Continuing Education Activity

Stroke is a major cause of death and disability worldwide, with a prevalence of about 2.5%. A stroke is called ischemic when caused by an interruption of the blood supply to the brain either through a blood clot called a thrombus or an embolus, which is a dislodged clot. Stroke is a major cause of death and disability worldwide, with a prevalence of about 2.5%. This activity also highlights the role of the interprofessional teams in the evaluation and treatment of patients.

Objectives:

• Identify the etiology of stroke reperfusion injury.
• Outline the evaluation of stroke reperfusion injury.
• Review the management options available for stroke reperfusion injury.

Introduction

Stroke is a major cause of death and disability worldwide, with a prevalence of about 2.5%.[1] A stroke is called ischemic when caused by an interruption of the blood supply to the brain either through a blood clot called a thrombus or an embolus, which is a dislodged clot. At the onset of an acute ischemic stroke, lack of oxygen and other nutrients trigger a series of events causing electrophysiological, metabolic, and molecular damage, leading to irreversible brain tissue damage. The most proximal part of the arterio-vascular occlusion sustains maximal damage and is usually called an ischemic core. Between the ischemic core and normal brain tissue lies the 'penumbra,' an area of mild to moderate hypoxia that may become irreversibly damaged lest blood flow is restored to normal levels within a 'critical' time period. Without therapeutic interventions and continued ischemia, brain tissue death is quantified as a loss of 1.9 million neurons, 14 billion synapses, and 12 km of myelinated fibers every minute.[2][3][4][5]

In other words, one hour of ischemic brain damage can be compared to 3.6 years of normal brain aging. Acute stroke therapeutics aim to contain the tissue damage happening at the 'penumbra' level and restore the penumbra's functionality. Alteplase, a tissue plasminogen activator (tPA), is the only United States FDA (Food and Drug Administration) approved clot-busting medication used to recanalize the thrombosed / occluded vasculature in an ischemic stroke.[6][7] Many studies have consistently shown better outcomes in acute ischemic stroke patients who received tPA. Intervention with tPA, newer proven endovascular interventions like mechanical thrombectomy aimed at recanalizing thrombosed vessels paradoxically may lead to deleterious consequences in the ischemic tissue due to many complexly woven biochemical and pathological events. In a subacute context, procedures like carotid endarterectomy and stenting may also lead to reperfusion injury.[8][9][10] Such functional, microscopic, and sometimes a macroscopic injury consequential to blood flow restoration is termed as ischemia-reperfusion (I/R) injury.

Etiology

Prolonged cerebral ischemia will deprive the brain cells of energy leading to physiological dysfunction and, eventually, cell death. Many biochemical, physiological, and morphological changes in the cell and its environment precede its eventual death, such as energy failure, lactic acidosis, increases in oxygen extraction fraction and glucose utilization, protein synthesis inhibition, chromatin condensation, cell organelle disruption, and cell shrinkage.[11] 

The multitude of events initiated by the ischemic insult plays a role in the molecular, microscopic, functional, and macroscopic derangements observed in ischemia-reperfusion (I/R) injury. Most notable are the oxidative stress, leukocyte recruitment, and breach of the blood-brain barrier.

Epidemiology

Due to the lack of worldwide assessment data on acute ischemic stroke (AIS) reperfusion injury after thrombolysis and mechanical thrombectomy, it is hard to predict the epidemiology of I/R injury. However, the term hemorrhagic transformation (HT) can be considered an expression of reperfusion injury. In the historic NINDS trial, HT occurred in 6.4% of r-tPA-treated patients compared to patients treated with a placebo where it was only 0.3%. Of note, this represents the symptomatic hemorrhagic transformation (sHT).[7][12]

In the large SITS-MOST observational study, sHT was seen in 7.3% of ischemic stroke patients undergoing thrombolysis.[13] A much better insight into the epidemiology of thrombectomy based reperfusion injury can be sourced from the Highly Effective Reperfusion evaluated in Multiple Endovascular Stroke Trials (HERMES) metaanalysis by Goyal et al., where sHT was 4.4%.[14] In the recently concluded extended window endovascular trials - DEFUSE 3 and DAWN sHT was 7% and 6%, respectively.[8][15]

