What Is A Stroke? - Narration and Animation by Dr. Cal Shipley, M.D.

this is dr. Cal Shipley and today we're going to be looking at the question what is a stroke and how does it happen simply stated stroke occurs when there is rapid death of brain tissue due to a disturbance in blood supply disturbances in blood flow to the brain may be divided into ischemic varieties in which there is a loss of blood flow to brain tissue and hemorrhagic varieties in which there is bleeding into the brain or onto the surface of the brain I'll return to the causes of stroke in a moment but first let's look at the anatomy of the blood supply to the brain the blood is supplied to the brain by two pairs of arteries the anterior supply supplies blood primarily to the middle and front portions of the brain and consists of the left and right carotid arteries the posterior supply supplies blood primarily to the brainstem and back portions of the brain and consists of the right and left vertebral arteries which then join together to form the single basilar artery rotating the brain so that we have a view from the left side we can now more clearly see how the anterior supply originating from the carotid arteries distributes blood primarily to the middle and front portions of the brain supply originating from the basilar artery supplies mostly the brainstem and the rear portions of the brain and in a prime example of nature's wonderful design there is a communicating artery which joins the two systems known as the posterior communicating artery this vessel is part of the Circle of Willis in the event that there is sudden impairment of blood flow to the brain from either the anterior posterior system the communicating arteries allow for crossover flow which may help to prevent damage to the portion of the brain affected by the impaired flow now let's return to the stroke starting with ischemia or loss of blood flow to the brain ischemic causes account for 80% of all strokes the first type of ischemia we're going to look at is thrombosis thrombosis refers to a local obstruction of an artery and may be due to disease of the arterial wall such as hardening of the arteries or arterioles chlorosis dissection or fibromuscular dysplasia in which abnormal thickening of the artery wall causes obstruction rhombuses may occur in large arteries or small arteries let's take a look at large arteries first large arteries that may be affected by thrombosis include the carotid vertebral or basilar artery or any of their major branches including the anterior middle and posterior cerebral arteries or their larger branches as an example let's take a look at a large artery thrombosis occurring in the left middle cerebral artery in a typical scenario atherosclerosis built up over many years along the artery wall eventually undergoes degenerative change which causes rupture of the plaque surface ruptured the interior of the plaque releases chemical substances which promote clot formation in the blood vessel the resulting combination of blood clot and plaque leads to thrombosis complete obstruction to blood flow within the vessel the subsequent loss of blood flow in arteries that are beyond the point of thrombosis starves the brain of critical nutrients including oxygen and glucose and eventually leads to rapid death of brain tissue also known as stroke upstream the thrombosis occurs that is the larger the artery affected by obstruction the greater the potential devastation to brain tissue the human brain is exquisitely sensitive to even brief periods of ischemia changes in brain cells may be observed within two to three hours of interruption of nutrient flow with complete death of brain tissue occurring within 6 to 24 hours after the onset of ischemia now let's take a look at the other variety of thrombosis and that is thrombosis affecting small arteries stroke occurring as a result of this type of thrombosis is also known as lacunar strokes or lacunar infarct the Kuna infox occur as a result of disease in small arteries which are coming directly off larger arteries such as the carotid basilar and anterior middle and posterior cerebral arteries these small branches or penetrating arteries as they are called supply blood flow to the central portions of the brain and the brain stem as a result of the very specific anatomic location of these penetrating arteries lacunar infarcts commonly involved the structures of the central brain particularly the basal ganglia and the pawns in the brainstem the thrombosis which occurs in the small arteries involved in lacunar infarcts appears to be primarily caused by prolonged elevation of blood pressure also known as hypertension in a typical scenario prolonged hypertension results in a process known as lipo hyolee gnosis the process of lipo hi as depicted here consists of damage to the inner wall of the small artery with progressive accumulation of debris within the vessel ultimately leading to complete obstruction to blood flow the keuner infarct SAR typically small affecting areas of brain tissue up to 15 millimeters in size long-term