Monocular Fixation

Monocular fixation -Past pointing test,
Visuoscopy, Haidinger’s brush, Fixation
disparity, After image transfer test
Manoj Aryal B. Optometry
manojaryal85@gmail.com

Past pointing Visuoscopy
Haidinger’s brush Fixation disparity
After image transfer test
Monocular fixation testing methods
Overview of diagnostic evaluation of strabismus
Diagnostic test sequences
Presentation layout

Overview of
Diagnostic
evaluation of
strabismus
Management
plan
Case H/O
Diagnostic
summery
Diagnostic
test
sequences

Refraction
MONOCULAR FIXATION
Visual acuity
Deviation variables Correspondence
Sensorimotor fusion
Visual efficiency
Diagnostic test
sequences

A failure of an eye in monocular vision to take up
fixation with the fovea, but with some other point.
This hardly occurs except in clinical conditions as the
patient is generally not fixing with that eye anyway.
It is only shown when the better eye is covered
(Exception = microtropia with identity)
Eccentric Fixation

Four Theories as to the cause of Eccentric
Fixation
Anomalous correspondence theory
(Chavasse, 1939, Cuppers, 1956)
Suppression Theory (Worth, 1906, Bangerter,1953)
Motor theory (Schor, 1978)
Pickwell (1981)

1. Suppression Theory
Occurs when central acuity has dropped to a level
below that of the surrounding area, so that better
acuity results.
Now thought to be unlikely as foveal VA still seems
to be better than in the rest of the retina.

2. Anomalous Correspondence theory
A change in the central area of localisation
resulting from a central scotoma in the
amblyopic eye
Major problem with this theory is that the
angle of anomaly is usually much greater than
angle of EF
EF secondary to the development of ARC

Failure of the EOM to relax from the deviation (in
strabismus) = MUSCLE POTENTIATION.
This is a likely cause as habitual strabismic deviation
causes an adaptive after-effect which modifies the
subsequent monocular localisation
3. Motor theory

4. Pickwell (1981)
A sequel to an enlargement of Panum’s fusional
area following decompensated heterophoria at an
early age – eventually leads to microtropia – a loss
of accurate correspondence

Presence of EF will account for :
A portion of the patients reduced visual acuity
Can contaminate a number of other test
results, such as
the magnitude of deviation, and
the angle of anomaly in correspondence
testing.
May mask or fool the practitioner when
evaluating the patient for strabismus and

When an amblyopic eye attempts to fixate
monocularly, an off-foveal site is often used, and the
patient is subsequently diagnosed as having eccentric
fixation.
Assessing Monocular fixation

Location Noted by identifying what portion of the
retina is being used to fixate: nasal,
temporal, superior, or inferior retina.
There may be combination of both
horizontal and vertical components
Direction of the EF is usually in the direction
of the presenting strabismus

Magnitude of EF is evaluated by:
 Using the calibrated target in the direct
ophthalmoscope
 By determining the distance between a perceived
entoptic phenomenon and a fixation test point
Weymouth and Flom (1961)
Discovered the formula for predicting visual acuity
from known EF.

•Most EF is 3 prism diopter or less
•When magnitude of EF equals the magnitude of the
strabismus, no movement will be seen on the unilateral
cover test and the strabismus may be missed.
Predicting visual acuity from known EF
MAR=E(pd) +1
where,
Nasal EF =+E
Temporal EF=-E

Tests for Monocular
Fixation

ANOMALIES OF EGOCENTRIC LOCALIZATION
were first described by Von Graefel in patients with
recent paralyses of the extra ocular muscles and are
referred to as
"past-pointing" or "false orientation."
It occurs in recent paresis when affected eye is
looking monocularly in the direction of action of
paretic muscle
Past-pointing test

Not found in muscle restriction and nonparetic
incomitancy
Misjudging the location of an object and pointing
too far in the same direction in which the object
was displaced
E.g.-in case of LLR palsy,px points a finger to see
object to left located further than its normal
position.
Past-pointing test Cont.

FIGURE Past-pointing in a patient with left sixth nerve
paresis. A, Normal egocentric localization in dextroversion. B, Past-
pointing to the left when viewing an object in the median plane,
and C, in the paretic field of gaze

Past-pointing Theories
Egocentric localization of visual objects in
subjective space is almost accurate as
long as there is proper correlation
between ocular innervation and its effect,
that is, as long as the amount of actual
eye movement corresponds to what is
intended
(Von Helmholtz,Bielschowsky, and Hofmann)

 Paretic eye cannot be moved sufficiently to
permit fixation of an object with the fovea when
looking into the direction of a paretic muscle
 According to this theory the distance between
the fovea and the eccentric retinal elements on
which the image is falling determines the angle
of past-pointing
Adler 1945
Cont.

