The anterior sylvian point and the suprasylvian operculum.pdf

Neurosurg Focus 18 (6b): E2, 2005

The anterior sylvian point and the suprasylvian operculum

G UILHERME C ARVALHAL R IBAS , M.D., E DUARDO C ARVALHAL R IBAS ,

AND C ONSUELO J UNQUEIRA R ODRIGUES , M.D.

Clinical Anatomy Discipline of the Department of Surgery, University of São Paulo Medical School,

Hospital Albert Einstein, São Paulo, Brazil

Object. The sylvian fissure or lateral sulcus is the most identifiable feature of the superolateral brain surface and

constitutes the main microneurosurgical corridor, given the high frequency of approachable intracranial lesions

through this route. The anterior sylvian point (ASyP) divides this fissure in its main anterior and posterior rami and

was evaluated in this study for its morphology, exact location, and sulcal and neural relationships to assess its suit-

ability as an initial, visually identifiable landmark for further neuroimaging and intraoperative estimation of its adjoin-

ing suprasylvian structures.

Methods. This study is based on 32 formalin-fixed cerebral hemispheres. The brains were removed from the skulls

of 16 cadavers after the introduction of plastic catheters through properly positioned burr holes; the number of speci-

mens for some of the analyzed data differed because of incorrect positioning of catheters or damage to the studied

structures caused by the initial steps of the study.

The ASyP had a cisternal aspect in 94% of the specimens and was always located inferior to the triangular part of

the inferior frontal gyrus, 2.3 6 0.5 cm in front of the inferior rolandic point. The ASyP was located underneath the

1.5-cm-diameter cranial area of the anterior aspect of the squamous suture. Its adjoining structures that compose the

suprasylvian operculum have constant basic morphological configurations.

Conclusions. The ASyP underlies the anterior aspect of squamous suture just behind the pterion, can be easily rec-

ognized, and constitutes a reliable initial sulcal landmark for further estimation of the suprasylvian sulcal and gyral

structures. The suprasylvian operculum can be understood as a series of convolutions roughly arranged as a V -shaped

convolution, with its vertex constituted by the ASyP, followed by three U -shaped convolutions and one C -shaped con-

volution.

K EY W ORDS • brain mapping • burr hole • cerebral cortex • prefrontal cortex •

cranial suture • craniometry • craniotomy

In their original description of the microsurgical anato-

my of the subarachnoid cisterns in 1976, Yasargil, et al., 32

emphasized the importance of the SyF, which then became

the main microneurosurgical corridor to the base of the

brain. In later publications Yasargil, et al., described in de-

tail the microanatomy of this fissure and its underlying cis-

tern 29,30,33 and the technique of its opening. In agreement

with other authors, 12,14,16–18 Ya sargil divides the SyF into a

proximal segment (stem, sphenoidal, anterior ramus) and a

distal segment (lateral, posterior ramus) separated by the

sylvian point, 27,33 which is located beneath the triangular

part of the IFG. The horizontal and the anterior ascending

branches of the SyF that delineate the triangular part of the

IFG arise at the sylvian point. 14

Taylor and Haugton, 23 in their study of the topography of

the convolutions and fissures of the brain published in

1900, used the term sylvian point, defining it as “the point

where the main stem of the fissure of Sylvius reaches the

use of the term sylvian point.

Despite the emphasis given by recent authors to the loca-

tion of the sylvian point, none of them referred to the fre-

quent enlargement of the fissure at the sylvian point, de-

spite its presence in their own illustrations. 12,14,16,27,29,30,33 This

cisternal aspect of the sylvian point is also evident in the

illustrations in many other recent publications 4,8,13,15,18–22 and

in old texts 10,24,25 (Fig. 2).

The constant location and striking cisternal appearance

of the sylvian point indicate that it can be used not only as

a starting site to open the SyF, but also intraoperatively as

an initial landmark to identify other important neural and

sulcal structures along the fissure that are usually hidden by

arachnoidal and vascular coverings. In this study the syl-

vian point is designated as the anterior as opposed to the

posterior one, 26 which corresponds to the distal extremity of

the posterior ramus of the SyF, from which the ascending

stressed the

Abbreviations used in this paper: ASqP = anterior squamous

point; ASyF = anterior sylvian fissure; ASyP = anterior sylvian

point; IFG = inferior frontal gyrus; IFS = inferior frontal sulcus;

IRP = inferior rolandic point; SyF = sylvian fissure.

