When to Treat Comitant Strabismus
The first stage in the investigation of strabismus is to discover whether there is any pathological cause for the strabismus. If pathology is present, the patient should be referred for medical investigation and treatment. The detection of pathology is described elsewhere in this book and is summarised in Table 14.1 .
| Step | Rationale | What to do |
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| Detect incomitancy | Any new or changing incomitancy requires prompt referral |
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| Look for orbital pathology | Orbital pathology can cause strabismus, although this is rare. |
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| Detect ocular pathology | Pathology that destroys or diminishes the vision in a significant part of the visual field of one eye can dissociate the eyes and cause strabismus. |
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| Look for neurological problems | Pathology in the brain can cause comitant as well as incomitant deviations. |
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| Look for obvious causes of the strabismus | There will be a reason why a patient develops a strabismus. If you find a nonpathological reason, a pathological reason is less likely. |
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| Monitor the size of the deviation | If the deviation is increasing, there must be a reason. |
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| Is the strabismus responding to treatment? | If you believe you are treating the cause of the strabismus (e.g., correcting hypermetropia), the situation should improve. |
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There are three good reasons for treating orthoptic anomalies: if they are causing problems; if they are likely to deteriorate if left untreated; or if treatment may be required but less effective when the patient is older. Cosmetically apparent strabismus is associated with social alienation in children as young as 5–6 years ( Lukman et al., 2010 ) and after surgery most cases have an improvement in appearance, self-esteem, and self-confidence ( Menon, Saha, Tandon, Mehta, & Khokhar, 2002 ). The minimum size of strabismus that is cosmetically apparent varies ( Larson, Keech, & Verdick, 2003 ), but is less for exotropia (typically, 8Δ) than esotropia (typically, 14.5Δ) ( Wiessberg, Suckow, & Thorn, 2004 ).
Some cases of strabismus do not result in any overt problems (e.g., symptoms, poor cosmesis), and are unlikely to deteriorate or to be harder to treat later. However, strabismic patients are likely to have a marked reduction in stereoacuity and this is undesirable, even though the patient may not be aware of the deficit. Reduced stereoacuity can impair performance in everyday activities, such as driving ( Bauer et al., 2000 ) and motor tasks ( Hrisos et al., 2006 ), and binocular reaction times are faster than monocular ( Justo, Bermudez, Perez, & Gonzalez, 2004 ). There are reports of individuals with no history of stereopsis (e.g., from strabismus) developing stereoperception in adult life, sometimes with dramatic perceptual gains ( Barry & Bridgeman, 2017 ). This study was of individual’s self-report by questionnaire and so is subject to bias. Some of the reported improvements followed vision therapy, but some were reported spontaneously and despite the eyes remaining misaligned. In some cases, these reports could be psychogenic effects and further research is required.
Binocular reading speed is impaired in amblyopia, even when binocular visual acuity is normal ( Stifter, Burggasser, Hirmann, Thaler, & Radner, 2005 ). So, if the strabismus can be treated, this deserves consideration. It should be stressed that these benefits of stereopsis are subtle: people do not report a sudden drop in the quality of their vision when they cover one eye. Even for driving, the loss of stereopsis only affects some tasks and there is evidence that at far distances (e.g., >40 m), where stereopsis ceases to be effective, patients with long-standing strabismus make better use of monocular cues than nonstrabismic people ( Bauer et al., 2001 ). However, this view is controversial as other research indicates that strabismic patients do not have an enhanced ability to use monocular cues ( Nuzzi & Cantu, 2003 ).
Strabismus with a small stable angle is often associated with deep harmonious anomalous retinal correspondence (HARC) and this can give the patient quite good ‘pseudo-binocular vision’, sometimes with a reasonable degree of stereopsis. If these cases are asymptomatic, have a good cosmesis, and have good visual function, it is hard to justify the disruption to the child and family that accompanies treatment with exercises. Although successful treatment might increase career possibilities, it must be acknowledged that the vast majority of cases would gain little benefit from treatment. Additionally, these well-adapted cases will be difficult to treat and there is always a possibility that treatment might make the situation worse. Some parents are keen to eliminate a strabismus at any cost, but they should be fully informed of the likely risks and benefits and few practitioners would take on the treatment of this type of case, other than treating amblyopia in children of a suitable age.
