The near triad and associated visual problems

Abstract

When a person changes fixation from one target, at a given fixation distance, to another target, at an alternate fixation distance, a number of ocular systems need to be altered in order to maintain clear single binocular vision. One such system is that of accommodation, involving either positive or negative accommodation 1 . When viewing an object at six meters, a distance known optometrically as optical infinity, it is assumed that parallel light rays enter the eye. For an emmetrope this results in zero accommodative demand and therefore zero accommodation. When viewing an object closer than 6 meters positive accommodation is required. This involves intraocular lens changes which ultimately increase the refractive power of the eye, allowing the rays to form a point focus on the retina. The opposite process, negative accommodation, occurs when an object is fixated further away from some near fixation point. Accommodation can therefore be thought of as the process by which the refractive power of the eye is altered to ensure a clear retinal image 1 .It should also be noted that a change in the position of the visual axes must also take place 1 . This occurs to ensure that a single image of the target is seen. The change in relative position of the visual axis is called convergence when the angle formed by the visual axes increases, and divergence when this angle decreases 1 .When a person fixates a near target an additional process takes place, that is, pupillary constriction. This occurs together with accommodation and convergence, and alters the depth of focus 1 . These three sub-systems work together as a synkinesis known as the near triad 1, 2 .One should not forget the fact that the processes mentioned above are very complex and involve numerous pathways, the details of which are little known. As a result it is easy to understand that many problems can occur in each process, resulting in visual problems. This paper serves to address some of those visual problems. AccommodationThe amplitude of accommodation represents the closest near focusing response that can be produced with maximal voluntary effort in the fully corrected eye 2 . From around five years of age to around 52 years of age, the accommodative amplitude progressively decreases at a rate of approximately 0.30 D per year 2 . Various theorists attribute the loss of accommodation to changes in the elastic properties of the lens, changes in the action of the ciliary muscle, or both 3 . The anterior chamber depth, shape of the lens, refractive index and position of the lens surfaces change with age, affecting the accommodative process 4 . This leads to the development of presbyopia. The amplitude of accommodation inevitably decreases from 11 D or more at age 10 to nearly 0 D by age 70 years. This means that an emmetropic child can focus an object as close as 10 cm, while an older adult must move objects at least a metre or more away to see them clearly 3 . †

A review of the classification of nonstrabismic binocular vision anomalies

“…A persistently lower accommodation than expected for age is accommodative insufficiency 20 and this does not result from crystalline lens sclerosis. 1 In this, the amplitude of accommodation is significantly lower than the lower limit of expected for age using Hofstetter's formula.…”

“…4,21 These patients will usually exhibit a reduction in the accommodative amplitude by 2D or more. 4,20 They also show a reduced positive relative accommodation, difficulty with minus lens of accommodative facility, and an increased lag of accommodation. 1,4,20 Accommodative insufficiency, though as the commonest accommodative disorder 1,4,21 has the main symptom being a general asthenopia related to near 20 although it also presents with other clinical signs and symptoms.…”

“…1 Ill-sustain accommodation has been categorized as a sub classification of accommodative insufficiency 1 and is also known as accommodative fatigue. 20,21 On occasion, a patient might have sufficient amplitude of accommodation, but is unable to maintain the response, which may be the first stage of accommodative insufficiency. 4,20,21 Patients with this disorder initially report the ability to do near work and experience discomforts after some time 20,21 because the accommodative apparatus fails to maintain its effort to accommodate.…”

“…20,21 On occasion, a patient might have sufficient amplitude of accommodation, but is unable to maintain the response, which may be the first stage of accommodative insufficiency. 4,20,21 Patients with this disorder initially report the ability to do near work and experience discomforts after some time 20,21 because the accommodative apparatus fails to maintain its effort to accommodate. With continuous near work, accommodative system loses power and near point recedes causing blur near vision; normal amplitudes of accommodation begin to deteriorate over time.…”

“…Accommodative excess refers to an incessantly higher accommodative amplitude than age expected norms 20 , and is due to spasms of the ciliary muscle. 21 Clinical signs include a lead of accommodation and reduced negative relative accommodation.…”