Reconstruction of the Skull Base and Management of Skull Base Surgery Complications
Summary
Over the past two decades, the use of endoscopic endonasal surgery to manage lesions at the skull base has increased exponentially. Reconstruction of defects resulting from the resection of skull base and intradural lesions represents one of the most significant obstacles to the standardization and acceptance of expanded endonasal approaches. Vascularized reconstruction techniques have reduced the incidence of life-threatening complication by reliably separating intracranial and extracranial compartments. This chapter reviews the principles of reconstruction in endonasal skull base surgery, summarizes the surgical techniques of the recently developed vascularized flaps, and discusses the management of postoperative cerebrospinal fluid (CSF) leaks.
Introduction
The exponential growth in endoscopic endonasal approaches (EEAs) to manage lesions in the skull base reflects the evolution of these types of approaches, from transsphenoidal surgery for sellar pathologies to expanded EEAs for pathologies located along the ventral skull base. Recently, various refinements in reconstructive techniques have allowed endoscopic skull base surgeons to pursue extensive approaches while minimizing morbidity and mortality. By reliably assuring the separation of the intracranial and extracranial compartments, vascularized reconstruction techniques have reduced the incidence of life-threatening complications, including postoperative CSF leaks, ascending bacterial meningitis, intracranial abscess, and tension pneumocephalus. Separation of the cranial cavity from the sinonasal tract also protects cranial nerves and major vessels against desiccation and infection that could lead to the blow-out of the vessel.
Generalities of Skull Base Reconstruction in Endonasal Skull Base Surgery
An effective reconstruction strategy helps to avoid postoperative complications. A review of the current literature reveals that after a standard microsurgical or endoscopic transsphenoidal approach, the rate of postoperative CSF leak ranges from 0.3 to 14%, but generally is < 5%.1–7 Expanded endonasal approaches have been associated with a rate of postoperative CSF leak as high as 65%, depending on the extent of dural opening and arachnoid dissection (cisterns and/or ventricle opening). After various modifications of our reconstruction technique and after adopting the use of pedicled flaps, the incidence of postoperative CSF leaks following EEA decreased from 20 to 30% to < 5%.8–13
Tips and Tricks
Reconstruction issues should be thought of from the beginning of any endonasal endoscopic procedure to assess what type of leak might occur, what type of reconstruction will be used (either free graft or vascularized pedicled graft), and if a vascularized flap is chosen, what tissue is available for reconstruction.
A wide variety of tumors may be accessed through an EEA; therefore, an equally wide variety of skull base defects may result. As a general rule, small intraoperative leaks can be reliably repaired using free tissue grafts. Defects within a bony cavity, such as the sphenoid sinus or the bony tunnel resulting from transodontoid approaches, also lend themselves to obliteration with autologous fat grafts. Larger skull base defects, however, require a more complex reconstruction. Other important factors to consider, independent of the size of the defect, are the presence of high ventricular pressure, direct communication with the ventricles, and poor blood supply around the defect (i.e., postradiotherapy).13
Note
Whatever the type of flap, attention to technical details during flap harvesting, storing throughout the tumor resection, positioning, and fixation over the defect is paramount to achieve the best possible outcome.
