Cranial nerves anatomy & pathology

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Information about Cranial nerves anatomy & pathology
Health & Medicine

Published on September 1, 2013

Author: vishsankpal


Dr. Vishal Sankpal

Cranial nerves  Imaging techniques  Normal anatomy, clinical importance and nerve specific pathologies  General Pathologies

Imaging recommendations  Best imaging modality for any simple or complex cranial neuropathy is MRI.  The only exception to this is imaging of distal vagal neuropathy where it is necessary to image the aorto- pulmonary window on left with CECT.  If a lesion is located in bony area such as skull base, sinuses or mandible, CT in bone window is recommended to provide complimentary bone anatomy & lesion related information.  Contrast CT is not necessary if T1, T2 and contrast T1 MR is available.

Imaging approach  Cranial nerves do not stop at the skull base !!  CN 2, 3, 4 & 6 – include focused orbital sequences  CN 5 – include entire face to inferior mandible if V3 affected  CN 7 – include CP angle, temporal bone and parotid space  CN 8 – include CPA, IAC and inner ear  CN 9-12 – include basal cisterns, skull base and nasopharyngeal carotid space

Why specific MRI sequences ??  Traditional magnetic resonance (MR) imaging sequences provide excellent soft-tissue resolution, they may lack the spatial resolution necessary to define smaller structures such as cranial nerves  Steady-state free precession (SSFP) sequences allow much higher spatial resolution and clearer depiction of tiny intracranial structures  An SSFP sequence is any gradient-echo sequence in which a nonzero steady state develops between pulse repetitions for both the longitudinal and transverse relaxation values of the interrogated tissues. A small flip angle and short relaxation time are required for this to occur.

Advantages of SSFP -  ability to generate a strong signal in tissues that have a high T2/T1 ratio, such as cerebrospinal fluid (CSF) and fat  particularly useful for visualizing the cisternal segments of cranial nerves because they provide excellent contrast resolution between CSF and nerves, as well as high spatial resolution with submillimetric section thicknesses

Disadvantages of SSFP –  reduced contrast resolution between different soft tissues  global landmarks may be poorly depicted because of the submillimetric section thicknesses Thus, SSFP sequences play a supplemental role alongside traditional sequences in MR imaging of the cranial nerves. Usually referred to by their trade names or acronyms (eg, constructive interference steady state, or CISS, and fast imaging employing steady-state acquisition, or FIESTA)

Cranial nerves  Imaging techniques  Normal anatomy, clinical importance and nerve specific pathologies  General Pathologies

Normal Anatomy  There are twelve cranial nerves and their defining feature is that they exit the cranial cavity through foramina or fissures.  All cranial nerves innervate structures in the head or neck.  In addition, the vagus nerve [X] descends through the neck and into the thorax and abdomen where it innervates viscera.

 Olfactory nerve: CNI  Optic nerve: CN2  Oculomotor nerve: CN3  Trochlear nerve: CN4  Trigeminal nerve: CN5  Abducens nerve: CN 6  Facial nerve: CN 7  Vestibulocochlear nerve: CN 8  Glossopharyngeal nerve: CN 9  Vagus nerve: CN l0  Accessory nerve: CN 11  Hypoglossal nerve: CN12

Imaging anatomy Cranial nerves can be grouped based on area of brainstem origin -  Diencephalon : CN2  Mesencephalon(mid-brain):CN3 & CN4  Pons:CN5,CN6,CN7 & CN8  Medulla:CN9,CN10,CN11 & CN12

Origin -

Exit points -

Cranial Nerve I: The Olfactory Nerve  Unlike most cranial nerves, the olfactory nerve consists of white-matter tracts and is not surrounded by Schwann cells.  The neurosensory cells for smell reside in the olfactory epithelium along the roof of the nasal cavity.  The axons of these cells extend through the cribriform plate of the ethmoid bone into the olfactory bulb at the anterior end of the olfactory nerve.

 Courses posteriorly through the anterior cranial fossa in the olfactory groove.  Posterior to the olfactory groove, the cisternal segment of the nerve runs below and between the gyrus rectus and the medial orbital gyrus.  These secondary axons in the olfactory nerve eventually terminate in the inferomedial temporal lobe, uncus, and entorhinal cortex.  To avoid confusing the olfactory nerve with the gyrus rectus on axial images, it is important to remember that the olfactory nerve is situated deep in the olfactory groove, inferior to the gyrus rectus.  Coronal images are easiest to interpret because the nerves are seen in cross section.

Olfactory nerve

SSFP MR images show the olfactory nerve (white arrow) within the CSF-filled olfactory groove and the optic nerve Coronal image shows the cisternal segment of the olfactory nerve (arrow), which is located inferior to and between the gyrus rectus (r) and the medial orbital gyrus (o).

Imaging recommendations -  Coronal sinus CT is best for isolated anosmia Identifies nasal vault and cribriform plate lesions  MRI of brain, anterior cranial fossa and sino- nasal region best for complex anosmia cases Identifies intracranial and dural lesions

Clinical importance  CN1 dysfunction produces unilateral anosmia.  Esthesioneuroblastoma arises from olfactory epithelium in the nasal vault  Head trauma may cause anosmia – cribriform plate fracture or anterior temporal lobe injury  Seizure activity in the olfactory area may produce „uncinate fits‟, imaginary odor, oroglossal automatisms and impaired awareness.

