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Interventional Neuroradiology 14: 259-266, 2008

Morphological Study of Aneurysms
at the Junctionof the Superior Cerebellar Artery
H. IIZUKA, S. MIYACHI, T. OHSHIMA, T. IZUMI, A. TSURUMI, J. YOSHIDA
Department of Neurosurgery, Nagoya University Graduate School of Medicine, Showa-ku Nagoya, Japan

Key words: cerebral aneurysm, superior cerebellar artery, morphology, embolization

Summary
Superior cerebellar artery (SCA) aneurysms
sometimes involve the origin of the SCA making treatment difficult. We focused on the morphological characteristics of SCA aneurysms
and adjacent vascular structures to apply clinical decision-making for the treatment strategy.
Sixty-nine SCA aneurysms, including 34 ruptured and 35 unruptured ones, had been treatedfor over 12 years. Multiple aneurysms were associated in 30 patients. The pattern of the neckposition of aneurysms was classified into three
types: Type A: no SCA-involved type; Type B:
half involved type with SCA originating fromthe aneurysmal neck; Type C: pure SCA aneurysm with all the neck mounting on SCA. Morphological and clinical analysis was done between ruptured and unruptured aneurysms andamong the three types.
There was no difference in patient profile between ruptured and unruptured aneurysms. The
angle formed by the posterior cerebral arteryand SCA on the aneurysm side was obtuse in 62(90%) patients. From the morphological pointof view the SCA-involved type (types B + C)
was significantly more prevalent in ruptured
aneurysms (77%). Bleb formation was particular in ruptured aneurysms. As for the treatment,
the risk of SCA occlusion and incomplete andattempted operation was particularly high incases with SCA-involved type.
Although SCA aneurysms may grow due tothe hemodynamic stress at the opened bifurca-tion between the PCA and SCA, the neck shifting to the origin of SCA, particularly in ruptured lesions, may suggest some other etiologicalmechanism. SCA-involved type aneurysms had
a high treatment risk of SCA occlusion andtended to incomplete treatment to avoid such ischemic complications.
Introduction
Cerebral aneurysms are broadly divided into
lateral and terminal type by adjacent branchingpattern. According to this concept, the aneurysms at the junction of the basilar artery (BA)and superior cerebellar artery (SCA) look likethe lateral type protruding from the BA trunk.
However some aneurysms appear to be a terminal type between the posterior cerebral
artery (PCA) and SCA. This variation may influence the treatment strategy and results, particularly in the case of embolization 1,2. We studied the specific morphology of SCA aneurysms
and the adjacent vascular complex to clarify
the characteristics of ruptured lesions and address the treatment risk by the location of theneck including a review in the literature.
Methods
Profile of patients
We treated 69 patients with BA-SCA aneurysms at Nagoya University hospital and our facilities over a twelve-year period from 1995 to2006. We retrospectively reviewed the medical259

Morphological Study of Aneurysms at the Junction of the Superior Cerebellar Artery

H. Iizuka

Table 1 Summary of the cases.

ruptured unruptured

Figure 1 The pattern of the neck position of aneurysms.
Type A: no SCA-involved type; Type B: half involved typewith SCA originating from the aneurysmal neck; Type C:
pure SCA aneurysm with all the neck mounting on SCA.

records, radiographic studies and operation
records of the patients (Table 1). Forty-sevenpatients were female (68%) and 22 were male.
Their ages ranged from 44 to 77 years (mean61.9±8.6 SD years). Thirty-four patients had
ruptured aneurysms and 35 had unruptured lesions. Hunt & Hess grade of the patients withruptured aneurysms was Grade I in 5, II in 14,
III in 9, IV in 6, respectively. Thirty patients had
31 multiple aneurysms in other locations, including the anterior circulation in 24 and posterior circulation in seven. The definition of ruptured aneurysm among the multiple aneurysms
was based on the distribution of subarachnoid
hemorrhage and clinical manifestations. Tenaneurysms were clipped, 47 were embolizedwith detachable coils. The choice of the treatment methods was based on the operator’s decision-making and the patient’s wishes. Five patients with failed clipping underwent embolization and two patients with failed embolization
were clipped. Six patients with tiny unruptured
aneurysms were observed because of the
anatomical difficulty or patients declined treatments. Treatment was only attempted in six
aneurysms because of failure of access or thehigh risk of procedural complications (Table 1).
Analysis of images
Digital subtraction angiography (DSA) andthree dimensional CT angiography were used
as a reference to examine the morphology ofaneurysms including laterality, size, shape (theexistence of bleb), the location of the neckbased on the relation with the SCA, as well as
the pattern of the adjacent vascular complex
including the angle between SCA and PCA,
and the existence of other aneurysms. The

260

total

case number

34

35

69

patient profile
male
femaleage
H&H grade 0
H&H grade I
H&H grade II
H&H grade III
H&H grade IV

