Introduction to accommodative and binocular anomalies
Prof (Mrs) Olajire.B Ajayi
OD, PhD, FNCO, FAAO
Accommodation refers to the process
whereby changes in the dioptric power of the
crystalline lens occur so that an in-focus
retinal image of an object of regard is
obtained and maintained at the high-
In other words, Accommodation is an
involuntary adjustment of the dioptric power
of the eye made to see objects clearly at any
To focus on a near object, it is necessary for
the eye to increase its dioptric power. This
change is referred to as Positive
When an eye changes its focus from near to
distance, a decrease in dioptric power is
required referred to as Negative
Accommodation is generally used when
referring to an increased in the eye’s dioptric
The total amount by which an eye can change
in power is known as Amplitude of
accommodation, which is simply the eye’s
maximum power minus the eye’s minimum
The components of the eye involved in the
process of accommodation are:
The third cranial nerve
The ciliary muscle and ciliary body
The crystalline lens
The elastic capsule of the crystalline lens
The traditional and most recognised theory
and process of accommodation is that
provided by Helmholtz, who suggested that
when looking into the distance the crystalline
lens is pulled and stretched into a thinner and
flatter form by the zonules, which are
themselves pulled by movement of the cilliary
Some years later, Fincham (1937) suggested
that the substance of the lens did not possess
the necessary elasticity to relax into a
naturally accommodated state, he believed
that, during accommodation, the crystalline
lens is moulded into more curved shape by
the elastic property of the capsule.
In this accommodated state, the cilliary
muscle is said to be relaxed.
During near vision, the ciliary muscle
contracts, causing it to move forward; this in
turn relaxes the tension of the zonules,
resulting in an increase in curvature of the
Recent data has, however, shown that the
crystalline lens is elastic and there is, almost
without doubt some involvement of the
Some authorities believe that there are two
types of zonules: Main zonule and Tension
zonule, the latter being placed under tension
Tunnacliffe (1993) summarises the theory of
accommodation as follows:
When the cilliary muscle is relaxed, the elastic
tissues in the cilliary body hold the zonule
under tension to focus the eye for distance
During accommodation for near, the ciliary
muscle contracts, moves the ciliary body
forwards and inwards, and reduces the tension
in the zonule.
The elastic capsule compresses the lens and
aided by the pressure from the vitreous, the
anterior lens surface bulges into a steeper
shape at the pole.
On returning to a distance focus, the ciliary
body relaxes, the elastic tissue in the ciliary
restores the tension in the zonule and, aided
by the elastic nature of the lens, the lens is
pulled into a flatter, weaker shape necessary
for distance focus.
Accommodation may be divided into various
functional groups as follows:
Tonic accommodation represents the resting
state of accommodation, being that amount
of accommodation present in the absence of
An eye is never totally unaccommodated
which is why we describe an
unaccommodated eye as an eye in its weakest
dioptric state. This is also referred to as
Convergence accommodation is that amount of
accommodation stimulated by convergence. In
youth, the accommodative response follows
convergence of the eye.
The reaction time for convergence is about
0.2seconds, which is almost twice as fast as
that for accommodation.
Accommodation lags behind and takes its cue
from convergence. Convergence is often said to
Proximal accommodation is the amount of
accommodation induced by the individual’s
awareness of the proximity of an object.
Reflex accommodation is the normal
involuntary response to blur in order to
maintain a clear retinal image.
Voluntary accommodation does not depend on
the presence of a stimulus. The ability to relax
accommodation from some near focus position
is easily learned
Accommodation may be stimulated in either of
1) By placing a test object closer than infinity (in
practice, closer than 6m).
2) By the use of minus lenses.
