The near point of convergence (NPC) of the eyes can be investigated by placing a fixation object at 30 to 40 cm in the mid-plane of the patient’s head. The patient asked to maintain fixation on the object as it is brought towards the patient until one of the eyes loses fixation and dissociates from the fellow eye (von Noorden, 1990). This has also been termed the push-up test and tests a combination of both reflex and voluntary convergence(Mallett, 1988). It has also been termed as tonic convergence (Daum, 1983b). According to von Noorden (1990), a normal NPC should be 8 - 10 cm with anything more remote than this being ‘defective’ or ‘remote’. The push up test can also be carried out subjectively with the practitioner asking the patient to report when the target is first seen in diplopia. The target should be a bold vertical line (Mallett, 1988).
Jump convergence(phasic or step convergence) is tested by asking the patient to change fixation from a distant target to a near target positioned in the mid-plane 15 cm from the patient’s head. The position of the near target can be varied to determine the near point (Rashbass & Westheimer, 1961; Pickwell & Stevens, 1975). This test is almost entirely voluntary in nature. The practitioner may observe either a smooth convergence of both eyes from distance to near or a variety of abnormal responses including a slow or hesitant movement; an over-convergence; versional movements followed by convergence; or no movement of either eye or movement of one eye only (Pickwell & Hampshire, 1981).
Whilst it is not possible to isolate completely reflex convergence, the Capobianco test, which employs a deep red filter placed before one eye to produce ‘partial dissociation’, is purported to provide a measure of reflex convergence with little or no voluntary input (Capobianco, 1952).
Anomalies of vergence
There are a number of ways of classifying vergence anomalies (von Noorden, 1990; Evans, 1997). A useful, but simplistic method is to classify anomalies according to the fixation distance at which they are manifest. The earliest description of this type is the Duane-White classification of divergence and convergence malfunction (Borisch, 1970). This classification applies to strabismus rather than heterophoria, and therefore, when used in the context of heterophoria tends to imply that heterophoria is a form of latent strabismus. Tait (1951) modified the original classification to incorporate the concept of accommodative convergence involvement and his concepts are included in the discussion to follow.
Divergence insufficiency (DI)describes the condition where the patient has a decompensated esophoria for distance vision with either no or a lesser amplitude of well compensated esophoria at near (Evans, 1997). Tait (1951) further categorised DI as either primary or secondary, and theircharacteristics are described in Table1. Both types may be associated with convergence excess.
The term divergence excess (DE) describes a decompensated exophoria for distance, with any exophoria at near being of less amplitude and well compensated (Evans, 1997). Tait (1951) described this as a marked exophoria at far with equal or less exophoria at near. If the NPC is normal this may be described as primary DE and if deficient, it may be described as secondary DE.
Convergence excess(CE) describes a decompensated esophoria at near with any esophoria for distance being of less amplitude and well compensated (Evans, 1997). Tait (1951) defines CE in similar terms, there being orthophoria or moderate esophoria for distance and more marked esophoria for near, with the cause being an excessive ACA ratio.
A common cause of CE is a high AC/A ratio and/or uncorrected hypermetropia e.g., the classical Donder’s type squint. Another cause can be of a psychosomatic nature.
Symptoms may be particularly associated with close work and may include frontal headache, ocular fatigue, blurred vision, inertia of accommodation (see notes on Anomalies of Accommodation).
Investigation may include a cycloplegic refraction, determination of AC/A ratio
This is very rare and may be associated with ciliary spasm. Can be psychosomatically induced. Refer any new cases.
Convergence cannot be elicited even with a prism. Causes include 3rd N anomalies, head trauma, Parinaud’s syndrome (convergence paralysis + vertical gaze palsy).
All new cases require referral for neuro-ophthalmological opinion.
Convergence insufficiency(CI) is a condition in which the patient has an inability to sustain sufficient convergence for comfortable near vision (Evans, 1997). Despite this simple definition, CI may probably be more correctly described as a syndrome. Although CI is frequently associated with convergence weakness exophoria, which presents as a decompensated exophoria for near with any distance exophoria being both well compensated and of less amplitude than for near (Evans, 1997), von Noorden (1990) pointed out that CI can occur in the presence of esophoria at near.
