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156 Theory and Practice of Squint and Orthoptics
4. Eyeport @ Vision Training System
Has revolutionized the way people look at their
vision. To read more comfortably, learn more
easily, work less painfully, and play sports more
effortlessly, people everywhere are doing daily
eye exercises with the EYEPORT.
5. Eyelights
Target the weaker functioning side of the brain
via the non-dominant eye. Light stimulation
directly to the non-dominant brain causes an
excitatory barrage to travel to the mesencephalon, the most metabolic area of the brain,
where an increase in cellular activity takes place.
The excitatory barrage travels also to the
parietal, temporal, and occipital lobes of the
brain, while collateral fibers lead to the pineal
gland, pituitary gland, and hypothalamus.
6. AmbP iNet Program
An amblyopia hand-eye coordination program
which uses principles of operant conditioning
and behavior modification to appropriately alter
stimuli characteristics to improve visual acuity
(Fig. 6.49A and B).
• Patients begin therapy with targets that are
easily seen and become progressively smaller
as therapy progresses. Correct responses are
reinforced with subsequent reduction in the
size of the stimuli. Therapy is directed to
improve resolving ability with concomitant
use of hand-eye coordination tasks. Therapy
can be preformed monocularly or monocularly in a binocular field (Fig. 6.50A and B).
• The AmbP iNet programme provides a
cumulative graph depicting each session’s
performance. The computer denotes date,
Fig. 6.49A and B AmbP iNet program.
Fig. 6.50A and B Protocol of therapy with AmbP iNet program.
157Evaluation of a Case of Strabismus and Orthoptic Instruments
time, duration and denotes when the patient
has performed the assigned tasks as well as if
they were performed correctly.
7. PTS II iNET
It is a home-based computerized perceptual
therapy program that has been designed to
address a variety of visual perceptual information
processing domains, including simultaneous
processing, sequential processing, speed-ofinformation processing, visual temporal
processing, and rapid automatized naming,
• Dyslexia
• Ordinary reading disability
• Word decoding difficulty
• Problems in reading comprehension
• Spelling problems
• Memory disorders
• Laterality-directionality deficits
• Slow speed of information processing
• Non-verbal learning disability
• Mathematics difficulty
• Above average intelligence but not achieving
up to potential
• Acquired brain injury with perceptual-
cognitive deficits
• Attention disorders
• Diagnosis of perceptual/visual information
processing deficits (Fig. 6.51A and B).
Therapy procedure. PTS II iNet presents the
patient with a grid that shows the different
difficulty levels for each procedure. Each time
the patient meets or exceeds a therapy goal they
receive a gold star. The program gives the
patient verbal reinforcement. It prompts them
to “Get Ready” before a stimulus is presented.
PTS II iNet also verbally reinforces correct and
incorrect responses. For example, it will
encourage them by saying “Good Job” after they
input a correct response.
8. Dynamic reader
The dynamic reader is a home Vision Training
program designed to improve reading eye
movements and thus reading fluency and
comprehension.
9. ADR iNet
Moving text, standard, and whole line dynamic
reading.
10. Vivid vision
Vivid vision offers virtual reality-based vision
therapy for a range of visual conditions, including
amblyopia, strabismus, and convergence
insufficiency. The immersive experience of
virtual reality can enhance engagement in
therapy exercises.
11. DigiVision
DigiVision offers computer-based vision therapy
exercises designed to improve visual skills,
including eye tracking, focusing, and convergence. The program provides customizable
exercises that can be tailored to individual needs.
12. RevitalVision
RevitalVision focuses on visual improvement for
conditions such as amblyopia and low vision.
The program uses adaptive algorithms to tailor
exercises to the individual's visual abilities.
Fig. 6.51A and B Diagnosis of perceptual/visual information processing deficit.
158 Theory and Practice of Squint and Orthoptics
13. Opto
Opto offers a range of interactive vision therapy
exercises that target various visual skills,
including eye coordination, tracking, and
convergence. The program adapts exercises
based on user performance.
14. Ultimate vision
Ultimate vision provides online vision therapy
exercises to improve binocular vision, focusing,
and eye coordination. The program includes a
variety of engaging exercises.
