 Devotees
of Bacchus have always believed that a corkscrew can solve
many of life's problems. But who'd have thought the tippler's
tool could save stroke victims?
A new clot-busting corkscrew device
— the loftily-titled Mechanical Embolus Removal
in Cerebral Ischemia (MERCI) Retriever — could
do just that. The Retriever, which draws blood clots
from brain tissue in the same way a corkscrew draws
a cork from a bottle, has just received regulatory approval
in Canada for treating ischemic strokes.
"Strokes are the number one cause
of acquired severe adult disability in the world," says
Dr Michael Hill, director of the University of Calgary
Stroke Unit and one of the investigators in the MERCI
trial. "As of 2015 we are going to see a huge number
of strokes as the baby boomers hit their seventies.
Without greater awareness and better tools, nursing
care facilities will be overflowing," he warns. "Ten
years down the line we are going to be thankful to have
more tools to help treat strokes."
STROKE
OF GENIUS?
The key to the Retriever is a corkscrew-shaped wire
filament made of a nitinol metal alloy that has a memory
for shape. When snuggled inside a microcatheter tube,
the filament stays straight — but when released,
it returns to its helical shape and snags blood clots.
The Retriever is inserted through
a larger delivery tube into the femoral artery in the
patient's groin and fed up to the carotid artery in
the neck. The inner microcatheter is then gingerly extended
through the blood clot to the other side of the blocked
blood vessel. Once the microcatheter is in place, a
balloon surrounding the outer delivery tube is inflated
to temporarily stop blood flow to the affected artery,
which could whisk pieces of the clot deeper into the
brain.
After sneaking past the clot, the
wire filament leaves the microcatheter and reverts to
its corkscrew shape. The physician then reels in the
corkscrew and dislodges the clot from the walls of the
blood vessel. A syringe sucks the blood clot into the
Retriever as the wire filament is retracted. The balloon
is deflated and normal blood flow resumes.
THE
DRUG ALTERNATIVE
The only other regulatory-approved treatment for ischemic
strokes is tissue plasminogen activator (t-PA), an intravenous
drug that dissolves blood clots. This treatment doesn't
work for patients who can't tolerate excessive bleeding
and it can only be used within three hours of stroke
onset. Unfortunately, less than 40% of patients get
to the hospital within three hours because they don't
recognize the symptoms.
The advantage of the MERCI Retriever
is that it can work within eight hours of symptom onset,
giving physicians more time to treat strokes. And there
are other advantages. "What about a stroke after surgery
or when a patient has suffered trauma or has cracked
their head?" says Dr Hill. "This device has niches that
allow us to extend our ability to treat strokes."
The Retriever re-established blood
flow in 53% of the 114 patients treated in the non-randomized,
multi-centre MERCI trial, sponsored by Concentric Medical
Inc of Mountain View, California, creator of the Retriever.
Device-related adverse events, including arterial perforation
and movement of the clot into other parts of the brain,
occurred in 3.5% of treated patients.
Although all this satisfied the
regulatory agencies' safety requirements, its effectiveness
as compared to t-PA treatment remains unproven. "The
proportion of patients with an excellent outcome from
this treatment was no different from those treated with
t-PA. We need to conduct a randomized trial to show
that it's a better choice," notes Dr Hill.
Future trials will show how the
MERCI Retriever can work with other therapies to increase
the options for stroke treatment.
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