VNG: Gaze Testing With Fixation

In this video, Dr. Barin explores the optimal way to perform gaze, spontaneous nystagmus, and static position tests. He then moves on to identify different types of nystagmus and their clinical significance. Lastly, he uses this information to differentiate between central and non-central types of nystagmus.

You can read the full transcript below.

Introduction

The title suggests that this is in VNG because that's where we, for the most part, assess gaze stabilization. But the concepts are actually applicable to any test where we are interested in assessing and figuring out the diagnostic meaning of nystagmus and gaze stabilization.

So just an overview of what we're going to talk about today. Healthy subjects, when they're asked to maintain steady gaze, they can do that in the presence or the absence of visual and vestibular stimuli. They do not have extraneous eye movements when they're asked to fixate. If they fail in that, for a variety of reasons that we will discuss later on, they will have gaze instability.

And the most common form of manifestation of this gaze instability is nystagmus, which we will be discussing the rest of this webinar. Gaze stabilization tests in the VNG test battery are designed to evaluate gaze stability and presence of nystagmus under different conditions.

The three most common conditions are first of all the gaze test, where we examine gaze stability in different off-center gaze positions. I know most of you who do gaze tests, you also include the center gaze. That's fine, but in my definition, that's not part of the gaze test. That's actually part of the spontaneous nystagmus test. But whichever way you're doing it it's fine. It's just that that's the official definition of what the gaze test is. It's off-center gaze positions.

In the spontaneous nystagmus test, we look at the gaze stability, primarily in the center gaze, in the presence and absence of visual fixation. So you normally have part of the test where the patient has a target to fixate on and part of the test where the target is removed. The patient is placed in complete darkness.

The next condition that we use to examine gaze stability is static position testing, in which we look at the gaze stability in different head positions. So in this two-part webinar, we examine the clinical significance of nystagmus both with fixation and without fixation. In part one, we are going to talk about the nystagmus during fixation.

Pathways involved in the VNG test battery

Just to review the pathways that are involved in the gaze stabilization... Okay, so on the left-hand side of the screen, you see the peripheral structures. These are the three semicircular canals, utricle, and saccule. The information from the peripheral structures is carried to the central vestibular pathways that are located in the vestibular nuclei through two branches of the vestibular nerve, superior and inferior.

At least until now, our goal with VNG testing was just to separate the peripheral and central abnormalities. So when we talk about peripheral abnormalities, we are talking about the canal and the two branches of the vestibular nerve. And then everything after that root entry zone of the vestibular nerve we consider central pathways.

The areas that are in red are the areas that are activated during gaze testing with fixation. So these will be the oculomotor nuclei muscles. The target is presented to the eyes and then that information is processed in the higher central nervous system and in the cerebellum.

And eventually, it is transmitted to the oculomotor nuclei, and the eye muscles then generate appropriate eye movements. This is true pretty much in any part of the gaze stabilization test that is performed with fixation.

There is one area that might be of some interest, and it's this connection of the cerebellum to the vestibular nuclei. You might ask what connection there is between the gaze stabilization with fixation and the vestibular pathways.

This is what causes suppression of vestibular nystagmus. This connection primarily blocks the information coming from the peripheral system at the vestibular nuclei. And this is what happens when you, for example during caloric testing, ask the patient to fixate. This pathway between the cerebellum and the vestibular nuclei gets activated and suppresses the vestibular nystagmus.

So as you can see, the boxes in red are primarily in the central portion of the vestibular pathways. So in general, if you have an abnormality in the gaze stabilization task with fixation, it usually means it's a central abnormality, which we're going to discuss today.

On the other hand, without fixation, now many of the pathways including the peripheral pathways here are involved in the gaze stabilization test. What that means is that primarily when we are testing the gaze, which is the spontaneous nystagmus and similar types of nystagmus in the gaze stabilization test, if you are testing without fixation, both peripheral and central pathways are involved in this test.

