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Accommodation in practice: This article outlines the tests and conditions related to the accommodation system that can be encountered in optometric practice.

Introduction

An accurate accommodation system is important in maintaining clear vision. Stress to this system causes many problems, both common and uncommon, that optometrists will encounter in practice. From the expected cases of presbyopia to less common presentations, including accommodative spasm and inertia, it is rare for an optometrist to go through a day of practice without coming across some form of accommodative condition. This article will give an overview of how accommodation works, the main conditions that affect it and the management options available.

Accommodation

Accommodation is the ability of the eye to bring objects into focus along a range of distances from far-to-near by altering the shape of the crystalline lens in the eye. (1,2) It is primarily controlled by the parasympathetic nervous system; however, the sympathetic nervous system does contribute to regression of the accommodative response when the stimulus to accommodate has been removed. (3)

A four-part classification system describing components of accommodation has been in use for over 70 years, (4) and includes the following:

* Reflex accommodation

* Tonic accommodation

* Vergence accommodation

* Proximal accommodation.

Reflex accommodation is thought to be stimulated mainly by retinal blur to keep vision in focus; this component does not usually exert an effect greater than 2.00D and is regarded as the most influential component in the accommodative system. (5,6) Tonic accommodation refers to the resting position of accommodation when there is no active visual stimulus, (7) for example, when a patient is in a completely darkened room. Proximal accommodation is stimulated by the recognition of a near object. Finally, vergence accommodation is stimulated by convergence depending on the working distance of a given task. (1)

The accommodative response still fluctuates when focusing on a target at a fixed distance. (8-10) The microfluctuations have been shown to have two frequencies: a high frequency component (HFC); and a low frequency component (LFC). (8) The LFC tends to have a frequency below 0.6Hz whereas the HFC is between 1.0 and 2.3Hz. (11) The cause of HFC microfluctuations has been connected with the arterial pulse. (9, 11) It is maintained that LFC microfluctuations are of a neurological source. (10)

Prevalence

The most prevalent accommodation problem we see in practice is presbyopia, which occurs in most patients above the age of 45 years but can occur sooner in some patients. Optometrists will also see accommodation issues in young children, particularly in cases such as accommodative esotropia, which is the most common cause of strabismus in children. (12) Accommodation problems can cause issues for computer users; however, it is not thought to be any more severe than if hardcopy were being used instead. (13) Nevertheless, it is still important to investigate possible accommodative causes of computer vision syndrome symptoms alongside other possible contributing factors including dry eye, uncorrected refractive error and computer ergonomic issues. (13)

Refractive error

Before any accommodation tests can be carried out, it is important to start with a good foundation in the form of an accurate refraction. A lot of accommodation problems tend to be down to uncorrected refractive error, which has a knock-on effect on the ability to accommodate appropriately. For instance, in the case of uncorrected hyperopes, the accommodation system is compromised through over-exertion of the ciliary muscle for long periods during close work. Common symptoms associated with this will be a frontal headache and, in some more marked cases, blurred vision at near that gets worse as the day goes on due to the accommodative system having to over-exert to maintain clear vision. (6)

Presbyopes also suffer with symptoms of blurred near vision and eyestrain. As with uncorrected hyperopes, the accommodation system is unable to maintain the focus required in completing close work tasks over a long duration. The ciliary muscle has been shown to be relatively unaffected by the ageing process, (14) with decreased flexibility of the lens considered to be the main cause of presbyopia. (15) In order to correct this, a reading addition is required which gradually increases as the patient gets older. (16) Working distance is an important factor to consider as the range of near vision decreases as the add increases. Therefore, it is important to measure the patient's habitual working distance(s). Failure to do this can lead to the patient being unable to tolerate their near prescription. In the case of VDU users, varifocals or occupational lenses should be used to allow for this, or two separate pairs for near and intermediate vision if there is poor tolerance with varifocals. (17) Contact lenses are a viable alternative to spectacles for correction of presbyopia, (17,18) alongside a number of different surgical options. (19)

Accommodation tests

If an accommodation problem is suspected there are a number of tests that can be carried out to determine the presence of an anomaly and identify its type. These tests are not exhaustive and work best as part of a full eye examination incorporating a detailed case history, an accurate refraction and a thorough check of ocular health. A cover test should be performed to highlight any deviations, which may be contributing to the symptoms reported. Due to reduction of useable accommodation with advancing age, tests used specifically for measuring accommodation tend to be less relevant as patients pass the age of 45 years.