In our experience, pedicled mucosal and fascial flaps provide the most reliable, reproducible, and optimal reconstruction. However, free tissue autografts, such as fat, dermis, muscle, and fascia; allografts, such as acellular dermis; xenografts, such as collagen matrix sheet; and prosthetics, such as titanium mesh, are still an important part of the armamentarium. Regardless of the type of reconstruction, fixation of the tissue against the defect is critical to its healing. Multiple buttress techniques and materials have been proposed to fixate the grafts and flaps against the defect, including autologous cartilage and/or bone, adaptation plates or mesh (absorbable and nonabsorbable), and packing (sponge or strip gauze or Foley catheter balloons).5,7,9,11,14–21 In addition, multiple reports advocate a variety of tissue sealants, such as fibrin glue, BioGlue (CryoLife, Inc., Atlanta, Georgia), and DuraSeal (Confluent Surgical, Inc., Waltham, Massachusetts).15,16,19–21
Clinical risks and benefits of each technique must be correlated with the physiologic differences in wound healing. Free tissue grafts depend on adherence (fibrin bonds) and serum imbibition for the first 2 to 4 days. Anything that separates the graft from the defect (e.g., hematoma, seroma, CSF, or foreign body) will interfere with this process and will preclude the next phase of healing, revascularization. Separation of the graft away from the defect will also allow the formation of a CSF fistula, which will self-perpetuate by preserving the breach between the tissues. This explains our policy for early exploration and repair for any postoperative CSF leak. Revascularization is critical for the survival of the graft and its eventual remodeling into normal tissue. Revascularization of a free tissue graft first proceeds through inosculation (connection of preexistent vessels), followed by revascularization and neovascularization. Any movement of the graft during this phase will shear off the new vessels, leading to death of the graft. When using an allograft, such as acellular dermis, the wound also heals by secondary intention, which involves neoangiogenesis, formation of granulation tissue, and reepithelialization before reaching the remodeling phase. A flap increases the perfusion to the wound site and promotes primary healing over the defect; therefore, although subject to the same phases of inflammation, it accelerates the proliferative (neoangiogenesis) and remodeling phases. It also tolerates some movement, as it does not rely on the blood supply from the recipient tissue.
We have tried to simplify our reconstruction techniques; nonetheless, our approach cannot be translated into a “one size fits all” process. Skull base reconstruction should be adapted to the needs of each specific case. A systematic assessment of the presence and extent of an intraoperative CSF leak, as well as the patient′s comorbidities, is important, as it may optimize the reconstruction planning and/or reorient a previous plan.
We have modified a previously reported CSF leak grading system ( Table 41.1 )4 to better conform to our EEA experience. Use of vascularized tissue does not appear to be critical in CSF leak grades 1 and 2. As previously stated, multiple clinical series have demonstrated that small CSF fistulas can be reliably reconstructed with free tissue grafts, which yield a success rate > 95%, independent of the use of a variety of techniques.22–29 However, large or high-flow intraoperative CSF leaks require a more elaborate repair that, whenever possible, includes the use of vascularized tissue.13 Following these principles, we have obtained a 94% successful reconstruction rate, even in patients with factors that are associated with a high incidence of postoperative CSF leak (high-flow intraoperative CSF leak, ventricular hypertension, history of radiation therapy, or large grade 3 or 4 defects).4,13
Pedicled Flaps
Pedicled flaps present several advantages, including conformation to the complex geometry of the skull base, wide arc of rotation, promotion of rapid primary healing, and immediate protection of exposed neurovascular structures.30 Their axial blood supply allows a length-to-width ratio that is not afforded with a random pattern of blood supply; therefore, a large surface area can be supplied by a thin and long pedicle. In addition, it is possible to harvest areas that are beyond the reach of the axial vessel. These areas will have a random blood supply, but the concept makes it possible to harvest an area larger than that predicted on the basis of just the vascular pattern (i.e., the dynamic blood supply extends beyond the anatomical prediction). These reasons contribute to our routine use of pedicled vascularized flaps for reconstruction of skull base defects resulting from EEA. Nonetheless, multiple factors come into consideration to choose the most appropriate flap ( Table 41.2 ). It is important to consider basic principles of flap surgery, as described by Chrysopoulo: replace like with like, think of the area to reconstruct in terms of units (while this concept was developed for cosmesis, it applies well to the different EEA modules), have a backup plan, steal from Peter to pay Paul but only when Peter can afford it, and remember the secondary defect produced at the donor site.
Hadad-Bassagasteguy Flap
The Hadad-Bassagasteguy flap (HBF; see Video 70, Nasoseptal [Hadad] Flap Reconstruction ) comprises the nasal septal mucoperichondrium and mucoperiosteum and has a posterior vascular pedicle based on the nasoseptal arteries.9 Tumors involving the septum, pterygopalatine fossa, or sphenoid sinus rostrum imply the need to sacrifice the blood supply, thereby precluding the use of an HBF. In addition, patients who had suffered disruption of the blood supply to the septal flap due to prior posterior septectomy or large sphenoidotomies are not candidates for an HBF. Other techniques, such as free tissue grafts and alternative endonasal or regional pedicled flaps, should be considered in patients in whom the HBF is not a suitable option or is not available ( Table 41.2 ).