Pathophysiology

Most treatment strategies of ischemic stroke aim to reopen the thrombosed vessels. However, time is of the essence here. Beyond a critical time, instead of preserving brain tissues, restoration of oxygen amplifies the destruction of an already deranged neurovascular and brain parenchymal environment. This topic has been a scientific intrigue ever since, even before thrombolytic therapies evolved.

Prolonged ischemia and hypoxia, secondary to a cerebral vessel's thrombosis, result in a change to anaerobic metabolism, leading to insufficient energy balances and ion dysregulation due to pump failures like Na+/K+ ATPase, amongst many others. This cascades to a Na+ and Ca+2 overload swelling the neuron, causing morphological and functional disruption of cellular organelles, most notably the mitochondria. Damage and swelling of the mitochondria further exacerbate the brain cells' energy dysfunction.[16][17] Continued ischemia of the brain cells leads to the activation of microglial cells, which initiate post-ischemic inflammation by producing pro-inflammatory chemokines and cytokines (like TNF-α or IL-1β). Activated neutrophils also contribute to the post-ischemic production of the reactive oxygen species (ROS). Beyond a critical period, oxygen restoration worsens an already deranged neurovascular and brain parenchymal milieu setting the stage for an imminent I/R injury.[11]

A. The most critical consequence of introducing oxygen by restoring blood flow to such an oxygen-deprived tissue is the aggravated generation of reactive oxygen species (ROS). The unchecked generation of ROS directly damages the neurons and indirectly sets the immune system into overdrive.

1. Formation of ROS: Ischemic tissue is a region where there is the formation of ROS. Hypoxanthine, which is built up during ischemia, is suddenly metabolized (due to oxygen in the reperfusion) by hypoxanthine oxidase, aiding in the formation of the reperfusion mediators (like O-, HOCl-, HO). Mitochondria also contribute to the formation of reactive oxygen species (ROS), paving the way for the 'oxidative stress.'
2. ROS in I/R injury: Reactive oxygen species causes peroxidation of the cell membranes and directly damages the cells. Unchecked ROS intensifies and abets the ongoing pro-inflammatory molecular cascades, along with the recruitment and activation of more leukocytes.[18][19]

B. Inflammatory responses:

1. Many animal studies established a temporal relationship between the activation of inflammatory cascades and ischemia. At the ischemic penumbra, there is upregulation of adhesion molecules like p-selectins, ICAM-1, which result in leucocyte-endothelial interaction. Unchecked ROS production helps in the enhanced P-selectin mediated rolling and ICAM-1 mediated adhesion of the leukocyte, eventually leading to PECAM-1 (platelet-endothelial cell adhesion molecule1) aided transmigration or diapedesis of the leukocyte into the affected tissues.[20][21][22]
2. Oxidative stress also leads to complement activation, involving both C3a and the more powerful anaphylatoxin C5a. C5a induces the formation of proinflammatory-cytokines like tumor necrosis factor-α(TNF-α), IL-1(interleukin-1), IL-6 (interleukin-6). The cytokine production helps the leukocytes' aggregation because of the increased upregulation of leukocyte adhesion molecules. C5b-9, along with other activated complement components, lead to the formation of a membrane attack complex (MAC), causing further activation of the leukocytes, lysing the cell membranes. IgM antibodies have been shown to deposit in ischemic tissues. When the flow is resumed, the complement proteins adhere to them and activate the complement pathway, thus exacerbating the injury.[23][24]
3. Ischemia-reperfusion plays a vital role in the activation of platelets. P-selectins which are upregulated during this cascade, play an essential role in the adhesion of activated platelet and leukocytes. Activated platelets and neutrophils aggregate and may clog the brain's microvasculature, a phenomenon called 'no-reflow' that may occur after reperfusion. Activated platelets lead to the formation of a plethora of biochemical molecules, further enhancing the leukotaxis, extravasation of the leukocytes, and exacerbating tissue injury. 

