cranial nerves 3,4,6-neuroradioology

CRANIAL NERVES III,IV AND VI

Dr Parvathy S Nair

CRANIAL NERVE III(OCCULOMOTOR)

• Occulomotor nucleus (in front of cerebral aqueduct at level of superior colliculus of midbrain and exits ventrally in interpeduncular fossa)

• Four lateral paired subnuclei that innervate the superior, inferior, and medial rectus, as well as the inferior oblique muscles.– Axons from one superior rectus (SR) nucleus cross and pass through

the opposite SR subnucleus; thus, a lesion of one SR subnucleus results in bilateral superior rectus palsy.

– LPS has also bilateral supply– All other EOM gets ipsilateral supply

• Edinger-Westphal (parasympathetic)nucleus, which contains preganglionic, parasympathetic neurons whose axons project to the ciliary ganglion and postganglionic paraympathetic fibres supply sphincter pupillae and ciliaris muscle

Course• The oculomotor nerve exits the ventral midbrain in

the interpeuncular fossa-> pierces the dura mater-> between PCA above and SCA below

• Through the lateral wall of the cavernous sinus, enters orbit through superior orbital fissure.

• Within the orbit it branches into a superior ramus (to the superior rectus and levator muscles) and an inferior ramus (to the medial and inferior rectus, the inferior oblique, and the ciliary ganglion).

• Postganglionic fibres from the ciliary ganglion innervate the sphincter pupillae muscle of the iris as well as the ciliary muscle.

Functions

• EOM- all movements except lateral and down-and-out movements

• Levator palpabrae superioris-same branch as that of SR- elevation of lid

• Parasymp. innervation- miosis,accomodation and light reflex

CRANIAL NERVE IV(Trochlear)

• Long course• Emerges from trochlear nu at the level of

inferior colliculus• Connections-same as CN3• COURSE-After winding around the

periaqueductal grey matter and comes out posteriorly through the opposite side

• Crosses the tentorium->enters middle cranial fossa->lat wall of cavernous sinus->enters SOF->supplies Sup. Oblique muscle.

Functions

• Down and out movement of eyeball• “Reading muscle” • Intorsion – axis of vision is kept parallel

CRANIAL NERVE VI(ABDUCENS) • The abducens nucleus is located in the pons, on the floor

of the fourth ventricle, at the level of the facial colliculus. • Connections-same as CN3– PPRF connections-lateral conjugate gaze

• Course- exits from pontomedullary jn->runs between AICA and labrynthine artery->At the tip of the petrous temporal bone it makes a sharp turn forward to enter the cavernous sinus.

• In the cavernous sinus it runs alongside the ICA. It then enters the orbit through the SOF and innervates the lateral rectus.

• Abducent palsy is seen in increased ICT- due to compression against the petrous apex causing “false localization sign”

• FUNCTION-– Lateral movement of eye– Lat conjugate gaze

Examination

• 3,4,6 can be examined together– Ask patient to look up, down, obliquely up and

down– Each eye separately

• Parasympathetic function-only for CN 3– Pupillary light reflex – Accomodation reflex

MR Imaging of normal CN3,4,6

• Difficult to evaluate the cisternal segments of the cranial nerves, which are small in diameter and are located in close proximity to many other anatomic structures.

• Thin T2WI(1mm thickness)• SSFP/PSIF sequences depict these nerve

segments in greater detail• Type of coherent GRE sequence• 0.8mm thick-ideal

LESIONS

CN III

• Diabetic occulopathy– External ophthalmoplegia– Pupils are spared

• Compressive lesions– Aneurysms –PCA,Pcom,SCA– Tumors– Outer fibres-pupillary paralysis

• Inflammation-CST, Tolosa Hunt• Demyelination• Infections-TB

• External ophthalmoplegia– All EOM except LR and SO are affected– Eye turned down and out– Diplopia becoming better on looking to paralysed

side• Ptosis- LPS• Lateral gaze palsy – Medial longitudinal

fasciculus• Parasympathetic loss- – Mydriasis– Cycloplegia

CN IV

• Aneurysm of internal carotid• Cavernous sinus thrombosis

• Diplopia-on looking down• Medial deviation of affected eye• Head tilted towards the normal side

CN VI

• Medial deviation of eye• Diplopia –becoming worse on looking to the

paralysed side

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