cranial nerves part ii

CRANIAL NERVESCRANIAL NERVESPART IIPART II

Appearance of Normal Cranial Nerves

(REVIEW)

Abbreviations

• Olfactory nerve: CN1• Optic nerve: CN2• Oculomotor nerve:

CN3• Trochlear nerve: CN4• Trigeminal nerve:

CN5• Abducens nerve: CN6• Facial nerve: CN7

• Vestibulocochlear nerve: CN8

• Glossopharyngeal nerve: CN9

• Vagus nerve: CN1O• Accessory nerve:

CN11• Hypoglossal nerve:

CN12

Overview

• Cranial nerve groupings based on area of brainstem origin - Diencephalon: CN2 - Mesencephalon (midbrain): CN3 and CN4 - Pons: CN5, CN6, CN7, and CN8 - Medulla: CN9, CN1O, CN11 and CN12

Imaging Approaches

• CN1, 2, 3, 4 and 6: Include focused orbital sequences• CNS: Include entire face to inferior mandible if V3

affected• CN7: Include CPA, temporal bone and parotid space• CN8: Include CPA-lAC and inner ear• CN9-12: Include basal cistern, skull base,

nasopharyngeal carotid space• CN10: Follow carotid space to aortopulmonic window on

left, cervicothoracic junction on right• CN12: Remember to reach hyoid bone to include distal

loop as it rises into sublingual space

CN7CN7Facial Nerve

Overview

• Seventh cranial nerve• Cranial nerve that carries motor nerves to muscles of

facial expression, parasympathetics to lacrimal, submandibular and sublingual glands and taste from anterior 2/3 of tongue

• Mixed nerve: Motor, parasympathetic and special sensory (taste)

• Two roots: Motor & sensory (nervus intermedius) roots - Nervus intermedius exits lateral brainstem between motor root of facial and vestibulocochlear nerves, hence its name

• Four segments: Intra-axial, cisternal, intratemporal and extracranial (parotid)

Intra-Axial Segment• Three nuclei (one motor, two sensory)• Motor nucleus of facial nerve

- Located in ventrolateral pontine tegmentum - Efferent fibers loop dorsally around CN6 nucleus in floor of fourth ventricle forming facial colliculus - Fibers then course anterolaterally to exit lateral brainstem at pontomedullary junction

• Superior salivatory nucleus - Located lateral to CN7 motor nucleus in pons - Efferent parasympathetic fibers exit brainstem posterior to CN7 as nervus intermedius

• To submandibular, sublingual and lacrimal glands• Solitarius tract nucleus

- Termination of taste sensation fibers from anterior 2/3 of tongue - Cell bodies of these fibers in geniculate ganglion - Fibers travel within nervus intermedius

Cisternal Segment

• Two roots in cisternal CN7 - Larger motor root anteriorly - Smaller sensory nervus intermedius posteriorly

• Emerge from lateral brainstem at root exit zone in pontomedullary junction to enter cerebellopontine angle (CPA) cistern - CN8 exits brainstem posterior to CN7

• 2 roots join together & pass anterolaterally through CPA cistern with CN8 to internal auditory canal (lAC)

Intratemporal Segment

• CN7 further divided in T-bone into 4 segments: lAC, labyrinthine, tympanic and mastoid

• lAC segment: Porus acusticus to lAC fundus; anterosuperior position above crista falciformis

• Labyrinthine segment: Connects fundal CN7 to geniculate ganglion (anterior genu)

• Tympanic segment: Connects anterior to posterior genu, passing under lateral semicircular canal

• Mastoid segment: Inferiorly directed from posterior genu to stylomastoid foramen

Extracranial Segment

• CN7 exits skull base through stylomastoid foramen to enter parotid space

• Parotid CN7 passes lateral to retromandibular vein

• Ramifies within parotid, passes anteriorly to innervate muscles of facial expression

CN7 Branches

• Greater superficial petrosal nerve - Arises at geniculate ganglion, passes anteromedially, exits temporal bone via facial hiatus - Carries parasympathetic fibers to lacrimal gland

• Stapedius nerve - Arises from high mastoid segment of CN7 - Provides motor innervation to stapedius muscle

• Chorda tympani nerve - Arises from lower mastoid segment - Courses across middle ear to exit anterior T-bone - Carries taste fibers from anterior 2/3 tongue

• These fibers travel with lingual branch of mandibular division of trigeminal nerve

• Terminal motor branches

Imaging Recommendations

• Bone CT best for intratemporal segment of CN7

• MR for intra-axial, cisternal, lAC and extracranial segments

• Do not image routine Bell palsy!

