Fig. 21.1
T1-weighted MR images with gadolinium contrast enhancement showing a planum sphenoidale meningioma. Preoperative coronal (a) and sagittal (b) scans; postoperative coronal (c) and sagittal (d) scans
In T1-weighted scans, the tumor is of equivalent signal intensity compared to the surrounding brain, while in the T2-weighted scans, the meningioma usually shows a “sunburst” pattern with or without necrosis, cysts, hemorrhage, trapped hyperintense CSF clefts, and vascular flow voids. In FLAIR-weighted images, the hyperintense peritumoral edema can be identified, while T2-star gradient-echo sequences may detect calcifications. After the gadolinium injection, the tumor gains homogeneous and intense contrast enhancement; moreover, the possible presence of the “dural tail” (35–80 % of cases) can be detected.
Magnetic resonance angiography may provide essential information about blood supply and displaced arteries or even arteries embedded within the tumor.
Angiography, generally, has not been indicated unless preoperative embolization is planned. At any rate, the classic angiographic appearance of a meningioma is that of increasing hypervascular tumor blush throughout the arterial phase, persisting well into the late venous phase with slow washout.
In conclusion, even if MRI has to be considered as the best diagnostic tool for the evaluation of a meningioma, cranial computed tomography (CT) and, in selected cases, angiography may be considered as important adjuncts to evaluate and characterize the involvement of the anterior skull base, the infiltration of the olfactory groove and ethmoid bone, the relationship of the major vessels, and the main vascular supply of the tumor, i.e., the ethmoidal arteries.
21.2.1 Surgical Planning
The knowledge of surgical anatomy is imperative for complex neurosurgical procedures in regions with vital structures nearby, as in skull base surgery.
In the field of neurosurgery, progress in neuroimaging studies, such as high-resolution CT scans, MRI studies, and digital subtraction angiography data, has certainly refined the visualization of the brain and skull base anatomy. On the other hand, the progress in computer technology and medical image processing techniques has enabled stereoscopic display of anatomical structures from computed imaging data. Indeed, three-dimensional (3D) imaging, which allows image manipulation and surgical simulation on-screen, has become an indispensable part of the neurosurgical planning and training.
Our surgical planning for skull base meningiomas generally provides two different methodologies: First of all, standard medical image data such as MRI and CT scan to obtain a general idea of the tumor and the surrounding structures is performed.
In a second step, a virtual preliminary exploration of patient’s anatomy using the 3D reconstruction modules supported by the OsiriX software (Osirix®, advanced open-source PACS workstation DICOM viewer) in order to analyze the individual variability of the anatomy is performed. At the end of such step, the segmented objects (representing skin, tumor mass, vascular system, ventricular cavities) are displayed by a combination of volume rendering and polygonal iso-surfaces, ready to be manipulated.
Such types of 3D models have been previously utilized from recently published works of our group concerning the anatomy of microscopic and endoscopic skull base approaches [12–14] (Fig. 21.2).
Fig. 21.2
Anatomical photographs (a, b) of a cadaveric dissection showing patient positioning and skin incision for a subfrontal unilateral approach. (c, d) Computed 3D virtual reality images showing CT reconstruction of the head (c) and the target area, i.e., anterior cranial fossa (d)
21.3 Anatomy of the Approach
In order to perform a subfrontal approach, either uni- or bilateral, the anatomy of the anterior skull base and of the subfrontal pathway must be clearly understood.
First of all, it has to be remarked that frontal, parietal, temporal, zygomatic, and sphenoid bones, connected through their respective sutures, form the anterolateral region of the skull.