Olfactory neuroblastoma  also known as an esthesioneuroblastoma  is a tumor arising from the basal layer of the olfactory epithelium in the superior recess of the nasal cavity Epidemiology  bimodal age distribution with one peak in young adult patients (~ 2nd decade) and another peak in the 5th to 6th decades  no recognized gender predilection Clinical presentation  usually secondary to nasal stuffiness and rhinorrhoea or epistaxis  Presentation is often delayed  Patients often present late with larger tumors which can extend into the intracranial compartment (25 - 30% at diagnosis) and usually result in anosmia

Radiographic features  slow growing  begin as masses in the superior olfactory recess and initially involve the anterior and middle ethmoid air-cells on one side.  As they grow, they tend to destroy surrounding bone, and can extend in any direction  This invasion may be superiorly into the anterior cranial fossa, laterally into the orbits and across the midline into the contralateral nasal cavity.  Particular attention should be paid to the presence of cervical and retropharyngeal nodal metastases which are present in 10 - 44% of cases at diagnosis

CT  particularly useful in assessing bony destruction, although it cannot distinguish olfactory neuroblastomas from other tumours that arise in the same region  soft tissue attenuation, with relatively homogeneous enhancement  Focal calcifications are occasionally present  relatively slow growing and thus, the bony margins are often remodelled and resorbed, rather than being aggressively destroyed MRI  T1 : heterogeneous intermediate signal  T2 : heterogeneous intermediate signal  T1 C+ (Gd) : variable enhancement (usually moderate to intense)  When intracranial extension is present, peritumoural cysts between it and the overlying brain are often present. This may be helpful in distinguishing it from other entities. The margins of these cysts sometimes enhance.

Angiography / DSA  Angiography demonstrates a prominent tumour blush with arteriovenous shunting, and persistent opacification. Nuclear medicine  As with other neuroblastomas, olfactory neuroblastomas are MIBG avid.  This potentially helps to differentiate them from other tumors that arise in the region

Treatment and prognosis  usually involves combined chemotherapy and / or radiotherapy with surgical excision  Prognosis is significantly affected by presence of distant metastases  no distant metastases : 60% 5-year survival  distant metastases : 0% 5-year survival  Small localised tumours have a high cure rate, up to 85 - 90%

Differential diagnosis  Unfortunately, imaging alone often struggles to distinguish between olfactory neuroblastomas and other aggressive malignancies in the region.  olfactory neuroepithelioma  rare and indistinguishable on imaging  olfactory groove meningioma / haemangiopericytoma  especially if inferior extension  sinonasal carcinoma  may appear identical  usually older patients  lack peritumoural cysts  Rhabdomyosarcoma  melanoma metastases  lymphoma  nasopharyngeal carcinoma  epicentre more posteriorly located  usually older patients  juvenile nasopharyngeal angiofibroma  epicentre more posteroinferiorly located  almost exclusively in males  often somewhat younger

 Like the olfactory nerve, the optic nerve is a white-matter tract without surrounding Schwann cells.  It includes four anatomic segments: Intra-ocular Intra-orbital Intra-canalicular & Intra-cisternal Cranial Nerve II: The Optic Nerve

Intra-ocular  The intra-ocular segment leaves the ocular globe through the lamina cribrosa sclerae (the optic foramen of the sclera).  It is 1mm in length.

Intra-orbital  20-30 mm in length  Extends postero-medially from back of globe to orbital apex with in the intraconal space.  Covered by 3 meningeal layers of brain & the subarachnoid space with CSF is continuous with the suprasellar cistern. So fluctuations in IC pressure are transmitted via SAS of optic nerve-sheath complex.  Central retinal artery(1st br of ophthalmic artery) enters the optic nerve 1cm posterior to globe with accompanying vein to run to retina.

Intra-canalicular  It is a 4-9 mm segment within the bony optic canal.  Ophthalmic artery lies inferior to CN2.  Dura of CN2 fuses with the periosteum of the orbit (periorbita).  This segment of the nerve is frequently overlooked on radiologic images, so it should be specifically sought when imaging for vision loss.

Intra-cranial /cisternal segment  About 10mm length from optic canal to chiasm.  Covered by pia & surrounded by CSF within the suprasellar cistern.  Ophthalmic artery runs infero-lateral to nerve.  The anterior cerebral artery passes over the superolateral aspect of the cisternal segment of the nerve.

Optic nerve. the retinal (black arrow), orbital (black arrowheads), and canalicular (white arrowhead) segments.

The cisternal segment of the optic nerve (white arrow) leads to the chiasm, which resembles the Greek letter X in this plane. The optic tract (white arrowheads) leads backward from the chiasm to the thalamus. Important anatomic landmarks include the mamillary bodies (black arrowhead) and the anterior cerebral artery (black arrow).

 Key anatomic landmarks - in the suprasellar cistern include the infundibulum (stalk) of the pituitary gland, the anterior cerebral artery, and, posterior to the chiasm, the mamillary bodies.  The optic nerve terminates at the optic chiasm, where the two nerves meet, decussate, and form the optic tracts.  The optic tracts travel around the cerebral peduncles, after which most axons enter the lateral geniculate body of the thalamus, loop around the inferior horns of the lateral ventricles (Meyer loop), and enter the visual cortex in the occipital lobe.

