Although it has been shown that divergence is actively stimulated ( Breinin, 1957 ), exophoria appears to be a much more passive condition than esophoria. There are several potential explanations for this: the position of anatomical rest is relatively divergent ( Fig. 1.2 ), divergence has been thought to be a relaxation of convergence associated with a relaxation of accommodation, and the eyes do not diverge beyond parallel in normal vision. High tonic impulses to the abductors do not seem to be considered such a major factor in most exophoria in the way that high muscle tonus of the adductors contribute to esophoria. For near vision, factors that produce excessive convergence in children can even mask a basic exophoric deviation.
Based on the Duane-White classification, exophoria is usually considered under three headings:
- 1.
Divergence excess exophoria: a moderate or large distance exophoria, typically sometimes decompensating to a distance intermittent divergent strabismus, with compensated exophoria for near vision.
- 2.
Convergence insufficiency exophoria: shows decompensated exophoria for near vision. For distance vision, there is usually a smaller degree of exophoria which is compensated. The condition is typically diagnosed as a syndrome based on symptoms, larger exophoria at near than distance, low convergent fusional reserves, and remote near point of convergence (pp. 118–123).
- 3.
Basic (or mixed) exophoria: where the degree of exophoria does not differ significantly with the fixation distance.
Confusingly, the term convergence insufficiency, often abbreviated to CI, is used by some authors to describe convergence insufficiency exophoria (diagnosed in a variety of ways; pp. 118–123) and by other authors to describe isolated findings of a remote near point of convergence ( Cacho-Martinez et al., 2010 ). To avoid confusion, in this book, two different acronyms will be used, CIES for convergence insufficiency exophoria syndrome and NPCI for near point of convergence insufficiency, an isolated finding of a remote near point of convergence.
The differential diagnosis of decompensated exophoria and intermittent exotropia is not always clear, and this is especially so for divergence excess. Typically, patients with this condition fluctuate from good control (no exotropia or symptoms) to no control (distance exotropia). This contrasts with other forms of exophoria and esophoria, where the patient may have symptomatic decompensated heterophoria with no episodes of intermittent strabismus. It is debatable whether intermittent exotropia should be included in this chapter or in the section of Chapter 15 that deals with exotropia. Topics most relevant to heterophoria will be covered in this chapter and those relating most to strabismus in Chapter 15 , but both sections should be read in conjunction with each other.
Intermittent exotropia is more likely to be associated with neurological conditions (e.g., developmental delay, cerebral palsy, attention deficit disorder, history of intracranial haemorrhage) if it is of the convergence weakness type rather than the other types listed below ( Phillips, Fray, & Brodsky, 2005 ); although one study did not find an association between attention deficit disorder and convergence insufficiency ( Mezer & Wygnanski-Jaffe, 2012 ).
Divergence Excess
Divergence excess shows a large degree of exophoria for distance vision, which in many cases will be found to break down into a divergent strabismus. For near vision, the heterophoria is less by at least 7Δ ( Duane, 1897 ), and is compensated. Sometimes it is defined as an exo-deviation of 15Δ greater for distance vision than for near. Most patients with divergence excess are female, and the condition commonly presents itself in the mid-teens ( Pickwell, 1979b ), often as distance intermittent exotropia.
Approximately 10% of patients with intermittent exotropia have amblyopia ( Santiago et al., 1999 ). A long-term follow-up study of intermittent exotropia found that 36% converted to exophoria or orthophoria ( Rutstein & Corliss, 2003 ). Another study found the deviation only resolved in 4%, and more than half had an increase of at least 10Δ within 20 years of their diagnosis ( Nusz, Mohney, & Diehl, 2006 ). In a heterogeneous group of children (aged 3–10 years) with intermittent exotropia (mostly, divergence excess) who were not considered to require surgery and received no treatment over a 3-year period, progression to constant exotropia was uncommon and binocular control, stereoacuity, and magnitude of deviation typically remained stable or improved slightly ( Mohney et al., 2019 ).