Two parallel incisions follow the sagittal plane of the septum ( Fig. 41.1 ). An inferior incision is placed over the maxillary crest and extends from the posterior free border of the nasal septum to the mucocutaneous junction of the columella. Its most posterior extent continues superiorly along the free edge of the nasal septum and then laterally along the arch of the posterior choana to reach the lateral nasal wall to a point just anterior to the torus tubarius. If additional width is warranted, the inferior incision may be shifted laterally to incorporate the mucoperiosteum of the nasal floor. The superior incision is placed 1 to 2 cm below the superior aspect of the septum to preserve the olfactory epithelium.9 It extends laterally, crossing the rostrum of the sphenoid sinus at the inferior level of the sphenoid ostium. As the incision is carried anterior to the middle turbinate (no olfactory epithelium is found anterior to this level), it drifts superiorly to include the entire height of the septum. As the inferior incision, the superior incision extends anteriorly to reach the mucocutaneous junction. A vertical incision unites these incisions anteriorly. According to the size and shape of the anticipated skull base defect, as well as the possible need for oncologic margins, the flap′s length or width may be modified.
The surgeon can use the preoperative imaging to estimate the dimensions of both the defect and the HBF, thus anticipating any potential mismatch.12,31 However, in most instances, it is best and more practical to harvest as much surface area as possible and trim it if needed. This scenario, however, is extremely rare, as the flap “excess” can be accommodated to the surgical defect. Importantly, in the pediatric setting, the cranium-to-face ratio needs to reach near 1:1 before the dimensions of the nasal septum are adequate to cover a large skull base defect. This ratio is reached around 12 years of age.12
Subperichondrial elevation of the flap begins anteriorly, releasing its paddle and pedicle. Once harvested, the flap may be stored in the nasopharynx or inside the maxillary sinus after a large antrostomy (for lesions of the lower clivus, nasopharynx, and craniocervical junction). As the flap is temporarily stored away, care must be taken not to rotate its vascular pedicle. Furthermore, the flap should be periodically removed from its storage site and extended to relieve vascular congestion. During the opening of the surgical corridor, especially when drilling the ipsilateral lateral recess, pterygoid canal, or sphenoid bone, it is important to protect the flap′s pedicle. Risks to the pedicle should be anticipated; if the petrous segment of the internal carotid artery or pterygopalatine fossa needs to be exposed on one side, the flap should be harvested on the opposite side to avoid injuring its pedicle.
We usually use an inlay collagen matrix (to reestablish the arachnoid barrier) in conjunction with the HBF or any other vascularized flap. A buttress comprising a tissue sealant, absorbable nonadherent material, and packing (expandable sponge or the balloon of a Foley catheter) reduces the risk of migration and enhances the water-tightness of the reconstruction.11 If a significant dead space results from the bony exposure, as, for example, in a transclival approach, abdominal free fat grafts may be used to obliterate this space, making sure it is not interposed between the margins of the denuded sinonasal cavity and the flap ( Fig. 41.2 ) (see Video 71, Reconstruction Following Removal of Tuberculum Sellae Meningioma ). Care must be taken to place the septal mucosa flap directly against the bony edges of the skull base defect (or at a minimum the dural edges).11 All the periosteal surface of the flap should be in contact with denuded tissue. If exposed to air, it will desiccate and heal by secondary intention, thus leading to contraction and separation from the defect (i.e., failure of the repair).31 A biocompatible sealant is used to secure the flap in place. Next, the entire surface is covered with gelatin sponge (Gelfoam) or other absorbable nonadherent material to prevent the nasal packing or the Foley balloon from sticking to the flap.11 We commonly use silicone splints to prevent synechiae between the septum and lateral nasal wall. Recently, we described the use of a reverse rotation flap for the reconstruction of the donor site following harvesting of an HBF.32 The Caicedo reverse flap requires the preservation of the opposite septal mucoperiosteum during the harvesting of the HBF and the subsequent removal of the bony component of the posterior septum ( Fig. 41.3 ). Following superior and inferior incisions that mirror the corresponding incisions of the HBF, a final posterior incision liberates the mucoperiosteum and allows its 180-degree rotation to the opposite side, thereby relining the denuded portion of the septum. It is then fixed with transseptal stitches using chromic 3–0 or 4–0 suture.32 This technique promotes primary healing and reduces the nasal crusting. This, in turn, preserves the nasal airway and decreases the need for or the frequency of nasal toilette.32 Healing of the nasal corridor is usually complete by 12 weeks.13
Tips and Tricks
It should be noted that if any nasal packing is inserted, it should be placed and expanded/inflated under direct endoscopic visualization. This avoids displacement of the reconstruction, compromise of the flap′s vascular supply, or transmission of the compression to the optic nerves or other intracranial structures.
