The trigeminal nerve: An illustrated review of its imaging anatomy and pathology

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1 Clinical Radiology 68 (2013) 203e213 Contents lists available at SciVerse ScienceDirect Clinical Radiology journal homepage: Pictorial Review The trigeminal nerve: An illustrated review of its imaging anatomy and pathology G. Bathla *, A.N. Hegde Department of Diagnostic Imaging, National University Hospital, Singapore article information Article history: Received 12 March 2012 Received in revised form 21 May 2012 Accepted 29 May 2012 The trigeminal nerve is the largest cranial nerve and has both sensory and motor components. Due to its extensive distribution in the head and neck, the nerve or its branches may be involved by a myriad of disease entities. Additionally, the nerve may act as a route of spread in various inflammatory and neoplastic diseases, underlining the need for a thorough understanding of its anatomy. A segmental division of the trigeminal system is preferred when interpreting imaging studies as both the type of lesion and symptoms may vary based on the site of involvement. These segments include the brainstem, cisternal, Meckel s cave, cavernous sinus, and peripheral divisions. In general, dedicated magnetic resonance imaging (MRI) is preferred to evaluate nerve dysfunction. In select cases, contrast medium administration, heavily T2-weighted sequences, or MR angiography may prove to be diagnostic. This review aims to review the anatomy of the trigeminal nerve briefly, followed by illustrations of various lesions that may present with trigeminal nerve dysfunction. Ó 2012 The Royal College of Radiologists. Published by Elsevier Ltd. All rights reserved. Introduction The trigeminal nerve (TGN), also known as the fifth cranial nerve, is the principal nerve of the first brachial arch. 1 It is the largest cranial nerve and is extensively distributed in the supra hyoid neck. 2,3 Magnetic resonance imaging (MRI), supplemented with contrast-enhanced sequences in selected cases, is the preferred imaging technique to evaluate the TGN. However, computed tomography (CT) may be used in cases of trauma, to map the bony anatomy of skull base preoperatively, or in critically ill patients. 4 Additionally, lesions causing bony destruction (malignant otitis externa, skull base osteomyelitis, Langerhans cells histiocytosis), proliferation (fibrous * Guarantor and correspondent: G. Bathla, Department of Diagnostic Imaging, National University Hospital, 5 Lower Kent Ridge Road, Singapore , Singapore. Tel.: þ address: girishmamc@gmail.com (G. Bathla). dysplasia, chondrosarcoma), or remodelling (schwannomas, meningiomas) may be better assessed by a combination of CT and MRI. Imaging anatomy of the TGN The TGN is formed from three divisions: ophthalmic (V1), maxillary (V2), and mandibular (V3). V1 and V2 are purely sensory, while V3 has both sensory and motor components. 2 The extra-cranial branches of these divisions are summarized in Table 1. V1 is the smallest division and is formed through coalescence of V1 branches posterior to the orbital apex. 2 More proximally, V1 enters the cavernous sinus. Similarly, V2 is formed by coalescence of V2 branches in the infraorbital canal and pterygopalatine fossa. It traverses the foramen rotundum to enter the cavernous sinus. Both V1 and V2 travel within the lateral wall of the cavernous sinus and enter Meckel s cave posteriorly, where they join V3. The /$ e see front matter Ó 2012 The Royal College of Radiologists. Published by Elsevier Ltd. All rights reserved.

