High-Resolution 7T MR Imaging of the Trochlear Nerve.

BACKGROUND AND PURPOSE: The trochlear nerve has traditionally been difficult to identify on MR imaging. The advent of 7T MR imaging promises to greatly benefit visualization of small structures due to gains in the signal-to-noise ratio allowing improved spatial resolution. We investigated the utility of a clinically feasible ultra-high-resolution 7T MR imaging protocol for identification of the trochlear nerve, as well as assessment of normal trochlear nerve anatomy. MATERIALS AND METHODS: Coronal high-resolution 2D T2-weighted TSE images used in a 7T epilepsy protocol of 50 subjects at our institution were reviewed by 2 independent radiologists for visualization of the trochlear nerve at the nerve origin and cisternal, tentorial, and cavernous segments. The frequency of nerve visibility within these segments and their anatomy were documented, and disagreements were resolved by joint review. RESULTS: Of the 100 nerves reviewed in 50 subjects, at least 2 segments of the trochlear nerve from the brainstem to the cavernous sinus were identified in 100% of cases. The origins from the brainstem and cisternal segment were visible in 65% and 93% of nerves, respectively. The trochlear nerve was identified at the trochlear groove in 100% of cases and in the posterior wall of the cavernous sinus in 74% of cases. CONCLUSIONS: Coronal high-resolution 2D TSE at 7T reliably identified the trochlear nerve throughout its course and is a promising tool for imaging patients with suspected trochlear nerve pathology.

Disorders of the Fourth Cranial Nerve.

ABSTRACT: This review of disorders of the fourth cranial nerve includes discussion on anatomy, examination techniques, congenital and acquired etiologies, differential diagnosis, and management options. The findings of the superior oblique muscle on orbital MRI in patients with fourth nerve palsy have had a major impact on our understanding of this cranial neuropathy. In addition, briefly reviewed are rare disorders of the fourth nerve: superior oblique myokymia, Brown syndrome, and ocular neuromyotonia. It behooves the clinician to have a clear understanding of the role that the fourth cranial nerve plays in a variety of neuro-ophthalmic conditions.

Case Report: Unilateral Cranial Nerve IV Palsy Secondary to Trochlear Schwannoma.

SIGNIFICANCE: Cranial nerve IV palsy is the most common cranial nerve affected in both pediatric and adult patients with vertical and/or torsional diplopia. The condition has multiple known etiologies, including schwannoma, which is rarely reported in the literature. Schwannoma should be considered when the most common etiologies have been ruled out. PURPOSE: This report documents a rare case of cranial nerve IV palsy secondary to a trochlear schwannoma. Treatment and management considerations will be discussed. CASE REPORT: A 57-year-old man presented to the clinic for evaluation of his recent-onset vertical diplopia. He was diagnosed with left cranial nerve IV palsy. MRI of the brain and orbits revealed the presence of a schwannoma along the course of his left fourth cranial nerve. It did not compress any other cranial nerves or the brainstem. The patient was referred to the neuro-ophthalmology clinic for further evaluation. He was managed conservatively with prismatic spectacle correction to relieve his diplopia. Repeat MRI of the brain and orbits was recommended every 6 months. CONCLUSIONS: Although rare, schwannoma of the fourth cranial nerve should be considered in cases of cranial nerve IV palsies without an obvious etiology. Neuroimaging of the brain and orbits is warranted in cases where more common etiologies have been ruled out or when other cranial nerves and/or the brainstem are involved.

Trochlear cistern of the cavernous sinus: an anatomical study using magnetic resonance imaging.

