15.1.1 Benign Lesions

Although benign lesions generally have a better prognosis than malignant lesions, many of these lesions can compress important neuronal structures, requiring surgery to relieve the pressure. In the pediatric population, however, the age of the patient can affect the decision of whether to perform surgery. The risks of surgery and anesthesia on the developing brain must be considered, along with the benefits of performing surgery while the skull is still developing.

The most common benign tumors are craniopharyngiomas, which originate in Rathke’s pouch and can lead to increased intracranial pressure, bitemporal hemianopsia, and hypothalamic disturbances.2 Juvenile nasal angiofibromas (JNAs) are benign vascular tumors caused by an incomplete regression of the first branchial arch artery, affecting only males.3 Fibro-osseous lesions are a group of tumors in which normal bone is replaced by connective tissue composed of fibroblasts excreting matrix and cementlike elements. Conditions for development of these lesions include fibrous dysplasia, aneurysmal bone cysts, giant granulomas, ossifying fibromas, giant cell tumors, osteoma, osteoblastoma, osteosarcoma, and the like.4 ^,​ 5 ^,​ 6 Langerhans cell histiocytosis (LCH), a clonal proliferation of dendritic cells, is the most common type of histiocytosis.7

Some of these tumors are related to genetic conditions, such as neurofibromatosis types 1 and 2 (NF1 and NF2, respectively). NF1 may present with optic gliomas or neurofibromas. Optic gliomas are low-grade pilocytic or pilomyxoid astrocytomas, with rare instances of more advanced neoplasms in young adults.8 Neurofibromas arise from nonmyelinating Schwann cells. The majority of NF1 patients will develop neurofibromas, which can be either cutaneous or plexiform with various morphological types.9 NF2 may present with schwannomas—slow-growing tumors originating from the nerve sheath of peripheral nerves. Schwannomas of the vestibular nerve are the most common type, almost all of which are related to neurofibromatosis type 2 (NF2).10

Not all the lesions seen in the pediatric population result from neoplasms, however; many are congenital abnormalities. These include encephaloceles—neural tube defects in which cranial vault content protrudes through gaps in the cranium. The content of the sac may include the meninges (meningocele), brain matter (meningoencephalocele), and parts of the ventricles (hydroencephalocele). Dermoid and epidermoid cysts are benign, low-grade lesions originating in remnants of ectodermal tissue and infiltrating the neural tube or pharyngeal arches during the third to fifth weeks of development.11 Finally, mucoceles—cystic lesions of the paranasal sinuses that form due to blockage or stricture of the ostia, causing an accumulation of secretions in a cyst lined by upper respiratory epithelium—are rare in children and are usually associated with cystic fibrosis, nasal polyposis, trauma, or surgery.12

15.1.2 Malignant Lesions

The most common of the malignant lesions in children are sarcomas. These include the following subtypes: rhabdomyosarcoma (RMS; 48%), malignant fibrous histiocytoma (11%), osteosarcoma (8%), Ewing’s sarcoma (6%), chordoma (4%), malignant nerve sheath tumor (4%), chondrosarcoma (3%), and synovial sarcoma (2%).13

RMS, a sarcoma derived from striated muscle, is the most common soft tissue sarcoma of childhood. Fifty percent of patients are diagnosed in the first decade of life, and they often present with advanced disease.14 ^,​ 15 RMS can be classified histologically as either embryonal or alveolar. Embryonal RMS typically occurs in children under age 10, often in the head and neck. Alveolar RMS occurs more commonly in adolescents and young adults, affecting the trunk and extremities. Embryonal RMS has a more favorable prognosis than alveolar RMS.16 Chordomas are slow-growing, locally aggressive, malignant bone tumors arising from notochord remnants along the spinal cord. Clival chordomas are locally aggressive and commonly involve adjacent structure such as the suprasellar area, cavernous sinus, parapharyngeal space, temporal bone, and fossa.17

Neuroectodermal tumors include esthesioneuroblastoma, Ewing’s sarcoma, and malignant nerve sheath tumor (a.k.a. malignant schwannoma). Esthesioneuroblastoma (or olfactory neuroblastoma) is a rare malignant tumor of the olfactory nerve found in the nasal cavity. Eight percent of cases occur before the age of 25, but very rarely (< 1%) before the age of 10.18 Ewing’s sarcoma, a highly malignant neoplasm of the bone, includes classic Ewing’s sarcoma, Askin’s tumor, and peripheral primitive neuroectodermal tumor.19 Although they accounts for 10% of all primary bone malignancies, only 1 to 6% of Ewing’s sarcomas occur in the skull.20 Schwannomas have already been described, as they are usually benign. In rare instances, however, they may become malignant, in which case they have a poor prognosis and a high rate of recurrence (40–50%).21

15.1.3 Cytogenetic Variations

Malignant pathologies can arise from genetic aberrations in the cell’s DNA, which can be tumor-specific or nonspecific, resulting in deletions and gains. In stepwise genetic transformations, one genetic alteration may increase a cell’s growth and replication abilities, encouraging more genetic changes that can increase tumor invasion and metastases.22 These cytogenetic variations can be diagnostic and can help determine the prognosis for some tumors.

