27.1 Introduction to Vascularized Flaps

Over recent years, endoscopic endonasal surgery has dramatically evolved and expanded to provide diverse options for the resection of skull base lesions. Extensive understanding of anatomy, combined with advances in instrumentation and technique, have allowed for significant evolution in these procedures. These expanded endonasal approaches (EEA) are being used to treat a multitude of complex intradural and extradural processes; although EEA were pioneered in the adult population, they have been progressively expanded to address the pediatric population.

A thorough understanding of anatomy has allowed surgery to encompass not only the paramedian skull base but also the orbit and upper cervical spine. With the use of more extensive approaches, evolution of reconstructive techniques has comparably followed. Initially, the bulk of skull base reconstruction was performed with cellular or acellular grafts. In a meta-analysis by Hegazy et al, ¹ cerebrospinal fluid (CSF) leaks resulting from trauma were repaired with cellular and acellular grafts. The study included 289 patients, with an overall success rate of 90%. As techniques expanded, reconstructive efforts began to encompass techniques using vascularized tissue. In a study by Thorp et al, ² 152 patients underwent skull base reconstruction with vascularized tissue, including primarily the nasoseptal flap (NSF) and the pericranial flap (PCF), but also several other vascularized tissue reconstructions. Overall, the study found a CSF leak rate of 3.3%, demonstrating success of these reconstructions. Further discussion of these reconstructive techniques will take place later in this chapter.

As with adult surgical procedures, approaches to the pediatric skull base through an endoscopic endonasal route have significantly evolved over the recent years. Advantages include the absence of external skin incisions and the need for craniotomy. While endonasal surgery overall has multiple advantages, careful attention to the differences between the pediatric and adult skull base anatomy is vital to success, as the developing cranium undergoes progressive pneumatization, further emphasizing thorough understanding of the relationship of critical structures. The literature includes several studies describing successful approaches to the midline skull base through EEA in the pediatric population. ^{3 ,​ 4 ,​ 5} This chapter will describe the differences between adult and pediatric endonasal surgeries and will discuss available surgical reconstructive techniques.

27.2 The Pediatric Skull Base

In the pediatric patient, the cranium and endonasal skull base are constantly changing. Pneumatization of the sinuses generally begins around the age of 2 years and continues into puberty, with different sinuses pneumatizing at different times and rates. In a study by Waitzman et al, ⁶ CT scans were used to demonstrate that cranial growth increased rapidly in the first few years of life, leveling off at about the age of 10 years. In comparison, this same group found that upper midface structures did not increase dramatically early in life, instead increasing when children were older. In a retrospective radioanatomical study by Shah et al, CT scans were used to assess the anatomic limitations for transsphenoidal EEA in pediatric patients. ⁵ All the patients included in the study were divided by age and compared to adult controls. CT scans were assessed, and measurements for expected skull base defects including those for transsellar, transcribriform, and transclival approaches were made by both a neuroradiologist and an otolaryngologist. For each of the defects, calculations compared the NSF area for skull base reconstruction. Patients included in the study were divided by age. The study concluded that before the age of 10 years, the NSF should be used with caution, as its size may not permit its use in skull base reconstruction, depending on the defect size. The group emphasized that careful assessment of the predicted defect for each patient should be performed prior to surgery for optimal reconstructive considerations. ⁵

Relative anatomic differences in the pediatric skull base compared to adults may make pediatric endonasal work more challenging. In a separate study, Tatreau et al assessed the role of developmental immaturity of the skull base and its relationship to the surrounding neurovasculature. ⁷ In this study, three separate bony anatomic limitations were discussed. The first anatomic limitation is the piriform aperture, which is the first superficial bony structure encountered during skull base surgery. In the developing pediatric population, soft tissues may be displaced; however, it is important to avoid damage to this area to avoid problems with normal development. Moreover, the size of this area is important in the passage of instruments during skull base surgery. Tatreau et al found that this area was prohibitively small for expanded EEA in patients younger than 24 months. From this age to approximately age 6 or 7 years, the space is narrow but not prohibitive, and after that age, it resembles the size of an adult nose. Because of this, sublabial approaches may be required to approach cases in the youngest of patients. ⁷