Moreover, the normal anatomy of the anterior cranial fossa should be deeply recognized. From the endocranial view, the anterior cranial base has a flat surface that comprises the anterior border of the sphenoid wings and the roof of the orbita, laterally, and the planum sphenoidale, medially. The anterior cranial fossa is principally formed by the orbital process of the frontal bone that is convex and has a variable number of orbital crests. A most anterior bony ridge, i.e., the frontal crest, is located in the midline and separates the two sides and gives attachment to the falx cerebri, which contains the origin of the superior and inferior sagittal sinuses. The central portion of the anterior fossa is deeper and is composed of the ethmoid bone, with the medial portion represented by the cribriform plate – that shows multiple perforations transmitting the olfactory nerve filaments – and the lateral one that is the fovea ethmoidalis, i.e., the roof of the ethmoid sinus. The crista galli is positioned at the center. The foramen cecum, crossed by an emissary anterior nasal vein, is located between the frontal crest and the crista galli. Lateral to the cribriform plate, the cribroethmoid foramina gives passage to the anterior and posterior ethmoidal arteries. On the other hand, the posterior portion of the anterior fossa is formed by the upper part of the sphenoid bone, namely, its body and lesser wings. Centrally, lies the planum or jugum sphenoidale, which constitutes the roof of the sphenoid sinus, bordered posteriorly by the anterior chiasmatic sulcus. Laterally, the lesser wing of the sphenoid roofs the optic canal, which contains the optic nerve and its dural sheath. The anterior clinoid process, i.e., the medial end of the lesser wings of the sphenoid, gives attachment to the tentorium cerebelli and covers the anteromedial portion of the cavernous sinus which contains the supraclinoid portion of the internal carotid artery (ICA). Other key neurovascular structures that can be exposed during the access to the anterior cranial fossa are the following: the olfactory bulb and tract, optic nerves, optic chiasm and lamina terminalis, the anterior cerebral arteries and the anterior communicating artery, the posterior communicating artery, the anterior choroidal artery, the third cranial nerve, the superior hypophyseal artery, the pituitary stalk and the diaphragma sellae, the ophthalmic arteries, and Heubner’s recurrent arteries. Moreover, the opening of the Liliequist’s membrane permits to get inside the interpeduncular cistern in order to expose the basilar artery, the posterior cerebral arteries, the superior cerebellar arteries, and the origin of the third cranial nerve.
It has to be reminded that before performing a subfrontal approach, unilateral or bilateral, a precise knowledge of the main frontal anatomic landmarks must be obtained (i.e., midline, orbital rim, supraorbital foramen, temporal line, and zygomatic arch). The orbital rim is the frontal bone part that forms the roof of the orbits, the zygomatic process of temporal bone, and the temporal process of the zygomatic bone form the zygomatic arch. The supraorbital foramen is situated along the supraorbital margin, which is entirely formed by the frontal bone and is crossed by the supraorbital nerve and vessels (supraorbital artery and supraorbital vein). Finally, the superior temporal line of the parietal bone gives attachment to the temporal fascia, indicating the origin of the temporalis muscle.
21.3.1 Brief Consideration of Surgical Neuroanatomy
As already said, meningiomas of the midline anterior cranial base are classified based on an anteroposterior direction: olfactory groove, planum sphenoidale, and tuberculum sellae meningiomas.
Olfactory groove meningiomas arise in the midline of the anterior cranial fossa over the cribriform plate and frontosphenoid suture. Those tumors generally grow in a symmetric fashion around the crista galli and, subsequently, may involve any part of the planum of the sphenoid bone and/or, less frequently, extend predominantly to one side. The anterior and posterior ethmoid arteries drive the primary blood supply to these tumors. However, these tumors may be also vascularized by the meningeal branches from the ophthalmic artery, anterior cerebral arteries, anterior communicating artery, pial collaterals, and external carotid circulation, such as anterior branches of the middle meningeal artery. Generally, in these cases, the olfactory nerves either are displaced laterally on the lower surface of the tumor or are adherent, compressed, or even not visible due to a diffuse spread within the tumor capsule. It has to be minded that in smaller tumors, the post-communicating segments of the anterior cerebral arteries usually are not involved in the tumor capsule. However, in large tumors, these and additional segments, i.e., the frontopolar or other small branches originating from the anterior cerebral arteries, may adhere to the tumor capsule.