13
21
61
–
5
14
9
6

9
26
62,7
35
–
–
–
–

22
47
61,9
35
5
14
9
6

aneurysm profileside rightside left
size < 4 mmsize 4 - 6 mmsize 7 - 9 mmsize ⱖ10 mm
leb formation

18
16
16
12
6
2
10

18
17
13
13
7
2
2

36
33
29
25
13
4
12

another aneurysms
anterior circulation
ICA
MCA
AcomA
posterior circulation
BA-tip
AICA
PICA

9
5
2
0
3
4
2
1
2

22
19
8
9
2
3
1
0
2

31
24
10
9
5
7
3
1
4

branching profile
open angled ipsialteral
PCA-SCA junction
31
neck involvement to SCA
type A
8
type B16
type C
10

31

62

16
15
4

24
31
14

treatment methodclipping, wrapping
coil embolization
attempt
observation

3
21
5
6

10
48
6
6

7
27
1
0

ICA: internal carotid artery, MCA: middle cerebral artery;
AcomA: anterior communicating artery, BA: basilar artery;
AICA: anterior inferior cerebellar artery,
PICA: posterior inferior cerebellar artery;
PCA: posterior cerebral artery, SCA: superior cerebellar artery.

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Interventional Neuroradiology 14: 259-266, 2008

A

B

C

D

Figure 2 A case of Type A: before (A) and after (B) embolization. Recurrent aneurysm 3 years after the initial treatment (C)was re-embolized with coils (D). Note the counterclockwise inclination to the right of BA course and the obtuse angle between posterior cerebral artery and SCA.

longest diameter in the three dimensionalviews was adopted as the size of the aneurysm.
The angle between the SCA and PCA was calculated in the most opened angle among theanteroposterior and various oblique views of
3D-CT angiography or 3D-DSA. The locationof the neck was classified based on the degreeof involvement of the SCA into the following

three types: Type A: no SCA-involved type;
SCA directly originates from BA and the necklocates on the junction of BA and SCA; Type
B: half involved type; most part of the neck isshifted to SCA and SCA originates from thelateral side of the aneurysmal neck; Type C:
pure SCA aneurysm; all the neck mounts on
the first segment of the SCA (Figure 1). Two

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Morphological Study of Aneurysms at the Junction of the Superior Cerebellar Artery

H. Iizuka

A

B

Figure 3 A case of Type B: before (A) and after (B) embolization.

neurosurgeons and neuroradiologists examined
and judged the images. We analyzed the data toclarify the relation between the morphologicalpattern and the rupture or treatment difficulties. Statistical evaluation was done by χ2 test.
Results
Differences between ruptured
and unruptured aneurysms
There was no significant difference as to thepatient age and sex between the two groups
with ruptured and unruptured aneurysms(Table 1). Female dominance was found in patients with unruptured aneurysms; 62% in ruptured cases and 74% in unruptured cases.
As for the aneurysm profile, there was no
statistical difference in the size of aneurysms,
however the rate of very small aneurysms (< 4mm) was higher in the ruptured ones (47%versus 37%). The laterality of the aneurysms
was slightly shifted to the right (52%). Bleb
formation was frequently found in ruptured
aneurysms (10/34; 29%) significantly different
from that of unruptured lesions (2/35; 6%)(p = 0.038).
The branching pattern of the BA-tip in caseswith aneurysms was left-right asymmetry in allcases. In the A-P view in DSA, 62 cases (90%)
displayed an obtuse angle between the PCA
and SCA on the aneurysm side (Figure 1). Thisparticular finding was similar in both groups.

262

As for the relation between the aneurysmal
neck and SCA, Type A in 24 cases (35%), TypeB in 31 cases (45%), and Type C in 14 cases(20%). In cases of ruptured aneurysms, it wasType A in eight cases (24%), Type B in 16 cases (47%), and Type C in ten cases (29%). Whilein cases of unruptured lesions, it was Type A in
16 cases (46%), Type B in 15 cases (42%), andType C in four cases (11%) (Table 2). Overall,the SCA involved type (Types B and C) accounted for 76% of cases of ruptured aneurysms and 53% of the unruptured lesions. The
distribution of Type A and Type C was significantly different between ruptures and unruptured aneurysms (p<0.05).
Relationship between the lateralityof the aneurysm and the inclination of the BA
Among 37 patients who had the aneurysm on
the right side 19 patients showed the course of
the BA trunk shifted with a counterclockwise
inclination in the anteroposterior view of theangiography. In contrast, 17 out of 33 patients
who had the SCA aneurysms on the left side
showed a clockwise rotation of the BA. Thirtypatients showed a vertical positioned BA, inwhom16 aneurysms were located on the rightside and 14 were on the left. In other words, 19
out of 21 BA of counterclockwise inclination to
the right formed aneurysms on the inclined
side, and 17 out of 19 of clockwise inclination to
the left showed the same tendency (Table 3).

