Participants with moderate or severe CRPS of between 1 and 5 years duration will be randomly allocated to receive IVIg 0.5 g/kg ( IntratectTM 50 g/l solution for infusion ) or matching placebo administered day 1 and day 22 after randomisation , followed by two optional doses of open-label medication on day 43 after randomisation and on day 64 after randomisation .