And generally, the outcome is a non-localizing finding. Oftentimes, with things like spontaneous nystagmus, you have other findings that will clarify the localization of spontaneous nystagmus. But if you're interpreting them on their own, these are non-localizing findings.

You should be aware that anytime you test the patient without fixation, you will need to have some alerting tasks. And the reason for that is that generally, the nystagmus can become suppressed if the eyes drift to one side and get stuck there. So unless you have cortical level involvement, you may not see the nystagmus and then the nystagmus can get suppressed without alerting.

So in tests without fixation, some level of alerting is usually necessary. So just to reiterate what I just said, that in tests without fixation, by themselves most of the findings are non-localizing. But this is the topic for our next webinar.

Gaze stabilization tests

So let's go through each one of these tests and review the procedures and then we'll go to the findings in the gaze test. You primarily want to record the eye movements when the patient is looking at a target about 25 to 30 degrees to the right or left and about 20 to 25 degrees up or down.

Sometimes in some laboratories, they perform this test both with fixation and without fixation. I usually do this with fixation only for two reasons. It's hard to maintain an off-center gaze position if you're in complete darkness, so very often, the patient's eyes drift back to the center.

And we do examine the eyes without fixation in the position test and other parts of the VNG, so I don't find it necessary to do this here. But it's of course up to you. You can do it; it doesn't cause any type of artifact or doesn't cause misinterpretation. But generally, it's enough just to do with fixation.

So in each case position, you really need to record as long as necessary. There's really no time limit until you're satisfied whether you're seeing nystagmus or not. Generally though, at least you need to measure for 20 seconds or so. And then you need to inspect the tracings to see if there's any type of nystagmus.

It's always a good idea in these cases to record the video as well. If it's not remarkable, you can delete it later. But if it's something remarkable, you want to keep the videos as well. So anytime you find an abnormality, it's a good idea to come back to that to make sure that you have a consistent and repeatable test result.

And remember, if you've done a visual exam prior to your VNG, which I highly recommend to do, your results should match your visual inspection. If you see something a discrepancy between the two that's not explainable by physiology or pathology, you need to resolve that discrepancy.

So in the second part of this webinar, we will talk about the spontaneous nystagmus and position tests. So I will leave that for the next part.

Normal gaze and spontaneous nystagmus results

When you have a normal result as you can see in this case, you have the patient gaze to the right and gaze to the left, you might see here a few bumps on the tracings. These are really not significant. It's mostly just eye blinks and things like that. If it's a nystagmus, you'll see it hopefully clearly and repeatedly. And then you can see that again in gaze up and gaze down, the patient doesn't have nystagmus or any significant nystagmus.

Now, as a part of the spontaneous nystagmus, you usually do the gaze at center gaze. And then you see the effective fixation here. You see the area that's in the yellow is where the patient is given a target. Before that, the patient is in complete darkness. But again, we'll talk about this in more detail in the next webinar.

What is nystagmus?

So as I mentioned, what we are looking for in these tests is evidence of nystagmus. And nystagmus by definition is a repetitive, involuntary, and to-and-from movement of the eyes. Nystagmus can originate from multiple sources. Some of them are actually not any type of pathology.

We can have physiologic nystagmus. For example, vestibular nystagmus that's induced by caloric irrigation or by rotation. These are normal manifestations of nystagmus. Optokinetic nystagmus is another form of physiologic nystagmus.

And then finally, end-point nystagmus is if you look far enough to the right or left. And I'm talking about angles of over 40 degrees. Even normal people have some level of nystagmus. And if you want to explore the origin of it, the eye muscles are like any other muscle.

If you stretch them far enough, they will not perform optimally. So if you're at 40 degrees right or left, your eye muscles are stretched to the limit. And so occasionally, they don't have enough tonic contraction to keep the eyes there.