• Imaging "Sweet Spots“: Include brainstem, CPA cistern, lAC, T-bone and parotid when MR completed for CN7 palsy

Imaging Pitfalls

• Mild enhancement of labyrinthine segment, geniculate ganglion and proximal tympanic segments of CN7 can be normal on post-contrast T1 MR - Secondary to circum neural arteriovenous plexus

• Always check parotid in peripheral CN? paralysis

Clinical Issues

Facial nerve paralysis can be central or peripheral• Central: Supranuclear injury resulting in paralysis of

contralateral muscles of facial expression with forehead sparing

• Peripheral: Injury to CN7 from brainstem nucleus peripherally, resulting in paralysis of all ipsilateral muscles of facial expression - If lesion proximal to geniculate ganglion, lacrimation, sound dampening and taste affected - If CN6 involved, check pons for lesion - If CN8 involved, check CPA-lAC for lesion - If lacrimation, sound dampening and taste are variably affected, T-bone lesion possible - If lacrimation, sound dampening and taste are spared, extracranial CN7 implicated

CN8CN8Vestibulocochlear nerve

Overview

• Eighth cranial nerve

• Afferent sensory nerve of hearing & balance

• Sensory nerve consisting of two parts - Vestibular part: Balance - Cochlear part: Hearing

• CN8 best described from peripheral to central

Cochlear Nerve

• Arises from bipolar neurons located in spiral ganglion within modiolus of cochlea - Peripheral fibers pass to organ of Corti in cochlear duct (scala media) within cochlea - Central fibers coalesce and pass as auditory component of CN8 (cochlear nerve) to brainstem

• Central fibers pass from modiolus through cochlear aperture into internal auditory canal (lAC) - Cochlear aperture defined as bony opening into anteroinferior quadrant of fundus of lAC - Maximum diameter of cochlear aperture - 2 mm

• Cochlear nerve passes from lAC fundus to porus acusticus within anteroinferior quadrant of lAC

Cochlear Nerve

• Near porus acusticus cochlear nerve joins together with superior & inferior vestibular nerves to form vestibulocochlear nerve (CN8)

• CN8 crosses cerebellopontine angle (CPA) cistern posterior to facial nerve

• CN8 enters lateral brainstem at pontomedullary junction posterior to facial nerve

• Cochlear nerve fibers bifurcate, ending in dorsal & ventral cochlear nuclei

• Dorsal & ventral cochlear nuclei - Cochlear nuclei found on lateral surface of inferior cerebellar peduncle (restiform body)

Vestibular NerveArises from bipolar neurons located in vestibular (Scarpa) ganglion

located within vestibular nerve in fundal portion of lAC• Vestibular ganglion not visible on imaging• Peripheral fibers pass to sensory epithelium of utricle, saccule &

semicircular canals - Traverse multiple foramina in cribriform plate in lateral wall of lAC fundus

• Central fibers coalesce to form superior & inferior vestibular nerves that pass medially to brainstem

Fundus of lAC• Superior & inferior vestibular nerves are separated by falciform crest

(transverse crest)• Superior vestibular nerve separated from facial nerve anteriorly by

vertical bony structure called Bill bar - Bill bar not visible on imaging (CT or MR)

Vestibular Nerve• Superior & inferior vestibular nerves pass medially from lAC fundus

to porus acusticus within posterosuperior & posteroinferior quadrants of lAC

• Near porus acusticus superior & inferior vestibular nerves join together with cochlear nerve to form vestibulocochlear nerve (CN8)