The investigation and management of comitant strabismus can be broadly divided into two parts: sensory and motor. Patients who have not managed to achieve a sensory adaptation to their strabismus (usually, this is because they are too old) will have diplopia and this is discussed next. Suppression and HARC are adaptations in the visual sensory mechanisms which occur in strabismus. These adaptations have been reviewed in Chapter 12 and are further described in this chapter with particular reference to the clinical investigation and treatment. As explained in Chapter 12 , small suppression areas can coexist with HARC, especially in small-angle strabismus. In strabismus over 25Δ, suppression seems to dominate.
In cases of strabismus where treatment is appropriate, sensory factors (suppression, HARC) are generally treated first. Throughout this period some form of occlusion is maintained during the intervals between treatment to prevent diplopia and confusion. In cases where the patient is diplopic, treatment of the motor deviation can be started straight away. The motor deviation sometimes spontaneously resolves when sensory factors have been corrected. When this does not occur, refractive correction or fusional reserve exercises are required to treat the motor component. Treatment of sensory factors should only be attempted in cases where the practitioner is sure the motor deviation will respond to treatment (see later).
Diplopia
It has already been noted that most patients with strabismus develop a sensory adaptation (HARC or suppression) to avoid diplopia and confusion. In some cases, this is not possible, usually because the patient is too old, and the patient develops diplopia and confusion ( Chapter 12 ). The distinction between diplopia and confusion is illustrated in Fig. 12.1 , and both phenomena usually occur together (p. 166). Throughout this section, the word diplopia is used to describe the problems of diplopia and confusion.
Investigation of Diplopia
Diagnosis: the Worth Four Dot Test
In most cases, diplopia is detected from the symptom of double vision in everyday life. Occasionally, it may be necessary to formally test for diplopia (e.g., patients who may be denying diplopia to enter certain vocations). In these cases, the Worth Four Dot Test can be used. The test is carried out in room illumination and the patient should not be shown the test targets until they are wearing red-green glasses and should be told not to close or cover one eye. The patient is asked to describe what they see, and the possible responses are illustrated in Fig. 14.1 . The figure illustrates a stable situation, but often patients experience a dynamic perception: the monocular inputs may move or disappear.
The red-green glasses create an artificial viewing condition, so it is possible that a patient reports diplopia with the Worth test but does not usually experience diplopia in everyday life ( Bagolini, 1999 ). The test is also sometimes used to investigate HARC and suppression, but there are better tests ( Bagolini, 1999 ) which use more natural viewing conditions (see later).
Investigation
In addition to strabismus, other conditions can lead to reports of ‘double vision’ and the investigation of diplopia is summarised in Fig. 14.2 . The investigation of diplopia is simplifying by asking the patient to view an isolated target. Covering each eye is essential to determine whether the diplopia is monocular or binocular, and a pinhole will further help to determine the aetiology ( Finlay, 2000 ). Monocular diplopia accounts for one-quarter of cases of diplopia presenting to an eye hospital ( Morris, 1991 ). For nearly all these cases a genuine cause can be found, usually lenticular or corneal pathology. Monocular diplopia can result from an epiretinal membrane ( Veverka et al., 2017 ), for example after cataract surgery ( Foroozan & Arnold, 2005 ). Monocular diplopia in children can be due to refractive errors, cataracts, corneal disease, or occasionally retinal disease ( Taylor, 1997 ). Sensory causes of monocular diplopia and polyopia (more than two images) include brain trauma, cerebrovascular accidents, and migraine.
When diplopia is binocular, orthoptic tests should be used to detect the presence of strabismus. The direction of the diplopia (horizontal, vertical, oblique, torsional) should be determined by questioning the patient. By introducing a red filter in front of one eye, or by covering an eye, the practitioner can determine whether horizontal diplopia is crossed (heteronymous, suggesting an exotropia) or uncrossed (homonymous, suggesting an esotropia). If diplopia occurs after surgery, it should be determined whether it is in accordance with the postoperative deviation or paradoxical (crossed with esotropia and uncrossed with exotropia), in which case there is a persistence of the preoperative sensory adaptation (von Noorden, 1996 ). The practitioner should detect and investigate incomitancy as outlined in Chapter 17 and any comitant deviation as outlined elsewhere in this chapter and in Chapter 15, Chapter 16 .
