R. Timothy Bentley Purdue University, West Lafayette, IN, USA Meningiomas are the most common primary brain tumor of dogs (especially non‐brachycephalic breeds) and cats [1, 2], and they are the most responsive to surgery. Gross‐total resection (GTR) refers to the complete removal of a brain tumor, as assessed by a combination of intraoperative inspection and postoperative Magnetic Resonance Imaging (MRI) [3]. In cats, as in humans, meningiomas tend to be low grade and surgery alone is often curative [1, 2]. In dogs, surgical biopsy often results in a low grade, yet meningiomas may recur despite GTR [2, 4]. Necropsy may reveal that when the histological features of a canine meningioma itself are worthy of a low grade, there is microscopic invasion of the adjacent brain surface [2, 5]. Meningiomas are believed to arise from the arachnoid cap cells within arachnoid villi [1, 2], and as such they are located deep to the dura mater and attached to its underside (Figure 22.1). They are superficial to the pial surface of the brain. Particularly in cats, the meningioma may be very well encapsulated and peel off the pial surface of the brain very easily. In dogs, meningiomas can be much more adherent to the brain surface, even invasive [2]. The subarachnoid space containing cerebrospinal fluid (CSF) is deep to the meningioma, but it is typically collapsed, with the meningioma tight against the brain surface. When incising the dura adjacent to a meningioma, CSF should leak out, but when incising the dura immediately over a meningioma often no CSF appears. The vascular supply of meningiomas appears to arise from a combination of the overlying dura mater and the pial surface of the brain [6–8]. In humans, it is often dural and pial. In cats, it is often dural. In dogs, it is often pial. This may correlate to the ease with which many feline meningiomas can be physically removed from the brain surface at surgery and/or their lower rate of postoperative recurrence [2]. This also pertains to the recurrence rate as in many canine meningiomas, a majority of the vasculature arises from the pia, and thus has a close association with the brain parenchyma. Most meningiomas occur in the rostral cranial fossa, which is the location most amenable to surgery. Caudal fossa meningiomas are less common, but cerebellar tumors in particular can be operable (see section Suboccipital craniectomy below, and Chapter 24 Surgery of the Caudal Fossa in this book). Middle fossa meningiomas are also less common but can be operable [see Chapter 20 Surgical Management of Sellar Masses in this book]). The dura mater is a single surgical layer; however, it is comprised of two layers: the outer periosteal layer and the inner meningeal layer. These two layers divide around venous sinuses such as the dorsal sagittal sinus (DSS). These veins are valve‐less and must be ligated on both sides of any intended incision. The dura mater is fused with the periosteum of the skull (there is no epidural fat). The epidural space is a potential space that can be filled with hematoma postoperatively. The falx cerebri sits within the longitudinal fissure, immediately under the skull and between the two cerebral hemispheres. It attaches rostrally to the internal frontal crest (frontal bone), the crista galli (the midline ridge in the cribriform plate, ethmoid bone), and the presphenoid bone. It is continuous with the membranous tentorium cerebelli caudally. The DSS is within the dorsal part of the falx (Figure 22.2), fully enclosed within the falcine dura. The DSS flows from rostral to caudal, becoming progressively wider in diameter as it accepts dorsal cerebral veins (vv. cerebri dorsales) and diploic veins from the skull. Ventral to the falx is the vein of the corpus callosum (v. corporis callosi), which joins with other veins to form the straight sinus (sinus rectus). The straight sinus enters the DSS at the level of the occipital lobes. Caudal to the occipital lobes, the DSS passes through the periosteal dura mater to enter the foramen for the DSS (syn. impar foramen) where it is about 3 mm in diameter in dogs. The DSS ends at the confluence of the sinuses (confluens sinuum) located inside the caudal skull, which is drained by the paired transverse sinuses. The straight sinus sometimes joins the confluence within the skull, rather than joining the DSS in the falx. Each transverse sinus runs laterally in the transverse canal (fully enclosed in bone) then in the transverse groove (incompletely enclosed in bone), then continues as the temporal sinus