PURPOSE: The present study aimed to explore the trochlear cistern (TC) of the cavernous sinus using magnetic resonance imaging (MRI). METHODS: Following conventional MRI examination, a total of 73 patients underwent the constructive interference steady-state (CISS) sequence in thin-sliced coronal sections. Moreover, three injected cadaver heads were dissected. RESULTS: In the cadaver specimens, the extent of the TC was difficult to identify on any dissected side. On the CISS images, the TC was identified in 98.6% on the right side and 94.5% on the left, while transmitting the trochlear nerve (TN) was identified in 83.6% on the right and 79.5% on the left. Most TNs were delineated as a single trunk, while duplication of the nerve was found in 3% of cases. The TC, commonly located inferior or inferolateral aspect of the oculomotor trigone. The size and extent of TC were highly variable. The TN location in the TC was also variable and was identified throughout the upper, middle, and lower parts of the TC. Moreover, relationships between the TC and Meckel's cave were highly variable. CONCLUSIONS: TC shows morphological variability. The coronal CISS sequence is useful for exploring TC and TN in clinical practices.

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. 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. CONCLUSIONS: A segmental approach to high-resolution 3-dimensional MRI for the study of the trochlear nerve is suggested.

Trochlear nerve palsy associated with Moyamoya disease.

A 48-year-old woman with intracranial hemorrhage at the right hemisphere and the right midbrain was diagnosed as Moyamoya disease (MMD). While restoring consciousness, she complained of diplopia. The left hypertropia with a compensatory right head tilt was noted. The follow up brain image showed the evidence of past hemorrhage at the level of the right inferior colliculus. In this case, the hemorrhage directly damaged the right trochlear nucleus or intra-axial trochlear nerve before decussation and caused left (contralateral) hypertropia. To our knowledge, no report has been described of the trochlear nerve palsy in hemorrhagic Moyamoya disease. Here, we present a case of the patient with trochlear nerve palsy after hemorrhagic MMD and summarize the characteristics of trochlear nerve palsy according to its lesion sites.

Transventricular Transvelar Approach to Trochlear Nerve Schwannoma: Novel Technique to Lesions of Inferior Pineal Region.

OBJECTIVES: The schwannoma of the trochlear nerve is rare and originates mostly from the distal parts in the interpeduncular cistern. A lesion on the proximal segment in the inferior pineal region is extremely rare. Because of the rarity of the disease, the surgical approach to this region for the resection of trochlear nerve schwannoma has not been well documented in the literature. We herein describe a novel approach to successfully resect the trochlear nerve schwannoma. METHODS: A 12-year-old boy presented with occipital headache, abnormal gait, and disturbed conjoined eye movement. He was diagnosed with a lesion in the inferior pineal region compressing the superior medullary velum into the roof of the fourth ventricle. A bilateral midline suboccipital craniotomy was performed, and the fourth ventricle was exposed. The lesion was approached through the fourth ventricle superior medullary velum (transventricular transvelar approach). The lesion was totally resected, and his histopathology examination revealed trochlear schwannoma. RESULTS: The patient's symptoms resolved, and he had no recurrence at 12-year follow-up with normal eye movement and vision. CONCLUSION: The transventricular transvelar approach is feasible and safe to treat a lesion of the lower part of the pineal region being pushed through the superior medullary velum.

Ocular Torsion According to Trochlear Nerve Absence in Unilateral Superior Oblique Palsy.

Purpose: To investigate the relationship between objective ocular torsion and the presence or absence of the trochlear nerve in subjects with unilateral superior oblique palsy (SOP). Methods: A total of 159 subjects with congenital and acquired unilateral SOP were reviewed. Eighty-four subjects who had a normal trochlear nerve (present group) and 75 subjects without a trochlear nerve (absent group) were included. Cyclovertical motility parameters and objective ocular torsion were compared between groups, and factors related to ocular torsion were evaluated. Results: The degree of "net" excyclotorsion in the paretic eye was larger in the absent group compared to the present group (P = 0.002). The proportion of net excyclotorsion in the paretic eye was greater in the absent group (11% vs. 37%), while net incyclotorsion was greater in the present group (41% vs. 23%) (P < 0.001). Net excyclotorsion of the paretic eye was associated with absence of the trochlear nerve (P < 0.001) and smaller size of the paretic SO (P < 0.001). Net incyclotorsion of the paretic eye was related with a normal trochlear nerve (P = 0.005), larger size of the paretic SO (P = 0.002), and greater hypertropia during ipsilateral gaze (P = 0.024). Conclusions: The status of the trochlear nerve, paretic SO size, and hypertropia during ipsilateral gaze which reflects the tensile strength of the ipsilateral superior rectus, significantly contribute to ocular torsion in unilateral SOP.