2 204 G. Bathla, A.N. Hegde / Clinical Radiology 68 (2013) 203e213 Table 1 Branches arising from the three principal divisions of the trigeminal nerve. Ophthalmic nerve (sensory) Frontal Lacrimal Nasociliary Tentorial Dural Maxillary nerve (sensory) Infraorbital Zygomatic Greater and lesser palatine Posterior superior alveolar Meningeal Mandibular nerve (mixed) Sensory: Meningeal Lingual Auriculotemporal Inferior alveolar Buccal Motor: Masseteric Deep temporal Medial pterygoid Lateral pterygoid Mylohyoid sensory branches of V3, the largest division of TGN, merge together in the parapharyngeal space below the skull base to form the V3 trunk, which then enters Meckel s cave through the foramen ovale. 2,4 Within Meckel s cave, the three divisions coalesce to form the trigeminal or semilunar ganglion. The ganglion houses the cell bodies of first-order sensory neurons of all three divisions with the exception of proprioceptive fibres from V3, the first-order neurons of which are located in the mesencephalic nucleus itself. 1,3 The trigeminal ganglion then divides into multiple individual rootlets, which course through the prepontine cistern posteriorly to reach the brainstem. Within the brainstem, the fibres spread out to reach three different sensory nuclei (Fig 1). The principal sensory nucleus is located in the pontine tegmentum and mediates pressure and light touch from V1eV3. 5,3 The mesencephalic nucleus at the junction of pons and mid-brain receives predominantly proprioceptive input from V3. The spinal trigeminal nucleus extends from the pontomedullary junction to the upper cervical cord. 3 It mediates pain and temperature from V1eV3. The second-order neurons from these nuclei project to the thalamus, and the third-order neurons from there project to the cerebral cortex. 3 The motor component of TGN arises from the motor nucleus located in the floor of fourth ventricle. 2 After exiting the brainstem, it travels medial to the sensory component in the prepontine cistern and enters Meckel s cave where it bypasses the trigeminal ganglion and joins the sensory component of V3 at the skull base to form the mandibular nerve. 4 The motor component supplies the muscles of mastication, tensor tympani and tensor veli palatini. 5 Figure 1 (a) Axial constructive interference in steady state (CISS) image and (b) sagittal T1-weighted image with superimposed colour drawings to depict the anatomy of the TGN. The motor (light blue), principal sensory (yellow) and part of mesencephalic nucleus (green) are shown in relation to the TGN (orange). Distally, the nerve enters Meckel s cave (white arrow). The sagittal T1-weighted image (b) depicts the course of the spinal trigeminal nucleus (white), which may extend up to C4 level in some cases.

3 G. Bathla, A.N. Hegde / Clinical Radiology 68 (2013) 203e Conditions affecting the TGN Lesions affecting TGN function may involve the supratentorial compartment or it s nuclei in the brainstem. Alternately, the nerve may be affected in the prepontine cistern, Meckel s cave, cavernous sinus, or within the peripheral divisions or terminal branches. The clinical presentation varies depending on the site of involvement and may necessitate additional sequences or modification of routine protocol (Table 2). Brainstem Lesions that involve the brainstem rarely present with isolated TGN deficits, owing to the compact distribution of neural structures in this region. Other cranial nerve deficits are usually associated, with or without ipsilateral or contralateral symptoms. 3,4 In the brainstem, multiple sclerosis (MS) is the single most common cause of trigeminal neuropathy. 6 Trigeminal neuralgia may be seen in 1e2% of patients with MS and is often bilateral. 5 It may be the presenting symptom or may occur in association with other cranial nerve deficits. On imaging, these usually appear as areas of T2 prolongation (Fig 2), which may show contrast enhancement in the active phase. Occasional thin rim of T1 hyperintensity may also be seen and is usually attributed to free radicals and protein accumulations. 6 About one-fourth of acute strokes involve the brainstem. 6 Small infarcts involving the TGN nuclei are uncommon. Infarcts involving the posterior inferior cerebellar artery territory may affect the dorsal trigeminal nucleus, leading to cranial nerve (CN) V symptoms (Fig 3). Figure 2 Axial T2-weighted image though the level of the mid-pons in a patient with known MS who presented with new onset of rightsided facial paraesthesias. There is presence of an ill-defined hyperintense lesion involving the pons on right side, close to the site of entry of the TGN. They usually present clinically with Wallenberg syndrome and may have ipsilateral facial pain and temperature loss. 3,4 Brainstem gliomas may involve the trigeminal nuclei but rarely present with isolated TGN deficits. 4 These are often infiltrative and low grade with variable enhancement after contrast medium administration. 6 Similarly, Table 2 Different clinical presentations of trigeminal nerve dysfunction may help to localize the involved compartment and warrant a more tailored approach to imaging. Segments Clinical signs Preferred imaging technique Brainstem Cranial nerve deficits involving III, IV, VI, VIIeXII Wallenberg, medial medullary and Parinaud syndrome Ipsilateral hemiparesis/plegia with contralateral facial symptoms MRI brain a Include upper cervical cord Cisternal and Meckel s cave Trigeminal neuralgia/dysfunction Atypical trigeminal neuralgia Facial weakness (VII CN), hearing loss (VIII CN) Cavernous sinus Ophthalmoplegia, mydriasis, proptosis, cranial nerve III, IV, V1, V2, and VI deficits Peripheral segments Pain, paraesthesia, or analgesia in distribution of involved nerve Weakness in chewing, mouth opening, deviation of jaw Absent corneal, blink, lacrimal or oculocardiac reflexes (V1) Absent jaw jerk reflex, numb chin syndrome (V3) MRI brain Additionally, CISS (ST 0.5 mm) and high-resolution time of flight MR angiogram (ST 0.5 mm) in suspected neurovascular conflicts MRI brain High-resolution T2W and CISS axial images MRI brain Axial T1W (unenhanced) and axial and coronal T1W, fat-suppressed contrast-enhanced sequences through skull base in suspected perineural spread Additional axial and coronal T1W, T2W fat-suppressed and tri-plane T1W fat-suppressed contrast-enhanced sequences in suspected orbital disease CT in cases of trauma and base of skull disease The sequences listed here refer to those performed at the author s institute. CN, cranial nerve; MRI, magnetic resonance imaging; MR, magnetic resonance; CISS, constructive interference in steady state; CT, computed tomography. a MRI brain refers to the routine protocol followed at the National University Hospital, Singapore, and includes sagittal T1-weighted (T1W), axial T2- weighted (T2W), gradient echo (GRE), diffusion-weighted imaging (DWI), coronal fluid-attenuated inversion recovery (FLAIR) [section thickness (ST) 5 mm], and contrast-enhanced three-dimensional (3D) T1W (ST 3 mm).

