Oculomotor Nerve Palsy (OSCE Guide)


Oculomotor Nerve Palsy (OSCE Guide)

Oculomotor nerve (CN III) palsy is a common short case at the neurology station and it is usually evident with a distant because of unilateral complete ptosis.

As the name implies, the oculomotor nerve supplies the majority of the extraocular muscles apart from Lateral Rectus (supplied by VI nerve) and Superior Oblique (supplied by IV nerve). In addition, it supplies Levator Palpebrae Superioris muscle of upper eyelid and Sphincter Pupillae muscles which is responsible for pupillary constriction. This innervation is vital for understanding the clinical signs in III CN palsy, namely ptosis (often complete), dilated pupil and ophthalmoplegia.

There are two clinical entities, “Medical” and “Surgical” third nerve palsiesIn a case of Surgical third nerve palsy, you are expected to do an extended examination to clinically locate the site of nerve compression to obtain full allocated marks.



1. Unilateral Ptosis (Often Complete Ptosis)

This is obvious! You have to manually and gently elevate the upper eyelid when you carry on your examination to look for ophthalmoplegia.

2. Divergent Strabismus

Due to Medial Rectus palsy and unopposed action of Lateral Rectus supplied by the VI nerve.

In fact, the eye will be “Down & Out” because the Superior Oblique (supplied by IV nerve) is unantagonized by the paralyzed Superior Rectus, Inferior Rectus and Inferior Oblique muscles.

3. Ophthalmoplegia

Impaired adduction of eye due to paralysis of Medial Rectus.

4. Mydriasis (Dilated Pupil)

Due to the involvement of the parasympathetic nerve supply from the Edinger-Westphal nucleus. These fibers are located superficially, thus in external compression, they are affected first, making the pupil dilated.

  • Surgical Third Nerve palsy – When Pupil is affected (dilated)
  • Medical Third Nerve palsy – When pupil is spared.
5. Loss of Accommodation Reflex

Due to the involvement of the Ciliary muscle.


This is especially important when the pupil is affected (Surgical Third Nerve palsy) which would indicate an external compression of the Oculomotor nerve somewhere along its cause. You should do a targeted neurological examination to find out the possible location of the nerve.

1. At Midbrain – Contralateral Hemiplegia (Weber Syndrome)

Due to the involvement of Corticospinal tracts usually due to a Brainstem infarction.
Sometimes associated with tremor and involuntary movements (Benedikt Syndrome) when the red nucleus of the midbrain is involved.

2. After emerging from Midbrain – Isolated Surgical Third Nerve Palsy

It is seen without the involvement of other adjacent nerves. Here, the nerve is in close relationship with the posterior communicating artery and can be compressed with aneurysms of the above-mentioned artery.

3. At Cavernous Sinus – Associated IV & VI Nerve Palsies and Sensory Loss in V1 & V2.

At the cavernous sinus the oculomotor nerve is closely related to Trochlear and Abducens nerves and ophthalmic and Maxillary branches of Trigeminal nerves. Those nerves are affected together in case of Cavernous sinus thrombosis.

4. At Orbit – Associated IV & VI Nerve Palsies and Sensory Loss in V1 (NOT V2).

At the orbit, the Maxillary branch of the Trigeminal nerve is not in close relationship with the Oculomotor nerve, hence unaffected. It can occur in intraorbital cellulitis.


This patient has right complete ptosis and a divergent strabismus at neutral position. The right eye movements are impaired especially the adduction and it is fixed in down & out position. The right pupil is fixed and dilated. The accommodation reflex of the right eye is lost.

On my extended limited neurological examination, there are no associated IV or VI nerve palsies on the right side. There is no sensory deficit over the areas supplied by the maxillary and ophthalmic divisions of the Trigeminal nerve. The patient is having left hemiplegia. There are no hand tremors or involuntary movements.

So, my diagnosis is right oculomotor nerve palsy secondary to brainstem (midbrain) stroke. So, this is a case of Weber Syndrome.


1. From where the Oculomotor nerve originate?

It arises from the anterior aspect of the midbrain and originates from two nuclei. • Oculomotor nucleus – Originates at the level of the superior colliculus. • Edinger-Westphal nucleus – supplies parasympathetic fibres via the ciliary ganglion.

2. Describe the anatomical pathway of the Oculomotor nerve?

It originates at the midbrain at the level of superior colliculus —> passes between superior cerebellar and posterior cerebral arteries —> pierces the dura matter anterior and lateral to the posterior clinoid process —> transverses the cavernous sinus —> divides into two branches (Superior and inferior) at the orbit.

3. What are the structures supplied by the Oculomotor nerve?

◦ Superior branch supplies the superior rectus and levator palpebrae superioris. ◦ Inferior branch divides into three divisions and supplies to medial rectus, inferior rectus, inferior oblique and ciliary ganglion (Sphincter pupillae & Ciliary muscle)

4. What are the eponymous syndromes associated with oculomotor nerve palsy?

1. Weber Syndrome – Third nerve palsy + Contralateral Hemiplegia 2. Benedikt Syndrome – Third nerve palsy + Contralateral Involuntary Movements

5. What are the causes of oculomotor nerve palsy?

1. Brainstem Tumours 2. Brainstem Strokes (Ischemic/ Haemorrhagic) 3. Brainstem Demyelination 4. Cavernous Sinus Thrombosis 5. Tentorial Herniation 6. Posterior Communicating Artery Aneurysms 7. Superior Orbital Fissure Lesions 8. Subacute Meningitis 9. Mononeuritis Multiplex (in Diabetes)



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