Strabismus: non-surgical treatment

Optical Correction

Refractive errors, whether corrected or uncorrected, play a significant role in strabismus and its management. Correcting a patient’s refractive error may result in better control of a misalignment solely by gaining optimal visual acuity. Conversely, previously asymptomatic strabismus can become symptomatic due to changes in refractive error, changes in glasses prescription, refractive or cataract surgery. For example, non-alternating strabismus patients can report new onset of diplopia and other symptoms if a change or correction of their refractive error results in spontaneous alternation or fixation switch to their non-dominant eye outside of a longstanding functional suppression scotoma. Optically or surgically induced monovision, as a treatment for presbyopia, is another example where a previously compensated strabismus can manifest symptoms due to fixation switch, decompensation of fusional mechanisms or disruption of suppression.

All children and young adults with strabismus or suspected strabismus require a cycloplegic refraction to determine the basic refractive state without the influence of accommodation. This can be done with 0.5%–2% cyclopentolate (±1% tropicamide in dark irides), or 1% atropine. Cyclopentolate is commonly preferred over atropine due to its shorter onset (30–50 minutes for maximal cycloplegia) and duration of action (up to 24 hours). The mean residual accommodation of these agents is less than 0.1 D for a 3-day administration of 1% atropine, and between 0.4 and 0.6 D for 1% cyclopentolate with 1% tropicamide. The difference between atropine and cyclopentolate cycloplegia has consistently been measured to be between 0.4 and 0.7 D, but it is important to recognize that 15%−20% of children who have a cycloplegic refraction with 1% cyclopentolate alone will have a hyperopic undercorrection of 1 D or more. Therefore, in accommodative esotropia with a residual deviation, atropine refraction is still essential to uncover the maximal amount of hyperopia that needs to be corrected. The pupillary dilation achieved with any of these agents allows for an adequate funduscopic exam, which is mandatory during the initial evaluation of a strabismus patient to exclude any abnormalities of the visual axis or posterior segment.

Considering the optimal optical correction for each patient is essential in the management of their strabismus. In acquired esotropia, particularly accommodative esotropia, any uncorrected hypermetropia may be significant, and the hyperopic refractive error should be fully corrected with spectacles. The full hyperopic correction should be prescribed initially in order to remove all accommodative convergence (some ophthalmologists prefer to start by correcting the minimal amount of hyperopia necessary to control the deviation and facilitate acceptance of glasses with gradual increases of plus). In most forms of esotropia (including essential infantile esotropia), following surgical alignment, spectacle correction may be of considerable value in improving a small residual deviation. However, in patients with surgical overcorrections of partially accommodative esotropia, a need to reduce the hypermetropic correction by more than +2.50 D may be associated with long-term instability of the alignment; such undercorrections should be avoided.

Bifocals may be indicated in children with a high accommodative convergence to accommodation ratio (AC/A) accommodative esotropia (convergence excess esotropia), if the full hypermetropic spectacle correction aligns the eyes for distance fixation, but a residual esotropia is seen at near. Although it is known that bifocals are often effective in reducing the near deviation sufficiently to allow fusion, there is lack of evidence to suggest that bifocals provide a long-term effective treatment. Arnoldi and Shainberg compared the long-term outcome of bifocals, single-vision lenses and extraocular muscle surgery in patients with high AC/A esotropia and found that approximately one-third of patients in each of the treatment groups achieved success at a minimum follow-up of 5 years, though success was achieved at a significantly earlier age in patients that had surgery. Patients in the bifocal group were less likely to undergo emmetropization and tended to an increase in the AC/A ratio with time. There was a 5 prism diopter (PD) reduction in near deviation in patients with bifocals or single-vision lenses. Surgery resulted in a significant reduction of the near deviation and collapse of the distance–near disparity. The surgical reduction of the AC/A ratio was not permanent, and more patients lost stereoacuity in the surgery group than in the other two groups combined. They concluded that any of the three treatments can be effective if used under the right circumstances. More recently, Whitman et al. found that bifocals did not improve stereopsis outcomes compared to single-vision lenses in patients with accommodative esotropia with high AC/A ratio. Stereopsis was similar at initial and final visits, with similar improvement in both groups. In addition, they found that children in the bifocal group had a higher rate of strabismus surgery than children in the single vision-lens group. However, Whitman’s study was an observational non-randomized design, and therefore cannot answer the question whether treatment with bifocals is superior to treatment with single-vision lenses. Although the literature shows lack of evidence to suggest that bifocals provide an effective treatment for high AC/A ratio esotropia, it is known that children often lift their chins to fixate through the near segment suggesting that there is a visual benefit of bifocals for some children. Because this is a topic with continuing debate, ophthalmologists should weigh carefully the advantages and disadvantages of each treatment on a case-by-case basis.