4 206 G. Bathla, A.N. Hegde / Clinical Radiology 68 (2013) 203e213 Figure 3 Axial T2-weighted (a) and diffusion-weighted (b) images of a patient who presented with a partial Wallenberg syndrome and associated paraesthesias in the distribution of left TGN. There is a tiny focus of restricted diffusion involving the dorso-lateral medulla, in keeping with acute infarct. metastases and lymphoma may also cause CN V dysfunction but are rarely the initial manifestation of disease. 4,7 Overall, MS, glioma, and infarction are the most common brainstem and upper cord lesions that may result in CN V symptoms. 5,8 Cavernous haemangiomas may involve the TGN nuclei and present with trigeminal neuropathy. 7 They often show a typical popcorn appearance on T2-weighted images with blooming on T2* and susceptibility-weighted images (Fig 4). They almost never enhance unless when associated with a venous haemangioma. 4 Hypertensive haemorrhage and vascular malformations may also involve the CN V nuclei. 6 Rhomboencephalitis refers to involvement of the brainstem by an inflammatory process and may occur secondary to herpes simplex and rhino-cerebral fungal infections, mostly due to Mucor or Aspergillus species (Fig 5), although a variety of other infections are also implicated. 3,4,8 As the spinal trigeminal tract extends at least to C2 and sometimes to C4, involvement of the upper cervical cord by trauma, demyelination, disc herniations, and syrinx formation may also present with CN V dysfunction. 3,4,7 For Figure 4 Axial T2-weighted image (a) in a patient with sensory symptoms in trigeminal nerve distribution reveals presence of a popcorn lesion involving the pons on right side (black arrow). There is slight perilesional oedema. Axial T1-weighted image at the same level (b) reveals the presence of hyperintense foci within the lesion, in keeping with a subacute bleed.

5 G. Bathla, A.N. Hegde / Clinical Radiology 68 (2013) 203e Figure 5 Rhomboencephalitis secondary to Aspergillus sp. infection. Axial T2-weighted image (a) in a patient who was previously operated for fungal sinusitis. There is altered signal in the right cavernous sinus, extending into the brainstem. Axial, contrast-enhanced image (b) reveals corresponding heterogeneous enhancement in the involved regions (black arrows). this reason, the evaluation of CN V should also include the upper cervical spinal cord. 4 Cisternal segment and Meckel s cave Lesions involving the cisternal segment typically present with trigeminal neuralgia. This is likely because the segment corresponds to the transition between central and peripheral myelin, referred to as the root entry zone. It is about 2 mm in length and is located about 5e7 mm away from the pons. A thinner myelin layer in this region renders the nerve more vulnerable to extrinsic compression. 4,6 The most common cause of trigeminal neuralgia is compression of the root entry zone by a vascular loop, which is usually of the superior cerebellar artery (SCA), but Figure 6 Trigeminal neuralgia. Axial CISS images (a and b) in a patient with right-sided trigeminal neuralgia reveal indentation of the right TGN by the superior cerebellar artery. Incidentally, the left vertebral artery is ectatic and partly thrombosed, and is seen to indent the pontine belly.

