This review describes the sequential learning from initial free tissue grafting reconstructive techniques to the current use of vascularized flaps. Outcomes and limitations of current endoscopic reconstructive techniques are discussed, including a systematic review of the outcomes of endoscopic endonasal techniques to reconstruct large skull base defects (ESBR). The various endoscopic techniques for local and regional flaps in skull base reconstruction are described. Additionally, EMBASE (1980-December 7, 2010) and Medline (1950 – November 14, 2010) were searched using a search strategy designed to include any endoscopic endonasal reconstruction of the skull base. The manuscripts selected were subject to full text review to extract data on perioperative outcomes for ESBR. Surgical technique was used for sub-group analysis.
| EBM Question | Level of Evidence | Grade of Recommendation |
|---|---|---|
| How does endoscopic compare to open reconstruction of the skull base? | 3a | C |
This article describes the sequential learning from initial free tissue grafting reconstructive techniques to the current use of vascularized flaps. Outcomes and limitations of current endoscopic reconstructive techniques are discussed. The pathophysiology of idiopathic CSF leak as treatment is well documented and differs from that of surgical skull base defects, thus is not discussed here.
Pathophysiology
Reconstruction of the skull base directly relates to the nature of the surgical defect with differing goals between surgical groups. For example, many extradural tumor resections necessitate reconstruction to promote healing (especially in the setting of radiation therapy). In these cases, primary reconstructive goals are not avoidance of postoperative CSF leak and potential intracranial infection, but rather defect coverage to facilitate healing. This is in contrast to intradural surgery, as postoperative CSF leak and potential intracranial infection must be taken into consideration. Intradural tumor surgery can be divided into 2 main groups:
- 1.
Intradural, but extra-arachnoidal, as is the case with pituitary surgery when the diaphragm is not violated
- 2.
Intra-arachnoidal surgery where by definition an intraoperative CSF leak is appreciated 100% of the time.
Intra-arachnoidal surgery can be further divided into high-flow and low-flow leaks depending on whether a cistern was directly opened into the sinonasal defect. In addition to anatomic considerations, other important factors that must be noted when approaching reconstruction are the size of the dural defect, the nature of the patient’s CSF pressure, obesity, and states of poor healing such as Cushing disease and prior irradiation.
The underlying, foundational goals of surgical defect reconstruction in endonasal skull base surgery are identical to those of conventional external approaches, ie, to completely separate the cranial cavity from the sinonasal tract, eliminate dead space, and preserve neurovascular and ocular function. The principle of a multilayered reconstruction to reestablish tissue barriers is also preserved. Using endonasal pedicled vascular flaps and a reconstruction based on the aforementioned principles, postoperative CSF leak rates are now below 5%, a figure comparable to that reported for open cranial base reconstructive techniques.
Clinical presentation
Postoperative CSF leaks are typically noted within a week following surgery. The patient’s history is most suggestive and often diagnostic. Primary symptoms include persistent, often salty tasting, rhinorrhea. Primary physical examination findings include a positive reservoir sign and increased rhinorrhea with Valsava maneuver. Sinonasal endoscopy should be performed and at times the leak can be confirmed or localized; however, in the early postoperative period absorbable packing often remains in place rendering visualization of the skull base defect difficult. Mental status changes are not common findings unless the patient has worsening pneumocephalous, a finding identified on computed tomography (CT) scan. It is important to note that Beta-2 transferrin can be used as a confirmatory test; however, in the early postoperative period the history and physical examination are diagnostic. Intermediate timed CSF leaks (2 to 6 weeks postoperative) are less common and usually present with intermittent low flow leaking episodes from a pinpoint dural opening. Late CSF leaks (more than 6 weeks postoperative) are very rare and are usually seen with patient noncompliance (hard nose blowing) or radionecrosis.
