OPERATIVE TECHNIQUE

A Novel Approach to Microvascular Decompressionfor Hemifacial Spasm: Method Description and
Associated Outcomes
Shinji Nagahiro, MD, PhD
Yoshifumi Mizobuchi, MD,
PhD
Kohei Nakajima, MD, PhD
Yasushi Takagi, MD, PhD
Department of Neurosurgery, Graduate
School of Biomedical Sciences, Tokushima University, Tokushima, Japan
Correspondence:
Yoshifumi Mizobuchi, MD, PhD,
Department of Neurosurgery,
Graduate School of Biomedical Sciences,
Tokushima University,
3-18-15, Kuramoto-cho,
Tokushima 770-8503, Japan.
Email: y.mizobuchi1203@gmail.com
Received, November 11, 2021.
Accepted, January 27, 2022.
Published Online, April 29, 2022.
© Congress of Neurological Surgeons2022. All rights reserved.

BACKGROUND: Microvascular decompression (MVD) is the only potential cure forhemifacial spasm (HFS). However, traditional techniques such as the interposition method
may have limited effect in some cases. Alternative techniques have been proposed;
however, they can be more complex or difﬁcult to perform than the standard approach.
OBJECTIVE: To describe a safe decompression technique—the “shelter method”—which
involves creating a shelter-like space around the facial nerve root exit zone and present
associated outcomes.
METHODS: Medical records and intraoperative ﬁndings of 92 patients with HFS who underwent MVD using the shelter method between April 1997 and March 2017 were retrospectively reviewed. As a historical control group, we included 53 patients who had
undergone MVD by the traditional interposition method before March 1989. The patientswere divided into 3 subgroups according to the arteries involved and degree or direction ofarterial compression to the seventh nerve. Patient outcomes were assessed as excellent, good,
fair, and poor according to the MVD scoring system of the Japan Society for MVD Surgery.
RESULTS: In the shelter method group, complete disappearance of HFS was achieved in
87 patients (94.6%). The curative rate of the shelter method group was signiﬁcantly higher
than that of the interposition method group. The overall complication rates were signiﬁcantly lower in the shelter method group than in the interposition method group.
CONCLUSION: Our ﬁndings indicate high curative and low complication rates of the
shelter method, suggesting that it helps treat HFS caused by various types of arterial
compression.
KEY WORDS: Hemifacial spasm, Microvascular decompression, Shelter method
Operative Neurosurgery 00:1–7, 2022

H

emifacial spasm (HFS) may result fromthe compression of a facial nerve, most
likely at the nerve root exit zone (REZ).1
Microvascular decompression (MVD) is an effective treatment method for HFS.2 Since Jannetta’s
original description,3 there have been several developments in the techniques for keeping the offending vessel away from the nerve. In many cases,
2 different MVD techniques for displacing theoffending vessels have been used, speciﬁcally, theinterposition and transposition methods. Recent
advances in microsurgical techniques have resulted
in further improvements in patient outcomes,
ABBREVIATIONS: HFS, hemifacial spasm; MVD, microvascular decompression; PICA, posterior inferior
cerebellar artery; REZ, root exit zone; VA, vertebral
artery.

OPERATIVE NEUROSURGERY

https://doi.org/10.1227/ons.0000000000000223

reducing the associated morbidity and mortality
rates.4 However, some traditional techniquessuch as the interposition method have beenreported to have limited effects in some cases.
Therefore, several techniques involving thetransposition method have been introducedfor MVD; these procedures are often more
complicated and time-consuming than the interposition method.5-13 Herein, we aimed todescribe our simple and safe transposition
technique known as the “shelter method” and toreport the associated patient outcomes in thetreatment of HFS.

METHODS
Between April 1997 and March 2017, 104 patientswith HFS underwent MVD at our institution. This

