Stroke Flashcards
(77 cards)
What tissue compartments do stroke pathology involved?
The tissue compartments involved are: the vascular endothelium, astrocytes as well as neurones. A stroke begins as an occlusion to a cerebral artery due to a thrombus or tromboembolism.
What is the role of inflammatory cells in stroke pathology?
Inflammatory cells such as neutrophils, monocytes and macrophages have a role early on. Neutrophils enter the brain within 30 minutes of ischaemic stroke. They are able to enter the parenchyma because of the disrupted BBB and cell adhesion molecules on the endothelial cells.
These cells cause harm and further damage by releasing oxygen free radicals and proteolytic enzymes. Inflammatory cells also activate proteases such as MMP (matrix metalloproteinases) as well as endogenous tPA (tissue plasminogen activator).
5-7 days later, lymphocytes initiate a prolonged inflammatory response, and also promote damage. They promote inflammation by releasing IL-1B, TNFa and PAP (Platelet Activating Factor). They also promote the experession of adhesion molecules on endothelial cells such as ICAM-1 and E-selectins.
The effect of the inflammatory cells tend to be detrimental to the patient.
Describe the concept of the ischaemic core and penumbra
Stroke involves an ischaemic core and the ischaemic penumbra. The core is usually unsalvageable, whereas the penumbra has collateral blood supply, and thus can be salvaged at the onset of stroke and if treatment is administered on time.
Penumbra represents tissue at risk of infarction where perfusion is adequate to maintain cell viability but not adequate for normal neuronal function. This is the area we are trying to save with new therapies.
Lo et al 2003 showed that saving the penumbra with tPA resulted in a low overall damage.
What are normal, and stroke cerebral blood flow rates?
Normal cerebral blood flow is > 50 ml/100g/min
- > 22 < 50 ml/100g/min - Hypoperfusion but likely to survive depending on factors such as collateral flow
- < 22 ml/100g/min - Misery perfusion likely to progress to infarction - this is what is experienced by the penumbra.
- < 10 ml/100g/min rapid cell death - Effect of focal ischaemic on brain metabolism.
What are the metabolic effects of decreasing cerebral blood flow?
The decrease in cerebral blood flow has a range of metabolic effects. The diagram shows that as ml/100g/min (perfusion) decreases:
- ATP stores deplete rapidly at less than 20ml/100g/min
- Glucose utilisation decreases
- Protein synthesis decreases rapidly (conservation of energy)
- Intracellular water content (Na+/K+ uptake) increases
- HSP 70 (Heat Shock Protein 70) up-regulation
- Lactic acidosis
Describe the concept of therapeutic window in stroke
Penumbra represents tissue at risk of infarction where perfusion is adequate to maintain cell viability but not adequate for normal neuronal function.
The window where treatment can have any sort of beneficial effect is around 3h. However, the earlier the administration of treatment, the lower the overall infarct size.
What are the time-dependent stages in stroke pathophysiology?
- Energy failure (minutes) - very rapidly uses ATP
- Excitotoxicity (minutes) - due to buildup of glutamate
- Induction of immediate early genes (hours) - transcriptional upregulation
- Inflammation (hours/ days) - long-term and delated
- Programmed cell death / apoptosis (days)
- Late stage repair
Explain how energy failure contributes to stroke pathophysiology.
Reduced blood flow reduces the amount of ATP that can be generated. Remember the brain uses 20% of total O2 consumption used by the brain which is ~ 2% body weight). This interferes with ion gradients. The Na+ pump fails and hence membrane potential NOT maintained.
Extracellular glutamate (GLU) is elevated - this is usually maintained at very low levels in the synapse through energy dependent GLU transporters. These mechanisms are energy dependent, therefore ATP depletion leads to elevated GLU. Under 20ml/100g/min, extracellular GLU rises, which is dangerous as it activates GLU receptors. Same for GABA and Adenosine.
These changes can even be found in the plasma in the rest of the body (which is far away from the local brain tissue) - in quite a short timescale, only 6 hours.
Ability of GLU to cause damage is mostly via NMDA receptor - this is the crucial one because it is the Na/Ca channel that is the mediator of the next stage of cell death.
Explain how excitotoxicity contributes to stroke pathophysiology.
Excitotoxicity is the pathological process by which nerve cells are damaged or killed by excessive stimulation by neurotransmitters such as glutamate and similar substances. This occurs when receptors for the excitatory neurotransmitter glutamate such as the NMDA receptor and AMPA receptor are overactivated by glutamatergic storm. Glutamate excess is dangerous as it causes cell death by over-exciting neurones.
