6th nerve palsy Flashcards
what is the name of the 6th nerve
abducens
describe the anatomy of the abducens nerve and what it can be at risk of because of this
- The nerve has a long tortuous course
- It is predisposed to lesions all along its course from brainstem to the orbit
list all 7 different aetiologies of a 6th nerve palsy in adults
- 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)
what is the prognosis of a 6th nerve palsy caused by a vascular aetiology
gradual recovery between 6-12 months
list the 5 most common aetiologies of a 6th nerve palsy in children
most common:
- congenital
- intra cranial tumours
- trauma
- idiopathic
- post viral
list the 3 main features of a 6th nerve palsy
- 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
- AHP of face turn to affected side – greater when looking in the distance
- Field of BSV displaced to the unaffected/opposite side
list the 4 stages of muscle sequelae of a 6th nerve palsy
- Under action of affected lateral rectus
- Over action/contracture of contralateral medial rectus
- Over action/contracture of ipsilateral medial rectus
- Secondary inhibitional palsy of contralateral lateral rectus
list 7 things you can do to investigate a 6th nerve palsy
- History
- Cover test
- Ocular movements
- Hess chart
- Field of BSV
- Assessment of binocular vision
- Measurement of the deviation
what will the history of a recent onset 6th nerve palsy tell
Clear, concise time of onset of palsy as main presenting symptom is horizontal diplopia binocularly, greater in distance than near fixation
what may a patient with a partial 6th nerve palsy complain of
only of diplopia in the distance and worse to the affected side
what may a patient with a total 6th nerve palsy complain of
constant diplopia or a marked AHP
list 5 things you will want to ask/find out about an adult with a 6th nerve palsy
- The diplopia may have changed since the onset - ? better/worse
- 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
why should you ask about if the diplopia has changed and how long have they had it for
- because if the diplopia is of recent onset, then you must refer the patient to A and E on the same day
- but if the diplopia has been present for a few days later, then you do need to ask if the diplopia has progressed or gotten worse, or is it the same as when they first got/noticed it
list 3 characteristics of a 6th nerve palsy
The LR muscle abducts the eye and has increased function in the distance and on elevation, so a 6th nerve palsy causes:
- An esotropia results - may be esophoric for near
- Larger angle in the distance
- Should be larger on elevation (A) but in practice often larger on depression (V)
what will you see in partial 6th nerve palsy and compare this to a total 6th nerve palsy
some movement of that eye
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.
But 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
what will you see in a cover test when investigating a 6th nerve palsy and why
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 esot 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
what type of AHP will you see with a 6th nerve palsy and what may you see with a bilateral 6th nerve palsy
- May adopt an AHP of face turn to affected side to achieve BSV in pp
- If the VIN palsy is bilateral – usually a very large esotropia and in order to fix with either eye in the pp the patient may have to adopt an AHP of face turn to either side so px can fix with each eye individually
name 2 possible outcomes of ocular movements when investigating a 6th nerve palsy
- In a palsy there is increased movement of the affected eye when tested monocularly i.e. on abduction
- In a total VIN 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
what 2 things does a Hess chart provide when investigating a 6th nerve palsy
- Pictorial record
- Repeatable under the same conditions, of the degree of the palsy and the development of muscle sequelae
what can’t you tell on a Hess chart that shows a full muscle sequelae
which eye was the effected eye
what does a field of BSV do and when is it done
- Plots the size and area of single vision
- Plotted at each visit and the results compared
where will the field of BSV be plotted for a px with a unilateral 6th nerve palsy and how must the field of BSV be carried out to obtain valid results
- the field of BSV will be deviated to the opposite side of the deviated eye
- for the patients who adopt a AHP, you need to make sure the px is looking with their head straight
explain what tests are done in assessment of binocular vision and why
- CT: done for DV and NV, with and without AHP. It is done without the AHP to identify if they are straight and binocular at near fixation or if they are manifest and it is also done in the distance to prove that they have a manifest esotropia.
It is done with the AHP to make sure that with the AHP, they are corrected and single and to prove any binocular functions in the presence of the AHP - In constant deviations the eso must be corrected first with prisms or on the Synoptophore to measure the angle of deviation and and to see if it gets rid of the double vision at the corrected angle
- field of BSV
- Hess chart
- Full history: px will tell you when they see single and when they see double
are all done
when may you find that during assessment of binocular function, there may be no demonstrable BV
due to:
- pre-existing squint where px may not complain of double vision
- significant head trauma as it is possible to lose BV/fusion ability following head injury = px will have complete total double vision, so even at the corrected angle of deviation
if you measure a 20 dioptre prism esotropia and put up a 20 dioptre prism base out, the patient will still have double vision as they lost their fusion as a result of the trauma
