OPERATIVE NUANCES

Long-term Patient Outcomes After Microsurgical
Treatment of Blister-Like Aneurysms of the
Basilar Artery
BACKGROUND: Blister-like aneurysms (BLAs) are challenging lesions that require
unique microsurgical strategies. BLAs are predominantly found along the internal
carotid artery; however, BLAs of the basilar artery are a rare subset that requires
a modified treatment strategy.
OBJECTIVE: To discuss the microsurgical management and review the long-term
outcomes of patients with BLAs of the basilar artery.
METHODS: We retrospectively reviewed the surgical technique, postoperative results,
and long-term outcomes of all patients with basilar artery BLAs treated at our institution
from 2005 to 2011.
RESULTS: Four patients with basilar artery BLAs were identified over this 6-year
interval. All 4 patients were treated by direct microsurgical clipping. A thin layer of
cotton reinforcement was used beneath the clip tines to minimize the risk of clip
slippage in 2 of 4 patients; 1 patient required adjunctive endovascular stent placement
for residual aneurysm after clipping. Complete obliteration of all aneurysms was achieved, and there has been no recurrence at mean clinical follow-up of 72 months
(median, 74.5; range, 37-103) and imaging follow-up of 48 months (median, 54; range
12-72).
CONCLUSION: Direct clipping with or without cotton reinforcement can obliterate
basilar BLAs with excellent long-term outcomes. Clip wrapping is not an option for
these lesions given the proximity to perforating branches. Endovascular techniques
provide a useful adjunctive strategy; however, risks with antiplatelet therapy in the
acute subarachnoid hemorrhage period must be considered.

Michael A. Mooney, MD
M. Yashar S. Kalani, MD, PhD
Peter Nakaji, MD
Felipe C. Albuquerque, MD
Cameron G. McDougall, MD
Robert F. Spetzler, MD
Joseph M. Zabramski, MD
Division of Neurological Surgery, Barrow
Neurological Institute, St. Joseph’s
Hospital and Medical Center, Phoenix,
Arizona
Correspondence:
Joseph M. Zabramski, MD,
c/o Neuroscience Publications,
Barrow Neurological Institute,
St. Joseph’s Hospital and Medical Center,
350 W Thomas Rd,
Phoenix, AZ 85013.
E-mail: neuropub@dignityhealth.org
Received, December 29, 2014.
Accepted, May 20, 2015.
Published Online, June 25, 2015.

KEY WORDS: Aneurysm, Basilar artery, Blister, Clipping, Endovascular

Copyright © 2015 by the
Congress of Neurological Surgeons.

Operative Neurosurgery 11:387–393, 2015

B

lister-like aneurysms (BLAs) are small,
hemisphere-shaped defects that arise from
nonbranching points of the cerebral arteries
and are associated with a high risk of rupture.1-3 In
contrast to saccular aneurysms, they consist of
a platelet plug covering a thin layer of adventitia,
which overlies a defect in the intima and media,

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OPERATIVE NEUROSURGERY

ABBREVIATIONS: BLA, blister-like aneurysms;
GOS, Glasgow Outcome Scale; ICA, internal carotid
artery; PED, Pipeline Embolization Device; SAH,
subarachnoid hemorrhage
Supplemental digital content is available for this article.
Direct URL citations appear in the printed text and are
provided in the HTML and PDF versions of this article on the
journal’s Web site (www.operativeneurosurgery-online.com).

DOI: 10.1227/NEU.0000000000000866

and they are thought to be secondary to atherosclerosis, hemodynamic stress, dissection, or
a combination of these factors.4,5 These lesions
are challenging for the cerebrovascular surgeon
because of the fragility of the aneurysm wall, as
well as their poorly defined necks.3,6-10
Sundt and Murphey originally described BLAs
in 1969 in relation to the internal carotid artery
(ICA), but BLAs have since been described
throughout the cerebral vasculature.11,12 Basilar
artery BLAs make up a small subset of all BLAs,
and their treatment is further complicated by the
anatomy of the region and the proximity of
perforating branches.13 Surgical and endovascular approaches to these lesions have been
sporadically reported in the literature14-17; however, to our knowledge, no series of these rare

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MOONEY ET AL

aneurysms with long-term patient follow-up has been reported to
date. Herein, we report the outcomes for 4 patients with basilar
BLAs treated at our institution and describe our approach for
managing these lesions.