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NAGAHIRO ET AL

retrospective analysis included 96 consecutive patients who were followed up for more than 1 year. Four patients were excluded because of
the presence of short branches of the perforating arteries around the
REZ, precluding them from undergoing MVD using the shelter
method. MVD using the shelter method was used in 92 patients(95.8%) with the mean age of 55.6 years (range, 22-78). In total, 33
men and 59 women were included in the shelter method group. As ahistorical control group, we included 53 consecutive patients who had
undergone MVD using the interposition method for HFS, performedby the same surgeons, before March 1989, and who were followed up
for at least 1 year after surgery. Before March 1989, HFS was performed
using the interposition method in all patients who underwent MVD at
our institution.
This study included 18 men and 35 women. The mean age of thepatients at the time of surgery was 53 years (range, 26-74). Data frommedical records, details of surgical techniques, complication rates, andsurgical outcomes were retrospectively reviewed.
All the patients in the shelter and interposition method groups weredivided into 3 subgroups based on the arteries involved and the degree
or direction of the arterial compression to the seventh nerve upon
intraoperative observation. In group A, most common, simple type ofcompression is seen at the REZ caused by the anterior inferior cerebellar artery (AICA) or posterior inferior cerebellar artery (PICA),
from the anterior caudal direction (Figure 1A1). In this type ofcompression, a nerve indentation is usually observed at the compressed REZ. In group B, tortuous redundant vertebral artery (VA)
can be observed causing compression of the facial nerve directly or
indirectly, and there is an interposed AICA or PICA between the VA
and nerve (Figure 1B1). Compression of the seventh nerve was more
severe in this group than in group A. In group C, the AICA coursed
between the seventh and eighth cranial nerves and caused compressionof the dorsal aspect of the seventh nerve. The proximal portion of the
AICA or PICA compressed the ventral or caudal part of the seventh
nerve (Figure 1C1). Patient outcomes were assessed according to the
MVD scoring system of the Japan Society for MVD Surgery.14 Basedon this system, the efﬁcacy (E) of the surgery is designated as E0 when
the surgical outcome is complete disappearance of spasm, E1 when theoutcome is an occasional slight spasm, E2 when the outcome is anoccasional moderate spasm or apparently persisting spasm, and E3
when the surgical outcome is not cured. The complication score was
C0 when no complications were observed after surgery; C1 whenslight cranial nerve or cerebellar dysfunction, not affecting daily life,
was present; and C2 if both subjective and objective cranial nerves or
cerebellar dysfunction problematic for daily life were observed.
Follow-up data on the postoperative grade of involuntary movement
and complications were determined based on the patient’s self-reportthrough a direct interview during an outpatient visit, by a double
postal card, or by telephone contact. The effectiveness of MVD using
the interposition method or shelter method was compared among
different offending vessels. This study was approved by the ethics
committee of our hospital (details redacted for peer review). Becauseof the retrospective nature of the study, patient consent was not
required.

Surgical Procedures of the Shelter Method
The shelter method for HFS was performed as follows: With thepatient in the park bench position with an auditory brainstem response and abnormal muscle response and with the help of an evoked

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potential system, abnormal muscle response recordings were obtained.
Electrical stimulation of the temporal branch of the facial nerve
generated mentalis muscle recordings and that of the marginal
mandibular branch generated recordings from orbicularis oculi
muscles. A suboccipital craniectomy was performed, approximately
3.5 × 3.5 cm in size (Figure 2). The supracondylar area was removed tominimize retraction and to approach the REZ of the facial nerve
directly from the caudal direction. After dural incision, the arachnoid
membrane of the cisterna magna was incised, and the cerebrospinal
ﬂuid was sufﬁciently drained to provide adequate cerebellar relaxation. The REZ of the facial nerve and offending arteries was easily
observed with slight elevation of the ﬂocculus and choroid plexus awayfrom the ninth cranial nerve, without any tension on the eighth cranial
nerve.

Type ATo decompress the offending vessels, we created a space around the
REZ with no artery within it, which was moved using a prosthesis,
such as a polyurethane sponge or Teﬂon felt, placed between theartery and the brainstem around the REZ. To create space around the
REZ, we created shelter with a polyurethane sponge for the offending
artery to surround the REZ. During the surgery, the space between
the offending vessels and REZ may look adequate; however, thepostoperative changes in the brain position may bring the vessel up
against the REZ again. The “shelter” technique is more likely toensure that the vessel does not compress the facial nerve owing to thespace created in comparison with the transposition technique (Figure
1A3) (Video 1).

Type B
Decompression involved 2 processes. First, we transposed the VA on
the pons. The VA was safely lifted from the brainstem and transposedcaudally using a few Teﬂon pledges or by attaching the VA to the duramater of the petrous bone by ﬁbrin glue placed medial to the facial nerve.
Second, transposition of the directly offending artery was performed. The
directly offending artery was carefully lifted, and shelter was created with apolyurethane sponge for the offending artery to surround the REZ(Figure 1B3) (Video 2).