So you occasionally see intermittent nystagmus of a very small amplitude in normal people, especially older people, just because the muscle tones are affected by age the same way that any other muscle is affected.

So you see end-point nystagmus more commonly in older individuals. The only clinical precaution here is that we should not mistake end-point nystagmus with gaze nystagmus, which is abnormal. So we have to make sure that we're not taking the eyes so far to the extremes that you're generating end-point nystagmus.

The other thing that you should know is that the end-point nystagmus is affected by alcohol level and by other substances. So you could start seeing the end-point nystagmus sooner than 40 or 45 degrees off-center. So again, it's just a matter of distinguishing this, which is a normal variation from pathologies that you'll see in a few minutes.

So, pathological types of nystagmus can originate from peripheral vestibular abnormalities or central, whether it's vestibular or non-vestibular. So you can see them in both types of abnormalities.

The types of pathologic nystagmus. The most common one we see in the clinic is obviously spontaneous nystagmus, which by definition is present in the absence of visual and vestibular stimulation. Again, we'll discuss that in the next part of the webinar.

Gaze-evoked nystagmus is another pathological manifestation of gaze instability, and it's defined by a nystagmus that's present only in specific gaze positions. And finally, the positional nystagmus is a type of nystagmus that's present in specific head positions.

Finally, there's congenital nystagmus, which by definition is present at birth or shortly after birth. And because of that, we kind of categorize that differently. But it is a form of a pathologic nystagmus. We'll discuss that today at the end of the presentation.

Nystagmus characteristics

So when you see nystagmus, there are important characteristics that you have to remember. In order to interpret the nystagmus and determine the clinical significance of that, you should pay attention to these characteristics.

The shape of the nystagmus. Direction of the movement, whether it's fast phases or slow phases. Intensity of the nystagmus, which is defined by the slow phase velocity of the nystagmus. You do not usually need to measure intensity in the test with fixation because as we'll discuss in a minute, almost any nystagmus with fixation is abnormal. It doesn't have any strength or intensity criteria.

But for the nystagmus that's seen without fixation, the intensity needs to be determined in order to identify whether it's a significant pathologic nystagmus or not. And finally, the effect of fixation, which is a very important factor in identifying or distinguishing central from peripheral nystagmus.

And finally, the latency and duration. This type of parameter, latency and duration, is usually reserved for a transient type of nystagmus, which you might see during the Dix-Hallpike in patients with BPPV. But on occasion, you'll see it in the gaze test. And I will discuss that in a minute again.

Shape of nystagmus

Now, let's talk about the classification of nystagmus based on its shape. The nystagmus slow phase can be different shapes. The most common one we see in the clinic is where the slow phase of the nystagmus is a line like what you see here. This is a constant velocity, linear slow phase. And it's common in peripheral or central vestibular pathways.

The other type of nystagmus, this slow phase is not a line but actually the velocity is increasing. So as you can see, starts slowly and spikes toward the end of the slow phase. These are increasing-velocity nystagmus. It's common in congenital nystagmus.

The other type of slow phase is a decreasing velocity. So as you can see, it starts very fast and plateaus toward the end of the slow phase. And this is a decrease in velocity slow-phase nystagmus. This is common in cerebellar lesions.

All three of these types of nystagmus are categorized as jerk nystagmus because there's a distinct fast phase and slow phase. So you can see the slow phase here is the arm of the nystagmus that's taking a long time. And then there's a reset in the opposite direction, which is the fast phase of the nystagmus.

So any jerk-type nystagmus always has a slow phase and a fast phase. There are types of nystagmus where that is not true. For example, pendular nystagmus, which you can see a video of. I'm going to click on this. So as you can see in the eye movements, there is no slow or fast phase. The eyes are moving with equal speed right and left. This is called a pendular nystagmus. And again, it's usually seen in patients with congenital nystagmus.

