The science of stroke

What is a stroke?

Stroke shatters the lives of 15 million people worldwide each year, killing five million and leaving five million survivors with a permanent disability. Stroke can rob survivors of their ability to see, hear, speak, eat, move independently, and recognise familiar faces and things. It can also affect personality and memory.

So what exactly happens to cause such devastating consequences? The simple answer is the death and damage of brain cells due to a restriction or blockage of blood flow to the brain. Without a sufficient blood supply, brain cells are quickly depleted of the oxygen and vital nutrients that they require to function and survive.

The hours following an initial stroke insult are unpredictable, and clinicians don’t have a crystal ball to foresee long-term outcomes. Will there be any loss of cognitive or motor function beyond that which is immediately apparent? Is there any risk of stroke recurrence in the days, years or months that follow? The only option is to watch, wait and hope.

Nonetheless, the locality of a stroke can determine just how and what area of a person’s functioning may be affected. For example, brain stem strokes are the most catastrophic as they can affect involuntary functions such as breathing and heart rate.

Cerebellum strokes affect balance and coordination, while strokes restricted to the right or left hemisphere of the brain can cause paralysis on opposing sides of the body.

What causes a stroke?

There are two major causes of stroke. The majority of strokes result from reduced blood flow to the brain due to a blocked or narrowed blood vessel, with the remainder resulting from a burst blood vessel or brain bleed.

Blocked artery – ischemic stroke

Stroke resulting from an obstruction or constriction in the blood vessels that supply oxygen to the brain is known as ischemic stroke. In ischemic stroke, blood supply can be restricted by either:

1. An atherosclerotic thrombus - a blood clot that forms at a fatty/cholesterol plaque on the blood vessel wall, restricting or completely blocking blood flow; or
2. An embolism – a travelling blood clot (usually broken away from a distant thrombus) that has become lodged in a blood vessel supplying the brain.

When blood flow to the brain is temporarily restricted, the result is a transient ischemic attack or mini-stroke, which can be an early warning sign that a more severe stroke is on its way.

Brain bleed - haemorrhagic stroke

In rare cases, stroke results when blood vessels break and blood leaks onto the brain surface or into brain tissue. This is called a haemorrhagic stroke. But what causes our blood vessels

to burst? The answer to this question is anything that puts stress on blood vessel walls, including cholesterol plaques and blood clots, chronic high blood pressure, malformations in blood vessels present from birth, or a thin, weakened area of a blood vessel that balloons out, called an aneurysm.

Haemorrhagic strokes are usually far more destructive and difficult to treat than ischemic strokes. Not only do they deprive the brain of oxygen due to blood loss, but the accumulation of blood puts pressure on the brain, and to make matters worse, the blood itself behaves like an irritant, further damaging surrounding blood vessels and brain cells.

What happens in the brain when a stroke happens?

The brain expends more energy than any other organ in our body, which is why it is so vulnerable to injury during stroke. When brain cells are starved of oxygen they cannot sufficiently process their primary energy source — glucose — leading to harmful changes in brain pH and electrolyte balance, as well as inflammation and oxidative. These changes cause brain cells to swell and rupture, releasing their contents and damaging healthy neighbouring brain cells.

Brain injury from stroke usually extends further than the immediately affected region, which is known as the ischemic core. This ischemic core is the area of the brain that is directly affected by disturbances in blood supply and where brain injury is instantaneous and often irreversible.

The area of brain tissue on the edge of this inner ischemic core is called the ischemic penumbra or peri-infarct zone. Brain cells in the penumbra remain viable for several hours following stroke, as they can still draw in oxygen and nutrients from nearby blood vessels. However, this supply of oxygen and nutrients is limited, and without hasty intervention to restore blood flow, cells in this outer zone will also die.

There is a narrow window of opportunity to restore blood supply and salvage cells in the penumbra. Currently, the only available treatment for ischemic stroke victims is intravenous delivery of tissue plasminogen activator (tPA), which acts to dissolve blood clots and restore blood flow. However, tPA is only effective when administered within around four hours of a stroke, and in some cases, its blood-thinning properties can cause a brain bleed or haemorrhagic stroke.

Promising new stroke treatments

Unfortunately, those inflicted by stroke are rarely left unscathed. Oxygen-starved brain cells die within minutes, and given the sudden and unexpected nature of most strokes, preventing this immediate damage is challenging. However, researchers are dedicated to finding new treatments that limit the spread of brain damage in the hours immediately following a stroke.

In a world first, Professor Glenn King and his team of researchers here at the Institute for Molecular Bioscience, University of Queensland, discovered a small protein called Hi1a from venom of the Australian funnel-web spider that can block the brain injury caused by stroke.

Remarkably, preclinical trials have shown that Hi1a administered up to 8 hours after a stroke can reduce brain damage by 65 per cent. Researchers hope that the administration of this class of molecules, known as ‘neuroprotectants’, will one day reduce the incidence of death, brain damage and permanent disability from stroke. For now, our best cure is prevention, so be sure to maintain a healthy diet, exercise regularly and keep your blood pressure under control.