Physiology of torsional eye movements

In this video, you will review torsional eye movements, and their peripheral and central pathways. You will learn about the use of torsional eye movements for the evaluation ofthe vertical semicircular canals, otoliths, and their central pathways.

You can read the full transcript below.

Introduction

Hello, everyone. Welcome to this presentation of Trends in Balance. Today we'll be exploring two new developments that enhance the diagnosis and treatment of patients with BPPV.

One is the availability of measuring torsional eye movements. And the other is the availability of motion sensors that measure the trajectory of head movements during the maneuvers.

Establishing the direction of torsional movements is important in identifying the involved canals and BPPV. In general, torsional movements can also help with the evaluation of the vertical semicircular canals, the otoliths and their central connections.

Until recently, most VNG systems were limited to measuring just the horizontal and vertical components of the eye movements. But the new version of VisualEyes™ is now capable of measuring and analyzing torsional eye movements.

In this presentation, I will start with the review of torsional eye movements and their peripheral and central connections.

Dr. Whitney and Dr. Petrak will describe the practical application of measuring torsional eye movements and the head trajectories in BPPV patients.

Eye movements

Just as the head can move in three dimensions, the eyes can also move in three independent planes. They include the horizontal or yaw plane, the vertical or pitch plane, and the torsional or roll plane. As a side note, when we consider eye movements in different planes, we should interpret each component independently because they are mediated by different peripheral and central pathways.

Horizontal and vertical eye movements

First, let's quickly review horizontal and vertical eye movements, which can help us with establishing conventions for torsional movements. The center of rotation for horizontal and vertical eye movements is approximately 13 to 15 millimeters behind the cornea.

The reference for measuring horizontal and vertical eye movements is the line that connects the center of rotation to the center of the pupil.

When the eyes move horizontally, the angle of the line that connects the center of rotation to the new pupil center is used to represent horizontal eye movements expressed in degrees. The convention is that positive numbers are used when the eyes move to the right of the patient and negative numbers are used when the eyes move to the left of the patient.

Similarly, when the eyes move vertically, the angle of the line that connects the center of rotation to the new pupil center is used to represent vertical eye movements also expressed in degrees. The convention is that the positive numbers are used when the eyes move up and negative numbers are used for when the eyes move down.

Here are the horizontal tracings. When the eyes start to center gaze, the red dots represent the center of the pupil in the horizontal plane. After resting at the center gaze, the eyes move to the right and stay there for a short period of time, then move to the left and stay there for another short pause. And then finally they move back to the center.

The animation is the slowed down so we can pay attention to the details. Obviously, these are slow eye movements and if we are describing nystagmus, every slow phase is followed by a fast phase in the opposite direction. The tracing for vertical eye movements will be similar.

Torsional eye movements

In contrast, the center of rotation for torsional eye movements is the center of the pupil, which itself can move with horizontal and vertical line movements. The reference line is the line that connects the upper pole of the eye to the center of the pupil when the eyes looking straight ahead.

When the eye rotates in the role plane, the angle of the line that connects the center of the pupil to the new location of the upper pole is used to measure torsional eye movements to be consistent with the convention for horizontal eye movements.

The angle of the movement of the upper pole to the right of the subject is considered as right torsion and to the left of the subject as left torsion. Similar positive numbers are used for rightward torsion, and negative numbers are used for leftward torsion.

Torsional eye movements can also be described using the terms clockwise and counterclockwise. These terms describe the movement of any point on the iris from the examiners point of view. As a result, clockwise torsion is equivalent to leftward torsion, and counterclockwise motions are equivalent to the rightward torsion.

Here are the torsional movement tracings. When the upper pole of the eye starts at the upright position, then the eyes torsion to the right of the subject and stay there for a short period of time, then move to the left of the subject and stay there for another pause and finally move back to the upright position.

These movements can also be described as counterclockwise torsion, followed by clockwise torsion, and then followed by another counterclockwise torsion. Again, these are slow eye movements, and for nystagmus, slow eye movements are always followed by fast phases in the opposite direction.

Eye muscles

Now, let's consider the eye muscles that are involved in different types of eye movements. Each eye is moved by six muscles that form three extraocular muscle pairs. Under normal conditions, these muscle pairs work in a push pull manner, which means whenever one is contracting, its synergetic pair is relaxing.

Two of these muscles, the lateral and medial rectus muscles, are responsible for generating horizontal eye movements. The second pair, superior rectus and inferior rectus muscles, are primarily responsible for vertical eye movements, but they have a second role in producing torsional eye movements. The amount of torsion that they generate depends on the gaze direction.

The final pair, superior oblique and inferior oblique muscles, are primarily responsible for torsional eye movements, but they have a second roll in producing vertical eye movements. Again, the level of vertical line movement depends on the gaze direction.

Horizontal eye muscle function

This slide shows the muscles that are involved in horizontal eye movements to move the eyes leftward. The left lateral rectus and the right medial rectus muscles must contract.

