45 The Occipito-polar Approach: Study of the Occipital lnterhemispheric Supratentorial Transtentorial Transfalcine Approach
Abstract
Introduction: Lesions located in the posterior incisural space are difficult to treat due to limited exposure and because it contains important neurovascular structures. The classical approaches to this area are limited, especially when the lesion extends bilaterally and/or below the tentorium as it may occur with meningiomas. The parahippocampal gyrus and pulvinar of the thalamus region (PHG-PTR) is frequently affected by tumor and vascular lesions.
Patient series and anatomical study: An anatomical study of the occipito-polar approach and clinical evaluation of patients subjected to this access were performed. The surgical head position used in this surgical technique is the inverted park-bench position, that is, oblique and/or semi-prone on the side of the lesion with the head flexed and facing the floor.
Results: Tumor gross-total resection was attained in eighth cases and subtotal removal in nine. All the arteriovenous malformations were completely resected, the midbrain cavernoma was removed and the aneurysms were successfully trapped and/or clipped. The anatomical study revealed that after dislocation of the occipital lobe and opening of the quadrigeminal and the ambient cisterns, the following topographic anatomy of the posterior incisural space can be observed: on the superior portion of the splenium of the corpus callosum and immediately below is the isthmus of the cingulate gyrus, which continues downward and forward with the medial portion of the parahippocampal gyrus. Below the splenium of the corpus callosum and anterior and medially to the isthmus of the cingulate gyrus is the pulvinar of the thalamus.
Discussion: Occipito-polar approach has some major advantages over transcortical accesses. It is performed by dissecting along natural spaces—space between the medial surface of the occipital lobe and the falx, the transverse fissure of the brain and ambient cistern.
The surgical results and the anatomical study indicate that the occipito-polar approach makes it possible to treat the lesions of the posterior incisural space and regions of the parahippocampal gyrus and the pulvinar of the thalamus through a wide approach and with minimal cerebral clearance.
Keywords: infraoccipital supratentorial approach, occipital approach, transtentorial approach, transfalcine approach, posterior incisural space, falcotentorial meningioma, glioma, thalamus
45.1 Introduction
Tumors and vascular lesions frequently affect the parahippocampal gyrus and pulvinar of the thalamus region (PHG-PTR). In the present study, the PHG-PTR is defined as the region located lateral to the quadrigeminal cistern and below the splenium of the corpus callosum, which encompasses the pulvinar of the thalamus, the posteromedial portion of the parahippocampal gyrus, and the isthmus of the cingulate gyrus.
Lesions in this region are difficult to approach because they contain important neurovascular structures. Anterior (frontotemporal or orbitzygomatic) or lateral (subtemporal) approaches to the posterior mesial temporal lobe region and/or mesial parahippocampal gyrus require major retraction and/or damage to the brain structures related to this part of the temporal lobe. Transcortical accesses may lead to deficits, which are more deleterious in approaches to the dominant hemisphere. With supracerebellar infratentorial approach it may be difficult to access some of the lesions situated more superiorly and laterally, because of veins of the Galen’s ampoule can obstruct the view to the posterolateral incisural space.
In 1995, Smith and Spetzler proposed a posterior surgical approach, infraoccipital-supratentorial access for approaching lesions involving the PHG-PTR.1 The fundamental step in this access is the lateral retraction of the occipital pole. Since before 1995, we are still going about the same approach in a parallel way.
The objective of this study is to evaluate, through clinical and anatomical study, the occipito-polar approach in the treatment of lesions of the PHG-PTR in the posterior incisural space.
45.2 Patient Series, Anatomical Study, and Methods
45.2.1 Anatomical Study and Patient Series
Anatomical study of the occipito-polar approach and clinical evaluation of patients subjected to this access were performed.
The anatomical study was performed at the Neurosurgery Laboratory of the UFMG School of Medicine in five cephalic segments (10 sides), preserved in formaldehyde and whose vessels were injected with colored silicone.
The anatomical study was performed in both sides of five formalized cephalic segments (10 approaches), and later stored in alcohol solution. To decrease the rigidity of the formalized anatomical parts, quaternary ammonium compound in the form of “fabric softener” was added to the alcohol solution. The vertebral, carotid arteries and internal jugular veins were dissected in the neck and injected with colored silicone. The 10 anatomic occipito-polar approaches were performed in the five cephalic segments trying to simulate the surgical technique used in the present series and described below (Fig. 45.1).
