*Department of Neurosurgery, Ondokuz Mayis University, School of Medicine, Samsun, Turkey; ‡

Department of Neurosurgery, University of Health Sciences, Bakirkoy Prof. Dr.

Mazhar Osman Training and Research Hospital for Neurology, Neurosurgery and Psychiatry, Istanbul, Turkey; §

Department of Neurosurgery, Ankara University, School of Medicine,

Ankara, Turkey; ||Department of Neurosurgery, Hacettepe University, School of Medicine, Ankara, Turkey; ¶

Department of Neurosurgery, University of Health Sciences, Umraniye

Teaching and Research Hospital, ̇

Istanbul, Turkey; #

Department of Neurosurgery, General University Hospital Alicante, Alicante, Spain; **Department of Neurosurgery, Indiana

University School of Medicine, Indianapolis, Indiana, USA; ††The Neurosurgical Atlas, Carmel, Indianapolis, Indiana, USA; ‡‡Department of Neurosurgery, Microsurgical Neuro-
anatomy Laboratory, Yeditepe University School of Medicine, Istanbul, Turkey

Correspondence: Abuzer Güngor, MD, Department of Neurosurgery, Yeditepe University Kosuyolu Hospital, Kos ̈ ̧uyolu Mah. Kos ̧uyolu Cad. No: 168 34718, Uskudar, ̇
Istanbul,

Turkey. Email: abuzergungor@gmail.com
Received, January 15, 2023; Accepted, March 16, 2023; Published Online, May 31, 2023.
© Congress of Neurological Surgeons 2023. All rights reserved.
BACKGROUND: It has always been a matter of debate which position is ideal for the supracerebellar approach. The risk
of venous air embolism (VAE) is the major deterrent for surgeons and anesthesiologists, despite the fact that sitting and
semisitting positions are commonly used in these operations.
OBJECTIVE: To demonstrate a reduction on the risk of VAE and tension pneumocephalus throughout the operation
period while taking advantages of the semisitting position.
METHODS: In this study, 11 patients with various diagnoses were operated in our department using the supracerebellar
approach in the dynamic lateral semisitting position. We used end-tidal carbon dioxide and arterial blood pressure
monitoring to detect venous air embolism.
RESULTS: None of the patients had clinically significant VAE in this study. No tension pneumocephalus or major
complications were observed. All the patients were extubated safely after surgery.
CONCLUSION: The ideal position, with which to apply the supracerebellar approach, is still a challenge. In our study, we
presented an alternative position that has advantages of the sitting and semisitting positions with a lower risk of venous
air embolism.
KEY WORDS: Supracerebellar approach, Air embolism, Sitting position, Semisitting position, Tension pneumocephalus
Operative Neurosurgery 25:103–111, 2023 https://doi.org/10.1227/ons.0000000000000758
The supracerebellar approach is commonly used to operate
on pineal region lesions.1-3 This approach can also be used

for the midbrain, thalamus, third ventricle, superior cere-
bellar surface, cerebral peduncle, ambient cistern, and aqueduct

and mesial temporal lobe lesions.1,4-10 These areas are often
challenging for surgery because of their deep location and close
relationship with important neural and vascular structures.11 The

supracerebellar approach can be applied in the form of in-
fratentorial and transtentorial.10,12-14

It has always been a matter of debate which position is most

ideal for the supracerebellar approach. Therefore, it can be per-
formed in a prone, sitting or semisitting position.15-26 Although

sitting or semisitting positions are frequently used in these op-
erations, the risk of venous air embolism (VAE) is the most critical

deterrent for surgeons and anesthesiologists. Many different
methods have been developed to prevent, detect, and treat VAE,

but these methods require a lot of experience, and when hap-
pening, their prognosis still remains poor.16,17,22,27-33

VAE occurs as a result of air inflex into the systemic circulation
from the surgical area when the patient’s central venous pressure

decreases below atmospheric pressure.23,30,34 Air can enter the cir-
culation through any open vein or dural sinus in the surgical area.17

Although all surgical positions have a potential VAE risk, the sitting
and semisitting positions present higher risk than the prone position.35

ABBREVIATIONS: ETCO2, end-tidal carbon dioxide; MR, magnetic
resonance; TEE, transesophageal echocardiography; VAE, venous air
embolism.
Operative Neurosurgery Speaks! Audio abstracts available for this article at
operativeneurosurgery-online.com.