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Interventional Neuroradiology 14: 259-266, 2008

Results and treatment complications
Complete occlusion of the aneurysms was achievedin 43 patients (62%). According to the analysis by typeof aneurysms, there was no relation among the distribution of the aneurysms, size, and side with types. However, SCA involved type (Types B and C) had a relatively higher tendency to fail the complete treatments.
The rate of neck remnant was 12% in Type A, 35% inType B and 31% in Type C (Table 2).
As for the clinical results, there was no treatmentrelated mortality. Treatment-related morbidity was
4.3%. The patients with permanent deficits (more thanmRS 3) included two patients with intraoperative rupture of other aneurysms and one patient with cerebellar infarction due to the SCA occlusion. Four patientsdied from the initial damage of subarachnoid hemorrhage in three and rerupture of the aneuryrm in one.
In our endovascular treatment there were six technical complications. Extravasation occurred in one patient without neurological deficits. SCA occlusion or
embolism occurred in five patients: three of them weresuccessfully recanalized with local fibrinolytic therapy,
another two patients in whom SCA occlusion occurred
several hours after the embolization were managedwith medical therapy. All the patients except one witha large cerebellar infarction recovered well with orwithout mild cerebellar symptoms. All thrombotic
complications of SCA occurred in the SCA involved
type (Table 2).
Recurrence of aneurysms occurred in four cases.
The recurrent style was coil sinking and regrowth after
incomplete clipping in two small ruptured aneurysms,
and coil compaction with persistent growth in two unruptured aneurysms. With regard to coil compaction
and regrowth, additional coil embolization was needed
for three patients and clipping was performed in one.

A

B

C

Discussion
Generally aneurysms form at the junction of arteries 3,4, and hemodynamic stress serves to enlarge
them 3,5,6. In particular, the hemodynamic stress exerted
on the bifurcation may be one of the factors generating terminal type aneurysms. Thus, aneurysms of theBA-tip may be readily formed by hemodynamic stress.
Tanaka et Al.1 indicated with regard to nine cases of
SCA aneurysms that the angle between the PCA andSCA on the side of an aneurysm was obtuse in an anteroposterior view in DSA, while that on the opposite
side was acute. In other words, the fact that the angle
between the PCA and SCA is obtuse may be one of
the mechanisms of aneurysm formation at the BASCA junction. According to our results, three out offour recurrent cases were Type A aneurysms. Type A

Figure 4 A case of Type C: 3D-DSA (A) before(B) and after (C) embolization.

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Morphological Study of Aneurysms at the Junction of the Superior Cerebellar ArteryA

H. Iizuka

B

C

Figure 5 A case of intraoperative branch occlusion: Pre-embolization of Type B unruptured aneurysm (A). SCA was occluded after the THIRD coil insertion (B). Successful recanalization with local fibrinolytic therapy (C).
Table 2 Characteristics by type of neck involvement to SCA.

type A

type B

type C

total

rupture

9

16

9

34

unrupture

16

15

4

35

right

12 (5)

16 (7)8 (6)

36 (18)left

13 (4)

15 (9)5 (3)

33 (16)

<4 mm

10

8

4

22

4 - 6 mm

10

14

7

31

7 - 9 mm

2

6

2

10

ⱖ10 mm2

2

0

4

bleb formation

5

3

4

12

2

6

2

10

attempt

1

4 [1]

1

6

observation

5

1

0

6

total occlusion

19

15

9

43

body filing

3

5

1

9

neck remnant

3 [1]

11 [1]

4

17 [2]

extravasation

0

1

0

1

SCA occlusion

0

4

1

5

recurrence

3

1 [1]

0

4 [1]