• Vestibulocochlear nerve crosses CPA cistern posterior to facial nerve

• Enters lateral brainstem at junction pons & medulla posterior to facial nerve

• Vestibular nerve fibers divide into ascending & descending branches which mainly terminate in vestibular nuclear complex

• Vestibular nuclear complex - Four nuclei (lateral, superior, medial & inferior) - Located beneath lateral recess along floor of fourth ventricle (rhomboid fossa) in lower pons - Complex connections exist between vestibular nuclei, cerebellum, spinal cord (vestibulospinal tract) & nuclei controlling eye movement

Imaging RecommendationsSensorineural hearing loss (SNHL)• Intracochlear lesion suspected

- CT & MR both useful for imaging - Congenital lesions of membranous labyrinth seen as abnormalities of fluid spaces on MR or in bony labyrinth shape on bone CT - Bone CT better for otosclerosis, Paget disease, labyrinthine ossificans or if trauma suspected - Only MR will demonstrate labyrinthitis or intralabyrinthine tumor

• CN8 lesion suspected (CPA-lAC) - MR imaging method of choice - Thin-section, high-resolution T2 sequence in axial & coronal planes may be used to screen patients with unilateral sensorineural hearing loss - Tl C+ MR remains gold standard

Imaging "Sweet Spots"

• Unilateral sensorineural hearing loss - Focus on brainstem (inferior cerebellar peduncle )-CPA-lAC-cochlea - Central acoustic pathway (intra-axial pathways above cochlear nuclei) rarely site of offending lesion

• Cisternal & lAC segments of CNS routinely visualized on high-resolution T2 MR

Imaging Pitfalls:• Beware small lesions of lAC (≤ 2 mm)!

- Follow-up imaging recommended as may be transient finding where surgery not needed

Clinical Importance

• Vestibular nerve dysfunction (dizziness, vertigo, imbalance) alone usually has negative MR

• 95% of lesions causing unilateral SNHL found by MR, are vestibulocochlear schwannoma

Appearance of NormalCranial Nerves on

Steady-State Free PrecessionMR Images

RadioGraphics 2009; 29:1045–1055 • Sujay Sheth, BA • Barton F. Branstetter

IV, MD • Edward J. Escott, MD

Cranial Nerves VIIand 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.

• Next, the nerves cross the porus acusticus (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.

RadioGraphics 2009; 29:1045–1055


Vestibulocochlear Nerves• 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 SSFP cross-sectional images.

• Images in that plane are most frequently used for the detection of cochlear nerve aplasia.

• On any single axial SSFP 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 SSFP images may signal

a nerve abnormality in a branch of the facial or vestibulocochlear nerve.



Vestibulocochlear Nerves• The facial nerve exits the internal auditory

canal and enters the facial canal via the fallopian aqueduct on the anterior aspect of the Bill bar.

• 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.


Axial 0.8-mm-thick SSFP MR 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).


Cerebellopontine angle meningioma in a 52-year-old womanwith left sensorineural hearing loss. (a) Axial 0.8-mm-thick SSFP MR image shows a tumor that fills the internal auditory canal (arrow) and extends into the cerebellopontine angle cistern. The relation of the tumor to the facial and vestibulocochlear nervesis not clearly shown. (b) Coronal oblique 0.8-mm-thick SSFP MR image shows direct involvement of the facial nerve (arrowhead), a contraindication against surgical resection. The tumor was treated instead with stereotactic radiosurgery. RadioGraphics 2009; 29:1045–1055

Cochlear nerve aplasia in a 4-year-old girl with congenital hearing loss who was under consideration for cochlear implantation. Sagittal oblique 1.0-mm-thick SSFP images, obtained in planes perpendicular to the left (a) and right (b) internal auditory canals, show the main branches of the facial and vestibulocochlear nerves in cross section. The anterior (A), posterior (P), superior (S), and inferior (I) aspects of the canals are labeled for ease of orientation. In both a and b, the facial (white arrow), superior vestibular (white arrowhead), and inferior vestibular (black arrowhead) nerves are depicted; 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.


