Management of Refractive Errors & Prescription of Spectacles Yogesh Shukla
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Accommodation1

 
Introduction
Accommodation is the ability of the eye to increase its power, by virtue of which, one is able to see clearly from distance to near. The accommodation is measured in diopters (D).
It is to be understood that accommodation does not only involve the change in the shape of the crystalline lens and thus its power, but the mechanism of accommodation also involves the neuro-muscular effort to accomplish this change in power. Thus, we have the accommodation ‘effort’ and the accommodation ‘effect’.
The neuromuscular ‘effort’ involves the nerves supplying the ciliary muscles and the efficiency of the ciliary muscles themselves. The ‘effect’ is manifested by the change in curvature of the lens and its capsule.
It is to be further noted that accommodation also involves the vergence and the size of the pupil. To be precise, the whole ‘near reflex’, involving accommodation, convergence and miosis, comes into play to enable us to focus and see clearly at near. You shall see in the subsequent pages of this chapter, as to how accommodation is affected in a number of neuromuscular conditions, of which we do 2not usually take cognizance. Let us first, become conversant with a few definitions.
The amplitude of accommodation (AA) is the maximum dioptric power attainable by an eye.
The range of accommodation is the linear distance over which the person can maintain clear vision by virtue of accommodation.
This ‘range’ lies between the ‘far point of accommodation’ and the ‘near point of accommodation’, which simply means that the range covers between infinity to the closest point where the object can be seen clearly.
For clinical practical purposes, what is more important is the range of accommodation rather than the amplitude of accommodation. In other words, a person may have perfect amplitude of accommodation to see clearly close to the eye, but he will not be able to maintain clear vision at this distance for long and ocular fatigue will ensue. Thus, he will have to hold the object (a book for example) a little further away, where he is comfortable. Therefore, in prescribing presbyopic corrections, one should not rely too much on amplitude measured by a Prince ruler, but at the distance where the patient can comfortably accommodate for long.
‘Accommodative insufficiency’ is a term implying the inability of a person to see clearly at near, at the designated age.
‘Accommodative inefficiency’ is the term specifying a person's inability to focus quickly from far to near objects. In other words, there is a time lag in focusing from distance to near.
The accommodative power or the amplitude of accommodation slowly but surely, decreases with advancing age. But this decrease does not follow the ‘rule of thumb’ in each case. There are ample examples, where a person of age 45 or 48 reads comfortably unaided while others complain of difficulty in seeing at near before 40 years of age. This wide physiologic variation is baffling sometimes but we must not forget the interaction of nerves, muscles and the crystalline lens in the mechanism of accommodation.
Even the authorities in ophthalmology like Duane and Donders’ differed on the amplitude. Donders found it was 4.5 D at age 40, while Duane found it to be 5.8 D at age 40.3
Examining the observations by a number of authorities, and as measured by Prince ruler, the following guidelines are recommended:
  • At age 40—Average amplitude 5.0 D
  • At age 44—Average amplitude 4.0 D
  • At age 48—Average amplitude 3.0 D.
Below the age of 40 years, add 1.0 D for every 4 years; above 40 years, subtract 1.0 D for every 4 years. For example, at 36, the average amplitude would be 6.0 D, at 32 it would be 7.0 D.
Similarly, at age 52, average amplitude would be 2.0 D, and at age of 56, it would be 1.0 D only. To all the above figures up to the age of 50 years, ± 1.0 D should be considered as range.
These figures become more relevant when a person reaches the presbyopic age. It is then when the actual clinical test of the doctor comes into effect. Since the amplitude varies so much, a correct age for bifocals is difficult to predict.
It is more prudent to know the amplitude, range and the near requirements of the patient, rather than to follow strictly the above nomogram.
 
How to test Accommodation
Three most common tests are narrated here. As accommodation varies from person to person, so does it varies according to the test performed. It should be kept in mind, that each test is not foolproof and accurate. Even with the variables kept constant, chances of error prevail. At best, these give a fair amount of result which can be incorporated in prescriptions. The tests are performed monocularly.
 
Near-point Accommodation Method
In an emmetropic patient, the far point is at infinity. The near point will then be the dioptric equivalent of the maximum accommodative capability of the patient. In other words, converting the near point distance into diopters, will be the measure of amplitude. In this test, the patient views a near target, say N6 type of letters, which is moved towards the eye. The distance at which it blurs, is noted as the near point. Suppose the target blurs at 10 cm (0.1 m), then the amplitude 4is 10 D. This is based on a standard law of optics where 1 D increase in power has focal point at 1 m. It we use a larger target, say N8 or N10 size the near point will be closer as the patient will be able to clearly even at 6 cm. This means that the near point distance alone does not determine the amplitude. Target size is an important determinant. The illumination and speed at which the target is brought closer will also influence the amplitude.
 
