MLF syndrome
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My understanding is that the CN VI nucleus is sending fibers through the MLF to the contralateral eye's CN III to coordinate with its medial rectus muscle.
If the MLF is interrupted, then the abducens nucleus will still be able to abduct the ipsilateral eye, but the signals through the MLF onward will no longer reach the contralateral eye's medial rectus muscle.
Now, when the abducens nucleus fires, the ipsilateral eye (whose tracts are intact) will abduct. The contralateral eye will not be able to adduct (no signals reach the medial rectus muscle) and will stay still.
So now, the eye ipsilateral to the abducens nucleus has already abducted and you will see a nystagmus as it tries to correct itself.
If the MLF is interrupted, then the abducens nucleus will still be able to abduct the ipsilateral eye, but the signals through the MLF onward will no longer reach the contralateral eye's medial rectus muscle.
Now, when the abducens nucleus fires, the ipsilateral eye (whose tracts are intact) will abduct. The contralateral eye will not be able to adduct (no signals reach the medial rectus muscle) and will stay still.
So now, the eye ipsilateral to the abducens nucleus has already abducted and you will see a nystagmus as it tries to correct itself.
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I think the above poster does an admirable job at explaining the problems. However, let me try to clarify a bit about MLF.
Studying focal lesions can help understand the disease associated with it but only if you have a solid understanding of the physiology involved. MLF can be complicated because you need to know specific words associated with it (nystagmus) as well as its physiology.
First, definitions: Nystagmus- is the correcting mechansim that is initiated by the brain in response to a pathology. Thus, nystagmus by itself is not pathological. It is a correction. So what is the pathologic movement? The pathologic movement is the slow drift of the eye in a direction. Nystagmus, however, is a fast correction.
So when can pathological slow drift take place? Usually due to two situations. (the second one I will have you answer).
One situation has the eye moving in a direction due to the intact fibers overpowering towards one side- due to a pathologic loss of function controlling (or countering) the other side. Example: The eyes are kept looking forward (known as parallel gaze) by both the lateral and medial rectus. That is, due to tonic constant low grade stimulus of both muscles, they counteract one another and allow you to look straight ahead (instead of either side). Thus, if the medial rectus is damaged- your eye will slow drift to the lateral aspect due to the relative overactivity of the lateral rectus muscle (- in reality the lateral rectus muscle may be acting normally- but since medial is not working it appears as if it is overactive relative to the lost medial rectus). When the brain realizes this (usually by comparing the positioning of the eye with respect to the head- and its delayed recognition that no voluntary nor involuntary stimulus activated the slow drift) it attempts to correct for it by rapidly moving the eye back to its original position- a nystagmus. Do not confuse nystagmus (a correction) vs. a saccade- also a rapid eye movement but constantly happening.
Now lets look at MLF.
MLF has two functions- one is voluntary and the other is involuntary.
While the above poster does a good job explaining the function of MLF- his diagram and his explanation do not go hand in hand. His diagram shows voluntary control via MLF while his explanation seems to describe involuntary control.
Let us look at involuntary control first.
How does this happen?
Here is an outline:
head turn to one side
vestibular nucleus of head turn activated
sends signal to the MLF- to the ipsilateral CNIII
and to the contralateral CNVI
causing eye to look to the opposite side of head turn.
Full explanation:
It is via the vestibular nucleus. The vestibular nucleus (not to confuse with cochlear) receives input from CN VIII to recognize head turning and control the eyes as a result. When you are turning your head - you naturally want to maintain parallel gaze. If no correction were made, then - your head would turn to the right (for example) but your eyes would remain in their original location and be stuck in position with no parallel gaze- what would be the use of that?.
So to correct this, the CNIII is stimulated when your head turns to the right. This is by recognizing the change in fluid motion in the horizontal, anterior and posterior planes in the semicircular ducts (how this happens is a story unto itself- if you are interested I will explain). Once the fluid moves in a direction the CNVIII is stimulated on that side of the head. So if you turn to the right your CNVIII nerve is activated. It sends signals to the cochlear nucleus. The cochlear nucleus then sends a signal to the IPSILATERAL MLF.
This MLF now sends signal to the CNIII on the same side (same side as the head turning so in this case the right MLF sends signals to the right CNIII). The CNIII stimulates the medial rectus to look medially and to the left. Likewise, the MLF sends a contralateral fiber to the LEFT abducens nucleus. This causes your eye to abduct to the left.
Thus, your head turns to the right and to maintain parallel gaze- your eyes turn to the left (via abduction on the opposite eye and adduction on the medial eye).
Now, the diagram by the above poster is NOT for the involuntary (described above) movement. It is instead, a description of the voluntary gaze.
What is voluntary gaze? It is controlled by the FEF or frontal eye field nucleus. Where do you think the frontal EYE field nucleus would be? If you thought- hmm its in the occipital lobe because that is where the vision center is you are incorrect. The FEF is located in front of the precentral gyrus.
The precentral gyrus contains the motor control system (you know with the humunculus) and in front of this is the planning center. These two are located on the frontal lobe.
When you DECIDE to look to the right your FEF will send a signal fron the LEFT FEF (think- motor actions that cause your right arm to be raised originate from the left lobe) to the contralateral side to the RIGHT PPRF.
The right PPRF is located in the SAME area as the abducens - in fact 99% of the time a damage to the abducens or the PPRF will result in damage to the other one (can you think of another nucleus in this region that would also be damaged- its a cranial nerve nucleus). The abducens nucleus is most often located at the level of the lower pons. The abducens nucleus then causes the same lateral rectus muscle on the right side to abduct to the right.
