The Anterior Sylvian Point and the Suprasylvian Operculum
The Anterior Sylvian Point and the Suprasylvian Operculum
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 suitability as an initial, visually identifiable landmark for further neuroimaging and intraoperative estimation of its adjoining 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 specimens 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 ± 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 recognized, 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 convolution.
In their original description of the microsurgical anatomy of the subarachnoid cisterns in 1976, Ya¸sargil, et al., emphasized the importance of the SyF, which then became the main microneurosurgical corridor to the base of the brain. In later publications Ya¸sargil, et al., described in detail the microanatomy of this fissure and its underlying cistern and the technique of its opening. In agreement with other authors, 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, 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.
Taylor and Haugton, 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 outer aspect of the hemisphere." In his textbook published in 1912, Krause reproduced illustrations of the German anatomist August von Froriep (18491917) with identification of the sylvian point and also illustrated an anatomical opening of the SyF for the exposure of a superficial insular lesion (Fig. 1). Recently Türe, et al., stressed the use of the term sylvian point.
(Enlarge Image)
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.)
Despite the emphasis given by recent authors to the location of the sylvian point, none of them referred to the frequent enlargement of the fissure at the sylvian point, despite its presence in their own illustrations. This cisternal aspect of the sylvian point is also evident in the illustrations in many other recent publications and in old texts (Fig. 2).
(Enlarge Image)
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 Topogrαfica, ed 5. Barcelona: Salvat, 1932.)
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 sylvian point is designated as the anterior as opposed to the posterior one, which corresponds to the distal extremity of the posterior ramus of the SyF, from which the ascending terminal ramus and the occasional descending terminal ramus originate. 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 relationships 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.
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 suitability as an initial, visually identifiable landmark for further neuroimaging and intraoperative estimation of its adjoining 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 specimens 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 ± 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 recognized, 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 convolution.
In their original description of the microsurgical anatomy of the subarachnoid cisterns in 1976, Ya¸sargil, et al., emphasized the importance of the SyF, which then became the main microneurosurgical corridor to the base of the brain. In later publications Ya¸sargil, et al., described in detail the microanatomy of this fissure and its underlying cistern and the technique of its opening. In agreement with other authors, 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, 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.
Taylor and Haugton, 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 outer aspect of the hemisphere." In his textbook published in 1912, Krause reproduced illustrations of the German anatomist August von Froriep (18491917) with identification of the sylvian point and also illustrated an anatomical opening of the SyF for the exposure of a superficial insular lesion (Fig. 1). Recently Türe, et al., stressed the use of the term sylvian point.
(Enlarge Image)
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.)
Despite the emphasis given by recent authors to the location of the sylvian point, none of them referred to the frequent enlargement of the fissure at the sylvian point, despite its presence in their own illustrations. This cisternal aspect of the sylvian point is also evident in the illustrations in many other recent publications and in old texts (Fig. 2).
(Enlarge Image)
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 Topogrαfica, ed 5. Barcelona: Salvat, 1932.)
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 sylvian point is designated as the anterior as opposed to the posterior one, which corresponds to the distal extremity of the posterior ramus of the SyF, from which the ascending terminal ramus and the occasional descending terminal ramus originate. 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 relationships 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.