Neurosurg. Focus / Volume 18 / June, 2005

outer aspect of the hemisphere.” In his textbook published

in 1912, Krause 10 reproduced illustrations of the German

anatomist August von Froriep (1849–1917) 11 with identifi-

cation of the sylvian point and also illustrated an anatomi-

cal opening of the SyF for the exposure of a superficial

insular lesion (Fig. 1). Recently Türe, et al., 27

G. C. Ribas, E. C. Ribas, and C. J. Rodrigues

Fig. 1. Reproduction of an old illustration of the sylvian point by August von Froriep (A), and of the SyF opening by F. Krause (B).

(Reprinted from Krause F: Chirurgie du Cerveau et de la Moelle Epiniere. Paris: Societé D’Editions Scientifiques et Medicales, 1912,

Vols I and II.)

terminal ramus and the occasional descending terminal

ramus originate. 14

Our aims in this study were as follows: 1) to evaluate and

confirm the aforementioned general features of the anterior

sylvian point; 2) to evaluate and establish the main relation-

ships of the ASyP with other important neural and sulcal

structures along the sylvian fissure; and 3) to establish the

relationships of the ASyP with external cranial landmarks

to orient its surgical exposure.

This study was conducted in two parts: Part I was a study

of the surface anatomy of the ASyP and of its related neur-

al and sulcal structures; and Part II was a study of the rela-

tionships of the ASyP with its external cranial landmarks.

After proper identification of the cadaver and with the path-

ologist’s consent, the study was conducted according to the

following steps: 1) exposure of the external cranial surface

by the standard biauricular necroscopic incision and detach-

ment of both temporal muscles, with special attention paid

to exposure of the cranial sutures in particular; 2) placement

of a 1.5-cm burr hole at the anterior part of the squamous

suture, just behind the sphenoparietal suture, with an elec-

trical drill (Dremel Moto-Tool; Dremel, Racine, WI); 3)

opening of the dura mater with a No. 11 blade scalpel; 4)

perpendicular introduction of an approximately 7-cm-long

(2.5-mm-diameter) plastic tracheal aspiration tube (model

sonda-suga No. 08; Embramed, São Paulo, Brazil) with the

aid of a metallic guide; 5) circumferential opening of the

skull with the appropriate saw and of the dura mater with

MATERIALS AND METHODS

Data were collected from 32 cerebral hemispheres ob-

tained in 16 adult cadavers at the Death Verification In-

stitute of the Department of Pathology and at the Clinical

Anatomy Discipline of the Department of Surgery at the

University of São Paulo Medical School, after authoriza-

tion from the Ethical Committee for Analysis of Research

Projects.

Fig. 2. Variations of the IFG illustrated by Testut and Jacob, showing the typical enlargement of the SyF beneath its triangular part.

(Reprinted from Testut L, Jacob O: Tratado de Anatomia Topográfica, ed 5. Barcelona: Salvat, 1932.)

Neurosurg. Focus / Volume 18 / June, 2005

The anterior sylvian point and the suprasylvian operculum

scissors; 6) careful removal of the whole encephalon after

basal divisions of the intracranial vessels and the cranial

nerves; 7) replacement of the calvaria and closure of the

scalp for necroscopic final procedures by the expert person-

nel; 8) storage of the resected encephalons in 10% formalin

solution, with the specimens suspended by a string tied to

the basilar artery to avoid brain deformations; 9) after ade-

quate fixation (at least 2 months), section of the brainstem

at the midbrain and its removal together with the cerebel-

lum; 10) removal of the arachnoidal membranes and the

superficial vessels of the cerebral hemispheres with the aid

of microsurgical loupes (3.5 3 ; Designs for Vision, Inc.,

Ronkonkoma, NY) and/or an operating microscope (model

MDM; Carl Zeiss, Inc., Oberkochen, Germany); 11) micro-

scopic evaluation and acquisition of measurements perti-

nent to the introduced catheter sites; and 12) further micro-

scopic evaluation after removal of the catheters and

acquisition of measurements relative to the neural and sul-

cal structures of interest for our study.

For didactic purposes, the first part of this study concerns

the surface anatomy of the brain, whereas the second part

deals with the relationship of the ASyP for cranial land-

marks. Because some of the hemispheres were damaged by

the catheter that was required for the second part of this

study, the first part is based on fewer specimens than the

second part. The measurements were recorded in millime-

ters and were completed with the aid of compasses and/or

flexible millimetric tapes by the first author, at least twice.

Statistical Analysis

All continuous variables were summarized by the mean

and standard deviation. For the evaluation of the cranial–

cerebral relationships, the 90th percentiles were also calcu-

lated.