uncontrolled elevations in blood pressure affect all the arteries in the human body and the vessels responsible for lacunar infarcts are no exception to this rule as a result the number of areas of lacunae or stroke in affected individuals tends to increase over time it should be noted that in recent years there has been some controversy over the classification of small vessel disease of the brain and subsequent lacunar infarcts as distinct entities in the causes of stroke many of the opponents of the theory of lacunar infarcts feel that these strokes are simply caused by clots which have traveled to these arteries from elsewhere also known as emboli we will be discussing the concept of emboli and embolism in our next segment so now we've looked at the most common cause of ischemic strokes in the brain thrombosis the second most common cause is embolism embolism occurs when clots or particles originating from upstream of a particular artery travel to that artery and obstruct it this is of course in contrast to thrombosis where the thrombus forms at the point of obstruction in the artery the heart is a common source of emboli to the brain let's take a look at a little bit of cardiac anatomy to set things up here we see the left ventricle and the left atrium small chamber on top with the atrial appendage and in cross-section we see the left atrium and we've unfolded the atrium here for clarity to show how the atrial appendage is kind of an extended chamber off the atrium itself the blood flow goes from the left atrium into the left ventricle and then the left ventricle pushes blood up into the aorta and hence to the brain via the carotid and vertebral and Basler arteries as we discussed previously and so the anatomy and flow characteristics make clear any clots or particles which are formed in or enter into the heart have a really good chance of being pumped right up into the vessels of the brain conditions that can result in formation of clot in the left ventricle and then embolization of clot to the brain are congestive heart failure and myocardial infarction also known as heart attack the physiological mechanism which gives rise to clot formation in both conditions is similar let's take a look at our heart model to better understand how this occurs taking a look at the heart and cross-section we can isolate the left ventricle and then freeze it at its point of maximum contraction percentage of blood ejected from the left ventricle with each contraction is known as the ejection fraction a normal ejection fraction is 50 to 65% in other words 50 to 65% of the blood contained in the left ventricle just prior to each contraction is ejected into the aorta in both congestive heart failure and myocardial infarction also known as heart attack there may be marked impairment of left ventricular contraction and therefore a marked reduction in ejection fraction this impairment in contraction can lead to a condition known as blood stasis stasis refers to pooling of blood in the base also known as the apex of the left ventricle when blood pools any human body and the left ventricle is no exception it tends to form clots the medical term for a clot is a thrombus immediately after formation a thrombus may be adherent to the wall of the left ventricle unpredictably a thrombus may break free and be ejected into the aorta a thrombus has now become an embolus once in motion an embolus has a tendency to break up into smaller pieces the tiniest pieces may not cause significant impairment of blood flow to the brain however larger fragments may cause significant obstruction of blood flow to brain tissue resulting in stroke in this example a significantly sized embolus has lodged in a branch of the right middle cerebral artery on CT scan the area of stroke can be readily viewed as this dark oval area this particular patient had had a previous stroke as noted here on the right side of the screen another common condition in which thrombi may form and embolize to the brain is atrial fibrillation unlike normal heart rhythm innate REO fibrillation the atrial chamber Quivers or fibrillate s-- at a much higher rate than the ventricles moreover the fibrillating atrium is much less effective in injecting blood into the ventricle just prior to ventricular contraction also known as systole the beating left ventricle continues to draw blood through the main chamber of the atrium however in the appendage blood movement is much slow to condition known as blood stasis stasis in the left atrial appendage greatly increases the chance for clot formation as demonstrated here many clots are initially adherent to the wall of the fibrillating atrium an adherent clot also known as a thrombus may unpredictably detach itself from the wall of the atrium enter the left ventricle and then be injected up into the Horta and hence to the brain the tendency for thrombus detachment and ejection increases if the atrium suddenly reverts to a normal rhythm once detached the thrombus now travels or embolize --is to the arteries of the brain along the way the embolus as it is now