Type CThe AICA coursed between the seventh and eighth cranial nerves andcaused compression of the dorsal aspect of the seventh nerve. The
proximal portion of the AICA or PICA compressed the ventral or caudal
part of the seventh nerve. This double compression made transposition
from the ventral region of the facial nerve REZ likely to be unsuccessful.
Consequently, we ﬁrst decompressed the normal typical pressure part of
the REZ. The offending artery from the ventral side was carefully lifted,
and a polyurethane sponge was inserted between the offending artery andbrainstem, allowing us to observe the meatal branch of the AICA pressing
the ventral side of the REZ between the facial and cochlear nerves,
making decompression straightforward. Next, we separated the meatalbranch between the facial and cochlear nerves from the REZ and moved
toward the internal auditory canal. The meatal branch was then transposed to the petrous bone and attached by ﬁbrin glue. At this stage,
it was important to prevent pulling on the perforator because thisbranch penetrates the nerve and brainstem in the internal acoustic meatus(Figure 1C3) (Video 3).

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SHELTER METHOD FOR HEMIFACIAL SPASM

FIGURE 1. A1, Most common, simple type of compression is seen at the REZ caused by the AICA or PICA, from the anterior caudal direction. A2, The pressure on the REZ is notrelieved sufﬁciently by the interposition method. A3, To decompress the offending vessels, the artery is moved by polyurethane sponges or a Teﬂon felt, placed between the artery andthe brainstem around the REZ. B1, Tortuous redundant VA can be observed causing compression of the facial nerve directly or indirectly, and there is an interposed AICA or
PICA between the vertebral artery and nerve. B2, The pressure on the REZ by a VA cannot be effectively treated by direct decompression using a prosthesis. B3, To decompress, weﬁrst transposed the vertebral artery onto the pons. The vertebral artery was lifted from the brainstem and transposed caudally with a few Teﬂon pledges or by attaching the vertebral
artery to the dura matter of the petrous bone by ﬁbrin glue placed medial to the facial nerve. Subsequently, the offending artery was transposed, lifted, and sheltered with apolyurethane sponge to surround the REZ. The AICA coursed between the seventh and eighth cranial nerves and caused compression of the dorsal aspect of the seventh nerve. C1,
The proximal portion of the AICA or PICA compressed the ventral or caudal part of the seventh nerve. C2, In patients with type C hemifacial spasm, the pressure on the REZ is notrelieved by the interposition method. C3, We decompressed a typical pressure part of the REZ. The AICA or PICA offending arteries on the ventral side were carefully lifted fromthe brainstem and a polyurethane sponge was inserted between the offending artery and the brainstem, allowing visualization of the meatal branch of the AICA pressing theventral side of REZ between the facial and cochlear nerves, ensuring straightforward decompression. We separated the meatal branch between the facial and cochlear nerves fromthe REZ and moved toward the internal auditory canal. The meatal branch was transposed onto the petrous bone and attached with ﬁbrin glue. AICA, anterior inferior cerebellar
artery; PICA, posterior inferior cerebellar artery; REZ, root exit zone; VA, vertebral artery.

Statistical Analysis

RESULTSSPSS software (version 17.0 SPSS Inc.) was used for analyses. The
Fisher exact test was used to compare categorical data. P-values of <.05
indicated statistical signiﬁcance.

The patients’ demographic characteristics are summarized inTable 1.

OPERATIVE NEUROSURGERY

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NAGAHIRO ET AL

Complications
No deaths were recorded in either group. Various complications were experienced by 18.9% of the patients with the interposition method and by 5.4% of the patients with the shelter
method at discharge. Complication rates at discharge were signiﬁcantly lower when using the shelter method than when using
the interposition method (P = .02) (Table 3). Six patients in theinterposition method group and 1 in the shelter method groupexperienced long-term hearing disturbance. The overall complication rates were signiﬁcantly lower when using the shelter
method than when using the interposition method (P = .0098).
Meanwhile, in type B, fewer complications were observed with the
shelter method than with the interposition method (P = .0037)(Table 4).

DISCUSSIONFIGURE 2. With the patient in the park bench position, a suboccipitalcraniectomy was performed, approximately 3.5 × 3.5 cm in size. The supracondylar area was removed to minimize retraction and to approach the root exitzone of the facial nerve directly from the caudal direction.

