Segmental Imaging of the Trochlear Nerve: Anatomic and Pathologic Considerations

Background: The trochlear nerve (the fourth cranial nerve) is the only cranial nerve that arises from the dorsal aspect of the midbrain. The nerve has a lengthy course making it highly susceptible to injury. It is also the smallest cranial nerve and is often difficult to identify on neuroimaging.

Trainees’ Corner Section Editors: Vivek R. Patel, MD Prem Subramanian, MD, PhD Segmental Imaging of the Trochlear Nerve: Anatomic and Pathologic Considerations Nivedita Agarwal, MD, Ali Karim Ahmed, MD, Richard H. Wiggins III, MD, Timothy J. McCulley, MD, Marinos Kontzialis, MD, Leonardo L. Macedo, MD, Asim F. Choudhri, MD, Lauren C. Ditta, MD, Masaru Ishii, MD, Gary L. Gallia, MD, PhD, Nafi Aygun, MD, Ari M. Blitz, MD Downloaded from http://journals.lww.com/jneuro-ophthalmology by BhDMf5ePHKav1zEoum1tQfN4a+kJLhEZgbsIHo4XMi0hCywCX1AWnYQp/IlQrHD3i3D0OdRyi7TvSFl4Cf3VC1y0abggQZXdtwnfKZBYtws= on 05/04/2022 Background: The trochlear nerve (the fourth cranial nerve) is the only cranial nerve that arises from the dorsal aspect of the midbrain. The nerve has a lengthy course making it highly susceptible to injury. It is also the smallest cranial nerve and is often difficult to identify on neuroimaging. Evidence Acquisition: High-resolution 3-dimensional skull base MRI allows for submillimeter isotropic acquisition and is optimal for cranial nerve evaluation. In this text, the detailed anatomy of the fourth cranial nerve applicable to imaging will be reviewed. Results: Detailed anatomic knowledge of each segment of the trochlear nerve is necessary in patients with trochlear nerve palsy. A systematic approach to identification and assessment of each trochlear nerve segment is essential. Pathologic cases are provided for each segment. Section of Radiology (Nivedita Agarwal), Hospital Santa Maria del Carmine, Rovereto, Italy; Division of Neuroradiology (Nivedita Agarwal, RHW), Department of Radiology. University of Utah, Salt Lake City, Utah; Department of Neurosurgery (AKA, GLG), the Johns Hopkins School of Medicine, Baltimore, Maryland; Division of Neuro-ophthalmology (TJM), Department of Ophthalmology, the Johns Hopkins School of Medicine, Baltimore, Maryland; Division of Neuroradiology (MK), Department of Diagnostic Radiology, Rush University Medical Center, Chicago, Illinois; Department of Neuroradiology (LLM), Cedimagem/Alliar Diagnostic Center, Juiz de Fora, Brazil; Department of Radiology (AFC), Le Bonheur Children’s Hospital, the University of Tennessee Health Sciences Center, Memphis, Tennessee; Department of Opththalmology (LCD), St. Jude Children’s Research Hospital, Memphis, Tennessee; Department of Otolaryngology Head and Neck Surgery (MI), the Johns Hopkins School of Medicine, Baltimore, Maryland; Division of Neuroradiology (Nafi Aygun), Department of Radiology, the Johns Hopkins School of Medicine, Baltimore, Maryland; and Division of Neuroradiology (AMB), Department of Radiology, University Hospitals, Case Western Reserve University School of Medicine, Cleveland, Ohio. The authors report no conflicts of interest. Address correspondence to Ari M. Blitz, MD, Division of Neuroradiology, Department of Radiology, University Hospitals, Case Western Reserve University School of Medicine, 2061 Cornell Road, Cleveland, OH 44106; E-mail: [e-mail] Agarwal et al: J Neuro-Ophthalmol 2021; 41: e7-e15 Conclusions: A segmental approach to high-resolution 3-dimensional MRI for the study of the trochlear nerve is suggested. Journal of Neuro-Ophthalmology 2021;41:e7–15 doi: 10.1097/WNO.0000000000001125 © 2020 by North American Neuro-Ophthalmology Society T rochlear nerve, the fourth cranial nerve (CN IV), is derived from the Greek word “troxilέa” (trokhiléa, “pulley”). It is a general somatic efferent nerve that innervates only 1 extraocular muscle, the superior oblique. The superior oblique muscle (SOM) originates from the orbital apex, above the annulus of Zinn, courses anteriorly in the orbit superomedial to the globe, loops around the trochlea, and attaches just temporal to the superior rectus onto the sclera of the eye. The primary function of the muscle is internal rotation (intorsion) of the eye, the secondary function is depression, and the tertiary function is abduction of the eye. CN IV palsy, unlike oculomotor (CN III) and abducens (CN VI) palsies, may go unnoticed by the patient because of subtle compensatory mechanisms such as a slight turn or tilt of the head to the unaffected side. We previously described the segmental