RESULTS: PART I

Surface Anatomy of the ASyP and the Suprasylvian

Operculum

Morphology and Location of the ASyP. The ASyP was

located inferior to the triangular part and anterior/inferior to

the opercular part of the IFG in all 18 specimens studied.

Due to the usual retraction of the triangular part of the IFG

in relation to the SyF, the ASyP was characterized as an

enlargement of the SyF, with a mean diameter of 3 to 4 mm

in nine specimens, larger than 5 mm in five specimens,

between 2 and 3 mm in three specimens, and smaller than

2 mm in one specimen (Fig. 3).

Relationship of the ASyP With the IRP. The IRP was de-

fined as the projection point of the inferior extremity of the

central sulcus over the SyF, and the mean distance along the

SyF between the ASyP and the IRP was 2.36 ± 0.5 cm.

The Suprasylvian Operculum.

The IFG Triangular Part. Despite its variable shape, the

IFG triangular part was delimited anteriorly and posteriorly

by the horizontal and anterior ascending rami of the SyF 14

in all 20 specimens evaluated. The IFG triangular part was

divided superiorly by a small sulcal segment of the IFS in

17 specimens, by a noncontinuous short IFS branch in two

specimens, and one specimen contained no sulcal segment.

Fig. 3. A: Photograph of a cadaveric suprasylvian operculum

specimen. B: Sketch of the neural and sulcal morphology (modi-

fied from Ono, et al.). C and D: Different magnetic resonance

imaging sagittal views, with illustrations of the V -shaped convolu-

tion (1) constituted by the triangular part of the IFG located just

superiorly to the ASyP, and usually containing a descending

branch of the IFS; of its following three U -shaped convolutions

respectively comprised by (2) the opercular part of the IFG, which

is always intersected by the inferior part of the precentral sulcus;

the subcentral gyrus or rolandic operculum (3) composed by the

inferior connection of the pre- and postcentral gyri enclosing the

inferior part of the central sulcus; the connection arm (4) between

the postcentral and supramarginal gyri that contains the inferior

part of the postcentral sulcus; and finally the C -shaped convolution

(5) constituted by the connection of the supramarginal and superi-

or temporal gyri that encircles the posterior end of the SyF. The

bottoms of the U -shaped convolutions and their related sulcal

extremities can be situated either superior to the fissure or inside its

cleft. Stars designate the areas as labeled. AAR = anterior ascend-

ing ramus of SyF; ASCR = anterior subcentral ramus; CS = central

sulcus; HR = horizontal ramus of SyF; IFS/PreCS = IFS and pre-

central sulcus meeting point; PAR = posterior ascending ramus of

SyF; PostCS = postcentral sulcus; PreCS = precentral sulcus;

PSCR = posterior subcentral ramus; PSyP = posterior sylvian

point.

Neurosurg. Focus / Volume 18 / June, 2005

G. C. Ribas, E. C. Ribas, and C. J. Rodrigues

The IFG Opercular Part. The IFG opercular part always

had a U -shaped appearance. It was divided by the inferior

segment of the precentral sulcus, and it was delimited ante-

riorly by the anterior ascending ramus and posteriorly by

the anterior subcentral ramus of the SyF. The bottom of this

U -shaped convolution was superior and adjacent to the SyF

in eight of the 18 specimens studied, and was enclosed in-

side the SyF in the 10 specimens in which the inferior ex-

tremity of the precentral sulcus was located at the level of

or inside this fissure.

Rolandic Operculum. The posteriorly following inferior

connection of the pre- and postcentral gyri (subcentral

gyrus, rolandic operculum) also always presented as a U -

shaped convolution, which was contained inside the inferi-

or segment of the central sulcus and was always delimited

by the anterior and posterior subcentral sulci and the rami

of the SyF. According to the position of the inferior extrem-

ity of the central sulcus in relation to the SyF, the bottom of

the U was superior and adjacent to the SyF in 25 of the 30

specimens studied and enclosed inside the SyF in the other

five.

Postcentral and Supramarginal Gyri Connection. The post-

central and supramarginal gyri connection arm was always

present and was disposed between the posterior subcentral

ramus and the posterior ascending ramus of the SyF and was

also configured as a U -shaped convolution contained inside

the inferior segment of the postcentral sulcus in the 18 spec-

imens. The bottom of the U was found to be superior to the

SyF in 11 of the 18 specimens and inside the SyF in the other

seven, according to the positioning of the inferior extremity

of the postcentral sulcus in relation to the SyF. In all 18 spec-

imens, the supramarginal gyrus was continuous with the

superior temporal gyrus through a C -shaped gyral connec-

tion that encircled the distal end of the SyF.

to the ASyP. The 90th percentile for probability values per-

tinent to the vertical positioning of the ASqP in relation to

the ASyP (total 0.00 cm, superior 0.00 cm, inferior 0.00

cm) disclose the very close vertical relationship between

the ASyP and the squamous suture.