Prince Ruler
In this, a scaled ruler marked with cm, and equivalent diopters, having a movable target riding on the ruler is used to measure the amplitude. It is usually 1/2 m in length, having a reading card mounted on the scale. A +3.0 sphere is usually added to the distance correction to pull up the range of accommodation to 40 cm.
The patient holds the prince ruler with one end resting on the nose and slowly moves the reading card towards and away on the Prince ruler, to locate both the near and far points of accommodation. The difference between the far point and near point gives the AA in diopters.
 
Spherical Add
This test is also performed monocularly. The patient fixates at a reading card at a convenient distance of 33 cm or 40 cm. Plus lenses are then added to relax the accommodation until the target blurs. Minus lenses are then added to stimulate the accommodation, until the target blurs again. The difference between the maximum plus and maximum minus gives the amplitude of accommodation.
The first two tests have a major drawback in the sense that when the object is moved closer, the angular size of image on the retina increases, and makes the letters easier to read, requiring less accommodation.
The spherical add method obviates this problem, but it also has a fallacy. The incorporation of minus 1.0 D to stimulate accommodation does not actually stimulate 1.0 D of accommodation and addition of minus 2.0 D stimulates only 1.8 and minus 3.0 D even less.5
Because of these inherent deficits, measurements by all these methods is only close approximates of amplitude.
In my experience and by other authorities, prince ruler is the recommended tool used for evaluating amplitude.
There are a few clinical pearls to be kept in mind while measuring AA:
  • Accommodative amplitude varies with the position of eyes. With the eyes down in reading position, the amplitude increases measurably. If evaluation is done in straight gaze position, which is normally done and the patients most of the activities are for near, then the patient will be over corrected for near. The patient may find working range too close with the correction given.
  • The amplitude also is greater when the frontal plane of the face is down, parallel to the floor. This is due to gravitational forward shifting of lens owing to relaxed zonules when ciliary muscles are contracting to induce accommodation. The effect can add 0.5 D to 1.0 D in elderly to even 3.0 D in younger person.
 
Accommodation and convergence
Every individual has a fixed neuromuscular relationship between the amount of accommodation exerted and the accompanying convergence. This is accommodative–convergence to accommodation (AC/A) ratio.
It is this finely balanced AC/A alliance that keeps the eye in alignment during all visual tasks at all distances. The alignment incorporates ‘fusional vergence’ by virtue of which, there is elasticity in the physiologic bonding between accommodation and convergence.
These are a few terms which are relevant in context with the accommodation–convergence relationship. They are more of academic interest but the clinician must be conversant with them.
‘Relative accommodation’ describes binocular accommodative function and concerns with the ability to sustain fusion, when accommodation demands changes. The essence of this is very clear. In normal physiologic conditions, both eyes accommodate equally and the demand for convergence and fusion is equally met, but there are conditions where both eyes do not accommodate equally, leading convergence and fusion in disarray.6
 
Test for Relative Accommodation
The patients gaze is binocularly fixed on a stationary near target. Spherical lenses are then simultaneously placed before both eyes till the target blurs. The maximum plus lens the patient can overcome to relax accommodation keeping the target clear is called ‘negative relative accommodation’ and the maximum minus lenses that the patient can overcome to stimulate accommodation is called ‘positive relative accommodation’.
 
Excessive Accommodation Amplitude
There are innumerable examples in clinical practice where patients boast that they can see clearly at near, at even at 50–60 years of age.
Parents, who come for refraction of their children, grumble as to how come such a small child is requiring glasses while one of its grandparent is reading newspaper without any glasses! This is a difficult situation, as the parents may not understand the scientific oddity behind the grandparents still good near vision.
There are a few clear-cut mechanisms by which the patient has good uncorrected near vision. Myopes, early nuclear cataracts, early cortical cataracts (with some clear area having greater diopteric power), pinhole affect from use of miotics in glaucoma therapy, are some examples which the clinicians should keep in mind while giving presbyopic correction.
 