Regarding the planum sphenoidal meningiomas, it has to be taken into consideration that planum sphenoidale and tuberculum sellae are part of the sphenoid bone. However, it is often difficult to clearly separate these tumors simply based on their bony covering. Rather, their relationship to the optic nerves and chiasm can distinguish these tumors as to their most likely origin. While planum sphenoidale meningiomas usually push the optic nerves dorsally and caudally, tuberculum sellae meningiomas lead to an upward and/or lateral bulging of these structures. At any rate, both entities might grow between, around, and beyond the optic nerves.
Tuberculum sellae meningiomas arise from the dura of the tuberculum sellae, chiasmatic sulcus, limbus sphenoidale, and diaphragma sellae. The tuberculum sellae is a bony elevation ridge that lines up the anterior aspect of the hypophyseal fossa, dividing it from the chiasmatic sulcus. The lateral end of this structure is just inferomedial to the intracranial outlet of the optic canal, through which the optic nerve runs to join the contralateral optic nerve at the chiasm. Behind the optic foramen, the anterior clinoid process is directed posteriorly and medially. The primary blood supply to these tumors is principally from the posterior ethmoid arteries. According to a recent proposed classification of suprasellar meningiomas, based on the origin and location of the tumor, Liu et al. [15] identified four groups: (a) tumor originating from the planum sphenoidale, rarely involves the optic pathway or pituitary stalk; (b) tumor located at the tuberculum sellae, mainly involves the optic pathway but rarely involves pituitary stalk; (c1) tumor located at the diaphragma sellae, which involves both the optic pathway and the pituitary stalk, pushing the chiasm anteriorly in to “prefixed chiasm” position, resulting in minimal pre-chiasmatic working area; (c2) tumor located at the diaphragma sellae but pushing the optic chiasm posteriorly, putting it in to “postfixed chiasm” position, resulting in expansion of the pre-chiasmatic area.
21.4 Technique
Generally, as the majority of meningiomas are benign tumors (WHO I), extra-axial and well-defined, complete surgical removal (Simpson Grade I) should be the primary goal in most cases. For anterior midline meningiomas, complete tumor excision even with resection of infiltrated dura or removal of hyperostotic bone might be achieved with low morbidity in most instances. However, when tumors are hardly adherent to the anterior brain circulation vessels, the optic apparatus, or within and near the pituitary gland and stalk, complete removal might represent a high-risk procedure for damage of those important neurovascular and endocrine structures [4, 16, 17].
21.4.1 Subfrontal Unilateral Approach
The patient is placed in the supine on the operating table with the head fixed in a three-pin Mayfield head holder. The positioning largely depends on the involvement of the midline structures, the displacement and orientation of the main vessels, and the optic nerves. As a general rule, the head has to be rotated toward the contralateral side of 20–40° and slightly extended posteriorly, as the frontal lobes follow gravity, thus making a natural exposure of the anterior cranial base and facilitates good venous drainage during surgery. In other words, the patient’s neck has to be retroflexed, in order to form an angle of approximately 20° between the plane of the anterior cranial base and the vertical plane of the axis (Fig. 21.2). Fine adjustments of the patient’s position can be obtained by tilting the operating table.
After a precise definition of the frontal anatomic landmarks, already described in the “anatomy of the approach section,” the line of the incision is marked on the skin.
A curved frontal skin incision, beginning at the level of the top of the helix of the ear or slightly anterior to it on the side of the craniotomy and behind the hairline, is performed and extends until the midline in a curvilinear fashion above the superior temporal line. The skin incision is posterior to the superficial temporal artery, in order to include the artery in the skin flap. The incision should not be extended below the zygomatic arch to avoid injury of the frontal branch of the facial nerve (Fig. 21.3).
Fig. 21.3
Anatomical pictures of a cadaveric dissection showing the skin flap preparation; pericranium before (a) and after (b) its incision along the superior temporal line; corresponding intraoperative images (c, d). TM temporalis muscle, G galea capitis, STL superior temporal line
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