OPERATIVE NEUROSURGERY VOLUME 25 | NUMBER 2 | AUGUST 2023 | 103
NEUROSURGICAL ATLAS SERIES

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The sitting and semisitting positions provide a better gravi-
tational drainage of cerebrospinal fluid (CSF) and blood out of the

surgical area, allowing the surgeon to use both hands for mi-
crosurgical manipulation.17,23,36-38 In addition, by the effect of

gravity, it creates an unobstructed corridor between the cere-
bellum and the tentorium cerebelli, without the need to retract the

cerebellar parenchyma.6,8,10,13,14,17,19,22,38,39 These positions
give a direct view of the patient’s chest and face enabling to easier
manipulation in case of emergency and better observation of
neurostimulation responses.17 Despite all these advantages, many
surgeons avoid using the sitting or semisitting positions because of
the risks of VAE and tension pneumocephalus.23 Although these
risks are lower in the prone position, it lacks benefits such as
gravitational drainage of blood and CSF, avoidance of cerebellar
retraction, and surgeon’s comfort.17,19,23,36,37
The dynamic lateral semisitting position is a combination of
the semisitting and lateral decubitus positions.40 In case of VAE
in the dynamic lateral semisitting position, the surgery can be
continued in the lateral decubitus position by placing the back
section of the operating table parallel to the ground. This
maneuver prevents venous air embolism by increasing central
venous pressure, and the surgeon can continue the surgery and
close the air entrance area.
We aimed to demonstrate our dynamic lateral semisitting
position with prevention of VAE and tension pneumocephalus

throughout the operation, while taking advantage of the semi-
sitting position.

METHODS
Patient Selection
All patients consented to the procedure, and the patient in the figures
consented to the publication of his image. This study was approved by the
institutional review board. We investigated 11 patients with various
diagnoses who were operated in our department using the supracerebellar
approach in the dynamic lateral semisitting position between 2020 and

2022. Right-left cardiac shunt and cervical instability were our exclusion
      criteria, but none were found in our patients during the preoperative
      examinations.
      Preoperative Management
      Preoperative evaluation of all patients included transthoracic

echocardiography and cervical flexion-extension radiographs to in-
vestigate right-left cardiac shunt and cervical instability. Any sign of a

right-to-left cardiac shunt was considered as a contraindication for
surgery in a semisitting position.17,21,37,41,42 Cranial lesion evaluation
varies according to the pathology the patients presented with. T1/T2-
weighted magnetic resonance (MR) with/without contrast, diffusion
tensor imaging (DTI), and fiber tractography were performed in
patients with mass lesions. MR angiography and digital subtraction
angiography (DSA) were performed in a patient with vascular lesion. In
addition, MR venography was performed to visualize the dominant
transverse sinus and determine the safer (nondominant) side for
midline lesions.

Positioning
After endotracheal intubation and placing a central venous catheter
with the tip at the right atrium, the Mayfield 3-pin head holder was
applied. While the patient’s head was held securely, the patient was
turned to a lateral decubitus position. Then the head was rotated 15°
toward the lower shoulder and elevated 40°, and the Mayfield 3-pin head
holder was fixed. This position is maintained throughout the extradural
part of the surgery (Figures 1A, 1B, and 2A).
After dura mater incision and hemostasis, the back section of the
operation table was tilted up by 40°, so the head angle becomes 80°
(Figures 1C, 1D, and 2B). Thus, the patient was placed in the lateral

semisitting position. In case of VAE, the neck area would be easily ac-
cessible for the anesthesiologist to apply compression on the jugular veins

if necessary.
After the intradural part of the surgery was completed, the back
section of the operating table was returned to a parallel position to
the ground and this is maintained till the end of the surgery to
prevent VAE and pneumocephalus (Figures 1E, 1F, and 2A). The
operating table was covered with a viscoelastic foam mattress, and
all extremities were supported with gel pads to prevent pressure
sores.
Monitoring and Management of Intraoperative VAE