CN9CN9Glossopharyngeal nerve

Overview• Ninth cranial nerve• Complex cranial nerve functions

- Taste & sensation to posterior 1/3 tongue - Sensory nerve to middle ear & pharynx - Parasympathetic to parotid gland - Motor to stylopharyngeus muscle - Viscerosensory to carotid body & sinus

• Mixed nerve (sensory, taste, motor, parasympathetic)

• Four segments: Intra-axial, cisternal, skull base & extracranial

Intra-Axial Segment

• Glossopharyngeal nuclei are in upper & middle medulla - Motor fibers to stylopharyngeus muscle originate in nucleus ambiguus - Sensory fibers from tympanic membrane, soft palate, tongue base & pharynx terminate in spinal nucleus CNS - Taste fibers from posterior 1/3 tongue terminate in solitary tract nucleus - Parasympathetic fibers to parotid gland originate in inferior salivatory nucleus

Cisternal Segment

• Exits lateral medulla in postolivary sulcus just above vagus nerve

• Travel anterolaterally through basal cistern together with vagus nerve & bulbar portion of accessory nerve

• Passes through glossopharyngeal meatus into pars nervosa portion of jugular foramen

Skull Base Segment

• Passes through anterior pars nervosa portion of jugular foramen - Accompanied by inferior petrosal sinus - Vagus (CN10) & spinal accessory (CN11) nerves are posterior within pars vascularis portion of jugular foramen - Superior & inferior sensory ganglia of CN9 are found within jugular foramen


• 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)

Extracranial Branches• Tympanic branch (Jacobsen nerve)

- Sensation from middle ear & parasympathetic to parotid gland - Arises from inferior sensory ganglion within jugular foramen - Passes through inferior tympanic canaliculus into middle ear (Aberrant internal carotid artery enters middle ear via this canal) - Forms tympanic plexus on cochlear promontory (Glomus bodies associated with this nerve form glomus tympanicum paraganglioma)

• Stylopharyngeus branch - Motor to stylopharyngeus muscle

• • Sinus nerve - Supplies viscerosensory fibers to carotid sinus & carotid body - Conducts impulses from mechanoreceptors of sinus & chemoreceptors of carotid body to medulla

• Pharyngeal branches - Sensory to posterior oropharynx & soft palate

• Lingual branch - Sensory & taste to posterior 1/3 of tongue


• MR imaging method of choice - Superior sensitivity to skull base, meningeal, cisternal & brainstem pathology - Sequences should include a combination of T2, T1 without fat-saturation & contrast-enhanced T1 with fat-saturation in axial & coronal planes

• Bone CT used to supplement MR when complex skull base pathology discovered


• Focused enhanced MR imaging extends from pontomedullary junction above to hyoid bone below

• CN9 nuclei & intra-axial segment not directly visualized - Position inferred by identifying upper medulla, posterior to postolivary sulcus - Cisternal segment is not always visualized on routine MR imaging (High-resolution thin-section T2 sequences usually demonstrate CN9, 10, 11 nerve complex passing through basal cisterns; Bone CT with bone algorithm clearly demonstrates bony anatomy of pars nervosa)

• Extracranial segment not visualized• Imaging Pitfalls: Remember to image entire extracranial

course of CN9; do not just stop at skull base!

Clinical Importance

• Glossopharyngeal nerve dysfunction usually associated with CN1O & CN11 neuropathy - Isolated glossopharyngeal neuropathy exceedingly rare

Cranial Nerve IX: TheGlossopharyngeal 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.


Cranial Nerve IX: TheGlossopharyngeal Nerve

• 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).


Coronal oblique 0.8-mm-thick 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.


