Reperfusion injury/ Reperfusion Hemorrhage/ Hemorrhage
Large Vessel Occlusion (LVO) if treated successfully can undoubtedly improve patient outcomes as evidenced by the current literature. The restoration of blood flow to viable brain tissue permits for salvage of the penumbra but at the same time opens the door for a process known as cerebral re-perfusion injury. Cerebral reperfusion injury may be defined as deterioration of salvageable brain tissue, which was initially ischemic, after reperfusion. (\cite{Pan_2006}) Although research is ongoing with the mechanisms by which the injury occurs, several studies have demonstrated various processes involved. Inflammatory response and release of free radicals to injured tissue by restored blood flow is one mechanism in addition to the involvement of leukocyte infiltration and breakdown of the blood brain barrier. Reperfusion injury occurs on a microvascular or cellular level, but is certainly related to a more overt and conspicuous phenomena which is reperfusion hemorrhage or hemorrhagic transformation.
An endpoint in clinical trials, symptomatic intracranial hemorrhage (SICH) after ischemic stroke is not an uncommon consequence of recanalization of LVO by either tPA or mechanical thrombectomy.\cite{Hao2017} determined that a number of factors may predispose patients to SICH following thrombectomy. These factors included cardioembolic stroke, poor collateral circulation, delayed endovascular treatment, multiple passes with stent retriever device, lower pretreatment ASPECTS, and a high baseline neutrophil ratio. SICH has potentially devastating ramifications for patients depending on the severity of the bleed. The potential of worsening a patient’s poor clinical exam or increasing their post procedure NIHSS is a risk undertaken.  Therefore, patient selection and evaluation of the risk benefit profile is imperative. Of the modifiable factors studied by Hao et. al, the neurointeventionist should pay close attention to the time at which endovascular treatment is rendered with regard to the onset of stroke and the number of passes attempted with the stent retriever as these can be directly impacted. Delays in treatment are modifiable only to a certain extent, however. Expediting door to groin times and streamlining the process by which patients receive interventions is a constant effort by providers to prevent delays. With the introduction of the ADAPT technique, intervention may not require the use of stent retrievers. Stent retrievers may have the deleterious effect of endothelial damage and weakening of what in many cases might be unhealthy atherosclerotic vessel lumens. New clinical trials such as the DAWN trial demonstrate that the therapeutic time of window of 6 hours may be extended for intervention while still providing benefit to patients. However, with longer periods of ischemia, cerebral tissue may be exposed to a higher risk of injury and potential SICH (DO WE HAVE REFERENCE TO BACK THIS UP?).