Clinical presentation
Postoperative CSF leaks are typically noted within a week following surgery. The patient’s history is most suggestive and often diagnostic. Primary symptoms include persistent, often salty tasting, rhinorrhea. Primary physical examination findings include a positive reservoir sign and increased rhinorrhea with Valsava maneuver. Sinonasal endoscopy should be performed and at times the leak can be confirmed or localized; however, in the early postoperative period absorbable packing often remains in place rendering visualization of the skull base defect difficult. Mental status changes are not common findings unless the patient has worsening pneumocephalous, a finding identified on computed tomography (CT) scan. It is important to note that Beta-2 transferrin can be used as a confirmatory test; however, in the early postoperative period the history and physical examination are diagnostic. Intermediate timed CSF leaks (2 to 6 weeks postoperative) are less common and usually present with intermittent low flow leaking episodes from a pinpoint dural opening. Late CSF leaks (more than 6 weeks postoperative) are very rare and are usually seen with patient noncompliance (hard nose blowing) or radionecrosis.
Management
Endoscopic Reconstruction with Free Tissue Grafts
Before the adoption of vascularized tissue flaps as our primary reconstructive technique (see later in this article) skull base reconstruction after EEA was limited to the use of free tissue grafts. These techniques were adapted from experience accumulated with the endoscopic repair of CSF leaks associated with endoscopic sinus surgery and trauma, and then expanded to repair larger dural defects as well as defects over high-flow intraoperative CSF leaks. As with any reconstruction, a multilayer approach with complete defect coverage is the key to an endoscopic dural reconstruction.
First, a subdural inlay graft (between the brain and the dura) of collagen matrix (Duragen, Integra Life Sciences, Plainsboro, NJ, USA) is placed; this helps to obliterate the intradural dead space. Its pliability and texture allows for safe manipulation around neurovascular structures. Ideally, this subdural graft should extend 5 to 10 mm beyond the dural margins in all directions. A subsequent inlay graft of acellular dermis (AlloDerm Life Cell, Branchburg, NJ, USA) is then placed in the epidural space (between the dura and the skull base). Occasionally, the bony ledges of the defect are not adequate to support an inlay graft; therefore, the acellular dermal graft is placed extracranially (at the nasal side of the defect) as an onlay graft. All the edges of the defect should be denuded of mucosa to allow for revascularization of the graft and to avoid mucocele formation. Alternatively, this graft can be sutured to the dura with nitinol U-clips (Medtronic U-Clips, Memphis, TN), placement of which is technically challenging. Importantly, U-clips provide anchor points to prevent migration of the graft, but do not result in a water-tight suture line.
Although this is an off-label indication for the acellular dermal graft, we have found that the handling characteristics, availability (no need for skin graft harvesting), and ingrowth of the patient’s own tissue with rapid epithelialization are advantages that outweigh its cost. When using this technique, it is important to use a single graft with dimensions extending beyond the defect margins in all directions, and ensure the graft is adequately hydrated in normal saline solution before its insertion. In our experience, a thinner graft offers the best take, although it is somewhat difficult to manipulate endonasally.
Once both grafts are in place, the edges of the AlloDerm are bolstered intranasally with oxidized cellulose (absorbable packing). A biologic or synthetic glue is then sprayed or applied over the edges and absorbable gelatin sponge squares are used to further bolster the reconstruction. These layers of absorbable packing accomplish 3 goals. First, they fix the grafts in place and protect them from changes in airflow within the nose. Second, these layers allow for “filling” of the concavities and convexities of the skull base to better distribute the pressure of removable packing on the underlying grafts. Third, the absorbable packing protects the grafts from movement during removal of nonabsorbable packing that typically occurs 3 to 5 days postoperatively, as later described. We then use the balloon of a 12-French Foley catheter or 10-cm expandable sponge packing to stabilize and bolster the inlay/onlay grafts, further preventing early brain herniation. Placement and inflation of the balloon catheter using 10 mL of saline is performed under direct endoscopic visualization. Care is taken to avoid overinflation, as this may result in compressive effects over the intracranial structures. Moreover, if the optic nerves or chiasm are exposed during the dissection, Foley placement is avoided, as the balloon may exert too much pressure on these structures. Instead, expandable sponge packing is used. Any nonabsorbable nasal packing or balloon is removed 3 to 5 days after the EEA.
