Most esophoria is ‘accommodative’, in that it largely results from excessive accommodation due to uncorrected hypermetropia or prolonged close work. As a result of the accommodation/convergence linkage, the overactive accommodation produces excessive convergence. Some eso-deviations do not have this accommodative factor and are then known as ‘nonaccommodative’ or sometimes as anatomical esophoria.
Esophoria can be classified (Duane-White classification) according to whether the esophoria is greater for distance or for near vision, or if it is the same for both:
Divergence weakness: shows decompensated esophoria for distance vision. In near vision, the heterophoria will be smaller and compensated.
Convergence excess: characterised by an increase in the degree of esophoria for near vision. There is usually a small degree of compensated heterophoria for distance vision, and a higher degree of esophoria which is decompensated for near vision. This is in contrast to the normal physiological exophoria.
Basic (or mixed or nonspecific) type: shows decompensated esophoria of about the same degree in distance and in near vision. The methods of investigation and management that apply to both divergence weakness esophoria and to convergence excess esophoria will apply to basic esophoria. A separate section on basic esophoria is therefore not included.
Divergence Weakness Esophoria
This is the most common cause for decompensated esophoria in distance vision. It is usually decreased by the refractive correction to the extent that it becomes compensated.
Factors such as abnormal orbital shape, lengths of check ligaments, muscles insertions, etc., are thought to contribute to esophoria in some patients. There is no evidence to show that these change in adult life, except after injury. If esophoria becomes decompensated, therefore, it is because other factors have intervened: poor health, deteriorated working conditions, etc. The anatomical factors may, however, explain why some patients have a predisposition for their esophoria to decompensate.
Excitable or ‘Neurotic’ Temperaments
The esophoria in these cases may be variable with the emotional state and level of anxiety, being compensated one day and decompensated the next. It may be aggravated by stimulants.
Patients with uncontrolled HIV/AIDS have more esophoria/less exophoria at distance and near than a control group ( Espana-Gregori, Montes-Mico, Bueno-Gimeno, Diaz-Llopi, & Menezo-Rozalen, 2001 ) and may therefore be more likely to suffer from decompensated esophoria.
Pathological disturbances, particularly those affecting the central nervous system, can cause incomitant esophoria which will tend to break down into strabismus in one direction of gaze. A retrospective study found a high correlation between adult onset distance esophoria or esotropia and cerebellar disease ( Hufner et al., 2015 ). It is important to detect lateral rectus palsy, which causes an eso-deviation which is worse for distance vision and when the patient looks to the side of the affected muscle ( Chapter 17 ).
A routine examination of the eye and vision is carried out in each case. In this type of esophoria, particular attention can be given to the undermentioned factors:
Symptoms , usually associated with distance vision and with prolonged use of the eyes. Symptoms will usually be less or absent in the morning, except headaches which may occur on the day after prolonged use of the eyes ( Rabbetts, 2007 ). The symptoms are likely to be headaches in the frontal area, sometimes intermittent diplopia and blurred near vision if uncorrected hypermetropia is present.
Refraction , which is very important because of the association with uncorrected hypermetropia. In young patients, significant esophoria is an indication for cycloplegia ( Table 2.11 ).
Decompensation tests , which will be the most important aspect in the investigation and are fully described in Chapter 4 . Measurement of the heterophoria by the cover test or subjective dissociation tests will show a higher degree for distance than for near vision.
Removal of Cause of Decompensation
The factors likely to put stress on the visual system or on the general well-being of the patient should be considered ( Chapter 4 ). Consider particularly the patient’s visual working conditions in this type of esophoria.
Significant hypermetropia should be corrected and, in many cases, no other form of treatment is required. To encourage emmetropisation in young children ( Hung et al., 1995 ), it is advisable to give the weakest correction that renders the esophoria compensated and which provides good visual acuities. Some patients will require the full refractive correction to prevent decompensation. In children and young adults, a cycloplegic refraction is necessary if variable refractive findings make it difficult to assess the refractive error or if latent hypermetropia is suspected.
