VS 117 Notes
1/26/99

Handout #3 was given out in the beginning of class
Demonstration of eye movement was performed at the end of lecture
Outline
Measure Eye Position
A. Applications
B. Subjective Methods
Direct Observation
Corneal Reflex
Distortion Tests
Haploscopic Tests
Entoptic Tests
C. Objective Methods
Cover Tests
EOG
Limbal
Pupil
Purkinje Tracker

Clinical Measurements of Eye Movements
1) Steadiness of binocular fixation
Shine pen light into patient's eye and see a reflection of the light or have
patient follow finger up and down and make a loop. These two methods are called confrontational tests because doctor sits directly opposite of the patient and observes eye movements while patient follows the finger to see if there's restriction of gaze. This direct observation is the most commonly used method for measuring eye movements. What we are looking for is to see how steady and accurate monocular fixation can be. Have someone look at a small spot and if you see any movements at all, then they are abnormal. Normally, there are tiny movements of the eye but they are so small that you can't see them by direct observation.
2) Binocular alignment
To see if one eye is deviating up/down or right/left. If the eye is turned this is called a strabismus or tropia. Usually one eye is looking at you and other eye is aimed somewhere else. Convergent strabismus is found in young children up to ages 5-6. Unilateral cover test: Cover one eye with cover paddle, you disrupt binocular vision. When you do this the covered eye goes to resting position. Alternate cover test: Alternately patch the left and right eye. If there is no movement on the unilateral cover test but there is on the alternate CT, The error between uncovered eye alignment and resting position of covered eye is called phoria. Almost everyone has a phoria. It is normal. If there is movement on both the alternate and unilateral cover tests, the result is a strabismus.
3) Fixation Disparity
Fixation inaccuracy with both eyes open. Even if you have normal straight eyes, the eye tends to turn in or out a little too much. This is called fixation disparity. It is usually associated with a large underlying phoria. Measure with subjective method here on the order of less than 1 degree and with both eyes open.
4)Accommodative Lag
Another motor phenomenon we measure clinically that is similar to fixation disparity. We try to accommodate a certain distance and our eyes tend to accommodate behind that distance. This is called accommodative lag. Lag because we accommodate less than we should and its due in part to the depth of focus. Abnormally large lags are a big problem for some youngsters having difficulty learning to read. It is a symptom of inattention however occasionally patients will have a genuine weakness of accommodation or accommodative lag. Basically, if your trying to accommodate and your far point or conjugate focus is farther away than what your looking at this is accommodative lag. We all have it, small amounts of it is fine, about three-quarters of diopter is normal.
5)Eye Tracking
This is a confrontational test. Ask patient to follow pen left/right and up/down and what we're looking for is for one eye to suddenly stop moving while the other eye is still following. This indicates a gaze restriction which suggests weakened of one or more ocular muscles. These tests look for neurological disorders.

This is the standard procedure for each of your patients. Measure phoria, look for fixation disparity, binocular alignment, tracking pursuits, saccades, making quick movements left and right. Looking for deviations from normal pattern.

6) Reading Eye Movements
Lots of kids have reading problems. Subspecialty of optometry.

Subjective Measurements
1) DIRECT OBSERVATION
What we're looking for is binocular alignment and steady fixation. Usually if there is abnormal movement of the eye it is on the order of 1/2 degree for detection.

Purkinje Images
Aim a pen light at the eye and observe the corneal reflex to judge eye position. Reflection is referred to as catoptric image (reflected image). Reflected images are from all optical surfaces of the eye. There are four optical surfaces in the eye; front and back surface of the cornea and front and back surface of the lens. They were first documented by Purkinje. We call the images first (P1) , second (P2) , third (P3) , and fourth (P4) Purkinje Images:

P1 is reflected from the front surface of the cornea.
P2 is reflected from the back surface of the cornea.
P3 is reflected from the front surface of the lens.
P4 is reflected from the back surface of the lens.

