21 Third Nerve Palsy

Where is the oculomotor (CN III) nuclear complex located, and what is its general organization?

Located in the midbrain, anterior to the cerebral aqueduct. Each CN III-innervated muscle is controlled by a specific subnucleus within the oculomotor nuclear complex.

Which CN III subnuclei have unique innervation patterns that are commonly tested?

• Central caudal subnucleus → innervates both levator palpebrae superioris muscles (bilateral eyelid elevation).

• Superior rectus subnucleus → innervates the contralateral superior rectus muscle.

• Edinger-Westphal nucleus → supplies preganglionic parasympathetic fibers to the pupil.

What is the function of the Edinger-Westphal nucleus?

Provides preganglionic cholinergic (parasympathetic) fibers that ultimately control pupillary constriction (miosis) and accommodation.

Describe the course of the oculomotor fascicles through the brainstem.

CN III fascicles travel ventrally through the midbrain, pass through the red nucleus, traverse the medial cerebral peduncle, and exit into the interpeduncular fossa.

What are the classic clinical findings of a CN III palsy?

• Diplopia (may produce exotropia, hypertropia, or hypotropia)

• Ptosis due to levator palpebrae superioris weakness

• Pupil enlargement (or sometimes normal pupil)

• Periorbital/brow pain

Why does a CN III palsy cause ptosis?

CN III innervates the levator palpebrae superioris (LPS). Weakness of this muscle leads to drooping of the upper eyelid (ptosis).

What pupil findings may occur in a CN III palsy?

The pupil may be dilated ("blown pupil") if parasympathetic fibers are involved, or normal if those fibers are spared.

What symptom of a CN III palsy should raise concern for an aneurysm?

Severe or persistent pain around the eye and brow, especially when associated with pupillary involvement, is more suggestive of an aneurysmal compression than an ischemic palsy.

What are the hallmark ocular motor findings of a CN III palsy?

• Ptosis from levator palpebrae superioris weakness

• Eye positioned "down and out" (unopposed LR and SO muscles)

• Limitations of adduction, elevation, and depression

• Often associated with diplopia

Why is the eye classically "down and out" in a complete CN III palsy?

CN III normally innervates most extraocular muscles. When CN III is lost, the intact lateral rectus (CN VI) abducts the eye and the superior oblique (CN IV) depresses/intorts the eye, resulting in a down-and-out position.

Which eye movements are impaired in a CN III palsy?

Deficits can include:

• Adduction (medial rectus)

• Elevation (superior rectus, inferior oblique)

• Depression (inferior rectus)
  Severity depends on whether the palsy is complete or partial.

What physical exam findings may be seen in a complete CN III palsy?

• Ptosis

• Dilated pupil (if parasympathetic fibers involved)

• Down-and-out eye position

• Compensatory raised eyebrow/wrinkled forehead due to frontalis muscle activation attempting to overcome ptosis

What is the difference between internal and external ophthalmoplegia in CN III palsy?

• Internal ophthalmoplegia: Parasympathetic (pupil) involvement → abnormal pupil function.

• External ophthalmoplegia: Somatic motor involvement → extraocular muscle weakness and ptosis.

What is the difference between a complete and incomplete CN III palsy?

• Complete: All CN III functions affected (EOMs, eyelid, ± pupil).

• Incomplete: Only some CN III functions are affected.

Why is the distinction between pupil-involving and pupil-sparing CN III palsy clinically important?

• Pupil-involving CN III palsy → concerning for a compressive lesion (especially posterior communicating artery aneurysm).

• Pupil-sparing CN III palsy → more commonly due to microvascular ischemia (e.g., diabetes, hypertension).

How does CN III divide, and what structures are supplied by each division?

CN III divides in the anterior cavernous sinus/orbit into:

• Superior division:

  • Superior rectus (SR)

  • Levator palpebrae superioris (LPS)

• Inferior division:

  • Medial rectus (MR)

  • Inferior rectus (IR)

  • Inferior oblique (IO)

  • Parasympathetic fibers to the pupil (via ciliary ganglion)

What is the "Rule of the Pupil" in CN III palsy?

A pupil-sparing CN III palsy is usually due to ischemia (microvascular infarction) rather than compression and can often be observed clinically.

When does the Rule of the Pupil apply?

When there is:

• No pupil involvement (normal pupil)

• Complete external ophthalmoplegia (EOM weakness and ptosis)

Why are ischemic CN III palsies often pupil-sparing?

