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1 Trigeminal Nerve natomy: Illustrated Using Examples of bnormalities T he trigeminal nerve has an extensive anatomic course. Comprehensive knowledge of trigeminal nerve anatomy facilitates understanding of the relationship between the brainstem, skull base, and facial area. In our article we attempt to solidify this knowledge using examples of abnormalities. We will describe the anatomy from the peripheral branches toward the brainstem in keeping with the direction of sensory Fig. 1. Diagram shows trigeminal nerve (TGN), trigeminal ganglion, and peripheral divisions and their branches. From foramen rotundum ossis sphenoidalis, maxillary nerve (thin underline) gains access to pterygopalatine fossa and continues in floor of orbit as infraorbital nerve. Inferior alveolar and lingual nerves (thick underline) are branches of mandibular nerve. propagation and perineural spread of malignant and inflammatory disease. Peripheral Divisions of the Trigeminal Nerve The trigeminal nerve trifurcates into ophthalmic, maxillary, and mandibular nerves distal to the trigeminal ganglion. The ophthalmic nerve passes forward in the lateral wall of the cavernous sinus. It gains access Pictorial Essay H.. M. Kamel 1,2 and J. Toland 3 into the orbit via the superior orbital fissure (Fig. 1). The ophthalmic nerve then divides to supply sensation to the eyeball, lachrymal glands, conjunctiva, part of the nasal mucosa, skin of the nose, eyelid, and forehead [1]. The maxillary nerve exits the skull base through the foramen rotundum ossis sphenoidalis inferolateral to the cavernous sinus. It then enters the pterygopalatine fossa where it gives off several branches. Its main Received ugust 17, 1999; accepted after revision March 2, Department of Neuroradiology, The Royal Victoria Hospital, Grosvenor Rd., elfast T12 6, United Kingdom. 2 Present address: Radiology Department, Hamad Medical Corporation, P. O. ox 3050, Doha, Qatar. ddress correspondence to H.. M. Kamel. 3 Department of Neuroradiology, eaumont Hospital, Dublin 9, Ireland. JR 2001;176: X/01/ merican Roentgen Ray Society JR:176, January

2 Kamel and Toland Fig. 2. Sagittal diagram shows three peripheral divisions of trigeminal nerve entering convexity and root bundles leaving concavity of sickle-shaped trigeminal ganglion. Motor root (solid arrowhead) bypasses ganglion and reunites with mandibular nerve in foramen ovale basis cranii. Open arrowhead indicates descending spinal trigeminal tract. Diagram also shows motor and sensory trigeminal nuclei (underline) in brainstem and cervical cord.,, and C track nuclear origin of fibers contributing to opthalmic; D and E, the maxillary; and F, H, and I, mandibular divisions of trigeminal nerve. Fig year-old man with nasopharyngeal carcinoma. Coronal contrast-enhanced T1-weighted spin-echo MR image with fat suppression shows enhancement and thickening of mandibular nerve extending into trigeminal ganglion (arrow). Note widening of foramen ovale basis cranii and enhancing soft-tissue mass causing destruction of right side of sphenoid bone. trunk continues anteriorly in the orbital floor (Fig. 1) and emerges onto the face as the infraorbital nerve to innervate the middle third of the face and upper teeth [1]. The mandibular nerve runs laterally along the skull base then exits the cranium by descending through the foramen ovale into the masticator space. The motor root of the trigeminal nerve bypasses the trigeminal ganglion and reunites with the mandibular nerve in the foramen ovale basis cranii (Fig. 2). s the mandibular nerve enters the masticator space, it divides into several sensory branches to supply sensation to the lower third of the face and the tongue, floor of the mouth, and the jaw (Fig. 1). The motor root of the mandibular nerve innervates the four muscles of mastication: the mylohyoid, the anterior belly of digastric, the tensor muscle of the tympanic membranes, and tensor muscle of valum palatinum [2]. Infection and neoplasia most commonly involve the peripheral divisions of the trigeminal nerve. Direct spread from local tumors (Figs. 3 and 4) or metastases from distant malignancies are the most common causes of malignant involvement [1]. Trigeminal Ganglion and Preganglionic Trigeminal Nerve The trigeminal ganglion is contained within the Meckel s cavity posterolateral to the cavernous sinus on either side of the sphenoid bone. The Meckel s cavity is a cerebrospinal fluid containing arachnoidal pouch protruding from the posterior cranial fossa. Medial to the ganglion in Meckel s cavity is the internal carotid artery in the posterior portion of the cavernous sinus. Inferior is the motor root of the trigeminal nerve (Fig. 2) and the apex of the petrous temporal with the internal carotid artery in its bony canal [1]. The Meckel s cavity can be involved either by extrinsic or intrinsic disease. Extrinsic le- Fig year-old man with squamous cell carcinoma of skin., xial T2-weighted spin-echo MR image shows thickening of skin and subcutaneous tissue of right cheek and band of intermediate signal intensity extending along course of infraorbital nerve (arrow). High signal is noted in thickened mucosa of both maxillary sinuses., Coronal T1-weighted spin-echo MR image after IV contrast injection shows spread of irregular enhancing mass (arrow) along infraorbital nerve. Note normal mucosal enhancement in both maxillary sinuses. 248 JR:176, January 2001