METHODS
Patient Selection
We queried a database for all patients with aneurysms treated at our
institution between 2005 and 2011 to identify BLAs of the basilar
circulation treated with a microsurgical technique. BLAs were defined as
shallow, broad-based aneurysms arising from nonbranching sites of the
basilar artery, which were diagnosed with cerebral angiograms and
confirmed at operative inspection. We limited our search of the database
to those treated before 2011 to obtain long-term follow-up on these
patients. We reviewed the sex, age, lesion size, symptoms at presentation,
treatment modalities used, complications, and outcomes for each patient.
The patients’ clinical examinations and imaging studies, operative and
angiographic reports, and in- and outpatient records were reviewed.
Preoperative and postoperative vascular imaging was obtained by using
conventional angiography, computed tomography angiography, or
magnetic resonance angiography.

Treatment
All patients presented here were treated using microsurgery. Electrophysiological monitoring, including electroencephalography and somatosensory evoked potentials, were recorded in all cases. Angiographic
intraoperative evaluation was performed by using indocyanine green
video angiography. Postoperative angiography was obtained to document
aneurysm occlusion and degree of parent vessel stenosis, if present.
Aneurysm occlusion was classified as complete with no residual or as
incomplete when any untreated residual remained. We do not routinely

use temporary clips, intraoperative hypotension, or intraoperative cardiac
arrest. Outcomes were measured by using Glasgow Outcome Scale (GOS)
scores. Imaging follow-up time points are determined on a case-by-case
basis.
We use “cotton reinforcement” to indicate the technique of placing
cotton beneath the clip tines, as previously described by Barrow and
Spetzler.18 Although this technique was originally described to repair
intraoperative tears of the aneurysm neck, we have expanded its use to
cases where an added layer of friction between the vessel wall and clip
tines can help prevent slippage of the clip on these wide-necked, fragile
aneurysms.3

RESULTS
We identified 4 patients (3 male, 1 female) with a mean age of
52.5 years (range, 35-66 years), each with a diagnosis of a basilar
artery BLA. No other aneurysms were detected in any of the
patients in this series (Table).
The mean size of the base of the BLAs was 2.25 mm (range, 1-4
mm). The mean size of the length of the aneurysm dome was 2 mm
(range, 1-2.5 mm). All BLAs were located at nonbranching points
on the basilar artery. All patients presented with subarachnoid
hemorrhage (SAH). The Hunt-Hess grade at presentation ranged
from 1 to 3. Fisher grade was not available for 1 patient who was
transferred from an outside hospital; the other 3 patients had scores
of 3 or 4. The admission GOS score was used to assess and grade
patients at presentation. The mean GOS score on presentation was
4 (median, 4; range, 3-5). The definitive diagnosis of BLA was
made using angiography in 4 of 4 patients.
In 3 patients, the aneurysms were treated by using a microsurgical technique, and, in 1 patient, a combined microsurgical/
endovascular approach was used. All aneurysms were confirmed to

TABLE. Patient Characteristics and Outcomesa
Case

Age/sex
Presenting symptom
Dimension (length · width) (mm)
Tool for 1 BLA diagnosis
Hunt-Hess grade
Fisher grade
Treatment
VPS
Vasospasm
Admission GOS
Discharge GOS
GOS last F/U
Imaging F/U (mo)
Clinical F/U (mo)

1

2

3

4

60/M
Headache
2·3
DSA
2
3
Direct clip
1
2
3
4
5
42
83

66/M
Headache
1·1
DSA
3
Unknownb
Direct clip
1
1
4
3
5
66
66

35/M
Seizure
2.5 · 4
CTA
2
4
Cotton clip
1
2
5
5
5
12
37

49/F
Headache
2.5 · 1
DSA
1
3
Cotton clip 1 stent
2
2
4
5
5
72
103

a
CTA, computerized tomographic angiography; DSA, digital subtraction angiography; F, female; F/U, follow-up; GOS, Glasgow Outcome Scale; M, male; VPS,
ventriculoperitoneal shunt.
b
Original CT from outside facility was not available for this patient.