Finally, another type where there's no distinct slow phase is square-wave jerk nystagmus, which you can see here. This type of nystagmus doesn't have a slow phase, it only consists of fast phases. By real definition, this is not a nystagmus.

Even though we call it the square-wave jerk nystagmus, it's really not a nystagmus. It is due to the saccadic intrusion. So normally, your circuits are under voluntary control. But in some abnormalities, you lose that voluntary control. So the eyes are getting thrown off the target and then you try to come back to the target.

And the width of these square-wave jerk nystagmus is about 200 milliseconds. That's the inter-saccadic latency. So the types where the eyes are staying constant, usually right around 200-250 milliseconds or so. And that's how you can separate somebody who has a square-wave jerk nystagmus from somebody who has voluntary eye movements.

Voluntary eye movements usually have a much larger width than 200 milliseconds. We'll discuss this again a little bit more. But these are the different types of nystagmus as far as their shape is concerned.

Direction of nystagmus

The next characteristic of the nystagmus that you should determine is the direction. Remember that it's not correct to say the nystagmus is right or left. You always have to specify whether you're talking about fast phase or slow phase. Traditionally, we characterized nystagmus by its fast phases.

This is because in the old days, when you looked at the patient's eyes, what caught your eyes was the fast phase of the nystagmus. In reality, most of the abnormalities affect the slow phase of the nystagmus. For example, if you have vestibular a lesion on one side, that asymmetry makes the eyes move slowly in one direction.

The fast phase is just serving to bring your eyes back to the center. But because historically we have classified the nystagmus based on the fast phase, we continue to do that. The word or the terminology that we use to identify this, the fact that we're talking about this fast phase, is the term beats.

So when you say right beating, left beating, up beating, or down beating, you are clarifying that you are talking about the fast phase of the nystagmus. If you want to, for some reason, give the slow phase, you have to specifically say slow phase of the nystagmus is to the right or left. So giving a single direction usually is not enough. You have to specify which direction or which phase of the nystagmus you're talking about.

So as for the direction, the nystagmus can be horizontal, which are right-left oscillations. This is the most common type of nystagmus we see in the clinic. It could be vertical, which is an up-down oscillation of the eye movement. And then it could be torsional.

Now first of all, I want to just clarify there are times that people think that the combination of horizontal and vertical nystagmus equates to torsional nystagmus. That's not the case. Torsional nystagmus can be purely torsional with no horizontal and vertical component. In fact, you can identify a pure torsional nystagmus.

If you stand in front of the mirror and tilt your head right or left, you can see that your eyes make a perfectly torsional movement without much of a horizontal or vertical component. So torsional nystagmus is usually classified based on the movement of the upper pole of the eye.

So if the fast phase of the torsional nystagmus moves toward the right ear, that's called the counterclockwise rotation. And then if the eye moves the opposite direction, that's a clockwise rotation.

I prefer to use the terms torsion toward the left or torsion toward the right. The reason for that is that whenever we talk about the direction of nystagmus, we talk about the direction with respect to the patient. So it's the patient's right or patient's left. And that's consistent throughout the vestibular evaluation.

When you talk about clockwise or counterclockwise, that changes. Clockwise and counterclockwise are based on the examiner's point of view. So it's you looking at the patient's eyes to see which direction it's moving. To eliminate that inconsistency, if you say torsion toward the left or torsion toward the right, again you're talking about the patient's point of view.

Now, when we analyze the nystagmus, we have to interpret each component of the nystagmus on its own and then combine to look at what effect or what source it could come from. So generally, break down the nystagmus to three primary components if you see a combination of these, and then interpret each one separately.

In the old days, we were taught that if you have vertical nystagmus or torsional nystagmus, it's always central. We know from our experience with BPPV patients that that's probably not quite right. What we should say is that if you have purely vertical or purely torsional nystagmus, then that's central.