Haggerty and colleagues described a grading system for intermittent exotropia ( Haggerty, Richardson, Hrisos, Strong, & Clarke, 2004 ). Consulting room assessment is problematic because the presentation is sometimes highly variable ( Hatt et al., 2007 ), although in approximately two-thirds of children the condition is stable ( Buck et al., 2007 ).
Aetiology
The causes of divergence excess are uncertain ( Cooper, 1977 ). It has been argued that accommodative convergence is the mechanism that maintains ocular alignment ( Ahn, Yang, & Hwang, 2012 ). A different view is that intermittent distance exotropia is controlled by convergence rather than accommodation, with the convergence inducing accommodation via the AC/A link ( Horwood & Riddell, 2012 ). These authors argued that minus lenses help in this condition not because they induce accommodation which induces convergence, but rather the minus lenses allow more controlling convergence to be recruited by correcting any secondary excessive accommodative (pseudo-myopia) blur.
True and Simulated Divergence Excess
A distinction has been made between ‘true divergence excess’ and ‘simulated divergence excess’ ( Burian & von Noorden, 1974 ). In simulated divergence excess, unilateral occlusion for 30–45 minutes causes an increase in the near deviation revealing a basic exo-deviation, not divergence excess. It seems likely that in these cases, high tonic, accommodative, or proximal convergence obscures the real nature of the deviation for near vision. The high convergence lessens as the patient reaches adulthood, and simulated divergence excess then reveals itself to be a basic exo-deviation. This may be important where surgery is to be considered, but nonsurgical management may be the same for true and simulated divergence excess in the initial stages. In simulated divergence excess, the management may have to be modified as the patient gets older.
Ansons and Davis (2001) further classified the condition, mainly based on the response to occlusion and on the size of the AC/A ratio. Their classification is summarised in Table 8.1 .
| Classification | Response of near deviation to occlusion | AC/A ratio |
|---|---|---|
| True divergence excess | No significant increase | Normal or low |
| Simulated with high AC/A ratio | Increases to be similar to distance deviation | High |
| Simulated with normal AC/A | Increases to be similar to distance deviation | Normal or low |
Investigation
The investigation of divergence excess should follow the routine eye examination, giving particular attention to the following:
- 1.
Symptoms: patients with divergence excess do not usually complain of any marked symptoms. If asked, they may report that intermittent diplopia has been present for as long as they can remember, but often there is established suppression and no diplopia. Some patients learn to control the deviation for distance by accommodating and will report blurred vision. The most usual reason given for presenting for eye examination is that their friends and relatives have noticed the divergence of one eye. This deviation becomes apparent with inattention, tiredness, emotional stress, poor health, and alcohol. Bright sunlight is also reported to produce the deviation ( Eustace, Weston, & Druby, 1973 ). Patients may therefore report that they close one eye in bright light. This is more likely to be related to photophobia ( Wiggins and von Noorden, 1990 ) than to avoiding diplopia and confusion ( Wang and Chryssanthou, 1988 ). Intermittent exotropia in children can be a cause of excessive blinking ( Coats, Paysse, & Kim, 2001 ).
- 2.
Visual acuity: In intermittent exotropia, worse binocular than monocular visual acuity can be a sign of diminishing fusional control ( Ahn et al., 2012 ).
- 3.
A cover test , which may show decompensated exophoria for distance vision, but sometimes this can appear compensated if the patient is exercising a high level of concentration. If the cover test is repeated, or the alternating cover test carried out, the distance vision deviation increases and the exophoria may break down into a divergent strabismus. Cover test recovery can be graded using a general grading system for all types of heterophoria and intermittent heterotropia ( Table 2.4 ) or using a scale specifically developed for intermittent exotropia ( Kim et al., 2017 ). A V-syndrome often accompanies divergence excess ( Chapter 17 ).
An important diagnostic sign is the deviation increases for true distance vision, that is fixation distances much greater than 6 m. This can be detected by repeating the cover test when the patient looks through a window.
- 4.
Refractive error , which in divergence excess is usually either low hypermetropia or myopia ( Pickwell, 1979b ).
- 5.
