EEA in pediatric patients requires entry into the immature nasal cavity and sphenoid sinus through the developing splanchnocranium. Therefore, anatomical boundaries based on the exact age-related differences in midline skull base anatomy play a critical role. Tatreau et al.32) measured important anatomical data about key structures in EEA based on computed tomography (CT) scans. First, the anterior nasal cavity is the point of entry for EEA and the distance between the bilateral piriform apertures, which are the most superficial bony structures, is the determining factor of the width of the anterior aperture of the nose because the surrounding soft tissue such as the nasal ala can be retracted to facilitate the entrance of the endoscope and instruments. The pyriform aperture width was significantly narrower in patients up to 6 to 7 years of age (17.2±0.5 mm) compared to adults (22.2±1.3 mm). However, there was no significant difference in pyriform aperture width among patients 9 to 10 years of age and older. The second concern is sphenoid sinus pneumatization, although the accuracy of neurosurgical navigation and advances in drill technology have made this concern less clinically significant. Pneumatization of the anterior wall of the sphenoid sinus is not fully completed until 6 to 7 years of age, and it then progresses posteriorly to the planum sphenoidale and anterior sellar wall. At 6 to 7 years old, pneumatization of the anterior sellar wall and the sellar floor was complete in 77% and 32% of patients, respectively. Pneumatization did not occur before 10 years of age in the clival recess but it reached the anterior clivus at 15 years of age. The last concern is the intercarotid distance, which determines the width of the skull base corridor for transtubercular and sellar approaches. The cavernous intercarotid distance was not significantly different between patients 9 to 10 years of age and adults (12.6±0.9), although it was significantly shorter in patients <24 months old (8.8±1.2 mm) than in those up to 6 to 7 years old (10.2±1.0 mm). Interestingly, intercarotid distance at the level of the superior clivus did not change after 24 months of age, although bony prominences of the clival carotid arteries are clearly associated with age. This report showed that, compared to adults, the development of critical nasal structures affecting EEA was 80–90% completed by the age of 8 years and that caution should be taken in patients under 3 to 4 years old due to the narrow aperture and intercarotid distance.

Knowledge of the anatomic location of sinonasal and skull base growth centers is important because disturbance of these centers can impair long-term craniofacial growth27). Compared with transcranial approaches, EEA does not require surgical manipulation of cranial vaults, but it is necessary to perform a relatively wide resection of the skull base sometimes. The growth of the calvaria and skull base is quite different. The growth of the cranial vault grows is rapid and driven primarily by the volume of neural tissue, whereas growth of the skull base is slow and partially independent of neural tissue. For example, frontal lobe growth slows between 5 and 6 years of age, but the anterior fossa and nasomaxillary complex continue to be drawn anteriorly for 3–4 more years because of middle fossa expansion10). Sgouros et al.28) showed that skull base growth is uniform until the age of 5 years and a change in the growth rate typically occurs after that age. Gruber and Brockmeyer10) showed that the skull base of children remains underdeveloped for at least 10 years after birth and the spheno-occipital synchondrosis, one of the primary skull base growth centers, closes at the completion of skull base ossification, around age 12 to 16 years10,28).

Therefore, it is the policy of the authors’ institution that EEA be recommended to patients 8 years or older, keeping in mind the surgical space and long-term impact on craniofacial growth such as underdevelopment of central facial bones.

The learning curve of EEA, attributable to differences in surgical visualization, long surgical corridor, complex anatomy and different instrumentation, is well known. A stepwise progression, starting with mastering the more straightforward procedures before attempting more complicated approaches, has been recommended for surgical teams31). Extensive practice and experience are required to execute these types of surgeries and it has been well established that the rates of favorable outcomes and complications in EEA are largely dependent on surgical skill and experience. However, the representative tumors for which EEA is indicated in pediatric patients, such as craniopharyngiomas and pituitary adenomas, are relatively infrequent; this is one of the main causes of the prolonged learning curve, similar to transcranial skull base surgeries1). Epidemiologic studies have shown that the age-adjusted incidence rate of tumors in the sellar region varies from 3.68 per 100000 to 0.78 per 100000 between ages 0 to 1926). Zhan et al.35) recently described a surgical retrospective cohort of 11 pediatric sellar tumors over 2 years and Chivukula et al. published an EEA series of only 133 patients (including 21 skull base bony lesions) who underwent surgery at the University of Pittsburgh Medical Center over 13 years4). A multidisciplinary team approach including various specialists with different backgrounds in a collaborative environment has been recommended to establish the safety and feasibility of surgery and shorten the duration of the learning curve30). Stepwise progression, starting and mastering the more simple procedures before attempting more complicated procedures, has been recommended31).