15. AmblyoPlay
AmblyoPlay offers vision therapy exercises
designed to improve amblyopia (lazy eye) and
other visual skills. The program includes
gamified exercises for both children and adults.
16. NovaVision
NovaVision focuses on neurorehabilitation and
visual field training. It is often used for
individuals with visual field deficits due to
conditions like stroke or traumatic brain injury.
17. VTWorks
VTWorks provides computer-based vision
therapy exercises that target eye teaming,
focusing, and tracking. The program offers a
variety of activities and progress tracking
features.
18. NeuroTracker
While originally designed to enhance cognitive
performance, NeuroTracker has also been used
in sports vision training and rehabilitation. It
involves tracking multiple moving objects on a
computer screen, which can help improve visual
tracking and attention skills.
19. EyeLeo
EyeLeo is a computer program designed to
remind users to take regular breaks and perform
eye exercises. While not a comprehensive vision
therapy program, it can be useful for preventing
digital eye strain and maintaining good visual
habits.
20. Dynavision D2
Although primarily used in sports training and
rehabilitation, the Dynavision D2 can be used
to improve visual-motor coordination, reaction
time, and peripheral awareness. It features a
board with lights that users must touch in
response to visual cues.
21. NovaVision VRT
NovaVision's Vision Restoration Therapy (VRT)
is designed to help individuals with visual field
loss, often caused by conditions like stroke or
brain injury. The program uses visual stimuli
presented on a computer screen to stimulate the
brain and improve visual field perception.
22. Brainware safari
Therapy for 14 cognitive skills in six areas.
23. Play attention
Improves attention.
24. Sanet vision untegrator
Procedures for saccadic trainer, tactile feedback,
hand speed function, “visual search” saccadic,
metronome, tachistoscope functions.
25. Sub iNet
Addresses subitizing deficits to improve math
skills.
26. Track and read
Twelve therapy procedures for developing
saccadic eye movements, span of recognition, and
visual sequential memory skills.
NEUROVISION THERAPY PROGRAMS
1. Revital vision. Concept of neurovision
therapy is based on the visual plasticity, which
is the ability of the visual system to change its
responses in order to adapt to the changes in
the visual input. Revital vision is a perceptual
Learning therapy program developed by
“Revital Vision Technology”. Perceptual
learning is an alternative treatment option
which modifies visual function in adult
amblyopia. The perceptual learning therapy
program (Revital Vision) is a non-invasive
software-based patient-specific, interactive
perceptual learning tool based on visual
stimulation. It facilitates neural connections at
the cortical level through a computerized
training regimen using Gabor patches to
improve contrast sensitivity and visual acuity.
The term perceptual learning describes a process
whereby practicing certain visual tasks leads to
159Evaluation of a Case of Strabismus and Orthoptic Instruments
an improvement in visual performance. As
visual perception depends on both the optical
input received from the eye and the neural
processing of that input in the visual cortex,
Revital vision technology improves quality of
vision (visual acuity and contrast sensitivity) by
enhancing neural processing in the primary
visual cortex.
The typical building blocks of the visual stimulus
in the field of visual neuroscience are:
• Gabor patch,
• Neuronal lateral interactions,
• Brain (neural plasticity), and
• Perceptual learning.
Effects of revital vision–perceptual learning.
Neurovision technology (Revital) is formed
around proprietary algorithms and has proved
successful in the following ways:
• Improvement of lateral interactions in
amblyopia
• Improvement of CSF in amblyopia
• Improvement of CSF in non-amblyopic
groups
• Improvement of VA
• Transfer to improvement of binocular vision.
2. Neurovision rehabilitator used for those with
brain injury is useful as ocular vestibular
integrator, visual motor enhancer, visuomotor
integrator, dynamic ocular motor processing,
fixation anomalies.
III. COMPUTER-BASED COMBINED DIAGNOSTIC
AND THERAPEUTIC PROGRAMS
1. TrYe (Train your eyes) vision therapy software
TrYe (Train your eyes) vision therapy software
has been designed and developed by ‘Digital
Works Technology Private Ltd’, using the
domain expertise of ‘Sankra Nethralya,
Chennai, India’. This program has the ability to
run on multiple platforms, such as PC, Laptop,
Mobile and Tablet.