Postoperative magnetic resonance imaging (MRI) allows assessment of the position of the flap in respect to the defect and its vascularity (i.e., viability).33 The HBF has a characteristic appearance on the immediate postoperative, contrasted MRI,33 most frequently assuming a C shape and enhancing brightly on the contrasted images ( Fig. 41.4 ). However, the enhancement and average thickness of the flap in the immediate postoperative period can be variable. Absence of enhancement on the immediate postoperative MRI mandates careful observation. If there is a strong suspicion of a compromised flap, we advocate partial deflation of the Foley catheter balloon or even early exploration. Lack of enhancement, however, does not necessarily equate to a compromised vascular supply, as some of our nonenhancing flaps subsequently enhanced on delayed postoperative MRI (3–7 months). However, as previously mentioned, careful observation of the patient is required. Removal of the nasal packing to relieve the pressure and/or surgical reexploration may be warranted.
Note
In regards to the nasoseptal flap, postoperative MRI allows the position of the flap to be verified in respect to the defect and its vascularity, as a viable flap will enhance brightly on the contrasted images. Careful observation is mandated in the absence of enhancement on the immediate postoperative MRI.
The HBF offers a surface area of vascularized tissue of ~25 cm14,29,31 ( Table 41.3 ). Its dimensions allow coverage of two contiguous EEA modules in both the rostral and caudal orientation (e.g., cribriform plate and planum sphenoidale or the sella and clivus). Its large surface area, long reach, and wide arc of rotation make it the most versatile and reliable among all current endonasal pedicled flaps. After more than 400 nasoseptal HBFs, we have experienced the loss of only 2 flaps and an overall postoperative CSF leak rate < 5%.11,13 Loss of the HBF occurred in two patients who had undergone extensive preoperative radiation therapy to the area of the posterior choana, a phenomenon that has been noted by others.8
One of the prior drawbacks of the HBF was that its harvesting had to occur at the beginning of the surgery, as its vascular supply is compromised during the sphenoidotomy and posterior septectomy ( Table 41.4 ). Currently, we can preserve both vascular pedicles when performing the sphenoidotomies by preserving the mucosa around the posterior nasal arteries (“rescue flap”) for cases in which the risk for an intraoperative CSF leak is low. A rescue flap involves performing the HBF posterior incisions to then release its pedicle and to expose the posterior bony septum by elevating the mucoperiosteum ( Fig. 41.5 ). A freed pedicle allows its mobilization and displacement away from the surgical target. In selected patients with loose tissues and a short distance between the natural ostium of the sphenoid and the posterior choana, the pedicle can be mobilized without the need for the incisions. In case of a significant intraoperative CSF leak, a rescue flap can then be elevated and used in the standard fashion. This technique also ensures the availability of the nasoseptal flap if a subsequent surgery is potentially required.
Note
Alternative flaps may be useful if a nasoseptal flap is not available, if it is insufficient to entirely cover the repair, or if it fails. Knowledge of how to harvest these alternative flaps is important in an endoscopic skull base practice.
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