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BASILAR BLISTER-LIKE ANEURYSMS

be ruptured on direct intraoperative inspection. Among the
patients treated with microsurgical technique alone, 2 aneurysms
were directly clipped, and 1 aneurysm was clipped with cotton
reinforcement beneath the clip tines. In the single case of
combined microsurgical/endovascular treatment, the aneurysm
was clipped with cotton reinforcement and the vessel was stented
(Figure 1).
None of the patients experienced perioperative mortality. Three
patients required ventricular shunting and 1 patient experienced

clinically significant vasospasm. There was 1 case of treatmentrelated morbidity in a patient who required a subdural-peritoneal
shunt for a persistent subdural hygroma. The mean GOS score at
discharge was 4.25 (median, 4.5; range, 3-5). All patients were
contacted by phone at the time of the study to assess long-term
outcomes. At a mean clinical follow-up of 72 months (median,
74.5; range, 37-103), the GOS was 5 in all 4 patients.
Postoperative imaging revealed complete obliteration of the
aneurysm in 3 of the 4 cases. In 1 patient (case 4), residual

FIGURE 1. A, B, case 1, a basilar apex blister-like aneurysm (BLA) that was directly clipped. C, D, case 3, a midbasilar trunk BLA clipped with cotton reinforcement and mild parent artery stenosis after clipping. E, F, case 4,
a basilar BLA near the origin of the posterior cerebral artery directly clipped with cotton reinforcement and underwent
subsequent Neuroform stenting. Used with permission from Barrow Neurological Institute, Phoenix, Arizona.

OPERATIVE NEUROSURGERY

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MOONEY ET AL

aneurysm was treated by endovascular placement of a Neuroform
stent (Boston Scientific, Natick, Massachusetts). A small residual
aneurysm persisted immediately after stent placement; however,
repeat angiography 1 month later revealed complete obliteration.
One patient (case 3) was noted to have mild stenosis of the basilar
artery at the site of clipping. Follow-up imaging at a mean of 48
months (median, 54; range, 12-72) revealed no cases of aneurysm
recurrence/regrowth.

ILLUSTRATIVE CASE
Case 3
A 35-year-old man presented to an outside hospital after a tonicclonic seizure, and he was found to have a Fisher grade 4 SAH. An

external ventricular drain was placed at the outside hospital, and he
was transferred to our institution for further management. He was
neurologically intact on admission to our institute. Computed
tomography angiogram on admission demonstrated a mid-basilar
BLA (Figure 2A and 2B). The patient underwent a right
retrosigmoid craniotomy for exploration of the aneurysm. After
an attempt at direct clipping, the aneurysm was reinforced with
cotton and clipped (see Video, Supplemental Digital Content,
http://links.lww.com/NEU/A742). He required ventriculoperitoneal shunt placement for failure to wean the drain, and he was
discharged to home on postoperative day 19. On discharge he
was neurologically intact. Postoperative computed tomography
angiography and follow-up digital subtraction angiography at 1
year demonstrated complete obliteration of the aneurysm with

FIGURE 2. A, sagittal computed tomography angiography (CTA) on admission demonstrating an aneurysm along the basilar
trunk. B, 3-dimensional reconstruction of the aneurysm from the initial CTA. C, postoperative sagittal CTA demonstrating clip
placement and complete obliteration of the aneurysm. D, postoperative digital subtraction angiography of a left vertebral artery
injection, Townes view, demonstrating complete obliteration and mild parent artery stenosis from clip placement. Used with
permission from Barrow Neurological Institute, Phoenix, Arizona.

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BASILAR BLISTER-LIKE ANEURYSMS

mild parent artery stenosis adjacent to the aneurysm clip (Figure
2C and 2D). At long-term follow-up of 3 years, the patient is
doing well and has returned to work; he is scheduled for repeat
imaging follow-up at 5 years.