Is based on the discovery of sensory receptors in the
extra ocular muscles.
There is proprioceptive feedback from the extra
ocular muscles to the central nervous system that
influences egocentric localization of objects
Cont.

Thus, past-pointing is felt to be caused by a disparity
between the information received by the brain from
the extra ocular muscles and the amount of motor
impulses required to produce adequate movement
into the field of action of a paretic muscle

How to perform past-pointing test clinically?
this result indicates that fixation does not coincide with the
centre of localisation
If finger goes a few cm to the side then past pointing has been
demonstrated (do not repeat too many times as PX adapt)
repeat with the non-amblyopic eye occluded.
occlude amblyopic eye, hold pen 25cm in front and ask patient
to touch pen with the tip of their finger
carry out test initially with good eye (checks normal ability and
increases confidence)

ii. Visuoscopy
Visuoscope, a modified ophthalmoscope that
projects a fixation target on the fundus
The examiner projects the fixation mark close to the fovea
The eye not tested is to be occluded.
The patient is asked to look directly at the asterisk
The position of the fixation target on the fundus is noted

Poor patient cooperation
Lack of distinct foveal reflex

Haidinger’s brushes are entoptically perceived as a
pattern of closely radiating lines that resembles a
propeller
Haidinger’s brush is due to
birefringence induced by
Xanthophyll which is
radially polarizing.
iii. Haidinger’s brushes

Haidinger’s brushes can be used to detect macular
edema.
The effect is less pronounced or absent in macular
edema. This can occur even before ophthalmoscopic
signs of macular edema.
Because Haidinger's brush corresponds to the macula, it
is sometimes used as a gross subjective test of macular
function.
Haidinger’s brushes Cont.

Haidinger’s brush can determine whether amblyopic
patients fixate with their fovea or not (eccentric fixation)
since the fovea always corresponds to the center of the
hourglass and the center of rotation
Haidinger’s brushes Cont.
Pointing task with HB. Patient is
instructed to align the point P with HB
superimposed on the fovea.
A-central fixation with brush, pointer
and fixation point aligned
B-eccentric fixation with primary
visual direction at the eccentric point

When the patient views the reference dot on the MIT, they
should see the brush centered on the dot
The Macular Integrity Tester (MIT) generates Haidinger’s
brushes.

The practitioner can then measure the distance
between these two points and convert the millimeter
displacement into prism diopter, based on the testing
distance
In eccentric fixation, this is not the case because a
retinal locus other than the fovea is used for fixation
The direction and magnitude of the EF can be assessed
by asking the patient to report the location of the
propeller in relationship to the fixation point.

Maxwell's spot. A round, dark-purplish spot
perceived when the eye is exposed to a homogenous
blue or purple field alternately with achromatic light,
can be used to evaluate the magnitude and direction
of EF, similarly to haidinger’s brush
Used in vision therapy to “tag” where the patient is
fixating
Can also be used to measure the density of macular
pigment. The darker Maxwell’s spot, the denser the
pigment

Visual sensation persisting after the original
stimulus has been removed
Once created in each eye, their position in relation
to each other persists in an open or closed eye
conditions
Subjective test to determine and detect the
presence or absence of ARC
After Image Tests
Introduction

Positive after image-which appear the same as the
original stimulus
Negative after image-in which the light areas of the
original stimulus appear dark
Introduction Cont.
After Image Tests

After Image Tests
Principle
To produce after images by presenting a bright light
to each eye in turn.
The images can be obtained by using special slides in
the major amblyoscope or with a hand-held flash
apparatus.

The apparatus consists of a linear light with central
black band within a circular black background
mounted on the flash apparatus.
The lines can be presented horizontally or vertically
The apparatus is flashed while the patient fixates the
black band
After Image Tests
Principle
Cont.

After Image Tests
Procedures
First presented horizontally to the eye with better VA
and then vertically to the poorer eye for 15-20
seconds in a darkened room
Each eye fixates on the central black mark of a
glowing filament

Looking the after image of two filaments on a
uniform surface(wall),px. sees a cross which shows
NRC, or the 2 filaments will be separated indicating
ARC.
Fellow eye is occluded.
Cont.
After Image Tests
Procedures

Cont.
After Image Tests
Procedures
Alternatively the position of positive after image
can be assessed in dark
The patient looks at a black wall in normal
illumination and should see two negative linear
after images, each with a central gap corresponding
to the fixation and therefore representing the visual
direction of fovea.

