Before dealing with the individual heterophoric conditions in the next three chapters, this chapter outlines the basic principles of management. There are two reasons to treat a heterophoria: to alleviate symptoms and to prevent the heterophoria breaking down into a strabismus. It is easier to treat a heterophoria than a strabismus and if a heterophoria breaks down into a strabismus, this can lead to serious problems such as diplopia and amblyopia.
Following the investigation of binocular vision and the total findings to reach a diagnosis, a decision must be made regarding the best course of action to assist the patient: the management of the case. In general, there are five possible lines of action which may help in alleviating symptoms:
Remove the cause of decompensation.
Refractive correction or modification.
Give eye exercises.
Prescribe prism relief.
Refer to another practitioner.
Although it is logical to consider them in this order, it may be that some are not appropriate or possible in a particular case. Sometimes one course of action is going to comprise the primary or sole treatment of the case. For example, in many cases of decompensated heterophoria, the refractive correction by itself will result in the phoria becoming compensated and no further action will be necessary. In other cases, where there is the possibility of active disease or pathology, or of recent injury, referral will be the first priority and other possibilities may not be pursued until appropriate medical attention has been given.
In the healthcare sciences there are several levels of the type of evidence that may be produced in support of an intervention or treatment ( Evans, 1997a ). The initial evidence is often in the form of anecdotal clinical observations. These may be supported by open trials (e.g., Dalziel, 1981 ) but these types of evidence are influenced by the placebo effect. The placebo effect should not be underestimated ( Evans, 1997b ) and a therapy can only be convincingly proven by double-masked placebo-controlled trials (RCT). In recent years, strong RCT evidence supporting eye exercises to treat convergence insufficiency exophoria syndrome ( Chapter 8 ) has emerged ( Chapter 10 ), but other interventions for decompensated heterophoria still lack a strong evidence base ( Rowe & Evans, 2018 ). If there is an obstacle to fusion, removing that obstacle has face validity. Likewise, providing a patient with optimised vision through refractive correction would seem to be an obviously sensible approach.
Removal of Cause of Decompensation
Consideration must be given to those general factors that put stress on the visual system or on the general well-being of the patient. These factors are discussed in Chapter 4 . It will be obvious that all treatments will aim at removing the cause of the decompensation, and therefore the other four options may also contribute to this. However, there are some factors that contribute to binocular anomalies which do not come under the other headings. For example, a patient working long hours at excessively close work in poor illumination will need to give consideration to proper working conditions and should be advised accordingly. In some cases, improving the visual working environment will be all that is required to restore compensation of the heterophoria.
Immediate removal of some of these general factors of decompensation may not be possible, as in some instances of poor general health, in old age, or in some vocations. Greater reliance must then be placed on the other options.
The importance of the refractive correction has already been discussed in the section on refraction and visual acuity in Chapter 4 . In many cases, decompensated heterophoria and binocular instability become compensated when a refractive correction is given. This may be explained by one or more of the following factors:
Accommodation-convergence relationship. Uncorrected spherical error may result in an abnormal degree of accommodation. This will be excessive in hypermetropia and, for near vision, it will be less than normal in myopia. Because of the link of accommodation to convergence, this can result in stress on convergence.
If significant esophoria is found, the practitioner should search carefully for hypermetropia. Significant esophoria in a young patient is an indication for cycloplegia ( Table 2.11 ). Some cases will require multifocal lenses and these types of cases are discussed further in Chapter 7 .
For esophoria with myopia, a myopic correction is required to give clear distance vision, but care must be taken not to give an overcorrection; an undercorrection of 0.50D may be tolerated.
In cases of decompensated exophoria and myopia, an overcorrection can be considered if the patient’s amplitude of accommodation is adequate. The patient should be given the minimum overcorrection (‘negative add’) for the exophoria to become compensated. The negative add is then gradually reduced over a period of months so that the patient’s fusional reserves increasingly compensate for more of the deviation. For exophoric patients with hypermetropia, care must be taken that the correction does not contribute to the phoria becoming decompensated; a partial correction can be considered if this is likely.
Blurring. If it occurs in one or both eyes, blurring will make binocular vision more difficult. This is particularly important in high astigmatism, and care must be taken to ensure an accurate astigmatic correction. Dwyer and Wick (1995) suggested that the correction of even small refractive errors can dramatically improve binocular function, although other research suggests that this may be unlikely ( Ukwade and Bedell, 1993 ). Dwyer and Wick (1995) argued that, even in low hypermetropia, spectacles might eliminate slight blur and aid the compensation of phorias. It would be interesting for placebo-controlled trials to investigate this hypothesis. In the meantime, this is a matter for professional judgement and reasons for prescribing should be carefully documented in the clinical records ( College of Optometrists, 2012 ).