The ischemic injury preferentially affects the central portion of the nerve, while the superficial parasympathetic fibers controlling the pupil are relatively spared.

What are the major causes of a CN III palsy?

• Ischemia (most common overall)

• Aneurysm/compression (especially PCom aneurysm)

• Tumor/compression

• Trauma

• Multiple sclerosis (rare)

What aneurysm is classically associated with a compressive CN III palsy?

A posterior communicating artery (PCom) aneurysm, usually near its junction with the internal carotid artery.

Why is recognition of an aneurysmal CN III palsy critical?

An enlarging aneurysm may rupture, causing a potentially fatal subarachnoid hemorrhage.

What CN III palsy pattern should make you worry about compression from an aneurysm?

• Painful CN III palsy

• Pupil involvement (dilated pupil)

• Progressive symptoms

What is the most common cause of a CN III palsy?

Ischemic infarction of the oculomotor nerve (microvascular disease).

What pupil findings are typical of an ischemic CN III palsy?

Usually pupil-sparing or only minimal pupil involvement.

Where can ischemic CN III lesions occur?

Along various portions of the nerve, including:

• Brainstem

• Subarachnoid space

• Cavernous sinus (intracavernous portion)

What is the expected prognosis for an ischemic CN III palsy?

Most patients show spontaneous recovery over several months.

What are the two classic midbrain syndromes associated with CN III palsy?

• Benedikt syndrome = CN III palsy + contralateral ataxia/involuntary movements (± hemiplegia) due to red nucleus involvement.

• Weber syndrome = CN III palsy + contralateral hemiplegia due to cerebral peduncle involvement.

How can you distinguish Benedikt syndrome from Weber syndrome?

• Benedikt = Red nucleus → movement disorder/ataxia.

• Weber = Cerebral peduncle → corticospinal tract involvement causing hemiplegia.

A patient has ipsilateral CN III palsy and contralateral hemiplegia. What syndrome is present and where is the lesion?

Weber syndrome; lesion involves the cerebral peduncle in the midbrain.

A patient has ipsilateral CN III palsy with contralateral ataxia and involuntary movements. What syndrome is present and where is the lesion?

Benedikt syndrome; lesion involves the red nucleus in the midbrain.

How can uncal (transtentorial) herniation cause a CN III palsy?

The uncus of the temporal lobe herniates across the tentorial edge and compresses CN III, producing a compressive third nerve palsy.

What findings suggest CN III palsy due to uncal herniation?

• Ipsilateral CN III palsy (often with a dilated pupil)

• Altered mental status/decreased consciousness

• Signs of increased intracranial pressure

Why is altered mental status an important clue in a compressive CN III palsy from herniation?

Uncal herniation reflects significant intracranial mass effect and brain compression, which commonly impairs consciousness in addition to causing CN III dysfunction.

What is aberrant regeneration (oculomotor synkinesis)?

Misdirected regrowth of CN III fibers following traumatic or chronic compressive CN III injury, causing abnormal linked eye, eyelid, and pupil movements.

Which types of CN III palsies are most likely to develop aberrant regeneration?

• Traumatic CN III palsy

• Chronic compressive CN III palsy

Not typical of ischemic CN III palsy.

What is the pseudo-Graefe sign?

Lid elevation during adduction or depression of the eye due to aberrant regeneration of CN III fibers.

What abnormal pupillary finding can occur with oculomotor synkinesis?

During adduction, the pupil may show miosis despite being poorly reactive to light, reflecting miswiring of parasympathetic fibers.

What are the two types of aberrant regeneration in CN III palsy?

• Secondary aberrant regeneration → develops after an acute CN III palsy.

• Primary aberrant regeneration → occurs without a preceding acute CN III paresis.

Why is primary aberrant regeneration clinically important?

It may be the first sign of an expanding lesion in or around the cavernous sinus, even without a prior history of acute CN III palsy.

How does the presence of aberrant regeneration help distinguish ischemic from compressive CN III palsy?

If aberrant regeneration is present, the cause is unlikely to be ischemic and a compressive or traumatic etiology should be suspected.

What is the typical recovery pattern of an ischemic CN III palsy?

Recovery generally occurs over several months, and aberrant regeneration is usually absent.

What is the first step in evaluating a patient with a CN III palsy?