Uses of TrYe software include:
• Binocular vision assessment, and thus addresses
to the binocular vision dysfunctions. Binocular
vision assessment programme in TrYe is based
on aperant conditioning using random dot
stereograms (RDS) as targets. RDS targets are
devoid of monocular cues and have been
recommended for fusional vergence testing
and training.
• ‘Office vision therapy’ (TrYe-Doctor’s module)
program for dysfunctions of binocular vision,
accommodation and ocular motor system.
• Home based vision therapy program (TrYe-
Patient’s module) for vision therapy at
home.
2. Computer orthoptics program by HTS INC
solutionsTM. HTS iNet/Computer Vergence
System (CVS) is a sophisticated yet easy-to-use
vergence exercise computer application that the
patients can run on Windows and Mac
computers. It encourages and motivates patients
by continuously evaluating their progress and
making the exercises more challenging
whenever the goals are achieved.
To assure compliance, CVS uses random-dot
stereograms. Each exercise requires bifovial
fixation for correct responses. The responses
required are a simple choice of four arrows keys;
up, down, left or right. The results of each
exercise session are documented as the program
notes the date, duration and vergence demand
of each exercise. All results are available for
your review via the Internet. The HTS iNet
program is fully automated and easy to use.
Procedures include:
• Pursuits
• Saccades
• Base-in/base-out vergence
• Auto Slide Vergence
• Jump ductions
• Base-up/base-down vergence
• Accommodative rock
3. Computer orthoptics. It includes both
orthoptic therapy and diagnostic procedure:
• Therapy procedures include smooth vergence,
rotations, jump ductions, multiple choice
vergence, accommodative rock, pursuits,
saccades, visual memory, cheiroscope and
amblyopia therapy procedures.
• Diagnostic procedures for heterophobia,
fusional ranges, accommodative facility,
pursuits, saccades, Worth 4 dot, motor
160 Theory and Practice of Squint and Orthoptics
field, fixation disparity, visual memory,
aniseikonia.
4. Bynocs Amblygo and DeStrain-Dichoptic
vision therapy in which both eyes are made to
view contrast adjusted images to improve hand
eye coordination.
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162 Theory and Practice of Squint and Orthoptics
7
Anomalies of Convergence,
Divergence and Accommodation
CONVERGENCE
Types of convergence
•
Angle of convergence
•
Near point, far point, range and amplitude of
•
convergence
Anomalies of convergence
Convergence insufficiency
•
Convergence insufficiency
•
associated with accommodative
insufficiency
Convergence paralysis
•
Convergence spasm
•
CONVERGENCE
Convergence is a disjugate movement in which
both eyes rotate inward so that the lines of sight
intersect in front of the eyes. It allows bifoveal
single vision to be maintained at any fixation
distance. Convergence remains more or less
same throughout life. It does not deteriorate
with increasing age, unlike accommodation, but
may deteriorate under certain abnormal
circumstances. The power of convergence can
be increased by exercises.
TYPES OF CONVERGENCE
Convergence is a very complex process. It may
be voluntary or reflex.
I. Voluntary convergence
Voluntary convergence is the ability to
consciously and intentionally control the
convergence of the eyes when looking at a near
object. It is not a part of normal convergence
movement as it occurs in everyday vision.
DIVERGENCE
Fusional divergence
•
Anomalies of divergence
Divergence insufficiency
•
Divergence paralysis
•
Acquired motor fusion deficiency
•
ACCOMMODATION
Accommodation and related terms
•
Assessment of accommodation
•
Age-related changes in accommodation
•
Anomalies of accommodation
•
Insufficiency of accommodation
•
Paralysis of accommodation
•
Voluntary convergence is thus a separate
phenomenon from the reflex convergence
involved in normal visual activities; and that not
every one is capable of doing or learning this.
• Comedians learn to invoke voluntary
convergence to cross their eyes.
• Some patients learn to use voluntary
convergence to produce convergence
nystagmus.
II. Reflex convergence
Reflexive convergence is an automatic eye
movement that occurs in response to the
accommodation (focusing) effort when
switching from a distant target to a near target.
This type of convergence helps to maintain
single binocular vision at different distances.
Reflex convergence has four components: Tonic,
fusional, accommodative and proximal convergence.