3 Trigeminal Nerve natomy Fig year-old girl with known diagnosis of leukemia., Expansion and reduction of normal high signal intensity in Meckel s cavity (arrows) on T2-weighted spin-echo MR image. Note signal void from internal carotid artery medial to Meckel's cave., Coronal T1-weighted spin-echo MR image after IV contrast injection shows bilateral Meckel s cavity enhancement (arrows), particularly of left side, as result of leukemic deposits. sions, usually bony metastasis, chordoma, or chondrosarcoma, destroy adjacent bone as they extend toward the Meckel s cavity. Intrinsic lesions simply expand the Meckel s cavity (Fig. 5). When the tumor is large enough, the pressure exerted by it leads to erosion of the surrounding bone. Tumors may also extend away from the Meckel s cavity with enlargement of the foramen ovale basis cranii, foramen rotundum ossis sphenoidalis, or the superior orbital fissure. Intrinsic lesions include primary tumors of the Meckel s cavity as well as secondary neoplasms from perineural spread of local tumors, leptomeningeal, or he- Fig year-old man with pial arteriovenous malformation., Tangle of dilated blood vessels (arrow) at root entry zone of right trigeminal nerve on T2-weighted spinecho MR image., Late arterial phase of vertebral angiogram shows arteriovenous malformation supplied by dilated pontine artery (small arrow). Note also early shunting into dilated draining vein (large arrow). Fig year-old woman with petrous apex meningioma. Homogenous enhancing mass involving root entry zone and Meckel s cavity. Mass forms obtuse angle with dura on axial contrast-enhanced T1-weighted spin-echo MR image. rrow indicates normal right-sided Meckel s cavity and dural tail on posterior surface of clivus. Fig year-old man with trigeminal schwannoma. Dumbbell-shaped high-signal-intensity mass extending between middle and posterior cranial fossa, along course of trigeminal nerve, on axial T2-weighted spin-echo MR image. Trigeminal schwannomas have smooth margins and appear iso- to hyperintense to brain on T2-weighted MR imaging. Larger tumors may show heterogeneous signal intensity. Fig year-old woman with pituitary adenoma. Spread of large enhancing pituitary mass through cavernous sinus to involve trigeminal ganglion in Meckel s cavity on T1- weighted contrast-enhanced coronal spin-echo MR image. Left-sided Meckel s cavity (arrow) is normal. Signal void associated with flowing blood differentiates carotid artery within enhancing pituitary adenoma. 8 9 JR:176, January

4 Kamel and Toland matogenous metastasis. Primary tumors of the Meckel s cavity include trigeminal schwannoma, meningioma, and epidermoid cyst [1]. Pituitary fossa and cavernous sinus lesions (Fig. 6) may extend to the Meckel s cavity or involve the cavernous portion of the trigeminal nerve divisions. s many as one third of patients with intracavernous carotid aneurysms have trigeminal nerve manifestations [1, 2]. Metastatic disease is the second most common lesion to present as a cavernous sinus mass with trigeminal neuropathy [1]. The root entry zone is the cisternal part of the trigeminal nerve just as it enters the pons (Fig. 2). Lesions affecting the root entry zone include vascular compression, primary and secondary neoplasms, and infection. Vascular contact with the root entry zone is thought to represent the most common cause of idiopathic trigeminal neuralgia. Other conditions leading to vascular compression include aneurysms, arteriovenous malformations (Fig. 7), dural arteriovenous fistulas, and vertebrobasilar ectasia. Primary tumors involving the prepontine cistern include meningioma (Fig. 8), trigeminal schwannoma (Fig. 9), epidermoid cyst (Fig. 10), vestibular schwannoma, and lipoma [1]. Secondary neoplasms affecting the root entry zone include perineural spread of tumors from head and neck malignancy, hematogenous metastasis, and leptomeningeal spread of tumors [3] (Fig. 11). enign inflammatory or infectious conditions such as sarcoidosis (Fig. 12), viral encephalitis, herpes neuritis [4], and Lyme disease can also affect the root entry zone [1]. Fig year-old man with histologic diagnosis of epidermoid cyst. (Courtesy of McKinstry CS, elfast, United Kingdom), T2-weighted spin-echo MR image shows spread of slow-growing smooth mass of high signal intensity from cerebellopontine cistern to prepontine cistern across root entry zone of trigeminal nerve. Note septa and focal areas of high signal intensity