Either of these procedures has the effect of
increasing the vergence of the rays of light in
Assuming that a given eye has no refractive
error or that the refractive error is
compensated by lenses, an object located at
an infinite distance would form a sharp image
on the retina without the need for
However, if an object is placed at a distance
of 40cm from the eye’s spectacle plane (note
that object distances, for specification of
accommodation, are measured form the
spectacle plane located 13mm in front of the
corneal apex), rays of light form the object
would diverge and if no accommodation took
place, would focus at an imaginary point
behind the eye.
However, with sufficient accommodation, the
rays will focus on the retina. The amount of
accommodation required at 40cm is 1/0.40 D
or 2.50 D. The 2.50 D is the stimulus to
For the same eye, instead of moving the test
object to a distance of 40cm from the spectacle
plane, another way to stimulate 2.50 D of
accommodation would be to allow the test
object to remain at infinity and to place a -2.50
D lens in the spectacle plane of the eye.
The two methods
may be used in
example, a test
object may be at a
distance of 1m,
stimulating 1.00 D
and a -2.00 D lens
may be placed in
front of the eye,
additional 2.00 D,
for a total of 3.00 D
The amplitude of accommodation is the
maximum amount of accommodation or
focusing ability that the patient can exert in
response to a near target.
The near target is moved closer to the
patient’s eyes until it first blurs (the push-up
amplitude) and then moved away from the
eyes until it becomes clear (the push-down
The amplitude of accommodation declines
from about 14 D at age 10 to about 0.50D at
Accommodation or focusing allows targets to
be made clear over a large range of distances.
The amplitude of accommodation measures
the full range of accommodation: from the far
point, where accommodation is fully relaxed,
to the near point, with maximum
If the far point is at infinity (as in the case of
emmetropes and those wearing optimal
refractive correction for distance vision), then
measurement of the near point allows the
amplitude of accommodation to be determined
The amplitude is calculated simply by taking
the inverse of the near point of
accommodation, which is expressed in metres.
For example, if the near point was 10cm, the
amplitude of accommodation is 1/0.10 = 10 D.
The amplitude of accommodation gradually
falls with age, and causes patients over the age
of about 45 years to have difficulty with near
work and require reading glasses.
Measurement of the amplitude of
accommodation can help to identify the
appropriate reading add required to alleviate
the patient’s near visual problems.
The amplitude of accommodation becomes
zero at age 55–60 (Charman 1989).
FIG 3: The RAF Near point rule in use. (Haag-Streit UK,
The usual method of measuring the A of A for
a subject involves a technique known as the
blur technique. It requires the use of the RAF
near point rule which possess a centimetre
scale, a dioptric scale and an age scale.
The A of A is measured with the distance
correction in place, so any spherical and/or
astigmatic ametropia is corrected and the
individual has a real artificial near point (i.e. in
front of the eye).
The RAF rule should be placed below the
patient’s nose in a slightly depressed position
to mimic the action of the eyes when reading.
This method measures the spectacle A of A (A
of A measured in the spectacle plane), which
is the value required when calculating the
power of the near spectacle correction.
Monocular amplitudes are measured first (to
screen for anomalies of the 3rd cranial nerve)
and should be approximately equal.
The binocular amplitude is usually greater than
either monocular value because convergence
drives accommodation. Either the N5 letters or
the telephone numbers are used as a target.
The amplitude of accommodation is read off
the scale from the back of the slider.
There are two variations on the blur technique:
The individual moves the near print along the
sliding scale from the remote end of the rule
until the print just blurs. The A of A is read
off at this point.
This is called the ‘push-up-to-blur’ method.
The individual starts with the print at the end
of the scale closest to the face and moves the
print away until it is just legible. The A of A is
read off at this point.
This is called the push-down-to-clear
If both techniques are used, the recorded
amplitude of accommodation should be the
average of the readings obtained with both
methods. Possible subjective variations in
recording the clinical amplitude of
accommodation may be caused by the
The depth of field: when accommodation
occurs, the pupil constricts, so increasing the
depth of field.
Target luminance: the individual may have
trouble reading a certain size print in low or
poor illumination, but may read it easily in
This may also constrict the pupil causing the
further increase in the depth of field.