 Because a single image obtained with an SSFP sequence usually depicts only a short segment of the optic nerve, thick-section reconstruction of SSFP acquisitions may be needed to allow examination of the entire length of the nerve on a single image.  Standard T2-weighted images also are useful for this purpose.

Clinical Importance  Lesion location  Optic nerve pathology: Monocular visual loss  Optic chiasm pathology: Bitemporal heteronymous hemianopia (loss of bilateral temporal visual fields)  Retrochiasmal pathology: Homonymous hemianopsia  Increased intracranial pressure transmitted along SAS of optic nerve-sheath complex  Manifests clinically as papilledema  Imaging shows flattening of posterior sclera, tortuosity and elongation of intraorbital optic nerves and dilatation of perioptic SAS.

Optic nerve sheath meningioma  benign tumour arising from the arachnoid cap sells of optic nerve sheath  represents ~ 10 - 30% of all orbital meningiomas  majority are direct extensions from intracranial meningiomas Epidemiology -  approximately 1/3rd of all optic nerve neoplasms (gliomas MC)  are usually seen in adults (mean age at presentation 40 years)  However up to 25% present in children, in which case they tend to be more aggressive  female predilection  The vast majority of cases are sporadic, although patients with neurofibromatosis type II (NF2) are at increased risk.

Clinical presentation –  visual loss  95% of cases  painless and progressive  more pronounced with lesions that affect the orbital apex  exacerbated during pregnancy  proptosis  60-90% of cases  more pronounced with anterior lesions near the globe  occurs later, as the tumour enlarges  may be exacerbated by associated hyperostosis

Radiographic features  same imaging characteristics as meningiomas elsewhere  tubular : 65%  exophytic : 25%  fusiform : 10%  Perioptic cysts- occasionally cysts filled with arachnoid fluid between the globe and the anterior margin of the tumour as a result of impaired CSF flow backwards

CT  On axial or oblique sagittal imaging the enhancing tumour surrounding the non-enhancing optic nerve results in the so-called tram-track sign.  On coronal imaging the tumour appears as a cuff of enhancing tumour around a central non-enhancing dot (optic nerve)  Tumour extending into the optic canal may lead to canal widening, or alternatively hyperostosis

MRI  greater ability to delineate posterior extension  T1 : isointense to somewhat hypointense compared to the optic nerve  T2 : isointense to somewhat hyperintense compared to the optic nerve  T1 C+ (GAD) : homogeneous enhancement  Careful examination of the orbital apex and optic nerve canals is essential if small intracanalicular tumours are to be identified

Optic nerve meningioma

Cranial Nerve III: The Oculomotor Nerve It is divided in to  Intra-axial segment  Cisternal segment  Cavernous segment  Extra-cranial segment  Thin sections in Axial & Coronal section in the region of periaqueductal white matter.

Intra-axial segment  The oculomotor nerve originates from nuclei deep to the superior colliculus, ventral to the cerebral aqueduct, and inferior to the pineal gland.  The nerve then travels across the midbrain from posterior to anterior.  The oculomotor nerve root emerges into the interpeduncular cistern, and this root entry zone in the cistern is a good way to identify the oculomotor nerve on axial FIESTA images.

Cisternal segment  Courses anterolaterally through interpeduncular & pre- pontine cisterns, passes between the posterior cerebral artery & superior cerebellar artery which makes it easy to identify on coronal FIESTA images.  Crosses the petro-clinoid ligament & penetrates the dura to enter roof of cavernous sinus.

Cavernous segment  The cavernous segment of the oculomotor nerve runs along the lateral wall of the cavernous sinus and is the most superior of the nerves in this sinus.  It is surrounded by narrow oculomotor CSF cistern.

Extra-cranial segment  The oculomotor nerve then enters the orbit through the superior orbital fissure, before splitting into superior and inferior divisions lateral to the optic nerve.  Superior branch- LPS & superior rectus  Inferior branch – inferior rectus, medial rectus & inferior oblique muscles  The inferior branch also gives fibers to ciliary ganglion.

Oculomotor nerve. FIESTA image shows the nerve (small arrows) where it emerges from the inter- peduncular cistern (large arrow) SSFP MR image shows the oculomotor nerve (white arrow) in cross section between the posterior cerebral artery (white arrowhead) and the superior cerebellar artery (black arrowhead).

Oculomotor nerve compression in an 82-year-old woman with ptosis of the right eye. Axial 0.8-mm-thick FIESTA MR image shows displacement and compression of the right oculomotor nerve in the root entry zone (long arrow) by the distal basilar artery (short arrow). The left oculomotor nerve (arrowhead), in comparison, appears normal.

Clinical Importance  Uncal herniation pushes CN3 on petro-clinoid ligament.  During trauma downward shift of brainstem upon impact can stretch CN3 over petroclinoid ligament.  CN3 is susceptible to compression by PCA aneurysms. Clinical Findings  Oculomotor ophthalmoplegia  Strabismus, ptosis, pupillary dilatation, downward abducted globe and paralysis of accommodation

Cranial Nerve IV: The Trochlear Nerve  The trochlear nerve is the only nerve with a root entry zone arising from the dorsal (posterior) brainstem.  After exiting the pons, the trochlear nerve curves forward over the superior cerebellar peduncle, then runs alongside the oculomotor nerve between the posterior cerebral and superior cerebellar arteries.  The trochlear nerve then pierces the dura to enter the cisterna basalis between the free and attached borders of the cerebellar tentorium.  After completing its cisternal course, the trochlear nerve runs through the lateral cavernous sinus just below the oculomotor nerve and enters the orbit through the superior orbital fissure to innervate the superior oblique muscle.