Long-Term Outcomes
In the interposition method group, complete disappearance of
spasm (E0) was achieved in 39 patients (73.6%). For types A andB, E0 was achieved in 30 of 32 (93.8%) and 9 of 14 patients(64.3%), respectively; no patient (0 of 7) achieved E0 in type C. By
contrast, in the shelter method group, E0 was achieved in 87(94.6%) patients of which 40 (95.2%), 36 (94.7%), and 11(91.7%) patients, respectively, were classiﬁed under types A, B, andC. The overall complete cure rate in the shelter method group was
higher than that in the interposition method group (P = .0005); thecorresponding values for type A cases were comparable between thegroups. In types B and C, complete cure rates in the shelter method
group were signiﬁcantly higher than those in the interposition
method group (P = .0114 and P = .0001, respectively) (Table 2).

MVD is an interposition technique with pad placement that
was ﬁrst described by Gardner15 and largely popularized byJannetta et al.1 MVD has become the treatment of choice forpatients with HFS because of its high success rate and low
operative morbidity.16 However, the curative effect of MVD
decreases depending on the type of offending vessels while
neurovascular compression by a large vessel such as a VA-rerated
cannot be effectively treated with direct decompression by
prosthesis (Figure 1B2).6,17,18 Sindou et al19-21 suggested that
MVD performed when an implant is not in contact with thenerve root has a signiﬁcantly better long-term outcome than
MVD with pad placement. The transposition technique, rather
than the interposition technique with pad placement, has become the technique of choice. In this study, long-term outcomesof the interposition method were satisfactory, and the complication rate was low in patients with AICA or PICA involvement(Figure 1A2). By contrast, the outcomes were poorer in VAassociated cases than in non–VA-associated cases. Decompression of the facial nerve by the insertion of some material between
the REZ of the facial nerve and VA may be effective only in the
short term, as this material may gradually harden because ofadhesion or granulation, losing its shock absorbency.22,23 Theinterposition technique does not provide adequate decompression of the neural structure in VA-associated cases. Several

TABLE 1. Demographic and Clinical Characteristics of Patients Who Underwent Microvascular Decompression Using the Shelter and Traditional
Methods
Characteristic

No. of patients

Shelter method

92

1:1.8

22-78 (mean, 55.6)

Interposition method

53

1:1.9

26-74 (mean, 53.0)

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Sex (M/F)

Age (y)Compressive pattern
Type A 42
Type B 38
Type C 12
Type A 32
Type B 14
Type C 7

Follow-up (y)1.0-14.4 (mean, 6.1)

1.0-5.3 (mean, 3.0)

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SHELTER METHOD FOR HEMIFACIAL SPASMTABLE 2. Long-Term Effect of Patients Who Underwent Microvascular Decompression Using Interposition and Shelter Methods
Complete effect
E0/E0 + E1 + E2 + E3

Long-term effectCompressive pattern
Type A

Type B

Type C

Total

EffectInterposition

ShelterInterposition (%)

Shelter (%)

P-value

E0
E1
E2
E3
E0
E1
E2
E3
E0
E1
E2
E3
E0
E1
E2
E3

30 (93.8%)
2 (6.2%)00
9 (64.3%)
3 (21.4%)
2 (14.3%)002 (28.6%)
5 (71.4%)
039 (73.6%)
7 (13.2%)
7 (13.2%)
040 (95.2%)
1 (2.4%)0
1 (2.4%)36 (94.7%)
2 (5.3%)00
11 (91.7%)
00
1 (8.3%)87 (94.6%)
3 (3.3%)0
2 (2.1%)

93.8

95.2

1.0000

64.3

94.7

.0114

0

91.7

.0001

73.6

94.6

.0005

E0, complete disappearance of spasm; E1, occasional slight spasm; E2, occasional moderate spasm or apparently persistent spasm; E3, recurrence to a degree equivalent to thecondition before surgery; P-values were calculated using the Fisher exact test. Bold indicates statistical signiﬁcance.

alternative surgical techniques have been introduced, and various materials have been reported to achieve successful transposition in VA-associated cases.5-13 Some of them involve
complicated manipulations, which require signiﬁcant time andskill to be successfully and safely achieved within such a small
space. Our shelter method requires neither surgical needle nor
clips; thus, it is less complex and less hazardous and may bequicker than previously reported techniques. In addition, theefﬁcacy of MVD in the shelter method group was signiﬁcantly
better than that in the interposition method group of VAassociated HFS. A favorable surgical outcome in the VAassociated cases required creating space by permanent andcomplete transposition of the offending vessels.
In patients with type C HFS, the pressure on the REZ is notrelieved by the interposition method (Figure 1C2) because thepressure comes from both the ventral and dorsal directions. With
the shelter method, the pressure from the dorsal side is relievedby moving the compressing artery that is ﬁxed between the facial
and auditory nerves more distally and thereafter deﬂecting it
dorsally.