But a combination of torsion, horizontal, or vertical can come from abnormalities and focal lesions of the anterior or posterior canal. So any combination should be interpreted more cautiously. Don't just assume any type of vertical is always central. If there's a combination of vertical and torsion, it probably means one of the vertical canals or their pathways are affected.

Intensity of nystagmus

So the next classification, as I mentioned, is the intensity of the nystagmus. Intensity of the nystagmus is basically measuring the slow phase velocity of the nystagmus. You do not need to do this if you're just looking at nystagmus with fixation.

Anytime you're looking at nystagmus without fixation, this becomes an important factor. Let me just briefly deviate from the PowerPoint content and just give you the reason for this. There are other classifications for intensity of nystagmus. For example, people use frequency or actually the amplitude of the eye movement.

The reason that the slow phase velocity is the most important feature of nystagmus as far as the intensity is concerned is because it directly relates to how fast the patient thinks their head is moving.

So for example, in the caloric test, if the peak nystagmus is about 30-40 degrees during that phase of the caloric test, the patient feels like the head is moving 30-40 degrees per second. That directly relates to the patient's symptoms and how much activation of the vestibular pathways the patient is experiencing. The other parameters that are used sometimes like frequency or amplitude, they do not have this feature.

So back to what we're discussing here. In the old days, when you did not have recordings and you just relied on the examination of the eye movements, the intensity was classified based on how many different gaze positions you could see the nystagmus.

So first-degree nystagmus was present when the gaze was directed toward the fast phase. So for example, if the patient had left-beating nystagmus in left gaze and no nystagmus in any other gaze position, that was called first-degree nystagmus.

Second-degree nystagmus is the one that's present in the direction of the fast phases but also in the center gaze. So somebody who has left-beating nystagmus in left gaze and left-beating in the center gaze has second-degree nystagmus.

And finally, third-degree nystagmus is when nystagmus is present in all three gaze positions: right, left, and center.

Based on this old classification, the rough interpretation was that if it's a first-degree nystagmus, then it's a peripheral disease. If it's a second-degree nystagmus, we don't know. If it's a third-degree nystagmus, it's usually central.

There's a lot of truth to that but it's insufficient for accurate interpretation. So now that we have recording, we can do a lot better. And in fact, once we test the effect of fixation, that interpretation becomes much clearer. So I don't usually rely on this first, second, or third-degree nystagmus for interpretation.

Symmetric gaze-evoked nystagmus

So the first abnormality we'll discuss is called symmetric gaze-evoked nystagmus. So I'm going to show the videos here. So here's the nystagmus. And you see that the patient has right-beating gaze nystagmus. Let's click on. Now you can see that the patient has left-beating gaze nystagmus.

When we look at the tracings, the intensity of both nystagmus is about the same. That's why it's called a symmetrical gaze-evoked nystagmus.

Asymmetric gaze-evoked nystagmus

A counterexample to that is this patient has right-beating nystagmus in the right gaze and left-beating nystagmus in the left gaze. But they're not quite the same intensity. It seems to be much more intense in the right gaze compared to the left gaze.

This is an interesting nystagmus. It's called Brun's nystagmus. This nystagmus is generated by a large acoustic neuroma. And just think for a second and decide for yourself which side is the acoustic neuroma? Since you have a 50-50 chance of getting it correctly, I'll just go ahead and explain which one it is.

This is a right-sided acoustic neuroma. It's a very large tumor. This is an old slide. We don't get these types of nystagmus very often nowadays because before we get to this stage, the acoustic neuroma has to be removed.

An interesting feature of this nystagmus is that when you look at the left gaze, this nystagmus has a linear slow phase. So this is actually the vestibular component of the acoustic neuroma. It's there because of the compression of the vestibular nerve by the vestibular schwannoma tumors that's generating this left-beating nystagmus.

If we have caught this patient early on, let's say before the tumor became this large, this will be the only thing you will see. The other side, the right side. This is the effect of the tumor compressing on the brainstem. And if you recall from the shape of the nystagmus, I said if you have a slow phase that's decreasing in velocity it's usually a cerebellar sign. That's what you see here.