Fig. 45.1(a) Sagittal view of posterior incisural space. (b) Arrow showing the pathway to the posterior incisural space following the inferior and medial face of the occipital lobe. (c) Pathway to the posterior incisural space along the straight sinus. (d) Outlines of the craniotomy and the three craniometric points that determine the limits of the craniotomy (I = Inion, A = Asterion, L = Lambda). (e) Bilateral occipito-polar approach with superior sagittal sinus and transverse sinuses fully exposed. (f) Shows the angle of view in a right side occipito-polar approach. (g) Left side occipito-polar approach with superior sagital sinus (SSS) at the top, transverse sinus at the right. (h) Opening of the dura mater and natural dislocation of occipital pole due to the gravity. (i) Retraction of occipital lobe and exposure of neurovascular structures in the PHG-PTR. PHG-PTR, parahippocampal gyrus and pulvinar of the thalamus region.
At the end of the approach, the following measurements were taken: from the torcula to splenium of the corpus callosum, torcula to pineal gland, torcula to calcarine artery origin, torcula to isthmus of the corpus callosum, torcula to pulvinar, torcula to quadrigeminal plate, torcula to falcotentorial junction, and torcula to occipital pole (Table 45.1).
Out of the 17 patients with tumor lesions, 10 were males and 7 females. The age of these patients ranged from 15 to 65 years, with a mean of 48 years. Linear accelerator radiotherapy was given to 13 patients (8 astrocytomas, 1 solitary fibrous tumor, and 4 metastasis), and only 1 underwent radiotherapy before surgical treatment.
In the group of vascular lesions, six were females and two males, with a mean age of 29 years (range of 4 to 50 years). In patients with arteriovenous malformations, vascular injury was manifested by intraventricular hemorrhage in three and subarachnoid hemorrhage in two, the two aneurysms and the cavernoma presented with bleeding in one.
45.2.2 Surgical Technique
The surgical position used was an inverted park-bench position, that is, semi-prone on the side of the lesion with the head flexed and facing the floor. The cephalic segment was slightly flexed and rotated approximately 30 degrees in the same direction to the side to be approached. In this position, the external occipital protuberance occupies the highest position and the occipital region to be approached is located in an inferior plane to the contralateral occipital region. The medial face of the occipital lobe on the approach side tends, by force of gravity, to move away from the falx cerebri, eliminating the need for major retraction over the occipital lobe. It is essential to include the external occipital protuberance in the bone flap to expose the torcula, as this structure defines the line of sight toward the falcotentorial junction (straight sinus) to reach the transverse fissure of the brain and the cistern ambient.
In anatomical position of the specimens, the falcotentorial junction is almost parallel to the horizontal plane. The cerebral falx is in the median (sagittal) plane and the cerebellum tent in the horizontal (axial) plane. In the surgical position, the falcotentorial junction is rotated approximately 30 degrees to the side to be approached, allowing, under the force of gravity, the downward displacement of the occipital lobe with consequent opening of the space between the medial face of this lobe and the falx cerebri.
The skin flap over the occipital region is U-shaped, with inferior base in the nuchal region and/or the temporal side. Alternatively, the skin flap can be pedicled toward the ear on the temporal region in order to try to avoid ischemic areas in the skin. Skin incision should extend 2 cm beyond midline on the contralateral side, 7 cm from midline toward the asterion, 2 cm below the external protuberance, and 8 cm above (Fig. 45.2).
Fig. 45.2(a, b) “U” shaped skin incision for occipito-polar approach (I = Inion, A = Asterion, L = Lambda). (c, d) “C” shaped skin incision for occipito-polar approach (I = Inion, A = Asterion, L = Lambda). Note that whether using an “U” or “C” shaped incision it must extend past the midline by 2 cm, inferiorly to the inion by 2 cm, above the inion by 8 cm, and laterally to the inion by 7 cm.
Such a skin flap allows craniotomy with complete exposure of the torcula (confluence of the sinuses), the superior sagittal sinus, the transverse sinuses, and the occipital lobe on the approached hemisphere. Sinus exposures are crucial to allow adequate vision of the occipital lobe pole through retraction from the falx cerebri and the cerebellum tentorium.