The second anatomic limitation in pediatric skull base surgery is incomplete pneumatization of the sphenoid sinus. The patterns of pneumatization of the sphenoid sinus have previously been described in the literature. ^{8 ,​ 9 ,​ 10} Studies have found that the pneumatization patterns can be classified into three categories presellar/conchal, sellar, and postsellar and follow a pattern of aeration from anterior to posterior. Tatreau et al found that by the age of 6 or 7 years, the anterior wall of the sphenoid sinus was fully pneumatized. They also found that by this same age, 77% of patients had anterior sellar wall pneumatization and 32% had sellar floor pneumatization; additionally, 88 ± 17% of the planum was pneumatized as well. Other findings included the pneumatization of the dorsum sellae, which was not evident in 84% of patients younger than 16 years. Spread of pneumatization to the clival recess was not seen before the age of 10 years, but was present in 89% of patients older than 15 years. ⁷ While incomplete sphenoid sinus pneumatization does correlate with the need for more drilling during surgery, this is not prohibitive to endonasal approaches. In a study by Cavallo et al, several anatomical conditions that limit the use of endoscopic surgery were also discussed. This includes thicker bone overlying the planum and tuberculum, making approaches more complex. ¹¹ However, careful surgical planning and adequate imaging can overcome these challenges, allowing EEA in the pediatric population.

With the advantages of pediatric endonasal skull base surgery clearly evident, it is important to reflect on the risks and morbidity associated with injuries to neurovascular structures in this area. There is an inherent relationship between the internal carotid arteries (ICAs) and ongoing sphenoid sinus pneumatization that occurs in a developing skull base. Tatreau et al assessed the pediatric patients included in their study, measuring minimum distances between the carotid arteries at the level of the cavernous sinus and at the level of the superior clivus, immediately below the sellar floor. Around age 12 years, 89% of patients were found to have some degree of prominence in the vertical portion of the ICA. ⁷ Based on CT measurements, the study found that intercarotid distances at the superior clivus were relatively fixed after 24 months. At the cavernous sinus, the intercarotid distance was significantly smaller in patients younger than 24 months and up to 6 to 7 years, compared to 9- to 10-year-old patients. There was no statistically significant difference in patients after age 9 to 10 years compared to adults. Overall, the average distance at the level of the cavernous sinus in adults is approximately 12 to 18 mm, which decreases only to about 10 mm in 3- and 4-year-old patients. Because of this, endoscopic transsellar surgery could be performed in all except the youngest of patients. Yilmazlar et al discussed how a narrow intercarotid distance at the level of the cavernous sinus could be considered a relative contraindication to transsphenoidal surgery. ¹² Several studies emphasize that this largely depends on the experience and confidence of the surgeon.

27.3 Approaches to Skull Base Reconstruction

With the anatomical considerations outlined earlier, skull base surgery in the pediatric population has the same goals and objectives as it does in the adult population. These goals include safe resection of pathology followed by watertight closure of the defect separating the nasal cavity from the intracranial space/structures. These goals prevent potential complications that result in postoperative morbidity including pneumocephalus, CSF leak, meningitis, or death. Preoperative planning and careful patient selection are critical for safe surgical outcomes. Meticulous surgical planning for skull base surgery in the pediatric population is especially important, and while pediatric patients have a decreased incidence of comorbid conditions, including smoking and obesity, prior radiation remains an important factor to be considered. Kassam et al also emphasize the importance of considering the size of pediatric patients, as younger children may have smaller blood volumes. Because of this, procedures may require staging to keep the patient safe. ³ In addition, because children are still growing, surgical effects on craniofacial growth should also be considered. While endonasal work typically does not disrupt growth centers, combined open procedures may disrupt growth plates and dentition in pediatric patients. Even with special considerations, care for pediatric patients should be as standardized as possible for optimization of outcomes.

Historically, skull base surgery has involved the use of lumbar drains. As described by Stokken et al, lumbar drains were frequently used in skull base surgery for CSF diversion postoperatively. ¹³ As the field has evolved and changed, the literature now reflects a preference to not use drains in skull base surgery unless patient-specific factors require its use. As described by Ransom et al, ¹⁴ lumbar drains were independently assessed and found to have a complication rate of 12.3%. A combination of improved reconstructive techniques and associated lumbar drain complications have driven the field to use drains more judiciously, specifically in patients with recurrent leaks requiring multiple reconstruction or those with confounders that elevate intracranial pressure. Zanation et al identified that patients with defects along both the clivus and anterior skull base were more likely to have leaks after surgery. ¹⁵ In the pediatric population, lumbar drains are further complicated due to patient discomfort, associated complications, and difficulty in placement. While no true consensus exists for which patients require lumbar drain use, the surgical team should carefully assess the patient and expected procedures prior to surgery to determine the need for lumbar drainage concurrently.