The PPRF then sends a contralateral signal to the LEFT MLF nucleus causing the CNIII to adduct to the right.
Questions you should be able to answer if you fully understand the above:
Now let us look at what happens to lesions in voluntary and involuntary control.
if the left MLF nucleus is damaged the right eye will be able to still abduct to the right- however, the left eye won't abe able to adduct. Thus you will have a nystagmus of the contralateral MLF eye because the abducted eye will attempt to revert gaze back to the center (remember no nystagmus is possible on the left eye since your medial rectus muscle cant work).
For this same lesion will you abe able to look to the left?
Suppose you have a lesion to the FEF on the left side can you abduct to the right? The answer is no. Would you ever be able to abduct to the right? (think about this one)
How would a patient present with a FEF lesion on the left? Why?
How would a patient present with a PPRF lesion on the right? Why?
Okay, lemme take a shot at this (please correct me if i am wrong on anything!);
first question: Yes you would be able to look to the left because the abducens nerve would still be working to abduct the left eye, and then the right MLF would adduct the right eye. Second question: If you turn your head to the left, the ipilateral MLF gets stimulated (in this case, the left side). Then that causes the left eye to adduct (from the chochlear nucleus innervation of the left MLF) and the right eye to abduct (through a contralateral fiber from the left MLF). You can bypass the affected FEF this way. Third question: So FEF lesion on the left, patient wont be able to abduct to the right and adduct to the right (left eye wont adduct to the right), due to the pathology you already described before. Fourth Question: PPRF lesion on the right, same features as an FEF lesion on the left (cant abduct to the right, and adduct to the right (left eye wont adduct to the right).
first question: Yes you would be able to look to the left because the abducens nerve would still be working to abduct the left eye, and then the right MLF would adduct the right eye. Second question: If you turn your head to the left, the ipilateral MLF gets stimulated (in this case, the left side). Then that causes the left eye to adduct (from the chochlear nucleus innervation of the left MLF) and the right eye to abduct (through a contralateral fiber from the left MLF). You can bypass the affected FEF this way. Third question: So FEF lesion on the left, patient wont be able to abduct to the right and adduct to the right (left eye wont adduct to the right), due to the pathology you already described before. Fourth Question: PPRF lesion on the right, same features as an FEF lesion on the left (cant abduct to the right, and adduct to the right (left eye wont adduct to the right).
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Thanks for the help.
How would a patient present with a FEF lesion on the left? Why?
The patient would be unable to look/ saccades to the right.
Would you ever be able to abduct to the right? (think about this one)
Maybe if the patient was following an object right as opposed to just performing a saccade (smooth pursuit).
How would a patient present with a PPRF lesion on the right? Why?
Right eye can't abduct and left eye can't adduct with lateral gaze to right. left eye can still converge on object approaching.
Heres something that still bothers me:
An individual with left MLF syndrome is asked to look to the right.
Explanation:
The right PPRF/Abduc nuc is still intact and the right eye can abduct, but the MLF going to the left eye is lesioned so this eye is unable to adduct for right parallel gaze. Would the right eye have nystagmus to the left because of a pathologic slow phase movement to the right? So in other words my error may have been not separating the voluntary movement of the clinician have the pt look to the right vs... the nystagmus (slow phase right - pathologic/fast phase left - right FEF correction)
So if the above patient had a right FEF lesion along with his left MLF lesion would the right eye nystagmus be absent (purely hypothetical)?
How would a patient present with a FEF lesion on the left? Why?
The patient would be unable to look/ saccades to the right.
Would you ever be able to abduct to the right? (think about this one)
Maybe if the patient was following an object right as opposed to just performing a saccade (smooth pursuit).
How would a patient present with a PPRF lesion on the right? Why?
Right eye can't abduct and left eye can't adduct with lateral gaze to right. left eye can still converge on object approaching.
Heres something that still bothers me:
An individual with left MLF syndrome is asked to look to the right.
Explanation:
The right PPRF/Abduc nuc is still intact and the right eye can abduct, but the MLF going to the left eye is lesioned so this eye is unable to adduct for right parallel gaze. Would the right eye have nystagmus to the left because of a pathologic slow phase movement to the right? So in other words my error may have been not separating the voluntary movement of the clinician have the pt look to the right vs... the nystagmus (slow phase right - pathologic/fast phase left - right FEF correction)
So if the above patient had a right FEF lesion along with his left MLF lesion would the right eye nystagmus be absent (purely hypothetical)?
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You are correct on everything but the last two. You may be up in arms but it was a bit of a trick question. I asked how the patient would present- not if he could look in a certain direction. Due to his FEF lesion on the left his eyes would slowly deviate to the left- because the right FEF is functioning and active (baseline active) so since he can't balance that with a functioning left FEF he would end up looking to the left. The same with the fourth question- I think this is a legit question as they can show you images and ask you what is going on-
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Even I can't answer the question to the nystagmus without retracing my steps- and I am too tired to do that. I doubt this would be a test question.
You are correct about the rest though.
okay so the opposite side will compensate for the lesioned side? Is that what the concept is?
got it 🙂. Step 1 is next friday for me, if something like this gets asked, hopefully i can answer it correctly 🙂
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Awesome explanation, sanj.
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Ok well since no one answered my other questions I'll answer them for you
PPRF is located with abducens nucleus. This is located in lower pons near cranial 7. Sometimes 7 is also destroyed.
Also- parallel gaze- if you are looking at point A and you turn your head- the involuntary control allows your eyes to maintain fixed visualization of this point A. It does not mean that your eyes are looking straight ahead in a new location because your head moved- just in case you were confused.
i figured it was cranial nerve 7.