anatomy and approaches to imaging of the cranial nerves (1,2). In this article, we describe the anatomy of CN IV, current limitations in the visualization of the segments of CN IV using high-resolution MRI (Fig. 1), as well as pathologic entities affecting each segment of CN IV (Table 1). CN IV is unique among the cranial nerves in several ways. It is the only nerve that arises from the dorsal aspect of the brainstem, and the only nerve that crosses the midline, and it does so within the brain parenchyma. CN IV is the thinnest cranial nerve (0.26–0.28 mm2) (3), which makes it difficult to detect on MRI. e7 Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited. Trainees’ Corner TABLE 1. Summary of trochlear nerve by segment, vascular supply, and imaging modalities CN IV Segment Nuclear (CN IV.a) Parenchymal fascicular (CN IV.b) Cisternal (CN IV.c) Dural cave (CN IV.d) Interdural (CN IV.e) Foraminal (CN IV.f) Extraforaminal (CN IV.g) Vascular Supply Clinical Correlation Imaging The posterior and lateral Lacunar ischemic and focal 3D fluid-attenuated inversion mesencephalic arteries hemorrhagic infarcts can recovery image and/or DTI. arising from the medial result in contralateral branch of the superior superior oblique muscle cerebellar artery. palsy. Bilateral superior oblique muscleCN IV.b is not visible on routine Mesencephalic perforating palsy may be seen with lesions MRI, but its approximate arteries of the medial location can be inferred by at the level of the superior superior cerebellar trunk of identifying the superior medullary velum. This may the superior cerebellar medullary velum. include demyelination and artery. hemorrhagic lesions. Vermian and the collicular Perimesencephalic High-resolution constructive arteries and more distally by subarachnoid hemorrhage, interference in steady state. superior cerebellar artery transtentorial herniation, branches. aneurysmal/cavernoma compression, pineal tumors, and tentorial meningioma. CN IV.d is difficult to identify Dural-based infection, Branches of the maxillary even on high-resolution inflammation, masses, and artery and branches of the constructive interference in dural venous sinus internal carotid artery steady state images. It is (medial tentorial artery a.k.a thrombosis. lateral and inferior to CN III.d Bernasconi–Cassinari in the anterior tip of the artery). oculomotor cistern when visualized. The inferolateral trunk and Cavernous sinus syndrome (i.e., High-resolution constructive interference in steady state branches of the cavernous sinus thrombosis, meningohypophyseal artery. cavernous meningioma, head images before and after and neck cancer, and carotid- administration of intravenous contrast. cavernous fistula). Rarely, cavernous CN IV schwannomas have been described. Craniofacial fractures, orbital Not readily identified even on Inferolateral trunk and the high-resolution MRI. infection, bone neoplasms, supraorbital branch of the and thrombosis of the lacrimal artery. cavernous sinus including the inferior and superior ophthalmic veins. Inflammatory, infectious, and Muscle volume and signal Terminal branches of the traumatic lesions may cause intensity changes on STIR ophthalmic artery and the images are important damage to the CN IV.g. anterior meningeal artery. indirect measures of muscle denervation. DTI, diffusion tensor imaging; STIR, short TI inversion recovery. GENERAL CONSIDERATIONS FOR IMAGING High-resolution 3-dimensional (3D) MRI is optimal for detection of certain CN IV segments because it emerges from the midbrain and courses through the quadrigeminal and ambient cisterns. Our standard MRI cranial nerve protocol includes isotropic 3D 0.6-mm constructive interference in steady state (CISS) images that allow for post hoc e8 reconstruction in any arbitrary plane. In cases of trochlear pathology, thinner slices are acquired (0.4 mm) to enhance visualization of CN IV, which remains limited. As with the other CNs, the differential diagnosis of lesions affecting CN IV varies by segment, although certain pathologic conditions do not have a known predilection for any particular anatomic segment. Once in the cavernous sinus, identifying CN IV is difficult even when high-resolution images are acquired. Agarwal et al: J Neuro-Ophthalmol 2021; 41: e7-e15 Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited. Trainees’ Corner FIG. 1. Segmental anatomy approach to cranial nerves with surrounding tissue (Reprint permissions from Blitz AM, Choudhri AF, Chonka ZD, Ilica AT, Macedo LL, Chhabra A, Gallia GL, Aygun N. Anatomic