6 208 G. Bathla, A.N. Hegde / Clinical Radiology 68 (2013) 203e213 Figure 7 Axial 3D time-of-flight (TOF) MR angiography image in a patient with left-sided trigeminal neuralgia reveals presence of flow-related enhancement in the persistent left trigeminal artery. may also arise from the anterior inferior cerebellar artery (AICA) or basilar artery 4,7 (Fig 6). Besides vascular loops, aneurysms, arteriovenous malformations, persistent trigeminal arteries (Fig 7), dural fistulas, petrous vein, and vascular ectasias may also present with trigeminal neuralgia. 6,8 High-resolution MR angiography and threedimensional (3D) gradient echo steady-state free-procession (SSFP) sequences are often used to visualize the exact site of compression and identify the offending vessel. 6 Clinically, the maxillary branch is most commonly affected, often along with mandibular division. 3 On imaging, the diagnosis should be considered only in an appropriate clinical setting when the offending vessel crosses the nerve perpendicularly in the region of the root entry zone, with consequent deviation, indentation, or encasement of nerve. 4,6 In patients with severe trigeminal neuralgia, atrophy of the nerve may also be seen. 6 The cisternal segment may also be involved by neoplasms. Schwannoma is the most common primary TGN neoplasm and frequently affects the cisternal segment and trigeminal ganglion, although it may involve any segment of the nerve 4,9 (Fig 8). On imaging, the lesions grow along the nerve and may be dumbbell or saddle shaped. Larger lesions have a heterogeneous appearance secondary to necrosis, cystic degeneration, and haemorrhage. 4,5,9 CT may also show associated bony remodelling or erosion at the trigeminal recess. Aggressive features, including rapid growth and bony destruction, may point towards underlying malignant schwannoma. 4 The nerve may also be impinged upon by various benign and malignant lesions involving the cerebellopontine angle cistern and skull base 4,7 (Fig 9). Similarly, petroclival meningiomas, when they extend in to the Meckel s cave, may also present with TGN symptoms (Fig 10). Since it traverses the cerebrospinal fluid (CSF), this segment is also susceptible to leptomeningeal diseases, such as carcinomatosis, lymphoma (Fig 11), sarcoidosis, and meningitis. Nerve inflammation secondary to petrous apicitis may also occur as in Gradenigo s syndrome. 3,9 Figure 8 Axial (a) and coronal (b and c) T2-weighted images in a patient with a right-sided trigeminal schwannoma. There is presence of a slightly hyperintense mass lesion along the course of the right TGN (a), extending into the superior orbital fissure. The mass is also seen to extend into the right foramen rotundum (white arrow in b) and foramen ovale (black arrow in c).

7 G. Bathla, A.N. Hegde / Clinical Radiology 68 (2013) 203e Figure 9 Axial CISS image (a) in a patient with left-sided trigeminal symptoms. There is presence of a small heterogeneously hyperintense lesion involving the prepontine cistern (long black arrow), causing indentation of left trigeminal nerve (short black arrow). There is restricted diffusion on the diffusion-weighted image (b), in keeping with an epidermoid. Figure 10 Axial T2-weighted image in a patient with right-sided facial paraesthesias reveals presence of a dural-based lesion in the region of petrous apex. There is mass effect on the brainstem with extension of the lesion into Meckel s cave. Imaging features were in keeping with meningioma. Figure 11 Coronal T1-weighted contrast-enhanced image in a patient with secondary relapse of systemic lymphoma. There is thickening and abnormal enhancement of both TGNs (black arrows). Associated abnormal leptomeningeal enhancement over both cerebral convexities is also evident.

8 210 G. Bathla, A.N. Hegde / Clinical Radiology 68 (2013) 203e213 Figure 12 TolosaeHunt syndrome. Axial T2-weighted (a) and contrast-enhanced (b) images of a patient with multiple right-sided cranial nerve dysfunctions. The right cavernous is bulky and shows abnormal T2 hypointense soft tissue within (white arrow). Subtle attenuation of right cavernous ICA is also evident (arrowhead). Homogeneous enhancement is noted on post-contrast image (b). (Image courtesy of Dr Narayan Lath, Singapore General Hospital, Singapore.) Cavernous segment Only V1 and V2 traverse the cavernous sinus with V3 exiting the cranial cavity through the foramen ovale. Most of the disease entities involving the cisternal and Meckel s cave segments can also affect the cavernous segment. In addition, this segment may also be affected by diseases that specifically involve the cavernous sinus. These include Figure 13 Rhabdomyosarcoma with perineural spread. Coronal T2-weighted image (a) reveals bulky, soft-tissue infiltration of the left cavernous sinus, extending into the foramen rotundum (white arrow). The foramen rotundum on the right side appears normal (arrowhead). Axial contrast-enhanced image (b) reveals homogeneous enhancement after contrast medium administration.