27.3.1 Reconstructive Options

Acellular Grafts

In the skull base reconstructive ladder, there is extensive literature describing multiple reconstructive options. As they were traditionally developed and studied in the adult population, many of the techniques that will be described here have limited data in the pediatric skull base surgery literature. However, these techniques serve as the workhorse for pediatric skull base reconstruction and will be carefully delineated.

Acellular grafts have always played a vital role in skull base reconstruction. Several products, including acellular dermal matrix (AlloDerm LifeCell, Branchburg, NJ) and collagen matrix (Duragen, Integra Life Sciences, Plainsboro, NJ), have been used as adjuncts to skull base reconstruction. For inlay techniques, Alloderm has been used in the subdural or epidural plane. If used as an onlay over the bony skull base, all underlying mucosa must be removed to prevent mucocele formation, and it must be fully hydrated prior to use. For surgical procedures that require the removal of dura, collagen matrix Duragen may be used as either an inlay, between the brain and dura in a subdural plane, or in the epidural plane, between the dura and bony skull base. This graft has been used to seal the defect and eliminate CSF leakage from dural resection. With surgical resection in which bony margins are limited, this may also be used as an onlay graft. All acellular grafting techniques must be bolstered into place using a combination of packing tissues that will be delineated further in the text.

Cellular Grafts

In addition to acellular options, cellular grafts were among the first techniques used for skull base reconstruction. As with all reconstruction, it is important to emphasize the importance placed on multilayer closure. In a study by Harvey et al, free tissue was used to repair smaller defects (<1 cm) in conjunction with multilayer closure and was found to have a success rate of greater than 90%. ¹⁶ Several options can be used, which are described here.

Free Mucosal Graft

Mucosal grafts are available from tissue throughout the nasal cavity and can be used for reconstruction of skull base defects. Grafts can be taken from the septum, nasal floor, and/or middle turbinate. This reconstructive technique does not require a second surgical site. However, nasal malignancies may preclude the use of mucosa from within the nose for reconstruction due to concerns for underlying disease. During surgery, surgeons may elect to remove the middle turbinate and/or the septum as part of the approach or resection. With these resections, harvest of the mucosa from these structures may serve as free mucosal grafts. In addition, surgical approach and resection provides wide access to the nasal floor, which may also be used for reconstruction. To harvest nasal floor mucosa, needle-tip bovie electrocautery is bent to 45 degrees, and used to make circumferential incisions. Variations in size of the mucosal graft can be made with extension onto the septum and under the inferior turbinate. The graft is then carefully elevated with a Cottle elevator, and the graft is kept in saline until it is required for reconstruction. Meticulous hemostasis is confirmed along the donor site prior to use of the graft for reconstruction. As previously described, it is vital to remove all underlying mucosa from the reconstruction site in order to prevent mucocele formation. Multilayer bolstering for closure is used following graft placement as described below.

Abdominal Fat

Abdominal fat grafts are frequently used as a component of skull base reconstruction. These grafts are traditionally used to obliterate space within a defect in order to create a more laminar reconstructive site. This graft may be used in conjunction with other reconstruction tools, including acellular grafts and/or vascularized reconstructions. The incision must be made at a second surgical site, either the periumbilical region, lower abdominal area, or lateral hip and must be prepped and draped separately. The incision is made, and circumferential dissection is performed in order to obtain the adequate volume required for reconstruction. Once complete, the wound is irrigated, hemostasis is confirmed, and it is closed in multiple layers. A drain may be used if the size of the defect requires it; however, this is rarely required in children. Caution must be undertaken in this harvest, as many children are thin and have only small amounts of fat. Also, with extensive harvest, cosmetic deformity may result, thereby precluding large periumbilical grafts. Recently, dermal fat grafts have been used in skull base reconstruction. To harvest, an ellipse incision is made. Leaving the dermis attached to the fat, the epidermis is removed to obtain the composite graft. Fat can still be circumferentially dissected around the dermal component, but it remains attached for use. The dermis allows for a stronger reconstruction that is easier to manipulate during insertion and further graft placement. Just like traditional fat grafts, the dermal fat graft can be used with further acellular and cellular reconstructions as part of the multilayer closure.

It should be noted that in a study by Hadad et al, resections that were reconstructed with free tissue grafts, which resulted in a greater than 3-cm defect, were found to have a CSF leak rate of 20 to 30%, a rate that was unacceptable. Because of this, they recommended vascularized reconstruction techniques. ¹⁷ These will be further described in the next section.

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