Learning Objectives: After participating in this CME activity, the neurosurgeon should be better able to:1. Identify the clinical presentation of trigeminal schwannomas.2. Define trigeminal schwannomas in terms of clinical and radiologic features.3. Explain the microsurgical and topographical anatomy of the trigeminal nerve.This article is the first of 2 parts.The first case of trigeminal schwannoma (TS) was reported by Dixon in 1846. The first clinical series was reported by Cuneo and Rand in 1927. Since then, many case series have been reported, with high mortality and morbidity rates due to the difficulty of tumor removal. However, the advances in microsurgical techniques and skull base approaches have led to improved surgical outcomes and long-term tumor control rates in recent years.TSs generally are benign, slow-growing, and well-circumscribed tumors and are the second most common type of intracranial schwannomas, representing less than 1% of all intracranial tumors and 0.8% to 8% of all intracranial schwannomas. The peak incidence is between the third and fifth decades, with a slight female predominance with no statistical difference. However, they may also occur in young children and elderly patients. TS may arise anywhere along the course of the trigeminal nerve (TN). Tumors of the root are located in the cerebellopontine angle, whereas tumors of the trigeminal (gasserian) ganglion (TG) are located in Meckel’s cave and the lateral wall of the cavernous sinus. The orbit, the pterygopalatine fossa, and the infratemporal fossa are involved when TSs originate from 1 of the 3 branches of the nerve. Therefore, schwannomas of the root, ganglion, and peripheral divisions are intradural, interdural, and extradural, respectively. Several cases of malignant TS have been reported in the literature.Back to TopTopographical and Surgical Anatomy of the Trigeminal NerveTN is the fifth and largest of all cranial nerves, serving the major sensory supply of the face and motor supply of the mastication muscles. The name “trigeminal” literally means “3 born together,” referring to the 3 divisions of the nerve: the ophthalmic (V1), maxillary (V2), and mandibular (V3) divisions (Figure 1). In terms of neurosurgical anatomy, TN’s location in the brainstem, cistern, Meckel’s cave, TG, and the peripheral divisions have been discussed in the text.Figure 1Back to TopBrainstemTN has 3 sensory and 1 motor central nuclei located in the brainstem: the mesencephalic nucleus, the principal sensory nucleus, the spinal trigeminal nucleus, and the motor nucleus:The mesencephalic nucleus. The mesencephalic nucleus is located anterolateral to the fourth ventricle and periaqueductal gray matter within the lower midbrain and upper pons, and medial to the sensory nucleus descending to the level of the motor nucleus. The mesencephalic nucleus receives pressure and kinesthesis sense from the teeth, hard palate, periodontium, and temporomandibular joints and stretch sense from the muscles of mastication. The relevant afferent fibers enter the pons from the cisternal segment to control the degree and force of bite.The principal sensory nucleus. The principal sensory nucleus is located in the upper pons above the spinal trigeminal tract and nucleus, is lateral to the entering trigeminal root fibers, and conveys impulses for tactile and pressure sense. Trigeminal fibers from the principal sensory nucleus contain both crossed fibers originating from the ventral part of the nucleus and uncrossed fibers originating from the dorsomedial part of the nucleus to terminate in the ventral posteromedial nucleus of the thalamus.The spinal trigeminal nucleus. The spinal trigeminal nucleus is located anterolateral to the fourth ventricle and extends from the midpons down to the upper cervical spine as far as C2 to C4, transmitting the pain, temperature, and some tactile sensation from the face. The 3 parts of the spinal trigeminal nucleus are named pars oralis, pars interpolaris, and pars caudalis. The pars oralis receives sense impulses from the nose and mouth. The pars interpolaris is related to cutaneous facial regions, whereas the pars caudalis receives impulses from the forehead, cheek, and jaw.The motor nucleus. The trigeminal motor nucleus is located medial to the principal sensory nucleus in the tegmentum of the pons. The axons of the trigeminal motor nucleus exit the brainstem medial to the axons of the sensory nuclei along the cisternal segment, extend to Meckel’s cave, and pass underneath the TG without synapsing. The fibers innervate the muscles of mastication and mylohyoid muscles, the anterior bellies of the digastric muscles, tensor tympani and tensor veli palatini.Back to TopCisternal Segment (Trigeminal Root)TN emerges on the lateral surface of the midpons, crosses the prepontine cistern and extends anteriorly along the medial aspect of the petrous apex as a large sensory root and a smaller motor root. The