Px. indicates the relative position of two gaps in
the center of each after image
The gaps correspond to the visual direction of
each fovea if central fixation is present
Interpretation depends on the fixation behavior
Cont.
After Image Tests
Procedures

Interpretation with central fixation
First assess with visuoscope
In px with NRC, two fovea have a common visual
direction, sees the gap superimposed
After Image Tests

Px. with ARC and right exotropia ,vertical after
image is displaced to the right
In ARC, two fovea no longer have a common visual
direction.Px with right esotropia sees vertical after
image displaced to the left

Interpretation with Eccentric fixation
After Image Tests
Asterisk lies between fovea and disc with
visuoscopy
With NRC(rare in px. with EF)or when the angle of
anomaly is not identical with the degree of
eccentricity, the after image in right eye will be
localized uncrossed to right

In small angle strabismus, px may employ the same
extra-foveal retinal element for fixation that forms
a common visual direction with fovea of sound eye
under binocular conditions (ARC)
When degree of eccentricity of fixation is equal to
angle of anomaly, superimposition of gaps occur
indicating ARC.
Interpretation with Eccentric fixation Cont.

Indications for use
After Image Tests
The test is difficult for the patient to perform and to
interpret, which limits it’s application
Testing conditions are entirely artificial and
abnormal retinal correspondence is only diagnosed if
it is well established
Other tests have largely superseded this test in
routine clinical examination.

Fixation Disparity Test
The underconvergence or overconvergence with
respect to the plane of regard called fixation
disparity is measured in minutes of arc and
depends on the size of Panum’s fusional area.
Fixation disparity (FD) is the difference between
the convergence angle under binocular viewing
and the angle subtended by the target at the
centers of rotation.
OR

It is a failure of the two visual axes to
simultaneously intersect the object of regard
during attempted binocular fixation.
It is also referred to as a micro strabismus because
the eyes are not binocularly aligned during
attempted fusion.
Retinal slip is another synonym

The main difference between a strabismus and FD is
that the patient is not fusing with a strabismus but are
with FD.
Fixation disparity is the residual error of a partially
corrected phoria.
 Usually a large phoria underlies a small fixation
disparity

Fixation disparity in which
the eyes underconverse in
relation to plane of regard
is called exo fixation
disparity, whereas fixation
disparity in which the eyes
overconverse Is called eso
fixation disparity
Fig. Exo FD
Fig. Eso FD manojaryal85@gmail.com

Components of fixation disparity curve
The horizontal axis intercept(associated
phoria)
The vertical axis intercept(FD amount)
Slope at the vertical axis intercept
The center of symmetry (inflection point)
Shape/type of curve type

The horizontal axis intercept(Associated Phoria)

The vertical axis intercept(FD amount)

Steep fixation disparity curve
Indicates:
SVA mechanism that is too
slow or that has too little gain
given the stimulus time
constraint
slope - measured between 3 base-in & 3 base-out

Cont.
Flat fixation disparity curve
Indicates as a pair the disparity
vergence controller and the
SVA mechanism are operating
in comfortable ranges with
regard to both amplification
ability and speed

The center of symmetry
(inflection point)
Signal the passage of the disparity vergence system from control
of the crossed disparity detector system to the uncrossed
disparity detector system(or vice versa)
It moves up & down(vertically) according to need for a
maintenance level of innervation( fixation disparity)
Moves side to
side(horizontally) according
to the direction and amount
of phoria and state of
adaptation.

Shape/type of curve type
Clinically identical(most common) curve
Consist of relatively flat
central portion with an
upsweep on the left and
down sweep on the
right
The central portion of
the curve owes it’s
flatness to the action of
the SVA mechanism

The flatness of the central portion is very much dependent on
the manner in which curve is generated
Faster the curve generated- the steeper the curve
When they are symptomatic the slope of FDC is typically steeper
than the average.
Usually asymptomatic.
Slope Value of approx. 1’ of arc per  discriminate b/w
symptomatic & asymptomatic Pts.
Pts. With steeper slopes respond well to Orthoptics when they
are symptomatic

The cortical images from the two eyes
increasingly flip and the fusion of these images
become increasingly less exact as the prism
power increases, finally diplopia occurs

Fixation disparity
changed little over a
wide range of prism
power, then suddenly
made a large change
at one or both of the
extremes

Most type II occur with Esophoria.
Respond better to prisms & to lens adds than to
Orthoptics.
An irregular FDC may be indicative of an
accommodative problem.

Can also be managed
with Prism
Can be trained with Orthoptics but not as easily as Type I
Most often found in high
Exophoria

Fixation disparity
changed with forced
vergence only within a
narrow and outside the
interval levelled off at
nearly constant values
Indicating poorly developed binocular coordination

Methods for measurement of
Fixation Disparity
Mallett FD test
It was designed not to measure FD but not determine
the amount of prism necessary to eliminate the FD and
is therefore an associated phoria test
Two FD units(mallet boxes) are available
A wall mounted unit for d/s testing
A hand held unit for near testing

Both near and distance FD units incorporate a foveal
fusion lock as well as a peripheral fusion lock: the only
monocularly seen stimuli are in the para-foveal area
Both units required polaroid filter in front of the patient
eye
Illumination:
Strong enough to allow for the filters

