**4.3 Regional pathology**

The most common tumors of the cerebellopontine angle in pediatric patients are schwannomas (65%), meningiomas (5%), and epidermoid cysts (5%). Up to 10% of tumors of the cerebellopontine angle in pediatric patients may be malignant, which is significantly greater than in adults (**Figure 6**) [30]. Schwannomas are

#### **Figure 6.**

*A 21 month old male developed upgazepalsy and gross motor regression. MRI brain with contrast demonstrated large fourth ventricular ependymoma extending into the cerebellopontine angle (A-C). Gross total resection was achieved with a far lateral craniotomy (D).*

#### *Pediatric Skull Base Tumors DOI: http://dx.doi.org/10.5772/intechopen.95605*

rare tumors and rarer still in pediatric patients, with about 10% of cases diagnosed in patients younger than 21. These are considered benign masses that arise from Schwann cells of the nerve sheath and may be found anywhere in the body, with 16–45% reported to be in the head and neck [31]. The vestibular nerve is thought to be the most common location for intracranial schwannomas, though they can also occur on the trigeminal nerve, facial nerve, and lower cranial nerves in decreasing frequency.

Vestibular schwannomas present with unilateral hearing loss, tinnitus, headache, and disequilibrium, and they can cause cerebellar ataxia and brainstem compression with significant extension into the cerebellopontine angle [32]. Pediatric vestibular schwannomas tend to be diagnosed in adolescence and the tumors tend to be larger at presentation than in adults [32, 33]. Surgical resection, stereotactic radiosurgery, and observation are all valid treatment options for schwannomas, however the long life-span of pediatric patients argues for more aggressive definitive management. In one series of 148 pediatric vestibular schwannomas 82 (55.4%) patients were treated with surgery only, 45 (30.4%) with observation, 6 (4.1%) with radiation only, and 12 (8.1%) with surgery and radiation [33]. Residual tumors after surgery may grow at a faster rate than in adults, therefore the surgeon should safely attempt gross total resection or should consider radiosurgery in cases with significant residual tumor [32].

Sporadic schwannomas are rare in children, but they are the hallmark of genetic conditions, neurofibromatosis 2 (NF2) and schwannomatosis, with bilateral vestibular schwannomas affecting 95% of individuals with NF2 [34]. Overall treatment goals change in these patients due to the high lifetime number of tumors. Efforts should be made to avoid surgery unless necessary to preserve vital cranial nerve function and radiation should be avoided. Recent clinical trials have demonstrated hearing and quality of life improvement in both adult and pediatric NF2 patients with bevacizumab therapy. Pediatric patients did not demonstrate tumor regression unlike their adult counterparts in this trial [35].

Epidermoid cysts are rare dysontogenic lesions with a predilection for the cerebellopontine angle in 40–60% of cases and are the third most common lesion in the cerebellopontine angle. They tend to grow along arachnoid planes and frequently extend into neighboring compartments. These tumors tend to encase neurovascular structures and are quite adhesive. There is a characteristic appearance on MRI of a lesion filling and expanding the subarachnoid space that is dark on T1, bright on T2 and is avidly diffusion restricting. Patients present with cranial nerve impairments, most commonly of the trigeminal and the vestibulocochlear nerve.

Trigeminal neuralgia may be more common than sensory impairment when the trigeminal nerve is involved. Depending on the size of the tumor, almost all cranial nerve impairments have been described. Complete surgical excision can be curative; however, the surgeon must take great care to reduce morbidity and injury to cranial nerves. Complex or combined approaches may be necessary for tumors that span multiple compartments. Aseptic meningitis may occur post operatively and can be treated with a course of dexamethasone [36, 37].

At the craniocervical junction, chordomas are rare bony tumors that arise from notochordal remnants. These tumors are slow growing but locally aggressive. Only 5% of chordomas present in children, and they tend to be more aggressive in younger children. The most common location overall is the sacrum, with a minority of tumors occurring at the skull base. However, the more frequent location in pediatric patients is midline at the spheno-occipital synchondrosis of the clivus. Tumors present with headaches, other signs of increased intracranial pressure, diplopia, or mixed cranial neuropathies. Optimal treatment includes maximal safe resection followed by high dose radiotherapy, though there is still controversy regarding the

type and dose of radiation. Proton beam may have benefits in pediatric patients over conventional radiotherapy due to a reduced radiation dose to neighboring structures. There is little role for chemotherapy due to the slow-growing nature of the disease [38, 39]. The future of chordoma treatment will likely be in molecular and targeted therapies and there are active clinical trials investigating a drug targeting programmed cell death ligand 1 (PD-L1), an immune checkpoint inhibitor expressed in over 90% of chordomas [40].

