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.