considerations, nomenclature, and advanced crosssectional imaging techniques for visualization of the cranial nerve segments by MRI. Neuroimaging Clin N Am. 2014;24:1–15). ROLE OF HIGH-RESOLUTION 3-DIMENSIONAL MRI IN SKULL BASE SURGERY In a region with critical adjacent neurovascular structures, high-resolution 3D imaging of the skull base optimizes preoperative planning, identification of normal microanatomic structures, and facilitates unparalleled spatial appreciation of skull base pathologies (4). This modality has substantial advantages over traditional 2D imaging—in which visualization of structures of interest is heavily dependent on the angle of acquisition, slice position, and plane. This spatial knowledge is necessary for thorough preoperative surgical planning, to choose the best surgical approach, minimize morbidity, preserve critical adjacent structures, and optimize prognosis (1,4,5). As the only CN emerging from the dorsal mesencephalon, CN IV may be surgically encountered with the median supracerebellar infratentorial, subtemporal, fronto-temporo-orbitozygomatic, and endonasal transphenoidal approaches. the absence of ptosis. Hypertropia is a manifest vertical misalignment of the eye relative to the fixating fellow eye. Excyclotorsion is a rotated position of the eye, such that the superior pole of the vertical meridian is torted temporally and the inferior pole of the vertical meridian is torted nasally, which can create slanted or oblique double vision. Accompanying tinnitus has been reported in a case of focal infarction in CN IV.a possibly because of its proximity to auditory/cochlear nuclei in the inferior colliculus (8). Pathologic Conditions Lacunar ischemic and focal hemorrhagic infarcts can result in contralateral SOM palsy (9) (Fig. 2). Congenital CN IV palsy with secondary hypoplasia or atrophy of the SOM is well-established, either as part of the spectrum of genetic congenital cranial dysinnervation disorders or in isolation (10,11). NUCLEAR SEGMENT (CN IV.a) Anatomy The nuclei of CN IV.a are a group of oval-shaped, polygonal cell bodies (or perikarya) (5), centered in the dorsal part of the mesencephalon, ventral to the periaqueductal gray matter, immediately dorsal to the medial longitudinal fasciculi, and just below the inferior colliculus. CN IV.a are the smallest CN nuclei with approximately 2,115 cells (5). Vascular Supply There are several sources of blood supply to the midbrain. The posterior and lateral groups of mesencephalic arteries arising from the medial branch of the superior cerebellar artery (SCA) supply the inferior colliculi and most of the dorsal midbrain including CN IV.a (6,7). Clinical Presentation Lesions in CN IV.a will cause contralateral symptoms resulting in various grades of superior oblique palsy. Patients may present with acute onset of vertical or oblique diplopia, often worse in downgaze (the field of action of the SOM) in Agarwal et al: J Neuro-Ophthalmol 2021; 41: e7-e15 FIG. 2. Left paramedian mesencephalic acute stroke involving the left CN IV.a nucleus in a patient with right superior oblique muscle palsy. White arrow shows the area of infarct with restricted diffusion. e9 Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited. Trainees’ Corner Imaging Approach 3D fluid-attenuated inversion recovery image sequence may be helpful in identifying CN IV.a at the level of the inferior colliculi just below the level of red nuclei (12). In routine diffusion tensor imaging color maps, CN IV.a is dorsal to the decussation of the superior cerebellar peduncles. 3D anisotropy contrast-periodically rotated overlapping parallel lines with enhanced reconstruction (3DAC-PROPELLER) on 3T MRI may be useful in identifying CN IV.a (13). PARENCHYMAL FASCICULAR SEGMENT (CN IV. b) CISTERNAL SEGMENT (CN IV.c) Overview CN IV has the longest cisternal segment coursing anterolaterally in the quadrigeminal cistern and anteriorly in the ambient cistern, where the nerve may be or difficult to identify because of multiple adjacent vascular structures of similar caliber (19). CN IV.c may be further subdivided into a proximal segment in the quadrigeminal cistern (CN IV.c.I) and a more distal and lateral segment in the ambient cistern (CN IV.c.II). Quadrigeminal Cisternal Segment (CN IV.c.I) Anatomy The axons of the parenchymal fascicular segment (CN IV.b) wrap around the central gray matter and cross the midline dorsal to the cerebral aqueduct in the superior medullary velum just below the inferior colliculi. Early studies on axonal labeling with horseradish