DISCUSSION
Incidence and Characteristics
The true incidence and natural history of BLAs is difficult to
determine. In the absence of rupture, these aneurysms may appear
identical to small saccular aneurysms that would be treated
conservatively with observation; however, the fragility of their
walls and their propensity to rupture distinguishes them from their
saccular counterparts. Indeed, short-interval rupture of an incidentally discovered BLA that was thought to be a small saccular
aneurysm has been reported.3
Basilar artery aneurysms as a whole make up a small minority of
all intracranial aneurysms. In the Barrow Ruptured Aneurysm
Trial, patients with basilar trunk or basilar tip aneurysms made up
only 5.4% (22/408) of the cases of ruptured aneurysms enrolled in
the study.19 BLAs of the basilar artery are clearly a rare subset of
basilar artery aneurysms, and over the 7-year study period at our
institution, only 4 patients with these lesions were encountered.
Additionally, 3 of 4 (75%) of these patients were transferred from
outside hospitals for a higher level of care, further demonstrating
their rare occurrence.
Our limited knowledge regarding BLAs comes from both
pathological examination and practical experience. The wall of this
type of aneurysm has been described as consisting of a thin layer of
adventitia overlying a defect in the intima and media layers.4,20 The
fragile nature of BLAs has been associated with relatively high rates
of intraoperative or intraprocedural hemorrhage.1-3,9,10,21,22
Microsurgical techniques for treating these lesions have been
described, and options include direct clipping, clipping with cotton
reinforcement beneath the clip tines, clip wrapping, and vessel
sacrifice with or without bypass.3,8,10,11,23
Microsurgical Treatment
Early series of microsurgical treatment for BLAs consisted of
various clip techniques for ICA BLAs. Nakagawa et al9 published
one of the earliest series in 1986. In this report of 8 cases, they
were able to achieve direct clipping in 4 patients and clipping
with supplementary wrapping (with muscle or fascia) or coating
(with cellulose fabric and/or fibrin glue) in the remaining 4. A
more recent publication from our own institution highlights our
approach to these lesions.3 In the 18 ICA BLAs included in this
study, direct clipping was achieved in 12 patients. In cases where
this was not possible owing to aneurysm morphology or vessel
wall friability, cotton-reinforced clipping was successful in 3
patients, and clip wrapping with Gore-Tex (W.L. Gore &
Associates, Inc, Flagstaff, Arizona) was successful in the remaining 3. We recommend cotton-reinforced clipping of BLAs when
there is inadequate vessel diameter to allow standard clip
application without significant risk that the clip blades will slip

OPERATIVE NEUROSURGERY

off the aneurysm. In addition to minimizing slippage of the clip
blades, the cotton serves as a local reinforcement.18 More
recently, our technique for the management of ICA BLAs has
evolved to clip wrapping with Gore-Tex.3
BLAs of the basilar artery are exceedingly rare and present
a unique neurosurgical challenge. The proximity of these
aneurysms to perforating branches differentiates them from their
ICA counterparts. Literature describing treatment options for
basilar BLAs is extremely limited, likely because of both their rare
nature, as well as nonunified terminology to describe these lesions.
In 2010, Hauck et al24 described their microsurgical technique
for approaching 21 patients with small (,7 mm) basilar apex
aneurysms, all of which were directly clipped. Although
similarities exist in terms of the exposure of these lesions and
visualization of surrounding vessels, these lesions were not BLAs.
The majority of aneurysms in this series were unruptured (76%)
and all occurred at the basilar apex, with or without involvement
of the P1 segment. They were able to achieve favorable outcomes
with their approach to these aneurysms, with only 2 cases of
intraoperative rupture, and acceptable GOS scores in all patients.
In our case series, on the other hand, aneurysms occurred along
the course of the basilar trunk or at the basilar apex, and all
aneurysms were confirmed to be BLAs intraoperatively. The
largest diameter of any aneurysm in our series was 4 mm, and no
cases of intraoperative rupture occurred (Table). Our approach to
these lesions is a slight variation from our approach to BLAs of the
ICA, given the unique anatomic challenges of the region. Clip
wrapping is not an option for these aneurysms, as it is for ICA
BLAs, because of the extremely narrow window of operative
exposure and the presence of perforating branches in proximity to
the aneurysm. We continue to attempt direct clipping for all
lesions of this type but have found that cotton-reinforced clipping
is an excellent strategy for complete obliteration of these lesions.
Cotton reinforcement prevents slipping of the clip blades along
the often wide base of these aneurysms, while minimizing the
degree of parent vessel stenosis necessary for secure clip
placement. This strategy was used in 2 of the 4 patients in this
series. Clipping with a small degree of parent artery stenosis also
remains an acceptable strategy, as illustrated in case 3 (Figure 2).
Endovascular Treatment
Advances in endovascular treatment strategies have led to
alternative approaches to BLAs. However, direct coiling or stentassisted coiling is limited secondary to the wide-based morphology
and fragile wall associated with these aneurysms, and high rates of
intraprocedural rupture and postprocedure recurrence have been
observed.21,22,25,26 Neuroform stent reconstruction and flowdiverting stenting have been explored as potential treatment
options. Fiorella et al17 demonstrated the feasibility of Neuroform stent reconstruction for a variety of uncoilable “intradural
pseudoaneurysms,” which included 4 cases involving a dissecting
aneurysm, postsurgical aneurysm, or pseudoaneurysm of the
basilar artery, all of which had ruptured either acutely or
subacutely. In the setting of SAH, antiplatelet therapy is

