Stroke

A condition where poor blood flow to the Brain causes cell death. There are two types of stroke (both of which cause the brain to stop functioning properly):

1. Ischemic - due to lack of blood flow
2. Hemorrhagic - due to bleeding

About 87% of strokes are ischemic, the rest being hemorrhagic.

Signs and Symptoms §

Stroke symptoms typically start suddenly, over seconds to minutes, and in most cases do not progress further. The symptoms depend on the area of the brain affected.

Signs and symptoms appear soon after the stroke has occurred, they include:

• Inability to move or feel on one side of the body
• Problems understanding or speaking
• Dizziness
• Loss of vision to one side
• A hemorrhagic stroke may also be associated with a severe headache

If the symptoms last less than two hours the stroke is a transient ischemic attack (a mini-stroke).

The symptoms of a stroke may be permanent as long term complications also include pneumonia and loss of bladder control.

Ischemic §

• Typically caused by a blockage of a blood vessel which initiates an Ischemic Cascade (though there exist less common causes)
• Blockage causes a limited blood flow to the brain which in turn causes cell death
• If detected within three to four hours, may be treatable with a medication that can break down the clot
• There are four reasons why this might happen:
  1. Thrombosis (obstruction of a blood vessel by a blood clot forming locally)
  2. Embolism (obstruction due to an embolus from elsewhere in the body),
  3. Systemic hypoperfusion (general decrease in blood supply, e.g., in shock)
  4. Cerebral venous sinus thrombosis.

Process §

Since blood vessels in the brain are now blocked, the brain becomes low in energy, and thus it resorts to using anaerobic metabolism within the region of brain tissue affected by ischemia.

Anaerobic metabolism releases lactic acid which is an irritant capable of (eventually) destroying cells.

As oxygen or glucose become depleted in Ischemic brain tissue, the production of high energy phosphates fails leading to failure of energy-dependent processes (such as ion pumping) necessary for tissue cell survival. (This sets off a series of interrelated events that result in cellular injury and death.)

A major cause of neuronal injury is the release of the excitatory neurotransmitter glutamate. gradients. As a result, the transmembrane ion gradients run down, and glutamate transporters reverse their direction, releasing glutamate into the extracellular space.

Glutamate acts on receptors in nerve cells (especially NMDA receptors), producing an influx of calcium which activates enzymes that digest the cells’ proteins, lipids, and nuclear material. Calcium influx can also lead to the failure of mitochondria, which can lead further toward energy depletion and may trigger cell death due to Programmed Cell Death.

Brain tissue is especially vulnerable to ischemia since it has little respiratory reserve and is completely dependent on aerobic metabolism, unlike most other organs.

Hemorrhagic §

• Typically caused by either bleeding directly into the brain or into the space between the brain’s membranes. Bleeding may occur due to a raptured brain aneurysm.
• There are two main types of hemorrhagic stroke:
  1. Intracerebral hemorrhage, which is basically bleeding within the brain itself (when an artery in the brain bursts, flooding the surrounding tissue with blood), due to either intraparenchymal hemorrhage (bleeding within the brain tissue) or intraventricular hemorrhage (bleeding within the brain’s ventricular system).
  2. Subarachnoid hemorrhage, which is basically bleeding that occurs outside of the brain tissue but still within the skull, and precisely between the arachnoid mater and pia mater (the delicate innermost layer of the three layers of the meninges that surround the brain).