Imaging of Cranial Nerves III, IV, VI in Congenital Cranial Dysinnervation Disorders.

Congenital cranial dysinnervation disorders are a group of diseases caused by abnormal development of cranial nerve nuclei or their axonal connections, resulting in aberrant innervation of the ocular and facial musculature. Its diagnosis could be facilitated by the development of high resolution thin-section magnetic resonance imaging. The purpose of this review is to describe the method to visualize cranial nerves III, IV, and VI and to present the imaging findings of congenital cranial dysinnervation disorders including congenital oculomotor nerve palsy, congenital trochlear nerve palsy, Duane retraction syndrome, Möbius syndrome, congenital fibrosis of the extraocular muscles, synergistic divergence, and synergistic convergence.

Trochlear Groove and Trochlear Cistern: Useful Anatomic Landmarks for Identifying the Tentorial Segment of Cranial Nerve IV on MRI.

BACKGROUND AND PURPOSE: The trochlear groove and trochlear cistern are anatomic landmarks closely associated with the tentorial segment of cranial nerve IV. The purposes of this study were to describe the MR imaging appearances of the trochlear groove and trochlear cistern and to test our hypothesis that knowledge of these anatomic landmarks facilitates identification of cranial nerve IV in routine clinical practice. MATERIALS AND METHODS: For this retrospective study, consecutive MR imaging examinations of the sinuses performed in 25 patients (50 sides) at our institution were reviewed. Patient characteristics and study indications were recorded. Three readers performed independent assessments of trochlear groove, cistern, and nerve visibility on coronal images obtained by using a T2-weighted driven equilibrium radiofrequency reset pulse sequence. RESULTS: Interobserver agreement was 78% for visibility of the trochlear groove, 56% for the trochlear cistern, and 68% for cranial nerve IV. Following consensus review, the trochlear groove was present in 44/50 sides (88%), the trochlear cistern was present in 25/50 sides (50%), and cranial nerve IV was identified in 36/50 sides (72%). When the trochlear groove was present, cranial nerve IV was identified in 35/44 sides (80%), in contrast to 1/6 sides (17%) with no groove (P = .0013). When the trochlear cistern was present, cranial nerve IV was identified in 23/25 sides (92%), in contrast to 13/25 sides (52%) with no cistern (P = .0016). CONCLUSIONS: The trochlear groove and trochlear cistern are anatomic landmarks that facilitate identification of cranial nerve IV in routine clinical practice.

Synchronous ipsilateral cavernous malformations of the trochlear nerve.

BACKGROUND: Cranial nerve cavernous malformations (CM) are rare benign congenital vascular anomalies, with approximately 44 preceding cases in the literature. We report the fifth case of trochlear CM, as well as the first instance of two discrete CM occurring simultaneously along the same cranial nerve. METHODS: Case report. RESULTS: A fifty-seven year-old man presented with several years of diplopia; physical examination identified a complete left trochlear nerve paralysis. MRI revealed a 1cm enhancing lesion within the left ambient cistern, and the patient underwent left pretemporal transcavernous resection. Intraoperatively, a second, discrete CM of the trochlear nerve was also discovered; wide excision of the intrinsic trochlear lesions was completed, allowing both tumors to be removed en bloc with negative margins. Pathologic analysis confirmed both to be CM of the trochlear nerve. The patient recovered with a persistent left trochlear paralysis only, and follow-up MRI was negative for residual or recurrent disease. CONCLUSION: Cranial nerve CM are rare but potentially morbid mass lesions, with the capacity to precipitate significant neuropathies. Differential diagnosis includes schwannoma and hemangioblastoma. Definitive diagnosis may not be possible preoperatively; however, resection is recommended in symptomatic patients, potentially accompanied by nerve repair.

Pineal region schwannoma arising from the trochlear nerve.