Amplitude of accommodation is usually measured using an RAF rule monocularly and binocularly (see Figure 1). This measures the full range of clear vision from the far point to the near point. The near point is calculated in dioptres as the reciprocal of the near point in centimetres. The patient should be encouraged to keep the target as clear as possible and instructed to tell the optometrist when blur is first detected in order to find the most accurate measurement. The patient's optimum distance correction should be in place and as such it is advised to do this test following the refraction element of the examination. (20) Normal results are based on the age of the patient (see Table 1, see page 67). (21) There are many opinions on whether it is better to perform the test with the target being pushed towards or away from the patient. Due to the subjective nature of the test, the result tends to be overestimated when the target is moved towards the patient and underestimated when being moved away. (22) A compromise is found by finding the median point between the results of the push-up and pull-away methods.

Accommodative facility is a measurement of the eyes' ability to change focus from certain distances to another over the course of a minute. The test can give a good measure of how the accommodative system stands up to fatigue. It is measured using a set of flippers with plus and minus lenses that can vary depending on how much pressure is to be put on the accommodative system. It is measured in cycles per minute (cpm) with one cycle comprised of the eyes focusing through one set of lenses, then another. Lenses are changed to the next set every time the patient reports the target being clear (see Figure 2). The test should be carried out for the full minute. For example, halving the test duration and multiplying the cpm by two will not show the true result as the cycles tend to be longer as the test goes on. Normal values are reported as 8cpm binocularly and llcpm monocularly in adults when flippers comprising -2.00D and +2.00D lenses are used. The result is expected to be lower in children, with 5cpm binocularly and 7cpm monocularly considered normal. (23) Results lower than this may indicate symptomatic fatigue in the accommodative system.

Establishing the lead or lag of accommodation can prove helpful and is a test that can be done straight after refraction while the trial frame or phoropter is still in place. The lead of accommodation can be defined as by how much the eyes overaccommodate to a stimulus whereas a lag of accommodation is when the eyes underaccommodate. (24) This is easily calculated using dynamic retinoscopy by either changing the working distance (Nott retinoscopy) or changing lenses (MEM retinoscopy) to determine the lag or lead of accommodation. (25) In most patients there will tend to be a small lag as the full accommodative response is not required due to depth of focus. Lead of accommodation or larger lags of accommodation may prove to be symptomatic. (26)

It is important to remember the strong connection between accommodation and convergence. Therefore, in order to perform an adequate differential diagnosis of possible accommodative disorders it is necessary to carry out tests to evaluate convergence.

The near point of convergence (NPC) is a simple test to determine the nearest point at which binocular single vision can be maintained; this can be done using an RAF rule, similar to measuring the amplitude of accommodation, or using a budgie stick with the patient being asked to focus on either a line or a single letter. The target is gradually brought closer to the patient and they are asked to report when the target appears double. Typically, a result greater than 10cm can prove symptomatic. This test should be done objectively as well as subjectively as in some cases diplopia will not be reported due to suppression. Both eyes should be observed with one of the eyes moving out indicating a break in binocular fusion. (27)

Vergence facility is similar to the measurement of accommodative facility. However, in place of plus and minus lenses, base out and base in prisms of varying power are used to put stress on the vergence system. It is carried out in the same manner as accommodative facility with the patient observing a near target at their normal working distance and measuring how quickly an individual can maintain binocular vision when vergence demand is alternated. (28) Examples of common prism combinations used include 3 base in/12 base out and 8 base in/8 base out. (26)

Measuring the accommodative convergence/ accommodation (AC/ A) ratio is a good method of testing how well accommodation and convergence are working together. It calculates how many prism dioptres the eyes converge or diverge for each dioptre increase or decrease change in accommodation; this can be measured by adding lenses in front of the patient's corrected prescription using a Thorington near card or Maddox wing and calculating the change in phoria per dioptre increase or decrease in what is known as the gradient method. The AC/ A ratio is typically between three to four prism dioptres per accommodative dioptre in eyes with functionally normal binocularity. (29,30)

Accommodation and vergence conditions and management

There are a number of conditions that can cause accommodation to break down. Accommodative esotropia is most commonly found in children during their first decade of life. (31) A high degree of hyperopia is expected in these cases with the esotropia being caused by the need to overaccommodate to overcome the uncorrected refractive error; this results in overconvergence due to the AC/ A relationship and increases the more the patient tries to accommodate to correct the refractive error and respond to changes in working distance. (12) Cycloplegic refraction is advised in all cases and any possible onset of amblyopia should be investigated. The esotropia can be fully or partially accommodative. In fully accommodative esotropia the strabismus will be corrected using full refractive correction; however, a slightly reduced prescription tends to be given to prevent the onset of an exotropia. (23) In partially accommodative esotropia, a smaller misalignment of the eyes is still left over following refractive correction. (12,32) The full prescription is given and orthoptic intervention or surgery may be required depending on the severity of the residual strabismus.