1. Tonic convergence
It is that part of reflex convergence which results
from some inherent innervational tone of the
163Anomalies of Convergence, Divergence and Accommodation
extraocular muscles, when the patient is awake.
It is the sum of excitatory and inhibitory
influences from different sources such as cortical
centres, subcortical centres and vestibular
organs. It is independent of fusion or object
proximity. This is a form of proprioceptive eye
position control that keeps the eye converged
even after one eye is occluded for a while. It is
very important in determining the position of a
person's eyes, i.e. under the influence of tonic
convergence, the eye position will be more
convergent than before, but from an absolute
point of view, it will still be divergent. Tonic
convergence is most prominent in childhood
and decreases with age. The emotional energy
level of the individual may affect tonic
convergence. It disappears under deep general
anaesthesia and after patching one eye for 30 to
60 minutes.
2. Fusional convergence
Fusional convergence, also called positive
fusional convergence, is the convergence that is
produced to ensure that similar retinal images
are projected onto corresponding retinal areas.
It occurs without a change in refractive state of
the eye and is initiated by a bitemporal retinal
image disparity. In other words, fusional
convergence implies a responsiveness to
disparate stimuli lying outside the Panum's
fusional area. It is not a voluntary process, but
one of the optomotor reflexes and thus forms a
kind of fusion reflex or motor fusion. Fusional
vergence, in general, forms an important
mechanism for the achievement of bifoveal
single vision. And that a fusional vergence may
be a convergence, a divergence or a vertical
vergence movement.
Fusional convergence is the most important
type of convergence in the study of motor
anomalies. It has been found that the amplitude
of fusional convergence is greater, when attention
is directed between the two disparate retinal
images than when the attention is directed at only
one of the two images. The normal fusional
convergence amplitude for distance is about 18D
and for near it is 35 D. Fusional convergence helps
to control exophoria (latent divergent squint). The
fusional convergence may be decreased by
fatigue or illness, converting a phoria into a
tropia. The amplitude of fusional convergence
can be improved by orthoptic exercises.
3. Accommodative convergence
It is that component of reflex convergence which
occurs, when the eyes accommodate, or when a
nerve impulse to accommodate is discharged to
the eyes. Thus, the stimulus for accommodative
convergence is blurred retinal images rather
than the retinal disparity that stimulates fusional
convergence. In contradiction to fusional
convergence, accommodative convergence is
not dependent on binocular vision and occurs
even if one eye is occluded. Patients with one
blind eye still show convergence of the eyes,
when accommodating on near objects. In fact,
the accommodative convergence is a part of the
triad of synkinetic near reflex complex. Other two
components of this neurosynkinesis being
accommodation and miosis. The quantitative
relationship between the accommodative
convergence and accommodation is expressed
as the AC/A ratio. This relationship is a linear
one and is thought to be relatively stable
throughout life. In it, the accommodative
convergence is measured in prism dioptres and
the accommodation in lens dioptres. The AC/A
ratio, therefore, is expressed as so many prism
dioptres per one dioptre of accommodation. The
normal AC/A ratio is about 3 to 5 prism dioptres
for one dioptre of accommodation.
The fact that AC/A ratio remains almost normal
in presbyopic persons, indicates that it is the
stimulus for the accommodation that evokes the
response of accommodative convergence rather
than the amount of accommodation that actually
takes place. The majority of myopes have a high
AC/A ratio and hypermetropes have a low AC/
A ratio as compared with the emmetropes.
However, there is no correlation between the
degree of myopia, hypermetropia and the
magnitude of AC/A ratio. The pupillary distance
must also be considered in the determination of
the AC/A ratio, since the convergence requirement for an individual with a wide interpupillary
distance is greater than for a patient with a
narrow interpupillary distance looking at the
same fixation distance.
164 Theory and Practice of Squint and Orthoptics
Abnormalities of the AC/A ratio are very
important causes of strabismus. A high AC/A
ratio may cause excessive convergence and
produce a convergent squint (esotropia) during
accommodation on a near object. A low AC/A
ratio may cause a divergent squint (exotropia),
when the patient looks at a near object.