Variation in tolerance to blur: the individual
tolerates blur on a large-sized letter better
than on a small-sized letter, if the size of the
blur disc is assumed to be the same for both
The blur ratio (disc size: letter size) means
that a large letter may still be legible whereas
a small letter may not with the same amount
The push-up/push-down test is quick and
easy to perform and assessment of the near
point of clear vision relates to the typical
symptom reported by early presbyopes.
A combination of the push-up and push-
down measurements is preferred as it
provides a useful compromise between the
slight overestimate of the push-up technique
and the slight underestimate of the push-
The most commonly used alternative involves
using increasing amounts of minus spherical
lens power until distance vision blurs
This method typically provides lower
estimates of amplitude of accommodation
than those provided by the push-up method
and it can only be satisfactorily measured
using a phoropter.
In addition, the minus lens method provides a
less clinically relevant measure than the
push-up technique, which provides a direct
measurement of the near point of clear
Hofstetter" derived formulas for the expected
maximum, mean, and minimum
accommodative amplitudes in the population
from the normative data of “Duane and
Donders”. The formulas were based on age.
be suspected in
amplitudes less the
formula for the
Expected minimum amplitude = 15.00 -
[0.25 x (age in years)]
Expected mean amplitude = 18.5 0 - [0.30 x
(age in years)]
Expected maximum amplitude = 25.00 -
[0.40 x (age in years)]
The clinical measurement of accommodative
amplitudes may be affected by visual acuity,
target size and detail, depth of focus, patient
effort, blur interpretation, ability to converge,
refractive state, spectacle lens effects, and
Although full room lighting is desired,
excessive light should be avoided because of
pupil constriction with resulting increased
depth of focus, which can increase the
Uncorrected refractive errors will alter the
location of the near point of accommodation:
Uncorrected hyperopes will have erroneously
low amplitudes, and uncorrected myopes will
appear to have greater amplitudes, than
would be the case with the proper refractive
In addition to the effect of age, amplitudes
may also be reduced by disease, drug
reactions, or functional problems.
Illnesses such as mumps, measles, influenza,
anemia, and encephalitis may reduce
amplitudes. Multiple sclerosis and myotonic
dystrophy can have a similar effect.
Transient accommodative paresis may occur
in diabetics. Atrophy of the ciliary body in
some glaucomas may produce
A lesion in the Edinger-Westphal nucleus or
pineal tumours can cause reduced
Iridocyclitis, sinus problems, focal infections,
dental caries, or injections may be suspected in
Trauma to the craniocervical region, often seen
in whiplash, may also be responsible for
bilateral problems, whereas trauma, in the form
of a tear in the iris sphincter or the zonules of
Zinn, might reduce a monocular measurement.
Systemic drugs such as alcohol, central nervous
system stimulants and tranquilizers,
antihistamines, tricyclic antidepressants, and
phenothiazines may lead to bilateral
Topical agents such as cycloplegics may have
unilateral or bilateral effects, depending on
If a unilateral decrease in accommodation is
noted in conjunction with a dilated pupil, Adie's
tonic pupil and 3rd cranial nerve problems
need to be ruled out.
Accommodative facility is the ability of a
patient to rapidly change accommodation.
Accommodative facility is the ability of a
patient to rapidly change accommodation.
A reduced accommodative facility has been
shown to be related to symptoms experienced
in near viewing and it may exist even when
other accommodative measures, such as the
amplitude of accommodation are at normal
levels (Wick & Hall 1987).
The test can be performed rapidly with minimal
Measures of accommodative facility may be useful
in diagnosing binocular vision problems in
symptomatic patients whose phorias and visual
acuities are normal (Gall & Wick 2003).
It appears to have diagnostic value in that a
reduced facility correlates with near symptoms and
facility increases as symptoms are alleviated
Indeed, flippers can be part of the treatment.