Several instruments are required for the endoscopic skull base surgery, which are unique for transnasal approach or different design with those for transcranial approach. The endoscope of 4 mm diameter and 18 cm length is the most popularly adopted optic devices in the endoscopic skull base surgery, regardless of the age. The endoscope of 2 mm diameter could be used in young patients with the very narrow nasal cavity, however, its quality of visualization is not as high as it from larger one. In the nasal cavity manipulation, the microdebrider is the useful device to remove the unnecessary mucosa in the sphenoid sinus. The deliate bony work around the carotid protuberance and optic canal is performed with the diamond burrs with long and fine handpiece. The microsurgical instruments are modified to be suitable for the long and narrow surgical corridor of nasal cavity. The microdissectors with long shaft and the gun type of scissor, microforceps and bipolar forceps are popularly adopted. In addition, the angled suction are valuable devices in order to manipulate the skull base bony window.

Short-term sinonasal dysfunction includes epistaxis, sensation of nasal blockage due to mucosal swelling and crusting in the nasal cavity. One study showed that patients treated with the endoscopic technique were more likely to have postoperative nasal debridement compared with those who underwent microscopic surgery23). Sinonasal function worsened substantially for both groups at 2 weeks after surgery, but then returned to baseline at 3 months. At 3 months after surgery, patients treated with EEA reported statistically better sinonasal quality of life compared with patients treated by microscopic techniques. Experiences from the Massachusetts General Hospital show that chronic rhinosinusitis was the most common delayed complication25). Synechiae, septal perforation, and altered olfaction are also possible long-term complications. These can be prevented and managed by minimally invasive surgical manipulation of nasal structures, preservation of the upper septal mucosa, scheduled debridement and saline irrigation, and modification of surgical approach12,21).

Clear surgical visualization with proper control of bleeding is essential to achieve surgical goals and prevent avoidable complications. The robust vascularity created by the tight vascular network of the nasal cavity leads to diffuse oozing over the course of the operation. Low-pressure bleeding can be easily controlled by lowering blood pressure, gentle compression, hemostatic agents, electrocautery, and warm saline irrigation17). However, high-flow arterial bleeding often requires direct electrocautery or ligation with surgical clips and demands dynamic movement of the endoscope for the direct visualization of the bleeding focus and a team approach to control it27). ICA injury is the most catastrophic complication of EEA and in-depth anatomic knowledge and proper surgical technique are paramount to prevent it. The most commonly injured ICA segment was the cavernous, followed by the ophthalmic; this could occur at various surgical steps3). ICA injuries can sometimes be repaired but often require nasal packing and endovascular sacrifice3,9,20). Valentine et al. performed animal model experiments to compare the efficacy of packed material in ICA injury. They randomized sheep to receive 1 of 5 hemostatic techniques (Floseal, oxidized regenerated cellulose, Chitosan gel, muscle patch, or the U-Clip anastomotic device) and measured the time to hemostasis, time of sustained proper blood pressure, amount of blood loss, and survival time. This study revealed that the muscle patch and U-Clip anastomotic device significantly improve survival, reduce blood loss, and achieve primary hemostasis while maintaining vascular patency33).

CSF leakage and the associated infections are the Achilles’ heel of EEA and affected by various factors such as body-mass index, tumor characteristics, extent of exposure and the reconstructive techniques used. The skull base reconstruction techniques required in EEA are quite different from those used in trans-sellar approaches due to the degree of CSF leakage flow. Among the various surgical techniques, the nasoseptal flap (NSF) has become the workhorse for skull base reconstruction after endoscopic endonasal surgery, after it was first described by Hadad in 200611). The maximal possible size of an NSF depends on the size of the nasal septum, and it should be an important consideration with pediatric patients. A study with CT scans suggests that, although the width of the NSF is likely sufficient at any age, the length may be insufficient for a trans-sellar/transplanar defect until age 6 to 7 years, a transcribriform defect until age 9 to 10 years, and insufficient for a clival defect at all pediatric ages27,29). Therefore, adjuvant solutions including a free graft, turbinate flap, galeal flap, or temporalis rotation flap should be considered in young patients or revision surgery.