Fusional reserves , which are usually highly abnormal in that the divergent reserve for distance vision is very high: instead of the average value of 6–9Δ, it may exceed 20Δ. The very divergent position produced by measuring the base-in fusional reserve for distance vision is usually accompanied by suppression. This means that in some cases, when the limit of the divergent amplitude is reached, no diplopia is reported and this may give the appearance of a very much higher amplitude, unless the practitioner watches the patient’s eyes to note the point at which the divergence of one eye ceases. The very high divergent fusional reserve for distance vision is a major diagnostic feature.
Management
Removal of Cause of Decompensation
This is not usually possible in divergence excess.
Refractive Correction
Correction of any myopia assists by clearing the blurred distance vision and inducing accommodative convergence. In some cases, a negative distance addition can be used to correct the distance deviation and bifocals may be necessary to prevent excess accommodative convergence at near ( Percival, 1928 ). A prospective trial (nonrandomised) found 52% of patients with divergence excess intermittent exotropia achieve a good outcome with over-minus lenses alone and those who cannot be weaned out of lenses and ultimately require surgery also have a good outcome ( Rowe, Noonan, Freeman, & DeBell, 2009 ). These authors recommend over-minus lenses as the primary treatment and note they do not appear to induce myopia. Another study confirmed over-minus lens therapy for intermittent exotropia does not induce refractive errors ( Paula, Ibrahim, Martins, Bicas, & Velasco e Cruz, 2009 ).
In mild cases of divergence excess, a small adjustment to the prescription may be all that is required to alleviate symptoms. Case Study 8.1 is such a case.
Symptoms & History
Previous eye examination with local optometrist 6 months ago. Birth and first year normal; adenoidectomy at age 3 years. Spectacles for myopia since age 9 years, currently wearing MiSight contact lenses most of the time. Referred to the author because for the last 6 months reports transient diplopia with distance vision. Turning eye not seen by parents but noted by optometrist. The diplopia occurs three to four times a day and resolves immediately when the patient concentrates on his vision, but this often leads to a headache. Headaches are two to four times a week and patient believes result from straining to avoid diplopia. Diplopia is slightly less common with contact lenses. School progress good, spends a lot of time playing computer games. General health good and no medication. Family history: grandfather glaucoma and type 2 diabetes.
Relevant Clinical Findings
Normal: ocular health, pupil reactions, ocular motility, Amp. Acc., 25 degree visual fields, OCT scans, Ishihara.
| Spectacles & VA: | R −2.75DS 6/6+ | L −2.00/−0.75×35 6/6+B 6/5 |
| Retinoscopy: | R −2.25/−0.75×175 | L −2.00/−0.25×180 |
| Subjective: | R −3.00/−0.25×5 6/6+ | L −2.25/−0.50×35 6/6 |
| +1.00 blurring test: | R 6/12 | L 6/12+ |
| Stereoacuity (Randot, glasses):shapes – 250ʺ circles –16ʺ | ||
| Cover test (glasses): | D 10Δ XOP Grade 4 | N 5Δ XOP Grade 2 recovery |
| NPC (glasses): | 8 cm | |
| Mallett aligning prism (glas.): | D 1Δ in L, nil vertical | N 0.5Δ in, nil vertical |
| Mallett aligning sphere (gla.): | D −0.75 in addition to subjective findings | |
| D cover test with this ‘−ve Add’: D 6Δ XOP Grade 1 | ||
| Dissociation tests (glasses): | D 16 exo 0.5Δ down R | N 7Δ exo nil vertical/cyclo |
| AC/A (gradient, −1): | 4Δ /D | |
| D fusional reserves (glasses): | convergent −/4/2 | divergent −/18/16 |
Comment
The cover test recovery at D ( Table 2.4 ) supports decompensating distance exophoria from mild divergence excess. The aligning sphere and cover test indicate this will be compensated by a ‘negative add’ of −0.75 and this was prescribed. A letter was sent to the family optometrist explaining the management and asking them to increase the contact lens Rx similarly.
