New Paths in Endovascular Surgery
The development of endovascular surgery over the past
two decades has impacted methods for the management
of cerebrovascular disease dramatically. Endovascular
surgery is in its infancy and its potential for the
future is great. This paper describes our perspective
of the future of endovascular surgery in the management
of aneurysms.
Management of intracranial aneurysms
Endovascular techniques for the treatment of intracranial
aneurysms have evolved in the past two decades. Serbinenko
reported his experience with endovascular techniques
in 1979,(49) when he described embolization with intravascular
balloons. Balloon embolization became the endovascular
procedure of choice in the 1980s.(2,18-20,22,43,58)
However, it was not ideally suited to selective occlusion
of the aneurysm and preservation of the patency of the
parent artery. Although it is sometimes possible to
inflate a detachable balloon within the aneurysm while
preserving flow through the parent artery, the disadvantage
of this technique is that the size and shape of the
balloon may not conform to that of the aneurysm, resulting
in stretching of the aneurysm wall or incomplete filling
of the aneurysm. The inability to customize a balloon
to the configuration of an aneurysm led to the development
of coil systems for aneurysm embolization.
Coil treatment permits conformation of the coil mass
to the shape of the aneurysm, representing a significant
improvement over balloon embolization. Initially, pushable
coils were used for treatment of cerebrovascular lesions.(6,17)
The major disadvantage of this system was the inability
to remove coils that did not assume a favorable position
or configuration within the aneurysm.
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| 3D
platinum coil. |
This problem was addressed with the introduction of
mechanically detachable (24,36) and electrolytically
detachable(13-16) coils. First described by Guglielmi,
et al.(14,16) for the experimental treatment of cerebrovascular
lesions, electrolytically detachable coils were favored
by clinical interventionists because of concerns about
the forces applied within the aneurysm when detaching
mechanically detachable coils. The Guglielmi Detachable
Coil (GDC) design combines the advantages of soft compliant
platinum with retrievability (a coil can be withdrawn,
repositioned, or replaced before detachment), and atraumatic
detachment.
Subsequent to the approval of the GDC (GDC, Boston
Scientific/Target Therapeutics, Fremont, CA) by the
FDA in 1995, there has been a trend toward the preferential
use of endovascular therapy for the treatment of intracranial
aneurysms. Early series reported use of GDC embolization
solely for high-risk surgical cases (i.e., for patients
of poor clinical grade or those with aneurysms deemed
inoperable).(3,4,11,46,50,53) Since that time, however,
many centers have begun using endovascular treatment
as first-line therapy for intracranial aneurysms.(12,30,50)
Limitations
Evidence of the efficacy of endovascular treatment
for patients with subarachnoid hemorrhage presenting
in poor clinical condition(37) prompted some centers
to adopt a policy of reserving clip ligation for aneurysms
in patients felt to be at high risk for complications
from coil embolization. At these centers, the anatomy
of the aneurysm is evaluated with consideration for
the ability to fill the aneurysm with coils without
compromising the parent artery lumen. Favorable aneurysm
anatomy includes a dome-to-neck ratio of greater than
2 mm and a small aneurysm neck diameter, usually less
than 5 mm.(9) In addition, aneurysm location may be
a factor involved in treatment decisions. There have
been lower rates of technical success for coil embolization
for middle cerebral artery aneurysms.(45) The size of
the aneurysm dome and neck influences both the ability
to occlude the aneurysm with coils and the rate of subsequent
regrowth of the coil-treated aneurysm.(54) The presence
of a large intraparenchymal hematoma with mass effect
may favor a decision to perform open surgery to reduce
intracranial pressure. Conversely, evidence of significant
brain swelling without a mass lesion may increase the
risk of surgical retraction, resulting in reduction
in local blood flow and ischemic injury.(57) The overall
trend has been to consider endovascular treatment first,
reserving surgical therapy only for aneurysms with unfavorable
geometry or other clear surgical indications, such as
intraparenchymal hematoma.
Stent-assisted GDCs
 |
| Depiction
of stent across the neck of aneurysm. |
One of the major shortcomings of endovascular therapy,
despite the widespread enthusiasm for its indications,
was the inability to treat wide-necked aneurysms adequately.
The propensity for coil herniation and parent vessel
compromise made complete filling of the aneurysm nearly
impossible and coil compaction or aneurysm regrowth
a significant concern. In recent years, however, researchers
have described the treatment of wide-necked aneurysms
with stent-assisted coiling in experimental models.(10,52)
Hemodynamic evaluation in experimental aneurysm models
demonstrated significant flow alterations within the
aneurysm sac after stent placement across the ostium.(1,31,32,48,56)
Flow models may be used to quantify flow velocity and
bulk flow within the aneurysm or the parent vessel.
