A stroke occurs when the supply of blood to a part of the brain is blocked or a blood vessel bursts, causing bleeding in the brain. The former kind of stroke is also known as an ischemic stroke, and the latter as a brain hemorrhage. Both kinds of stroke result in decreased oxygen to a brain area, which causes the neurons in that area to malfunction or die. While all strokes can give rise to a depressed level of consciousness via increased intracranial pressure, the main kinds of strokes causing this particular condition are hemorrhages in the cerebral cortex. Strokes in the brain stem are likely to lead to a complete loss of consciousness.
Consciousness
As pointed out by NYU consciousness expert David J. Chalmers, there are two kinds of consciousness: state consciousness and creature consciousness. State consciousness is consciousness that pertains to a particular state, for example, a visual state or a belief state. Creature consciousness is consciousness that pertains to the organism as a whole. Blind people do not normally have any conscious visual states, but they are still conscious in the creature sense. We ordinarily lose consciousness in this latter sense only when we are in a dreamless state of sleep.
Strokes and the Loss of Consciousness
Strokes typically are associated with a reduced number of types of conscious states. For example, strokes in the temporal lobes on the sides of the brain can give rise to visual shape agnosia. As David Milner and Melvyn Goodale, authors of "The Visual Brain in Action," point out, visual agnosia patients cannot consciously recognize the shapes of objects through vision. However, the phrase "depressed levels of consciousness" is more commonly used to refer to the temporarily reduced levels of creature consciousness that can accompany strokes in the cerebral cortex.
Strokes in the Brain Stem
As the brain stem maintains a number of basic functions, including the regulation of sleep, heartbeat and breathing, a stroke in the brain stem can be deadly. However, it can also result in a persistent vegetative state, or a condition known as locked-in syndrome. People in a persistent vegetative state may laugh and cry, but they have no conscious sensations. In locked-in syndrome, motor cells in the lower brain stem and the spinal cord are disconnected, leaving the patient completely or almost completely mute and paralyzed but fully conscious.
Hemorrhage in the Cerebral Cortex
High blood pressure can cause arteries in the cerebral cortex to break, causing a hemorrhage. When blood leaks into the surrounding tissue, oxygen supplies are cut off, and pressure on the brain occurs. Neurons begin to die after a few minutes without oxygen. The increased pressure on the brain can cause a temporarily or permanently depressed state of consciousness. Normal conscious brainwaves lie in the 18 to 40 Hertz frequency range. Reduced conscious states have lower frequencies. Low frequency brainwaves often occur with increased pressure on the brain, but the altered state is normally reversed after stabilization.
Treatments
Temporarily depressed levels of consciousness require ventilatory assistance and airway support. Other acute forms of treatment may include surgery and administering blood-thinning medications for clots, dopamine for hypotension, Valium for convulsions and adrenaline for cardiac arrest. After stabilization, the patient is monitored for recurrent strokes or accompanying seizures and then sent to a rehabilitation facility, which aims at training new areas of the brain to perform the lost functions.
Prevention
Stroke patients with high levels of iron in their bodies are more likely to develop severe neurological damage and chronically depressed levels of consciousness, reports a Spanish research team in the April 25, 2000 issue of "Neurology." Iron levels increase as people get older, and a small fraction of the population are particularly prone to elevated iron levels. As a preventive measure, the researchers recommend that people prone to cardiovascular disease stay on a diet low in iron and cholesterol and undergo tests of these levels as part of their routine check-ups. Other preventive measures still being investigated consist of lowering blood pressure with beta-blockers, which block direct adrenaline attacks on the heart.
References
- "Neural Correlates of Consciousness:Empirical and Conceptual Questions"; What Is a Neural Correlate of Consciousness?; D. J. Chalmers; September 2000
- "The Visual Brain in Action"; D. Milner and M. Goodale; 2006
- "European Journal of Neurology"; Differences in Brain Metabolism Between Patients in Coma, Vegetative State, Minimally Conscious State and Locked-in Syndrome; Laureys, et al.; vol. 10, 2003
- "Hypertension"; Blood Pressure Lowering for the Secondary Prevention of Myocardial Infarction and Stroke; MacMahon, et al.; vol. 29, 1997
- "Neurology"; Body Iron Stores and Early Neurologic Deterioration in Acute Cerebral Infarction ; Davalos, et al.; April 25, 2000
