1 ORIGINAL ARTICLE Anatomy of the Trigeminal Nerve. Key Anatomical Facts for MRI Examination of Trigeminal Neuralgia. Sgarbi Nicolás, Saibene Andrés, Telis Osmar, Doassans Inés, Boschi Jorge, Soria Víctor Radiology Department – Hospital de Clínicas (Montevideo) Magnetic Resonance Service – Sanatorio Americano Anatomy Department School of Medicine – Universidad de la República Oriental del Uruguay Introduction A detailed knowledge of the trigeminal Trigeminal Neuralgia (TN) is currently a nerve (V) anatomy, including its origin, frequent reason for carrying out a craneo- course, relationships and distribution encephalic Magnetic Resonance Imaging branches, is essential for a correct (MRI) examination in Neuroradiology interpretation of the images, and Departments. correspondingly for an accurate diagnosis. It is a precise clinical diagnostic entity, although its pathophysiology and treatment Objectives – Materials – Methods are under discussion and permanently To revise the anatomy of the V cranial nerve revised. and its different segments, with a special Multiple disorders may produce TN, but in interest in those of fundamental most patients it is not possible to identify a importance for the correct interpretation of specific cause to account for the clinical high resolution MRI studies. condition. MRIs, carried out with a closed-system field Certain anatomical relationships have 1.5T power resonator with conventional likewise been indicated as probably related sequences and high resolution protocols with facial pain, giving rise to neurovascular (FIESTA), were used to analyze the conflict conditions. trigeminal nerve anatomy in its different sectors. Micro-dissections with multiple approaches and fine sections in the 3 planes of space in 5 adult cadaver heads (Department of Anatomy, School of Medicine, Neuroanatomy Laboratory), dissected with the aid of a neurosurgical microscope with x4 and x8 magnification, registered photographically the disposition and anatomical relationships of the structures in the 3 planes of space. Next, anatomical sections of the previous frozen sections, in the axial, coronal and sagital planes were carried out. Rev Imagenol, 2009, 12(2):28-33. SOCIEDAD DE RADIOLOGIA E IMAGENOLOGIA DEL URUGUAY 2 Anatomy or the Trigeminal Nerve. Key Anatomical Facts for MRI Examination of Trigeminal Neuralgia 2a 2b Results The sensitive nucleus is the largest, Descriptive Anatomy of the Trigeminal comprising 3 true sub-nuclei, each Nerve responsible for each aspect of the general The VTh is the most developed and sensitivity. extensive cranial nerve, with a broad The highest is the mesencephalic nucleus, distribution territory. It is a mixed nerve located in the tegmentum close to the conducting sensitive and motor somatic midline and to the grey matter close to the fibers to the face, and is conceptually Sylvian aqueduct. The neurons that form responsible for all its sensitive innervation this nucleus are in charge of the (touch, pain, temperature and propioceptive integration in the Vth nerve propioception) together with the motor territory, high level information for correct innervation of the mastication apparatus. mastication. Originating in the posterior fossa of the The main nucleus is in the pons, it is also brain stem, it follows a long and complex situated in the depth of the tegmentum, course towards its distribution territory, and is responsible for the tactile integration crossing several regions with a complex of the territory of this nerve. anatomy and establishing important Finally, the inferior nucleus occupies the relationships with several structures. tegmentum of the medulla, extending The nerve fibers originate in the brainstem caudally to the first segments of the cervical and are part of several grey matter nuclei spine, and is in charge of thermal and pain occupying all the brainstem and even the information. Its location explains the first spinal cervical segments. possible appearance of symptoms in the Each of these sensitive and motor nuclei facial territory in patients with a represents different processing centers, and degenerative/inflammatory disorder of the there is a true systematization of the upper cervical spine. information this nervous tract is responsible There is one single motor nucleus, located for conducting. Figure 1 in the pons tegmentum, supplying the Rev Imagenol, 2009, 12(2):28-33 3 ORIGINAL ARTICLE mastication muscles, and is correspondingly the temporal bone, where it finds Meckel´s called mastication nucleus. space region. During this course, The fibers related with all these nuclei surrounded by cerebrospinal fluid, it gather in the pons and emerge through the presents distant relations with cranial lateral sector of its anterior aspect, forming