 The nerve is named for the trochlea, the fibrous pulley through which the tendon of the superior oblique muscle passes.  The cisternal segment of this tiny nerve is most easily identifiable posterolateral to the brainstem.  Along part of its intracranial course, the trochlear nerve lies between dural layers, where it is difficult to visualize on radiologic images.  Particular attention should be given to the anterior aspect of the tentorium in patients in whom the presence of isolated trochlear nerve palsy is suspected.

 CN 4 is the smallest cranial nerve & has the longest intra-cranial course (~7.5 cm) Optimally visualized if thin sections are taken  Axial section margins : orbital roof - diencephalon to maxillary sinus roof- medulla  Coronal section margins: 4th ventricle to anterior globe

Trochlear nerve. FIESTA MR image shows both trochlear nerves (arrows) where they emerge from the dorsal midbrain to cross the ambient cisterns. The characteristic course of the trochlear nerves allows their differentiation from the nearby superior cerebellar artery (arrowheads).

Clinical Importance CN4 neuropathy divided into simple and complex  Simple CN4 neuropathy (isolated)  Most common form; usually secondary to trauma  Cisternal segment injury by free edge of tentorium cerebelli or from posterior cerebral or superior cerebellar artery aneurysm  Contusion of superior medullary velum  Complex CN4 neuropathy (associated with other CN injury, CN3 ± CN6)  Brainstem stoke or tumor  Cavernous sinus thrombosis, tumor  Orbital tumor

Clinical Findings  Paralysis of superior oblique muscle results in extorsion (outward rotation) of affected eye. Extorsion is secondary to unopposed action of inferior oblique muscle  Patient complaints: Diplopia, weakness of downward gaze, neck pain from head tilting.

Cranial Nerve V: The Trigeminal Nerve  largest (thickest) cranial nerve  composed of a large sensory root that runs medial to a smaller motor root  The roots emerge from the lateral mid-pons and travel anteriorly through the pre-pontine cistern.  Enters middle cranial fossa by passing beneath tentorium at the apex of the petrous temporal bone & passes through an opening in the dura called the porus trigeminus to enter the Meckel’s (trigeminal) cave.

Trigeminal nerve course and branches

 Meckel (trigeminal) cave is a CSF-containing pouch in the middle cranial fossa which is continuous with the pre- pontine sub-archnoid space.  Pia covers the CN 5 in Meckel‟s cave.  Because the trigeminal nerve is large and its course proceeds straight forward from the lateral pons, it is easy to recognize on most MR images.

Trigeminal nerve. FIESTA MR image shows the sensory (arrowhead) and motor (large arrow) roots of the trigeminal nerve where they cross the prepontine cistern and enter the Meckel cave (small arrows).

 In the Meckel cave, the nerve forms a mesh-like web that can be visualized only with high-resolution imaging.  Along the anterior aspect of the cavity, the trigeminal nerve forms the trigeminal (Gasserian / Semilunar) ganglion before splitting into three subdivisions. Imaging pitfall –  Trigeminal ganglion lacks blood-nerve barrier, so normally enhances in post contrast images.

 The ophthalmic (V1) and maxillary (V2) divisions of the nerve move medially into the cavernous sinus and exit the skull through the superior orbital fissure and foramen rotundum, respectively.  The mandibular division (V3), which includes the motor branches, exits the skull inferiorly through the foramen ovale.

Trigeminal nerve. FIESTA image at the level of the Meckel cave shows the complex web of trigeminal nerve branches (arrows), which coalesce anteriorly to form the Gasserian ganglion.

 Sagittal T2 MR along line of proximal trigeminal nerve shows the preganglionic segment between the root entry zone in the lateral pons and the trigeminal ganglion in the anteroinferior Meckel cave.  The cerebrospinal fluid within Meckel cave communicates with prepontine cistern through the porus trigeminus.

Trigeminal neuralgia / tic douloureux  abrupt unilateral shock like facial pain lasting seconds to minutes.  Neurovascular contact by an artery or a vein at the root entry zone of the trigeminal nerve is linked to trigeminal neuralgia.  The average diameter of the unaffected trigeminal nerve has been estimated on transverse MR images to be 4 mm, with the range being 2–6 mm .  In the majority of cases, there is atrophy of the nerve tissue which is secondary to chronic compression of the nerve by aging and tortuous vessels along the course of the nerve after its point of exit from the brainstem.  Up to 42% of symptomatic nerves have gross atrophy.

•Images in a 63-year-old man with trigeminal neuralgia, with NVC caused by the superior cerebellar artery. •Two adjacent transverse 3D CISS MR images show that the superior cerebellar artery (short arrow) has compressed the REZ of the right trigeminal nerve (long arrow) at the medial site.

