A STROKE can happen seemingly without warning. As blood clots block circulation to large portions of the brain, tissue begins to die and you can lose your senses, motor skills and memories.
Fortunately, our brains don鈥檛 give up without a fight. An emergency back-up system kicks in, opening dormant bridges between blocked arteries and healthy ones that normally supply different areas of the brain. This enables blood to flow around the blockage, resupplying starved brain tissue with oxygen and glucose (see Diagram).
Doctors have known about this 鈥渃ollateral鈥� system for decades, but precisely how it works, and why some stroke patients seem to have better collateral circulation than others, is still a mystery.
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Now a small but growing group of scientists believe that better understanding collateral circulation could hold the key to unlocking new stroke treatments.
Most stroke research focuses on developing surgical devices that pry out clots, or on 鈥渃lot-busting鈥� drugs that dissolve blockages, but David Liebeskind of the University of California at Los Angeles believes that the ability of collateral circulation to quickly restore blood flow to oxygen-starved areas of the brain makes it critical to reducing stroke damage. 鈥淐ollaterals may even be more important to patient recovery than whether the blocked artery is reopened,鈥� says Liebeskind.
鈥淐ollateral circulation may be even more important to patient recovery than whether the blocked artery is reopened鈥�
In a preliminary study of 62 people who had a type of stroke called persistent arterial occlusion, Liebeskind found that the death rate for patients with naturally good collateral circulation was 41 per cent, versus 65 per cent for patients with poor collateral circulation. They also recovered better 鈥� about a third of those with good collateral circulation escaped with moderate to light disabilities compared with just 7 per cent of those with poor collateral circulation. 鈥淲e鈥檝e seen the most dramatic shift towards good recoveries I think than anything I鈥檝e seen,鈥� says Liebeskind, who presented his results at the International Stroke Conference in San Francisco earlier this month.
Researchers are now looking at ways of manipulating collateral circulation to quickly restore the blood supply to areas of the brain damaged by a stroke. One device in clinical trials at 40 US hospitals is an experimental catheter called NeuroFlo, which sits in the aorta, the main blood vessel supplying the lower half of the body. Inflating its balloons enhances collateral circulation in the brain by reducing blood flow to the legs.
In a pilot study of 29 patients who had experienced a stroke less than 10 hours beforehand 鈥渁bout 60 per cent had significant neurological improvement, at least for the 24 hours after the device was used鈥�, says Rick Schallhorn, vice-president of marketing for CoAxia, the company that produces NeuroFlo.
Despite these advances, the arteries involved in collateral flow display a number of traits that researchers still don鈥檛 fully understand. They鈥檝e found that vessels diverting blood flow are able to grow in size so they can accommodate more blood and make the system more robust, but the flow remains highly sensitive to changes in blood pressure, and can collapse without warning.
鈥淰essels diverting blood flow are able to grow in size so they can accommodate more blood and make the system more robust鈥�
鈥淐ollaterals can go for a while then go bad. We don鈥檛 know why exactly,鈥� says Max Wintermark of the University of California, San Francisco.
It is also not clear why collateral circulation seems to be dormant except after strokes, and why it is not more extensive in the first place, which would allow the constant exchange of blood between the different arteries that supply the brain. 鈥淥ne thing you would think is if the body was smarter, you鈥檇 have larger [bridges],鈥� Liebeskind says.
To try and solve these mysteries, Liebeskind is now working with William Dillon of the University of California, San Francisco, to combine modified brain imaging techniques to measure collateral flow in near real time.
Despite the apparent importance of collaterals in minimising stroke damage, Liebeskind says the field is still in its infancy. 鈥淚f you open up a textbook on stroke, you won鈥檛 find anything on collaterals 鈥� it鈥檚 a whole missing chapter,鈥� he says.
