Week 3 - 6th nerve (Abducens Nerve)

Question 1

What is the pathway of the 6th cranial nerve?

Answer 1

• Abducens nucleus in the pons
• Exits the brainstem at the junction between the pons and medulla
• Enters the subarachnoid space and runs along dorello’s canal
• Passes above the tip of the petrous temporal bone (can bemain cause for palsy)
• Enters the cavernous sinus where it runs along side the internal carotid artery (disease of artery can induce palsy)
• Enters the lateral aspect of the superior orbital fissure and passes anteriorly to innervate the lateral rectus

Question 2

Cavernous sinus differentiations of size of tumour?

Answer 2

• Total ophthalmoplegia, due to all 3 nerves being in the sinus.
• The 6th is lowest down, therefore size of tumour can be differentiated based on which muscles paralised
• Ophthalmic nerve is final one at bottom of sinus : loss of cornea

Question 3

Aetiology of VI Palsy’s over 50’s?

Answer 3

Most Common Aetiology in over 50’s is a Microvascular Incident due to:
• Diabetes
• Hypertension
• High Cholesterol

Other aetiologies include:
• Trauma
• Giant cell arthritis- emergency !!
• Stroke

Question 4

Aetiology of VI Palsy’s - under 50’s?

Answer 4

Most common cause in under 50’s:
• Multiple Sclerosis
• Raised Intracranial pressure due to space occupying lesions
• Idiopathic increased intracranial pressure
• Trauma
• Cavernous sinus mass
• Viral infection

Question 5

Aetiology of VI Palsy’s - Children?

Answer 5

Most common cause in under 165
• Congenital VI Palsy
• Space occupying lesions
• Trauma
• Idiopathic
• Viral Infection affecting ears, nose or throat
• Hydrocephalus

Question 6

What are the clinical features or 6th nerve palsy?

Answer 6

• Esotropia in primary position greater on distance fixation
• Esotropia increasing in size on attempted abduction of affected eye
• Limited abduction of affected eye
• Patient complaining of uncrossed diplopia (more so on distance fixation)
• Patient may have face turn to affected side
• Field of BS moved towards unaffected eye

Question 7

What is the differential diagnosis for 6th nerve palsy?

Answer 7

• Myasthenia Gravis
• Duane’s Retraction Syndrome - Type 1
• Infantile Esotropia
• Mobeius Syndrome - combined 6th and 7th nerve palsy
• Medial Wall Fracture

Question 8

Investigations In order to differentiate VI Palsy?

Answer 8

• Assess Far Distance (cover test and measurements)
• Lateral Version measurements to compare varying size of esotropia
• Smooth Pursuits will show limitation of abduction of affected eye
• Saccades may show hypometria of affected eye
• Lee’s screen will support smooth pursuit findings and allow comparison of palsy at future visits

Question 9

What are some further investigations to do for a 6th cranial nerve?

Answer 9

• Unless patient has high risks factors indicating microvascular incident then patient most likely requires neuro-imaging to determine cause of palsy
• Important to establish aetiology of palsy to rule out anything sinister such as space occupying lesion or multiple sclerosis

Question 10

How is 6th nerve palsy managed?

Answer 10

Orthoptic
• Prisms to join diplopia and restore binocular single vision
• If deviation too large to control with prisms then occluding one eye will alleviate patients symptoms of diplopia
• Allow 6 month for recovery
• If only partial recovery then Botox or surgery can be used to regain BSV in primary position