Oculomotor CN III
- ✨ M ➡ The oculomotor nerve supplies medial rectus, superior rectus, inferior rectus, inferior oblique, and levator palpebrae superioris.
- 🌟 Pa ➡ It also carries parasympathetic fibres via the ciliary ganglion to the sphincter pupillae.
Features of CN III palsy
- Down-and-out eye due to paralysis of the medial rectus
- Ptosis due to paralysis of the levator palpebrae superioris
- Mydriasis due to failure of CN III's parasympathetic innervation of the sphincter pupillae via the ciliary ganglion to facilitate pupil constriction
Causes of CN III nerve palsy:
- False localising sign due to uncal herniation in high ICP due to an expanding extradural haematoma
- ⏰ This one comes up a lot! Head injury + Dilated pupil = Extradural haematoma with uncal herniation compressing the parasympathethetic fibres of CN III
- + headache = Posterior communicating artery aneurysm
- + contralateral hemiplegia = Weber’s syndrome
- Medical: Amyloid, multiple sclerosis, temporal arteritis, SLE, diabetes
CN III Palsy VS Horner's Syndrome
👁️ In Oculomotor CN III palsy, severe ptosis occurs due to loss of motor innervation to the levator palpebrae superioris, skeletal muscle that lifts the eyelids.
👁️ In Horner’s syndrome, mild ptosis occurs due to loss of sympathetic innervation to the superior tarsal muscle, smooth muscle that helps hold the eyelids elevated.
Pupillary light reflex pathway
- Retina detects light
- Optic CN II — afferent limb
- Bilateral pretectal nucleus in midbrain
- Bilateral Edinger-Westphal nuclei
- Parasympathetic fibres of Oculomotor CN III — efferent limb
- Ciliary ganglion
- Short ciliary nerves
- Sphincter pupillae muscle
- Direct and consensual pupillary constriction
📚 CT shows bilateral extradural haematomas. On the operating table, the anaesthetist notes one dilated pupil. Which side should the craniotomy first be performed on?
Ipsilateral side of the dilated pupil
👁️ A dilated pupil suggests compression of the ipsilateral oculomotor CN III, usually due to uncal herniation from mass effect on that side. In bilateral EDH, although both sides have bleeds, the first surgical priority is the side showing signs of herniation, i.e., the side of the dilated pupil. Prompt decompression can be life-saving.
👩⚕️ Go through these MCQs in detail and make sure you understand and learn the answers including why the incorrect options are incorrect!
A patient develops a down-and-out-eye after neck surgery. Which accompanying feature is likely?
- ((Exophthalmos::Typical of Graves’ orbitopathy due to inflammatory enlargement of extraocular muscles and orbital fat, not from an isolated oculomotor CN III palsy))
- ((Mydriasis::☑️ CN III carries parasympathetic fibres to the sphincter pupillae via the ciliary ganglion; damage prevents constriction and produces an ipsilateral dilated pupil))
- ((Nystagmus::Usually reflects vestibular or cerebellar pathology involving the vestibulocochlear CN VIII, not selective medial rectus and levator palpebrae weakness))
- ((Loss of corneal reflex::Corneal reflex depends on V1 (sensory) and VII (motor to orbicularis oculi); CN III palsy does not abolish it))
👁️🗨️ The eye’s deviation laterally, ‘down and out’, arises from medial rectus paralysis via oculomotor CN III injury.
CN III palsy △ down-and-out eye, ptosis, mydriasis.
A patient sustains head trauma and presents to A&E conscious. He is noted to have a right, fixed and dilated pupil. A few hours later, he deteriorates and dies. What is the likely cause?
- ((Left extradural haematoma::A left-sided extradural would compress the left oculomotor nerve, producing a left dilated pupil, not right-sided pupillary dilation))
- ((Right extradural haematoma::☑️ Oculomotor CN III nerve palsy: Eye is deviated ‘down and out, ptosis, dilated pupil - can suggest uncle herniation in extradural haematomas))
- ((Acute subdural haematoma::Tearing of the bridging veins leads to a more gradual decline))
- ((Subarachnoid haemorrhage::Presents with thunderclap headache and meningism))
👀 Remember that the oculomotor CN III facilitates pupil constriction via its parasympathetic innervation to the sphincter pupillae through the ciliary ganglion.
🤕 Extradural haematomas expand rapidly and focally, so they’re the classic cause of uncal (transtentorial) herniation. As the uncus herniates, it compresses the oculomotor CN III against the sharp edge of the tentorium. Parasympathetic fibres lie superficially, so they’re hit first resulting in an ipsilateral dilated, non-reactive pupil (“blown pupil”), followed by ptosis and a “down-and-out” eye.
After a head injury, a patient develops a fixed, dilated pupil. What is the cause?
- ((Unopposed sympathetic fibres::☑️ Loss of parasympathetic input from CN III leaves sympathetic dilation unopposed, producing a fixed, dilated pupil))
- ((Unopposed parasympathetic fibres:: Would cause pupillary constriction (miosis), not dilation))
- ((Direct iris sphincter muscle spasm:: Would produce a constricted pupil rather than a fixed, dilated one))
- ((Horner syndrome:: Causes miosis, ptosis, and anhidrosis — not a dilated pupil))
♦️ Compression of CN III (parasympathetic) leaves sympathetic supply unopposed → pupil dilation (mydriasis)
A patient has a head injury with initial GCS 15, then deteriorates with a blown pupil. Diagnosis?
- ((Hydrocephalus:: Causes raised intracranial pressure and typically bilateral signs, not an isolated blown pupil))
- ((Transtentorial (uncal) herniation::☑️ Medial temporal lobe herniates through the tentorium, compressing CN III))
- ((Diffuse axonal injury:: Produces reduced consciousness but does not cause an isolated unilateral dilated pupil))
- ((Subdural haematoma without herniation:: May cause deterioration but a blown pupil implies CN III compression from herniation))