Whilst the categories above may be useful for descriptive purposes, in practise clinical signs are not always clear cut and Evans (1997) also defines basic (or mixed) esophoria and exophoria in which the amplitude of heterophoria does not differ from distance to near.
CI is one of the most common causes of ocular discomfort. Von Noorden (1990) suggested that it is the most common cause of muscular asthenopia and that it frequently has an aetiological connection with accommodative difficulties.
von Graefe (1855), (cited by von Noorden, 1990), described symptoms arising from CI. Mentions of CI can also be found in textbooks published towards the turn of the 18th century (Berry, 1893; Clarke, 1893). Duane (1897) subsequently provided a clinical description of CI.
The description of Rouse et al (1998) further developed that of Duane (1897) and Tait (1951). For far there is either orthophoria or a slight ( 2 to 4 ) exophoria, normal versions, frequently subnormal abduction ( 8 to 10 and not more than 15 ), with prism induced convergence often decreased to 14 to 20 or less. For near vision there is a marked exophoria of 12 or greater, normal versions, and a NPC of 7.5 cm or greater.
Daum (1986a), using a criterion of a larger exo deviation for near compared to distance, reported other correlates which essentially concur with Duane’s group of diagnostic signs. However, despite these precise diagnostic descriptions, and the observation that most patients do exhibit exophoria at near, the disorder can occur in the presence of orthophoria or even esophoria (von Noorden, 1990). It is not clear whether Duane required asthenopia to be present to make a diagnosis of CI and studies have suggested that the presence of symptoms is not essential for a diagnosis of CI (Capobianco,1952; Cooper & Duckman, 1978). However, it is of interest to note that the clinicians that presented the classic descriptions of CI were not in a position at that time to differentiate between well compensated and poorly compensated heterophoria. Hence the definitions of Evans (1997) described above add a useful dimension to categorisation of vergence anomalies.
Some authors have defined CI in terms of a single sign such as a remote NPC. For example, Letourneau et al (1979) diagnosed CI when the NPC was more remote than 10 cm when measured with a penlight. Pickwell & Stevens (1975) made an initial diagnosis of CI if a standard NPC measurement was remote or jump convergence was inadequate. Others such as Letourneau & Ducic (1988) and Scheiman et al (1996) used more than one sign to diagnose CI. In a most comprehensive review, Daum (1988) analysed 58 published papers and noted considerable variation in the criteria used to define CI. He found that symptoms and decreased positive fusional reserves at near were the only criteria named in more than one half of the studies reviewed. An extended NPC and an exophoria that was larger for near were criteria in about one-third of the papers.
A review of the literature by Rouse et al (1998) reported that estimates of the prevalence of CI vary between 1.75 to 33.0% (Norn, 1966; Dwyer, 1992). This variability may be attributed to variations in the definition of CI and to differences between samples. Table 2 is adapted from Rouse et al (1998) and summarises some of the studies of CI reported in the literature.
In a study of 11,600 subjects, White & Brown (1939) suggested a 7.5% prevalence of CI. Although they used Duane’s criteria for diagnosing CI, they failed to provide a population description.
Kratka & Kratka (1956) examined 500 patients and found a prevalence of 25% (n = 125) manifesting at least one finding of CI and 12.5% exhibiting exophoria at near, remote NPC, and reduced positive fusional reserves. They found that 75% of the 125 were symptomatic and reported that in the asymptomatic patients, the large exophoria and remote NPC were accompanied by excellent positive fusional reserves.
Using the criterion of an NPC of > 9 cm, Norn (1966) found a prevalence of CI of 1.75% in a population of 10,022 aged from 6- to 70- years.
Mahto (1972) reported that 11% of his 310 patients under the age of 40 showed an NPC greater than 10cm. This study must be regarded with great caution as the target used to assess convergence was the examiner’s fingertip.