Trigeminal neuralgia. Male patient with left facial pain. Axial FIESTA image (A) and sagittal reconstruction (B) show that the root entry zone of the left trigeminal nerve is thinned and displaced by an adjacent vessel (thick white arrow points to the nerve and thin white arrow points to the vessel)

Cranial Nerve VI: The Abducens Nerve  Emerges from nuclei anterior to the fourth ventricle  Courses anteriorly through the pons to the pontomedullary junction and into the prepontine cistern  After crossing the prepontine cistern in a posterior-to- anterior direction, the abducens nerve runs vertically along the posterior aspect of the clivus, within a fibrous sheath called the ‘Dorello canal’.  Continues over the medial petrous apex and through the medial cavernous sinus, entering the orbit through the superior orbital fissure to innervate the lateral rectus muscle.

Abducens nerve

Abducens nerve. • The ponto-medullary junction • Cerebellopontine angle (CPA) and • Basilar artery (arrowhead) are important anatomic landmarks.

Abducens nerve. FIESTA image shows the abducens nerve where it enters the Dorello canal (arrow) along the posterior aspect of the clivus Vascular landmarks include the basilar artery (black arrowhead) and the anterior inferior cerebellar artery (white arrowhead).

 Axial and coronal MR sequences should include brainstem, fourth ventricle, cavernous sinus and orbit  Cisternal segment routinely visualized on high-resolution T2  CN6 entrance into Dorello canal is visualized due to invagination of cerebrospinal fluid into proximal canal  Abducens nerve is only cranial nerve to lie within cavernous sinus.

 It is important to note that the abducens nerve runs almost the entire length of the clivus.  Should be vigilant for clivus and petrous apex abnormalities in the setting of abducens nerve palsy.  Although the abducens nerve lies near the anterior inferior cerebellar artery and has a similar caliber, the two structures course in orthogonal directions and are thus easily distinguished.  In abducens neuropathy, affected eye will not abduct (rotate laterally)

Image of brainstem and prepontine cisterns shows proximal cisternal CN6 closely associated with the belly of the pons. CN3 is seen passing between posterior cerebral and superior cerebellar arteries.

 III (long black arrow),  IV (black arrowhead),  V1 (long white arrow),  V2 (white arrowhead), and  VI (short black arrow) are clearly demonstrated in the normal cavernous sinuses.

Cranial Nerves VII and VIII: The Facial and Vestibulocochlear Nerves  The facial and vestibulocochlear nerves have similar cisternal and canalicular courses .  They both emerge from the lateral aspect of the lower border of the pons and traverse the cerebellopontine angle cistern at an oblique angle.  There, they may be in close proximity to the anterior inferior cerebellar artery.

 The nerves cross the porus acousticus (an opening between the cerebellopontine angle cistern and the internal auditory canal; also known as the internal acoustic meatus) and traverse the length of the internal auditory canal.  Radiologic images that precisely depict the relationship of the nerves to masses in the cerebellopontine angle can help in surgical planning.

Facial nerve

FIESTA image shows the parallel courses of the facial (black arrowheads) and superior vestibular (white arrowheads) nerves as they cross the cerebellopontine angle to enter the internal auditory canal through the porus acusticus (double arrow).

Facial & vestibulocochlear nerves Facial nerve is anterior & superior to vestibulocochlear nerve within CPA& lAC. The anteroinferior cerebellar artery loop is a constant fixture in the normal anatomy of the CPA & lAC area.

Cerebellopontine angle meningioma in a 52-year-old woman with left sensorineural hearing loss. (a) Axial 0.8-mm-thick FIESTA image shows a tumor that fills the internal auditory canal (arrow) and extends into the CP angle cistern. (b) Coronal oblique 0.8-mm- thick FIESTA image shows direct involvement of the facial nerve (arrowhead), a contraindication against surgical resection. The tumor was treated instead with stereotactic radiosurgery.

 Within the internal auditory canal, the vestibulocochlear nerve splits into three parts (cochlear, superior vestibular, and inferior vestibular).  These three vestibulocochlear nerve branches, along with the facial nerve, have a characteristic appearance on sagittal oblique FIESTA cross-sectional images.  Images in that plane are most frequently used for the detection of cochlear nerve aplasia.

Cochlear nerve aplasia in a 4-year-old girl with congenital hearing loss who was under consideration for cochlear implantation. Sagittal oblique FIESTA images, obtained in planes perpendicular to the left (a) and right (b) internal auditory canals - facial (white arrow), superior vestibular (white arrowhead), inferior vestibular (black arrowhead) However, the cochlear nerve (black arrow in a) is absent in b, and that finding is a contraindication against cochlear implantation for the right ear. Incomplete separation of the superior and inferior vestibular nerves, also shown in b, is a normal variant.

 On any single axial FIESTA image, only two of the four nerves within the internal auditory canal typically are visible.  If one of the nerves is seen to enter the modiolus of the cochlea, then the two visible nerves are the cochlear and inferior vestibular nerves.  If the central modiolus is not depicted on the image, the visible nerves are the facial and superior vestibular nerves.  A filling defect within the membranous labyrinth on FIESTA images may signal a nerve abnormality in a branch of the facial or vestibulocochlear nerve.  The facial nerve exits the internal auditory canal and enters the facial canal.  After a complex course within the petrous bone, the facial nerve exits the skull base through the stylomastoid foramen and enters the substance of the parotid gland.