For moderate-sized skull base defects, we have also used an onlay free mucosal graft harvested from the resected middle turbinate instead of an AlloDerm graft. The remainder of the reconstructive procedure is as described previously. The take of the free mucosal grafts is excellent; however, use is limited by size and the fact that they can be used only as onlay grafts. Last, abdominal free fat is mainly used as a bolster or biologic dressing to the multilayered reconstruction. Fat can also be used to obliterate spaces such as the clival recess or a nasopharyngeal defect after a transnasal approach to the anterior spine. Harvesting abdominal fat has the added morbidity of an abdominal incision, scar, and the potential for infection, hematoma, or seroma formation.
Endoscopic reconstruction with multilayered free tissue grafts for larger dural defects during skull base tumor cases historically resulted in postoperative CSF leak rates of 20% to 30%. These leaks were usually managed with further endoscopic bolstering and some required CSF diversion for 3 to 5 days with lumbar drainage (see later in this article for further details). It is important to note that when managed with endoscopic revisions, it was clear that most of the CSF leaks were a result of graft migration or CSF fistula formation in the most dependent area of the flap. Given the unacceptably high CSF leak rate of 20% to 30% for skull base tumor resections with grafting reconstruction, vascularized tissue options were sought to reduce this incidence.
Endoscopic Reconstruction with Vascular Pedicled Flaps
Technique: nasoseptal flap (Hadad-Bassagastegay flap)
In most recent months, the use of a vascular pedicle flap has become the preferred skull base reconstruction. The most commonly used technique is a vascular flap of the nasal septum mucoperiosteum and mucoperichondrium that is pedicled on the nasoseptal artery, a branch of the posterior septal artery, which is one of the terminal branches of the internal maxillary artery.
The nasal cavity is decongested with 0.05% oxymetazoline on pledgets and the nasal septum is infiltrated with 0.5% to 1.0% lidocaine with 1:100,000 to 1:200,000 epinephrine. The inferior and middle turbinates are out-fractured to allow visualization of the entire height of the nasal septum from the olfactory sulcus to the nasal floor.
To facilitate a bimanual technique during the EEA, we usually elect to remove one of the middle turbinates, usually the right. Additionally, resection of the middle turbinate facilitates visualization of the nasoseptal flap (NSF) vascular pedicle and ipsilateral elevation of the septal flap.
The side of the flap (right or left) is determined by several factors:
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If the lesion requires dissection of the lateral pterygoid recess or the pterygomaxillary fossa, then the vascular supply of the flap will be compromised; thus, a flap on the opposite side is used.
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Similarly, lesions that invade the rostrum of the sphenoid or the septal mucoperiosteum will mandate harvesting a contralateral NSF.
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If the lesion is in the midline, and no significant lateral dissection is required, then sharp or large septal spurs (conferring a risk for perforation of the flap during dissection) may dictate the side from which the flap is harvested.
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Last, all things being equal, the right side is usually an easier dissection for the right-handed surgeon, especially in the setting of right middle turbinate removal.
The flap is designed according to the size and shape of the anticipated defect, although it is best to overestimate the size and then trim the flap if needed.
Two parallel incisions are performed following the sagittal plane of the septum, one over the maxillary crest and the other 1 to 2 cm below the most superior aspect of the septum, thereby preserving the olfactory epithelium ( Figs. 1 and 2 A, B). These incisions are joined anteriorly by a vertical incision at a level that is anterior to the plane of the anterior head of the inferior turbinate (see Fig. 2 C). These incisions may be modified to account for the specific area of reconstruction or to allow for adequate oncologic margins.
At the posterior septum, the superior incision is extended laterally with an inferior slant over the rostrum of the sphenoid sinus, crossing it horizontally at the level of the natural ostium (see Fig. 1 ).
The inferior incision is extended superiorly along the free posterior edge of the nasal septum and then laterally to cross the posterior choana below the floor of the sphenoid sinus.