The slow phase of this nystagmus is decreasing velocity, whereas this one is a linear velocity. So this is a vestibular pathway nystagmus, this is a central pathway nystagmus, which is what the definition of Brun's nystagmus is.

Unilateral gaze-evoked nystagmus

All right, so on occasions, you get nystagmus that is just present in only a single gaze position. As you can see here, it's a unilateral one. So here, you see nystagmus which is left beating in the left gaze and it's not present any other place.

Now the question we need to answer here: Is it a central nystagmus or not? You cannot answer that question without first looking at what effect fixation has on this patient. So if you look at the nystagmus in the absence of fixation, you'll see that the nystagmus is very intense.

So this is what's called the Alexander's law. In patients who have vestibular nystagmus, we're not talking about necessarily peripheral nystagmus, it could be in the central pathways. But it's in the vestibular pathways whether it's the canal, the vestibular nerve, or the vestibular nuclei.

If you have an asymmetry, this is the type of nystagmus you will generate. And when you look in the direction of the fast phases, the intensity of the nystagmus is the greatest. So that's when the patient is looking to the left, the left-beating nystagmus has the greatest intensity.

So when we look at the difference between what happens without fixation and what happens with fixation, you can determine that this is a form of spontaneous nystagmus and not necessarily a central abnormality.

So if you were to interpret this finding on its own, it just says that it's a non-localizing spontaneous type nystagmus. Now, if you have a caloric test, vHIT, or something that says this person has a right sided peripheral lesion, then this is a completely clear and explainable result that the patient has an acute vestibular abnormality on the right. And this is the manifestation of the nystagmus in the gaze test.

Rebound nystagmus

So one of the forms of what you might see in the gaze test is called a rebound nystagmus. It's in fact very close to gaze-evoked nystagmus. The only difference is that it's characterized by a transient type of gaze-evoked nystagmus.

So let's give an example. So if you ask the patient to look at the center gaze and you just wait long enough. And if you look at the patient's eyes, you may not see anything. If you ask the patient to look to the left for about 20 seconds or so, you'll see the nystagmus that beats toward the direction of the movement but then it goes away.

So if you ask the patient to look back to the center, you see now nystagmus that beats again in the direction of the movement. So at this time, you'll see right-beating nystagmus. Then it goes away after about 20 seconds or so. So any time the patient changes gaze position, you see a transient type of nystagmus that beats in that same direction of movement for about 20-30 seconds.

Patients who have gaze-evoked nystagmus may have this in conjunction with their gaze nystagmus and the significance of it is the same as the gaze nystagmus.

Gaze-evoked nystagmus interpretation and etiologies

So to interpret and to localize the gaze-evoked nystagmus, including the rebound nystagmus, all forms denote a central lesion in the cerebellum or brainstem. And it's quite common in lesions of the cerebellar flocculus.

So the only thing you have to be careful is that you do not misinterpret spontaneous nystagmus that's following Alexander's law. Because you do see that with fixation in a single gaze position. And you should not misinterpret end-point nystagmus as we discussed earlier.

The etiologies that can cause this are quite varied. Symmetric types of nystagmus can be caused by drug ingestion like lithium intoxication, metabolic disorders, myasthenia gravis, multiple sclerosis, and cerebellar atrophy. If you have an asymmetric one, it has a more localizing meaning. It's usually a cerebellar or brainstem abnormality that could be caused by tumors or infarcts.

Brun's nystagmus is a specific type of asymmetric nystagmus, which is often due to a mass or tumor in the posterior fossa on the side of the gaze-evoked nystagmus. Rebound nystagmus is often time seen with gaze nystagmus and it indicates cerebellar atrophy or more focal lesions of the cerebellum.