peroxidase solution in cats and rabbits showed that up to about 5% of trochlear nerve fibers may be ipsilateral, providing for a bilateral innervation of the SOM (14). Eight-three percentage of the fibers are myelinated, and approximately 17% are unmyelinated. It has been suggested that the unmyelinated fibers may carry sensory pain information from the SOM and the trigeminal territory (15). Vascular Supply After intraparenchymal decussation, the CN IV.c.I emerges from the dorsal aspect of the midbrain approximately 0.7 mm (range 0–1 mm) inferior to the inferior colliculus (20) and 4.0 mm lateral to the frenulum of the superior medullary velum (Fig. 3). On MRI, a single root is often visible; however, on anatomic studies, a variable number of rootlets, usually 2, seem to emerge and unite to form the CN IV.c (21,22). Imaging Approach CN IV.c.I can be well seen on high-resolution CISS axial images as a thin linear hypointense structure adjacent to the dorsal midbrain and outlined by hyperintense CSF (19). At this level, CN IV is surrounded by collicular arteries (branches of the P1 segment of the posterior cerebral artery Mesencephalic perforating arteries of the medial superior cerebellar trunk arising from the SCA supply CN IV.b (5). Clinical Presentation Isolated CN IV.b damage is rare and difficult to differentiate from CN IV.a lesions because of it very short course. Associated Horner syndrome may be seen because of its proximity to parasympathetic fascicles of the CN III (16). Bilateral SOM palsy may be seen with lesions at the level of the superior medullary velum. Pathology Reported causes that may involve CN IV.b include demyelination and hemorrhagic lesions. Hemorrhagic lesions because of small malformations or trauma (including neurosurgical) and focal ischemia have been reported (8,9). Patients with multiple sclerosis may rarely present with CN IV palsy (17). More recently, bilateral CN IV palsy was documented in a patient with neuromyelitis optica and lesions involving bilateral CN IV.b (18). Imaging Approach CN IV.b is not visible on routine MRI, but its approximate location can be inferred by identifying the superior medullary velum. e10 FIG. 3. CN IV.C anatomic cadaveric dissection CN IV.C.I macroscopic dissection of the brainstem shows the origin of CN IV.c.I from the dorsal surface of the brainstem below the inferior colliculi. F. SMV, frenulum of the superior medullary velum; lingula (of the cerebellum); IC, inferior colliculus; SC, superior colliculus; SCP, superior cerebellar peduncle. Agarwal et al: J Neuro-Ophthalmol 2021; 41: e7-e15 Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited. Trainees’ Corner [PCA]), the superior cerebellar vein, and branches of the SCA (Fig. 4A–C) (22). Ambient Cisternal Segment (CN IV.c.II) CN IV.c.II wraps around the midbrain in the ambient cistern at variable distance from the lateral border of the midbrain and demonstrates an oblique anterolateral course toward the undersurface of the medial free margin of the tentorial leaflet. This segment usually lies superior to the main trunk of the SCA, inferior to the PCA, and lateral to the oculomotor nerve (CN III) (20). However, this conventional position of CN IV.c.II above the SCA may vary, and the nerve may lie between the main rostral and caudal branches of the SCA (23). In the ambient cistern, several vascular structures such as branches of the collicular arteries, the transverse pontine arteries, and the basal veins of Rosenthal may surround CN IV. IV schwannomas are extremely rare, and when encountered, the diagnosis of neurofibromatosis should be considered (31–33). Occlusion of SCA or its branches, surgical manipulation of CN IV.c.II, and transtentorial herniation may compromise blood supply to the trochlear nerve (34) (Fig. 6). Imaging Approach CISS and MRA without and with contrast enable accurate identification of the proximal cisternal segment of the trochlear nerve and its neurovascular relationships (20). Arteries are hyperintense on unenhanced MRA images, whereas veins seem hyperintense on contrast-enhanced MRA (Fig. 7). Anatomic landmarks, including the trochlear cistern and trochlear groove, may aid in the identification of the tentorial segment of CN IV (35). Vascular Supply DURAL CAVE SEGMENT (CN IV.d) The CN IV.c.I segment is supplied by the vermian artery or its branches. CN IV.c.II is supplied by the vermian and the collicular arteries and more distally by SCA branches (22). Anatomy Clinical Presentation Isolated contralateral SOM palsy without associated neurologic findings