Elevation of the NSF begins anteriorly with a Cottle dissector or a small suction dissector (see Fig. 2 D, E). It is advantageous to complete all incisions before elevating the flap because it becomes difficult to orient the tissue and maintain it at tension once it has been elevated. Septal incisions may be completed with scissors or other sharp instruments as necessary. Elevation of the flap from the anterior face of the sphenoid sinus is completed with preservation of its posterolateral neurovascular pedicle (see Fig. 2 F).
The flap is then placed in the nasopharynx or maxillary sinus until it is needed at the end of the procedure for reconstruction. Multiple dimensional modifications are possible. The entire ipsilateral mucoperiosteum and mucoperichondrium may be harvested to cover anterior skull base defects as extensive as those that include the area from the posterior wall of the frontal sinus to the sella turcica and from orbit to orbit.
Additionally, a wider flap may be harvested by extending the incision to include the mucoperiosteum of the floor of the nose.
A radioanatomic study of the nasoseptal flap compared the relative size of the anterior skull base to the size of potential nasoseptal flaps, and demonstrated that most anterior skull base defects can be completely covered with the nasoseptal flap ( Fig. 3 ).
Additional considerations must be taken into account when reconstructing pediatric skull base defects with the NSF. It is clear that before age 10 years, the NSF area is significantly smaller than the area of larger, age-corresponding skull base defects (such as a transcribriform defect). Between the ages of 10 and 14, the NSF area approaches 100% of the size but not larger. This size discrepancy is because cranial growth occurs earlier in life and septal growth does not occur until puberty when midface growth accelerates. Bilateral flaps are conceptually possible; however, they are rarely used. During surgery, it is important to be careful with bone removal lateral to the pterygoid canal so that the vascular pedicle is not injured.
Once the extirpative portion of the procedure is complete ( Fig. 4 A), a multilayer cranial base repair is performed (see Fig. 4 ). An inlay collagen matrix (Duragen, Integra Life Sciences or DuraMatrix Onlay, Styker, Kalamazoo, MI, USA) is placed as described previously (see Fig. 4 B). The NSF is then placed as an onlay, over the bony edges of the cranial base defect (see Fig. 4 C). All mucosa is removed from around the defect before the flap is placed to promote healing and prevent mucocele formation. After the collagen graft and flap are in place, the edges of the flap are bolstered intranasally with oxidized cellulose absorbable packing (see Fig. 4 D); Duraseal (Confluent Surgical Inc, Waltham, MA, USA) is then placed (see Fig. 4 E). It is critical to separate the grafts from the nonabsorbable packing using some type of nonadherent material, such as absorbable gelatin sponge or absorbable gelatin film, as this will prevent traction on the grafts when the packing is removed (see Fig. 4 F). Shifting of the underlying inlay/onlay grafts may occur during the placement of the packing; thus, the surgeon must be vigilant and perform placement under direct visualization with the endoscope. Packing materials include either a Foley balloon or expandable tampon-type packing sponges as described previously. Sealants are never used between the grafts or under the flap, as this prevents direct tissue contact and healing. Packing is kept in place for 3 to 5 days postoperatively.
After adoption of the NSF for the reconstruction of EEA defects, postoperative CSF leak rates dropped considerably to less than 5% for all endonasal defects, a rate that compares with that of traditional open techniques. In fact, in a prospective series of 70 NSF skull base reconstructions for high-flow leaks with large dural defects (either the cistern or ventricle was widely opened into the nasal cavity during the dissection), the observed postoperative leak rate was 5.7% (4/70). A clear advantage of the NSF as a reconstructive option is endoscopic graft harvest, thereby avoiding a second approach or incision. A major drawback of the NSF is that its need must be anticipated before embarking on the resection, as the vascular pedicle of this flap is frequently compromised during sphenoidotomy and/or posterior septectomy. In addition, if a revision procedure is necessary, the flap may have been used previously or the pedicle previously damaged; therefore, 2 other pedicled flap options have been described and are reviewed later in this article. In selected revision cases, the NSF can be dissected from the defect and reused. In a series of 20 NSF takedowns and reuses for staged procedures or recurrence, 1 (5%) of 20 reconstructions leaked. There were no flap deaths during takedown and reuse. Healing of the intranasal dissection and nasoseptal flap reconstruction is usually completed in 6 to 12 weeks (see Fig. 3 , Fig. 5 ).