Monocular diplopia or binocular triplopia can occur through a persistence of the sensory state preceding a surgical intervention. The strabismic eye sees two images of a fixation point, as a result of competition between the innate normal retinal correspondence (NRC) and long-standing anomalous retinal correspondence (ARC) that existed before surgery. During binocular viewing, the NRC in the dominant eye can cause triplopia (von Noorden, 1996 ). Rarely, binocular diplopia can result when a previously dominant eye becomes the more myopic causing a change in ocular dominance. Such cases are resolved by correction of the myopia.
Intractable diplopia (see later) from strabismus suggests that either the patient was unable to develop sensory adaptations (e.g., because they were too old when the strabismus occurred) or that there has been a change in their sensory or motor status.
A particularly troublesome form of binocular sensory diplopia occurs in nonstrabismic patients who have developed a macular or retinal lesion (e.g., epiretinal membrane) causing metamorphopsia. Bifoveal fusion may be impossible, yet peripheral fusion is likely to be normal. This is sometimes called central-peripheral rivalry (CPR)-type diplopia ( Hatt et al., 2019 ). A useful way of investigating this dragged fovea syndrome is with the lights on-off test ( de Pool, Campbell, Broome, & Guyton, 2005 ). The patient fixates a small isolated target (e.g., dot or single 6/18 letter) that should be white in the centre of a black computer screen. With the room lights on, this will be seen doubled. When the room lights are suddenly extinguished, then within 2–10 seconds the letter should become single. Occasionally, patients need a partial prism correction to achieve this central fusion. There is no complete cure, but some cases benefit from monovision and others require occlusion ( de Pool et al., 2005 ). These authors cautioned that surgery for the epiretinal membrane should not be thought of as a cure, as this can trigger or worsen the problem. A low density Bangerter foil (p. 190) sometimes helps ( Silverberg et al., 1999 ).
It is possible that sensory diplopia might also occur as one of the anomalous visual effects that can accompany sensory visual stress (p. 64). These visual perceptual distortions probably result from hyperexcitability of the visual cortex ( Wilkins, 1995 ). Covering one eye halves the sensory input to the visual cortex and thus reduces the probability of these effects ( Wilkins, 1995 ). Hence, sensory diplopia from this source could conceivably present as binocular diplopia which resolves on covering one eye, although the diplopia is not likely to occur with isolated targets. The treatment of the nonbinocular types of diplopia classified in Fig. 14.2 was summarised by Evans (2001c) and the treatment of binocular cases is now described.
Can the Patient Achieve Binocular Single Vision?
In most cases, the complaint of binocular diplopia suggests that there is the potential for binocular single vision, especially if the patient can consciously control the diplopia by adopting a compensatory head posture. Exceptions are the intractable cases described next. In every case, prisms should be used to establish whether the diplopia can be eliminated before surgery is considered. Loose prisms, rotary prisms (e.g., in a phoropter), or prism bars can be used. Errors can occur when prisms are stacked (e.g., several loose prisms placed in a trial frame; Firth & Whittle, 1994 ).
The effect of prisms on diplopia from comitant strabismus can be investigated using the Mallett OXO test ( Fig. 14.3 ). If the diplopia is predominantly horizontal, the horizontal OXO should be used. Even incomitant cases, if the incomitancy is subtle, sometimes benefit from the prism suggested by testing with the Mallett unit in the primary position ( Chapter 17 ). For patients with good visual acuity, the usual small OXO can be used. Patients with poor acuity can use the large OXO which is included on modern near Mallett units.
If a patient with horizontal diplopia views the test whilst wearing the polarised visors, they should report seeing two OXO s, one with a line above the X and another with a line below the X ( Fig. 14.3A ). The position of the OXO s reveals the type of diplopia (e.g., horizontal in Fig. 14.3A ). Prisms are introduced and adjusted to bring the OXO s closer together ( Fig. 14.3B ). It should be determined whether the patient can fuse the diplopic OXO s (as in Fig. 14.3C ). If they cannot fuse the two OXO s, then there may be sensory fusion disruption syndrome or horror fusionis , described later. If the patient can fuse the OXO s, the prism should be refined to eliminate any fixation disparity ( Fig. 14.3D ). In patients with horizontal diplopia and with adequate accommodation, spheres (minus for exotropia, or plus at near for esotropia) can be used to try and eliminate the diplopia by altering the accommodative convergence. It should be noted that whilst the normal sized OXO test is usually called a fixation disparity test, its use for diplopia correction only involves fixation disparity in the last step ( Fig. 14.3C,D ).