suggest injury to the cisternal segment of CN IV.c.I and c.II. Pathologic Conditions CN IV.c can be affected by neurovascular compression, as are other cranial nerves (24–28). Perimesencephalic subarachnoid hemorrhage due to rupture of an aneurysm and superficial siderosis due to repetitive hemorrhage from a periventricular cavernous hemangioma (29) can cause isolated CN IV.c.II nerve palsy (21) (Fig. 5A, B). In instances of trauma, the nerve may be injured against the free edge of the tentorium (16). Aneurysmatic compression of the nerve and in general any space-occupying lesion (pineal region tumors, tentorial meningioma, etc.) may cause CN IV.c palsy (Fig. 6A, B). Pseudotumor cerebri has been very rarely reported to cause vertical diplopia due to trochlear nerve palsy (30). CN The nerve pierces the dura mater at the level of the free and attached segments of the tentorium in the posterolateral margin of the oculomotor triangle (22). Vascular Supply Branches of the maxillary artery and branches of the internal carotid artery (medial tentorial artery a.k.a Bernasconi– Cassinari artery) (22). Clinical Presentation Isolated CN IV.d palsy is rare and is frequently associated with variable degree of CN III palsy. Superior oblique myokymia is very rare and when seen, usually it is in isolation without any identifiable structural abnormality (24–27). Pathologic Conditions Dural-based infection, inflammation, and masses as well as dural venous sinus thrombosis may affect this portion of the trochlear nerve (Fig. 8). FIG. 4. A–C. Normal high-resolution constructive interference in steady state showing numerous of arteries in the cisternal space and the course of CN IV.c.I (white arrow). The left trochlear nucleus CN IV.a (white circle) and parenchymal fascicular segments CN IV.b (white dashed line) are also labeled. Agarwal et al: J Neuro-Ophthalmol 2021; 41: e7-e15 e11 Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited. Trainees’ Corner FIG. 5. A 38-year-old woman presenting with a left CN IV palsy. CT demonstrating traumatic subarachnoid hemorrhage. A. Coronal constructive interference in steady state without contrast demonstrates an asymmetric filling defect within the subarachnoid space (arrow) caused by a mass measuring up to 5 mm along the undersurface of the free edge of left aspect of the tentorium cerebelli. B. On postcontrast constructive interference in steady state in the axial plane (3T, 0.5-mm isotropic resolution), the left CN IV.c (arrowheads) can be seen extending through the ambient cistern into the region of the mass, which demonstrates enhancement (arrow). From (Blitz. 2014 Neuroimaging Clin N Am). Adaptations are themselves works protected by copyright. So in order to publish this adaptation, authorization must be obtained both from the owner of the copyright in the original work and from the owner of copyright in the translation or adaptation. CN IV.d is extremely hard to identify even on high-resolution CISS images. It is lateral and inferior to CN III.d in the anterior tip of the oculomotor cistern when visualized. ally in the interdural space (37,38). In the posterior cavernous sinus, CN IV.e is approximately 1 mm below CN III.e and approximately 2.5 mm above CN V.1.e (22). In the anterior cavernous sinus just proximal to the superior orbital fissure, CN IV.e is above the CN III.e. INTERDURAL SEGMENT (CN IV.e) Clinical Presentation Anatomy Isolated CN IV.e segment palsy is uncommon because of its close proximity to CN III.e and the ophthalmic division of the trigeminal nerve (CN V.1.e) in the cavernous sinus. Imaging Approach Two layers of dura mater line the lateral wall of the cavernous sinus: the outer meningeal dura and the inner periosteal dura (35). The inner periosteal dura separates the venous channels of the cavernous sinus and the internal carotid artery from the interdural space where cranial nerves lie (36). After crossing over the superior petrosal sinus, the CN IV pierces the meningeal dura and courses anterolater- FIG. 6. MRI through the orbits of a patient with a right-sided tentorial meningioma. The meningioma compresses CN IV.c.II causing ipsilateral superior oblique muscle atrophy and loss of right superior oblique muscle volume, shown by the white arrow. e12 FIG. 7. Uncal herniation (asterisk) past the tentorial incisura, causing compression of the ipsilateral CN IV.c.II. Courtesy R. Wiggins. Agarwal et al: J Neuro-Ophthalmol 2021; 41: e7-e15 Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited. Trainees’ Corner vein, lacrimal and frontal nerves, but