1/ Is trying to understand diplopia making you crossed-eyed?!
When a patient has diplopia, do you know where on the brain MRI to focus your eyes?
Here’s a thread to help you understand horizontal diplopia, so you’ll always know where to look & never miss the finding in these patients
2/ Horizontal gaze is controlled by the medial & lateral rectus musculature. How the eye is positioned is determined by the balance of force between the 2 muscles. It’s like a tug-of-war: whichever side is pulling harder determines which side wins & where the eye will go
3/ But it’s a little more complicated bc we have 2 eyes, so they must move in unison to maintain focus. Eyes turn like a car turns. Wheels on either side must move in unison for the car to go in the desired direction. But that means the wheels must move opposite w/respect to midline
4/ Same w/the eyes. For conjugate movement, one eye must move away from midline & the other move toward it, like the wheels on the car axial. If both eyes move identically, then you get crossed eyed & stuck, just like how a car would be stuck if both wheels moved identically
5/ So for conjugate horizontal movement, we need the opposite muscles to win the tug of war on each side—on one side the medial rectus needs to win & on the other side, the lateral rectus needs to win—this way the eyes have conjugate movement in the same direction
6/ For this to happen, we need activate the medial rectus on one side. Medial rectus is innervated by CN3. On the other side, we want the medial rectus at rest & the lateral rectus to be activated. CN6 activates the lateral rectus. So we want CN3 on one side & CN6 on the other
7/ You can think of horizontal gaze like 2 people dancing the tango. Eye movement is like a dance. Going in the same direction on the dance floor actually requires opposite movement by the partners—as one goes back, the other moves forward to keep them together
8/ You can also remember the horizontal gaze pathway by remembering how we dance. The dance starts when you hear the music & you feel the rhythm—and you start to sway your hips a little. It’s the same w/the eyes. Movement starts in the hips—the hips of the brainstem that is!
9/ So where are the hips of the brainstem? Well, I always think that the facial colliculus in the brainstem looks like a baby butt. So if the facial colliculus is the butt of brainstem—the hips must be directly lateral to it
10/ Directly lateral to the facial colliculus—the brainstem “hips”--is the paramedian pontine reticular formation (PPRF). This is an amorphous structure extending along the pons. Portion next to the facial colliculus is called the paraabducens nucleus & controls horizontal gaze.
11/ PPRF does for gaze what your hips do for dancing. Your hips start to feel the rhythm & help you figure out what the dance rhythm is & how you should move. Same for the PPRF. PPRF feels the input from higher supratentorial structures & figures out how it should move the eyes
12/ Next, after you feel the rhythm in your hips, it’s time to move. You start by moving your butt in the direction of your first move. Same with horizontal gaze. Next stop in the pathway is at the butt of the brainstem—the facial colliculus, where CN6 is located
13/ Now when you try to take your first step—you must pull your partner w/you, otherwise you will get separated. So you use your long arm to wrap around your partner & pull them w/you. This is the medial longitudinal fasciculus (MLF). It pulls CN6’s partner, CN3, along for the dance.
14/ But for conjugate gaze, it’s the medial rectus on the OPPOSITE side that needs to contract. So we need to activate CN3 on the opposite side.
15/ So just how your arms wraps around the back of your partner, the MLF wraps around from the ipsilateral CN6 to the contralateral CN3
16/ Finally, as you pull, your partner opposite you follows your move, so that you dance in the same direction together. Same w/gaze. Opposite CN3 is activated by the MLF wrapped around it from CN6. Now the medial rectus muscle will help the contralateral eye to follow
17/ You can remember to look for CN3 in the midbrain bc the midbrain looks like the head of a tango partner—with a big bun and flowers
18/ In fact, you can even remember where CN3 is in the midbrain bc it is right around where the EYE would be in the tango dancer’s head
19/ So the horizontal gaze starts w/decoding of supratentorial signals in the PPRF, then activation of the ipsilateral CN6/lateral rectus, followed by a cross over to the other side to activate the contralateral CN3/medial rectus
20/ So now you know the components of the horizontal gaze pathway & where they are located on MRI.
You will now always know where to look when a patient has horizontal diplopia.
Hopefully, this thread has helped you to see double the knowledge!
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