within mass. ssociated brainstem compression is causing hydrocephalus.,t1-weighted spin-echo MR image shows cauliflower-shaped contours and nonhomogeneous low to intermediate signal intensity within mass. rainstem The trigeminal nerve has three sensory and one motor nuclei. The sensory nuclei are the principal, mesencephalic, and spinal sensory (Fig. 2). The spinal trigeminal tract emerges from the sensory root in the pons and extends downward into the upper cervical cord. Fibers of this tract end in the spinal trigeminal nucleus, which merges rostrally with the principal sensory nucleus. Cervical extension of the spinal tract explains why some patients with upper cervical disk herniation present with trigeminal sensory neuropathy [5]. The motor nucleus of the trigeminal nerve forms an oval column of cell anteromedial to the motor root and the principal sensory nucleus in the pons (Fig. 12). The principal sensory nucleus lies lateral to the entering trigeminal root. The mesencephalic trigeminal nucleus forms a slender cell column near the lateral margin of the central gray matter anterior to the upper fourth ventricle and aqueduct. fferent fibers of the mesencephalic nucleus convey proprioception from teeth, hard palate, and temperomandibular joint. Cells of the mesencephalic nucleus form the sickle-shaped mesencephalic tract, which descends to the level of the motor nucleus and conveys impulses that control mastication and the force of a bite. Eventually all tracts from the principle sensory and spinal trigeminal nuclei project to the posteromedial nucleus of the thalamus, from which they track through the most posterior as- Fig year-old boy with leptomeningeal metastasis from cerebellar medulloblastoma. Contrast-enhanced T1-weighted spin-echo MR image shows bilateral symmetric enhancement and thickening of root entry zone of both Meckel s cavitys. In addition, nodular enhancement and thickening are seen on undersurface of both temporal lobes, superior surface of cerebellum, anterior surface of pons, and lining of lateral ventricles. (Courtesy of McKinstry CS, elfast, United Kingdom) Fig year-old man with diagnosis of neurosarcoidosis. Coronal T1-weighted spin-echo MR image shows bilateral asymmetric thickening and enhancement of trigeminal (solid straight arrow) and occulomotor (curved arrow) nerves and hypothalamus (open straight arrow). Lateral ventricles are dilated. 250 JR:176, January 2001

5 Trigeminal Nerve natomy Fig year-old man with diagnosis of multiple sclerosis. Oblong plaque of high signal intensity (arrow) involving right trigeminal sensory nucleus on T2-weighted spin-echo MR image. Note small area of high signal intensity adjacent to fourth ventricle and high signal intensity in white matter of temporal lobes. Fig year-old woman with lateral medullary syndrome caused by spontaneous vertebral artery dissection. Localized high signal intensity caused by area of infarction on right side of medulla oblongata (arrow) on T2-weighted spin-echo MR image. Note absence of flow void in right vertebral artery. pect of the posterior limb of the internal capsule and project to the postcentral gyrus [1]. Multiple sclerosis (Fig. 13), glioma, and infarction (Fig. 14) are the most common brainstem and upper cervical cord lesions resulting in fifth cranial nerve symptom. Less common lesions include metastasis, cavernous hemangiomas [6], hemorrhage, and arteriovenous malformation [1]. Rarely, rhombencephalitis may develop as a result of retrograde extension of herpes simplex virus type 1 from the trigeminal ganglion into the brainstem [4] (Fig. 15). In conclusion, a variety of conditions may involve the different segments of the trigeminal nerve. Knowledge of its anatomic course allows an understanding of disorders involving the brainstem and adjacent skull base. cknowledgment We thank C. S. McKinstry, elfast, United Kingdom, for his assistance in revising this paper and for providing Figures 10 and 11 of this article. References 1. De Marco JK, Hesselink JR. Trigeminal neuropathy. Neuroimag Clin N m 1993;3: Majoie CLM, Verbeeten, Dol J, Peters Fig year-old woman with herpes zoster. xial contrast-enhanced T1-weighted spin-echo MR image shows enhancement along pontine course of trigeminal nerve and low signal intensity at site of main trigeminal sensory nucleus (arrow). FLM. Trigeminal neuropathy: evaluation with MR imaging. RadioGraphics 1995;15: Donnet, Moullin G, Tubiana N, Gras R, Robert JL. Lymphomatous meningitis: neuroradiological appearances. Neuroradiology 1992;34: Tien RD, Dillon WP. Herpes trigeminal neuritis and rhombencephalitis on Gd-DTP-enhanced MR imaging. JNR 1990;11: arakos J, D mour P, Dillon WP, Newton TH. Trigeminal sensory neuropathy caused by cervical disc herniation. JNR 1990;11: Saito N, Wamakawa K, Sasaki T, Saito I, Takakura K. Intra-medullary cavernous angioma with trigeminal neuralgia: a case report and review of literature. Neurosurgery 1989;95: JR:176, January