Follow-up 9 Months Later
No problems. Diplopia now rare, just when exceptionally tired and not a problem as easy to control. No turning eye seen. Distance and near vison clear. Headaches now once a week or fewer, when too much computer gaming. Findings similar to before, except improved cover test and aligning prism:
| Cover test (glasses): | D 8Δ XOP Grade 2 | N 3Δ XOP Grade 1 recovery |
| Mallett aligning prism (glass.) | D 0.5Δ in R, nil vertical | N 0.5Δ in R, 0.5Δ up L |
Management
No change recommended.
Comment
This case only required a small over-minus to render the divergence excess compensated.
Where there is a low degree of hypermetropia, the correction of this is unlikely to assist, unless it is required to equalise the acuities. Sunglasses or tinted prescription lenses sometimes assist compensation ( Eustace et al., 1973 ).
Eye Exercises
With teenage patients, eye exercises can be helpful for divergence excess. The incentive of the patient may not be very high, as there are often no marked symptoms, but where there is a reasonable level of cooperation, exercises may be an appropriate form of management. Exercises are less likely to work in cases where there is a vertical deviation, high AC/A ratio, or large angle ( Daum, 1984 ).
Where eye exercises are given, the three main aims are to: treat any suppression; develop the convergent fusional reserves and/or negative relative accommodation; and develop a correct appreciation of physiological diplopia. These aims may be achieved by some of the exercises described in Chapter 10 . They may be taken in the preceding order, or an exercise used which incorporates more than one aim. For example, physiological diplopia can be used in such a way that it develops convergence and relative accommodation, and at the same time will, by its nature, help in checking suppression. This type of exercise has been found particularly useful in divergence excess ( Pickwell, 1979b ).
Relieving Prisms
These are seldom satisfactory in divergence excess, as they disturb near vision.
Referral
Surgery may be considered in cases of simulated divergence excess as the patient ages, particularly if an exo-deviation occurs at all distances of fixation.
Convergence Insufficiency Exophoria Syndrome (CIES)
Convergence insufficiency exophoria syndrome (CIES) is found relatively infrequently in children aged under 6 years, but in approximately 5% of those aged 6–18 years ( Scheiman et al., 1996 ). A borderline result across a variety of tests may be as indicative of a disorder as one definitely abnormal result ( Rae, 2015 ). A sensible approach is to combine different test results in a diagnostic algorithm and this approach for CIES is described later, and more generally for decompensated heterophoria in Chapter 5 .
Aetiology
Anatomical and Physiological Factors
Anatomical factors seem to play a large part in most cases of exophoria. When uncorrected, myopia may build up a false accommodation-convergence relationship for near vision.
Age
The average phoria for near vision increases with age from the early twenties, in a steady progression, becoming on average about 6Δ exophoria by the age of about 60 years. With normal patients, this increasing physiological exophoria for near vision does not seem to be caused by the reading addition ( Freier and Pickwell, 1983 ). Elderly patients often have decompensated exophoria for near vision.
Investigation
A routine eye examination should be carried out in each case, as described in Chapter 2 . In addition to appropriate tests of binocular function, three points should be noted, particularly, in this type of exophoria:
- 1.
Symptoms , which are not usually as marked in exophoria as in esophoria. Suppression is more likely to be associated with exophoria, which exists to lessen the symptoms. It is unclear whether this is why, in old age, there is often a high degree of exophoria for near vision which is not accompanied by symptoms. The symptoms are likely to include frontal headache associated with prolonged use of the eyes ( Rabbetts, 2007 ), ocular fatigue, and sometimes intermittent diplopia for near vision.
A prototype symptom questionnaire was developed ( Borsting, Rouse, & De Land, 1999 ) and later modified ( Fig. 8.1 ) into the Convergence Insufficiency Symptom Survey (CISS) questionnaire ( Rouse, Borsting et al., 2004 ). The questionnaire has been criticised because 5 of the 15 items could relate to non-ocular difficulties ( Horwood, Toor, & Riddell, 2014 ) and its emphasis on reading leads to overdiagnosis of NPCI ( Clark & Clark, 2015 ). This questionnaire was not designed as a screening tool and, not surprisingly, used in isolation it has a low sensitivity for detecting CIES ( Horwood et al., 2014 ). The overlap between the symptoms measured by CISS and those of dry eye was noted on p. 16.