Cochlear schwannoma – Patient presented with right-sided sensorineural hearing loss Thin T1W fat saturated contrast-enhanced image (A) shows a well-defined enhancing mass lesion (white arrow) of 2-mm size, confined to the right internal auditory canal. Oblique axial FIESTA image (B) reveals that the lesion is confined to the cochlear nerve (antero-inferior) (white arrow)

Case of NF2 -  Bilateral acoustic schwannomas  Thickened left trigeminal nerve , possibility of schwannoma

Cranial Nerve IX: The Glossopharyngeal Nerve  The glossopharyngeal nerve emerges from the lateral medulla into the lateral cerebellomedullary cistern, above the vagus nerve and at the level of the facial nerve.  In the lateral cerebellomedullary cistern, the glossopharyngeal nerve is closely associated with the flocculus of the cerebellum.  The flocculus is a lobule of cerebellar tissue that is directly adjacent to the glossopharyngeal nerve, and it should not be mistaken for an abnormality.

 From the lateral cerebellomedullary cistern, the nerve plunges into the jugular fossa and exits the skull through the jugular foramen.  In the jugular foramen, the glossopharyngeal nerve is anterior to the vagus and accessory nerves and is surrounded by its own dural sheath (the glossopharyngeal canal).  Exits jugular foramen into anterior nasopharyngeal carotid space  Passes lateral to internal carotid artery & stylopharyngeus muscle  Terminates in posterior sublingual space in floor of mouth (posterior 1/3 taste function)

9,10, 11 & 12 CN origin and course

Coronal oblique FIESTA image through the cerebellopontine angle shows the glossopharyngeal nerve (arrow) just beneath the flocculus (f) of the cerebellum. The two roots of the vagus nerve (arrowheads) are visible in the same plane, and the superior and inferior vestibular nerves can be seen above the flocculus.

The glossopharyngeal nerve (CN9), vagus nerve (CNl0) and bulbar accessory nerve (CNll) all exit the medulla laterally CN9 is the most cephalad of these. With routine MR imaging it is not possible to see these three cranial nerves individually. In the upper medulla the vagus nerve is well seen leaving the brainstem via the postolivary sulcus. The glossopharyngeal nerve is seen more laterally as it has already exited the brainstem above the vagus nerve.

 Cisternal segments is not always visualized on routine MR imaging.  High-resolution thin-section T2 / FIESTA sequences usually demonstrate CN9, 10, 11 nerve complex passing through basal cisterns.

Cranial Nerve X: The Vagus Nerve  The vagus nerve comprises two roots that emerge from the side of the medulla, from a groove called the posterolateral sulcus.  Leaving the medulla, the nerve roots enter the lateral cerebellomedullary cistern in a position inferior to the glossopharyngeal nerve and run parallel to it through the cistern.  Because of their parallel course, it may be difficult to distinguish between the glossopharyngeal and vagus nerves on axial FIESTA images; coronal or oblique coronal views along the course of the nerves are best for that purpose.

 After obliquely traversing the lateral cerebellomedullary cistern, the vagus nerve enters the jugular fossa and exits the skull through the jugular foramen, between the glossopharyngeal and accessory nerves.  In the neck, the vagus nerve lies within the carotid sheath, behind and between the internal jugular vein and common carotid artery.

Vagus nerve course

Coronal oblique SSFP MR image through the cerebellopontine angle shows the glossopharyngeal nerve (arrow) just beneath the flocculus (f) of the cerebellum. The two roots of the vagus nerve (arrowheads) are visible in the same plane, and the superior and inferior vestibular nerves can be seen above the flocculus.

The vagus and glossopharyngeal nerves, which are difficult to distinguish in this plane, are clearly distinguishable in the coronal oblique plane.

Cranial Nerve XI: The Accessory Nerve  The accessory nerve is composed of multiple cranial and spinal rootlets.  The cranial rootlets emerge into the lateral cerebellomedullary cistern below the vagus nerve.  The spinal rootlets emerge from upper cervical segments of the spinal cord.

 Accessory nerve course

SSFP MR image at the level of the cervicomedullary junction (CMJ) shows the cranial rootlets (arrowheads) of the accessory nerve.

Coronal oblique 0.8-mm-thick SSFP MR image shows the spinal rootlets (arrows) of the accessory nerve arising from the upper spinal cord to cross the foramen magnum and join the cranial rootlets.

 After leaving the spinal cord, the spinal rootlets pass superiorly through the foramen magnum into the cisterna magna (ie, the posterior cerebellomedullary cistern), in a position posterior to the vertebral artery, and join the cranial rootlets in the lateral cerebellomedullary cistern.  The conjoined nerve fibers then leave the skull through the jugular foramen, posterior to the glossopharyngeal and vagus nerves. Segmental spinal nerve roots at the C1 and C2 levels are distinguishable from accessory nerve rootlets at these levels because the spinal nerve roots are larger and extend to the neural foramina instead of continuing superiorly.

 CN 11 supply muscles of pharynx & larynx  Innervates sternomastoid muscle continues across floor of posterior cervical space in cervical neck & terminate in & innervate trapezius muscle.

Cranial Nerve XII: The Hypoglossal Nerve  The hypoglossal nerve arises from nuclei in front of the fourth ventricle, within the medulla, and emerges as a series of rootlets extending from the ventrolateral sulcus of the medulla into the lateral cerebellomedullary cistern .  The combined rootlets then cross the lateral cerebellomedullary cistern, where the nerve is surrounded anteriorly by the vertebral artery and posteriorly by the posterior inferior cerebellar artery.