after giving off the sigmoid sinus. The single DSS and paired transverse sinuses provide the only venous drainage of the dorsal cerebral hemispheres. Surgically, the rostral third of the DSS can be ligated (e.g. rostral to the cruciate sulcus or ventral to the frontal nasal sinus). The caudal two thirds cannot be acutely occluded, nor can the confluence within the bone, without risking venous stroke of both dorsal cerebral hemispheres. The caudal third of the falx also contains the straight sinus. If the caudal falx cerebri is traumatized leading to hemorrhage (e.g. debriding a meningioma away from the falx), it cannot be ligated. Pressure must be applied to the falx (e.g. gelatin sponge) until the bleeding arrests. The surgeon must hope that the sinuses do not thrombose. The dura cannot be elevated from the caudal skull on the midline (the vertical bone caudal to the occipital lobes), as this would tear the DSS between the dura mater and the foramen for the DSS. One transverse sinus can be occluded, but not both. For extremely ventral hemispheric exposure, the temporal sinus can be occluded within the temporal bone using bone wax. Figure 22.1 Caudal cerebellar meningioma in a dog. The white dura mater (Bishop‐Harmon forceps) is being reflected back from the cerebellum, revealing a meningioma (asterisk) and the cerebellar vermis. Note that the meningioma is stuck to the underside of the dura mater. The defect in the dura (durectomy) is purposefully being made larger than the meningioma. Figure 22.2 Placement of three ligatures around the dorsal sagittal sinus (DSS) before incising the rostral falx. Note that ligatures can only be placed around the rostral one‐third of the DSS, unless a preoperative angiogram has documented occlusion of the DSS by a meningioma in caudal two‐thirds of the falx. To remove a meningioma and all associated dura mater from the rostral falx, the DSS must be ligated. First, durotomy is performed on both sides of the falx cerebri (or a durectomy on the side of the tumor). If possible, the intended falcine incision (white arrow) should be 1–2 cm caudal to the meningioma (M). At least one simple and one transfixing (or three simple) sutures must be placed caudal to the intended incision. At least one suture should be placed rostral to the intended incision. The falx can then be pre‐cauterized (bipolar) and incised (e.g. Metzenbaum scissors). For a rostral falcine meningioma, either additional ligatures can be placed rostral to the tumor (with multiple sutures rostral to the incision). Or the falx can simply be removed from its attachment to the rostral skull (internal frontal crest, crista galli, and the presphenoid bone) using pre‐cautery (bipolar) and scissors. Lifting up the caudal edge of the falcine strip facilitates access to the falcine attachment to the skullbase. Inset: the falx cerebri is immediately ventral to the skull. The DSS is within the dorsal falx cerebri, and fully enveloped by dura mater in the normal subject. Surgical locations of rostral fossa meningiomas include convexity tumors, on the surface of one cerebral hemisphere [9]. Falcine tumors, arising from the falx cerebri, are especially common in the fronto‐olfactory region [1]. Variations of meningiomas recognized in human neurosurgery include parasagittal (to one side of the dura [9], e.g. contacting both the dorsal dura and the falcine dura), olfactory groove (the depression in the ethmoid bone for the olfactory bulb), and sphenoid wing (the ridge of the sphenoidal bone on the skullbase that is highly developed in humans, separating the anterior and middle cranial fossae). Of these, parasagittal and olfactory groove meningiomas are frequently operated in dogs and cats, but often classified simply as falcine meningiomas. Sphenoidal wing meningiomas necessitate a different surgical approach in humans (e.g. pterional craniotomy), but in dogs and cats most would be accessed through a rostrotentorial or transzygomatic approach (see Chapter 25 Transzygomatic Approach to Ventrolateral Craniotomy/Craniectomy in this book) and would often be classified simply as convexity meningiomas. This ridge of bone is poorly developed in dogs and cats and referred to simply as the sphenofrontal suture rather than the sphenoidal wing. As such, adoption of the sphenoidal wing terminology used in humans does not seem appropriate. Tentorial meningiomas may be accessible by a caudal rostrotentorial craniectomy and working under the occipital lobe or a transtentorial craniectomy [10]; note the transverse sinus anatomy. It