4. Proximal convergence
Proximal convergence, also known as
instrument convergence, is that component of
reflex convergence which is induced by the
proximity of the object of regard or the
awareness of the proximity of a near object. It
appears to be initiated by psychological factors,
since it occurs also when a subject just believes
that he/she is looking at a near object although
he/she actually is not. For example, while using
the haploscope optically set at infinity, proximal
convergence is often induced.
There exists a linear relationship between
proximal convergence and the changes in
fixation distance, similar to accommodative
convergence. Here the change in fixation
distance is expressed as changes in the vergence
of light, i.e. in dioptres. Thus a change in fixation
from infinity to 1 metre is a change of 1D; as is a
change from 1 metre to 0.5 metre. It has been
found that for about each dioptre of change of
fixation distance, an approximate change of 1.5
occurs in proximal convergence.
ANGLE OF CONVERGENCE
It refers to the angle that is formed between the
primary lines of sight during convergence
(Fig. 7.1A). Its size depends on the fixation
distance, becoming smaller with increasing
distance of fixation object (Fig. 7.1B) and on the
interpupillary distance (IPD), becoming larger
with increasing IPD (Fig. 7.1C).
The effect of IPD on the angle of convergence
is usually negligible and so practically not taken
into consideration while measuring the
convergence angle. Convergence angle can be
measured in metre angles or in prism dioptres.
Metre angle
One metre angle convergence is exerted by each
eye, when the eyes are directed to an object at a
distance of one metre of the meridian line
between the two eyes (Fig. 7.2). The convergence
exerted in metre angle (ma) by each eye is inversely
proportional to the distance in metres the object is
infront of the eyes, i.e. it would be 0.5 ma at
2 metres and 2 ma at half metre (Fig. 7.2).
In an emmetropic eye, the number of dioptres
of accommodation required to see an object
clearly is equal to the number of metre angles
through which each eye must converge to see
the object singly. Thus, one dioptre of accommodation is associated with one metre angle of
convergence of each eye.
Convergence in prism dioptres
When a converging prism (base out) is placed
in front of the eye, it will deviate the rays of light
A B C
Fig. 7.1 Angle of convergence (A) which becomes smaller with increasing fixation distance (B), and becomes larger with
increasing interpupillary distance (C).
Fig. 7.2 Convergence in metre angles.
165Anomalies of Convergence, Divergence and Accommodation
NEAR POINT, FAR POINT, RANGE AND
AMPLITUDE OF CONVERGENCE
Near point of convergence (NPC) is the closest
point at which an object can be seen singly during
bifoveal vision. In other words, it is the point at
which the two foveal lines of sight intersect,
when maximum convergence is exerted. It is
always closer than the near point of
accommodation and is usually less than 8 cm.
Far point of convergence refers to relative
position of the eyes when they are completely
at rest. It is usually infinity. At rest, eyes may be
in slight divergence and so the far point of
convergence may be in negative (behind the
eyes).
Range of convergence is the distance between
far point of convergence and near point of
convergence. The part of the range of convergence between the eye and infinity is called
positive convergence, and the part beyond infinity,
i.e. behind the eye (when eyes are in slight
divergence) is called negative convergence or
divergence.
Amplitude of convergence refers to difference in
the convergence power exerted to maintain the
eye in a position of rest and in a position of
maximum convergence.
Fig. 7.3 Convergence in prism dioptres.
entering the eyeball outwards and will tend to
produce diplopia. To maintain a binocular single
vision, the eye will turn inwards (converged)
through a corresponding degree (Fig. 7.3). The
convergence required to see singly an object
placed at one metre distance from the eyes with
a prism of one dioptre placed in front of one
eye is called one prism dioptre convergence. It
has been estimated that roughly 1 metre angle
convergence is equal to 3 convergence.
Measurement of amplitude of convergence
and near point of convergence
See pages 124 and 125 respectively.
ACCOMMODATIVE CONVERGENCE/
ACCOMMODATION (AC/A) RATIO
See page 126.
ANOMALIES OF CONVERGENCE
CONVERGENCE INSUFFICIENCY
Convergence insufficiency is the inability to
obtain and/or maintain adequate binocular
convergence for any length of time without
undue effort. It is the most common cause of
ocular asthenopic symptoms.
Etiology
1. Primary or idiopathic. In many cases, exact
etiology of the convergence insufficiency is not
known. It may be associated with a wide interpupillary distance and delayed or inadequate