INTRODUCTION TO ACCOMMODATIVE AND

BINOCULAR ANOMALIES

BY

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-resolution fovea.'

In other words, Accommodation is an involuntary adjustment of the dioptric power of the eye made to see objects clearly at any distance.

The Accommodative Process

To focus on a near object, it is necessary for the eye to increase its dioptric power. This change is referred to as Positive accommodation.

When an eye changes its focus from near to distance, a decrease in dioptric power is required referred to as Negative accommodation.

Accommodation is generally used when referring to an increased in the eye’s dioptric power.

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 power.

The components of the eye involved in the process of accommodation are: The third cranial nerve The ciliary muscle and ciliary body The zonules The crystalline lens The elastic capsule of the crystalline lens The vitreous

FIG 1: The Accommodative components of the Eye

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 muscle.

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 crystalline lens.

Recent data has, however, shown that the crystalline lens is elastic and there is, almost without doubt some involvement of the vitreous.

Some authorities believe that there are two types of zonules: Main zonule and Tension zonule, the latter being placed under tension during accommodation.

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 vision.

During accommodation for near, the ciliary muscle contracts, moves the ciliary body forwards and inwards, and reduces the tension in the zonule.

FIG 2: Illustration of Accommodative processes at near

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 Convergence accommodation Proximal accommodation Reflex accommodation Voluntary accommodation

 

Classification of Accommodation

Tonic accommodation represents the resting state of accommodation, being that amount of accommodation present in the absence of a stimulus.

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 accommodative tonus.

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 drive accommodation

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 two ways: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 the eye.

The Accommodative Stimulus

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 accommodation.

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 accommodation.

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 combination. For example, a test object may be at a distance of 1m, stimulating 1.00 D of accommodation, and a -2.00 D lens may be placed in front of the eye, stimulating an additional 2.00 D, for a total of 3.00 D stimulus to accommodation.

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 amplitude).

The Amplitude of Accommodation

 

The amplitude of accommodation declines from about 14 D at age 10 to about 0.50D at age 60.

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 accommodation exerted.

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 with ease.

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, Ltd.)

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:

Method 1 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.

Method 2

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 method.

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 following:

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 improved illumination.

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 letters.

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 of blur.

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-down technique.

The most commonly used alternative involves using increasing amounts of minus spherical lens power until distance vision blurs (‘Sheard’s technique’).

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 vision.

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.

Accommodative insufficiency should be suspected in patients with amplitudes less the values calculated from Hofstetter's formula for the minimum accommodative amplitude (Table 10-4):

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 examiner technique.

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 measured amplitude.

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 correction.

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 accommodative problems.

A lesion in the Edinger-Westphal nucleus or pineal tumours can cause reduced accommodation.

Iridocyclitis, sinus problems, focal infections, dental caries, or injections may be suspected in unilateral deficiencies.

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 accommodative insufficiencies.

Topical agents such as cycloplegics may have unilateral or bilateral effects, depending on their administration.

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).

Accommodative Facility

The test can be performed rapidly with minimal additional equipment.

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 through treatment.

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 D.

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.

Definitions of relevant to Accommodation

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.

Spectacle Accommodation

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 object.

Ocular Accommodation

FIG 4: The Spectacle and Ocular accommodations: Aspec = L1 with the sign changed.Aoc = K – L2 

It is the convergence that is associated with accommodation.

It may be brought into play by stimulating accommodation either by the use of a near-testing distance or by the use of minus lenses.

Accommodative Convergence

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 absent.

BINOCULAR VISION ANOMALIES

Anomalies of binocular vision in which fusion is maintained include heterophorias (usually simply called phorias), anomalies of fusional vergence, fixation disparity, and anomalies of accommodation.

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 correspondence.

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.

Heterophoria

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 disparity.

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 distant object.

There basically four phoria positions that can be assumed;

Exophoria – the covered eye will deviate outward.

Esophoria - the covered eye will deviate inward.

Hyperphoria- the covered eye will deviate upward or downward.

Orthophoria - the covered eye will remain in the original position.

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 hypertropia.

Strabismus (Tropia)

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 strabismus.

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..

Classifications of Strabismus

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 fusional vergence.

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.

suppression.

amblyopia.

eccentric fixation and

anomalous retinal correspondence.

Adaptations to Strabismus

Diplopia is a normal phenomenon which occurs in binocular vision for non-fixated objects whose images fall on the disparate retinal points.

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.

Diplopia and Confusion

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.

FIG 5a: Diplopia and Confusion

FIG 5b: Diplopia and Confusion

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 confusion.

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 coinciding.

FIG 6: Strabismic Amblyopia

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 retinal correspondence.

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 diplopia).

Suppression

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 non use).

Amblyopia

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 difficulty.

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 presented.

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 astigmatism.

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 amblyopia.

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 amblyopes.

Eccentric Fixation

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 eccentric fixation.

Corresponding retinal points are pairs of points, one in each retina, having the same visual direction.

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.

Anomalous Retinal Correspondence

ARC is classified as either Harmonious,

Unharmonious or

Paradoxical.

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 (strabismus).

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 or tropia.

Vertical or Cyclovertical Deviations

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 palsy.

Categorisation Cyclovertical deviation by Etiology

Vergences are binocular eye movements that are not conjugate.

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 movements.

The function of lateral (horizontal) vergences is to maintain bifoveal fixation of targets at various distances.

Fusional Vergence Anomalies

Therefore, lateral phorias and vergences are evaluated during fixation of a distant target and a near target during the typical eye examination.

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 corresponding meridians.

All three vergence motions are necessary for attainment and maintenance of bifoveal fixation.

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 functional strabismus.

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).

Accommodative Convergence/Accommodation Ratio (AC/A ratio)

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 diopters/diopter.

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 ratio.

It is also measured in prism diopters per diopter.

Here the change in accommodative stimulus is compared with the change in convergence response.

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 to accommodation.

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 accommodative convergence.

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 convergence,

AC/A = 20/2.5 = 8/1.