There is little justification for the use of the
2.00 DS flippers other than that they are the
power traditionally used.
Indeed, it may be that what is required is a
range of flipper powers that relate to the
patient’s amplitude of accommodation.
For example, for a young patient with an
amplitude of 12.00 D, the 2.00 DS represent
only a 33% range of the amplitude, whereas
they represent a 67% range of the amplitude
in an older patient with an amplitude of 6.00
Accommodation and accommodation
amplitude having been defined, the following
are other terms of interest.
The true far point - That point conjugate with
centre of the macula by refraction at the
uncorrected and unaccommodated eye. in
other words, how far an uncorrected and
unaccommodated eye can see.
The true near point – That point conjugate with
the centre of the macula by refraction at the
uncorrected but fully accommodated eye. i.e.
How close an uncorrected and fully
accommodated eye can see.
The range of accommodation – The distance
between the true far point and the true near
point. i.e. As this a distance between two true
points, the eye is uncorrected.
The artificial far point – that point conjugate
with the centre of the macula by refraction at
the corrected and unaccommodated eye.
This translates into how far a corrected and
unaccommodated eye can see. With regard to
reading spectacles, the position of the
artificial far point is found by taking the
reciprocal of the reading addition.
The artificial near point - That point
conjugate with the centre of the macula by
refraction at the corrected but fully
accommodated eye. i.e. how close a corrected
and fully accommodated eye can see.
The range of clear vision – The distance
between the artificial far point and the
artificial near point.
The accommodation required to neutralise
negative vergence arising from a near object
measured in the plane of the spectacle lens is
known as Spectacle accommodation.
The symbol for spectacle accommodation is
Aspec and its value is simply the object
vergence L with its sign changed.
The accommodation required to neutralise
negative vergence from a near object
measured in the plane of the eye.
Aoc is the symbol for ocular accommodation.
Aoc = K – L2
Where K is the ocular refraction and L2 the
vergence arriving at the eye from the near
FIG 4: The Spectacle and Ocular accommodations: Aspec =
L1 with the sign changed.
Aoc = K – L2
It is the convergence that is associated with
It may be brought into play by stimulating
accommodation either by the use of a near-
testing distance or by the use of minus
Binocular vision anomalies can be considered
in terms of two broad categories:
1. Anomalies in which binocular vision is
maintained, but often at the cost of a
considerable amount of stress.
2. Anomalies in which binocular vision is
Anomalies of binocular vision in which fusion is
maintained include heterophorias (usually
simply called phorias), anomalies of fusional
vergence, fixation disparity, and anomalies of
Many of these anomalies occur in association
with a deficiency or an excess of
accommodative convergence and when this
occurs, an easily recognizable binocular vision
syndrome may be present.
When binocular vision is absent, the condition
is known variously as heterotropia,
strabismus or squint.
Strabismus may be accompanied by one or
more of a variety of adaptive phenomena,
including suppression, amblyopia, eccentric
fixation and anomalous retinal
Heterophoria can be defined as a latent
deviation of the eyes while Heterotropia –also
called strabismus or squint is a manifest
deviation of the eyes.
Phoria position, also called the physiological
position of rest, is the position that the visual
axes take with respect to one another in the
absence of all stimuli to fusion.
For the majority of people, for distance
fixation the visual axes are parallel or slightly
divergent but due to the presence of tonic
convergence stimuli, they are less divergent
than in the anatomical position of rest.
Unless an individual has strabismus, when a
distant object is viewed, the visual axes will
be parallel, being aimed at the object of
regard within the limits of fixation disparity.
This accurate aiming of the visual axes occurs
as a result of information provided by retinal
However if retinal disparity information is
temporarily removed, a process known as
dissociation of the eyes, the eyes will assume
the phoria position.
One of the simple methods of dissociating
the eyes is to place a cover (an occlude) in
front of one eye while a person is fixating a
There basically four phoria positions that can be
Exophoria – the covered eye will deviate outward.