Clinical manifestations

Aneurysm side

Aneurysm size

Treatment methodclipping
coiling

angiographic result

complications( ): number of the ruptured cases

264[ ]: number of the clipping cases

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aneurysm has the orifice directly opening to
the BA, therefore they might be exposed byhigher hemodynamic stress than other branchoriented types (Types B and C). Obvious distribution of the laterality of the aneurysms by BA
inclination may also suggest participation of
the hemodynamic effect. It is well known thataneurysms with blebs tend to rupture 7-9. Our
study showing that bleb formation was particularly seen in the ruptured aneurysms confirms
previous evidence. As for our case, there wasno significant difference in type, but Type B
aneurysms had a tendency to less bleb formation. It is unclear whether this fact may be correlated with the hemodynamic stress.
In our series, 58% of the aneurysms belonged
to the SCA involved type (Types B and C)
rather than the BA-originated type (Type A).
Type C aneurysms were relatively found particularly in ruptured aneurysms, in contrast about
half of the unruptured aneurysm were Type A.
This difference was statistically significant(p<0.05). The reason for this difference was unclear, but we speculate that the fragility of theaneurysm wall may be caused by the thinner
structure of the branches rather than the main
trunk. We cannot address the monistic explanation that the extension of the aneurysm to thebranches may cause enlargement and rupture.
With regard to the size of the aneurysm, several reports disclosed that larger aneurysms(especially more than 10 mm) tend to rupture
more easily 10. The International Study of Unruptured Intracranial Aneurysms Investigators(ISUIA) 11 reported in 2003 that aneurysms
more than 13 mm in size tend to rupture. According to our initial speculation, the larger theaneurysms become, the more frequently the
SCA should have been involved in theaneurysm, and should have ruptured, but ourstudy failed to prove this hypothesis. There wasno correlation among size, type and rupture(Tables 1 and 2). This may suggest that aneurysms may enlarge as concentric circles justfrom their origin and may not have an alternative direction to extend the orifice.
According to a review of 22 studies on theincidence of technical complications in aneurysm embolization, Park et Al 12 found that 23%
occurred in cases of rupture and 8% in cases
without rupture. Robert et Al 13 reported one
case with an embolized BA-SCA aneurysm inwhich the shape of coils changed, and the SCA
was occluded at the two year follow-up. Al-

Interventional Neuroradiology 14: 259-266, 2008

bayram et Al 14 indicated that in cases of BASCA aneurysms thrombotic complications occurred in three out of five cases and one caseoccluded the SCA completely. In the present
study, technical complications such as SCA occlusion and ischemic complications occurred in
8.7%, which is comparable to that in other reports 2. However, the rate of ischemic complication (7.2%) due to the branch occlusion wasmuch higher than that of aneurysms in otherlocations, mean 2.4% in our 625 experiences. In
our series, all five cases where the SCA wascompletely occluded belonged to SCA-involved type. Lubics et Al 15 addressed the difficultyof endovascular treatment for aneurysms witha branch arising from the sac. They had no experiences of branch occlusion, but complete occlusion of the aneurysm was achieved in only
22% of cases. Therefore we have to make theutmost effort to preserve the SCA using various adjunctive techniques for these high-risk
aneurysms. In other words, we must evaluate
small, asymptomatic SCA-involved type aneurysms based on the risk-benefit assessment including the choice of observation, and this discreet tendency was expressed in the recent negative inclination to embolize these aneurysms(Table 1). Attempted rate (8.7%) was slightly
higher than that of the report of the International Subarachnoid Aneurysm Trial (ISAT) 16.
In our series, coil compaction occurred in
19% of embolized aneurysms. One of the reasons is the incomplete packing in several former cases using the mechanical detachable
coils, which have a poor size line-up to achieve
suitable coil embolization. Further, a terminal
type of aneurysm with a wide neck readily affected by hemodynamic stress may result incoil compaction and regrowth, as proven in experimental studies 17. Hope et Al 18 reviewed 63
Table 3 The relationship between the laterality of theaneurysm and the inclination of basilar artery.

BA inlination*aneurysm
side

counterclockwize(right)clockwize(right)vertical(left)

right

18

2

16

left

2

17

14

total

20

19

30

: see Fig. 1

265

Morphological Study of Aneurysms at the Junction of the Superior Cerebellar Arteryaneurysm cases after seven months follow-up
and showed 28% had coil compaction and 11%
had aneurysmal growth. They stressed that the
major cause of recurrence was wide-neck size.
It is known that rough packing of a large aneurysm frequently causes coil compaction14,19-21.
Thus, caution is especially needed in cases witha terminal type of relatively large SCA aneurysms and an obtuse angle of the branching pattern of the BA-tip because of possible post-embolization coil compaction and recurrence.
Conclusions
BA-SCA aneurysms particularly tended tooriginate closer to the SCA and were significantly frequent in cases of ruptured aneurysm.

H. Iizuka

Although SCA aneurysms may grow due to thehemodynamic stress at the opened bifurcation
between the PCA and SCA, the right side dominance and the neck shifting to the origin of
SCA, particularly in ruptured aneurysms, maysuggest some other etiological mechanism.
SCA-involved type aneurysms had a high treatment risk of SCA occlusion and tend to incomplete treatment to avoid ischemic complications. When embolizing SCA aneurysms, particularly those in cases with a broad neck, either aballoon catheter or some other microcatheters
should be used to preserve the SCA and shouldbe packed tightly to avoid later recanalization.
Further perioperative management focusing on
antiplatelet agents may be important to avoid
ischemic complications.

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Shigeru Miyachi, M.D.,
Department of Neurosurgery
Nagoya University
Graduate School of Medicine
65 Tsurumai-cho, Showa-ku Nagoya
466-8550 Japan
E-mail: smiyachi@med.nagoya-u.ac.jp