If a prismatic or spherical correction eliminates the diplopia, this can be prescribed. Some patients adapt to the correction and require a stronger prescription, but this is not usually the case (p. 104). With larger angles, which may require surgical intervention, a prism adaptation test or a trial with botulinum toxin is advisable before surgery, as described in Chapter 17 . Patients with diplopia or suppression should have a postoperative diplopia test carried out before surgery, to assess the risk of inducing intractable diplopia after surgery. This will usually be carried out at the hospital and is also described in Chapter 17 .
Causes of Intractable Diplopia
Intractable diplopia can be very distressing for patients, sometimes greatly impairing their quality of life. Some cases can be managed surgically, and an ophthalmologist will advise on this. Other cases cannot be managed surgically, and these patients may turn to the optometrist to manage the diplopia through optical or other means.
Table 14.2 lists the main causes of intractable diplopia. An additional category might be patients who have received inappropriate orthoptic treatment. For example, intractable diplopia might occur if long-standing deep HARC is broken down with full-time occlusion in an adult or if an attempt was made to treat, in a strabismic patient, either the sensory adaptation or the motor deviation in isolation. So far, the author has not seen any patients whose intractable diplopia results from inappropriate orthoptic treatment. A prospective observational study in the UK ( Newsham, O’Connor, & Harrad, 2018 ) found the commonest cause of intractable diplopia is strabismus surgery (32%), with other main causes being spontaneous (25%), severe head trauma (8%), cataract surgery (6%), and vitrectomy (6%).
| Causes of Intractable Diplopia |
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The risk of intractable diplopia for people undergoing strabismus surgery has been estimated to be between 0.2% ( Newsham et al., 2018 ) and 0.8% of cases ( Kushner, 2002 ). Intractable diplopia from refractive surgery can be traced to one of five mechanisms ( Kushner & Kowal, 2003 ): technical problems, prior need for prisms, aniseikonia, iatrogenic monovision, and improper control of accommodation in patients with strabismus. Screening methods to detect these problems have been advocated for patients considering refractive surgery ( Kowal, Battu, & Kushner, 2005 , Kushner & Kowal, 2003 ). Attempts to induce monovision in a patient with long-standing strabismus or incomitancy is another possible cause of diplopia ( Godts, Tassignon, & Gobin, 2004 ; Evans, 2007a ). Indeed, it seems unwise to prescribe monovision with refractive surgery before a temporary trial of monovision with contact lenses ( Vogt, 2003 ), although this will not detect all cases at risk of strabismus ( Pollard, Greenberg, Bordenca, Elliott, & Hsu, 2011 ). The cause of diplopia in these cases has been attributed to fixation switch diplopia , when the patient is forced to fixate with a previously strabismic eye ( Kushner, 1995 ).
One uncommon risk factor for intractable diplopia is when patients with unilateral aphakia are fitted with a secondary posterior chamber intraocular lens ( Khan, 2008 ). Pre-surgical prism testing is useful in these cases (Khan, 2007).
Horror Fusionis and Sensory Fusion Disruption Syndrome
Heterotropic patients with horror fusionis cannot demonstrate fusion, even when the deviation is corrected with prisms or in a haploscopic instrument. These patients report a ‘jumping over’ phenomenon: as the prism is increased and the diplopic images move together, they suddenly ‘jump’ and, for example, crossed diplopia suddenly changes to uncrossed diplopia. The same phenomenon occurs when the prisms are changed in the other direction. It appears that the patient is unable to achieve motor fusion. Caloroso and Rouse (1993) advise the condition should be differentially diagnosed from aniseikonia, undetected small-angle HARC, and deep foveal suppression (when horror fusionis would not be present for large targets). Many affected patients are congenital esotropes and Kirschen (1999) stated that horror fusionis is only seen in occasional patients who have had a strabismus since early childhood. Treatment is usually aimed at alleviating any intractable diplopia, and this may require occlusion (see later).