Square wave jerk nystagmus

Now, as we discussed, we have another type of nystagmus which in reality is not nystagmus. That is a square wave jerk nystagmus. As you can see here in the video, the patient eye movements have no slow phase at all. It's just the back-and-forth eye movements.

And if you look at the tracings, the width of these squares are around 200 milliseconds. So that's clearly indicating that this is not a voluntary eye movement. This is a square wave jerk nystagmus. It's considered a form of saccadic intrusion, which is an involuntary firing of the saccade pulses.

It throws the patient's eyes off the target and then the patient needs to correct and come back. And that takes about 200 milliseconds or so. If that inter-saccadic latency is much bigger than 200 milliseconds, that usually means that these are involuntary eye movements.

There's some controversy about the normal limits. What you just saw is definitely abnormal. But if you see something much less frequent, especially in older people, you will probably not call that abnormal.

If you see for example ten square wave jerk nystagmus in a period of 60 seconds in a patient who's 78 years old, that usually is not considered significant. In a young person, even 10 beats of square wave jerk nystagmus could be considered abnormal. So the normal limits are not fully established.

There's one paper that has a nice description of these involuntary eye movements and their normal limits. We'll reference to that in the follow-up to this webinar. But generally, if you see just a few beats over a one-minute period, you should be cautious about calling that abnormal.

If you do have a significant square wave jerk nystagmus, it is definitely a central lesion. And it usually implicates the cerebellum or the basal ganglia. Most of the etiologies that cause this are neurodegenerative diseases like multiple sclerosis or spinal cerebral degeneration.

Periodic alternating nystagmus

One form of the nystagmus that you may encounter, and it's definitely a central abnormality, is a periodic alternating nystagmus. And many people abbreviate that to PAN which is fine, but then you have to make sure that you're not misunderstanding that with positional alcohol nystagmus.

But when positional alcohol nystagmus is abbreviated to PAN, there's usually a 1 or 2 after that which distinguishes whether you talk about phase one or phase two of the positional alcohol nystagmus. PAN is used for periodic alternating nystagmus and does not have a secondary number to it.

As you can see here, this patient initially has right-beating nystagmus. Then at some point in time, that nystagmus changes direction and it goes left-beating. So this is a nystagmus that changes direction in a single case or head position.

Sometimes, people use the terminology direction changing gaze. And really what they mean is in the left gaze and right gaze. They don't mean in a single gaze position. So you have to be sure what terminologies you are using.

So in periodic alternating nystagmus, the nystagmus changes direction in a single gaze or head position. The characteristic of this nystagmus is that it changes direction every two to four minutes. It's usually horizontal and conjugate. Almost always, the ones I have seen are present both with and without fixation. So you see this in the gaze test and in the position testing.

This form of nystagmus can be either congenital or acquired. If it's acquired, there's a medication that's given to the patients called Baclofen that abolishes this type of nystagmus. It's quite a rare condition, so it's not that you will encounter this often in the clinic. But it causes complication in our interpretation of the tests.

So because the nystagmus changes direction every two to four minutes, technically, if you want to identify this on each gaze or each head position, you have to wait for two to four minutes to see if the gaze changes direction, which is completely impractical. You will have a very long test.

So instead of trying to do something like that, you should really look for inconsistencies. For example, in the position testing, it's very rare to see nystagmus beating in one direction in the sitting position and other direction in the supine position. Or at the beginning of the caloric test, it's very rare to see nystagmus beating in different directions in different irrigations (at the beginning in the first five seconds or so).

So if you see any inconsistencies like this, then you may want to keep the patient in one gaze position or one head position for an extended period of time to see if periodic alternating nystagmus is present. In general, we don't look for this every time in our tests because it will make the test too long.

If you do find it, it's a central abnormality. And it could happen in Chiari malformation and other craniocervical junction abnormalities. It could happen in MS patients. It's quite common in patients who are blind. It's also a complication of anticonvulsant medications. So people are on seizure medications, you could see them. And as I mentioned in the acquired type of nystagmus, generally a medication Baclofen abolishes nystagmus.