known may fragment into many smaller pieces with only those of significant size causing obstruction to blood flow in this example one embolus has lodged in the right middle cerebral artery resulting in a stroke as noted on this CT scan the dark oval shape indicates the area of stroke another condition in which embody may occur from the heart to the brain is endocarditis endocarditis can occur when there is fungal or more commonly bacterial infection in the bloodstream a condition known as septicemia or sepsis as the organisms travel through the heart they attach themselves to the valve leaflets between the heart chambers the oxygen a nutrient rich environment found in the left side of the heart results in rapid growth of organisms which form clumps known as vegetations unpredictably the vegetations may detach from the valve leaflets the vegetations are then injected into the aorta where they may travel up into the arteries of the brain causing obstruction to blood flow and subsequent stroke you the final category of ischemic stroke which from look at is global loss of blood flow to the brain global blood flow loss may occur in any condition in the body in which there is a generalised loss of blood flow to all organs conditions which can lead to global blood flow loss include severe loss of blood volume as in hemorrhagic shock rhythm disturbances of the heart such as ventricular fibrillation or cardiac arrest infectious states with blood borne infection also known as septicemia and narcotics overdose the typical brain injury that occurs with a global loss of blood flow is known as a watershed stroke this occurs let's take a look at the blood flow patterns in the brain here's a view of the brain in cross-section looking from above the arterial blood flow to the brain is such that there is a flow from the outer edge towards the center of each cerebral hemisphere as well as a flow from the inner edge towards the center of each cerebral hemisphere where the inner and outer flows meet is known as the watershed area it therefore follows that when there is a generalized loss of blood flow to the brain such as in narcotic overdose egde the watershed areas of each hemisphere are particularly susceptible to ischemia and stroke seen from above animation and in an MRI watershed stroke has a very characteristic appearance as you may recall from the beginning of the review the second major classification of stroke causes is hemorrhagic hemorrhagic causes account for 20% of all strokes hemorrhagic causes may be further subdivided into interest cerebral hemorrhage that is bleeding within the substance of the brain itself and subarachnoid hemorrhage or bleeding onto the surface of the brain the frequency of occurrence is equally split between the two different forms of hemorrhage amongst the causes of interest cerebral hemorrhage are uncontrolled high blood pressure also known as hypertension trauma bleeding disorders such as hemophilia vascular malformations such as aneurysms and arteriovenous malformations amyloid angiopathy which is an acquired disease resulting in weakness of blood vessel walls and drug abuse primarily involving the use of cocaine or amphetamines the mechanism of brain tissue damage in interest cerebral hemorrhage is very similar irrespective of the specific cause so for purposes of illustration today let's take a look at interest cerebral hemorrhage occurring as a result of an arteriovenous malformation an arteriovenous malformation represents a dramatic abnormality between the arterial or feeder side of the vascular system and the venous or drainage side of the vascular system as this segment demonstrates prior to interfacing with the venous side of the system arteries divide into smaller and smaller branches with the very smallest branches paying capillaries the size of the capillaries is such that red blood cells may only flow through them in single-file this branching structure from largest to smallest vessels results in a steady decrease in the blood pressure at each level until by the time one gets to the capillaries the blood pressure is only a fraction of that in the major arteries this reduction in pressure as vessels become smaller is critically important if capillaries were to be exposed to the much higher pressures present in the larger arteries they would immediately rupture on the venous side of the system capillaries interface with tiny venules which joined together to form larger and larger veins in an arteriovenous malformation this normal branching structure is absent instead we see a large feeder artery entering a nest or nidus of small abnormal arterioles on the venous side the normal branching structure is also absent with one large vein draining the nidus here is a view of the AVM as seen on angiogram the lack of a normal structure in the AVM results in extremely high flows and pressures across the abnormal small vessels of the nidus as a consequence AVMs have a high potential for rupture and hemorrhage of knidos vessels as mentioned previously interest cerebral hemorrhage takes place within