C. Blood-Brain Barrier (BBB), composed of the vascular endothelium, basement membrane, pericytes, and astrocyte foot process, bestows a unique and environment to the brain, protecting it from the fluctuations in plasma. The barrier is unique because of the non-fenestrated basement membrane, minimal pinocytic transport, and tight junctions(TJ). Nevertheless, sometimes there is a minimal opening of BBB in certain physiologic states.[25]

A heady mix of ROS mediated lipid peroxidation, leukocyte-endothelial adhesion, activated complement, aggregation of activated platelets, and leukocytes (mostly neutrophils) leads to the interruption of the TJ and a breach of the BBB. This breach leads to access to the leukocytes, particularly the activated neutrophils and their toxins, which deranges the brain tissue physiologic milieu's fidelity. Reperfusion also leads to the activation of proteases like Matrix metalloproteinases(MMP), affecting the capillary basal lamina's integrity, leading to increased capillary permeability and ending in the opening of BBB.[26]

History and Physical

'Time is brain', and delay in care, either with thrombolysis or endovascular therapy, can be detrimental in patients with acute ischemic stroke.

Evaluation

The time from symptom onset to disruption of the blood-brain barrier (after recanalization using rtPA) was observed to be around 12.9 hours.[27] The time for BBB disruption is also associated with age. Usually, imaging of penumbra is not indicated for regular rtPA usage if the patient has met all the criteria. Penumbral imaging becomes crucial after prolonged ischemia (beyond 6 hours). To pursue further treatment options, including thrombectomy, and to prevent any iatrogenic breach of BBB or HT, it is necessary to know the lesion age. CT and MRI based perfusion studies are used to identify the ischemic core and penumbra and assess the risk-benefit aspects of reperfusion therapies and prevent any unintended consequences of recanalizing a thrombosed cerebral vessel.[28][29][30][5]

The hyperintense acute reperfusion marker (HARM) is a hyperintense radiologic signal within the CSF spaces visualized on postcontrast fluid-attenuated inversion recovery (FLAIR) sequences. This radiologic finding is associated with permeability changes to the blood-brain barrier in acute stroke. SPECT imaging with 99mTc-duramycin has proved useful in detecting apoptotic neuronal cell death in a rat model of ischemia-reperfusion injury.[31][32][16][33]

No-Reflow: Leukocyte recruitment, platelet activation, increased viscosity of the blood due to movement of water from the plasma to the perivascular space in the ischemic tissue will clog the microvasculature of the downstream tissue more. Reperfusion just severely exacerbates this phenomenon, termed as NO-reflow.[34][35] The clogged microvasculature proves detrimental to an already oxygen-nutrient starved tissue. In essence, despite recanalization and blood flow resumption, it leads to continued ischemia and possibly ischemic expansion because of the clogged microvasculature.[36][16]

Treatment / Management

Based on the pathophysiology, there are many potential targets for treatment in ischemia-reperfusion injury. The therapeutic targets include complement depletion, attenuating the excess ROS, mitigating the effects of inflammatory cascades, and inhibiting leukocyte activation and platelet recruitment.

1. Since the prolonged ischemia and the subsequent I/R injury associated with recanalization are closely intertwined with the proinflammatory conditions, it is a theoretical possibility to deal with this proinflammatory state with glucocorticoids.[22] Dexamethasone was useful in rat models of ischemic stroke but was not beneficial in human clinical trials.[37]