Some tumors that show complex karyotype changes include squamous cell carcinoma, chordomas, and RMS.22 It has been shown that half of nonkeratinizing squamous cell carcinomas have an abnormal karyotype.23 In chordomas, frequent chromosomal aberrations have been discovered, often involving losses in chromosomes 3, 4, 10, and 13.24 Approximately 75% of RMS tumors are estimated to involve complex karyotypes. These frequently involve translocation breakpoints in the 1p11–q11 region, translocations of 13q14, gains of chromosomes 2, 7, 8, 12, and 13, and losses at chromosome 11p15.5.23 ^,​ 25 ^,​ 26

Additionally, some tumors are associated with specific chromosomal changes. Approximately 78% of patients who have Ewing’s sarcoma have a translocation of t(11;22)(q24;q12).22 Synovial sarcoma has a translocation of t(X;18) in more than 90% of cases, with a third of cases having this as the sole chromosomal abnormality.27 With such specific tumor markers, a simple biopsy of a suspicious mass can make the diagnosis and treatment plan easy to determine.

15.2.1 Surgical Techniques

Although numerous surgical techniques are available to treat skull base tumors, most such techniques have been developed for use in adults, most likely due to the paucity of tumors in the pediatric population. Nonetheless, various approaches have been successfully performed in children and are described here.

Anterior Skull Base Open Approaches

Several open approaches are tried and true methods for anterior skull base tumor extirpation, including the following: subcranial, subfrontal, transfacial/transmaxillary, midface degloving, orbitozygomatic, and fronto-orbital. The subcranial approach is often combined with other approaches if a large tumor invades adjacent structures such as the orbit, pterygomaxillary fossa, or maxilla.

Subcranial

The goal of this approach is to gain access to the anterior skull base from the medial orbital rims along the ethmoid roof to the planum sphenoidale and superior clivus.28 ^,​ 29 The subcranial approach is applied when lesions involve the central anterior skull base, such as in the cribriform plate and the frontal sinus, and for lesions extending far into the paranasal sinus, nasal cavity, or orbits, including esthesioneuroblastomas, meningoencephaloceles, gliomas, and nasal dermoid cysts. This technique can only be performed if the lesion does not have extensions to adjacent bony structures or fossae. If such extensions are found, additional approaches are combined, such as transfacial, midface degloving, orbitozygomatic, transorbital, Le Fort I, pterional, and the like.

A bicoronal incision is made from one supra-auricular area to the other on the contralateral side 2 cm behind the hairline (Fig. 15.1; Fig. 15.2). A subperiosteal pericranial flap is raised anteriorly, exposing the periorbita down to the nasal bone and laterally to the temporalis fascia. Then the flap is reflected anteriorly to expose the area to be resected. An osteotomy may be performed either by type A osteotomy, which includes the anterior wall of the frontal sinus, or by type B osteotomy, which includes the posterior wall of the frontal sinus. In both types the segment is extracted en bloc, with the nasal bone and superior medial walls of the orbits as a single segment that is stored in saline for reinsertion after tumor extirpation. Bilateral ethmoidectomy and sphenoidotomy are performed to complete the exposure with possible unilateral preservation of the cribriform plate so as to preserve olfaction.

Subfrontal

The subfrontal approach gains access to anterior skull base tumors with intracranial extension while providing wide exposure, enabling minimal brain manipulation.30 ^,​ 31 This can be useful for large tumors, such as craniopharyngiomas and JNAs. A bicoronal incision is made from the zygoma along the hairline to the contralateral zygoma. A subperiosteal pericranial flap is raised and reflected anteriorly, followed by a craniotomy involving both anterior and posterior walls of the frontal sinus, osteotomy of the nasolacrimal suture, and the roof of the orbits in one or two segments.

Transfacial

The transfacial approach is typically used to provide adequate access to the skull base, giving a wide operative view of deep lesions. The benefits include minimizing intraoperative brain traction, avoiding piecemeal resection, and reducing damage to neural and vascular structures.32 ^,​ 33 The transfacial approach includes transfacial swing osteotomy, transnasomaxillary, and transpalatal, as well as a few others. Transfacial swing osteotomy and frontopterional approach can be used to access the clivus, requiring a Weber-Ferguson incision, which can result in a large scar.32 The transnasomaxillary approach allows for exposure of the entire midline skull base region for large nasopharyngeal and clival lesions, such as chondrosarcomas, meningoencephaloceles, and gliomas. It is performed through a modified Weber-Ferguson incision, directing the incision across the radix and along the opposite subciliary margin of the lower lid.34 The transpalatal approach is often used for skull base herniations, including meningoceles and encephaloceles, particularly in neonates who are unable to undergo transnasal endoscopic approaches. The transpalatal approach can provide better visualization in neonates than the transnasal endoscopic approach can and is associated with less morbidity than open craniotomy is.35 The procedure begins with a U-shaped incision in the hard palate, 5 mm from the dental arch. The subperiosteal flap is raised to gain access to the nasopharynx, allowing visualization of the vomer for its removal. The lateral atretic plates are removed, and the flap is closed using a two-layered closure.36