Horizontal Relationship. The ASqP was anterior to the

ASyP in six of the 27 specimens, at the same level as the

ASyP along the SyF in 15, and posterior to the ASyP in the

other six specimens, with a mean distance of 0.02 6 0.53

cm anterior to the ASyP. The 90th percentile for probabili-

ty values pertinent to the horizontal positioning of the

ASqP in relation to the ASyP (total 0.68 cm, anterior 0.00

cm, posterior 0.92 cm), disclosed a slight predominance of

anterior positioning of the ASyP in relation to the ASqP.

DISCUSSION

The sylvian fissure is the single most identifiable feature

of the superolateral face of the brain, and together with the

underlying sylvian cistern it constitutes the most frequent-

ly used microneurosurgical corridor because of the high

proportion of intracranial lesions that are accessible

through its opening. The transsylvian approach 32 through a

pterional craniotomy 31,33 is particularly useful for anterior

basal extrinsic lesions and for frontobasal, mesial temporal,

and insular intrinsic intracranial lesions. 29,30,33 Other fron-

totemporal craniotomies derived from the pterional 29,31 and

supraorbital 9 craniotomies, as are the combined epi- and

subdural approach with anterior clinoid removal 2,3 and the

orbitozygomatic extension of the pterional craniotomy, 6,7

enhance the basal approaches and minimize the brain re-

traction but do not disregard the opening of the SyF to op-

timize its ideal exposures.

Our findings relative to the ASyP indicate that it consti-

tutes an easily identifiable sulcal landmark with consistent

topographic relationships. Hence, it constitutes a good mi-

croneurosurgical starting point for anatomical orientation

and for the SyF opening.

The superior and inferior margins of the SyF constitute

the frontoparietal and temporal operculi, which cover the

superior and inferior aspects of the insula. 27 The frontopari-

etal operculum extends from the anterior to the posterior

ascending branch of the SyF. 28 With the orbital part of the

IFG disposed anteriorly, the suprasylvian structures can be

understood as a series of convolutions roughly arranged in

a V shape with its vertex constituted by the ASyP, followed

by three U -shaped convolutions and one C -shaped convo-

lution. The bottoms of the three U -shaped convolutions and

their related sulcal extremities can be situated either supe-

rior to the SyF or inside it, thereby giving the false visual

impression that their related sulci end at the SyF.

The anterior V -shaped convolution is constituted by the

triangular part of the IFG and is located just superior to the

ASyP. The horizontal and the anterior ascending rami of

the SyF delineate this convolution. Usually the triangular

part of the IFG contains a descending branch of the inferi-

or frontal sulcus (IFS).

The most anterior U -shaped convolution is the opercular

part of the IFG, which encloses the inferior aspect of the

precentral sulcus. In agreement with Ebeling, et al., 5 and

Ono, et al., 14 we also found that the precentral sulcus al-

ways ends inside the opercular part of the IFG, superiorly

RESULTS: PART II

Relationship of the ASyP With the External Cranial

Surface

The relationship of the ASyP with the external cranial

surface was evaluated based on the study of topographic

correlations between the ASyP and a skull point that was

designated as the ASqP. The ASqP was defined as the cen-

tral point of a 1.5-cm-diameter burr hole located on the

most anterior segment of the squamous suture, superior to

the sphenosquamous suture and just posterior to the sphe-

noparietal suture, and hence over the squamous suture just

posterior to the H -shaped central bar that characterizes the

pterion.

Pterion and ASqP Identification. After its exposure, the

pterion had an evident H -shaped morphology in 23 of the

32 specimens, and it had a dissimilar shape in the other

nine, but allowed an easy and proper ASqP identification in

all specimens studied.

Topographic Relationships Between the ASqP and the

ASyP

Vertical Relationship. The ASqP was superior to the

ASyP in one of the 27 specimens studied, was situated at

the ASyP level in 19, and was inferior to the ASyP in the

other seven, at a mean distance of 0.18 6 0.41 cm inferior

Neurosurg. Focus / Volume 18 / June, 2005

The anterior sylvian point and the suprasylvian operculum

or adjacent to the SyF (44%) or inside the SyF (56%). The

U -shaped convolution that encloses the inferior segment of

the precentral sulcus corresponds anteriorly to the opercu-

lar part itself and posteriorly to its connection with the pre-

central gyrus. Anteriorly the opercular part of the IFG is de-

limited by the anterior ascending ramus of the SyF, and

posteriorly by the anterior subcentral ramus of the SyF.

