tolerance