Esophoria - the covered eye will deviate inward.
Hyperphoria- the covered eye will deviate upward
Orthophoria - the covered eye will remain in the
This is a manifest deviation of the eyes.
Strabismus may be manifest in just one eye
as a unilateral strabismus or in either eye as
an alternating strabismus.
Unilatereal strabismus is designated in terms
of the deviating eye. if the right eye turns
outward, the condition is right exotropia; if
the left eye turns inward, it is left esotropia
and if the right eye turns upward, it is right
1. Strabismus is classified in terms of
constancy, being a constant strabismus if it
occurs at all times and intermittent if it
occurs only part of the time. If it occurs at
only one testing distance (at 6m but not at
40cm, or vice versa), it is called periodic
2. Strabismus may also be classified in terms
of comitancy and etiology. Strabismus is
called concomitant if the angle of squint is
the same in all directions of gaze, and is
called incomitant if the angle differs in
different directions of gaze..
Functional strabismus is always concomitant
and may be due to pre-existing condition ( or
a combination of conditions) such as an
uncorrected refractive error, an abnormally
high or low AC/A ratio, or a deficiency in
Paralytic strabismus is typically incomitant,
although longstanding cases tend to develop
comitancy. Congenital paralytic strabismus is
usually due to an ocular muscle anomaly such
as a long or short muscle or a misplaced
muscle insertion or tendon.
Adaptations that occur in strabismus with the
passage of time include:
diplopia and confusion.
eccentric fixation and
anomalous retinal correspondence.
Diplopia is a normal phenomenon which
occurs in binocular vision for non-fixated
objects whose images fall on the disparate
It is easily demonstrated to persons with
normal binocular vision: fixate binocularly a
distant object and place a pencil vertically
some 25cm in front of your nose.
Two blurred pencils will be seen. An
individual with strabismus is beset by two
concurrent problems; diplopia and confusion.
To illustrate using the diagram below; (figure
5-27 Page 90 Grosvenor) when the good eye
foveally fixates the object ( a house, in this
case)the object is imaged on a nonfoveal area
of the deviating eye resulting in diplopia.
At the same time, whatever object happens to be
on the foveal line of sight for the deviating eye (a
tree in this case) will appear to occupy the same
position in space as the house, causing
Assuming that the house and the tree are the
only objects in the individual’s visual field, the
individual’s percept is that shown in figure 5-27.
Two houses and two trees are seen with the
middle images of the house and the tree
When strabismus is present at birth or
develops early in life, the problems presented
by diplopia and confusion are often solved by
the development of one or more of a number
of adaptive mechanisms.
These adaptations include suppression,
amblyopia, eccentric fixation and anomalous
Suppression is a phenomenon in which there
is a cortical inhibition of the information
arriving from specific regions of the retina of
the deviating eye.
The regions in which suppression areas
develop are the macular areas (thus avoiding
confusion) and the peripheral area of the
retina corresponding to the direction of
fixation for the normal eye (thus avoiding
According to Parks (1979), macular and
peripheral suppression differ in that macular
suppression exists in both binocular and
monocular vision, whereas peripheral
suppression exists only in binocular vision.
The worth dot test provides the one of the
simplest methods of testing for suppression.
Amblyopia is considered to exists when VA is
poor (less than 6/6 or 20/20) with corrective
lenses, without obvious cause.
Amblyopia can be considered as either
organic or functional.
Functional amblyopia includes strabismic,
refractive and hysterical (psychogenic)
amblyopia. Functional amblyopia due to
strabismus or refractive anomalies was
formerly referred to as suppression amblyopia
or as amblyopia exanopsia (amblyopia due to
Strabismic Amblyopia may occur in the
deviating eye of a strabismic patient as a
result of long-continued suppression.
The VA in the strabismic eye may be as poor
as 6/60 (20/200) or as good as 6/6 (20/20).
