Oculomotor Nerve Palsy

Basics

Description

  • There are 6 muscles responsible for controlling eye movement, which are innervated by 3 cranial nerves (CN):
    • CN III (oculomotor nerve) innervates 4 of the 6 eye muscles, as well as the eyelid and pupil:
      • Medial rectus:
        • Adduction – moves eye medially toward nose
      • Superior rectus:
        • Elevation – moves eye upward
        • Intorsion – rotates top of eye toward nose
        • Slight adduction
      • Inferior rectus:
        • Depression – moves eye inferiorly
        • Extorsion – rotates top of eye away from nose
        • Slight adduction
      • Inferior oblique:
        • Extorsion – rotates top of eye away from nose
        • Slight elevation and abduction
      • Levator palpebrae superioris:
        • Raises eyelid
      • Iris sphincter pupillae and ciliary muscles
        • Constricts pupil (miosis)
        • Innervated by parasympathetic fibers of CN III
    • CN IV (trochlear nerve) innervates the superior oblique muscle:
      • Depression and slight abduction (lateral rotation)
      • Intorsion – rotates the top of the eye toward the nose
    • CN VI (abducens nerve) innervates the lateral rectus:
      • Moves eye laterally (abduction)
      • Longest intracranial nerve and most commonly affected
  • Oculomotor nerve palsy results from damage to CN III, IV, or VI, resulting in abnormal eye movements, lid ptosis, and/or changes to the pupil:
    • Complete oculomotor nerve palsy:
      • Eye “down and out,” ptosis, pupil dilated
      • Most often caused by compressive lesions
        • 95–97% of compressive lesions involve the pupil
        • Parasympathetic fibers sit peripherally in CN III
        • Mydriasis is often 1st symptom of compression
      • Pupil-sparing complete oculomotor nerve palsy:
        • Almost always ischemic from microvascular disease due to underlying diabetes, hypertension, and/or hyperlipidemia
        • Ischemic injuries often spare the pupil because the outer parasympathetics are not affected
        • Often benign and fully resolves in 3 mo
    • Incomplete oculomotor nerve palsy (more common):
      • Partial loss of function of CN III
      • Most often caused by ischemia of vasa vasorum
  • Pathophysiology of oculomotor nerve palsy:
    • Midbrain lesions of the oculomotor nucleus lead to bilateral CN III palsy (ischemia of the basilar artery)
    • Lesions leaving CN III nucleus are often associated with other neurologic findings such as hemiplegia or ataxia (Weber syndrome, Benedikt syndrome)
    • Lesions in the subarachnoid space cause complete palsy with pupil involvement (compressive aneurysms) or complete palsy with pupil sparing (ischemia due to risk factors)
    • Lesions in the cavernous sinus and superior orbital fissure can cause isolated CN III palsy, but often are associated with CN IV, CN VI, and maxillary division of CN V dysfunctions
    • Lesions in the orbit are associated with visual loss (CN II), ophthalmoplegia (CN III, IV, VI), and proptosis and caused by trauma, mass, inflammation:
      • Incomplete CN III palsy originates here as the nerve divides into superior and inferior divisions

Etiology

  • Acquired etiologies:
    • Vascular disorders:
      • Diabetes mellitus (DM), hypertension, atherosclerosis, microvascular ischemia
      • Aneurysm (esp posterior communicating artery)
      • Arteriovenous malformation (AVM)
      • Intracranial hemorrhage (nonaneurysmal)
      • Cerebral vascular accident
      • Cavernous sinus thrombosis
    • Infectious:
      • Meningitis, syphilis, herpes zoster, brain abscess, tuberculosis, SARS-CoV-2
    • Inflammatory:
      • Sarcoidosis, giant cell arteritis, systemic lupus erythematous
    • Neoplastic:
      • Intracranial tumor, pituitary tumor, orbital tumor, leukemia, schwannoma
    • Degenerative:
      • Myasthenia gravis, Guillain–Barré
    • Trauma:
      • Head injury, recent or remote, trauma during childbirth
    • Migraine headache
    • Iatrogenic:
      • Chemotherapy, radiation therapy
    • Idiopathic:
      • Idiopathic intracranial hypertension (IIH)
    • Congenital etiologies are often unknown but some with familial tendency:
      • Aplasia or hypoplasia of CN III

Pediatric Considerations

  • Trauma is the most common cause of acquired oculomotor nerve palsies:
    • Includes birth trauma
  • Can also be seen in neurofibromatosis, ophthalmologic migraine with recurrent CN III palsy, and spontaneous intracranial hypotension

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