Midfacial Degloving

Midfacial degloving offers access to nasal cavity and paranasal sinus lesions without leaving an external scar on the face.37 In this procedure, a transfixion incision and a bilateral intercartilaginous incision are performed, through which nasal dorsum tissues, anterior wall of the maxillary sinus, glabella, and frontal bone are lifted. The surgeon then extends them laterally toward the nasal cavity floor to touch the caudal portion of the transfixion incision. A sublabial incision is made from the first molar to the contralateral tooth, incising the mucoperiosteum. The tissues are raised bilaterally to the inferior orbital rim using a periosteum elevator. The flap is then raised to the glabella, medial cantus region, and forehead so as to expose the midfacial skeleton, allowing for removal of the tumor, which can include inverted papillomas and nasopharyngeal angiofibromas.38

Maxillectomy

A maxillectomy, which may be partial or total, involves removal of the maxilla to access tumors in the hard palate. Suprastructure maxillectomy involves an osteotomy above the malar eminence and through the inferior–lateral wall of the orbit. If combined with the subcranial approach, reconstruction of the orbit is warranted either by titanium mesh or split calvarial bone graft and a vascularized flap. In cases involving extensive loss of volume, a large complex defect, total maxillectomy with orbital exenteration, or radiation therapy (adjuvant or neoadjuvant), a vascularized free flap is preferred, usually from the anterolateral thigh or rectus abdominis.

Transorbital

Tumors invading the content of the anterior orbit or the orbital apex necessitate orbital exenteration.39 If the tumor does not involve the maxilla, a transorbital approach may suffice with or without sparing of the eyelid, oncological margins permitting. If a tumor involves the skull base, orbital content, and walls of the maxilla, a combined subcranial–total maxillectomy is indicated, which includes orbital exenteration.40

Le Fort I

The combined subcranial–Le Fort I osteotomy entails freeing of the alveolus ridge so as to gain access to the lower clivus when it is not attainable by other open approaches or endoscopically.41 A gingivobuccal incision is performed and a facial flap raised to the level of the infraorbital nerves and laterally to the pterygomaxillary junction. Then a Le Fort I osteotomy is performed, followed by a partial maxillectomy.40 This approach may harm the buds of the permanent denture in children younger than 10 years old and thus should be avoided in such patients if possible. Some tumors that can be removed through this technique include angiofibromas, hemangiomas, giant cell tumors, and malignant fibrous histiocytoma.42

Lateral Approaches

Lateral approaches are used when lesions, often chordomas, invade the temporal bone, middle cranial fossa, or cavernous sinus and are subdivided into anterolateral (subtemporal), lateral (transpetrous), and posterolateral.43 The anterolateral approach entails a question mark incision following the hairline (Fig. 15.3; Fig. 15.4), removal of the zygomatic process, and raising of a temporalis muscle flap, thus exposing the infratemporal fossa and pterionic area, followed by a craniotomy with anterior (pterygoid) or posterior (petrous) extension as needed.30 The lateral (transpetrous) approaches are commonly used for tumors of the petrous bone and include presigmoid, retrolabyrinthine, translabyrinthine, transcochlear, and total or radical temporal bone resection (petrosectomy), depending on the area of the temporal bone involved.

Preoperative hearing and facial nerve function are crucial considerations when assessing the appropriate lateral approach, as these may need to be sacrificed. Benign lesions usually do not necessitate such measures.44 The skin incision is a C-shaped retroauricular incision with possible extension and obliteration of the external auditory canal. The posterior–lateral or retrosigmoid approach gains access to cerebellopontine angle tumors by exposing the lateral aspect of the posterior fossa at the junction of the sigmoid and transverse sinuses. Exposure is gained superiorly to the level of the tentorium and inferiorly to the jugular foramen, with possible extension by drilling of the petrous bone into the internal acoustic meatus, Meckel’s cave, and cavernous sinus.

Related posts:

5 Anesthesia and Intraoperative Monitoring of Patients with Skull Base Tumors 4 Head, Neck, and Neuro-otologic Assessment of Patients with Skull Base Tumors 3 Imaging of Skull Base Neoplasms 10 Management of Benign Skull Base Tumors in Neuro-oncology: Systemic Cytotoxic and Targeted Therapies 9 Clinical Applications of Radiotherapy for Skull Base Tumors 14 Quality of Life and Measures of Outcome for Patients with Skull Base Tumors

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