In this study, spasms disappeared completely in 94.6% of thepatients. The efﬁcacy of MVD in treating HFS in this study was
superior to that reported in other studies.4 Moreover, the overall
complication rate was lower with the shelter method group(1.1%) than with the interposition method group (11.3%). In
type B cases, the incidence of complications was signiﬁcantly
lower in the shelter method group (0.0%) than in the interposition method group (28.6%). Masuoka et al24 reported theincidence of lower cranial nerve palsy to be higher in VAassociated HFS than in non–VA-associated HFS cases. In the
shelter method, the risk of neural tissue injury was decreased by athoughtful and deliberate manipulation. The shelter method may
help reduce the risk of complications associated with MVD.
Limitations
This study has several limitations. First, this was a retrospective
study, and a historical cohort was used for comparisons. Second, theshelter method may not be suitable for all cases because its success may
depend on the type of perforating branches involved. The involvementof many short perforating arteries may preclude obtaining space

TABLE 3. Postoperative Immediate Complications of Patients Who Underwent Microvascular Decompression for Hemifacial Spasm
Complication
Facial paresis
Hearing disturbance
Lower cranial nerve palsy
Total

OPERATIVE NEUROSURGERYInterposition method, No. (%)

Shelter method, No. (%)

P-value

4 (7.5)5 (9.4)
1 (1.9)10 (18.9)

2 (2.1)3 (3.3)0
5 (5.4)

.191
.142
.366
.020

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NAGAHIRO ET AL

TABLE 4. Permanent Complication of Patients Who Underwent Microvascular Decompression Using Interposition and Shelter Methods
Total complication rate
C1 + C2/C0 + C1 + C2

Complication
Compressive pattern
Type A

Type B

Type C

Total

Complication

Interposition

ShelterInterposition (%)

Shelter (%)

P-value

C0
C1
C2
C0
C1
C2
C0
C1
C2
C0
C1
C2

30 (93.8%)
2 (6.2%)0
10 (71.4%)4 (28.6%)0
7 (100%)0047 (88.7%)
6 (11.3%)
042 (100%)00
38 (100.0%)
00
11 (91.7%)
1 (8.3%)0
91 (98.9%)
1 (1.1%)
0

6.2

3.8

.1840

28.6

0

.0037

0

0

1.0000

11.3

1.5

.0098

C0, no complications; C1, slight cranial nerve or cerebellar dysfunction, not bothersome for daily life; C2, both subjective and objective cranial nerve or cerebellar dysfunction
problematic for daily life; P-values were calculated using the Fisher exact test. Bold indicates statistical signiﬁcance.

around the REZ by the shelter method. Finally, patients undergoing
MVD using the interposition technique were encountered in a period
when the operator was in the learning curve, and the microscopic
surgical instruments and techniques were in the process of development; therefore, there is a possibility of a selection bias. However, theshelter method was used owing to the poor results in the early period,
resulting in good outcomes and a low rate of complications.

CONCLUSION
Herein, we described a novel decompression technique thatcreates free shelter-like space around the REZ. Our single-center
retrospective study indicated that the rate of long-term curative
effect in MVD with the shelter method was higher than that with
the interposition method. MVD with the shelter method may
help treat HFS caused by various types of arterial compression.
Funding
This study did not receive any funding or ﬁnancial support.

Disclosures
The authors have no personal, ﬁnancial, or institutional interest in any of thedrugs, materials, or devices described in this article.

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OPERATIVE NEUROSURGERYCOMMENTI

congratulate the authors on this addition to the literature on MVD for
hemifacial spasm. The paper addresses the issue of how to correct avascular compression of a cranial nerve: interposition or transposition.
Placing padding between the vessel and nerve (interposition) was popularized by Janetta and taught to his students who have promulgated thistechnique. Moving the vessel away from the nerve (transposition) has beenchampioned by Sindou (and some of the reviewers of this paper). The
authors describe the “shelter method” which is essentially another methodof transposition. Those of us who have successfully re-operated on patientsand removed whatever padding someone has placed against the nerve havelong since learned the rule “leave nothing touching the nerve.” The authors’
work supports this concept with large numbers from excellent surgeons.
Chris Honey
Vancouver, BC, CANADAVIDEO 1. Shelter method for type A.
VIDEO 2. Shelter method for type B.
VIDEO 3. Shelter method for type C.

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