is possible to have multiple meningiomas, especially in cats, and it may be appropriate to remove two or more in a single surgery [11]. Uncommon locations include intraventricular meningiomas in cats, and the optic nerve (which is sheathed by dura mater rather than epineurium) [1]. A summary of the surgical approaches to the differing locations of intracranial meningiomas is shown in Table 22.1. Table 22.1 Main approaches to the cranial vault for the extirpation of meningiomas in dogs and cats [4, 12, 13]. This provides access to the olfactory bulbs, the rostral frontal lobes and up to one third of the falx cerebri. Typically, the upper plate of the frontal bone is removed (and replaced during closure), then the lower plate of the frontal bone is removed (and not replaced) to access the cranial vault [4, 14]. A dorsal midline skin incision is made, starting rostral to the medial canthi and ending over the external sagittal crest. The subcutaneous frontalis muscle is sharply incised on midline. A vein often exits through a small midline foramen in the nasal bone; it can be cauterized or bone‐waxed. The sharp midline incision is continued to the periosteum. The periosteum and subcutaneous tissues are elevated (e.g. Freer elevator) in a lateral direction, to either side of the incision. A diamond‐shaped bone flap is now removed from the upper plate of bone, to enter the frontal sinus [4]. The outline of this bone flap is the diamond shape of bone, rostral to the temporalis muscles. It is a flat segment of bone bordered by the left and right frontal crests, the zygomatic processes, and the orbits. It is primarily the squamous part of the frontal bone, but the rostral section is nasal bone. A sagittal saw (e.g. 3–5 mm blades, E‐pen) can be used to cut the bone flap at a 45° angle [4]. This angulation allows the bone flap to sit in position during closure (Figure 22.3). Leaving a small ledge laterally facilitates closure (see below). Once this cut has been made, the plate of bone will be rigidly held in place by the many osseous septa within the frontal sinus, and especially by the midline septum (septum sinuum frontalium) that divides the frontal sinus into left and right compartments. A small burr (e.g. 3 mm) can be used on the external sagittal crest and the most caudal corner of the diamond‐shaped flap to gain access to the septum. Next, a small osteotome is driven down the midline, passing ventral to the bone flap and roughly parallel with it in a rostro‐ventral direction, to destroy the midline septum (Figures 22.4 and 22.5). Often, it becomes impossible to drive the osteotome any further rostrally. Even smaller osteotomes can now be used either side of midline, to crack the osseous septa that connect the upper and lower plates of frontal bone. As the bone flap becomes progressively more mobile, the first osteotome can be driven further down the midline septum and the bone flap will come free. Figure 22.3 A conventional diamond‐shaped bone flap for a transfrontal craniotomy. Note the wide access to the olfactory bulbs (white triangles) but the narrow access to the frontal lobes. Additional bone is being burred away to improve access to the left frontal lobe, resulting in a bone gap at closure (white arrow). In this brachycephalic cat skull conformation, the caudal half of the diamond is reduced in size and made even smaller by burring bone to pass osteotomes (black triangles). Matching pairs of 1 mm burr holes were made in a small ledge created by making the bone flap slightly less wide than the frontal sinus itself. Note that none of the horizontal mattress sutures (0 PDS) are tied down until the suture has been passed through all the burr holes. Bone sutures are often placed on all 4 sides of a bone flap, but suturing three sides is acceptable. Note that the rostral bone sutures were placed by first passing both ends of the suture down through the skull and then up through the bone flap. This leaves the knots dorsal to the bone flap and medial to sagittal saw incision. In this case, the caudal suture was placed by passing the suture ends down through the bone flap and up through the skull, but this can be very awkward. In extremely small patients with very thin plates of bone, it is especially important to make the sagittal saw cut at a steep angle (far from vertical). The bone flap would fall down into the frontal sinus if vertical cuts were made. The osseous septa are removed with rongeurs from the underside of the bone flap, as are the septa remaining within the sinus. The bone flap is