Cranial nerve anatomy and pathology

Oculomotor CN III

Oculomotor CN III palsy △ 

• Down-and-out eye medial rectus paralysis
• Ptosis levator palpebrae superioris paralysis
• Mydriasis loss of parasympathetic to sphincter pupillae

Remember that the oculomotor CN III facilitates pupil constriction via its parasympathetic innervation to the sphincter pupillae through the ciliary ganglion.

Causes of oculomotor CN III nerve palsy: 

• False localising sign due to uncal herniation in high ICP due to an expanding extradural haematoma
• + headache = Posterior communicating artery aneurysm 
• + contralateral hemiplegia = Weber’s syndrome 
• Medical: Amyloid, multiple sclerosis, temporal arteritis, SLE, diabetes

Oculomotor 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, mild ptosis occurs due to loss of sympathetic innervation to the superior tarsal muscle, smooth muscle that helps hold the eyelids elevated.

• A patient develops a down-and-out-eye after neck surgery. Which accompanying feature is likely?
  • Exophthalmos
  • Mydriasis
  • Nystagmus
• 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 
  • Right extradural haematoma
  • Acute subdural
  • Subarachnoid haemorrhage