Accommodation Spasm
To see clearly at near a person invokes accommodation or relaxes accommodation to see at far. Some persons, after using their eyes for long at near work, are unable to relax their accommodation to view at distance. Over time, this assumes a fixed nature of their accommodation and presents a ‘pseudomyopia,’ with all the features of true myopia. Eventually, the patients begins to manifest symptoms of ocular asthenopia and an excessive near work in this condition, may result in blurring for near also. This is ‘accommodation spasm’ and refraction in this state, will lead to overestimating a myopic error 7or underestimating hyperopic error. Cycloplegic refraction, will however, uncover the truth.
Accommodation spasm is frequently seen in persons with psychogenic stress. They are under pressure at school, workplace or home. Other ailments like spastic colon, duodenal ulcer, or migraine, is commonly identified in these patients.
Symptoms of this syndrome complex are headaches, blurred distance vision, abnormally close near point, and most commonly a fluctuating visual acuity. The hallmark of diagnosis is the marked difference in manifest and cycloplegic refractions. It would be understandable that as hyperopes have more accommodation demand, ciliary spasms should be more common in these patients. But surprisingly, this is not true. Accommodation spasms are as common in emmetropes as in byperopes.
When ciliary spasm occurs in a true myope, though uncommon, it may be easily misread as rapidly increasing myopia. A cycloplegic refraction again is the best diagnostic tool.
Patients with uncorrected astigmatism tend to make constantly varying accommodative effort in an attempt to see clearly and succumb to ciliary spasms.
 
Management of Accommodation Spasm
Patients with ciliary spasms are not easy to treat but one of the following can be tried.
 
Cycloplegia
This is the best and an easy treatment. An appropriate cycloplegic agent may be used to break the spasm and the patient advised not to indulge in near work for a few days.
 
Prescription of Plus Lenses
In a postcycloplegic refraction, the maximum plus tolerated for a clear distant vision, even if this is fraction of plus found during full cycloplegic refraction, should be given. Gradually, the spherical power can be increased in of +0.5 D to +1.0 D, and can be worn for months or even years.8
 
In Severe Cases
Cycloplegics like cyclopentolate 1%, can be instilled twice daily. Simultaneously, plus lenses or even bifocals can be prescribed to ensure clear vision for distance and near. After a period varying from few weeks to few months, the cycloplegic is discontinued and the patient continues wearing the distance correction.
 
Unequal Accommodation
Accommodation amplitudes in the two eyes are usually equal. If a difference is noticed with glasses, then the difference may be with refraction done.
A perfect refraction correction means that when two eyes are working together, the accommodation is equally relaxed in both eyes. This is known as ‘binocular balancing’ and is one of the most important and probably least used in routine refractions; a perfectly balanced refraction provides comfort and satisfaction to the patient.
Following are some useful tests for assessing the binocular balance. They are simple and quick to perform.
 
Partial Fogging Method
After giving the correction lenses in the trial from, blur the vision by inserting +0.75 D in each eye. Instruct the patient to concentrate on the line he is now seeing which may be 6/9 or 6/12. Alternate an occluder rapidly between the two eyes. Ask the patient each time which is better? Then, fog the better seeing eye with +0.25 D and again ask which is better? Continue this process, with + 0.25 D increments until both eyes are equally fogged. At this point, the monocular refractions of the two eyes are balanced and the accommodation is equally suppressed.
Now, remove the monocular cover to allow the eyes to see binocularly the line in question, and remove plus lenses simultaneously from both eyes, in increments of +0.25, until he sees the 6/6 line clearly with both eyes.
Though the description seems long, it will be found that if the original refraction is perfect the difference between the two eyes will not be more than +0.25 D.9
 
Full Accommodation Suppression
In this test +2.0 D lenses are used to fog the vision, instead of + 0.75 D, which reduces the vision instead to 6/60 or 6/36. The rest of the procedure is same as the partial fogging method.
Though some patients will find it difficult to explain the difference in vision with so extreme fogging but if the patient is able to feel the difference in acuity with this test, then it means that the manifest refraction is grossly faulty with undercorrection in plus or over-correction in minus.
 
Prism—Dissociation Test
In this test, vision is fogged with +0.75 D, but the dissociation of images in the two eyes is accomplished by vertical prisms, rather than by alternate occlusion. The patient is asked to concentrate on a single letter with both eyes open. Vertical prisms are introduced before the two eyes, of usually 4PD to 6PD, which will maintain vertical dissociation. Then fogging is done by introducing plus lenses in increments of +0.25 D, to blur the clearer line until both are equally blurred. At this point, prisms are removed and the extra plus lenses also.
This test is supposed to be more sensitive and gives more consistent results.
 