kept clean and secure (e.g. a bowl of flush) until closure. Ectoturbinates 2 and 3 extend into the most rostral aspect of the fontal sinus. To access the most rostral cranial vault (olfactory bulbs), these can be removed with rongeurs. The resulting mucosal hemorrhage is controlled by packing gauzes or lap sponges rostrally in the frontal sinus. Ectoturbinate removal maximizes frontal sinus drainage postoperatively, minimizing mucocele formation. The cribriform plate itself can even be resected unilaterally, but it is usually possible to resect even the most rostral meningiomas through a bone defect that ends immediately caudal to the cribriform plate, reaching behind an intact cribriform plate. Intraoperative mucosal hemorrhage and postoperative hyposmia arise from cribriform plate resection, severely impacting postoperative appetite. The mucoperiosteum is removed from the entire frontal sinus, to reduce postoperative mucocele (e.g. dry gauze is driven over every surface and into every crevice using a Freer elevator). A burr (e.g. 3 mm, E‐pen or Surgairtome) is used to remove the inner plate of the frontal bone, entering the cranial vault. This second bone flap will also be diamond‐shaped, but it can be discarded. Ipsilateral to the tumor, it is often beneficial to make this bone flap as wide as possible. To do so, burr exactly where the horizontal bone over the brain meets the vertical bone that makes up the lateral aspects of the frontal sinus. The bone will be of varying thickness as it follows the underlying gyri and sulci, and there are larger indents of bone, on the midline over the falx cerebri (frontal suture), and between the frontal lobe and the olfactory bulb. When encountering spots with thicker bone, it can be useful to change the direction of burr movement. Briefly stop moving the burr parallel to the bone cut being made and oscillate it perpendicularly until the ridge of thicker bone gives way. For example, burr rostro‐caudally through the midline frontal suture at the caudal most point of the diamond but be especially careful not to damage the dura mater and underlying DSS. It is usually possible to remove the inner plate of the frontal bone in a single piece, taking care to patiently and carefully work it free from the dura mater. Some surgeons first divide the bone in two, burring down the midline over the falx cerebri, to increase mobilization. The older the patient, the more the dura mater and periosteum will have become fused, frustrating attempts to remove the bone flap without tearing the dura mater. For meningioma surgery, if necessary, a segment of torn dura can be removed along with the bone flap, as the dura needs to be removed anyway. Iris scissors can facilitate this. Resultant dural bleeding can be controlled with bipolar cautery. However, it is not permitted to accidentally tear or purposefully incise the midline dura mater while elevating the bone flap. This would cause hemorrhage from the DSS. Figure 22.4 Using osteotomes to elevate a Purdue diamond. A Purdue diamond bone flap was made with a sagittal saw. Just enough bone is being burred away from the caudal edge of the diamond and the external sagittal crest to pass a small osteotome into the septum sinuum frontalium. When this osteotome would not advance any further, an even smaller osteotome was driven either side of midline, to crack the many osseous septa within the frontal sinus. The first osteotome was again advanced, cracking the bone flap free. Osteotomes are driven rostro‐ventrally, parallel with the underside of the bone flap. At closure, bone sutures have been placed at multiple locations (black triangles). A suture was also placed in the third side of the bone flap (Figure 22.5). The left and right temporalis muscles have been apposed to one another where the external sagittal crest was sacrificed (white triangle), and the bilateral incisions in the temporalis fascia alongside the frontal crests are now being sutured. The dura mater is opened by making a stab incision (e.g. #11 blade or a bent needle, Figure 22.6) then performing a durectomy (e.g. iris scissors and DeBakey forceps). The widest possible margin of normal dura should be excised (maximum 1–2 cm). To prevent iatrogenic trauma to healthy brain, the initial stab incision can be made over the tumor itself, then scissors used to create a durectomy larger than the tumor. For a convexity tumor, the dura mater is opened unilaterally, preserving the falx cerebri and DSS. For a falcine tumor, a “falcine strip” can be removed (see below). The