Transesophageal echocardiography (TEE), precordial Doppler, end-
tidal carbon dioxide (ETCO2), and arterial blood pressure monitoring are

frequently used to detect VAE.24
Early detection and cessation of the endovascular air entrance is crucial
in VAE. In this study, we used ETCO2 and arterial blood pressure
monitoring to detect VAE. A decrease of more than 5 mm Hg in the
ETCO2 level or hemodynamic instability is considered clinically
significant VAE.
In the case of VAE, the surgeon needs to find and stop the air entrance

while the anesthesiologist compresses both jugular veins simulta-
neously.17,18 In the meantime, reducing the back section angle of the

operating table helps by increasing the venous pressure. Air bubbles can
be removed from the right atrium by gentle aspiration through the central
venous catheter placed preoperatively.17,18,36,41

RESULTS
We reviewed 11 patients who were operated with dynamic

lateral semisitting position between 2020 and 2022. Character-
istic features of the patients are described in Table. The preop-
erative and postoperative GCS of all patients was 15.

ETCO2 and arterial blood pressure monitoring were used to
diagnose VAE. A decrease of more than 5 mm Hg in the
ETCO2 level or hemodynamic instability was considered
clinically significant VAE.17 None of the patients presented
intraoperative clinically significant VAE in this study. No
tension pneumocephalus or other major complications were
observed. All the patients were extubated safely after surgery.
Postoperative imaging showed that total removal tumors and
cavernomas. Postoperative DSA confirmed that the aneurysm
had been treated successfully. All patients had an uneventful
postoperative course.

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FIGURE 1. Dynamic lateral semisitting position steps. A, The patient is placed in the lateral decubitus position until the dura mater
incision is made to avoid venous air embolism; B, placement of incision and craniotomy; C, after the dura mater incision and hemostasis,
the patient is placed in the dynamic lateral semisitting position to take advantage of the gravitational effects; D, dural opening; E, after the
intradural phase, the patient is placed in the lateral decubitus position again and the dura mater is sutured; F, dural closure.

OPERATIVE NEUROSURGERY VOLUME 25 | NUMBER 2 | AUGUST 2023 | 105
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DISCUSSION
The supracerebellar approach has a very important role in
neurosurgery and allows safe access to deep brain structures. This
approach can be performed in prone, sitting, semisitting, or as we

promote in this study in the dynamic lateral semisitting position.
We present and discuss the clinical results of 11 patients operated
with the dynamic lateral semisitting position in this single-center
study.
The sitting position in neurosurgery was more popular in the
1970s and 1980s than it is today.17,19 Over time, it has been

modified to the semisitting position to reduce associated com-
plications such as hemodynamic instability, VAE, and tension

pneumocephalus. Hence, the semisitting position is more fre-
quently used now than the sitting position.17,26,37,43-46 Although

the semisitting position has advantages over the sitting position
and reduces the risk of many complications, new-generation
neurosurgeons and anesthesiologists still avoid using this position.
As it is known, the most essential benefits of the semisitting
position are gravitational drainage of CSF and blood of the
surgical area, creating an unobstructed corridor between the
cerebellum and the tentorium cerebelli without the need for

cerebellar retraction (Figure 3E and 3F) and allowing two-
handed microsurgical dissection. All these benefits are related

to the intradural part of the surgery. However, the risk of VAE
exists at all stages of surgery, including during the application
and removal of 3-pin head holder. Using the dynamic lateral
semisitting position, we aim to reduce the risk of VAE by
keeping the patient in the lateral decubitus position during the
dural and extradural phases of the surgery, which are at higher
risk of VAE. The patient can be easily placed into the lateral
semisitting position with a single move in the intradural phase

of the surgery to take advantage of the benefits of the semi-
sitting position, as previously mentioned.