Together, the triangular and opercular parts of the IFG con-

stitute the motor speech area of Broca in the dominant

hemisphere 1,8 and the intraoperative estimation of its local-

ization can then be done based on the initial identification

of the ASyP.

The middle U -shaped convolution is called the subcen-

tral gyrus, and it connects the precentral and postcentral gy-

ri. The subcentral gyrus is also called the inferior frontopar-

ietal plis de passage of Broca 1 or the rolandic operculum,

because it encircles the inferior part of the central sulcus.

The location of the subcentral gyrus in relation to the SyF

varied in accordance with the positioning of the IRP; it

could be found either superior or adjacent to the SyF (83%)

or inside it (17%). The subcentral gyrus is delimited by the

anterior and posterior subcentral rami of the SyF.

The third U -shaped convolution, which is composed of

the arm connecting the postcentral and supramarginal gyri,

contains the inferior part of the postcentral sulcus. It is de-

limited anteriorly by the posterior subcentral ramus of the

SyF, and posteriorly by the posterior ascending ramus of

the SyF. According to the position of the inferior extremity

of the postcentral sulcus in relation to the SyF, the bottom

of this third U -shaped convolution can be superior to the

SyF (61%) or inside it (39%).

The C -shaped convolution that completes the frontopari-

etal or suprasylvian operculum is constituted by the con-

nection arm between the supramarginal gyrus and the supe-

rior temporal gyri, which encircles the posterior end of the

SyF. The inferior margin of the SyF is related only to the

superior temporal gyrus that constitutes the temporal oper-

culum.

Aside from its usually evident cisternal aspect due to the

fact that the IFG triangular part is generally retracted in re-

lation to its adjacent anterior orbital and posterior opercular

parts, the ASyP can also be intraoperatively identified as

the SyF segment located just posterior to the IFG orbital

part once this convolution bulges after the dural opening.

Ya sargil also emphasizes that “the Sylvian point is locat-

ed in the same plane of the IFG triangular part, and 10 to

15 mm anterior to the Sylvian venous confluence constitut-

ed by frontal and temporal tributaries veins,” and advises

“to begin opening the fissure immediately anterior to this

vein confluence at a point where a temporal or frontal ar-

tery or where both arteries appear at the surface of the fis-

sure,” 33 hence at the ASyP area. The opening of this fissure

site soon exposes the insular apex, 26 and the limen insula

and the middle cerebral artery bifurcation are located a lit-

tle deeper and more anterior, 10 to 20 mm perpendicular to

the ASyP itself. 33

Whereas the opening of the SyF posterior to the ASyP

exposes the lateral aspect of the insula, the opening of its

stem anterior to the ASyP leads to the suprasellar cisterns.

In conjunction with the opening of the sylvian ascending

ramus, this exposes the anterior periinsular sulcus, 26 and

thus the anterior aspect of the insula situated behind the

posterior orbital gyrus.

In regard to the ASyP projection on the external cranial

surface, Taylor and Haugton (1900) 23 described it in the

past as corresponding to the point of intersection of the syl-

vian line, constituted by a line from the junction of the third

and fourth segments of the nasion–inion curvature to the

orbitotemporal angle, with the line drawn from the junction

of the first and second segments of the nasion–inion curva-

ture to the external auditory meatus.

The consistently short distance between the ASqP and

the ASyP indicates that the ASyP is related to the 1.5-cm-

diameter burr hole centered on the anterior segment of the

squamous suture just behind the pterion (Fig. 4).

The knowledge of the ASyP distance to the IRP along

the SyF allows the neurosurgeon to estimate visually the

projection of the central sulcus over the SyF. The distance

of 2.3 6 0.5 cm is in accordance with the findings of Ono,

et al. 14

The knowledge of these basic anatomical features and

relationships can then lead us to a relatively precise initial

visual identification of the ASyP, both in preoperative neu-

roimaging studies and intraoperatively, and from it to fur-

Fig. 4. A: Photograph obtained at necropsy showing a 1.5-cm-diameter area (green ball) over the anterior segment of the squamous suture,

just behind the pterion. B: Photograph of cadaveric brain showing the horizontal and vertical mean distances of the ASyP from the central

point of the anterior squamous suture in a 1.5-cm-diameter area.

Neurosurg. Focus / Volume 18 / April, 2005

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