Determine whether it is an isolated CN III palsy or whether other cranial nerves/neurologic systems are involved.

What findings suggest the palsy is not an isolated CN III lesion?

Involvement of:

• CN IV (trochlear nerve)

• CN VI (abducens nerve)

• Abnormal corneal/facial sensation (CN V)

• Facial weakness or other neurologic deficits

These findings suggest a broader process (e.g., cavernous sinus lesion).

How can CN IV involvement be detected when evaluating a CN III palsy?

Look for intorsion of the eye on downgaze, which suggests trochlear nerve involvement.

After determining a CN III palsy is isolated, what is the next management question?

Ask whether it fulfills all criteria for an ischemic (microvascular) CN III palsy or whether there is evidence suggesting another cause requiring further workup.

What historical features support a diagnosis of ischemic (microvascular) CN III palsy?

• Age > 50 years

• Vascular risk factors (HTN, diabetes, etc.)

• Sudden onset diplopia and ptosis

• No significant hemifacial pain, hemicranial pain, paresthesias, or numbness

What type of patient is most likely to develop an ischemic CN III palsy?

An older patient (>50 years) with microvascular risk factors such as diabetes mellitus or hypertension who develops an acute pupil-sparing CN III palsy.

What examination findings are required for a classic ischemic CN III palsy?

• Complete external ophthalmoplegia + ptosis

• Normal pupil

• Normal CN IV function

• Normal CN VI function

• No aberrant regeneration

• Otherwise normal neuro-ophthalmic examination

What does “complete external ophthalmoplegia” mean in CN III palsy?

Ptosis plus impairment of all CN III-mediated extraocular movements while sparing the pupil.

Which cranial nerves should remain normal in a classic ischemic CN III palsy?

• CN IV (normal intorsion on downgaze in abduction)

• CN VI (full abduction)

Involvement of other ocular motor nerves suggests a non-ischemic process.

What additional examination findings should be normal in an ischemic CN III palsy?

• Normal visual acuity (VA)

• Normal visual fields (VF)

• No RAPD

• Normal eye appearance

• Normal corneal sensation (CN V)

• Normal facial nerve function (CN VII)

Under what circumstances does the “Rule of the Pupil” reliably apply?

Only when the CN III palsy is otherwise complete, meaning:

• Ptosis present

• All CN III extraocular muscle functions affected

• Pupil spared

Why does a pupil-sparing CN III palsy not completely exclude an aneurysm?

Early compressive lesions (especially aneurysms) may initially spare the pupil, with pupillary involvement developing later.

Why should acute CN III palsies generally undergo urgent neuroimaging?

Because:

• The Rule of the Pupil is not perfectly reliable.

• Some aneurysmal compressions may initially spare the pupil.

• Missing a compressive aneurysm can be life-threatening.

How should a presumed ischemic CN III palsy be followed initially?

• Observe closely for 5-7 days

• Monitor for new pupil involvement or progression

• Assess vascular risk factors (blood pressure, blood glucose)

• Re-evaluate in approximately 6 weeks

What systemic condition should always be considered in older patients with a presumed ischemic CN III palsy?

Giant Cell Arteritis (GCA) should be ruled out if signs or symptoms are present.

When should MRI be obtained in a presumed ischemic CN III palsy being observed clinically?

If:

• There is no recovery by ~3 months, or

• New neurologic symptoms develop

What is the expected course of a true ischemic CN III palsy?

Gradual spontaneous recovery over several months. Lack of improvement should prompt reconsideration of the diagnosis.

What are the major conditions that can mimic or be mistaken for a CN III palsy?

• Myasthenia gravis

• Giant cell arteritis (GCA)

• Thyroid eye disease

• Orbital trauma

• Orbital inflammation

Why is myasthenia gravis an important mimic of CN III palsy?

Myasthenia can cause ptosis and ophthalmoplegia that resemble CN III dysfunction, often with variable or fluctuating symptoms.

When should GCA be suspected in a patient presenting with diplopia or ocular motor palsy?

When diplopia occurs with:

• Headache

• Temporal symptoms

• Jaw claudication

• Scalp tenderness

• Other symptoms suggestive of GCA

These patients warrant ESR and CRP testing.

What is the relationship between GCA and diplopia?

Approximately 12% of patients with GCA develop diplopia, making GCA an important consideration in older patients with acute ocular motor symptoms.