If you held a candle in front of the eye, you would see reflected image.
P1=P2=brightest, upright
P3=reflected from front surface of lens, erect, larger than P1 and P2
P4=concave in the back, inverted and very small, brighter than P3

We are going to use P1 to measure some eye position, Later on use P1 and P4 together to measure eye positions. In general, P3 used to measure accommodation because when we accommodate curvature of the front surface of lens changes the most of all the optical surfaces. When we accommodate the P3 image is tiny. What is done is to measure size or intensity of P3 to measure accommodation. P2 and P1 are the same, they are too close together, can't tell apart.

Angle Lambda
In clinic we measure eye position by looking at some angle. The key angle we are going to look at is angle lambda. There are some key optical axes that define the eye. Two of them are very similar: 1. the line of sight: matches the center of pupil to what we're looking at 2. the visual axis: connects the nodal point of the eye to the fovea. These two are almost the same. What we're interested in clinically is looking at the separation between the pupillary axis (an axis that starts at the center of pupil and runs perpendicular to the cornea) and the line of sight. This is called angle lambda. Clinically, they call angle lambda, angle kappa. Angle kappa is the difference between pupillary axis and visual axis. We don't measure that clinically but for some reason it's called angle lambda/angle kappa. It is just a little misnomer. So in clinic, I suggest you call it angle lambda.

First Purkinje: Shine light into person's eye and see a little corneal. Hold your pen light and aim it at the patient's eye and see a little reflection of the pen light off to the nasal side of the pupil which means the eye is actually turned outward a little bit. So hold pen light and have patient track and move eye in until corneal reflex is centered in pupil. Now you are staring right down pupillary axis and the line between your finger and eye is the line of sight. To measure lambda, we find the separation between the corneal reflex (line of sight) and pupil center (pupillary axis). A prism diopter approximately equals reciprocal of viewing distance. Normally, 10 prisms diopter or 5 degrees is a normal angle lambda. The corneal reflex is usually a half mm nasal. Every mm is about 22 prism diopters. 1 mm displacement in adult eye is approximately 22 prism diopters. In infant eye, a child under 4 years of age, angle lambda is double that of adult eye with 44 prism diopter because eye is shorter. As eye grows longer, distance between pupillary axis and visual axis gets smaller. This is how you go about measuring angle lambda.

There are some conditions where patient do not use the fovea to fixate. This happens with a condition called amblyopia which means lazy eye. Usually find in children associated with eye turns. The deviating eye never has a chance to develop; it is not used, it is suppressed so there is a neurological impairment of that eye. When cover good eye, the amblyopic eye starts to fixate, and it tends to fixate nasally. This nasal eccentric fixation causes a temporal ward displacement of the corneal light reflex and a negative angle Kappa. Refer nasal ward reflex as positive angle lambda and refer temporal ward reflex as negative angle. Nonfoveal point for fixation is known as eccentric fixation.

Hirschberg Test
Binocular angle lambda with both eyes open. In this test have the patient fixate on the penlight binocularly and observe the symmetry of the corneal light reflections. In the case of strabismus, you compare the location of first Purkinje image reflex or corneal reflex on both eyes. Estimate the amount or magnitude of the eye turn. Let's say in one eye the corneal reflex is in the center and the other the reflex is about 2 mm temporal ward. Remember every mm is about 22 prism diopters. 2 mm is 44 prism diopter of eye turning in (esotropia) causing corneal reflex to be displaced temporal ward. Let's say for the sake of argument that the centered eye has a reflex of 1 mm nasal ward and the turned in eye is 2 mm temporal. Take the difference between the two eyes, so you have +1 in one eye and -2 in other eye: -2 -(+1) = -3 So, the difference between the eye is 3 mm and 3 mm (22 prism diopters/mm) = 66 prism diopters. We have 66 prism diopters of inward eye turn called esotropia. You can get a rough estimate of magnitude of eye turn by looking at corneal reflexes. There's another test which you can do by displacing that reflex by centering it by using a prism. You look at the amount prism it takes to center it and it's called Krimsky Test.