Glutamate binds to NMDA (and AMPA) and causes Na+ and Ca2+ influx.
- Na+ influx causes depolarisation and cell swelling (due to osmotic forces).
- Ca2+ influx mediates:
○ Proteases, Nucleases
○ XDH (Xanthine Dehydrogenase)
○ PLA2 (Phospholipase A2 – regulates inflammatory response)
○ NOS (Nitric Oxide Synthase)
This results in formation of reactive oxygen species, which can interact with lipids in cell membrane, leading to loss of membrane integrity.
Increased calcium levels intracellularly causes mitochondrial surface membrane to become more permeable. Leakage of Cytochrome C from mitochondria leads to mediation of programmed cell death - apoptosis.
What are the three different types of NOS?
There are three different forms of NOS. In general only the NOS in endothelial cells are beneficial in the event of CI:
- nNOS (neuronal)- acts as a retrograde messenger - leads to toxic levels of NO free radicals, causing neuronal lesions
- eNOS (endothelial)- vasodilatory action (relaxing smooth muscles) - beneficial - improves cerebral blood flow
- iNOS (inducible) - immune mediator - leads to toxic effects enhanced in ischaemia
What protects against free-radical damage?
There are endogenous antioxidants and free radical scavengers which are important in the ischaemic period and also in the subsequent reperfusion when tissue is exposed to high levels of oxygen (“oxidative stress”) - by NO and O2.
These protective antioxidant and free radical scavengers include:
- Superoxide dismutase (SOD)
- Catalase
- Glutathioneperoxidase
- Alpha-tocopherol
- Ascorbic acid
Explain how glutamate mediated excitotoxicity can influence the extent of necrosis or apoptosis
The severity of the insult influences the extent of apoptosis that occurs in comparison to necrosis. In a severe insult, Ca2+ influx causes free radical generation and uptake into mitochondria. There is severe ATP depletion which causes mitochondrial swelling which causes a necrotic lesion. This happens at the ischaemic core.
At the penumbra there is a more milder insult. Reduced oxygen and thus ATP causes a transient depolarisation. Ca2+ is able to be loaded into the mitochondria reducing formation of ROS (Reactive Oxygen Species). This causes activation of Cytochrome C and therefore apoptosis through the mitochondrial pathway.
Explain how excitotoxicity could be targeted for treatment, and drawbacks.
Glutamate antagonists and ion channel blockers in stroke models
- NMDA, AMPA antagonists - HIGHLY effective up to ~2h after insult BUT have psychotomimetic (NMDA) and respiratory depressive properties. Furthermore, the window of therapeutic opportunity is difficult to translate to application in man.
- Metabotropic receptors: Group 1 receptors antagonists (postsynaptic and associated with NMDAR action) and Group II / III mGluR agonists (presynaptic, inhibit glutamate release) are effective.
- Ca2+ channel (L, P/Q and N), Ca2+-dependent K+, channel and proton activated Ca2+ permeable channel (ASIC1a) blockers reduce brain injury.
Explain how transcriptional cascades contribute to stroke pathophysiology.
Glutamate activates a transcriptional cascade which occurs during normal neuronal activity (but also is a response to noxious insults). Ca2+ influx through NMDAR function, causes the activation of Ca2+-Calmodulin Kinase IV Pathway (CAMKIV). This leads to the phosphorylation of CREB (cAMP Response Element Binding protein). CREB/CREB binding protein (CBP) complex activates transcriptional and neurotrophic factors.
This leads to the activation of the multi-potential early response genes:
- Inducible transcription factors (IEGs) - which activate/repress other genes
- Enzymes such as COX-2 - which underlie developmental and behavioural responses
- Neuroprotective proteins - e.g. HSPs which counter damaging effects This pathway mediates an injury response which can contribute to cell survival or death.
What are some of the genes activated by glutamate mediated transcriptional cascades in the penumbra?
Elevated extracellular K+ and glutamate depolarisation in penumbra leads to upregulation of injury response genes (e.g. c-jun, ATF3, and heat shock proteins HSPs). Transcription of these genes are found all around the area of infarct. The upregulation of these genes is sensitive to glutamate antagonists - providing glutamate antagonists prevents this from happening.
Explain the role of heat-shock proteins in stroke pathophysiology.
Heat Shock Proteins act as protein chaperones facilitating the transfer of proteins between subcellular compartments. Following a noxious stimulus (heat, ischaemia) HSPs are induced which target abnormal proteins for degradation. HSPs are also anti-apoptotic and antioxidant (HSP27).
Overexpression of HSP70 and HSP27 reduces the infarct size in mice after MCAO (Middle Cerebral Artery Occlusion).
