VN - Gaze Disorders - Week 8 Flashcards
True or false
Lesions in the abducens nucleus results in similar pathology to lesions of the abducens nerve
Explain why this is the case using horizontal gazes as an example, noting what the abducens nuclei does and where the contraleteral eye receives eye movement command from.
False they do not have similar consequences
The abducens nucleus sends the final eye movement command to the lateral rectus
Interneurons in the nucleus are the source of the MLF,l anf project to the contralateral 3rd nerve nucleus to drive the contralateral medial rectus
This means that all commands for horizontal eye movement must pass through the abducens nucleus
Compare what a lesion of the abducens nucleus will result in vs a lesion of the abducens nerve, if they are different consequences or the same.
An abducens nucleus lesion will result in complete ipsilateral loss of conjugate movement (i.e. total loss of leftward or rightward gaze in both eyes) of all types.
An abducens nerve lesion only affects one lateral rectus muscle
Is there a loss of only some or all conjugate ipsilateral eye movement with an abducens nucleus lesion? Note whether there is sparing of vertical movement and vergence. Note what clinical finding you would expect with undamaged hemifield and also on contralateral gaze. What is this often seen with?
Loss of all conjugate ipsilateral eye movement
Sparing of vergence and vertical eye movement
In undamaged hemifield, ipsilateral movements present but slow
Hprizontal gaze evoked nystagmus on contralateral gaze
Often seen with ipsilateral facial palsy
Lesions in what region of the brain may at first glance seem like a lesion of the abducens nucleus? Describe why this is the case.
PPRF
It sends saccadic pulses to be integrated via the prepositus and medial vestibular nucleus
A lesion here eliminates both pulse and step, since they are combined in the abducens nucleus
What deficit would you expect of a PPRF lesion? What about on contralateral gaze (2)?
Loss of saccades towards the side of the lesion
Acutely, contralateral gaze deviation
Gaze-evoked nystagmus on contralateral gaze
Describe whether a PPRF lesion can affect pursuit and the VOR and why (noting what feature of the PPRF that makes it so).
The two can either be impaired or preserved
Dogma that these lesions only affect saccades has changed with the discovery that the PPRF contains multiple cell types as well as fibres of passage running through it
Describe why pursuit is sometimes, but not always, affected by PPRF lesions.
Both EBN and saccade/pursuit neurons have been identified in the PPRF. A lesion here can therefore impair both, including just one or the other.
What will damage to the PPRF result in? What about damage to the abducens nucleus? What about damage to the MLF at its origin? What would be the only remaining eye movement? What is this known as and what kind of lesion can cause it (2)?
PPRF - eliminates ipsilateral saccades
Abducens nucleus - eliminates ipsilateral eye movements
MLF - causes the contralateral eye to adduct
-only remaining eye movement is abduction of the contralateral eye
One and a half syndrome
-a lesion affecting the PPRF and MLF or abducens nucleus and MLF
What is there a potential for with one and a half syndrome and why?
Facial nerve involvement if the abducens nucleus is involved
-facial nerve passes over the abducens nucleus
What lesion can give rise to internuclear ophthalmoplegia?
MLF lesion
What will an acute lesion to the FEF result in (isilateral and contralateral) and how is it different to brainstem lesions? Make a note on whether these two are permanent or resolve.
FEF - may cause ipsilateral gaze deviation and contralateral saccade abolition
Unlike brainstem lesions, which may cause permanent deficits, these tend to resolve, leaving more subtle defects in saccades.
-especially non-reflexive ones
What may lesions of the parietal lobe lead to and especially lesions on which side? Explain what this condition is (2).
Hemispatial neglect, especially lesions on the right side
The patient is to a greater or lesser extent aware of contralateral space
Thus they dont perceive stimuli which should normally attract their gaze, especially if it competes with stimuli in their intact field
Describe what the MLF does, making note of whether or not it crosses the midline. What will damage to the MLF result in? Describe what happens with vergence and why. What is this condition known as?
It crosses the midline to innervate the contralateral medial rectus by driving motor neurons in the contralateral 3rd nerve nucleus in the midbrain
Damage will result in impaired adduction in the affected eye
-vergence is not affected because it has a separate input
This is internuclear ophthalmoplegia
Can internuclear ophthalmoplegia be unilateral or is it always bilateral?
Can be unilateral
What generally occurs to the adducting eye with internuclear ophthalmoplegia? Keeping this in mind, will seeing full range of motion as a clinical finding rule out internuclear ophthalmoplegia?
It will eventually reach its target
-full range of motion will not rule out internuclear ophthalmoplegia
