Short description of the protocol intended for the lay public. Include a brief statement of the study hypothesis (Limit : 5000 characters) The sickle cells anemia is a monogenic disease linked to the presence of Hemoglobin S due to a mutation in the Hemoglobin Beta chain. The lack of circulating oxygen induces a polymerization of the Hemoglobin S which change the red cell conformation into sickle. Those cells interact and causes vaso-occlusive crisis (CVO). The MEOPA is a medical gas used as an antalgic and a sedative especially in sickle cells disease patients. The nitrous oxide, oxide the cobalt ion in the vitamin B12 which inactivate it irreversibly creating a functional deficiency. During the metabolism of vitamin B12, homocysteine is transformed in methionine which is used in to form the myelin sheath and helped in producing DNA. Numerous studies already shown that the longer the exposition to MEOPA is the greater the functional deficiency of vitamin B12 occur. A few studies shown a symptomatic deficiency of vitamin B12 due to the exposition of MEOPA in sickle cells patient but there is no explanation on the necessary amount of exposure or if some patients are more at risk. When there is a deficiency of vitamin B12 the symptoms can go from a simple orthostatic hypotension to a combined spinal sclerosis. The participation to the study will be proposed to every patient hospitalized for a CVO in the follow up of the emergency room visit or directly in pediatric reanimation. During a usual blood test, a small amount of blood (4mL) will be collected in addition to dose the Vitamin B12, the vitamin B9, the homocysteine, and the methionine. A small amount of urine will also be collected to dose the methylmalonic acid, all those elements are a part of the metabolism of B12 vitamin. The same sample will be taken on the day of departure of the hospital. During the hospitalization the pain management, a daily neurological exam, and the exposition to the MEOPA will be assessed meticulously. An appointment will take place at 7 days and at one month after the hospital departure to evaluate the possible neurological defect. Each patient can only be included once.