Causes of Unequal Amplitudes
In spite of best of efforts, if unequal amplitude persists, then reasons for such situation should be looked for. Uniocular trauma, blunt or penetrating, is a frequent cause of unequal amplitude. Early cataracts and early presbyopia, long-standing anisometropia and incorrect refraction are the other common causes.
 
Subnormal Accommodation
Subnormal accommodation is a term used to denote accommodation amplitude recordably less than for the patient's age.
Presbyopia is a physiological insufficiency of accommodation that comes naturally with advancing age. Hyperopia is another 10condition, where the amplitude may be normal, but a premature presbyopia sets in. Therefore, before labeling a person having a truly subnormal accommodation power, cycloplegic refraction is mandatory to rule out hyperopia or hyperopic astigmatism. The following conditions should be kept in mind before labeling a true subnormal accommodation as idiopathic.
  • Previous illness, such as postviral or exanthematous encephalitis
  • Present illness, like hypothyroidism, severe anemia, diabetes, open angle glaucoma
  • Drug intake, like anticholinergics, phenothiazines, tranquilizers, chloroquine, etc.
Complete paralysis of accommodation is rare. But the above mentioned causes can lead to complete accommodative paralysis and can be caused by cycloplegic agents, midbrain disorders, Adie's syndrome, Panretinal photocoagulation, etc.
The most common causes for unilateral accommodation paralysis are trauma, uveitis, and acute angle-closure glaucoma.
Accommodation insufficiency may be manifested as poor amplitude or as poorly sustained accommodation or as accommodation ‘inefficiency’. Poorly sustained accommodation most frequently occurs in uncorrected hyperopes, where accommodation cannot be maintained for long and either asthenopic symptoms ensue or frequent blurring occurs in near work.
Accommodation inefficiency is a condition where a patient takes time to focus at a near object after seeing at a distant object. This may occur in some of the conditions where ciliary muscles are affected or can be idiopathic.
 
Refractive Errors and Accommodation
Let us see what relationship exists between refractive errors and accommodation. An emmetrope differs from an optically corrected ametrope, whose ametropia has been corrected with glasses or contact lenses, because each of them have a different accommodation demand on their ciliary-lens apparatus in performing near tasks. Here are some examples to be remembered in doing presbyopic corrections in different refractive errors:11
  • A 8 D or 10 D myope, who wears a full spectacle correction for distance will require less accommodation for near viewing than if he is wearing contact lenses. This is why a 40-year-old patient has difficulty in reading when he switches to contact lenses from spectacles
  • The 10 D hyperopic person requires more accommodation for near targets with glasses, than when he switches to contact lenses. The explanation of this phenomena is based on the optical effectivity of various corrective lenses, which incorporates the vertex distance effect, in viewing near objects.
Table 1.1 is a good guide for understanding the vertex distance effect the various corrections have on accommodation required for reading:
Table 1.1   Accommodation required
With spectacle correction
With contact lens correction
Difference
10 D myope
3.0
4.5
1.5 D less
10 D hyperope
6.5
4.5
2 D more
with reading distance at 30 cm, and vertex distance 20 mm
It may be surprising, but this is a true physical phenomena and the magnitude of the difference in power and magnification, depends on how far from the nodal point of eye a lens is placed and whether it is plus or minus.
For a perfect correction of refractive error, a lens would have to be placed inside the eye, at the nodal point. An intraocular lens subverts most of these problems; but since it is a pseudolens, the laws of accommodation does not apply here.
Further, to the above explanation, spectacle or contact lens, all correct the refractive error and are said to have equal optical effectivity, but only if the object is located at infinity; the optical effectivity changes for near objects as it will be imaged at a different distance by each type of correction, and therefore, the demand for accommodation will be different in each correction.12
 
Pearls
As has been seen, accommodation problems are common and varied. Different ages and different refractive errors have different demand on accommodation, and therefore, in the process of correcting a refractive error, the role of accommodation must always be kept in mind. The following pearls will guide through a smooth and hassle-free journey of correcting accommodative problems:
  • As mentioned in the introduction, an accurate history taking is paramount. Correlate its relevance and compatibility with the clinical findings.
  • Use an accommodation measuring ruler, especially in presbyopic correction, where most of surprises are lurking.
  • Cyclopegia should be used judiciously.
  • Do not rely on manifest refraction alone at any age, especially when refracting a first timer patient. Always perform cycloplegic refraction also.