Chondrosarcomas are often grouped together with chordomas as they are both locally destructive, slow growing bony lesions, but they are a distinct histopathologic entity and have a better prognosis than chordomas. Chondrosarcomas are typically found in paramedian locations, arise from chondrocytes, and comprise 5–12% of cases found at the skull base. The most common location is the clivus followed by the temporal-occipital junction. There are four histologic subtypes: conventional, mesenchymal, clear cell, and dedifferentiated, and tumors are graded I-III based on the level of differentiation. The mesenchymal subtype tends to be the most aggressive. Radical surgical resection is again the mainstay of treatment with adjuvant radiation therapy, specifically proton beam, for most patients due to high rates of residual tumors and locoregional recurrence. Radiation has been shown to significantly decrease recurrence rates [41–43].

#### **4.4 Surgical approaches**

Pediatric tumors have a predilection for the posterior cranial fossa, specifically the cerebellar hemispheres and the fourth ventricle. A majority of these common intrinsic brain tumors such as medulloblastomas, juvenile pilocytic astrocytomas, and ependymomas do not require complex skull base approaches and can be accessed by a suboccipital craniotomy with or without a C1 laminectomy. However, many extrinsic tumors or large exophytic intrinsic tumors that extend to the cerebellopontine angle, the craniocervical junction, and the jugular foramen require knowledge of skull base approaches to maximize exposure.

The workhorse of posterior fossa approaches is the suboccipital craniotomy for dorsal midline lesions. The suboccipital craniotomy is performed by creating a dorsal midline window from the foramen magnum to the confluence of sinuses. There are many methods to perform this craniotomy including drilling bur holes, using a craniotome, a cutting and diamond bur, and Kerrison rongeurs. In children, we find it is safe and efficient to perform using a craniotome by stripping the dura from the foramen magnum and inserting the footplate under the lip of foramen magnum. This can be augmented by a C1 laminectomy for a more inferior to superior view. In rare cases, we have extended our craniotomy superior to the transverse sinus to perform a concurrent interhemispheric transtentorial approach to posterior fossa tumors.

For more inferior and lateral exposure than a standard suboccipital approach, the far-lateral, or extreme lateral transcondylar (ELITE), craniotomy may be required. This provides additional access to lower clival lesions, the craniocervical junction, and lesions of the upper cervical spine (**Figure 7**). In addition to a lateral suboccipital craniotomy, the ipsilateral occipital condyle is drilled extradurally until the hypoglossal canal is skeletonized. The hypoglossal canal generally is a marker that the posterior third of the condyle has been removed, which should not result in craniocervical instability. The lateral mass of C1 is also removed and the vertebral artery is sometimes mobilized. Though rarely necessary, the jugular tubercle can be drilled at this point to provide greater access to the lower cranial nerves and clivus.

The cerebellopontine angle (CP angle) is almost exclusively accessed by lateral skull base approaches, namely retrosigmoid and transpetrosal craniotomies. The

**Figure 7.**

*A 13-month-old male presented to the emergency department with failure to thrive and eye movement abnormalities. A large atypical teratoid rhabdoid tumor (ATRT) was diagnosed with MRI brain (A-C). Tumor was resected using a suboccipital craniotomy with a modified far lateral approach to reach the tumor extending to the anteromedial brainstem.*

retrosigmoid craniotomy is a standard neurosurgical approach. It is performed by creating a bony window just inferior and posterior to the transverse sigmoid junction. This can be performed with craniotome, bur holes and Kerrison rongeurs, or a cutting and diamond bur. Additional intradural drilling to open the internal auditory canal can be performed for vestibular schwannomas that extend from the auditory canal into the CP angle. The retrosigmoid craniotomy has immense flexibility, however it does rely on cerebellar retraction and therefore puts neural elements at some risk.

Transpetrosal approaches involve varying degrees of removal of the petrous temporal bone in order to expose the anterolateral brainstem from the tegmen to the jugular tubercle. With greater bony removal, there is increasing exposure of lesions of the anterior brainstem. However, the more extensive the petrosectomy, the greater the risk to cranial nerves, so the preoperative cranial nerve function and the goals of surgery are critically important when choosing an approach. Presigmoid retrolabyrinthine craniotomy preserves hearing and facial function. Translabyrinthine craniectomy sacrifices hearing but preserves the facial nerve well.

Transcochlear approaches sacrifice hearing by removing the labyrinth and closing the ear canal and puts the facial nerve at risk by mobilizing it from its canal. It is recommended that all these approaches are performed with a neurotology partner, as the mastoid is not well pneumatized in children and it can be difficult to identify the labyrinthine and facial recess. Any of these approaches can be combined with middle fossa approaches for combined exposure of lesions that span multiple compartments.

Transnasal, transmaxillary and transoral approaches provide access from the sella to odontoid process and upper cervical spine. These are generally reserved for midline extradural lesions such as chordomas and chondrosarcomas. The morbidity of these approaches is significantly decreased since the advent of endoscopic surgery.
