CNVI Palsy

Question 1

Which cranial nerve is predisposed to lesions along its course from brainstem to orbit?

Answer 1

CNVI palsy (6th nerve)

Question 2

What is the motor function of CNVI?

Answer 2

Has a purely somatic motor function providing innervation to the lateral rectus muscle.

Question 3

Where does the CNVI arise from?

Answer 3

Arises from the abducens nucleus in the pons of the brainstem.

Question 4

Where does CNVI exit from the brainstem?

Answer 4

It exits the brainstem at the junction of the pons and the medulla (pontomedullary junction).

Question 5

Where does CNVI go after exiting the brainstem?

Answer 5

The 6th nerve then enters subarachnoid space, piercing the dura mater covering the basilar part of the occipital bone and travelling to Dorello’s canal.

It leaves Dorello’s canal and then enters the cavernous sinus at the petrous temporal bone.

Travelling through the cavernous sinus it enters the bony orbit through the SOF, passing through the common tendinous ring.

It then terminates by innervating the LR.

Question 6

What 5 parts can the course of CNVI be divided into?

Answer 6

It can be divided into five parts:
1. Nucleus and intraparenchymal portion
2. Cisternal portion
3. Dorello canal portion
4. Cavernous sinus portion
5. Orbital portion

Question 7

What are common aetiologies in CNVI palsy?

Answer 7

• Abducens nerve palsy can be caused by structural pathology leading to downward pressure on the brainstem which can stretch the nerve from its origin at the junction of the pons and medulla.
• Diabetic neuropathy (possibly the most common cause)
• Thrombophlebitis of the cavernous sinus (though often likely not isolated to the 6th)
• Fractures of the petrous temporal bone
• Aneurysms of the intracavernous carotid artery
• Mass lesion
• MS (demyelination)
• Infections (meningitis)
• Rare causes of isolated sixth nerve damage include Wernicke–Korsakoff syndrome and Tolosa–Hunt syndrome. Wernicke–Korsakoff syndrome is caused by thiamine deficiency, classically due to alcoholism. The characteristic ocular abnormalities are nystagmus and lateral rectus weakness. Tolosa-Hunt syndrome is an idiopathic granulomatous disease that causes painful oculomotor (especially sixth nerve) palsies

Question 8

How can the aetiologies of CNVI be split into?

Answer 8

• Neoplasm:
  Chordoma, meningioma, nasopharyngeal carcinoma, pituitary adenoma
• Trauma : head injury, skull fracture
• Systemic disorders:
  Diabetes, hypertension – most commonly seen cause of acquired VIN palsy in A and E
• Other vascular causes:
  Aneurysm, cerebrovascular insults, bleed from an arteriovenous malformation in the midbrain
• Associated neurological disorder: Migraine, demyelinating disease (MS), raised intracranial pressure
• Iatrogenic:
  Post lumbar puncture, post spinal or epidural anaesthesia - rare
• Other:
  Idiopathic, inflammatory, infections e.g. post viral (rare)

Question 9

What do nuclear lesions of the abducents nucleus cause?

Answer 9

Nuclear lesions (abducens nucleus) do not produce an isolated 6th nerve palsy, but rather a horizontal gaze palsy that affects both eyes simultaneously. The abducens nucleus contains two types of cells: motor neurons that control the lateral rectus muscle on the same side, and interneurons that cross the midline and connect to the contralateral oculomotor nucleus (which controls the medial rectus muscle of the opposite eye). Lesions of the abducens nucleus and the medial longitudinal fasciculus (MLF) produce observable sixth nerve problems, most notably internuclear ophthalmoplegia (INO).

Question 10

What type of CNVI palsy in children are more likely to have spontaneous recovery?

Answer 10

Post-Viral CNVI

Question 11

What are the muscle actions of the abducens nerve?

Answer 11

The abducens nerve is a purely somatic motor nerve, it has no sensory function. It innervates the lateral rectus muscle, an extraocular muscles of the eye, which is responsible for the abduction of the eyes on the same (ipsilateral) side.

The abducens nerve is also secondarily involved in innervation of the contralateral rectus muscle by way of the longitudinal fasciculus so that both eyes move laterally in a coordinated manner.

Question 12

Is the abducens nerve ipsilateral or contralateral?

Answer 12

Ipsilateral abduction of the eye but secondarily involved in innervation of the contralateral rectus muscle by way of the longitudinal fasciculus so that both eyes move laterally in a coordinated manner.

Question 13

When is diplopia worse in CNVI palsy?