Patients should be asked to wear the correction constantly for distance and near vision for about a month, and then tests for compensation repeated if symptoms persist. Where the spectacles or contact lenses resolve the symptoms and the esophoria becomes compensated, the refractive correction should continue to be worn.
In some cases, the esophoria is not changed by a hypermetropic correction or there is found to be no refractive error: the esophoria is nonaccommodative. Consideration should then be given to eye exercises or relieving prisms. Occasionally, myopes have decompensated esophoria for distance vision and in young patients the possibility of latent hypermetropia should be excluded with cycloplegia.
If the decompensation of divergence weakness type esophoria persists after consideration has been given to the general decompensating factors and the refractive correction, exercises may be considered.
Teaching an appreciation of physiological diplopia has been found to be useful in this condition ( Pickwell & Jenkins, 1982 ). The patient is asked to look at a small isolated object (not easily confused with background details) at a distance of 3–6 m. A second object, such as a pencil, is held on the median line at about 40 cm from the eyes. The patient is encouraged to notice that this second object is seen in physiological diplopia, providing fixation is maintained on the distance one. When this has been appreciated, fixation is changed to the near object, and physiological diplopia of the distance one is observed. The patient is then encouraged to alternate between fixating the distance object with crossed physiological diplopia of the near one, and fixating the near object with uncrossed physiological diplopia of the distant one. A pause of several seconds must be made with each change of fixation, or confusion results. This exercise in vergence coordination seems to be particularly helpful in young patients. This is a useful first stage in treatment, followed by exercises to increase the divergent amplitude of fusional reserves (the negative fusional reserve) and/or the positive relative accommodation. A range of suitable exercises is described in Chapter 10 .
Prism relief in esophoria is required only for a minority of cases. The symptoms in most cases are relieved by refractive correction or by eye exercises. Prisms may be considered when eye exercises have been tried and found not to be successful, or where it is inappropriate because of the patient’s age, poor health, unwillingness, or inability to give the time required. The power of the prism required is that which is likely to make the esophoria compensated, as assessed by the methods described in Chapter 6 . In general, it will be the lowest prism power which will give no disparity on the fixation disparity test, and/or a smooth prompt recovery on the cover test.
This will be the first consideration when a pathological cause is suspected, but it is unlikely that surgery will help in other cases. As noted earlier, lateral rectus underaction should be detected and referred ( Chapter 17 ).
Convergence Excess Esophoria
This type of esophoria is low in degree for distance vision but increases for near vision.
Excessive Accommodative Effort
This is usually the main factor, and may be caused by uncorrected hypermetropia, latent hypermetropia, early presbyopia, spasm of the near triad or of accommodation, or by pseudo-myopia. Another cause is prolonged work at an excessively close working distance.
High AC/A Ratio
The accommodative convergence/accommodation, or AC/A, ratio is often a factor in producing convergence excess esophoria. The ratio is a measure of the effect of a change in accommodation on the convergence and is expressed as the change in convergence (Δ) for each dioptre change in accommodation (pp. 42–43). This is normally about 4Δ/D ( Appendix 10 ) and when it is high (over ~6), accommodation for near vision will result in an excess of convergence. Convergence excess rarely occurs with a low AC/A ratio.
Visual Conversion Reaction (pp. 18–19)
Convergence excess can also be present as a visual conversion reaction. This typically occurs in young energetic patients, often accompanied by some psychological stress or anxiety; for example, school examination pressures or relationship difficulties.
This can occasionally result in convergence excess, due to the high ciliary muscle effort required to produce adequate accommodation.
Excessive Proximal Convergence
Of the three main cues that cause convergence during near vision (proximity, disparity, and accommodation; p. 3) the proximal cue is the most powerful ( Joubert & Bedell, 1990 ; North, Henson, & Smith, 1993 ). The magnitude of the proximal cue varies between individuals and it is quite likely that a convergence excess esophoria that is not caused by any of the previously listed causes will result from excessive proximal convergence.