Because of the risk of injury to the sinuses, with consequent bleeding and air embolism, skull landmarks should be followed to guide the full exposure of the torcula and the superior sagittal and transverse sinuses. Three craniometric points are used as landmarks: lambda, asterion, and inion.2 A line, approximately 7 cm long, that joins the inion to the lambda corresponds to the superior sagittal sinus and the upper edge of the occipital lobe. The asterion corresponds to the lateral portion of the transverse sinus. The inion–asterion line generally corresponds to the inferior margin of the transverse sinus groove. The inion–external occipital protuberance corresponds to the torcula internally in the skull. The posterior end of the superior sagittal sinus dilates, constituting the confluence of sinuses or torcular Herophili, which lies mainly on the right side of the internal occipital protuberance, where it continues with the most developed transverse sinus.
An occipital craniotomy is performed to expose the torcula, and superior sagittal and transverse sinuses. After craniotomy, in some cases, lumbar cerebrospinal fluid (CSF) drainage was initiated, through a previously implanted external lumbar drain, to facilitate exposure and decrease the need for occipital lobe retraction. Dura mater was incised along the superior and transverse sagittal sinuses and reflected laterally. When lumbar CSF drainage was contraindicated, the occipital horn of the lateral ventricle was punctured. The occipital lobe was gently retracted from the cerebellar tentorium and cerebral falx. The lack of anastomotic veins between the occipital lobe and the superior sagittal sinus facilitates this maneuver.
After that, dissection is carried out under the surgical microscope. After identifying one of the branches of the posterior cerebral artery (calcarine and/or posterior temporal arteries) it was used as a guide to reach the ambient cistern. Next, the ambient cistern was opened and CSF was drained. The resulting relaxation allows greater lateral displacement of the occipital lobe, permitting better exposure of the incisural space and no need of the cerebral retraction. Medially, within the ambient cistern, lies the basal vein (vein of Rosenthal) and the posterior cerebral artery.
Continuing, tumors were devascularized by coagulation of the feeding arteries. Internal decompression/debulking was performed with suction, bipolar coagulation, and microscissors. Dissection was carried out in the arachnoid plane or within the boundaries of normal brain tissue for radical resection of the lesion, thus reducing the risk of neurovascular structure damage.
Finally, the dura was sutured, the bone flap was replaced, and the muscle-aponeurotic plane, subcutaneous tissue, and skin were closed.
Following the surgical intervention, the patients were observed in an intensive care unit. All patients underwent postoperative CT and/or MRI, and patients with vascular lesions underwent cerebral angiography.
45.3 Illustrative Surgical Cases
45.3.1 Case 1
A 58-year-old female was admitted to our neurosurgical unit with headaches, mental confusion, and papilledema. A CT scan revealed a large mass in the incisural space compressing the midbrain and causing hydrocephalus. She underwent an emergency procedure to implant a ventriculoperitoneal shunt.
The patient recovered fully after the shunt, and an MRI showed a large hyperintense lesion, located bilaterally on the posterior incisural space, compressing the midbrain. The patient was operated using an occipito-polar approach that consisted of a unilateral occipital craniotomy exposing the torcula, superior sagittal sinus, transverse sinus, and occipital lobe. After opening the dura, the occipital lobe was retracted, exposing the tentorium, falx, and part of the tumor.
The ipsilateral supratentorial part of the tumor was then removed. An incision at the tentorium, lateral and parallel to the straight sinus, was performed (occipital transtentorial approach), allowing the removal of the ipsilateral infratentorial part of the tumor. Another incision in the falx was made parallel to and 1 cm above the straight sinus, exposing the contralateral supratentorial aspect of the lesion that was removed. Finally, the contralateral tentorium was incised, parallel and 1 cm laterally to the straight sinus, exposing the contralateral infratentorial aspect of the tumor.
After a brief confusion of the mental status that was resolved spontaneously, the patient was discharged from the hospital on day 12 postoperatively without any neurological deficit. A head CT scan was performed 24 hours and 24 months after surgery showing complete tumor removal. The histopathology was compatible with fibrous meningioma. In the follow-up the patient was intact and returned to her regular daily activities (Fig. 45.8).
45.3.2 Case 2
A 71-year-old female with progressive headache in the last 3 months, followed by episodes of mental confusion and vomiting, came to our institution. Initial head CT was followed by a contrast MRI that showed evidence of a large extra-axial enhancing lesion, with dural attachment to the cerebral falx and tentorium, suggesting a falcotentorial meningioma. After signing consent, the patient was taken to surgery (Fig. 45.3 and Fig. 45.4).