Anisometropia. Anisometropia produces interocular differences in blurring. It can be important in making the heterophoria decompensated and in causing binocular instability. This is especially the case in high anisometropia ( Chapter 11 ). In other cases, care must be taken to ensure that the refractive correction is properly balanced, either by a retinoscopic method or subjectively. The methods are described in Chapter 2 .
The Effect of Contact Lens Wear And Refractive Surgery
Theoretically, myopes have to exert more accommodation and convergence when wearing contact lenses than spectacles ( Rabbetts, 2007 ). Research validates this prediction but finds large interindividual variation, with some individuals showing the opposite effect ( Hunt, Wolffsohn, & Garcia-Resua, 2006 ). This is likely to explain why, when myopic children undergo refractive surgery, there is initially (at 1 week and 1 month) a reduction in mean near convergent fusional reserves and then a recovery to normal ( Han, Yang, & Hwang, 2014 ). It also probably explains why, on average, when myopes wear contact lenses the accommodative lag is higher (by about 0.25 to 0.50D) than with spectacles ( Jimenez, Martinez-Almeida, Salas, & Ortiz, 2011 ). These authors speculated that this could contribute to myopia progression.
It is possible that in some cases the improvement in peripheral fusion from the wider field of view with contact lenses improves the binocular status. In practice, if a patient with a binocular vision anomaly is motivated to try contact lenses, it may be sensible to undertake a trial and reassess the ocular motor status when wearing contact lenses.
The fact that hypermetropic spectacles require patients to exert a greater amount of accommodation for near fixation than an emmetrope can exacerbate, or even simulate, a convergence excess type of deviation ( Black, 2006 ). Contact lenses may be helpful in these cases and, as for other issues discussed in this section, the same applies to refractive surgery.
Conditions Amenable to Treatment Through Refractive Modification
It can be seen from the section above on the accommodation-convergence relationship that, even for an emmetropic patient, a refractive correction can be used to correct a decompensated heterophoria. The principle is to prescribe over-minus or under-plus (‘negative add’) in exophoria and over-plus or under-minus (‘positive add’) in esophoria. This form of treatment is sometimes described as refractive modification. The conditions that can be managed using refractive modification are summarised in Table 6.1 and in Chapter 7, Chapter 8 .
|Modification to refractive correction
|Basic esophoria (problematic esophoria at distance and near)
|Maximum plus, bifocals may help at near
|Divergence weakness esophoria (problematic esophoria at distance)
|Maximum plus at distance
|Convergence excess esophoria (problematic esophoria at near)
|Bifocals or varifocals
|Basic exophoria (problematic exophoria at distance and near)
|Over-minus at distance and near
|Divergence excess exophoria (problematic exophoria at distance)
|Over-minus at distance, maybe bifocals
|Convergence weakness exophoria (problematic exophoria at near)
|Upside down executive bifocals (p. 122)
For a ‘negative add’ to be effective, the patient must have adequate accommodation and a higher AC/A ratio will make refractive modification more likely to succeed. The only conditions that are not amenable to treatment by refractive modification are cases of esophoria that are producing symptoms with distance vision. This is because, in the absence of latent hypermetropia, there is clearly a limit to how much over-plussing a patient can tolerate before the blur produces problems.
Strength of Evidence
An understanding of basic physiology of vision provides intuitive support for the approach of providing clear vision and using the accommodative-convergence link to reduce a deviation. However, a narrative review noted a lack of strong evidence (e.g., RCTs) for this approach ( Rowe & Evans, 2018 ). The evidence-based approach indicates that clinicians should integrate the best available external evidence with their own clinical expertise and with the patient’s priorities ( Rowe & Evans, 2018 ). Tests, like the Mallett unit, which mimic everyday viewing, are well-suited to investigating whether refractive corrections are likely to improve compensation under natural viewing conditions, as described later. In some cases, glasses can be prescribed as a diagnostic tool ( Elliott, 2014 ), although the clinician should be mindful of the possibility of a placebo effect.
Clinical Approach to Treatment Through Refractive Modification
The clinical technique for this approach is very simple and is summarised in Table 6.2 . In most cases, the spherical correction that eliminates any fixation disparity on the Mallett unit, at the relevant distance(s), is determined. The result should be confirmed with a cover test where improved recovery ( Table 2.4 ) indicates the refractive modification is adequate. The required correction is the smallest that will eliminate a slip on the Mallett unit and give good cover test recovery (bearing in mind the effects of tiredness).