Hypoglossal nerve course

 The hypoglossal nerve then exits the skull via the hypoglossal canal, which runs obliquely in the axial plane, at an angle of approximately 45° between the coronal and sagittal planes.  After exiting the skull, the hypoglossal nerve runs medial to the glossopharyngeal, vagus, and accessory nerves and deep to the digastric muscle, looping over the hyoid bone to innervate a large part of the tongue.  It is the motor cranial nerve to intrinsic & extrinsic muscles of tongue(except Palatoglossus).

Coronal oblique 0.8-mm-thick FIESTA image shows multiple hypoglossal nerve roots (arrows) converging toward the hypoglossal foramen (arrowhead). The nerve roots are immediately posterior to the vertebral artery (V).

Axial 0.8-mm-thick FIESTA image shows the oblique course of the hypoglossal nerve (black arrowhead) as it crosses the lateral cerebellomedullary cistern toward the hypoglossal canal (white arrowheads). The vertebral arteries (white arrows) are anterior to the nerve, and the posterior inferior cerebellar artery (black arrow) is posterior to the nerve.

 The cisternal part of the nerve might be affected in a vertebrobasilar aneurysm or dolichoectasia, skull base neoplasms, basal skull fractures or fractures of the occipital condyle, basal meningitis or subarachnoid hemorrhages.  Primary tumors of the hypoglossal nerve are very rare but could include schwannomas.  Malignancies within the nasopharynx, oropharynx and sublingual spaces might also invade the hypoglossal canal and the peripheral aspect of the hypoglossal nerve.

Cranial nerves  Imaging techniques  Normal anatomy, clinical importance and nerve specific pathologies  General Pathologies

Cranial nerve pathologies -  Tumours  Infections  Post-infectious and Demyelinating Disorders  Post-radiation Neuritis

Tumors of cranial nerves  Schwannoma (Neurilemoma, neurinoma)  Neurofibroma

Schwannoma(Neurilemoma, neurinoma)  Benign encapsulated nerve sheath tumor composed of differentiated neoplastic Schwann cells.  All cranial nerves (exceptions: Olfactory, optic nerves) have myelinated schwann cell sheaths and are sites for intracranial schwannomas  98% of intracerebral schwannomas arise from cranial nerves.  Cranial nerve schwannoma:  Slow-growing extra-axial mass. Displaces ("buckles") cortex  A CSF-vascular "cleft" between tumor, brain may be seen.

NECT - Cranial nerve schwannoma –  Noncalcified extra-axial mass  Iso / slightly hyperdense compared to brain  May enlarge bony foramina ( lAC, foramen ovale, facial nerve canal) CECT: Strong, uniform enhancement

MR Findings - Tl  Usually iso-, sometimes mixed iso/hypointense T2  Hyperintense  Surrounding edema common DWI  Solid portion of schwannomas shows no restriction (isointense to normal brain parenchyma)  Elevated ADC values (reflect increased amount of extracellular water in tumor matrix) Post Contrast  Enhances strongly  2/3 solid; 1/3 ring or inhomogeneous

Acoustic schwannoma Trigeminal schwannoma

Neurofibroma  Head & neck neurofibromas are usually plexiform tumors  Plexiform NF‟s are a unique feature of Neurofibromatosis1  They are not native to intracranial cavity but occur as central extensions of the peripheral tumors  Commonly occur along the orbital division of the CN5  They are poorly delineated, diffusely infiltrating masses that can expand & erode bone.

CT  NCCT – Isodense  CECT - Moderate/strong enhancement MRI  T1WI: Isointense infiltrating mass  T2WI: Hyperintense  TI post contrast: Enhances strongly, somewhat heterogeneously

Contrast-enhanced T1-weighted fat- saturated axial MR image shows a poorly demarcated enhancing mass lesion that extends from the subcutaneous soft tissue into the orbit (arrow), middle cranial fossa (*), and cavernous sinus (arrowhead). Plexiform neurofibromas

Neurofibromatosis 1  Optic nerve gliomas are the most common CNS tumor in NF1  Can involve one or both the optic nerves & commonly extend in to the chiasma  Posterior extension in to optic tracts, LGB & optic radiation can also occur.  Most of these are histologically benign, low grade astrocytomas.  Most of them are appear  T1- hypo-isointense  T2-hyperintense  Variable contrast enhancement

MR images of a 5-year-old girl with NF-1. (a) a large chiasmal glioma (black arrow) and a probable glioma in the medulla (white arrow) (b) bilateral extension of the chiasmal glioma along the optic tracts. The high-signal tumor involves the lateral geniculate bodies and continues to the basal ganglia bilaterally. (c) Axial image at the level of the suprasellar cistern shows the chiasmal glioma (arrow), tumor extension into the left temporal lobe with a large cystic lesion (probably a trapped left temporal horn), and foci of increased intensity (“hamartomas”) in the bilateral cerebellar peduncles, cerebellar white matter, and pons.

Neurofibromatosis 2  Associated with tumors of Schwann cells.  Bilateral acoustic schwannomas are diagnostic  Trigeminal nerve is the next most frequently involved nerve after CN8  Involvement of more than one cranial nerve by schwannomas favors the diagnosis of NF2.

Bilateral acoustic schwannomas

MR images of a 17-year-old girl with NF-2. (a) Axial image at the level of the internal auditory canals shows isointense bilateral acoustic schwannomas. (b) Axial image at a slightly more rostral level reveals focal thickening of the left fifth nerve (probable fifth nerve schwannoma) (arrow).

