Exemplar of  damage to brain (motor cortex) or brain stem:  Traumatic Brain Injury (TBI)

Traumatic brain injury (TBI) is a leading cause of disability and death worldwide, and has a lasting impact on patients and those who care for them. TBIs occur on a spectrum, and their severity can range anywhere from a mild concussion to a severe TBI with lifelong consequences to a person’s function. In Canada, there are approximately 20 000 hospitalizations for TBI every year. TBI commonly arises from motor vehicle accidents, sports, and falls, but can occur anytime there is risk of trauma to the head. Immediate symptoms of a TBI may include nausea, dizziness, memory loss (also called amnesia), and headache. In more severe TBIs, long term symptoms may include altered cognitive and behavioral deficit. Some evidence suggests that severe TBIs may even contribute to the development of neurodegenerative diseases such as Alzheimer’s and Parkinson’s diseases.

Primary and Secondary Injury in TBI

The progression of a TBI is often divided into two stages: the immediate primary injury and the delayed secondary injury. The primary injury refers to the immediate damage caused to the brain by the traumatic event. In a “closed head” injury, the skull remains intact and injury occurs due to linear or rotational acceleration of the brain colliding with the inner surface of the skull. An example of this would be a head strike from an accident or fall.  In a “penetrating” injury an object is embedded into the skull, meninges, or brain tissue. A bullet embedded in the skull as a result of a gunshot wound is an example of a penetrating injury. Damage from a penetrating injury can come as a result of the delicate brain tissue becoming exposed to the external environment.

The secondary injury includes the long-term and slowly progressing changes that occur in the brain as results of the trauma. It is thought that the progression of the secondary injury is the most critical contributing factor to the patient’s recovery.

Pathophysiology of TBI

We certainly don’t know all of the pathophysiologic changes that occur during TBI, and those we have studied are not yet fully understood. The following table outlines some of the possible changes that occur in the brain and it’s surrounding structures, but understand that this list is incomplete and more research is required on this topic.

Table 12-4 Pathophysiological mechanisms of brain tissue is injured

Figure 12.50. Depiction of the meningeal layers and various types of hemorrhages and hematomas. Notice that the hematomas are named based off of their location within the meninges; a subdural hematoma is located below the dura mater, for example. Used and adapted under creative commons license, original author MyUpchar.

Figure 12.51. Some of the proposed pathophysiologic mechanisms of primary and secondary traumatic brain injury (TBI). Used and adapted under creative commons license, original author https://doi.org/10.3389/fendo.2021.634415

Exemplar of damage to brain (motor cortex) or brain stem:  Ischemic Stroke

A stroke is a life threatening health emergency that occurs when blood flow to a region of the brain stops, resulting in neuron cell damage and death. Strokes are one of the leading causes of death in Canada; in the year 2019 the incidence of stroke was estimated to be 280 per 100 000 adults aged 20 years and older. There are two broad types of strokes: ischemic stroke and hemorrhagic stroke. Ischemic stroke, which is the focus of this chapter, occurs when a blood vessel within the brain is suddenly blocked, interrupting blood flow. Hemorrhagic stroke, which is comparatively rarer and occurs in approximately 20% of acute strokes, is caused by damage to blood vessels within the brain that leads to significant bleeding into the brain tissue.

Blood Circulation in the CNS

To understand the consequences of a stroke, it’s important to understand the anatomy of the circulatory system within the CNS. Make sure you review the information in Circulation in the CNS.   There are three main arteries that supply the cerebral cortex with blood: the anterior, middle, and posterior cerebral arteries. We can use our knowledge of the function of different brain areas in combination with the perfusion area (the area of brain that is supplied with blood) of an artery to work through what the symptoms of an ischemic stroke may be. For example, as Figure 12.52 demonstrates below, the pre-central gyrus (which contains the motor cortex) is supplied with blood by the anterior and middle cerebral arteries. An ischemic stroke affecting the right middle or anterior cerebral artery will result in motor deficits to the left side of the body (recall what we learned in chapter XYZ: the motor neurons cross over the innervate muscles on the other side of the body).

Figure 12.52. Depiction of the perfusion areas of the three arteries that supply the cerebral cortex with blood; the posterior, middle, and anterior cerebral arteries. Used under creative commons license, original authors Frank Gaillard. Patrick J. Lynch, retrieved from Arterial Supply of the Brain | Circle of Willis | Geeky Medics

An important anatomical feature of the circulatory system with respect to strokes is a structure called the Circle of Willis (Figure 12.24). This structure is an anastomosis (connection) of arteries within the CNS that functions to create redundancy within the circulatory system. Essentially, if one artery within the circle becomes blocked, blood flow from other vessels within the circle can accommodate to  maintain blood flow and prevent significant ischemia from occurring. This important anatomical feature can prevent significant brain tissue damage in the case of an ischemic stroke.

Figure 12.24. The circulation of the CNS viewed from the base of the brain. Note the connection of the anterior cerebral arteries, anterior communicating arteries, internal carotid arteries, middle cerebral arteries, posterior communicating arteries, and posterior cerebral arteries (top to bottom). These form an important structure called the Circle of Willis.

Causes of Ischemic Stroke

Now that we’ve learned about what an ischemic stroke is, you might be wondering what are the causes of stroke- that is, what actually blocks the blood vessel? We’ll briefly discuss two common mechanisms that can cause an ischemic stroke- cardioembolism and large vessel disease.

Cardioembolism

An embolus is any material that may become lodged and cause a blockage within a blood vessel, called an embolism. Embolisms can be caused by many materials, including fat and air bubbles. When an embolism is caused by a blood clot, is it called a thrombus. In the case of a cardioembolism, the embolus originates in the heart, as suggested by its name. The embolus may travel into the circulation within the CNS, where it can become lodged in a vessel and interrupt blood flow, causing an ischemic stroke. Cardioembolism accounts for approximately 20% of ischemic strokes.

Large Vessel Disease

In large vessel disease, the blockage originates in large vessels such as the internal carotid arteries, aorta, vertebral, or basilar arteries. The most common cause is atherosclerosis, where fatty plaques of cholesterol and other lipids cause narrowing of the vessel.  Over time, plaques may rupture and travel to the CNS circulation, causing a stroke. Atherosclerotic plaques are generally caused by diseases of abnormal lipid levels, such as dyslipidemia (high or abnormal levels of blood lipids) or hypercholesterolemia (high blood cholesterol).

Refer to our previous chapter on atherosclerosis to learn about how these fatty plaques originate in the heart, rupture, and then travel to the brain Atherosclerosis and Angina.