In this case, contraction of the lateral rectus muscle pulls the eyes away from the center as the contraction of the medial rectus muscle pushes the eyes toward the center.

Simultaneously, the paired muscles, the right lateral rectus and the left medial rectus muscles, must relax for this motion to take place.

The amount of change in the neural firing is directly related to how fast the eyes move. For fast eye movements such as saccades, the change in the neural firing is very high.

Vertical eye muscle function

Vertical eye movements are mediated by the superior and inferior rectus muscles. They also generate slight torsion. Or the exact motion depends on the gaze direction.

Contraction of the superior rectus muscle pulls the eyes up with a slight clockwise torsion, whereas the contraction of the inferior rectus muscle pushes the eyes down with the slight counterclockwise torsion.

Again, the muscle pairs work in a push pull manner, and the amount of change in the neural firing rate is directly related to how fast the eyes move.

Torsional eye muscle function

Torsional eye movements are mediated by the superior and inferior oblique muscles. They also generate a slight amount of vertical movement.

Contraction of the superior oblique muscle torsions the eye clockwise with a slight downward movement, whereas contracts of the inferior oblique muscle torsions the eye counterclockwise with the slight upward movement. Again, the paired muscles have the opposite action.

Pathways for torsional eye movements

When we look at the pathways that mediate torsional eye movements, we realize that voluntary saccade and smooth pursuit mechanisms are not developed in humans. As a result, almost all torsional eye movements are involuntary.

Torsional eye movements can be generated through three different pathways.

1. Otoliths

First are the otoliths that can generate static torsion in response to head tilts. This can be measured in the ocular counter-rolling test, but that's a topic for another presentation.

In individuals with otolith lesions, a tonic torsion is generated toward the side of lesion during the acute phase, but it resolves with compensation.

2. Vertical canals

Second, when the head this moved in the plane of the vertical canals, they generate torsional and vertical nystagmus.

Damage to the vertical canals or their pathways, such as those that occurred during vestibular neuritis, generate persistent torsional and vertical nystagmus in the acute phase. They may resolve over an extended period of time with compensation.

BPPV can also stimulate the vertical canals during the diagnostic maneuvers, which will generate transient, torsional and vertical nystagmus.

3. Optokinetic system

Finally, optokinetic stimulations to the rotation of the full visual field about the visual axis can also generate torsional nystagmus. Remember that the optokinetic system is the visual counterpart to the vestibular system and supplements it for low frequency movements.

Horizontal canal-eye muscle connections

Now let's discuss the peripheral vestibular pathways that generate torsional eye movements.

One of the features of the vestibular pathways is that they connect to the eye muscles directly at the brainstem level and do not go through the higher cortical levels. That's the reason for lower latencies of vestibular responses compared to, for example, smooth pursuit responses.

The excitatory pathways from the horizontal canal go to the ipsilateral medial rectus muscle and to the contralateral rectus muscle. For example, excitation of the right lateral canal which can occur in lateral canal canalithiasis during the roll maneuver, will move the eyes slowly to the left, followed by a saccade directed toward the stimulated canal.

For this case, the slow phase velocity will be represented by negative numbers because distal eye movements are leftward. The inhibitory responses that occur when the lateral canal is damaged cause slow eye movements toward the damaged canal, followed by a fast phase that is directed away from the damaged canal.

For right sided lesions, the nystagmus will be left beating and the SPV will be represented by positive numbers for the anterior canal.

Torsional canal-eye muscle connections

The excitatory pathways go to the ipsilateral superior rectus muscle and to the contralateral, inferior oblique muscle. So the excitation of the anterior canal which can occur in anterior canal BPPV during the Dix-Hallpike maneuver, causes down beating and torsional nystagmus directed toward the involved canal.

For the excitation of the right anterior canal cases, the torsional component will be beating counterclockwise with negative SPVs, then inhibitory pathways from the anterior canal go to the contralateral superior rectus muscle and to the ipsilateral inferior oblique muscle.

So anterior canal lesions will cause persistent up beating and torsional nystagmus beating away from the side of lesion. For right anterior canal lesions, that will be clockwise beating torsion with positive SPVs.

For the posterior canal, the excitatory pathways go to the ipsilateral superior oblique muscle and to the contralateral inferior rectus muscle. So the excitation of the posterior canal, which can occur during the Dix-Hallpike maneuver in posterior canal BPPV, causes up beating and torsional nystagmus beating toward the involved canal.

For the right posterior canal BPPV cases during the Dix-Hallpike maneuver, the torsional component will be beating counterclockwise with negative SPVs.

The inhibitory pathways from the posterior canal go to the contralateral superior rectus muscle and to the ipsilateral inferior oblique muscle. So, posterior canal lesions will cause persistent down beating and torsional nystagmus, beating away from the side of lesion

For right posterior canal lesions, that will be clockwise beating torsion with SPVs.