Heterotropic patients with sensory fusion disruption syndrome ( Case Study 14.1 ) can achieve motor superimposition of their diplopic images, but sensory fusion cannot be attained. If appropriate prisms are placed before the eyes, the patient reports the targets are ‘on top of each other, but not together’. One of the images is often seen in constant motion ( Kirschen, 1999 ). The condition usually follows closed head trauma, sometimes associated with coma ( Case Study 14.1 ). Treatment includes monovision (London, cited by Evans, 1994 ) occlusion (full or central; Kirschen, 1999 ), or hypnosis.
Symptoms & History
Head injury 6 years ago resulting in coma for 10 months. Since then has recovered quite well, with rehabilitation. Physically good (takes antiepileptic medication), mentally agile (some memory problems), but intractable diplopia. Referred by ophthalmologist to see if hypnosis can help the diplopia. The diplopia is present all the time, oblique, same for D and N, worse when concentrates, at night, and when tired. The right eye’s image stays still, but the left eye’s image constantly moves.
Initial Results & Management
Low myope and corrected visual acuities 6/6 in each eye. Variable angle strabismus at distance and near, but no marked incomitancy seen on motility testing. Prisms were adjusted in a trial frame and with these the diplopic images could be brought together but the patient never obtained fusion. The left eye’s image oscillated and was never stationary. No stereoacuity could be demonstrated with any prismatic correction.
Outcome
Hypnosis was tried, but in this case, was unsuccessful. Patient was referred for an occlusive contact lens.
It is essential that horror fusionis and sensory fusion disruption syndrome are identified before surgery, which would not be able to eliminate the diplopia. Indeed, it has been suggested that some patients may find it easier to ignore diplopic images that are far apart, so surgery might make the symptoms worse through reducing the angle of the deviation. However, each patient is different, and it should not be concluded that patients will necessarily be helped by increasing the separation of the two images ( Case Study 14.2 ). In summary, if the diplopia cannot be eliminated, it is best to only change the angle if testing has indicated the patient will be more comfortable with a new angle or equally tolerant of a cosmetically improved angle of deviation.
Symptoms & History
Strabismus operation age 4 years and diplopia since. For many years, the diplopia was corrected with a prismatic correction in spectacles. At age 62 years, Parkinson disease diagnosed which now produces noticeable tremor and gait problems. Since age 70 years, constant diplopia that cannot be eliminated with spectacles, despite trying several prismatic corrections. Local optometrist referred to HES who tried convergence exercises and Fresnel prisms to no avail. Neurologist opines diplopia unlikely to be related to Parkinson disease. Age 71 years saw strabismus specialist and orthoptist who considered ‘vergence eye movements appeared to be absent’, but saccades, smooth pursuit, pupils all normal, as are spatial skills and face recognition. Recommended occlusion, but patient disturbed by cosmesis, so is unhappily living with diplopia. Situation has not changed since saw strabismus specialist and patient often closes right eye to eliminate diplopia. Local optometrist referred patient to the author.
Results & Management
Moderate hypermetropia and astigmatism with normal ocular health (including fields) except moderate cataracts in both eyes, worse in right, with commensurate acuities, R6/12 L6/9. Motility difficult to interpret owing to head tremor but appears to be moderate underaction right medial rectus. Vergence movements present but limited (NPC at times 20 cm, at other times exotropic at near). Ocular motor tests with current glasses reveal large variable exophoria/exotropia at distance and near, with variable small vertical deviation and 7.5-degree excyclotorsion. Tested with rotary prisms, but variable nature of ocular motor imbalance means no prismatic strength gives single vision for more than a few seconds.
Outcome
Discussed options. The cataract is worse in the right eye but surgery to this eye could make the diplopia more noticeable. Tested with Bangerter filters (p. 190) RE and no diplopia with filter of 0.1. Started with this and after a month progressed to 0.2, but unable to lighten filter further without diplopia recurring. Patient much happier with this than full occlusion. In view of the quite deep filter, at change of spectacles removed prism from spectacles, but continued with Bangerter filter. Interestingly, patient noticed more diplopia so remade with prism. This indicates some perception is occurring through the filter, even though it reduces the acuity to worse than 6/30.
Prognosis
Consider early cataract surgery in the left eye as cataract in right eye progresses, which may present a ‘natural’ equivalent of the Bangerter filter.
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