Studies using these models demonstrated that the greatest
shear stress occurs at the distal aneurysm neck, a common
site for aneurysm regrowth after coiling. Studies also
showed that bulk flow may be reduced after parent artery
stenting to as little as 5% of the pre-stenting baseline.56
Clinical application of these findings followed soon
afterward.(21,23,29,34,38,55)
 |
| Depiction
of stent and coil technique. |
Successful deployment of a stent across the aneurysm
ostium permits reconstruction of the parent vessel lumen
and protects against coil prolapse. With this protection
in place, coils may be packed more tightly within the
aneurysm without fear of parent vessel compromise, thereby
reducing the risk for residual aneurysm or aneurysm
regrowth.
One concern about stent-assisted coiling is the fate
of normal arterial branches after stents are placed
across them. As most intracranial aneurysms occur at
vessel bifurcations or branch points, a normal branch
often arises near the aneurysm and may be difficult
to avoid during stent positioning. In our experience,
however, these branches do not become occluded after
stenting. In a review of 10 patients with 10 branch
arteries crossed by stents and angiographic follow-up
ranging from four days to 35 months (average followup,
10 months), the branches remained angiographically patent,
and no patient experienced an associated clinical ischemic
episode.(33) Flow demand for angiographically demonstrable
branch arteries is sufficient to maintain patency after
stents are placed across them. Another concern about
stent-assisted coiling is the ability to deliver a stent
to the target site through the tortuous cerebral vessels.
Stent delivery devices tend to be rigid, more rigid
than balloon catheters without stents and much more
rigid than microcatheters. Accordingly, it may be quite
difficult to navigate a tortuous carotid siphon or cervical
vertebral artery loop. In our series of intracranial
stenting for all indications, we were able to deploy
a stent at the target site in 19 of 27 attempts.(47)
Unsuccessful attempts were related to inability to access
the proximal vessel from a highly tortuous arch (one
case), arterial injury (two cases), and inability to
navigate the stent into position (five cases). In the
difficult navigation cases, the loop of artery that
was proximal to the target lesion tended to have a small
radius of curvature, in comparison with those cases
in which stent deployment was successful. Therefore,
when we are considering treatment by intracranial stenting,
we give more consideration to proximal vascular anatomy
than to the presence of normal branches that may be
compromised during stent positioning.
Potential for aneurysm regrowth
Perhaps the most significant shortcoming of endovascular
treatment for intracranial aneurysms is the potential
for regrowth or recurrence of an aneurysm after coiling.
Large series have reported angiographic recurrence or
growth of a remnant after coiling in 10 - 15% of cases.(5,7,28)
Although these same series report a low rate of recurrent
hemorrhage for ruptured aneurysms treated with coiling,
and no cases of hemorrhage after coiling for unruptured
aneurysms, hemorrhage remains the primary concern in
cases of aneurysm regrowth. Current developments in
the field of endovascular therapy for aneurysms are
focused on ways to reduce the incidence of aneurysm
recurrence. Several design modifications primarily aimed
at increasing the biological activity of GDCs have been
proposed to stimulate the growth of endothelium over
the aneurysm ostium at the coil surface. In a swine
model, higher rates of aneurysm obliteration and re-endothelialization
across the aneurysm ostium were achieved with collagen-coated
platinum coils than with Dacron-fibered coils.(8) Additional
coil modifications have included ion implantation, protein
coating, and coating with growth-factor secreting tissue
grafts.(40,41) In vitro studies show increased endothelial
cell-to-cell adhesion after exposure of coils to ion-implanted
collagen coating than to non-implanted collagen coating.(39)
If the processes of cell adhesion and tissue growth
at the aneurysm-coil interface were enhanced by coil
modifications, improved tissue healing across the aneurysm
ostium would, in theory, reduce the risk of aneurysm
recurrence or regrowth. This technology awaits clinical
evaluation.
 |
| Depiction
of current technique for aneurysm embolization with
polymer. |
Another technique under consideration for prevention
of aneurysm growth is the use of liquid polymers to
fill aneurysms. The concept stems from the fact that
aneurysm recurrence is reduced when there is a greater
density of coil packing.(44) In an in vitro study, Piotin,
et al.44 demonstrated that dense coil packing represented
filling of only 30 to 40% of the volume of the aneurysm
dome. The implication is that the less residual volume
left unfilled after treatment (i.e., the greater the
density of packing), the lower he risk of aneurysm recurrence.
Since coil filling is incomplete even in the most densely
packed aneurysms, increased filling may require the
use of a different medium. Animal studies have demonstrated
excellent filling of aneurysms using cellulose acetate
polymer (CAP).(35,51) Subsequent clinical application
was promising as well.(25-27) Histological evaluation
suggests that re-endothelialization is more rapid and
complete with CAP than with GDC.(35) However, the same
study suggests that complete filling of the aneurysm
is more technically feasible with GDC than with CAP,
underscoring the relative benefits of each. Another
agent currently under clinical evaluation in the United
States is the ethylene vinyl alcohol (EVAL) copolymer
dissolved in dimethyl sulfoxide (DMSO). Clinical use
of the ethylene vinyl alcohol mixture for aneurysm embolization
was first reported by Nishi, et al.(42) The parent vessel
was occluded in two of three cases. Preservation of
the parent artery requires temporary balloon occlusion
during polymer delivery and precipitation, which may
limit the use of polymers. The technology remains promising,
however.
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