CRISE VASO OCCLUSIVE PDF

The altered form of hemoglobin in patients with this condition is called hemoglobin S or sickle hemoglobin. Sickle cell anemia patients often experience episodes of acute pain that are caused by vaso-occlusive crisis VOC. VOC is the most common complication of sickle cell anemia and a frequent reason for emergency department visits and hospitalization. What is VOC? Hemoglobin is a protein inside red blood cells and is responsible for transporting oxygen.

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Other adhesive interactions require a soluble bridge molecule eg, thrombospondin, VWF. Circulating endothelial cells characterized by an activated phenotype expression of VCAM-1 and E-selectin and increased levels of plasma sVCAM-1 have also been reported and are reflective of continuous endothelial activation.

SCD mice indeed exhibit a more dramatic VOC phenotype when the experiment is carried out at nighttime. Circulating leukocytes and platelets also have an activated phenotype. Ischemia-reperfusion injury, release of free hemoglobin and heme secondary to RBC lysis, and increased production of placental growth factor PlGF all may contribute to the inflammatory vasculopathy. Intravascular hemolysis results in release of cell free hemoglobin in the plasma, translocation of hemoglobin to the spaces between the endothelium and the smooth muscle cells, nitric oxide depletion in the plasma and subendothelial spaces, oxidative stress, and hemin release that contribute to the inflammation.

Reduction of plasma levels of tissue factor in a sickle cell transgenic mouse model results in decreased plasma levels of IL-6 and soluble VCAM Neutrophil adhesion and activation: critical steps for VOC Various studies have noted a surprising pivotal role of neutrophils in a disease caused by a mutation only expressed in the erythroid lineage.

In vivo evidence for this phenomenon was first reported using SCD mice that exclusively express human sickle hemoglobin. To observe dynamic interactions between the vascular wall and flowing blood cells, the cremasteric microvasculature was examined using intravital microscopy 6 In this model, more striking than the occasional direct interactions of the SS-RBC with the endothelium were the prominent interactions of circulating SS-RBCs with adherent leukocytes.

This suggested that the recruitment of the adherent leukocytes to activated endothelium was necessary for the VOC process. Mice deficient in the C3 complement protein, a ligand for Mac-1 integrin, have a partial reduction in RBC capture, suggesting the role of complement opsonization on the RBCs. Surprisingly, inhibition of E-selectin abrogates these effects, whereas inhibition of P-selectin has only a partial effect.

Chen and P. As predicted by the model above, its administration to SCD mice resulted in decreased leukocyte adhesion, increased leukocyte rolling velocity, dramatic reduction of RBC capture by the neutrophils, and improved blood flow rates. Intravascular hemolysis and VOC It is possible that severe, acute, or subacute hemolysis triggers VOC by overwhelming the adaptive mechanisms.

Clinical support for this concept comes, for example, from the observation that delayed hemolytic transfusion reactions can precipitate VOC events. Recent studies in a sickle cell murine model of hemolytic transfusion reaction have identified CXCL1 as a key inflammatory mediator of VOC. Targeted inhibition of this pathway may represent a new therapeutic approach for VOC. When infused into SCD mice, cell-free hemoglobin and its reactive ferric protoporphyrin-IX group hemin produce vascular stasis.

Although the exact mechanisms remain incompletely elucidated, one can speculate that the activated adherent leukocytes, which are rigid and larger than SS-RBCs, likely drive VOC in collecting venules, whereas the SS-RBCs may contribute in smaller vessels or in situations where there is no potent inflammatory trigger.

Triggers for VOC vary and can include inflammation, stress, increased viscosity, decreased flow, hemolysis, or a combination of factors as follows: 1 endothelial activation by SS-RBCs and other inflammatory mediators, 2 recruitment of adherent leukocytes, 3 activation of recruited neutrophils and of other leukocytes eg, monocytes or iNKT cells , 4 interactions of sickle erythrocytes with adherent neutrophils, 5 vascular clogging by heterotypic cell-cell aggregates composed of SS-RBCs, adherent leukocytes and possibly platelets, 6 increased transit time to greater than the delay time for deoxygenation-induced hemoglobin polymerization, propagating retrograde VOC, and 7 ischemia as a result of the obstruction that creates a feedback loop of worsening endothelial activation.

The process described above is highly dynamic and can potentially be reversed with targeted therapy.

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Vaso-occlusion in sickle cell disease: pathophysiology and novel targeted therapies

Other adhesive interactions require a soluble bridge molecule eg, thrombospondin, VWF. Circulating endothelial cells characterized by an activated phenotype expression of VCAM-1 and E-selectin and increased levels of plasma sVCAM-1 have also been reported and are reflective of continuous endothelial activation. SCD mice indeed exhibit a more dramatic VOC phenotype when the experiment is carried out at nighttime. Circulating leukocytes and platelets also have an activated phenotype.

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