In many studies using the semisitting position, it is aimed to
increase venous pressure by keeping the patient’s legs above the
head level.17,47 In the dynamic lateral semisitting position, it is
not possible to raise the legs above the head level, but we increase
the venous pressure by placing the patient in the lateral decubitus
position in the extradural part of the operation.
FIGURE 2. Head elevation degree of the dynamic lateral semisitting position. A, The lateral decubitus position
with the head angle is 40°; B, the dynamic lateral semisitting position with the head angle is 80°.

TABLE. Characteristics of the Patients
Characteristics
Female/male ratio 3/8
Age in y (mean ± SD) 35.2 ± 15.7
ASA status score (I/II/III) 3/5/3
Weight in kg (mean ± SD) 74.8 ± 12.8
Lesion location (no.)
Brainstem 5
Pineal region 2
Ambient cistern 2
Thalamus 1
Superior cerebellar surface 1
Surgical pathology (no.)
Cavernous malformation 3
Neuroepithelial tumor 2
Glial tumor 3
Epidermoid cyst 1
Metastasis 1
Aneurysm 1

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In case of VAE while performing the supracerebellar approach
in sitting or semisitting positions, if air entrapment continues even
after all manipulations, then the degree of the back section of the
operating table should be decreased temporarily. It may be very
difficult for the surgeon to continue the operation. In such a
situation in dynamic lateral semisitting position, the surgeon can
continue the operation comfortably and close the air entrance
area, even if the back section of the operating table is kept parallel
to the ground.
Although the risk of VAE is lower in other positions than sitting
or semisitting positions, it still represents a threat. VAE can occur
whenever the venous pressure reduces below the atmospheric

pressure.17,18,24,36 In addition, the prone position, which is of-
fered as an alternative to the sitting or semisitting positions to

prevent VAE, has its own potential risks. It also lacks the benefits
of the semisitting position such as gravitational drainage of blood
and CSF, avoidance of cerebellar retraction, and surgeon’s
comfort.23
There are significant differences between sensitivities of various
VAE monitoring modalities and the clinical significance of the

findings.18,24 Because of its high sensitivity, it is recommended to
use TEE for early diagnosis of venous air embolism, but this
method requires special equipment and experience. However,
owing to the high sensitivity of TEE, the incidence of VAE
and the risk of false positives are increased.18,24 VAE on only
TEE or precordial Doppler without ETCO2 or hemodynamic

changes may be clinically insignificant.18,24 In clinically signifi-
cant VAE, ETCO2 decreases and/or hemodynamic changes are

observed.16,17,24,36 There are different data about VAE in the

literature. Various monitoring methods were used for the diag-
nosis of VAE, and various severity classifications were used for

VAE in these studies, and most studies are retrospective.24 Many
studies have used different ETCO2 values for the diagnosis of
clinically significant VAE.37,47-50 Feigl et al determined this limit
as 3 mm Hg, while Ammirate et al determined it as 5 mm Hg.37,47
In this study, we decided to set the ETCO2 limit as 5 mm Hg. In
a retrospective study, M. Ammirati et al37 reported that ETCO2
decreased by more than 5 mm Hg in 26.8% of 41 patients who
were operated in the semisitting position. In a prospective study
by X. Wang et al,41 15.4% of 26 patients who underwent surgery
FIGURE 3. Midbrain cavernoma surgery steps. A, Burr hole opening; B, cerebrospinal fluid drainage from the cisterna magna; C, locating the transverse sinus with Doppler;
D, bridging vein dissection (bridging veins were maintained in all operations); E and F, brainstem view without the need to retract the cerebellar parenchyma.

