CHAPTER 144 Trigeminal Schwannomas
Epidemiology and History
Trigeminal schwannomas (TSs) are benign, slow-growing tumors arising from the peripheral nerve sheath of the trigeminal nerve. Although the trigeminal nerve is the second most common intracranial site for schwannomas after the vestibular nerve, TSs are very rare tumors. Schwannomas make up roughly 8% of intracranial tumors, and TSs account for 0.8% to 8.0% of all intracranial schwannomas.1–7 This corresponds to 0.07% to 0.36% of all intracranial tumors. The peak incidence is observed in the fourth and fifth decades.1–5,7,8 TSs are slightly more common in females.9 Dixon in 1846 and Smith in 1849 were the first authors to describe primary tumors arising from the gasserian ganglion.10 Frazier reported the first successful removal of a TS in 1918,11 and the first clinical series was presented by Cuneo and Rand in 1927.12 Several hundred cases have been reported since.6,9 Earlier studies had reported low total resection rates and high mortality and morbidity. However, the introduction of microsurgery and skull base techniques has improved results considerably (Table 144-1).1–3,5–9,13–24 Today, TSs can be diagnosed reliably with noninvasive techniques, and total surgical resection with virtually no mortality and very low permanent morbidity can be accomplished in benign cases. In the past few decades, radiosurgery has also been established as a safe and effective adjuvant therapy for residual or recurrent tumors or as primary treatment in carefully selected cases.
Pathology and Pathogenesis
Schwannomas are encapsulated, benign tumors and are classified as World Health Organization grade I.25 In the literature they are also commonly referred as neuromas,3 neurinomas,26 and neurilemmomas.15 Schwannomas are distinct from traumatic neuromas, which are a non-neoplastic proliferation of Schwann cells in response to nerve trauma, and from neurofibromas, which are well-circumscribed intraneural or infiltrative extraneural tumors of the peripheral nerve sheath. Rare malignant schwannomas can also be seen and represent 0% to 7.9% of cases in large series.1–3,6,7,9,15,27–31 Schwannomas arise from the peripheral nerve sheath, distal to the oligodendroglia–Schwann cell junction. The trigeminal nerve and the gasserian ganglion are the most common sites for intracranial schwannomas after the vestibular nerve. Although most schwannomas are sporadic tumors, they may also be seen in association with neurofibromatosis type 225 or in patients with schwannomatosis.32 In such cases, TSs can occur along with schwannomas of other cranial nerves.
Pathologic Anatomy and Classification Schemes
The trigeminal nerve is associated with motor, sensory, and proprioceptive function. Sensory fibers innervate the scalp, face, mucous membranes of the nose, nasal cavity, and mouth. Motor fibers innervate the muscles of mastication: the tensor, digastric, and mylohyoid muscles. The nuclei in the brainstem extend from the inferior colliculi to the second segment of the cervical spinal cord. The trigeminal nerve root originates on the lateral aspect of the rostral pons; courses cranially, laterally, and anteriorly toward the petrous apex; and enters Meckel’s cave through the trigeminal pore, just inferior to the superior petrosal sinus.33 This first portion, which extends from the brainstem to Meckel’s cave, is called the cisternal segment. The nerve root is myelinated by oligodendrocytes from its origin at the brainstem up to the central myelin–peripheral myelin transition zone, where this function is taken over by Schwann cells. The trigeminal nerve root sheath has its central myelin–peripheral myelin transition zone at a mean of 1.13 mm on the medial and 2.47 mm on the lateral side away from the root entry zone at the brainstem.34 Schwannomas arise from the peripheral myelin zone. In the cisternal segment the trigeminal nerve root resides inside the cerebellopontine angle. After entering the trigeminal pore, it courses intradurally in the trigeminal cistern within two leaflets of the dura, which is called Meckel’s cave. It lies posterolateral to the sella and the cavernous sinus.
TSs may arise from the gasserian ganglion, the trigeminal nerve root, and the three divisions of the nerve. According to the point of origin, the tumor may be localized to the middle fossa (commonly referred as the “ganglion type”), the posterior fossa (commonly referred as the “root type”), or the extracranial space or may extend into several of these compartments (Fig. 144-1).3 Schwannomas arising from the gasserian ganglion grow into Meckel’s cave. Growth from the root of cranial nerve V (CN V) in the posterior fossa occurs subdurally in the cerebellopontine angle, whereas growth into the extracranial branches in the orbit and infratemporal fossa occurs epidurally. The tumor may extend into the orbit through the superior orbital fissure, to the infratemporal fossa through the foramen ovale or foramen rotundum, and to the cavernous sinus through the lateral wall and the cerebellopontine angle through the trigeminal pore.
