Oct 16, 2011

Tech Talk – Stroke Treatment with Medical Devices

I thought I’d move into a newer area for medical devices, stroke treatment.  Stroke affects more than 700,000 people a year in the US alone, of these, over 150,000 die.  Most of the strokes are ischemic in nature.  Unfortunately the treatment options are very limited and time to treatment is absolutely critical to a good outcome.  Successful recanalization of the occluded cerebral vessel during the acute ischemic event is associated with lower three month mortality and improved functional outcome.  [Source]

Intravenous Tissue Plasminogen Activator (tPA) is the FDA approved drug for acute ischemic stroke for up to three hours after the stroke.  This drug can dissolve the clot and is sometimes applied right at the clot through a catheter.  This is generally the first form of treatment; however, tPA is not effective in all cases and can cause bleeding in the brain, particularly in older patients. 

Mechanical removal of the clot using a medical device is being performed more and more alone or in conjunction with tPA.  These devices have some advanatges over tPA, including more rapid achievement, ability to treat large vessels, and lower risk of hemorrhagic events.  Only two neurothrombectomy devices are currently cleared for use in the US.  You don’t have to be a rocket science to figure this one out- 700,000 people affected and two cleared devices, stroke treatment is a screaming opportunity for medical devices.

The FDA defines these devices as neurothrombectomy devices, these are devices intended to retrieve or destroy blood clots in the cerebral neurovasculature by mechanical, laser, ultrasound technologies, or combination of technologies.  The FDA has provided guidance on these devices, one note of interest is that a clinical trial must be performed due to the high risk of the device.

There are some general procedural steps common to both devices that I will cover quickly now.  Both devices must have access to the clot itself.  This means advancing a guide wire and catheter using angiography to the clot first. Don’t tell cardiologists, but this is more difficult in the head than in the heart- there are many more possible pathways and the vessels are generally smaller and more easily damaged.  If the patient has been given tPA any damage can be catastrophic.  Angiography is also used to measure the vessel diameter so the appropriate sized device can be chosen.

Another procedure common to both devices in certain situations is the use of a balloon guide catheter with aspiration.  The balloon guide catheter is inflated, which blocks blood flow in the blood vessel with the clot.  Aspiration is then applied, this means taking a large syringe (typically 60 ml) and pulling it back, sucking whatever you can out of the blood vessel, alternatively you can buy a pump to do this.  Minimizing balloon inflation time is important because the lack of blood flow is what causes a stroke, you don’t want to compound the problem.

The two approved devices are shown below:
                                            Merci Retreiver (left) and Penumbra System (right) Image source.


The first FDA approved neurothrombectomy device was the Merci Retriever by Concentric Medical.  The device was approved through the FDA 510(k) process in 2004, the current indication for use is:
Merci Retrievers are intended to restore blood flow in the neurovasculature by removing thrombus in patients experiencing ischemic stroke. Patients who are ineligible for intravenous tissue plasminogen activator (IV t-PA) or who fail IV t-PA therapy are candidates for treatment. Merci Retrievers are also indicated for use in the retrieval of foreign bodies misplaced during interventional radiological procedures in the neuro, peripheral and coronary vasculature.
The Merci Retriever system includes a flexible nitinol wire coil formed into what looks like a corkscrew.  The latest version of the device has filaments (made of suture material – I would guess nylon) that provide an additional mechanism for securing the clot during removal.

Basically, the device is advanced distal to the clot, deployed, turned, and when pulled back through the clot it captures the clot in the corkscrew and the device is then removed from the artery while under balloon aspiration.  The balloon aspiration (pulling a vacuum on the vessel while a balloon blocks it) minimizes pieces breaking off from the clot and causing additional issues.  [source]  You can watch a demonstration of the device here

Concentric Medical was recently bought by Stryker for $135million, which goes along with Stryker's previous purchase of Boston Scientific’s neurovascular division.  I don’t know what Concentric’s revenue was, but I think this sounds like a good acquisition, with the caveat that the Concentric team must remain focused on Stroke treatment and not get caught up in all the other things Stryker does.

The second device in use is the Penumbra System of Continuous Aspiration Thrombectomy by Penumbra. The Penumbra System is used for the “revascularization of patients with acute ischemic stroke secondary to intracranial large vessel occlusive disease…within 8 hours of symptom onset”.  The device is first advanced to the blood clot, the Penumbra Catheter’s tip is then placed at the proximal end of the clot.  The Penumbra “separator” is advanced to the clot, aspiration is started, then the separator is used to help to break up the clot (or “debulking”) and make it easier to suck into the guide catheter.  The separator has a straight tip and a cone (purple cone shown in the picture).  In theory, there should be less damage to the vessel with this system, this is important if the patient has received tPA and that has not worked. 

As reported in a State of the Evidence article, the clinical effectiveness of the devices, defined as the having a good outcome (modified Rankin Scale score 0 to 2), rates ranged from 21 to 36% with the MERCI and 20 to 48% with the Penumbra System.  These numbers are not necessarily comparable to each other as the devices can be used for different types of clot.  Presumably the Merci retriever is typically used for “hard” clots and the Penumbra system is used for softer clots (This is just my guess).

Other types of devices sometimes used off-label in the US, such as snares, exist, but I would expect their use will decline as more devices get approval for stroke treatment.  Additionally, stent retrievers are available in the EU, but not yet approved for the US, I assume these devices will be approved in the next year or so and if you were so inclined you could roadmap out which ones are doing well in the EU and make an investment on that.  The EU is currently two or so years ahead in types of devices available for stroke treatment.  Other devices are certainly under development, with ideas from coronary or peripheral vascular being expanded for use.  Startups include Insera Therapeutics who is developing a snare type device.   I wasn’t able to identify any more in a few minutes of Google searching- if you know of any, leave a comment and I’ll add them later.

Update:  In 2012 two more stroke treatment devices were approved by the FDA, thee Solitaire FR by Covidien and the Trevo by Stryker.  Clinical trials for both of these devices showed that they were superior to the Merci Retriever.  

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