divisions of TN are situated with the V1 being the most inferior, the V2 in the middle, and the V3 the most superior while exiting the brainstem. The motor root, lying on the anterosuperomedial side of the sensory root, consists of a primary superior and a secondary inferior motor rootlet. The rootlets of the motor root harbor numerous connections between each other and the sensory root. The single motor root formed by the 2 separate rootlets and the sensory root enter the middle cranial fossa through the trigeminal porus to reach Meckel’s cave.Back to TopMeckel’s Cave and the Trigeminal GanglionTN passes through the porus trigeminus, which is located at the trigeminal depression of the petrous apex, providing communication between the prepontine cistern and Meckel’s cave. The porus trigeminus is formed by the posterior petroclinoid ligament superiorly and the base of the posterior clinoid process medially. The greater superficial petrosal nerve (GSPN) along the lateral limit of the horizontal segment of the petrous segment of the internal carotid artery (ICA) is an important landmark for identification of the vessel, as well.The TG, trigeminal cistern, and postganglionic rootlets are located within Meckel’s cave, which is a cleft-like space between the meningeal and periosteal layers of dura, extending from the posterior fossa into the posteromedial portion of the middle fossa. The arachnoid membrane from the posterior fossa forms a pocket within the cave and fuses with the convex side of the ganglion. The TG and the overlying arachnoid membrane do not contain any potential subarachnoid space in the anterior aspect of the ganglion. Therefore, the term “trigeminal cistern” is the subarachnoid space behind the TG containing cerebrospinal fluid and trigeminal rootlets. The TG and trigeminal divisions have 2 layers. The outer layer is the meningeal dural layer of the middle fossa, whereas the inner layer constitutes the dorsolateral wall of Meckel’s cave. The cleavage plane between these 2 layers is the anatomic basis for interdural exposure of Meckel’s cave. The foramen ovale and the V3 are the most important landmarks for the identification of the MC in anterolateral or lateral extradural approaches. MC is best identified medial to the foramen spinosum and posterior to the foramen ovale, guiding the visualization of the lateral wall of Meckel’s cave.Back to TopPeripheral DivisionsThe sensory axons of the TG form the 3 major divisions of the TN. The motor roots travel within the V3.Ophthalmic division. The V1 is the first and smallest division of the TN (Figure 1). The branches of the V1 supply sensation to the cornea, the ciliary body, the iris, the conjunctiva, the lacrimal gland, some portions of the mucous membrane of the nasal cavity and sphenoid and frontal sinuses, the skin of the eyebrow, eyelids, forehead and nose, the tentorium cerebelli, and dura and the posterior part of the falx cerebri.The V1 courses upward and travels forward in the lower part of the lateral wall of the cavernous sinus to reach the superior orbital fissure from the TG. The oculomotor nerve, trochlear nerve, and V1 lie between the outer and inner dural layers of the lateral wall of the cavernous sinus from superomedial to inferolateral, respectively. The V1 branches into the lacrimal, the nasociliary, and the frontal nerves just before entering the superior orbital fissure with the oculomotor, trochlear, and abducens nerves.The superior orbital fissure is a narrow, triangular bony cleft, communicating the orbit with the cavernous sinus in the middle cranial fossa. The wide base of the superior orbital fissure is on the body of the sphenoid bone, whereas the narrow apex is situated between the greater and lesser wings. The superior orbital fissure can be divided into central, lateral, and inferior parts by the annular tendon from which the rectus muscles arise. The oculomotor foramen is the central part of the superior orbital fissure through which the oculomotor nerve passes with the nasociliary nerve, abducens nerve, and branches of the carotid sympathetic plexus. The frontal and lacrimal nerves pass lateral to the annular tendon with the trochlear nerve. The inferior part of the superior orbital fissure contains the orbital fat and inferior ophthalmic vein.Maxillary division. The V2 is the intermediate division of the TN, leaving the TG between the V1 and V3 (Figure 1). Classically, the V2 is thought to be traveling in the lateral wall of the cavernous sinus as the V1 does. However, the V2 has been shown to be traveling beneath the middle fossa dura below the level of junction of the medial and lateral walls of the cavernous sinus to the foramen rotundum of the sphenoid bone before entering the pterygopalatine fossa. The pterygopalatine fossa is a pyramidal-shaped, small space between the pterygoid, palatine, and maxillary bones and communicates with the middle cranial fossa via the