2. ROS, which is to be blamed for much oxidative stress associated with ischemia, also increases manifold during the reperfusion injury beyond the cellular milieu's ability to contain this excess production. Studies have shown that hydrogen gas's inhalation has attenuated the mitochondrial pore formation and eventual mitochondrial cell death and apoptosis. Inhalation of Hydrogen gas has been proposed to mop up these free ROS. The main premise behind Hydrogen gas supplementation as a treatment possibility is that the H2 gas will react with the ROS to form free water, thus attenuating the ROS's toxicity.[38][39] This has enjoyed success in the animal models but yet to be explored in human trials.[38][40] A clinical trial explored the possibility of an infusion of superoxide dismutase in preventing reperfusion injury in patients with hemorrhagic shock. It has shown promise but yet to be explored on a larger scale.[41]

3. A Rho-kinase inhibitor aimed at inhibiting the NADH oxidase and further limiting ROS production has shown promise in rat models of I/R injury, as did Apocynin.[42]

4. In animal models of cerebral ischemia, hyperbaric oxygen treatment has helped mitigate MMP-2 led injury and inhibited apoptosis. Hypothermia, inhalation of isoflurane, and non-invasive vagal nerve stimulation have all showed positive effects in mitigating the damage caused by activated MMP in ischemic tissue.[43][44] A review by Li, Yongchang, et al. has a detailed insight into the emerging possibilities to mitigate I/R injury[45][46][47][48]

The failure of the aforementioned substances in clinical trials versus their apparent successes in animal studies reverts us to a fundamental question of the validity of the animal models of ischemia and whether they can simulate and be compared to human cerebral ischemia. Nevertheless, expanding research in targeted treatments and modern, functional, and temporal neuroimaging abilities holds a future promise.

I/R injury could be prevented by proper assessment with perfusion studies to assess the risk-benefit aspects from reperfusion therapies and prevent any unintended consequences of recanalizing a thrombosed cerebral vessel. The finding of HARM is associated with permeability changes to the blood-brain barrier in acute stroke. Breach of the BBB may evolve into HT and may further and frequently lead to neurological instability. Better outcomes are associated with immediate admission to a dedicated neuroscience intensive care.[49] 

Supportive management of an I/R injury could include aiming for normovolemia, normotension, avoidance of hypoglycemia, antiepileptic drugs (can be considered if there are any seizures), and maintenance of cerebral perfusion pressure >70mmHg, amongst others. If there is significant edema associated with clinical decompensation, the use of osmotic therapies like mannitol and/or hypertonic saline can be considered. On the contrary, Mannitol could also increase hemorrhagic risk due to vessel damage and its osmotic effect.[50] In some clinical settings with impending herniation/herniation or clinical decompensation associated with elevated intracranial pressures, decompressive hemicraniectomy is utilized and has been shown to improve outcomes. Of note, the data above about the acute use of osmotic therapies in cerebral edema is from studies on stroke in general and not specifically related to reperfusion injury per se. Steroids are contraindicated in such presentations and have been associated with worse outcomes.

Differential Diagnosis

Deteriorating signs in the context of reperfusion in a patient admitted not too long ago will not yield many differentials. Bleeding from a tumor, arterial-venous malformations, infections, subarachnoid hemorrhage, subdural hematomas, meningitis can be considered in the differential and should be carefully ruled out based on findings, history, and evaluation in the context of the reperfusion but are unlikely.

Prognosis

Prognosis depends on age, size of the ischemic core, reperfusion strategies used, and other prognostic markers.

Complications

Complications from reperfusion injury include penumbral damage, ischemia expansion, HT, seizures, malignant cerebral edema, and herniation, all of which are associated with worse clinical outcomes in a stroke patient. 

Deterrence and Patient Education

Deterrence and patient education have a minor role for I/R injury in particular. Since the condition is consequential to ischemic stroke treatment, physicians should always advise their patients to engage in healthy lifestyles as a primary preventative measure to prevent cerebrovascular accidents.

Enhancing Healthcare Team Outcomes

Time sensitiveness is the most limiting factor in treating stroke and in treating the I/R injury. It requires a dedicated interprofessional team effort involving multiple specialists (neurologists, neurosurgeons, emergency physicians, critical care physicians, nurses, anesthetists) and medical staff personnel to enhance patient care to achieve good outcomes.[51]