Stroke: Etiology and Features

Below is an excerpt from, “Emerging Global Market for Neurointerventional Technologies in Stroke, 2014-2019.” Published Sept. 2015. Report #C310.

The adult brain weighs approximately 3 pounds and contains billions of nerve cells (neurons) and trillions of support cells (glia). It is composed of three parts: the cerebrum, the cerebellum, and the brain stem.

The cerebrum, the largest part of the human brain, is divided into left and right hemispheres that communicate with each other through a bundle of nerve fibers called the corpus callosum. The hemispheres are covered by a 2mm- to 6mm-thick layer of gray matter (composed of neurons) known as the cerebral cortex, which is responsible for the higher functions of language, perception, reasoning, thought, and voluntary movement. The inner portion of the cerebrum is white matter, which is composed of neural “processors” that carry information between neurons in the brain and spinal cord. The cerebral cortex consists of many folds that allow a large surface area to fit into a small volume. These folds divide the cerebral cortex into four lobes, which perform various specific functions, although no region of the brain functions alone.

The cerebellum is also divided into hemispheres and has a cortex; this portion of the brain plays an important role in balance, fine motor coordination, posture, and voluntary movement.

The brain stem, the smallest part of the brain, connects the cerebral hemispheres with the spinal cord. It contains the medulla oblongata, the pons, and the reticular activating system, structures that are responsible for basic life functions such as initiating and maintaining wakefulness; regulating blood pressure, breathing, and heart rate; and reflex actions such as coughing, sneezing, swallowing, and vomiting.

Other structures located in the brain include the basal ganglia, which consists of a number of components including two that are important in coordinating movement, called the globus pallidus and the substantia nigra; the hypothalamus, which is important in controlling body temperature, circadian rhythm, metabolism of fats and sugar, and water balance; the limbic system, a group of structures important for learning and memory as well as for controlling emotional behavior; the pituitary gland, which secretes many hormones that travel to, and regulate the function of, other organs and glands; and the thalamus, which contains nuclei that receive sensory input from spinal and brainstem circuits and process information from auditory, pain, somatic sensory, taste, thermal, and visual modalities.

Although the brain makes up only approximately 2% of body weight in humans, it receives between 15% and 20% of the body’s blood supply. The blood vessels, which enter the brain through holes in the skull called foramina, bring oxygen, nutrients, and hormones into the brain and carry away carbon dioxide and other waste products.

Häggström, Mikael. “Medical gallery of Mikael Häggström 2014”. Wikiversity Journal of Medicine 1 (2). DOI:10.15347/wjm/2014.008. ISSN 20018762.

The brain is supplied with blood by two pairs of major arteries called the internal carotid and vertebral arteries. The vertebral arteries meet at the base of the brain to form the basilar artery, which joins the internal carotid arteries to form the Circle of Willis; this ring structure is a safety mechanism that helps ensure blood flow to the brain if one of the arteries leading into it becomes blocked. Every minute, 600mL to 700mL of blood flows through the carotid arteries and their branches, while 100mL to 200mL flows through the vertebral-basilar system.

The internal carotid arteries and their branches, which are an integral part of the carotid system, supply the anterior two-thirds of the cerebral hemispheres, including the deep white matter and the basal ganglia. The vertebral arteries and basilar artery and their branches supply the remaining posterior and medial regions of the hemispheres, the brain stem, the cerebellum, and cervical spinal cord, and most of the diencephalon.

The carotid and vertebral-basilar systems are connected through the Circle of Willis; however, these connections may not be functionally significant if the connecting vessels are of small diameter, or if pressure gradients are too small to drive adequate blood flow.

The right common carotid artery originates from the bifurcation of the brachiocephalic trunk; the left common carotid artery originates directly from the aortic arch. Each common carotid artery then branches to form the internal and external carotid arteries. After the internal carotid artery ascends through the neck, traverses the temporal bone, and passes through the cavernous sinus, it reaches the subarachnoid space at the base of the brain. As the internal carotid artery leaves the cavernous sinus, it forms its first intracranial branch, the ophthalmic artery, which travels along the optic nerve into the orbit. There, its branches supply the retina and other structures of the eyeball, as well as other structures in and around the orbit. The internal carotid artery continues in a superior direction and usually branches into the posterior communicating and anterior choroidal arteries.

The posterior communicating arteries range from large to threadlike, and link the internal carotid vessel to the posterior cerebral arteries; however, in some individuals, one or both of the posterior cerebral arteries retain their embryological state as direct branches of the internal carotid artery itself. The anterior choroidal artery also varies greatly in size and importance in different individuals, and may branch from the middle cerebral arteries rather than the internal carotid artery.

The internal carotid artery also divides to produce the anterior and middle cerebral arteries. Atheromatous plaque tends to form at the branches and curves of the cerebral arteries. In carotid circulation, the most frequent sites for plaque formation are in the internal carotid artery at its origin from the common carotid artery, in the stem of the middle cerebral artery or its bifurcation into superior and inferior divisions, and in the anterior cerebral artery as it curves over the corpus callosum.

The vertebral arteries typically arise from the subclavian arteries and course through the cervical transverse foramina, run medially, and ascend into the foramen magnum, where they pierce the dura mater and enter the cranial cavity. The vertebral arteries run alongside the medulla oblongata and give rise to vessels that participate in supplying the spinal cord as well as the brain stem.

Cardiogenic emboli tend to enter the vertebral circulation far less frequently than they enter the carotid circulation. Several aspects of vascular anatomy may account for this tendency. Each vertebral artery arises from the subclavian at a sharp angle and has a much smaller diameter. By contrast, the internal carotid artery is approximately the same diameter as the common carotid artery, and makes only a slight bend at its origin. In addition, the vertebral-basilar system handles a much smaller percentage of the total cerebral blood flow than the carotid system.

Acute stroke – also known as “cerebrovascular accident” – represents a catastrophic manifestation of accumulated circulatory disorders that affect the vasculature of the brain described above. The two major subdivision of stroke are ischemia or lack of blood and oxygen supply typically resulting from occlusion of cerebral arteries, and hemorrhage or leakage of blood outside the normal cerebral vessel conduit. Both types of stroke cause necrosis of certain groups of brain cells, which leads to irreversible impairment of various neurological functions in about 22% to 25% of patients and death within one year in another 35% of stroke caseloads.

According to available clinical data, ischemic stroke accounts for approximately 87% of all stroke cases in the U.S., 85% in largest European states and up to 80% in major Asian-Pacific states and the rest of the world, with the remaining 13%, 15%, and 20% represented by episodes of hemorrhagic stroke. These two major groups or categories of stroke are further differentiated by the location of the brain hemorrhage and ischemia.

Clinical Features and Characteristics of Ischemic Stroke

 

Clinical Features and Characteristics of Hemorrhagic Stroke


“Emerging Global Market for Neurointerventional Technologies in Stroke, 2014-2019.”
Report #C310. 194 Pages. 34 Exhibits. Published September 2015