nerves VI, VII and VIII, and vascular a thick nervous tract with two roots: a relations, with the superior petrous vein thicker and lateral sensitive root and a (Dandy), and more especially arterial thinner more medial motor root. relations. Figures 2 and 4 The only intra-axial segment of the Vth ends This is the point where the variants in the there and initiates its long course to its trajectories and the resulting arterial loops, distribution territory; it is formed by may generate sites of close contact different sub-segments before dividing between the arterial, pulsating vessels and itself into its terminal branches (the the nervous tract which may result in cisternal and Gasserian or transdural neurovascular conflict. segments). Figure 2 Two arteries may be related: the middle The point where the roots emerge in the and the superior cerebellar arteries, brainstem is called “REZ” (Root Entry Zone), according to the height of its origin in the an anatomical landmark of great functional basilar trunk and the direction of its initial hierarchy. Figure 3 portion. Figure 5 3a 3b The cisternal (Vc) or pre-ganglionar It later enters Meckel´s space, the region segment occupies the upper floor sector of where the Gasserian ganglion is located, a the cerebellopontine angle, above the ganglionar station relaying sensitive acoustic-vestibulo-facial bundle, heading information, and following which its three upwards and laterally, from the posterior terminal branches originate: the ophthalmic fossa to the apex of the petrous portion of Rev Imagenol, 2009, 12(2):28-33. SOCIEDAD DE RADIOLOGIA E IMAGENOLOGIA DEL URUGUAY 4 Anatomy or the Trigeminal Nerve. Key Anatomical Facts for MRI Examination of Trigeminal Neuralgia (V1), maxillary (V2) and mandibular (V3) Prior to this, the three sensitive branches nerves. establish a close relationship with the This region is a space carved in a dural fold, lateral wall of the cavernous sinus, passing occupied in nearly all specimens by through the width of the duramater, cerebrospinal fluid. Here the nervous trunk especially V1 and V2. is formed by multiple small nerve roots and After going through this sector, V1 reaches the ganglion is formed actually by a true the superior orbital fissure (sphenoidal network of minute nerve filaments and cleft) and crosses it to reach the orbit therefore should be called plexus rather where it is distributed. than ganglion. Only the sensitive fibers form Its distribution territory encompasses the part of this structure, the small motor tract sensitivity of the eye-globe and conjunctiva, passes, in the majority of cases, below the eyelids, frontal region, the skin of the nose ganglion –as may be clearly observed in the and naso-sinusal mucosa, and the dissections carried out. Figure 6 vegetative innervation of the lacrimal After the Gasserian ganglion, the glands. distribution branches, already separated in The second branch, V2, goes towards the all cases, run forward seeking for the foramen rotundum in the middle fossa of different orifices in the skull base though the skull, reaching the pterygo-palatine which they reach their distribution territory. region where it subdivides into multiple Figure 7 collateral branches. 4a 4b 4c Rev Imagenol, 2009, 12(2):28-33 5 ORIGINAL ARTICLE 5a 5b The main tract of this branch continues in skull through the foramen ovale, and the floor of the orbit to emerge in the face reaches the deep masticator space. through the infraorbital orifice and is There it forms multiple sensitive branches distributed in the skin of the middle facial destined for the lower third of the face and territory and part of the oral cavity. part of the oral cavity, additionally to the Finally V3, which travels together with the temporo-mandibular joint, while the motor motor root of the nerve, crosses the base of branches supply the mastication muscles. 6a 6b Rev Imagenol, 2009, 12(2):28-33. SOCIEDAD DE RADIOLOGIA E IMAGENOLOGIA DEL URUGUAY 6 Anatomy or the Trigeminal Nerve. Key Anatomical Facts for MRI Examination of Trigeminal Neuralgia Trigeminal Imaging Anatomy The Gasserian ganglion was observed within Meckel’s space and the anatomical findings In all cases analyzed, each of the were corroborated in all cases. Thus, a components of the trigeminal system was plexiform structure where the branches of clearly identified, with the exception of the the nervous trunk converge and where the previously described nuclei. three terminal branches arise was observed A thick nerve tract surrounded by in 100% of cases. cerebrospinal fluid (cisternal portion of the V) was observed in all cases from its site of Discussion origin in the brainstem until it enters into The anatomy of the trigeminus is well Meckel´s space, and in all the specimens it known