foramen rotundum. The V2 is embedded in fat tissue with pterygopalatine ganglion, maxillary artery, and its branches in the pterygopalatine fossa.The V2 enters the infraorbital canal after crossing the pterygopalatine fossa, becoming the infraorbital nerve, and runs through the infraorbital canal situated below the floor of the orbit, in the roof of the maxillary sinus.Maxillary division also gives off the postganglionic parasympathetic ganglionic branches joining the V2, the zygomatic nerve, the posterior superior alveolar nerves, the palatine nerves, the nasopalatine nerve, the pharyngeal nerve, the middle and anterior superior alveolar nerves, the inferior palpebral, and the superior labial branches during its course.Mandibular division. The V3 is the largest of all divisions and consists of a large sensory root and a smaller motor root. The motor root arises in the pons and joins the sensory root just outside the foramen ovale. The meningeal nerve is a branch of the main trunk of the V3, passing through the foramen spinosum with middle meningeal artery to innervate the middle cranial fossa dura. The medial pterygoid nerve and the tensor veli palatine and tensor tympani nerves arise from the main trunk and pass through the otic ganglion to innervate the medial pterygoid and tensor muscles, respectively. The V3 then exits the middle cranial fossa through the foramen ovale to enter the infratemporal fossa (Figure 1) and divides into smaller anterior and large posterior divisions.The smaller anterior division of the V3 gives off the sensory buccal nerve, the masseteric nerve, the deep temporal nerves, and the lateral pterygoid nerve. The large posterior division of the V3 gives rise to the auriculotemporal nerve, the inferior alveolar nerve, and the lingual nerve.Back to TopVidian NerveThe vidian nerve is formed by the union of the GSPN and the deep petrosal nerve. The GSPN is a branch of the facial nerve originating at the geniculate ganglion and carries preganglionic parasympathetic fibers arising from the superior salivary nucleus in the brainstem. The nerve exits the superior surface of the temporal bone via the facial hiatus to reach the middle cranial fossa lying between the 2 layers of the dura. It passes under the V3 toward the foramen lacerum before joining the deep petrosal nerve carrying the postganglionic sympathetic fibers from the carotid plexus. The vidian nerve passes through the vidian canal with the vidian artery and the vidian vein to reach the pterygopalatine fossa. The vidian canal and nerve are important landmarks in directing the microsurgical approaches to the anteromedial part of the cavernous sinus and Meckel’s cave. The anterior opening of the vidian canal is located inferomedial to the foramen rotundum, and the posterior opening is located above the posterior edge of the medial pterygoid plate.Back to TopHistory, Presentation, and Physical Examination and Imaging FindingsLiterature about the natural history of TS is dramatically lacking. Several authors have reported minimal growth rates over limited follow-up periods in a majority of the limited number of cases. In addition, most of the TSs tend to reach large sizes before they are diagnosed, given the slow growth rate of the tumor and mildness of the symptoms. However, recent advances in neuroimaging have led to detection of small TS before the other cranial nerves and the surrounding structures are involved. Therefore, the increased number of relatively smaller tumors and asymptomatic patients may lead to natural history studies in the future.The signs and symptoms related to TS depend on the location of the tumor and direction/extension of the growth of the tumor. However, there are no clinical findings specific to TS. The early and mild symptoms may exist for a long time before the diagnosis. Numbness and/or paresthesia in the distribution of one or more trigeminal branches, the most common symptoms, are present in 70% to 95% of patients. Complete anesthesia in all 3 divisions may be an indicator of malignant invasion of the TG. Facial pain is encountered in 10% to 45% of cases. The intensity of the pain may vary from light to lancinating pain, which tends to last longer and does not respond to carbamazepine when compared with classical trigeminal neuralgia. Constant facial pain is suggested to be associated with the involvement of the TG due to its relatively fixed position on the petrous bone, whereas intermittent facial pain may indicate a posterior fossa TS compressing the more mobile trigeminal roots. Headache (28%) is the second most common symptom. Sudden onset of headache is reported to be related to intratumoral hemorrhage or subarachnoid hemorrhage from the tumor. Extension of the tumor into the cavernous sinus may lead to dysfunction of other cranial nerves. Diplopia is a common finding (10%–20%) associated with involvement of the abducens nerve. Nevertheless, dysfunction of