Third Nerve Pathway

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    • Cranial nerve that supplies the medial, superior, and inferior rectus muscles and the inferior oblique muscle to provide ipsilateral adduction, supraduction, infraduction; also supplies the levator palpebrae superioris to provide elevation of the upper lid, the ciliary muscle to provide accommodation, and the iris sphincter to provide pupil constriction
    • Five anatomic segments: nuclear, fascicular, subarachnoid, cavernous, and orbital
    • Signals originate in a cluster of paramedian midbrain nuclei
      • Somatic subnuclei mediate adduction (via the medial rectus), supraduction (via the superior rectus and inferior oblique), infraduction (via the inferior rectus), and upper lid elevation (via the levator palpebrae superioris)
      • Parasympathetic (Edinger-Westphal) subnucleus mediates accommodation (via the ciliary muscle) and pupil constriction (via the iris sphincter muscle)
    • Axons leave the third nerve nuclei and travel ventrolaterally, passing through the red nucleus, substantia nigra, and cerebral peduncle, gathering as a single nerve that exits the brainstem in the interpeduncular fossa
    • Nerve traverses the subarachnoid space between the posterior cerebral artery and the superior cerebral artery, passing inferolateral to the junction of the posterior communicating artery and the internal carotid artery
    • Nerve leaves the subarachnoid space to enter the cavernous sinus
      in its superior outer wall
    • In the cavernous sinus, the third nerve divides into a superior division,
      supplying the levator and superior rectus, and an inferior division,
      supplying the medial and inferior recti, the inferior oblique, the ciliary body, and the iris sphincter muscle
    • Nerve leaves the cavernous sinus to enter the orbit in the superior orbital fissure, entering the muscle cone
    • Parasympathetic portion of the third nerve splits off from its inferior division to synapse in the ciliary ganglion;
      short posterior ciliary nerves leave the ciliary ganglion to enter the posterior scleral wall, eventually supplying the iris sphincter for pupil constriction and the ciliary muscle for accommodation
    • Lesion of the nucleus
      • Produces ipsilateral deficits in adduction and infraduction
      • Produces a contralateral deficit in supraduction because of crossed innervation of the superior rectus muscle
      • Tip: lesions of the third nerve nucleus are extremely rare!
    • Lesion of the fascicles
      • Produces a complete or incomplete third nerve palsy
      • Tip: fascicular third nerve palsy is often accompanied by binocular vertical gaze deficits because of the proximity of the riMLF and INC, which mediate vertical gaze
      • Common cause: infarct of proximal posterior cerebral artery penetrating arteries to the midbrain and thalamus ("top of the basilar syndrome")
      • May be accompanied by an ipsilateral or contralateral limb tremor or ataxia because third nerve fascicles pass through the red nucleus and the superior cerebellar peduncle
      • May be accompanied by contralateral limb weakness because exiting fascicles pass near the cerebral peduncle, which carries corticopontine and corticospinal tracts ("Weber syndrome")
    • Lesion of the subarachnoid segment
      • Produces a complete or incomplete third nerve palsy
      • Tip: reversible ischemia—the most common cause of isolated third nerve palsy in adults--occurs here or in the cavernous segment
      • Tip: in ischemic lesions, the pupil may be spared because ischemia affects the core of the nerve while the pupillomotor axons lie on the periphery of the nerve
      • Nerve compression by aneurysm occurs at the junction of internal carotid and posterior communicating arteries,
        the apex of the basilar artery, and at the take-off of the superior cerebellar artery
      • Tip: in compressive third nerve palsies, including aneurysm and uncal herniation, a dilated and poorly constricting pupil often occurs because compression impacts the periphery of the nerve, where pupillomotor axons lie; however, compressive lesions may sometimes spare the pupil
      • Inflammation and intrinsic nerve tumors also affect this segment of the nerve
    • Lesion of the cavernous segment
      • Produces a complete or incomplete third nerve palsy, often in combination with other ocular motor palsies, trigeminopathy, or Horner syndrome
      • Common causes: pituitary tumors,
        intracavernous aneurysms, tumors, inflammations, and fistulas
      • Tip: cavernous lesions often damage the third nerve’s superior division
        more than its inferior division, so that ptosis and a supraduction deficit may occur selectively
    • Lesion of the orbital segment
      • Lesions confined to the orbit, even the posterior orbit, rarely produce deficits in third nerve function because the nerve enters the proximal portions of the extraocular muscles and levator; ductional deficits and ptosis from orbital lesions are more often the result of obstruction, weakening, or scarring of the extraocular muscles
      • Lesion of the ciliary ganglion or ciliary nerves
        • Produces ipsilateral mydriasis and impaired pupil constriction to light, but no ductional deficits or ptosis
        • Produces slow (“tonic”) pupil constriction to a near target and slow redilatation when gaze is redirected from near to far
        • Tonic pupil is usually idiopathic, but may be caused by orbital trauma (including surgery), inflammation, cyclocryotherapy, or dense retinal photocoagulation
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    Ocular Motor Cranial Nerve Pathways

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