2) Distortion Tests
Maddox Rod
Take a single target in space (usually a pen light) and distort the image of it in one eye so that each eye can see a unique percept of that target. Maddox rod takes a point source and turns it into a streak. As you change orientation of rod, the streak orientation changes too. Maddox rod is a series of high power cylinders that take that image of a point source and form it into a streak perpendicular to axis. The subject see a little pen light with one eye and if the Maddox bars are held horizontally the other eye sees a vertical streak. If the streak does not fall on the fovea of the eye, the patient will see a separation between the light source from the pen light and the streak. You might see vertical streak shifted to the right or left of pen light. The patient tells you how far apart she sees the penlight and the streak. From that you can actually estimate the magnitude of eye turn and you could put in prisms to align the streak with pen light. If you rotate streak so bars are vertical, you get a horizontal streak, and from that you can then measure vertically. Let's do an example: someone with nasal eye turn. A person fixating penlight with right eye on the fovea and left eye has big esotropia, but the pen light image of left eye is on nasal hemiretina. Now put Maddox rod on left eye causing a vertical streak. Foveas of two eyes see things in same direction. If I were to put after image on fovea of one eye and also in other eye, see after images together no matter where my eyes are turned. So, the streak is imaged on nasal retina which means the patient sees it in temporal view. Projection is opposite to deviation. Something on nasal retina projects temporally. This is used for single targets made to appear slightly different in two eyes.

3) Haploscopic Tests
This presents independent targets to right and left eye which you can move around independently . One way is that you have a red and a green flashlight projecting a red image and a green image. Patient aims images until they both appear at the same visual direction. Let's say here we have same condition as Maddox rod example: red image of soldier on right fovea and green image of house on left. We have to move image of house over so that patient can see two images superimposed on each other-soldier in house. Superimposing red and green images is called Anaglyphic Technique You can also separate images with Polarized Light. Red object is seen in one eye and different green object is seen in other eye. Put them on foveas so they appear superimposed and have identical visual direction. This is different from distortion test because distortion test has one object imaged on fovea in one eye and nonfovea in other eye (separate visual directions). Haploscopic test has two objects one imaged in fovea of each eye. The location of target is where the eye is pointing in the same direction whereas in distortion test the imaged seen on nonfovea is in the opposite direction. Clinically, the simplest haploscopic test is the Von Graefe which uses a vertical prism to produce two retinal images.

4) Entoptic Phenomenons
These are images you perceive that result from some structure of the eye. One of the structures of the eye is the projection of optic radiations from ganglion cells around the fovea as they project towards the optic disc. They fan out from fovea and as a result they tend to polarized light. If you put Polaroid that spins around in front of one eye, you'll see little bright area where the axis of Polaroid line up with axis of radiation and everything also appear dark , you'll see bright brush spinning around. This is called Haidinger's brush. It's associated with the fovea. It indicates where the patient's fovea intersects the screen you're looking at. Put a little letter A and ask patient to look at letter A and sometimes patient will see little brush to the side which indicates they have eccentric fixation: they are not using their fovea to fixate and you can estimate the amount of fixation error by how far foveal target is from brush. Eccentric fixation occurs a lot in amblyopia.

Another entoptic phenomenon is Maxwell's Spot. Macular pigment is a protection against UV. Yellow macular pigment is directly over the macula and nowhere else in retina. If you look at cobalt filter that has short and long wavelengths, red and blue imaged together, the blue gets filter out by yellow filter while red gets passed. You see a little pinkish spot called Maxwell's spot. What you are seeing is a region of macular pigment.

Purkinje also observed another entoptic phenomenon. When he got up in the morning the first time he opened his eyes, he would see the outline of retinal blood vessels. This Purkinje tree results from the shadow of blood vessels on retina before light adaptation.