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FIGURE 4. Preoperative (left columns) and postoperative (right columns) T2-weighted sagittal (upper rows) and T1-weighted
contrast-enhanced axial (lower rows) MR images of some cases. A, Thalamic cavernoma; B, midbrain cavernoma; C, midbrain
glioma; D, pineal papillary tumor; E, cerebellary metastasis; F, pineal neuroepithelial tumor.

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in the semisitting position reported VAE. In a series report by M.
Kurihara et al, 26% of the 23 patients operated in the semisitting
position had a VAE, whereby ETCO2 decreased by more than
5 mm Hg.18 In a study by Feigl et al47 about evaluation of the risk

of paradoxical venous air embolism in patients with patent fo-
ramen ovale, the rate of patients with an increase in ETCO2 of

more than 3 mm Hg was found to be 9.6% (5/52). In a pro-
spective study reported by H. Türe et al, investigating the effect of

the degree of head elevation in the semisitting position on the
VAE; clinically important VAE was detected in 8% of the group
with a head elevation of 30° and 50% in the group with a head
elevation of 45°.17 In this study, we use 40° head elevation, and we
did not detect clinically significant VAE that led to a decrease in
ETCO2 levels of more than 5 mm Hg or hemodynamic changes.
Gravity-dependent supine position for lateral supracerebellar
infratentorial approach was described by Awad et al.15 Although this
position may reduce the risk of VAE, the gravitational advantages are

limited compared with the semisitting position. The lateral su-
pracerebellar approach is not ideal for midline lesions. In the dy-
namic lateral semisitting position, lateral supracerebellar, midline

supracerebellar, and retrosigmoid approaches can be applied.

In a study about the occurrence and management of postop-
erative pneumocephalus using the semisitting position by Ma-
chetanz et al,46 tension pneumocephalus was reported in 3.3% of

429 patients in the semisitting position. In our study, we did not
detect tension pneumocephalus in any of the patients. It is
possible that the number of patients in our study is insufficient to
conclude on tension pneumocephalus; however, we think that
placing the patient from the lateral semisitting position to the
lateral decubitus position during the dura mater suturing stage
might play a role in preventing the tension pneumocephalus by
increasing the intracranial pressure in this final step of the surgery.
J. H. Palazón et al reported that mean cerebral perfusion
pressure values fell slightly when the head was elevated to 30°
(3.5 mm Hg) compared with the supine position, and a greater
reduction was achieved when the head was elevated 45° (7.1 mm
Hg) compared with the supine position.5 While performing the
supracerebellar approach using the dynamic lateral semisitting
position, placing the patient in the lateral decubitus position
during the dural and extradural stages, probably increases the
cerebral perfusion pressure. This manipulation can protect the
cerebral tissue against possible hemodynamic changes.

FIGURE 5. DSA images of P2 aneurism. A, Preoperative coronal DSA image; B, preoperative sagittal DSA
image; C, postoperative coronal DSA image; D, postoperative sagittal DSA image.

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Another benefit of the dynamic lateral semisitting position is
that the surgeon can combine retrosigmoid and supracerebellar
approaches with a simple manipulation on back section angle of
the operating table.
Although the number of our cases is small, we think that the
dynamic lateral semisitting position, which we apply safely in various
localizations and pathologies (Figures 4 and 5), can be an alternative
to other positions used in supracerebellar approaches. The safety of
this position can be tested by using it in more cases in the future.
Limitation
The small number of cases and the fact that a TEE was not used to
diagnose clinically insignificant VAE is a limitation of this study.
CONCLUSION
The ideal position, with which to apply the supracerebellar
approach is still a challenge. In our study, we presented dynamic

lateral semisitting position that has the advantages of the semi-
sitting position and potentially lower risk of VAE and tension

pneumocephalus. We completed this study with no clinically
significant VAE or tension pneumocephalus detected in any of our
patients. It would be beneficial to study these complications with
extensive series in the future.
Funding
This study did not receive any funding or financial support.
Disclosures

The authors have no personal, financial, or institutional in-
terest in any of the drugs, materials, or devices described in this

article.
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