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MOONEY ET AL

dangerous and often limited in these patients, and, indeed, 2 cases
of stent thrombosis occurred in this series owing to insufficient
antiplatelet therapy.
Aydin et al14 reported their experience with SILK flowdiverting stenting (Balt Extrusion, Montmorency, France) in 11
patients with BLAs, 2 of whom had basilar trunk BLAs. All of
these patients presented with SAH, and therapy was delayed 5
days in all patients to allow for dual-antiplatelet therapy before
stenting. Although most aneurysms (9/11) were not obliterated
with stent placement initially, all BLAs that were examined with
angiography (9/9) at the 3-month follow-up were obliterated at
that time with good patient outcomes reported. One patient in
this series, however, had an ischemic stroke with transient
hemiparesis due to stent thrombosis in the setting of antiplatelet
agent noncompliance, and 2 others were found to have mild instent stenosis at 3-month follow-up.
More recently, Chalouhi et al15 described their experience
using the Pipeline Embolization Device (PED, eV3, Inc, Irvine,
California) for 8 patients with BLAs, 1 of which arose from the
basilar artery. Five of these 8 patients presented with SAH, and all
were started on dual-antiplatelet therapy 1 to 13 days from their
sentinel bleed. Aneurysm obliteration was achieved in 5 of 6
patients who had relatively short-term follow-up data available (2
weeks to 6 months), and 1 case of “minimal” asymptomatic instent stenosis occurred. Yoon et al27 reported mixed results in
their experience with the PED for 11 patients with ruptured ICA
blister aneurysms. Although complete obliteration was achieved
in 7 of 8 patients with angiographic follow-up, 3 major and
2 minor perioperative complications were reported. Major
complications included 1 large ipsilateral middle cerebral artery
infarct after PED placement, 1 hemorrhagic infarct leading to
death, and 1 ophthalmic artery occlusion leading to blindness.
Minor complications included an asymptomatic carotid occlusion after stent thrombosis and an asymptomatic carotid
dissection from guide-wire placement. These authors concluded
that PED placement is a feasible treatment option for BLAs, even
in the setting of acute SAH; however, ischemic and hemorrhagic
complications can be encountered after stent placement, and this
technique should be further investigated for long-term safety and
efficacy.

CONCLUSION
BLAs are dangerous lesions that pose a significant treatment
challenge. BLAs involving the basilar artery are rare but require
specific surgical considerations. In our experience, microsurgical
clipping is a favorable approach to these aneurysms that can
achieve complete obliteration and excellent long-term outcomes.
Direct clipping with cotton reinforcement beneath the clip blades
is an acceptable strategy for basilar BLAs where direct clipping
cannot be achieved. Endovascular techniques may have a role in
the treatment of these lesions; however, their long-term safety and
efficacy has yet to be proven, and must be weighed against the risk
of dual-antiplatelet therapy in patients presenting with acute SAH.

392 | VOLUME 11 | NUMBER 3 | SEPTEMBER 2015

Disclosure
The authors have no personal, financial, or institutional interest in any of the
drugs, materials, or devices described in this article.

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OPERATIVE NEUROSURGERY

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Supplemental digital content is available for this article. Direct URL citations appear in
the printed text and are provided in the HTML and PDF versions of this article on the
journal’s Web site (www.operativeneurosurgery-online.com).

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