9 TolosaeHunt syndrome, which is characterized by lymphocytic dural infiltration of unknown aetiology (Fig 12). MRI reveals a bulky cavernous sinus that is iso- to hypointense on T1 and T2-weighted images and shows enhancement after contrast medium administration. Narrowing of the cavernous portion of the internal carotid artery is a common finding. 4,6 Vascular lesions involving this segment include carotid aneurysms and carotico-cavernous fistulas. Giant aneurysms of the cavernous segment are may present with multiple cranial nerve palsies. 6 Up to one-third of these patients may have TGN symptoms. 5,10 Similarly, lesions involving the skull base may have TGN dysfunction secondary to involvement of cavernous sinus. These include invasive pituitary adenomas, osteocartilaginous tumours, chordomas, and metastases. 4 G. Bathla, A.N. Hegde / Clinical Radiology 68 (2013) 203e Peripheral segments The terminal branches of the V1, V2, and V3 are the supra-orbital, infra-orbital, and inferior alveolar nerves, respectively. 4 These are most commonly involved in perineural spread of head and neck malignancies, which is most frequently seen along the maxillary division. 4 Adenoid cystic carcinoma has a strong predilection for perineural spread, although it may also be seen with squamous cell carcinoma, lymphoma, and melanoma. 9 In the paediatric age group, rhabdomyosarcomas are known to extend along the TGN 11 (Fig 13). Perineural spread along the V1 division is seen in patients with lacrimal gland or skin tumours. Perineural spread along V2 occurs in tumours involving the midface and paranasal sinuses and along V3 with tumours involving the nasopharynx, masticator space, mandible, parotid gland, and external auditory canal. 6,9 On imaging, perineural spread can be seen in both antegrade and retrograde directions and may also have skip lesions, hence underlining the need for evaluation of the entire TGN in such cases. 3,4 Imaging features include irregular thickening and enhancement of the nerve, obliteration of the juxta-foraminal fat pads, widening of the neural foramen, atrophy and altered signal of the supplied muscles 3,9 (Fig 14). Trigeminal schwannomas involving the peripheral branches are relatively rare, and usually involve the ophthalmic branch (Fig 15). Meningiomas can extend along the TGN divisions and mimic schwannomas (Fig 16). The peripheral nerves may also be affected by trauma, which is one of the common causes of impaired sensory function along CN V distribution. 12 Fractures involving the orbits and central skull base may result in distal branch neuropathy if they extend along the expected course of the nerve. Iatrogenic trauma due to dental procedures is a common cause of inferior alveolar/lingual nerve injury. 3 Fibro-osseous lesions of the skull base and facial bones may also compress the nerve roots and present with CN V symptoms. 6,9 Figure 14 Perineural spread in nasopharyngeal carcinoma. Coronal T1-weighted image (a) reveals abnormal isointense soft tissue involving the region of the nasopharynx and left masticator space. Cranially, it is seen to extend into the left cavernous sinus through the foramen ovale (small black arrow). Axial contrast-enhanced image (b) reveals enhancement of the mandibular division of the left TGN (long black arrow).

10 212 G. Bathla, A.N. Hegde / Clinical Radiology 68 (2013) 203e213 Figure 15 Axial T2-weighted (a) and contrast-enhanced (b) images of a patient with schwannoma involving the V1 division. There is presence of a partly cystic retro-bulbar mass with heterogeneous enhancement after contrast medium administration. Figure 16 Meningioma along the mandibular division. Coronal T2-weighted (a) and contrast-enhanced (b) images reveal the presence of a T2 hyperintense (a), dumbbell-shaped lesion in the left middle cranial fossa with extension through the left foramen ovale. There is involvement of the cavernous sinus with heterogeneous enhancement after contrast medium administration (b). Conclusion The TGN is the largest cranial nerve. It may be involved by various lesions and can serve as a pathway of disease spread. A thorough understanding of its anatomy and pathology is of vital importance, both in diagnosis and staging of diseases. References 1. Monkhouse S. The trigeminal nerve. In: Monkhouse S, editor. Cranial nerves functional anatomy. Cambridge: Cambridge University Press; p. 50e Williams LS, Schmalfuss IM, Sistrom CL, et al. MR imaging of the trigeminal ganglion, nerve, and the perineural vascular plexus: normal appearance and variants with correlation to cadaver specimens. AJNR Am J Neuroradiol 2003;24:1317e23.

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