The location of the tumor also directs the surgical approach. Therefore, several surgical classification schemes have been proposed to guide the surgical approach. Jefferson proposed a very useful scheme in 1953 and classified TSs into middle fossa (type A), posterior fossa (type B), and dumbbell (type C) tumors.30 This classification scheme has gained common acceptance in the neurosurgical community, and a slight modification was made by Day and Fukushima in their 1998 publication.3 Day and Fukushima described type D tumors, which arise from the extracranial portion of the trigeminal nerve.3 These tumors arise from either the maxillary or mandibular divisions and extend into the infratemporal fossa through the foramen ovale or rotundum, respectively.3 Dolenc,2 Samii and colleagues,7 Yoshida and Kawase,6 Al-Mefty and coworkers,5 Goel,4 and Gwak and associates22 have also proposed alternative classification schemes. The majority of TSs stay confined to a single cranial fossa (50% to 80%).9,13,15,16,24,35 The most common location is the middle cranial fossa (50%), followed by posterior fossa (30%) and dumbbell (20%) tumors.30,36 When 383 patients in 13 large series were analyzed together, type A schwannomas made up 36.6%; type B schwannomas, 18.3%; type C schwannomas, 34.5; and type D schwannomas, 10.7% of all cases (Table 144-1). The incidence of TSs that extend into the multiple cranial spaces is reported to range from 27% to 59%.24 Cavernous sinus involvement is frequent and seen in up to 38% of TSs.18,37 The tumor grows intradurally within Meckel’s cave and by expansion rather than invasion. The walls of Meckel’s cave are not rigid and can accommodate even a large tumor mass without disruption of the dural barrier. As expected, the majority of TSs do not invade the cavernous sinus proper or the internal carotid adventitia.18,37,38 The lateral wall of the cavernous sinus may be penetrated in rare cases with involvement of the vascular channels. This is also a frequent site of recurrence.18
Clinical Findings
Schwannomas may arise from any segment of the trigeminal nerve, from the root in the cisternal segment to the divisions exiting through cranial foramina. Symptoms may vary according to the point of origin and direction/extent of tumor growth. There are no specific findings diagnostic of TS. Trigeminal nerve–related complaints are the initial symptom in the vast majority of patients. Different studies have reported them in 90% to 100% of cases.1,3,9,15,31 Facial hypoesthesia is the most common trigeminal symptom and is present in 70% of patients.6 In most cases, all three divisions are affected. Facial pain or trigeminal motor dysfunction is less commonly encountered. The pain has a paroxysmal, lancinating character and is similar to trigeminal neuralgia. However, the absence of triggering mechanisms, its long duration, and unresponsiveness to drugs differ from classic trigeminal neuralgia.3 In 13% to 38.5% the pain syndrome may be identical to trigeminal neuralgia.3,9 In very rare cases of malignant invasion of the gasserian ganglion, complete anesthesia in all three divisions may be encountered.27,29,30 Other cranial nerve deficits are also seen, but trigeminal involvement is usually more severe. Diplopia is a common symptom and in most cases is related to abducens nerve compression by a middle fossa tumor. Abducens nerve palsy is detected in 26% of patients.6 Headaches, hemifacial spasm, and focal seizures may also be seen. Long-tract signs such as hemiparesis or gait disturbance may also occur. TSs in the cerebellopontine angle result in hearing loss, tinnitus, or gait disturbances (or any combination of these symptoms).15 Facial nerve involvement and long-tract and cerebellar signs may also be seen. Hearing loss and facial nerve dysfunction have been reported in patients with significant erosion of the petrous bone causing damage to the inner ear structures or creating conductive pathologies.15,30,31,39 Far less common symptoms include isolated sixth nerve palsy without trigeminal nerve involvement,40 which may be due to compression of CN VI in Dorello’s canal. The duration of symptoms may vary from a few months to several years.6 Sudden onset of headache secondary to intratumoral hemorrhage has been reported rarely.6,41,42
In the magnetic resonance imaging (MRI) era, incidental TSs have also been detected.6 Decision making for incidental meningiomas is more complicated, and most do agree that as long as the patient is asymptomatic, close follow-up with MRI is the optimal choice, especially in elderly patients. However, previous studies have indicated that nonvestibular intracranial schwannomas have a higher incidence of growth than do sporadic vestibular schwannomas but less so than in patients with neurofibromatosis type 2.43 Therefore, early intervention is advisable in symptomatic cases.
Neuroradiologic Evaluation
MRI and computed tomography (CT) are the “gold standards” for imaging of TSs, with MRI being the preferred mode. Schwannomas appear as well-circumscribed, heterogeneously enhancing masses that are isointense or hypointense on T1-weighted (T1W) images and, most commonly, hyperintense on T2-weighted (T2W) images.24,44–46 Cystic changes or rare intratumoral hemorrhage may be seen.24,47 Cystic changes were reported in 39% to 40% of the tumors.5,24 MRI also provides valuable information on cavernous sinus invasion. Magnetic resonance venography may be performed to demonstrate the vein of Labbé and other venous structures.5 On CT, schwannomas appear isointense to hyperintense in comparison to surrounding brain parenchyma. Most commonly they enhance intensely after injection of iodinated contrast material.45,48,49 Enhancement on CT studies was reported in 33% to 50% of the tumors.5,24 CT is the preferred method to demonstrate bone erosion. In our experience we have detected bone erosion in 70% of patients.24 High-resolution CT clearly demonstrates tumor location and extent and accompanying bony changes.
Plain films are of limited value in imaging TS. In advanced cases, erosion of the petrous apex may be seen, which is highly suggestive of TS.1,50 Yasargil, while reporting his surgical results on 18 predominantly large TSs, noted that petrous erosion was detected in 83.3% of his patients.8 Enlargement of the foramen ovale and spinosum may be found in tumors that invade these structures.
