Thrombotic Thrombocytopenic Purpura (TTP) is a potentially life-threatening thrombotic microangiopathy caused by a severe deficiency of ADAMTS13 (a disintegrin and metalloproteinase with a thrombospondin type 1 motif member 13). Decreased ADAMTS13 activity leads to an accumulation of ultralarge von Willebrand factor (vWF) multimers which induce aggregation of platelets and microthrombi. These microthrombi may involve the brain, heart, kidneys and lead to life-threatening organ failures.

In experimental models, magnesium sulfate increases cleavage of newly released vWF by ADAMTS13, decreases the endothelial secretion of ultralarge vWF and inhibits the interaction of vWF with platelets. In another thrombotic microangiopathy, magnesium sulfate has been shown to reduce the risk of seizures in women with severe pre-eclampsia. In analogy with its evidence-based therapeutic application in pre-eclampsia and based on a strong rationale for magnesium supplementation in TTP, we propose a phase 3, double blind, placebo controlled, and randomized study to evaluate the efficacy of magnesium sulfate in more rapidly restoring normal platelet counts as measure of prevention of further microvascular thrombosis in patients with Thrombotic Thrombocytopenic Purpura.

Inclusion Criteria:

• Age > or = 18 years
• Health insurance
• Signed inform consent by patient or relatives

Exclusion Criteria:

• Pregnancy
• No health insurance

For each day of treatment, 4 ampoules of 10 ml will be distributed to the patient's nurse (4 ampoules of 1.5g of magnesium sulphate or 3 ampoules of 5% glucose as placebo).

The bulbs will be labeled identically so that the blind can be maintained Moreover, in order to preserve the blind, the dosage of magnesemia should not be performed outside of a necessity judged by the clinician in charge of the patient