Sulindac is an NSAID which increases HSP27 and reduces infarct size [Modi et al 2014].
Heat shock proteins play a role in ischaemic preconditioning (IPC). IPC is a process in which brief exposure to ischaemia (such as after TIA) provides robust protection/tolerance to subsequent prolonged ischaemia. HSP involvement in IPC has been demonstrated in cardiac and cerebral ischaemia [Sun et al 2010] mediated through the NF-kB* pathway [Tranter et al 2010] .
Explain the role of cytokines in the pathophysiology of stroke.
Cytokines are produced by a range of activated cell types (endothelial cells, microglia, neurones, astrocytes, platelets, leukocytes, fibroblast) within the first few hours after ischaemia.
• IL-1 and TNFa upregulate adhesion molecules promoting neutrophil migration
○ CSF levels of IL-1, IL-6 and TNFa at 24h correlate with infarct size • Chemokines (e.g. CINC and MCP-1)* detected in the brain between 6 and 24h attract neutrophils & infiltration.
However, some cytokines are neuroprotective. IGFb and IL-10 produced by lymphocytes limit leukocyte invasion and reduce immune responses Complex protective/harmful effects are seen due to multiple sites of action
Describe how cytokines have been studied as a potential therapeutic target for stroke.
- IL-1b receptor antagonists are protective.
- TNFa neutralising antibodies and antisense nucleotides are protective.
- Preliminary studies suggested that neutrophil infiltration is correlated with infarct size.
However, Phase III anti-neutrophil drugs failed to improve stroke outcome and anti-ICAM (n=625) increased mortality (Becker et al 2002)
Describe the role of apoptosis in stroke pathophysiology.
This is most relevant to damage in the penumbra (e.g. delayed cell death). Damage in the core is primarily by necrotic pathways.
Apoptosis is triggered by free radicals, death receptor, DNA damage, protease action and ion imbalance. Release of cytochrome c from mitochondria activates the formation of an apoptosome complex (APAF1 + procaspase 9) and caspase 3 activation (detected at ~8h) leading to DNA fragmentation.
Describe how apoptosis have been studied as a potential therapeutic target for stroke.
- Caspase 3 selective inhibitors (zDEVD.FMK) are effective up to 9h after reversible ischaemia. Broad specificity caspase inhibitors (zVAD)/ caspase 1 deletion protects against ischaemia.
- Viral mediated gene transfer of Bcl-2 and Bcl-XL are neuroprotective
Explain the role of late-stage repair in stroke.
- Growth factors are secreted by neurones, astrocytes, microglia, macrophages, vascular and peripheral cells e.g. IGF1, and erythropoietin.
- Glutamate-mediated synaptic activity increases BDNF [Brain-derived neurotrophic factor] transcription and secretion.
- Neuronal sprouting occurs in an attempt to form contacts.
Explain the role of tPA in stroke treatment.
Restoration of blood flow by thrombolysis with tPA has had the greatest impact on stroke treatment. It is the most effective procedure for SALVAGING ischaemic brain tissue. BUT rapid administration is ESSENTIAL together with rapid assessment of risk. Not everyone benefits as there is a significant haemorrhage risk. Furthermore tPA has no effect on mortality.
Define Stroke
Definition of stroke: acute-onset neurological symptoms or signs indicative of focal central nervous system dysfunction, due to vascular cause (ischemia/ haemorrhage), lasting >24 hours. • If<24hours,it is a TIA. • You can get occlusions in the spinal cord and retina as well as the brain.
Describe the causal classification of stroke
There are two main types of stroke: ischaemic (80%) and haemorrhagic (20%). Ischaemic stroke can be: • Arterial: ○ AIS (Acute Ischaemic Stroke) - classically thought of stroke. Most common cause of stroke. ○ Lacunar - small vessel blockage. • Venous: ○ CVST (cerebral venous sinus thrombosis) - thrombus in the veins causing backpressure and oedema. Haemorrhagic stroke can also be: • Arterial: ○ Aneurysm - balloon-like bulge in the blood vessel. Most common cause of haemorrhagic stroke. ○ PICH (primary intracerebral haemorrhage) - within the brain tissue (not related to meninges). Also most common cause of haemorrhagic stroke. ○ AVM (arteriovenous malformation) ○ EDH (extradural haemorrhage) - usually in young patients after trauma ○ AIS transformed to haemorrhagic stroke with or without thrombolysis. • Venous: ○ SDH (subdural haemorrhage) - due to sheer forces; can be acute in young people, but often chronic in elderly patients. ○ CVST (cerebral venous sinus thrombosis) ○ Cavernoma