the substance of the brain tissue itself the accumulation of blood also known as a hematoma continues to grow until the pressure surrounding it limits expansion or until it decompresses itself by rupturing into the cerebral spinal fluid system either into the ventricles of the brain or onto the surface of the brain as seen here on an MRI image the size of the hematoma can be very significant interest cerebral hemorrhage can have a devastating effect on brain function in affected individuals the hemorrhage occurs within the substance where billions of neurons are densely packed together as the hematoma grows twists and ruptures neurons along its path causing massive irreversible damage one neurosurgeon has likened the process to turning on a fire hose in a bowl of jell-o switching from the microscopic to the macroscopic level we see how the hematoma fragments brain tissue as it continues to expand you let's take a look now at the other common car hemorrhagic stroke subarachnoid hemorrhage unlike interest cerebral hemorrhage in which bleeding occurs within the brain itself subarachnoid hemorrhage occurs on the surface of the brain the most common cause of subarachnoid hemorrhage is rupture of an aneurysm 85% of aneurysms in the brain occur in the anterior circulation predominantly in the Circle of Willis the risk factors for both formation and rupture of aneurysms overlap and include cigarette smoking moderate to heavy use of alcohol high blood pressure genetic risk use of sympathomimetic drugs such as diet pills cold remedies and cocaine and amphetamine abuse estrogen deficiency particularly in menopausal females anticoagulation therapy and possibly the use of cholesterol-lowering medications known as statins as an example of subarachnoid hemorrhage we're going to look at a ruptured berry aneurysm occurring in the Circle of Willis to begin let's take a look at the anatomy of the Circle of Willis the Circle of Willis is best seen by examining the underside of the brain the Circle of Willis is really a beautiful piece of anatomy and represents a central hub from which radiates the entire blood flow to the brain in flow is primarily via the internal carotid and vertebral arteries outflow of blood from the circle occurs via anterior middle and posterior cerebral arteries each with left and right branches on either side of the circle the basilar posterior communicating and interior communicating arteries complete the circle by merging the inflow of blood to the brain from the carotid and vertebral arteries the chance is minimized at an obstruction or impairment and flow in any one of these arteries will result in a significant impairment of blood supply to the outflowing arteries now that we have a better understanding of the anatomy of the Circle of Willis let's take a look at how it contributes to subarachnoid hemorrhage in the brain while beautiful and design and function the Circle of Willis is also the location of 85% of all aneurysms within the brain one common sight for the occurrence of an aneurysm is the junction between the anterior communicating and interior cerebral arteries aneurisms form as a result of an acquired structural weakness in the blood vessel wall arteries which carry very high flows such as those in the Circle of Willis are thought to be particularly susceptible to mechanical damage in the vessel wall the previously mentioned contributory factors such as cigarette smoking or high blood pressure may accelerate the process of damage to the vessel wall the strength of the aneurysm wall is inversely proportional to its size lesions 1 centimeter or larger are particularly susceptible to sudden increase in growth and rupture with rupture blood spreads rapidly across the surface of the brain via the cerebral spinal fluid bleeding generally only lasts a few seconds however rebleeding may occur and is most likely within 24 hours of rupture and initial hemorrhage the most common cause of death and disability after subarachnoid hemorrhage is vasospasm extreme narrowing of the arteries with impairment of blood flow to brain tissue and subsequent stroke vasospasm usually occurs within the first three days after hemorrhage and generally Peaks in effect about one week out basal spasm occurs as a result of chemical irritation of blood vessel walls by breakdown products of hemorrhaged blood the duration of vasospasm and the location of the vessels affected determines the size and depth of subsequent strokes you

Channel: Dr. Cal Shipley, M.D.

Views: 601,913

Keywords: stroke, CVA, cerebrovascular accident, cerebral ischemia, cerebral thrombosis, cerebral embolism, intracerebral hemorrhage, subarachnoid hemorrhage, endocarditis, global ischemia, narcotics overdosage, watershed stroke, brain blood supply, circle of willis, blood supply to the brain, carotid arteries, basilar artery, vertebral arteries, anterior cerebral artery, posterior cerebral artery, middle cerebral artery, vascular anatomy of the brain, AVM, arteriovenous malformation

Id: uLJewzJcCZ0

Length: 27min 25sec (1645 seconds)

Published: Tue Jan 29 2013