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Go the 7Th and 8Th Nerve The 7th and 8th nerve The 7th and 8th nerve John L. Go, MD, FACR University of Southern California John L. Go, MD Director of Head and Neck Imaging University of Southern California House Ear Clinic Los Angeles, CA [email protected] Objectives The 7th nerve • Review the anatomy of the 7th and 8th nerve The facial nerve is comprised of both a sensory and motor component as well as a parasympathetic component. The sensory component is responsible • Review pathological processes associated with these cranial nerves for taste of the anterior two thirds of the tongue, the sensory aspect for lacrimation, and the stapedial reflex. The motor division innervates the muscles of facial expression. The facial nucleus lies within the pontine tegmentum inferior and medial to the sixth nerve nucleus. Fibers which form the facial nerve travel cephalad and turn around the posterior aspect of the sixth nerve nucleus at the level of the facial colliculus of the fourth ventricle (termed the internal genu) and travel laterally to exit the brainstem laterally from the lower pons. The segment of the facial nerve traveling within the cerebellopontine angle cistern is the cistenal segment. As the facial nerve travels posterolatally in the Upon entering the porus acousticus of the internal transverse plane, the facial nerve enters its next audiory canal, the facial nerve lies within the segment, the tympanic segment located underneath anterosuperior Quadrant of the IAC and is called the the lateral semicircular canal. Posterior to the cisternal segment with the vestibulocochear nerve posterior limb of the semicircular canal, the facial traveling within the rest of the IAC. At the fundus of nerve then turns inferiorly, anterior to the mastoid the IAC, the facial nerve now enters the facial canal. process of the temporal bone. This represents the Within the facial canal, the facial nerve travels posterior genu of the facial nerve. The facial nerve anterolaterally and then makes a hairpin loop to now extends inferiorly and posterior to the facial recess, travel posterolaterally. This segment of the facial located as a depression along the posterior wall of canal is called the geniculate segment of the facial the middle ear cavity. This segment is called the nerve and is located superior to the cochlea. At the mastoid or vertical segment. From the proximal anterior genu, the facial nerve resides within the mastoid segment, the nerve to the stapedius muscle geniculate fossa, and the greater superficial petrosal leaves to innervate the stapedius muscle. Inferiorly, nerve leaves anteriorly through the facial hiatus to the chorda tympani nerve leaves the facial nerve to travel to the orbit to the lacrimal gland. Just distal to travel anteriorly to travel to the tongue. the anterior genu along the facial nerve is the geniculate ganglion. The facial nerve exits the lateral skull base at the stylomastoid foramen, and then travels inferolaterally to enter the parotid gland giving off its peripheral branches to innervate the muscles of facial expression. The 8th nerve The 8th nerve • Fibers from the cochlea converge to form the cochlear nerve situated in the anteroinferior Quadrant of the IAC. The nerve, with the vestibular component travels across the CPA cistern to the inferior cerebellar peduncle to the cochlear nucleus. Amirsys The 8th nerve 7th and 8th nerve • The vestibular has 3 components: • Leaves the brainstem along the antolateral upper medulla • Superior vestibular nerve • Travels across the CP angle cistern • Receives input from the lateral and • superior semicircular canal and Enters the porus acousticus utricle • The 8th nerve (cochleovestibular nerve) enters the IAC as a single nerve and • Inferior vestibular nerve is L shaped in appearance • Input from the saccule • Within the IAC the crista falciformis (vestibular crest) divides the IAC into a • Singular nerve: travels with the superior and inferior half inferior vestibular nerve • Facial nerve is anterosuperior, cochlear nerve is antero inferior • Receives input from the posterior semicircular canal • Superior and inferior vestibular nerve in posterior half of IAC • May have a separate canal, singular • Bill’s bar, a cartilaginous septum subdivides the superior half into an canal posterior to IAC anterior and posterior compartment. Anatomy Anatomy Amirsys Anatomy Anatomy S A P I CT: Imaging of the temporal bone MR Imaging of the Temporal bone • Obtain volumetric data set utilizing .4-1.25mm slices with 50% • Different from MRI of the head overlap for best results • Thinner sections with no interslice gap • Bone algorithm • Use of fat suppression • Head in neutral position • Thin section T2-weighted images, myelographic effect • Coronal and sagittal images reformatted • Diffusion imaging (non-EPI diffusion imaging if possible) • If necessary additional multiplanar reformatted images • Reconstruct at 1 mm or less slice thickness for hard copy to see ossicles MRI: Thin section T bone protocol CISS or FIESTA T2-weighted images (sub millimeter imaging) Heavily T2-weighted images: No T2 information provided. Diffusion Imaging MR Protocol • Spin echo T1-weighted images • 3mm slice thickness • no interslice gap • 256x256 matrix or higher • Fast spin echo T2-weighted images or FLAIR images • Post-contrast T1-weighted images (axial and coronal) with fat suppression • Thin section • Diffusion imaging • Thin section T2-weighted images (CISS/FIESTA) CP Angle Cistern Masses Vestibular schwannoma • Vestibular Schwannoma (80%) • Arise from Schwann cells that wrap around vestibular component of • Meningioma (10%) CN8 • Epidermoid (5%) • Occurs at glial-Schwann cell junction • Primary malignancy (2%) • Small intracanalicular lesions • Within the IAC: relationship to the vestibular crest • Arachnoid cyst, lipoma, glomus tumor, vascular lesion (1%) • Larger masses • Ice cream/cone configuration • Mass effect on the brainstem? Vestibular schwannoma Vestibular Schwannomas • CT • Post contrast images • Large lesions may cause smooth remodelling of bone • 100% enhance heterogeneously and strongly • MRI • 15% with intratumoral cyst • T1-weighted images • <5% with associated arachnoid cyst • Intermediate in signal intensity, isointense to grey matter • <1% with hemorrhage • T2-weighted images • Iso- to hyperintense to grey matter • Dependent on amount of water content: Antoni A and Antoni B cells, usually heterogeneously hyperintense • Most comprised of Antoni A cells Intracanalicular schwannoma Intracanalicular schwannoma Vestibular schwannoma with small CPA Vestibular scwhannoma with mild component compression of the pons Vestibular schwannoma with moderate Vestibular Schwannoma with marked compression of pons compression of the pons Bilateral vestibular schwannomas with varying Bilateral Vestibular Schwannoma degrees of compression of the pons Vestibular scwhannoma and associated Cystic vestibular schwannoma arachnoid cyst Facial Nerve Schwannomas Imaging • CT scans • Remodeling of fallopian canal • Intraneural tumor location difficult • MRI • Non-contrast • T1WI: mild hypointense or isointense signal • T2WI: Heterogeneously iso- to hyperintense in signal intensity • Contrast: tumor enhancement • Improved localization of facial nerve segment involvement Bilobed FN Schwannoma Facial Schwannoma Gd-T1WI T1WI Ossifying IAC hemangioma T1 post-gad noncontrast CT Cavernous IAC Exophytic cavernous hemangioma in IAC Hemangioma T1WI post Gd Meningioma Meningioma • 2nd most common CPA mass • Dural based in location • Asymmetrically centered on the porus acousticus • CT • 25% calcified on NECT • Bony hyperostosis • MRI • T1-weighted images • Isointense to grey matter signal intensity • T2-weighted images • Typically isointense to grey matter, may be mildly hyperintense in signal intensity Meningioma Meningioma • MRI • May see CSF cleft around the mass. Tubular signal flow voids may be seen within mass and on surface of mass • Post-contrast images • Homogeneous, avid enhancement • Presence of dural tail Meningioma Meningioma Epidermoid Cyst Epidermoid Cyst • Ectodermal inclusion cyst arising from ectodermal elements during neural tube • CT closure • Non-contrast CT: dirty CSF in appearance • Large lesions may cause cranial neuropathy • MRI • T1-weighted images • Vertigo may occur due to vascular compression • Similar to CSF in signal, heterogeneous in appearance • Pearly white mass upon visual inspection • T2-weighted images • Cyst wall: Stratified sQuamous epithelium covered by a fibrous capsule • Similar to CSF in signal intensity, may be heterogeneous in appearance • Post contrast images • No enhancement • Diffusion imaging • Strongly diffusion positive • May be used to determine residua after resection Right CP angle epidermoid Epidermoid Residual epidermoid ? White epidermoid Dermoid cyst • Rare <1% of epidermoids • Also considered ectodermal inclusion cyst • High signal intensity on T1 –weighted images • May contain fat, hair, teeth • Proteinaceous material and blood products • Similar to epidermoid, but fat content may help in diagnosis • No enhancement noted. • T1-weighted images • Other consideration: Neurenteric cyst. • Similar to epidermoid but with high signal intensity of fat • T2-weighted images • Fat signal intensity also present • Post contrast images • No enhancement • Diffusion positive Dermoid cyst Arachnoid cyst • Arachnoid duplication anomaly creating a CSF filled space • Well circumscribed in appearance • CT and MR imaging • EQual to CSF in signal intensity • No enhancement • No diffusion abnormality.
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