Answer 13

The diplopia is worse on attempts at looking laterally. The misalignment may make an affected eye point inward (esotropia). Can cause asthenopia

Patients usually present with binocular horizontal diplopia (double vision producing a side-by-side image with both eyes open), worse in the distance, an esotropia in primary gaze. Patients also may present with a head-turn to maintain binocularity and binocular fusion to minimize diplopia

A patient with a partial 6th may complain only of diplopia in the distance and worse to the affected side whereas those with a total 6th will complain of constant diplopia or may have marked AHP.

Question 14

What are isolated CNVI palsies in children assumed to be until proven otherwise?

Answer 14

Brain tumours

Question 15

What is the most common acquired ocular motor nerve palsy (OMNP)?

Answer 15

Abducens CNVI palsy

Question 16

What is diplopia like in a partial/incomplete CNVI palsy?

Answer 16

A patient with a partial 6th may complain only of diplopia in the distance and worse to the affected side whereas those with a total 6th will complain of constant diplopia or may have marked AHP.

Question 17

How long is recovery in CNVI palsy?

Answer 17

Usually a gradual recovery between 6-12 months.

Question 18

What will you see differently between a complete and partial CNVI palsy during OMs?

Answer 18

• In a palsy there is increased movement of the affected eye when tested monocularly i.e. on abduction
• In a total CNVI palsy the eye will not be able to abduct beyond the mid-line so abduction will not improve monocularly i.e. no further improvement, eye will still remain in the middle or slightly eso

If you try to test a patient with a 6th nerve palsy abduction monocularly i.e. by covering up their other eye to see if that eye does move any further, you will have a slightly improved abduction if you were to test monocularly. On the other hand, on a patient with a total 6th nerve palsy, where they got absolutely no abduction of that eye, whether you test them with both eyes open or monocularly, there will be no improvement to the movement of that eye at all = no further abduction of that eye.

Question 19

What are the 3 main features of CNVI palsy?

Answer 19

1. Esodeviation greater in the distance – may be binocular at near fixation with an esophoria, but this depends on the severity of the paresis they have
2. AHP of face turn to affected side – greater when looking in the distance
3. Field of BSV displaced to the unaffected/opposite side

Question 20

What are possible AHP’s in CNVI palsies?

Answer 20

Rotating their head to allow the eye to look sideways. Ipsilateral head turn.

Question 21

What specific tests can you use for testing CNVI palsy?

Answer 21

MRI scans are essential for diagnosing 6th nerve palsies, especially in people <50yo. It gives greater resolution of the orbits, cavernous sinus, posterior fossa and cranial nerves.
Patients have slow saccadic velocity in side gaze which can help with diagnoses.

Question 22

What will we see in a CT if someone has a CNVI palsy?

Answer 22

Limited abduction on the ipsilateral side. If muscle sequalae is complete there will be limited abduction on the contralateral side.

Small ET at near, moderate ET at distance with diplopia

Because the LR works better for distance and elevation, and because it tends to work better for elevation, you will expect these patients to have an A pattern as the degree of eso is greater in elevation compared to the depression for a 6th nerve palsy, but in clinic they are actually seen to have a V pattern instead

Question 23

Is the ET smaller at near or distance in CNVI palsy?

Answer 23

Small ET at near, moderate ET at distance with diplopia

Question 24

What pattern do we expect in CNVI palsy during OMs?

Answer 24

Because the LR works better for distance and elevation, and because it tends to work better for elevation, you will expect these patients to have an A pattern as the degree of eso is greater in elevation compared to the depression for a 6th nerve palsy, BUT in clinic they are actually seen to have a V pattern instead

Question 25

What is the muscle sequelae in Left CNVI palsy?

Answer 25

u/a LLR, o/a RMR, o/a LMR, u/a RLR

Question 26

What is the field of BSV like in a CNVI palsy?

Answer 26

Centrally placed field of BSV (depending on esotropia) and BSV when looking in away from affected side (depending on muscle sequalae development)

Question 27

Look at Hess Charts AND Ansons & Davies

Question 28

What should we ask in the case of a CNVI palsy?

Answer 28

• The diplopia may have changed since the onset - ? better/worse
  If the diplopia is recent onset then A&E referral needs to be made. If present for a few days then ask if progressed or worsened or if it’s the same as at onset
• Does the patient adopt an AHP to achieve BSV in pp?
• Details of the general health and age of patient may indicate the cause e.g.diabetes, hypertension
• May be a history of head injury
• It is important to establish the presence of any pre-existing squint as this will affect the management