CN10CN10Vagus nerve

Overview• Tenth cranial nerve• CN1O: Parasympathetic nerve supplying regions of head

and neck and thoracic and abdominal viscera• Additional vagus nerve components

- Motor to soft palate (except tensor veli palatini muscle), pharyngeal constrictor muscles, larynx and palatoglossus muscle of tongue - Visceral sensation from larynx, esophagus, trachea, thoracic and abdominal viscera - Sensory nerve to external tympanic membrane, external auditory canal (EAC) and external ear - Taste from epiglottis

• Mixed nerve (sensory, taste, motor, parasympathetic)• Segments: Intra-axial, cisternal, skull base and

extracranial

Intra-Axial Segment

• Vagal nuclei are in upper and middle medulla - Motor fibers originate in nucleus ambiguus - Taste from epiglottis goes to solitary tract nucleus - Sensory fibers from viscera terminate in dorsal vagal nucleus (afferent component) - Parasympathetic fibers project from dorsal vagal nucleus (efferent component) - Sensory from regional meninges and ear project to spinal nucleus CN5

• Fibers to and from these nuclei exit lateral medulla in postolivary sulcus inferior to CN9 and superior to bulbar portion of CN 11

Cisternal Segment

• Exits lateral medulla in postolivary sulcus between CN9 and bulbar portion of CN11

• Travel anterolaterally through basal cistern together with CN9 and bulbar portion of CN11

Skull Base Segment

• Passes through posterior pars vascularis portion of jugular foramen (JF) - Accompanied by CN11 and jugular bulb - Superior vagal ganglion is found within JF


• Exits JF into nasopharyngeal carotid space• Inferior vagal ganglion lies just below skull base• Descends along posterolateral aspect of internal carotid

artery into thorax - Passes anterior to aortic arch on left and subclavian artery on right

• Forms plexus around esophagus and major blood vessels to heart and lungs

• Gastric nerves emerge from esophageal plexus and provide parasympathetic innervation to stomach

• Innervation to intestines and visceral organs follows arterial blood supply to that organ

Extracranial Branches in Head & Neck

• Auricular branch (Arnold nerve) - Sensation from external surface of tympanic membrane, EAC and external ear - Arises from superior vagal ganglion within JF - Passes through mastoid canaliculus extending from posterolateral JF to mastoid segment CN7 canal - Enters EAC via tympanomastoid fissure

• Pharyngeal branches - Pharyngeal plexus exits just below skull base - Sensory to epiglottis, trachea and esophagus - Motor to soft palate (except tensor veli palatini muscle - CNV3) and pharyngeal constrictor muscles

Extracranial Branches in Head & Neck

• Superior laryngeal nerve - Motor to cricothyroid muscle - Sensory to mucosa of supraglottis

• Recurrent laryngeal nerve - On right recurs at cervicothoracic junction, passes posteriorly around subclavian artery - On left recurs in mediastinum by passing posteriorly under aorta at aortopulmonary window - Nerves recur in tracheoesophageal grooves (TEG) - Motor to all laryngeal muscles except cricothyroids - Sensory to mucosa of infraglottis

Imaging RecommendationsProximal vagal neuropathy• Image from medulla to hyoid bone• MR imaging method of choice• - Superior sensitivity to skull base, meningeal, cisternal and

brainstem pathology• - Sequences should include a combination of T2, T1 without fat-

saturation and contrast-enhanced T1 with fat-saturation in axial and coronal planes

• - Bone CT used to supplement MR when complex skull base pathology is present

• - Distal vagal neuropathy• Image from hyoid bone to mediastinum• Must reach carina if left vagal neuropathy• Key areas to evaluate are carotid space and TEG• CECT imaging method of choice

Clinical Importance

• Vagal nerve dysfunction separated into proximal and distal symptom complexes

• Proximal symptom complex - Injury site: Between medulla and hyoid bone - Multiple cranial nerves involved (CN9-12) with oropharyngeal and laryngeal dysfunction

• Distal symptom complex - Injury site: Below hyoid bone - Isolated CNlO involvement with laryngeal dysfunction only

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 SSFP images; coronal or oblique coronal views along the course of the nerves probably are best for that purpose.


Cranial Nerve X: The Vagus Nerve

• 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.


Axial oblique 0.8-mm-thick SSFP MR image shows the vagus nerve (arrow) where it crosses the lateral cerebellomedullary cistern (LCM) and approaches the jugular foramen. The vagus and glossopharyngeal nerves, which are difficult to distinguish in this plane, are clearly distinguishable in the coronal oblique plane (see Fig 17).