resides outside of the annulus of Zinn (41). Vascular Supply The blood supply is derived from the terminal branch of the inferolateral trunk and the supraorbital branch of the lacrimal artery (22). Clinical Presentation CN IV.f injury is usually associated with CN III and CN VI palsies. The presence of simultaneous vision loss and diplopia warrants study of the orbit apex, SOF, and skull base. Pathologic Conditions The SOF is reported to measure approximately 3 · 22 mm (42). Craniofacial fractures, orbital infection, bone neoplasms, and thrombosis of the cavernous sinus including the inferior and superior ophthalmic veins are major considerations. Inflammatory disease such as Tolosa–Hunt syndrome may cause congestion of the SOF and the orbital apex (43). Perineural spread of squamous cell carcinoma has been reported (44). FIG. 8. Precontrast constructive interference in steady state imaging in a case of infiltrating meningioma (asterisk), causing ipsilateral CN IV.c.II compression. CN IV.c is shown by the white arrow because it courses along the quadrigeminal and ambient cisterns. Imaging Approach Vascular Supply EXTRAFORAMINAL SEGMENT (CN IV.g) The inferolateral trunk and branches of the meningohypophyseal artery supply CN IV.e (6). Anatomy Pathologic Conditions Important diagnostic considerations resulting in cavernous sinus syndrome include cavernous sinus thrombosis and carotid-cavernous fistula. Rarely, CN IV schwannomas have been described in the cavernous sinus (30). Meningiomas, cavernomas, and head and neck cancer may infiltrate the cavernous sinus and involve CN IV.e (37,39). In our experience, CN IV.f at this level is not readily identified even on high-resolution MRI. CN IV.g bifurcates into a lateral and a medial division immediately before entering the SOM. Each division travels parallel to the fibers of the SOM in the medial and lateral belly of the muscle with very marginal territorial overlap. The compartmentalized innervation allows for independent contraction of different portions of the nerve allowing for primary and secondary eye movements (45). Clinical Presentation Imaging Approach High-resolution CISS images before and after administration of intravenous contrast may help to reveal the interdural segment better and to identify their relationships with pathological processes, although identification of CN IV may be difficult in the cavernous sinus even with high-resolution imaging (40). FORAMINAL SEGMENT (CN IV.f) Anatomy The tendons of the 4 recti muscles attach to the SOF. SOF can be divided into a superolateral and an inferomedial compartment. The CN IV.f passes through the superolateral compartment along with the superior ophthalmic Agarwal et al: J Neuro-Ophthalmol 2021; 41: e7-e15 Isolated lesion will cause vertical and excyclotorsional diplopia, often worse in downgaze. Vascular Supply Terminal branches of the ophthalmic artery and the anterior meningeal artery (6). Pathologic Conditions Inflammatory, infectious, and traumatic lesions may cause damage to the CN IV.g. Neoplastic considerations include meningioma, optic nerve glioma, perineural spread of head and neck cancer, and hematologic malignancies (46). Trochlear migraine presents with orbital pain, migraine, and limited eye movement because of inflammation of the soft tissue around the trochlea resulting in SOM tendinitis. Trochlear migraine e13 Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited. Trainees’ Corner may be idiopathic or less frequently secondary to autoimmune disorder (47). Trochlear migraine may be associated with superior oblique myokymia (48). Imaging Approach Muscle volume and signal intensity changes on short TI inversion recovery images are important indirect measures of muscle denervation that should prompt imaging of the entire course of the IV nerve (Fig. 5B). CONCLUSIONS CN IV is notoriously difficult to visualize because of its small size even on high-resolution CISS imaging. We present a segmental approach in the clinical and radiologic evaluation of trochlear pathology to improve diagnostic confidence and recognition of subtle imaging abnormalities. REFERENCES 1. Blitz AM, Choudhri AF, Chonka ZD, Ilica AT, Macedo LL, Chhabra A, Gallia GL, Aygun N. Anatomic considerations, nomenclature, and advanced cross-sectional imaging techniques for visualization of the cranial nerve segments by MR imaging. Neuroimaging Clin N Am. 2014;24:1–15. 2. Kontzialis M, Choudhri AF, Patel VR, Subramanian PS, Ishii M, Gallia GL, Aygun N, Blitz AM. 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