meningioma can now be removed (see section Meningioma resection). Lateral to the cribriform plate are one to two ethmoidal foramina (ethmoid bone), allowing passage of the ethmoidal vessels and nerve through the skull. These can be ablated during meningioma/dura resection, if necessary. Usually, olfactory bulb meningiomas are unilateral, separated from the contralateral olfactory bulb by an intact falx. It is often necessary to retract the olfactory bulb or debride the ipsilateral cribriform plate to remove such meningiomas. If the tumor involves the falx, resect a “falcine strip,” but protect the contralateral olfactory bulb and cribriform plate that are exposed upon doing so. With meningiomas covering the entire unilateral cribriform plate, complete ablation of one olfactory nerve is indicated, as adequate olfaction and appetite will be present postoperatively. The surgical site is copiously lavaged. A dural substitute (see section Substitutes for resected dura mater) is placed but gelatin sponge (e.g. Gelfoam®) is never left in the frontal sinus. The lower plate of frontal bone is not replaced. Figure 22.5 Bilateral Purdue diamond. Top left: a CT image of a dolichocephalic dog demonstrates that a conventional diamond‐shaped bone flap is created medial to the temporalis muscles (red lines) and provides narrow access to the frontal lobes. A Purdue diamond bone flap (green lines) is much wider. It is made by elevating the temporalis muscle, then incising the bone at its widest point. Top right: intraoperative photograph after bilaterally elevating the temporalis muscles. The dashed lines again demonstrate the bone cuts made for a traditional diamond (red) compared to a Purdue diamond (green). Bottom left: the access to the cranial vault after removing a bilateral Purdue diamond. The remaining bone making up the wall of the caudal right frontal sinus (black arrow) will be sacrificed as part of the craniectomy, to access the right frontal lobe. Note the remnant of the frontal sinus septum (black triangle). Bottom right: replacing the bone flap after a combined transfrontal and right rostrotentorial craniectomy. Bone sutures will be placed on both rostral edges of the diamond, and a single suture will be placed on the caudal‐left edge (white arrows indicate suitable burr hole locations). Note that the temporalis muscle fascia has been preserved on the frontal crest bilaterally, except where it was incised to be able to make the bone cut (white triangle). The fascia will be used for routine re‐apposition of the temporalis muscles, further securing the bone flap (see Figure 22.4). Compare the lower‐left image to the size of a traditional diamond bone flap. The exposure of the caudo‐lateral frontal sinus with a Purdue diamond is considerably greater. There are multiple techniques to replace the upper bone flap. The simplest is to make matching burr holes (e.g. 1 mm) in the skull and bone flap, then place bone sutures (e.g. size 0 PDS). Horizontal mattress sutures are recommended. As in Figure 22.3, it is much easier to pass suture down through the skull and up through the moveable bone flap. Sutures are temporarily secured with hemostats. Once sutures have been passed through all burr holes they are tied off. Typically, sutures are placed in all four sides of the diamond. If additional bone removal prevents this, sutures must be placed in three sides of the diamond. While using the sagittal saw during the approach, usually a small ledge is purposefully left lateral to the bone incision to facilitate burr hole placement during closure. However, a cut made at the extreme lateral edge of the frontal sinus improves access. If there is insufficient ledge to create a simple burr hole, an L‐hole can be made (see Figure 22.7).
22
Surgical Management of Intracranial Meningiomas
Introduction
Anatomy
Approach
Indications
Modifications
Transfrontal craniotomy
Meningiomas ventral to frontal nasal sinus
Olfactory bulbs; rostral frontal lobe Rostral falx cerebri
Rostrotentorial (lateral) craniotomy/craniectomy
Convexity meningiomas and parasagittal meningiomas
Parietal, temporal, caudal frontal lobes Middle – caudal falx cerebri
Bilateral rostrotentorial craniotomy
Falcine meningiomas that have occluded DSS flow
Middle – caudal falx cerebri
Suboccipital craniectomy
Caudal cerebellar meningiomas (see Chapter 24)
Transsphenoidal hypophysectomy
Pituitary fossa meningiomas (see Chapter 20)
Transfrontal Craniotomy (Bilateral Transfrontal Craniotomy)
Technique
Closure
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