CN11CN11Accessory nerve

Overview

• Eleventh cranial nerve

• Pure motor cranial nerve supplying sternocleidomastoid & trapezius muscles

• Motor cranial nerve only

• Four CN11 segments are defined - Intra-axial, cisternal, skull base & extracranial

Intra-Axial Segment

Two distinct nuclear origins• Bulbar (cranial) motor fibers originate in lower nucleus

ambiguus - Fibers course anterolaterally to exit lateral medulla in postolivary sulcus inferior to CN9 & 10

• Spinal motor fibers originate from spinal nucleus of accessory nerve - Narrow column of cells along lateral aspect of anterior horn from C1 to C5 - Nerve fibers emerge from lateral aspect of cervical spinal cord between anterior & posterior roots - Fibers combine forming a bundle that ascends entering skull base via foramen magnum

Cisternal Segment

• Bulbar portion travels anterolaterally through basal cistern together with CN9 & 10

• Bulbar & spinal portions join together within lateral basal cistern

Skull Base Segment

• Passes through posterior pars vascularis portion of jugular foramen - Vagus nerve (CN1O) & jugular bulb are also in pars vascularis

• Bulbar & spinal portions remain together in jugular foramen

Extracranial Segment• Combined CN11 exits jugular foramen into

nasopharyngeal carotid space• Fibers from bulbar portion which arose within nucleus

ambiguus transfer to vagus nerve - Travels via CN10 to supply muscles of pharynx & larynx • Larynx: except cricothyroid muscle via recurrent laryngeal nerve • Pharynx: superior constrictor & soft palate via pharyngeal plexus

• Fibers from spinal portion remain in extracranial CN11 - Diverges posterolaterally from carotid space - Descend along medial aspect of sternocleidomastoid muscle - Innervates sternomastoid muscle - Continues across floor of posterior cervical space in cervical neck - Terminate in & innervate trapezius muscle


• MR imaging method of choice - Superior sensitivity to skull base, meningeal, cisternal & brainstem pathology - Sequences should include a combination of T2, T1 without fat-saturation & contrast-enhanced TI with fat-saturation in axial & coronal planes

• Bone CT used to supplement MR when complex skull base pathology is present


• CN11 nuclei & intra-axial segment not directly visualized• Cisternal segment is often not visualized on routine MR

imaging - High-resolution thin-section T2 MR sequence usually demonstrates CN9, 10, 11 nerve complex passing through basal cisterns from post-olivary sulcus to pars vascularis of jugular foramen

• Bone CT clearly demonstrates bony anatomy of pars vascularis of jugular foramen

• Extracranial CN11 segment not directly visualized - Location inferred from its constant position deep to sternocleidomastoid muscle in floor of posterior cervical space

Imaging Pitfalls

• Hypertrophic levator scapulae muscle following serious CNll injury may mimic tumor

• Don't mistake this enlarged muscle for mass!

Clinical Importance• CNll innervates sternocleidomastoid & trapezius muscles

Function-Dysfunction• CN11 dysfunction: Isolated CN11 injury

- Most common cause is radical neck dissection because spinal accessory nodal chain intimately associated CN11 - Initial symptoms of spinal accessory neuropathy (downward & lateral rotation of scapula; shoulder droop resulting from loss of trapezius tone - Long term findings in spinal accessory neuropathy (within 6 months results in atrophy of ipsilateral sternocleidomastoid & trapezius muscles; (compensatory hypertrophy of ipsilateral levator scapulae muscle occurs over months)

• CN11 dysfunction: Complex CN11 dysfunction associated with CN9 & 10 neuropathy

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.

• 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.


Axial 0.8 mm-thick 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.