Is a self contained,
internally illuminated unit
The letters OXO are seen
by both eyes and
constitute a gross foveal
fusion lock

Two red strips are provided one above and one below
the X , are polarized so one strop is seen with each eye
Used to measure lateral associated phoria as well as
the vertical associated phoria
The letter X subtend an angle of 10min of arc at d/s of
6m(20/40)

Also presents OXO letters
Letter X subtends 18 min
of arc at 35 cm distance
Instead red, green stripes are provided

The patient is told to concentrate on the X in the
middle of OXO and is asked if he/she can
simultaneously see two red for distance, two green
strips for near

If top strip is directly above bottom strip i.e. if no
displacement between strips-patient has no lateral
phoria or has a compensated phoria.
If top strip (seen by LE) is to the right of bottom strip-
patient has Exo FD

If top strip is seen to the left of bottom- Eso FD
If both strips are displaced- FD is shared between
two eyes
If only one strip is displaced:
One eye is strongly dominant( undisplaced strip belong
to dominant eye)

The associated phoria or the
amount of plus sphere power
to be prescribed is the
smallest amount that will
bring about alignment.
Base out prism/plus sphere
can be introduced until the
strips are aligned.
Once FD Is eliminated, the
exophoria can be considered
fully compensated .
For convergence insufficiency
institute base out training, the
criterion being the elimination of
fixation disparity.
Introduce base in prism starting
with 1 prism diopter or minus
lens until two strips are aligned.
Exo –associated phoria Eso – associated phoria

Includes two stimuli
One for vertical disparity measurement
One for horizontal disparity measurement
Each test stimuli includes a black aperture
The Disparometer
Two reduced Snellen’s charts
seen with both eyes and two
polarized Vernier lines seen
in the center of the black
aperture(each eye seeing
only one Vernier lines)

First presents the patient with the target in
which Vernier lines have no disparity
If the patient reports that lines are not perfectly
aligned, the examiner presents stimuli having
varying amounts of disparity until the patient
reports alignment
By determining the amount of FD while forcing
vergence with base in and base out prisms
presented in steps 3 prism diopter, FD curve
can be plotted

Other methods …
Compare reflex position in each eye in turn (other
eye occluded). The relative displacement of the
reflex by 1mm = approx. 11degrees or 20 PD
Eccentricity is not usually this great however making
EF difficult to detect by this method.
Corneal reflex test

Bjerrum Screen
Method
In normal subjects the blind spot is the same angular
distance from fixation in both eyes.
Plot the blind spot carefully in both eyes and compare
positions
Degree of eccentricity can be measured by the
difference in angular distance of blind spot from
fixation in each eye

Amsler Chart
Amblyopes often have
small foveal scotoma
which shows up as a
disturbance on Amsler
 Occurs centrally if
central localisation
 Eccentrically if EF

Neutral density Filters
If a ND filter is added and no reduction in VA
occurs then EF is likely to be present

Centricity of Fixation (central vs eccentric)
Directional bias (nasal, temporal etc.)
Quality of fixation (steady vs unsteady)
Magnitude
Pattern of fixation (drifts, saccades, nystagmus
Percent foveation (30second visuoscopy)
Zero retinomotor point
Assessment of Fixation

Treatment of Monocular Fixation
As in amblyopia, have to encourage foveal fixation
Direct Occlusion alone may improve fixation but
often a slight eccentricity remains
Pleoptic Treatment – desensitises eccentrically
fixing area
After image transfer – use to locate foveal fixation

Inverse occlusion
Infants with eccentric fixation who are too young
for treatment with Haidinger’s brushes
Continued as long as improvement is made
Rarely suitable in adult – eccentric fixation is too
firmly established

Red filter
600 – 640 nm
Only stimulate cones
Patient will attempt to fix with area of the retina
which contains maximum no of cones

Pleoptics Therapy
Pleos=full,optikos=sight
Aim is to actively stimulate macula in dense
amblyopia with eccentric fixation
Two methods,either Bangerter or Cupper

Bangerter’s method
It was followed by intermittent stimulation of
macula with flashes of light
Performed by modified Gullstrand’s
Ophthalmoscope,called as Pleoptophore
Bangerter dazzled the extramacular retina including
the eccentric point by bright light protecting the
macula by a disc projected onto it.

Cupper’s method
Cupper used Euthyscope which had discs of varying
sizes to create a central after image apart from
dazzling the eccentric point
He used the alternate flashing of room
illumination(Alternascope) to perpetuate the after
images(forming negative after image in light and
positive after image in dark)
patient perceives an afterimage resembling the
white doughnut

Treat underlying amblyopia by occlusion of non-
squinting eye (if patient <6 years old).
In patients >6 years – correct refractive error,
otherwise do not treat the microtropia.
Surgery is not appropriate

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