Cranial nerve pathologies -  Tumours  Infections  Post-infectious and Demyelinating Disorders  Post-radiation Neuritis

Infections -  Infectious meningitis results from viral, bacterial, fungal, or parasitic infection. Tuberculosis –  Leptomeningitis is the most common form of intracranial tuberculosis, particularly in the pediatric population.  Nerve impairment has been attributed to ischemia of the nerve or entrapment of the nerve in basal exudates.

Fungal -  Cryptococcus neoformans is the most common fungus to involve the CNS.  Optic neuropathy is a rare complication of cryptococcal meningitis and usually occurs in non-AIDS patients.  Necrosis of the optic nerves and infiltration of the meninges around the optic tracts, nerves, and chiasm by cryptococcal organisms have been observed.

Lyme disease -  Multisystem inflammatory disorder caused by a spirochete (Borrelia burgdorferi).  Cranial neuritis in Lyme disease may involve any of cranial nerves III through VII, with the facial nerve most frequently affected and often associated with cochleovestibular nerve abnormalities.  The affected segments appear thickened and enhance.  Enhancement of cranial nerves including root entry zones is seen.

Lyme disease – Manifestation of palsy of left seventh cranial nerve in 10-year-old male patient with recent history of camping. (a) Transverse T1-weighted postcontrast MR image shows enhancement of left seventh cranial nerve (arrow). (b) Coronal T1-weighted postcontrast MR image demonstrates left trigeminal nerve enhancement (arrow)

Viral infections –  related to herpes simplex virus type 1, cytomegalovirus, and Varicella zoster organisms  manifest with cranial nerve involvement and show abnormal enhancement on MRI.

Cranial nerve pathologies -  Tumours  Infections  Post-infectious, inflammatory and Demyelinating Disorders  Post-radiation Neuritis

Bell‟s palsy -  A syndrome characterized by the acute onset of unilateral facial paralysis which progresses over a 2-5 day period.  There is pathologic enhancement of the intracanalicular– labyrinthine portion of CN 7.

The three criteria for pathologic enhancement of the facial nerve:  Enhancement outside the facial canal,  Extension of enhancement to cranial nerve VIII,  Intense enhancement of the labyrinthine and mastoid segments.

Ophthalmoplegic migraine –  is a rare condition characterized by headache and oculomotor nerve palsy lasting days to weeks. MRI findings-  include reversible enhancement of the cisternal segment of the oculomotor nerve  focal thickening at the exit of the nerve in the interpeduncular cistern.

57-year-old man with ophthalmoplegic migraine Unenhanced axial (A) and enhanced axial (B) T1-weighted images reveal smooth enlargement and homogeneous enhancement of cisternal segment of left oculomotor nerve (arrows).

Granulomatous diseases -  Intracranial neurosarcoidosis has a predilection for the basal leptomeninges, and involvement of every cranial nerve has been described.  MRI shows a spectrum of CNS abnormalities including diffuse or nodular thickening and abnormal enhancement of the leptomeninges in the basal cisterns and hypothalamic regions.  Facial nerve and optic nerve are most commonly affected  Perineural spread has also been reported in sarcoidosis.

•Enhancing lesion causing thickening and enhancement of cisternal segment of third cranial nerve (arrowheads). •Note enhancing suprasellar mass (m) and diffuse leptomeningeal enhancement (arrows). •Enhancement of entire visible portions of both optic nerves in orbit and optic canals (arrowheads). •Enhancement of extraocular muscles is normal finding.

Idiopathic hypertrophic cranial pachymeningitis -  rare disease  characterized by inflammation and fibrosis of the dura mater  It remains a diagnosis of exclusion but may be the presenting manifestation of granulomatous diseases such as sarcoidosis, Wegener‟s granulomatosis, or tuberculosis.  MRI - shows focal or diffuse thickening and enhancement of the dura that encase cranial nerves causing recurrent cranial neuropathies.  The oculomotor, abducens, and facial nerves are more frequently involved

Postradiation Neuritis -  uncommon, usually delayed, complication of radiation therapy or radiosurgery.  Cranial nerve deficits may be permanent or resolve spontaneously.  Pathology - Loss of the nerve–blood barrier due to demyelination and ischemia, coagulation necrosis, or peripheral fibrosis results in cranial nerve enhancement.  Radiation induced optic neuropathy occurs months to years after exposure of the anterior visual pathways to ionizing radiation.  MRI shows smooth enlargement and enhancement of the optic nerve and chiasm.

Recent advances in Cranial nerve imaging  The role of diffusion-tensor imaging (DTI) and tractography of the cranial and peripheral nerves being explored.

Conclusion  MRI is the imaging modality of choice in almost all cranial nerve pathologies  CT preferred to know the bony involvement  Precise knowledge about the origin and course of the cranial nerves is essential  Various vascular and bony landmarks on MRI and CT help in identification of cranial nerves and their pathologies  Relevant clinical details are necessary to image specific anatomical locations

References -  Diagnostic and surgical imaging anatomy – head, neck & brain – Harnsberger, Osborn et al.  Radiology review manual – Dahnert  Appearance of Normal Cranial Nerves on Steady-State Free Precession MR Images Sujay Sheth, BA, et al . RadioGraphics 2009;29:1045–1055  Internet –

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