the oculomotor nerve may be the cause of diplopia, as well. Compression of the abducens nerve at Dorello’s canal may cause isolated sixth nerve palsy even in the absence of TN dysfunction. Direct compression or traction on the geniculate ganglion may lead to facial nerve dysfunction in 20% of cases. Hearing loss is reported in 11% of the patients as a result of obstruction of the eustachian tube, or direct compression of the vestibulocochlear nerve or significant petrous bone erosion, causing damage to the inner ear structures. Extension of the tumor to the orbital apex may result in exophthalmos or even visual failure. The ipsilateral optic nerve may also be compressed by the tumors originating from the TG. Other symptoms include ataxia, dizziness, ear pain, and focal seizures.Facial hypoesthesia and diminished or absent corneal reflex are the most common findings in neurologic examination presenting in up to 72% of cases. Weakness of the mastication muscles is encountered in 25% to 35% of cases. Long-tract findings, cerebellar findings, and findings of increased intracranial pressure are encountered less frequently. Signs of involvement of cranial nerves II through XII may also present, the most common of which are the abducens, facial, and vestibulocochlear nerves.TSs appear as isodense or hypodense masses to the brain parenchyma, with significant enhancement after contrast agent injection on CT. Cystic components and fluid levels can be clearly demonstrated with CT scans when present. CT has a considerable role in preoperative evaluation, given its high capability of demonstration of bony structure, as well. Moreover, enlargement of the superior orbital fissure, foramen ovale and foramen spinosum, erosion of the middle fossa, and the petrous apex are of importance in the diagnosis of TS and easily demonstrated with CT.MRI is the modality of choice for TS in terms of demonstrating the anatomic relationship of the tumor with the surrounding structures and for differential diagnosis. TSs appear as isointense to brain structures on T1-weighted and hyperintense on T2-weighted images. The tumor significantly enhances after contrast injection, facilitating the demonstration of small tumors that would otherwise be overlooked (Figure 2). Differentiation based on MRI between malignant and benign TS is not always possible, given the similar enhancing and spreading patterns. However, rapid growth and aggressive and early bone destruction may be the indicators of malignant behavior, especially when inversely proportional to the size of the tumor. Larger tumors may exhibit heterogeneous features with or without fluid-fluid levels. Cystic and necrotic changes may present in up to 54% of cases.Figure 2Demonstration of the cerebral vasculature related to a TS is of consequence because the TS may displace the petrous segment of the ICA within the middle fossa or elevate and displace the proximal portions of the posterior and superior cerebellar arteries within the posterior fossa. Assessment of the tumor vascularity before surgery is essential, as well. Although TSs are relatively avascular tumors when compared with meningiomas, a small number of cases may have arterial feeders from the external carotid artery or the precavernosal segment of the ICA, requiring preoperative embolization. In rare cases, the ICA may be engulfed by the tumor. On the other hand, the need for an invasive, ordinary cerebral angiogram has vanished with the advent of MR angiography, which can be easily performed in every case.Primary tumors of bone, meningiomas, epidermoid tumors, and primary lymphoma of Meckel’s cave are the main differential diagnoses for TS. The margins of the bone destruction are smoother in TS when compared with the irregular borders of bone destruction associated with chordomas and chondrosarcomas. Epidermoid tumors cause sclerotic bone changes. Meningiomas are associated with calcification, dural tail, and hyperostosis of the adjacent bones. The presence of a dural tail in the absence of hyperostosis is the distinguishing characteristic of a lymphoma, which should be primarily considered in immunocompromised patients. Primitive neuroectodermal tumors, sarcoidosis, syphilis, and metastatic carcinomas represent the less common types of mass lesions, which can mimic a TS.Back to TopConclusionTSs are generally benign and rare lesions arising anywhere along the course of the TN, with a wide range of symptoms, depending on the location and extension of the tumor. The early and mild symptoms may exist for a long time, leading them to reach large size before the diagnosis, given the slow growth rate of the tumor. However, recent advances in modern imaging studies have led to detection of small TS before the surrounding neurovascular structures are involved. MRI is the modality of choice to demonstrate the relationship of the tumor with the surrounding structures and for differential diagnosis.