Technique: posterior pedicled inferior turbinate flap
In patients with prior septectomy or prior wide sphenoidotomies, the NSF blood supply has been interrupted and this option cannot be used. Other options such as free grafting or other locoregional vascular pedicle flaps have to be considered. The posterior pedicle inferior turbinate flap (PPITF) is based on the inferior turbinate artery, a terminal branch of the posterior lateral nasal artery (PLNA), which arises from the sphenopalatine artery (SPA). For the design and harvesting of the PPITF, the anatomic course of the PLNA has to be understood. The PLNA runs in a descending vertical or anteroinferior course over the perpendicular plate of the ascending process of the palatine bone, giving a branch medially to supply the middle turbinate. As the artery courses inferiorly, it enters the inferior turbinate on the superior aspect of its lateral attachment, approximately 1.0 to 1.5 cm from its posterior tip. Some have found that the artery may lie within the bone (50%), within the soft tissue (14%), or follow a mixed pattern (36%). The artery runs for some distance (mean of 1.2 cm) before piercing the bone and soft tissue and splitting off into between 2 and 6 branches.
The nasal cavity is decongested and the lateral nasal wall, just anterior to the inferior turbinate, is injected with a solution of 1% lidocaine with 1:100,000 epinephrine. We harvest the PPITF after completion of the EEA. This should be performed ipsilateral to the defect whenever possible to minimize the distance of the pedicle to the defect. Initially, the inferior turbinate is gently medialized to better expose the entire medial surface of the inferior turbinate. Given that this flap is substantially smaller when compared with the NSF, it is best to harvest the entire turbinate to ensure adequate coverage ( Fig. 6 A). A wider flap may be harvested by extending the lower incision to include the lateral mucoperiosteum of the turbinate and even the middle meatus. The first step is to identify the SPA as it exits the sphenopalatine foramen and follow it distally to identify the PLNA. The flap will be based solely on this vascular pedicle. Two parallel incisions are performed endoscopically following the sagittal plane of the inferior turbinate, one superiorly just above the inferior turbinate and the other inferiorly following the caudal margin of the inferior turbinate (see Fig. 6 A). A vertical incision, placed over the anterior head of the inferior turbinate, connects the 2 previously performed incisions (see Fig. 6 B). The mucoperiosteum is then elevated, starting at the anterior aspect of the inferior turbinate. A variable amount of bone may also be elevated, depending on the ease of dissecting the mucoperiosteum from the underlying bone. Care must be taken to avoid injuring the vascular pedicle as it enters at the superior aspect of its lateral attachment, approximately 1.0 to1.5 cm from its posterior tip. In addition, it is important to preserve the lateral nasal artery as it descends vertically over the ascending process of the palatine bone. It may course anteriorly to the posterior wall of the maxillary sinus; therefore, this should be considered when extending the maxillary antrostomy posteriorly. Once harvested, the flap is gently unrolled and mobilized to cover the skull base defect (see Fig. 6 C). The PPITF can be applied directly to dura or denuded bone or may be used over a fat graft. However, it is critical that the vascularized flap be in direct contact with the margins of the defect and any nonvascularized tissue between the margins of the defect and the flap must be meticulously removed. Biologic glue is applied over the flap, absorbable gelatin sponges are placed, and sponge nasal packing or the balloon of a 12-French Foley catheter is inserted to press the PPITF against the defect. Silicone nasal splints are used to protect the denuded lateral wall and are left in place for approximately 10 to 21 days postoperatively. According to an anatomic analysis of the anteriorly based inferior turbinate flap, the mucosal surface area is approximately 4.97 cm 2 (2.8 cm length with 1.7 cm width). Endoscopic analysis of the PPITF reveals that approximately 60% of the anterior cranial fossa can be covered. Although this may represent suboptimal coverage in some, the ability to bring vascular tissue into the repair and augment with free grafts may be sufficient in many cases ( Fig. 7 ). Bilateral flaps can be harvested to cover larger defects.