Vertical nystagmus

Everything that we have talked about so far has been related to horizontal nystagmus. You can also have vertical nystagmus in the presence of fixation. Here you see a down-beating nystagmus. So this is a clear down-beating nystagmus, and you can see in the tracings that it is a down-beating nystagmus.

Localization of vertical nystagmus, as long as it's a pure vertical nystagmus, is definitely a central lesion. From a diagnostic point of view, down-beating nystagmus has a slightly higher clinical significance than up-beating nystagmus for reasons that I will mention in a minute.

So if you see down-beating nystagmus, it's a central lesion in the posterior midline cerebellum or the underlying medulla. Up-beating nystagmus, almost the same localization. Medulla or anterior vermis of the cerebellum. Down-beating nystagmus can be caused by several different abnormalities including Chiari malformations and cerebellar degeneration.

It's quite common in cancer patients where the cancer has spread to the central nervous system. Or tumors or strokes affecting the brainstem or cerebellum. Also, neurogenerative diseases like multiple sclerosis. Vascular diseases like vertebrobasilar ischemia and cerebellar tumors.

The etiologies for up-beating nystagmus are very similar. But as I said, there's just a little bit less clinical significance for up-beating nystagmus because it could be a side effect of nicotine patches, alcohol intoxication, or some medications. For that reason, we generally give a little bit less diagnostic sensitivity or diagnostic importance to down-beating versus up-beating nystagmus. All the other etiologies are very common between the two.

Congenital nystagmus

Finally as I said, I will just mention something about the congenital nystagmus. Congenital nystagmus is called that because it's usually noticed at birth or shortly after birth. The most common manifestation is a pendular nystagmus or jerk nystagmus where the slow phase velocity increases. So oftentimes, you see this in the form of spikes in your tracings.

Generally, its conjugate, most often horizontal or torsional. As opposed to most types of nystagmus that can be suppressed by fixation, actually fixation enhances congenital nystagmus, and it's attenuated by convergence. All of those characteristics are hard to distinguish from other types of nystagmus.

The most common identifying factor for congenital nystagmus is that it has a null point. So for example, if you look at the patient's eye movements from the left gaze and/or right gaze and move all the way to the opposite side, there's a point where the nystagmus stops, and you don't see the nystagmus.

And that point is often not the central gaze. It's usually off-center gaze. So if you're in the train or in a bus and you see somebody reading the newspaper or reading a book and they tilt their head to one side to the other while they're reading, this is a good indication that they might have a congenital nystagmus.

So what they try to do to focus on the items is they tilt their head until what they're looking at is placed in their null point. That's where they can focus and read things. These days, children who have congenital nystagmus sometimes have a surgery to move the null point to the center gaze. This will make a functional difference in their life, so they don't have to tilt their head one way or the other.

It doesn't get rid of the nystagmus, but it does make the null point at the center point. In these cases, the nystagmus kind of loses the characteristics that I just described. So it's important when you're doing an interview with the patient to figure out or ask them if they've had eye surgery in their childhood. The example here is somebody who has had surgery, so the nystagmus is still there, but it doesn't quite match what I just described. The null point is closer to the center point.

Summary

So to just summarize what we talked about. In normal variations of gaze stabilization tests, you typically do not see any nystagmus with fixation in any gaze or head positions. If you have strong spontaneous nystagmus or significant nystagmus without fixation, sometimes that leaks through and you see a partially suppressed version of that, especially in the gaze toward the fast phases. So that's in spontaneous nystagmus with the Alexander's law component to that.

With that exception, everything else should be considered a central abnormality. And again, if you do not do the test the way it should be done and you have the patient gazing too far, like 40-45 degrees, then you might have end-point nystagmus. So whenever you have gaze abnormalities with fixation, that's an indication of a central abnormality.