Each case of convergence excess esophoria will require a full routine eye examination and probably a cycloplegic refraction. Particular attention should be given to the following factors:
Symptoms are usually associated with prolonged use of the eyes in near vision. Sometimes they are so severe as to render close work impossible for more than short periods. Frontal headache, ocular fatigue, and blurred near vision are usual symptoms. Sometimes difficulty is experienced in refocusing the eyes for distance vision after sustained close work.
Refraction , which may show variable and unreliable results. It may be seen during retinoscopy: neutralisation appearing at one moment and ‘with’ or ‘against’ movement the next, without any trial lens change. This is a sign of variable accommodation and may indicate the presence of latent hypermetropia. Another sign of latent error is markedly lower hypermetropia by subjective refraction than that shown in retinoscopy. These are clear indications that a cycloplegic refraction is required to reveal any latent error or spasm of accommodation that may accompany convergence excess esophoria ( Table 2.11 ). Occasionally, the spasm is such that pseudo-myopia occurs (pp. 35–36). This is usually of low degree, but can be as high as 10 D. Where myopia occurs in a young patient with high esophoria, the possibility of spasm should be explored by cycloplegic examination.
A gradient test gives useful information in convergence excess. It is one way of measuring the AC/A ratio, and is described on p. 42.
A cover test and fixation disparity test for near vision, which will indicate decompensation of the heterophoria at near ( Chapter 4 ).
Fusional reserves , especially at near.
Amplitude of accommodation is relevant for detecting decompensating near esophoria resulting from ‘pseudo-accommodative insufficiency’ (pp. 32, 35–36).
Removal of Cause of Decompensation
It may be necessary to restrict the patient’s close work and/or to increase the working distance. In many cases of convergence excess, the working distance has become unnecessarily close, due to bad visual habits. Patients acquire the habit of working excessively close during childhood, when the amplitude of accommodation was sufficient to permit this without symptoms. On reaching an age when the amplitude is reduced, the working distance causes motor visual stress and becomes the cause of convergence excess. The onset will vary with the amount of close work and the working distance, as well as with the degree of uncorrected refractive error. It may also be brought on by a marked increase in the amount of close work; for example, due to an approaching school examination period, a particularly engrossing computer game, or leaving school for an office job with longer hours of sustained near vision.
In some cases, changing the visual habits to require the patient to employ a more appropriate working distance will resolve the symptoms with no other treatment. A distance of 35–40 cm should be regarded as a minimum and with modern computer use in offices this is usually achievable. It is not always easy for patients to acquire new visual habits when the concentration is on the job in hand. It may be necessary for them to ask someone else to keep reminding them of the required working distance.
As noted earlier in this chapter, the minimum correction required to render the esophoria compensated and to allow clear and comfortable vision should be prescribed. Some cases require the full hypermetropic correction, which may initially blur distance vision. If it does not clear after a few days, a cycloplegic can be instilled to help the patient adjust to the glasses. As noted on pp. 97–98, contact lenses should induce less accommodative convergence than spectacles.
In any case, the patient should be seen again after wearing the correction for a few weeks, and the symptoms and decompensation reassessed. If the symptoms have cleared, the correction should continue to be worn for reading and other close work, as required to maintain relief of the symptoms. In cases of high hypermetropia, this may involve continued constant wear.
Multifocals, with a reading addition that relieves the decompensation of the esophoria for near vision, are sometimes prescribed ( Case Study 7.1 ). The addition can be found with the gradient test method, or by adding positive spheres until the cover test and fixation disparity test indicate compensation. This approach to convergence excess is seldom necessary in patients over the age of 14 years. The bifocal design should be large and set with the segment top at the same height as the pupil centre ( Chapter 14 ). Where convergence excess occurs in incipient presbyopia, reading glasses or multifocals are prescribed.