A common condition experienced by adults and children is accommodative insufficiency where the individual's accommodation is less than expected for someone of their age. (33) Causes include extreme fatigue or general health issues. Symptoms include blurred vision at near and frontal headaches. (34) Clinical tests show a reduction in the amplitude of accommodation, reduced accommodative facility, reduced near vision/ visual acuity and an exophoria at near on cover test that becomes relatively esophoric if the patient tries to exert more accommodation. (23)

Accommodative fatigue has many of the same signs and symptoms of accommodative insufficiency; however, it is more transient than the latter. While the accommodative system can be free of symptoms for a time it tires easily. For instance, in the case of VDU users they notice the condition deteriorating as their workday progresses with both the symptoms developing and their severity increasing. (35)

In both cases, any underlying conditions causing the insufficiency and fatigue should be treated. Positive lenses in the case of uncorrected refractive error will aid relaxation of the accommodative system and help to prevent overexertion during close work. A low addition has been shown to help in some cases; however, an excessive addition can prevent the accommodative system from functioning as it should. (36) Orthoptic exercises can be used if correcting the refractive error does not fix the problem. In some cases, there may be a connection with convergence insufficiency. For these patients push up exercises to strengthen convergence can help as may flipper exercises in the case of accommodative fatigue; (37) however, it has been recommended that larger studies are required to understand in which groups of patients these treatments are most effective. (38)

Accommodative inertia, also known as accommodative infacility, is a condition where the accommodative system has difficulty switching focus between distances. Most prevalent in adults over 30 years of age, intermittent blur is reported following changing focus from distance to near and vice versa. (23) Numerous causes include prolonged near work, poor general health, anisometropia, early presbyopia, and Holmes-Adie syndrome in unilateral cases. Reduced amplitude of accommodation and accommodative facility are likely findings in these cases. Any underlying conditions should be treated and refractive error corrected. Push up exercises or flipper exercises can be used to help with convergence and accommodative facility. (23,39)

Accommodative spasm, also known as accommodative excess, is caused by constant constriction of the ciliary muscle, which leads to exertion of accommodation. (40) There are many possible causes including uncorrected hyperopia, prolonged near work, an underlying emotional cause, lesions of the brain, multiple sclerosis, meningitis and head trauma. Commonly, the patient may present with pseudomyopia, headaches, and ocular discomfort. Cycloplegic refraction should show that myopia is not present. Esotropia and pupil miosis are present in more defined cases. In cases where hyperopia is present it should be gradually corrected. Cycloplegics can be used over the course of many weeks to treat more pronounced cases by breaking the spasm. Once this has been achieved, orthoptic exercises can be given to prevent a reoccurrence of the condition. (23,40)

Conclusion

The optometrist in practice can effectively deal with the vast majority of conditions affecting accommodation. As with any other condition, if the optometrist feels it is requires a more experienced practitioner, particularly in more complicated cases or those with systemic associations, it is important that the patient is referred on for appropriate investigation and management.

Exam questions

Under the enhanced CET rules of the GOC, MCQs for this exam appear online at www.optometry.co.uk. Please complete online by midnight on 9 February 2018. You will be unable to submit exams after this date. Please note that when taking an exam, the MCQs may require practitioners to apply additional knowledge that has not been covered in the related CET article.

CET points will be uploaded to the GOC within 10 working days. You will then need to log into your CET portfolio by clicking on 'MyGOC' on the GOC website (www.optical.org) to confirm your points.

References

Visit www.optometry.co.uk, and click on the 'Related CET article' title to view the article and accompanying 'references' in full.

About the author

* Niall Hynes is an optometrist and PhD candidate at the School of Optometry and Vision Science at the University of Bradford where he also studied his undergraduate optometry degree, graduating in 2011. Following this, he spent three years in a busy optometric practice in Sheffield. He returned to Bradford in 2014 to start his PhD researching factors that affect accommodative microfluctuations in addition to teaching on the undergraduate optometry programme. In his spare time Mr Hynes is a volunteer optometrist with Vision Aid Overseas primarily working in Zambia and is now part of the team that prepares new volunteers for assignments.

Course code: C-57770 Deadline: 9 February 2018

Learning objectives

* Be able to elicit relevant detail from patients presenting with accommodative anomalies (Group 1.1.2)

* Be able to assess accommodative function in children (Group 7.1.3) Understand the management of patients presenting with accommodative anomalies (Group 8.1.2)

* Understand the management of patients presenting with accommodative anomalies (Group 7.1.5)

Caption: Figure 1 Demonstration of the test for amplitude of accommodation using RAF rule following refraction

Caption: Figure 2 Demonstration of measurement of accommodative facility
Table 1 Reduction in amplitude of accommodation with
age (21)

Age (years)         Amplitude of
                  accommodation (D)

10                    13.40
15                    12.30
20                    11.10
25                     9.90
30                     8.70
35                     7.30
40                     5.80
45                     3.60
50                     1.90
55                     1.30
60                     1.20
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Title Annotation:Accommodation
Author:Hynes, Niall
Publication:Optometry Today
Date:Jan 1, 2018
Words:2889
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