OcuLearning › Neuro

Third Nerve Palsy

CRITICAL — Neuro

Symptoms

  • binocular diplopia: double vision that resolves when either eye is covered
  • ptosis: drooping of the upper eyelid, which may completely cover the eye
  • ocular pain: may be severe (aneurysm) or dull/aching (ischemic)
  • blurred vision: if the pupil is involved, causing loss of accommodation

Clinical Findings

Motility & position

  • "down and out": the eye is deviated outward (exotropia) and downward (hypotropia)
  • restricted gaze: inability to look up, down, or inward (adduction)
  • ptosis: levator palpebrae weakness

The pupil rule

  • pupil-involving: dilated, fixed, or sluggish; suggests compression (Aneurysm)
  • pupil-sparing: normal reactive pupil; favors microvascular ischemia (Diabetes/HTN)

Aberrant regeneration

  • synkinetic movements: e.g., the eyelid lifts when the patient tries to look down or in
  • significance: indicates a chronic compressive or traumatic cause; NEVER microvascular

Etiology

Compressive (surgical emergency)

  • p-com aneurysm: posterior communicating artery aneurysm compressing the nerve
  • uncal herniation: brain swelling forcing the temporal lobe onto the nerve
  • tumors: cavernous sinus masses or pituitary apoplexy

Ischemic (medical management)

  • microvascular: diabetes, hypertension, or GCA (vasculitis of small vessels)

Other

  • trauma, varicella zoster, or ophthalmoplegic migraine (in children)

Red Flags

  • painful pupil-involving palsy: a P-Com aneurysm until proven otherwise; requires emergent CTA/MRA
  • partial third nerve palsy: any "incomplete" palsy is high-risk and requires full neuroimaging
  • progression: a palsy that worsens over several days is highly suggestive of a growing tumor or aneurysm
  • age ≥55: check ESR/CRP to rule out Giant Cell Arteritis even if the pupil is spared

Investigations

Urgent imaging

  • cta/mra: emergent head and neck imaging to look for aneurysmal dilation
  • mri brain/orbits: with gadolinium to visualize the nerve and rule out mass lesions

Ocular exam

  • pupillary assessment: must be done carefully in a dark room; check for subtle anisocoria
  • motility: check all 9 cardinal positions for "hidden" weakness in other nerves

Lab work

  • vasculopathic screen: blood pressure, HbA1c, and lipid panel
  • gca screen: ESR, CRP, and platelets in patients over 55

Treatment

Etiological

  • aneurysm: neurosurgical clipping or endovascular coiling
  • ischemic: optimization of blood pressure and glucose control; aspirin

Symptomatic

  • diplopia: occlusion patching or Fresnel prisms (once the deviation is stable)
  • surgery: strabismus surgery or ptosis repair if the palsy does not resolve after 6 months

Clinical Pearl

Complete CN III palsy with pupil involvement: posterior communicating artery aneurysm until proven otherwise — emergent CTA/MRA. Pupil-sparing CN III: likely ischemic (diabetes, HTN). Aberrant regeneration does NOT occur after ischemic CN III.

Differential Diagnosis

  • • myasthenia gravis (the "great masquerader"; can mimic any eye movement deficit)
  • • thyroid eye disease (restrictive, usually causes proptosis or lid retraction)
  • • internuclear ophthalmoplegia (ino) (lesion in the MLF; affects adduction only)
  • • parinaud syndrome (dorsal midbrain lesion; affects upgaze and pupils)

Related Neuro Conditions