MLF lesion and nystagmus

[MudPhud20XX]

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According to Kaplan neuro, left MLF lesion which leads to internuclear ophthalmoplegia results in:
Lt. eye cannot look right with intact convergence.

1. So why would you still have intact convergence when there is a Lt MLF lesion which is still not going to the adduction which requires for the convergence right? What am I missing here?

It also says with Lt. MLF lesion, Rt. eye exhibits nystagmus.
2. Can anyone explain the Rt eye nystagmus?

Many thanks in advance.

 

[ChessMaster3000]

According to Kaplan neuro, left MLF lesion which leads to internuclear ophthalmoplegia results in:
Lt. eye cannot look right with intact convergence.

1. So why would you still have intact convergence when there is a Lt MLF lesion which is still not going to the adduction which requires for the convergence right? What am I missing here?

It also says with Lt. MLF lesion, Rt. eye exhibits nystagmus.
2. Can anyone explain the Rt eye nystagmus?

Many thanks in advance.

Good topics
1--the convergence is NOT mediated by MLF. MLF only mediates horizontal conjugate gaze when activated by the contralateral abducens nucleus. I can't remember which nucleus does convergence but I believe it is in the CN3 nucleus family (if you look this up please feel free to share!)

2--this may not be 100% correct, but when the right abducens nucleus fires to activates the right lateral rectus, as you know it also sends a signal via left MLF to the left medial rectus. when the brain realizes (somehow) that the medial rectus is not moving, the abducens nucleus hits overdrive (on the right side that is) and fires multiple times to try and get the MLF-left medial rectus going. This has the untoward effect of hyperactivating the lateral rectus, and that causes nystagmus on the right.

hope that helps

 

[MudPhud20XX]

Thanks ChestMaster, you are correct for #1. Kaplan does say as below:
Left MLF: Left eye cannot look right; convergence is intact (this is how to distinguish an INO (internuclear ophthalmoplegia) from oculomotor, which is CN3 lesion)

So you are saying the nystagmus is just a result of hyperactivation of the lateral rectus right? So it has nothing to do with VOR? Correct?

I also recall that cerebellum lesion can also trigger nystagmus and this is b/c cerebellum is actually involved in the VOR pathway, correct?

 

[Fatalis]

A cn3 lesion would present with a down and out motion of the eye [cn3 knocked out therefore cn4 and 6 take over]
also ChessMaster3000 is correct; convergence is done by cn3 and not by MLF.