What is the most likely symptom of right oculomotor (third cranial) nerve palsy?

Right Oculomotor Nerve Palsy: Clinical Manifestations

The most likely symptom of right oculomotor (third cranial) nerve palsy is diplopia looking to the left (option B).

Pathophysiology of Oculomotor Nerve Palsy

Oculomotor nerve (CN III) palsy affects the function of several extraocular muscles and other structures:

• Innervates four of six extraocular muscles: medial rectus, superior rectus, inferior rectus, and inferior oblique
• Controls levator palpebrae superioris (eyelid elevation)
• Carries parasympathetic fibers to the pupillary sphincter

Clinical Presentation

When the right oculomotor nerve is damaged, the following occurs:

1. Eye Position: The eye deviates "down and out" due to unopposed action of the lateral rectus (CN VI) and superior oblique (CN IV) muscles 1

   • The eye is typically abducted and infraducted
   • This contradicts option C (eye looking down), which is incomplete - the eye looks down AND out

2. Diplopia:

   • Patients experience horizontal and vertical diplopia
   • Diplopia is most pronounced when looking in the direction opposite to the affected side 1
   • With right CN III palsy, diplopia is worst when looking to the left, as the affected right eye cannot adduct properly due to medial rectus weakness

3. Pupillary Involvement:

   • In pupil-involving CN III palsy: pupil is dilated (mydriasis), not constricted 1
   • This directly contradicts option A (miosis), which describes pupillary constriction

4. Additional Findings:

   • Ptosis (drooping of the upper eyelid) due to levator palpebrae dysfunction
   • Impaired accommodation
   • Inability to elevate, depress, or adduct the eye

Differential Diagnosis

Option D (conjugate gaze palsy) is incorrect because:

• Conjugate gaze palsy refers to the inability to move both eyes together in the same direction
• This results from lesions in the brain stem or higher centers controlling eye movements
• Oculomotor nerve palsy causes an incomitant strabismus where one eye deviates from the other 1

Clinical Significance

The pattern of oculomotor nerve involvement has important diagnostic implications:

• Pupil-involving CN III palsy: Suggests compression, often by aneurysm (particularly posterior communicating artery aneurysm) 1, 2

  • Requires urgent neuroimaging with CTA or MRA
  • 90% of patients with isolated third nerve dysfunction caused by aneurysm have anisocoria 2

• Pupil-sparing CN III palsy: Often indicates microvascular ischemia, especially in patients with vascular risk factors like diabetes and hypertension 3

  • May resolve spontaneously within 3-6 months

Management Considerations

• Neuroimaging (MRI with contrast and vascular imaging) is indicated for most cases of acute oculomotor nerve palsy 1
• Patients with vascular risk factors and complete recovery within 3 months typically have ischemic etiology 3
• Surgical management may be required for persistent strabismus to improve ocular alignment 1

In conclusion, diplopia looking to the left is the most characteristic symptom of right oculomotor nerve palsy, as the affected right eye cannot adduct properly when the patient attempts left gaze.