Objective Measurements
Objective measurements: the doctor makes the decision/the patient doesn't tell you anything.
Subjective measurements: the patient has to indicate where he/she sees something.

1) Cover Paddle
Unilateral Cover Paddle: primarily used to detect eye turns or strabismus. If one eye is looking at you (fixing eye) and the other eye is not looking at you (deviating eye). Cover up the eye looking at you and the eye not looking at you has to move to fixate at target. To detect the presence of strabismus.
Alternate Cover Paddle: measure the magnitude of eye turn. Use prism to measure magnitude of strabismus.

2) Electro Oculography (EOG)
This measures voltage of eye. Normally the retina has positive voltage which means the back of the eye has a negative voltage due to pigment epithelium. The positive electrons show up on cornea and negative electrons show up in the back of the eye. Eye is like a battery or dipole. Positive charge in front of the eye and negative charge in back of the eye. We can measure that by putting little electrodes on either sides of eyes and then we measure and subtract the voltage sent from one to the other. Let's say the right electrode value is subtracted from the left electrode value. Eye points to right, the right electrode is more positive, so we get an end value that is positive. Eye points to left, we have positive subtracted to something negative. As eye moves from left to right we get plus value. Its very small but can measure accurately. This is used in non communicative patients. It acts as an objective test. Accuracy in the order of one degree. This is a good way for measuring nystagmus in non-communicative patients like children.

3) Limbus Tracker
Shine infrared red light on boundary between sclera and iris which is the limbus. As eye moves outwards, the light originally at the limbus is now being reflected off of the iris. Thus less light is being reflected. If the eye moves inwards, the light originally at the limbus is now being reflected off of the sclera. More light is being reflected off the sclera than iris. The reflected lights are measured in voltages, So, you take the voltage difference between the two to get either a positive or negative value.

Demonstration was performed in class of this method using the Ober visicorder that is located in the Binocular Vision Clinic: Erich Graf read a paragraph of a reading booklet and the tracker recorded the number of eye movements as regressions and fixations. This helps in determining how well subject can read. Regressions help in detecting lack of comprehension of subject.

4) Pupil Trackers
You get an image of an eye on video screen. The screen scans at 16 milliseconds for black area, pupil, and finds center of pupil. It specifies position of the eye with vertical and horizontal angles. It can also get torsional measurements. The iris has dark and light areas. Eye torts which then shifts profile of iris and this shift is used to measure torsion of eye.

5) Purkinje Image Tracking
A sensor that tracks the position of the first and fourth Purkinje images. The first Purkinje image is reflected off front of cornea and fourth Purkinje image reflected off of back of lens. As eye rotates, P1 rotates with the eye and P4 rotates in the opposite direction and you get a parallax. If we measure P1 and P4 locations, we can measure the eye rotation movement. If somebody translates head to left and to right which doesn't cause any parallax between the P1 and P4. This isn't confused with an eye movement; only time it senses an eye movement is when eye actually rotates and causes a parallax. This technique is still being exploited, pretty much a laboratory method. Some companies are working on it with video camera.

6) Magnetic Search Coil
This is a technique used in neurology clinics. Picture below is of an eye wearing an annular contact lens. It is like ERG electrode. It's a big coil of wire in annulus which generates a voltage as the eye rotates inside a big magnetic field. You sit inside a big box with alternating electrical fields setting up magnetic fields around you. Depending on angle of coil in the field, more or less voltage is generated. As eye rotates, the amount of voltage varies. Eye position is derived from measuring orientation of wire annulus inside magnetic field. This is probably the most sensitive method for measuring eye position and fast saccades, but it is also the most irritating. You have to anesthetize the front surface of eye, you can only wear this for about 20 minutes. They tend to pull aqueous out of the eye and elevate intraocular pressure in the eye. This is used for saccade anomalies.