Third Cranial Nerve Palsy (Oculomotor Nerve Palsy)

Third cranial nerve palsy affects the oculomotor nerve, which controls most eye movements, eyelid elevation, and pupil constriction. This causes droopy eyelid (ptosis), double vision, and a characteristic eye position. When the pupil is also affected, an aneurysm must be urgently ruled out.

What the Third Nerve Controls

The third cranial nerve (oculomotor nerve) controls:

• Eyelid elevation (levator muscle)
• Most eye movements (up, down, inward)
• Pupil constriction (parasympathetic fibers)
• Focusing (accommodation)

When the nerve is damaged, these functions are impaired.

Symptoms

Eye Appearance and Movement

• Complete ptosis—eyelid droops completely
• Eye turned outward and downward—"down and out" position
• Limited eye movement—can't look up, down, or inward
• When lid is lifted: double vision in most directions

Pupil Findings

Pupil-involving (dilated pupil):

• Larger pupil on affected side
• Pupil doesn't react to light
• Concerning for aneurysm—emergency

Pupil-sparing:

• Pupil normal size and reactive
• More likely microvascular cause
• Better prognosis

Causes

With Pupil Involvement (Emergency)

Other causes of pupil-involving third nerve palsy:

• Uncal herniation (brain swelling)
• Cavernous sinus lesions
• Compressive tumors
• Trauma

Pupil-Sparing (More Reassuring)

Microvascular/ischemic

• Most common in older adults
• Associated with diabetes, hypertension
• Typically pupil-sparing
• Usually recovers in 2-3 months
• "Microvascular cranial nerve palsy"

Other causes:

• Inflammation
• Infection
• Demyelination

Other Causes

• Trauma
• Tumors
• Inflammation (orbital, cavernous sinus)
• Post-surgical
• Giant cell arteritis (in older patients)

Pupil Involvement: Why It Matters

The pupil-controlling fibers run on the outside of the third nerve. They're affected by:

• Compression (aneurysm, tumor)—fibers are superficial, affected early
• Often spared in microvascular ischemia (affects center of nerve first)

This is why pupil involvement demands emergency evaluation for aneurysm.

However, exceptions exist—some aneurysms can initially spare the pupil, and some microvascular palsies can involve it. Clinical judgment is important.

Evaluation

Urgent Questions

1. Is the pupil involved?
2. Any severe headache?
3. Any other neurological symptoms?

Eye Examination

• Degree of ptosis
• Eye position and movements
• Pupil size and reactivity
• Other cranial nerves

Imaging

For pupil-involving third nerve palsy:

• Emergency CTA or MRA—looking for aneurysm
• If negative but suspicion high: catheter angiography

For pupil-sparing third nerve palsy in older adults with vascular risk factors:

• May observe carefully
• MRI/MRA often still performed
• Closer observation of pupil over first week

Blood Tests

• Blood sugar, HbA1c
• ESR, CRP (if GCA suspected in older patients)
• Other tests based on suspected cause

Treatment

For Aneurysm

• Neurosurgical or endovascular treatment
• Clipping or coiling to prevent rupture
• Urgent referral to neurosurgery/neurointerventional

For Microvascular

• Control underlying risk factors (diabetes, hypertension)
• Observation—usually recovers in 2-3 months
• Follow-up to confirm improvement
• If no improvement by 3 months, reconsider diagnosis

Symptomatic Management

For ptosis:

• May help diplopia by covering affected eye
• Ptosis crutch on glasses if needed long-term
• Surgery only if stable and persistent

For double vision:

• Eye patch when needed (alternating eyes)
• Fresnel prism if stable pattern
• Strabismus surgery only after stable for months

Recovery

Microvascular Third Nerve Palsy

• Most begin improving within weeks
• Full recovery typically by 3 months
• May have aberrant regeneration (rare in microvascular)

Compressive Causes

• Depends on treatment of underlying cause
• May have incomplete recovery
• May develop aberrant regeneration (misdirected nerve growth)

Aberrant Regeneration

Signs that nerve fibers regrew to wrong targets:

• Lid raises when looking down or inward
• Pupil constricts with eye movement
• Suggests prior compression—not typical of microvascular

Frequently Asked Questions

Why is pupil involvement so important?

The pupil fibers are on the outside of the nerve and are first affected by compression (like from an aneurysm). A dilated, unreactive pupil with third nerve palsy could mean a life-threatening aneurysm is pressing on the nerve.

If my pupil is normal, do I still need imaging?

Often yes, especially if you're younger, don't have typical vascular risk factors, or have other unusual features. The decision depends on your individual situation. Even "pupil-sparing" cases are sometimes imaged, especially initially.

How long until I recover?

Microvascular third nerve palsies typically improve over 2-3 months. If recovery doesn't begin within this timeframe, your doctor may want to repeat imaging or reconsider the diagnosis.

Can I drive with a third nerve palsy?

The ptosis and double vision make driving unsafe. Once recovered, or if symptoms are adequately managed (patch, prism), you may be able to drive—discuss with your doctor.

Will this happen again?

Microvascular palsies can recur, especially if risk factors (diabetes, hypertension) aren't controlled. However, most people don't have recurrence. Keeping blood sugar and blood pressure controlled reduces risk.

What is "aberrant regeneration"?

When the nerve regrows, fibers sometimes connect to wrong targets—for example, the lid may lift when looking down. This suggests the cause was compressive, not microvascular. It requires evaluation if discovered.

References