The typical response of a non amblyope in
reading the letters on the snellen acuity chart
is to read all letters in each line until a point
is reached at which some (or all) of the letters
in a given line cannot be distinguished
In contrast, the amblyope will often read one
or two letters in each of several lines, making
assessment of visual acuity difficult.
This is due to a phenomenon that has been
variously called contour interaction, the
crowding phenomenon or separation
Due to contour interaction, VA may be
relatively poor if a chart involving several
lines of letters is presented, but it tends to
improve if a single line of letters is presented
and improves still more if a single letter is
Therefore, in a routine vision screening of
school children, the whole chart method of
testing VA should be used. Otherwise, cases
of amblyopia may be missed.
Refractive amblyopia can occur as a result of
either uncorrected anisometropia or uncorrected
If one eye is much more hyperopic than the other
(for example, right eye +4.00 D, left eye +1.00
D), the more hyperopic eye will never experience a
sharply focused retinal image because the less
hyperopic eye will be required to accommodate a
lesser amount for vision at any distance.
Consequently, the more hyperopic eye routinely
suppresses, leading to amblyopia.
In the case of refractive amblyopia due to
uncorrected astigmatism, a child that has up
to 3.00 D or more of uncorrected
astigmatism may develop amblyopia in both
eyes because neither eye experiences
perfectly sharp retinal image.
This amblyopia, however tends to be small in
amount, with corrected VA of about 6/9
(20/30) and the vision often gradually
improves if corrective lenses are prescribed
during the early school years.
Eccentric fixation is another adaptation to
strabismus, which occurs along with
An off- foveal point in the retina of the
deviating eye is used for fixation both in
monocular and in binocular vision.
Eccentric fixation tends to occur if amblyopia
persists beyond early childhood, and it is
found in large proportion of strabismic
In esotropia, the eccentrically located retinal
point used for fixation is usually in the nasal
retina, whereas in exotropia it is usually in
the temporal retina.
The amount of eccentric fixation is closely
related to the depth of the patient’s
amblyopia: the greater the amblyopia (i.e the
poorer the VA), the larger the angle of
Corresponding retinal points are pairs of points,
one in each retina, having the same visual
Anomalous retinal correspondence (ARC) is a
condition in which an off-foveal point in the
retina of the deviating eye is associated, in
consciousness with the fovea of the fixing eye.
The angle between the fovea and the
anomalously corresponding point of the
deviating eye. subtended at the nodal point, is
known as the angle of anomaly.
ARC is classified as either
Harmonious ARC – the angle of anomaly is
equal to the angle of strabismus, so the
anomalous retinal correspondence serves to
fully compensate for the strabismus.
i.e. the anomalously corresponding point
serves to avoid both diplopia and confusion.
Unharmonious ARC - the angle of anomaly is
less than the angle of strabismus: strabismus
is not fully compensated.
Paradoxical ARC – the angle of anomaly is
such that the ARC not only does not
compensate for the strabismus, but it makes
the situation worse.
The term vertical deviation applies to either a
vertical phoria or a vertical tropia
A vertical phoria or tropia is often
accompanied by a cyclophoria or a
cyclotropia (a rotation of the deviating eye
around its anterior-posterior axis) thus the
term cyclovertical deviation is sometimes
used when referring to either a vertical phoria
As with esotropia or exotropia, a cyclovertical
deviation can be categorised as:
constant or intermittent.
comitant or incomitant and
functional or paralytic.
Idiopathic causes: Having no obvious
neurological or mechanical etiology.
Restrictive causes: Due to a mechanical
obstruction, including thyroid myopathy, blowout
fracture of the orbital floor, congenital fibrosis of
the extraocular muscles.