Using either a remote NPC (break > 10 cm) or a poor jump convergence as diagnostic signs, Pickwell & Stephens (1975) reported a CI prevalence of 36% in 200 consecutive patients aged 8- to 83- years. The prevalence of patients presenting solely with a remote NPC was 12%.
Letourneau et al (1979) examined 735 children aged 7- to 14-year-old. Using the single criterion of an NPC > 10 cm, they found that 8.3% of children
had CI. Letourneau & Ducic (1988) subsequently assessed 2054 children aged 6- to 13- years and found that 2.3% manifested at least two signs of CI.
Pickwell & Hampshire (1981) assessed 505 consecutive patients presenting to an optometric practice. 50 of these were excluded from the study because of manifest strabismus or active pathology. NPC was measured using a line on a black card mounted on a near point rule to obtain the precise distance. In addition, jump convergence movements were observed whilst the patient changed fixation from 6 m to 15 cm. Of the 455 patients included in the study, 110 (24.2%) showed poor convergence by one or both methods. 20% showed an abnormal jump-convergence response. Further analysis of the prevalence of various jump convergence responses is shown in Table 3.
Dwyer (1992) used Sheard’s criterion and characteristics of fixation disparity curves to diagnose CI and reported a prevalence of 33% of 144 consecutive patients, aged 7- to 18- years, presenting to his optometry practice. It should be noted that such a sample is likely to be highly selective.
Scheiman et al (1996) reported the prevalence of CI to be 5.3% of 1650 consecutive patients’ aged 6 to 18-years seen in an optometry clinic. The criterion used for diagnosis was based on the presence of multiple clinical signs.
Rouse et al (1998) examined the prevalence of convergence insufficiency amongst 8 to 12-year-old children in optometry clinic settings. They found that 17.6% of the children were either definite or high suspect CI according to their criteria that are shown in Table 4.
Binocular - Eye Teaming Problems
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Convergence Excess (Overconvergence)
The conventional definition is that the eyes turn inwards toward each other in an excessive amount when focusing on an object at close distance.
If your eyes tend to do this you may experience blurred vision, intermittent double vision, headaches, eye strain and fatigue.
Jill was a 17 year old high school student who was referred because she wasn't doing well on her standardized testing. Her grades in all her subjects were all good but when she had to take long timed tests she did poorly and got headaches. Her visual examination showed that she had 20/20 sight in both eyes at distance and near but when I checked her eye teaming ability at near she showed a tendency to over converge her eyes. A pair of reading glasses was prescribed to help take the strain off her eyes while doing close work along with visual therapy to help her eyes be more flexible. After receiving her glasses and doing her visual therapy her standardized test scores for college entrance went up 200 points
Convergence Insufficiency (Under convergence)
The conventional definition is that in doing close work it is necessary for the eyes to turn inward towards each other (convergence) as well as to focus on the object (accommodation or focusing). When the ability of the eyes to converge is inadequate it is called convergence insufficiency.
If your eyes tend to do this you may experience blurred vision, double vision, headaches, eye strain, burning of the eyes, excess tearing.
Scott was a 10 year old boy who was having problems in school. Though highly verbal he just didn't like to read. He would start to read and less than five minutes later he would get up and do something else. His mother was worried since he was becoming a behavior problem at school and he was failing reading. He had been to the eye doctor but had been given a clean bill of health saying he saw 20/20 and his eyes were healthy. After the visit with me, their next stop was to the pediatric neurologist to see if medication could help.
His examination revealed 20/20 sight at distance and near and healthy eyes, but he did also show a convergence insufficiency, this eye condition made it extremely difficult to focus on near tasks ex. reading for extended periods of time. No wonder he didn't like to read. I explained to his parents that glasses wouldn't help but that visual therapy to teach his eyes to be able to focus at near without strain was recommended
After a home based six month vision therapy program Scott's behavior in school and his grades both improved significantly as he now had the ability to focus his eyes on a near task for an extended period of time.