This is a double-blind, randomized, placebo-controlled study involving 30 patients, who will receive IVIg or placebo for 3 months and then will cross-over to the alternate therapy for another period of 3 monthly infusions. The monthly dose of IVIg is 2 GM/Kg divided into two daily doses. The drug will be considered effective if patients experience an increase of more than 15% in their baseline muscle strength. Muscle strength will be assessed with a series of objective dynamometric measurements performed before and at the end of each monthly infusion.