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    Friday, November 19, 2004

    Clot Busters: Combining ultrasound waves with clot-dissolving chemicals appears to improve stroke therapy:

    Dr. Maher Saqqur of the University of Alberta and his colleagues tested an experimental combination of TPA and ultrasound to treat stroke before brain tissues are starved of a blood supply.

    "What we find is that patients who receive the TPA plus the ultrasound do well compared to patients who get the TPA just by themselves," said Saqqur.

    The study looked at 126 patients. After three months, 42 per cent of patients who received the experimental treatment were fully recovered, compared to 30 per cent who had TPA alone.

    The study was limited to people who had clots in the middle cerebral artery, which is the only kind of stroke in which t-PA is recommended ( and even then, not without some controversy). The study did show improvement in outcome by combining ultrasound with t-PA compared to just using t-PA. But there's another question that needs to be asked. How would ultrasound alone compare to t-PA alone? A doctor in Scotland says he's seen it work wonders:

     A SCOTTISH doctor has accidentally found a remarkable treatment to ease the crippling effects of a stroke.

    Dr Paul Syme stumbled across the technique during routine examinations of stroke victims at Borders General Hospital in Melrose.

    He realised that ultrasound equipment used to pinpoint strokes could also help patients make a recovery.

    Dr Syme has used the technique, which sends sound waves through the body, to ease the symptoms of nearly 100 patients.

    He is now confident it could help tens of thousands of sufferers in Scotland alone.

    Speaking on BBC Radio Scotland’s Guinea Pigs programme, Dr Syme said: "This is a very exciting discovery, because we are encouraging the body to heal itself."

    He obtained an ultrasound machine, called a transcranial doppler, to help him locate the source of strokes. To his surprise, the devastating physical symptoms appeared to improve while patients used the machine and the effects lasted long enough to allow sufferers to lead more normal lives again.

    He's planning to collaborate with a couple of stroke centers in the UK to study it. Should be interesting. And it may be safer than using it with t-PA, which has the risk of devastating intracranial bleeding.

    How does this work, anyway? An accompanying article in the New England Journal of Medicine offers some theories:

    Ultrasound of sufficient amplitude, when applied to a fluid, causes the partly dissolved gases to form small bubbles. These bubbles then vibrate, absorbing the energy, and if enough energy is applied, they literally explode.

    ...However, this mechanism is unlikely to be responsible for the effect observed in the study by Alexandrov et al., because transcutaneous ultrasound devices are preset to prevent excessive deposition of ultrasound energy. Still, it is theoretically possible, although it is unlikely, that some gaseous bubbles trapped in the thrombus may be the right size and composition to meet the threshold for the onset of cavitation. The local effect would be to create gaps or symmetric "holes" in the fibrin mesh, thereby facilitating the permeation of t-PA into the thrombus.

    Other effects depend on the level of ultrasound energy applied. At very low energies, ultrasound has been shown to promote the motion of fluid, an effect called microstreaming. It is possible that the application of ultrasound energy agitates the blood close to the occluding thrombus and promotes the mixing of t-PA, effectively increasing the concentration of the agent that is in contact with the thrombus. The pressure waves that are generated may also increase the permeation of t-PA into the interior of the fibrin network. This phenomenon, however, is unlikely to explain all the beneficial results observed in this study. At slightly higher energies, ultrasound waves can have direct effects on the binding of t-PA to the fibrin mesh that forms the occlusive lesion. The binding of t-PA to the cross-linked fibrin and fibrin elements within a matrix is enhanced, in vitro, by ultrasound energy, and the fibrin cross-links are weakened, further increasing the binding of t-PA. These two mechanisms probably play key roles in vivo.

    Some authors have speculated that the heat generated by ultrasound is responsible for accelerating thrombolysis. Experiments have confirmed that the temperature elevation generated by ultrasound of sufficient power can increase the dissolution rate of thrombi.

    But, no one knows for sure.

    UPDATE: Journal Club has more details of the NEJM study.


    posted by Sydney on 11/19/2004 06:40:00 AM 0 comments


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