Schwannomas arising from the trochlear nerve are very rare and to our knowledge, less than 35 histologically documented cases have been reported in the literature. There are no reports of a schwannoma in the pineal region. We report a 24-year-old woman who underwent a para-occipital trans-tentorial approach and gross total excision of a pineal region schwannoma arising from the trochlear nerve. This is the first such reported case.

ANATOMICAL STUDY OF CRANIAL NERVE EMERGENCE AND SKULL FORAMINA IN THE HORSE USING MAGNETIC RESONANCE IMAGING AND COMPUTED TOMOGRAPHY.

For accurate interpretation of magnetic resonance (MR) images of the equine brain, knowledge of the normal cross-sectional anatomy of the brain and associated structures (such as the cranial nerves) is essential. The purpose of this prospective cadaver study was to describe and compare MRI and computed tomography (CT) anatomy of cranial nerves' origins and associated skull foramina in a sample of five horses. All horses were presented for euthanasia for reasons unrelated to the head. Heads were collected posteuthanasia and T2-weighted MR images were obtained in the transverse, sagittal, and dorsal planes. Thin-slice MR sequences were also acquired using transverse 3D-CISS sequences that allowed mutliplanar reformatting. Transverse thin-slice CT images were acquired and multiplanar reformatting was used to create comparative images. Magnetic resonance imaging consistently allowed visualization of cranial nerves II, V, VII, VIII, and XII in all horses. The cranial nerves III, IV, and VI were identifiable as a group despite difficulties in identification of individual nerves. The group of cranial nerves IX, X, and XI were identified in 4/5 horses although the region where they exited the skull was identified in all cases. The course of nerves II and V could be followed on several slices and the main divisions of cranial nerve V could be distinguished in all cases. In conclusion, CT allowed clear visualization of the skull foramina and occasionally the nerves themselves, facilitating identification of the nerves for comparison with MRI images.

Cystic schwannoma of the anterior tentorial hiatus. Case report and review of the literature.

Intracranial schwannomas not arising from the facial, trigeminal, or vestibular nerves in the absence of neurofibromatosis are extremely rare. We report a case of a schwannoma arising in the region of the anterior tentorial hiatus and posterior cavernous sinus. A 17-year old girl presented with headaches and intermittent diplopia. An MRI of the brain revealed a heterogeneously enhancing mass adjacent to the free edge of the tentorium, superior to the cerebellopontine angle. An orbitozygomatic pterional craniotomy was done with complete resection of the tumor. Postoperatively, the patient remained neurologically intact. The clinical presentation and treatment of schwannomas arising in this location are discussed.

Oculomotor motion disorders: current imaging of cranial nerves 3, 4, and 6.

The eye movements are controlled by the cranial nerves 3, 4, and 6 working in close cooperation under the supervision of the voluntary cortex. Clinically, the most common presentation of abnormal ocular motor motion is double vision. A thorough clinical examination can usually separate a local orbital cause which can produce a restriction of the muscles moving the eye from a neurogenic cause due to an abnormality of one of the three nerves or their association pathways. Recent articles in the scientific literature have described major advances in our understanding of the anatomy and vascular relationships of the three ocular motor nerves (cranial nerves 3, 4, and 6) and of the diagnosis and treatment of a variety of pathological processes that damage these nerves, including ischemia, inflammation, and compression.

Incidental asymptomatic orbital calcifications.

OBJECTIVE: To use modern computed tomography (CT) imaging to quantify the incidence of asymptomatic incidental orbital calcifications and describe their histological features. MATERIALS AND METHODS: One hundred orbital CT scans were reviewed. In addition, patients who had orbital calcification(s) detected on a brain CT scan were examined by the ophthalmology service. RESULTS: Of the orbital CT scans, 2% had bilateral drusen of the optic nerve head, 3% had calcified scleral plaques anterior to the medial or lateral rectus muscles, and 3% had bilateral ossification of the trochlear apparatus. Routine brain CT scans detected asymptomatic calcifications of the sclera and dura surrounding the proximal optic nerves. CONCLUSION: Incidental asymptomatic orbital calcifications are commonly encountered on modern high-resolution CT images of the brain and orbit. This article should help the clinician to confidently distinguish these densities from foreign bodies or pathological calcifications.