Neural dysfuction: Including fourth nerve palsy,
third nerve palsy, myesthenia gravis, multiple
sclerosis, skew deviation and double elevator
Vergences are binocular eye movements that are
Indeed, they are often called disconjugate eye
movements, because the lines of sight are
rotated toward or away from each other-not in
the same direction as occurs for conjugate eye
The function of lateral (horizontal) vergences is
to maintain bifoveal fixation of targets at various
Therefore, lateral phorias and vergences are
evaluated during fixation of a distant target
and a near target during the typical eye
There are vertical vergences, in which one eye
rotates up or down in the direction opposite
to that of the other eye, and torsional
vergences, in which an eye cyclorotates
relative to the other eye in order to achieve
All three vergence motions are necessary for
attainment and maintenance of bifoveal
The purpose of vergence eye movements is to
provide appropriate convergence and
divergence for the eyes.
Fusional movements are reflex movements of
the eyes occurring in response to retinal
disparity in order to produce a single image.
If the fusional movements are such that
although diplopia is eliminated there is still
some disparity; this is fixation disparity.
Fixation disparity is a small amount of
convergence or divergence that may be
present, relative to the plane of regard, when
an object is fixated.
The ability to make fusional movements on
the basis of disparity information prevents a
latent deviation from becoming manifest.
The importance of the ability to make
fusional movements may be demonstrated by
using as an example the development of
Fusional vergence therefore is the difference
between a phoria and a tropia.
The AC/A ratio describes the way
accommodation changes convergence when
the input to disparity vergence is cancelled.
The coupling of accommodation and
vergence allows clear stable single binocular
vision across a range of viewing distances.
A change in accommodation (A) is usually
accompanied by a change in vergence known
as accommodative convergence (AC).
When accommodation is exerted the eyes are
induced to converge. When accommodation is
relaxed the eyes are induced to diverge.
The amount of accommodative convergence
in prism dioptres (Δ) evoked by 1D of
accommodation is known as the AC/A ratio.
(i.e. The relationship between accommodative
convergence and accommodation is known as
the AC/A ratio).
The AC/A ratio can be thought of at three levels:
The first level is innervational. The
accommodative controller sends one signal to
the accommodative mechanism and another
proportional signal to the extraocular muscles,
commanding them to converge.
This signal must be encoded in the language of
nerves: action potentials. Therefore, the
innervational AC/A ratio must be measured in
units of impulses per unit of time.
The second level called the response AC/A ratio,
is the ratio of the actual outputs of the
accommodative and convergence mechanisms,
and it is measured in units of prism
It relates accommodative response with the
convergence that it causes.
However, the response AC/A is not often used
clinically, because its measurement is more time
consuming, requiring that the refractive state be
measured with each measurement of the phoria.
The third level - is the one that is most often
measured clinically - is the stimulus AC/A
It is also measured in prism diopters per
Here the change in accommodative stimulus
is compared with the change in convergence
As the actual accommodation response is
difficult to measure in clinical practice, it is
usual to measure the change in vergence
obtained with a fixed change in the stimulus
This is formally known as the stimulus AC/A
ratio but clinically it is usually just called the
AC/A ratio. The AC/A ratio is a useful
measure in the diagnosis and management of
binocular vision anomalies. AC/A ratios that
are abnormally high or low can give rise to
binocular vision problems.
The AC/A ratio remain fairly constant throughout
life until the onset of presbyopia. Measurements
of AC/A after the age of 45 years are of little
value (Ciuffreda et al. 1997).
If an individual is emmetropic or if any refractive
error is compensated with lenses, the stimulus to
accommodation for a distant object is zero.
If when a person fixates a distant object, the
visual axes are parallel when all stimuli to fusion
are eliminated (the condition of orthophoria), the
stimulus to convergence is also zero.
Note that for a person who is emmetropic
and orthophoric at distance, the AC/A ratio
will have a normal value only if 2.50 D of
accommodation is accompanied by 15∆ of
If 2.50 D of accommodation is accompanied
by only 10∆ of accommodative convergence,
AC/A = 10/2.5 = 4/1.
And if 2.50 D of accommodation is
accompanied by 20∆ of accommodative
AC/A = 20/2.5 = 8/1.