**2.2 Regional anatomy**

The anatomy of the anterior and parasellar skull base is complex and develops throughout childhood [5, 6]. Pathology that affects the skull base directly is relatively rare in children. Pathology and surgical intervention can significantly affect development of the pediatric skull base and potentially confer substantial morbidity. For this reason, surgical intervention is reserved for cases where neurologic or endocrinologic function are threatened or compromised, and where no other options for tumor control are possible. This should be determined by a multidisciplinary team.

We favor the endoscopic endonasal approach (EEA) for most pathology involving the anterior and parasellar skull base. Transcranial and transfacial approaches, or a combination of approaches are used for pathologies that span beyond the access of a single approach. Variability in the development of the pediatric skull base has relevance to all stages of the endonasal approach, including the (a) nasal phase (b) sphenoid phase (c) sellar and intradural phase and (d) closure and reconstruction. The anterior skull base and midfacies develop later than the posterior skull base and continue growing until 14 years of age.

The nasal aperture grows throughout childhood and can restrict access of endonasal instrumentation. We prefer to operate through a nasal aperture larger than 5 mm, which can be performed as early as 2 years [5]. The piriform aperture, limited by the nasal bones and maxilla, can limit the rostral-caudal extent of dissection. As a general rule, the working angle and distance to the sella from the piriform aperture increases with age. Endonasal approaches, particularly expanded approaches, may be more feasible in younger patients for this reason. The choanal aperture, limited by the middle and inferior turbinates, also has relevance to expanded approaches. We do not perform expanded approaches to the clivus in cases where choanal atresia is present, or where the choanal aperture is smaller than 10 mm [7].

The sphenoid sinus has conchal anatomy until 2 years of age [8, 9]. Pneumatization of the sphenoid sinus begins in the inferior-medial sphenoid bone and moves superiorly and laterally, ultimately determining the location of the carotid arteries. The inter-carotid distance is mature at 9 years, though pneumatization may continue beyond this age and affect the degree of protrusion of the carotid arteries into the sphenoid sinus [9]. Sellar lesions are best approached in sphenoid sinuses with greater than 10 mm between the cavernous carotid arteries. Pneumatization of the ethmoid sinuses begins anteriorly and moves posteriorly, with the posteromedial ethmoids pneumatizing last [9]. The degree of ethmoid sinus pneumatization has relevance to its communication with the maxillary sinus and the working trajectories available to the anterior skull base.

Growth of the nasal septum, and the potential use of a nasoseptal flap for reconstruction, lags behind the development of other anterior skull base structures [10]. Large defects in the anterior skull base may not be adequately covered by a nasoseptal flap before 10 years of age, and may limit the utility of the endonasal approach. Traditional sellar and tuberculum/planum approaches can be covered as early as 6 years, when needed. We do not routinely reconstruct with nasoseptal flaps, reserving their use only for cases where a high flow cerebrospinal fluid (CSF) leak is expected. Traditional reconstruction with fat, fibrin, collagen, and expandable polyvinyl acetate is performed routinely.