CN12CN12Hypoglossal nerve

Overview

• Twelfth cranial nerve• CN12: motor cranial nerve controlling intrinsic &

extrinsic muscles of tongue• Motor cranial nerve to intrinsic & extrinsic

muscles of tongue - Only extrinsic muscle not innervated by CN12 is palatoglossus muscle (Vagus nerve innervates palatoglossus muscle)

• Hypoglossal nerve anatomic segments: Intra-axial segment, Cisternal segment, Skull base segment, Extracranial

Intra-Axial Segment

• Hypoglossal nucleus - Located in medulla between dorsal vagal nucleus & midline - Long, thin nucleus that is about same length as the ventrolateral olive- Extends from level of hypoglossal eminence in floor of fourth ventricle just inferior to stria medullares to proximal medulla - In axial section, hypoglossal nucleus is located in dorsal medulla, medial to dorsal vagal nucleus

• Hypoglossal intra-axial axonal course - Efferent fibers from hypoglossal nucleus extend ventrally through medulla, lateral to medial lemniscus - Efferent fibers exit between olivary nucleus & pyramid at ventrolateral sulcus also called pre-olivary sulcus

Cisternal Segment

• Efferent fibers coalesce to form multiple rootlets

• Rootlets fuse into hypoglossal nerve just as it exits skull base through hypoglossal canal

• Hypoglossal filaments may merge with vagal fibers

Skull Base Segment

• Hypoglossal nerve exits the occipital bone via hypoglossal canal - Hypoglossal canal is located in inferior occipital bone caudal to jugular foramen - Variant anatomy of hypoglossal canal (osseous septa may bisect hypoglossal canal)

Extracranial Segment• Carotid space component of CN12

- Hypoglossal canal "empties" into medial nasopharyngeal carotid space

• Hypoglossal nerve immediately gives off dural branches after exiting hypoglossal canal - Hypoglossal nerve descends in posterior aspect of carotid space, closely apposed with vagus nerve - Exits carotid space anteriorly between jugular vein & internal carotid artery at inferior margin of posterior belly of digastric muscle

• Trans-spatial component of CN12 - After leaving carotid space, runs anteroinferiorly toward hyoid bone, lateral to carotid bifurcation - At level of occipital artery base, hypoglossal nerve turns anterior, continuing as muscular branch below posterior belly of digastric muscle - Gives off superior root of ansa cervicalis from horizontal segment CN12 to anastomose with lower root of ansa cervicalis


• Distal branches of imaging importance - Muscular branch travels on lateral margin of hyoglossus muscle in posterior sublingual space • Muscular branch innervates extrinsic (styloglossus, hyoglossus, genioglossus) and intrinsic tongue muscles • Geniohyoid innervated by C1 spinal nerve - Ansa cervicalis: Formed from superior and inferior (C1-C3 spinal nerves) roots • Innervates infrahyoid strap muscles (sternothyroid, sternohyoid, omohyoid)


• MR is preferred imaging study - Best delineates brainstem, cisterns, skull base & suprahyoid neck

• CECT with bone algorithm of skull base is excellent for skull base & suprahyoid neck


• Coverage of hypoglossal nerve requires CT or MR to visualize following anatomic areas - Brainstem, basal cistern & hypoglossal canal - Nasopharyngeal carotid space - Posterior belly digastric & carotid bifurcation - Hyoid bone & sublingual space

• Imaging Pitfalls: Failure to image to level of hyoid bone will result in missed diagnoses!

Clinical Importance

• Unilateral hypoglossal lesion causes tongue protrusion to "side of the lesion"

• Acute hypoglossal injury - Tongue fasciculates - Tongue deviates to side of injury when protruded

• Chronic hypoglossal injury - Tongue atrophy seen as fatty infiltration & volume loss on CT or MR - Infrahyoid strap muscles also atrophy

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.

• 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.


Coronal oblique 0.8-mm-thick SSFP MR 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 SSFP MR 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.


References

• Diagnostic and Surgical Imaging Anatomy. Brain, Head & Neck, Spine / H. Ric Harnsberger. [et al.] ; managing editor, Andre Macdonald. n 1st ed. I:174-I:259.

• RadioGraphics 2009; 29:1045–1055 • Sujay Sheth, BA • Barton F. Branstetter IV, MD • Edward J. Escott, MD. Appearance of Normal Cranial Nerves on Steady-State Free Precession MR Images.

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