Bifocal glasses can be prescribed either as +3.00 D add or with the minimum near addition that controls the near esotropia. The recommended strategy is then to gradually and incrementally reduce the bifocal correction once the patient has demonstrated stable alignment and fusional amplitudes. It is important to make sure that when the child looks down, as when reading, the line of sight will be through the bifocal segment (executive style or large flat-top segment bisecting the pupil) and that the glasses are properly fitted to prevent slippage on the nose.

In patients with intermittent exotropia (IXT), it may seem logical not to correct moderate to high hypermetropia since accommodative convergence has a beneficial effect on the control of the exodeviation. However, patients with high hyperopia and IXT may not be accommodating fully and may be experiencing blur, resulting in poor control of the deviation. In fact, a small study of children with moderate to high hyperopia (3−7 D) and IXT showed that full hyperopic prescription corrected the manifest deviation and improved the binocular status in all the patients. In contrast, a different study showed that partial spectacle correction of hyperopia increased the exodeviation by an average of 10 PD in one-third of patients with moderate hyperopia and IXT. This was not observed in patients with fully corrected hyperopia, emmetropia, or myopia. High hyperopia (>4 D), anisometropia (>1.5 D), significant astigmatism, and myopia should always be corrected in patients with IXT. It is unclear if moderate hyperopia needs to be corrected; age, degree of hyperopia, and amount of AC/A are factors to consider.

For the treatment of IXT, controversy still exists regarding what is the optimal non-surgical treatment and its timing. Ideally, non-surgical interventions improve the ability to control the deviation, preserve stereoacuity, and eliminate, or at least delay, surgical treatment. Most of these interventions have not been rigorously studied and their efficacy remains unclear. Non-surgical methods may be preferred for small angle deviations and very young children who may develop amblyopia or lose binocular function if overcorrected with surgery.

Overminus lens therapy (1.5−4 D additional minus) has been proposed to stimulate accommodative convergence and improve control of the exodeviation. An alternative theory for how overminus glasses improve exotropic control is by eliminating blur and therefore promoting fusional convergence rather than inducing blur and stimulating accommodative convergence. Recent research suggests that patients with intermittent exotropia converge in response to disparity-induced vergence cues triggering overaccommodation and blur during periods of binocular alignment. Therefore, overminus glasses may be, in fact, allowing the convergence response to disparity (resulting in optimal binocular vision) while neutralizing the blur induced by the accompanying excess of accommodation (resulting in optimal visual acuity). In small prospective studies of children treated with overminus glasses, some degree of improvement was seen in 45%−70% of patients. This improvement was seen independent of the initial angle of deviation and was maintained after discontinuation of therapy in some patients. One randomized clinical trial (RCT) in children with IXT treated with overminus spectacles (−2.50 D) versus observation was conducted by the Pediatric Eye Disease Investigator Group (PEDIG). After 8 weeks, distance control scores were better in children treated with overminus spectacles, without inducing other side effects like headaches and eyestrain. The long-term effects of overminus glasses, including after their discontinuation, are not well studied. A larger PEDIG RCT is currently underway (IXT5; ClinicalTrials.gov Identifier: NCT02807350). The aims of this randomized trial comparing overminus lens treatment to non-overminus glasses are to determine the long-term on- and off-treatment effect of overminus glasses in IXT distance control scores. An extension study is currently comparing the long-term refractive error changes between subjects originally treated with either overminus spectacles or non-overminus spectacles. The concern of inducing a myopic shift from increased accommodative demands in patient treated with overminus glasses remains unanswered with previous observational studies reporting no increase in myopia and others reporting higher myopia progression in all children with IXT regardless of treatment. Further studies are needed to clarify the relationship between accommodation, IXT, and myopia progression.

Occlusion Therapy

In children with IXT, part-time occlusion of either the preferred eye or alternate patching has been reported to aid in eliminating suppression, improving the frequency or decreasing the magnitude of the deviation. Published studies have varied in terms of patching dosage, duration, and outcome measures, and have reported varying success rates.

The short- and long-term effects of occlusion therapy and overminus glasses with base-in prism were investigated by Kushner in 279 children (2–10 years of age) who had poorly controlled intermittent exotropia. This conservative management was shown to delay surgery in most cases, sometimes for many years; in a small percentage of patients, it eliminated the need for surgery altogether. After initial occlusion therapy, 78% of children showed improvement. A progressive decreased response to occlusion was seen as age increased. Of children who initially improved with patching, 57% needed a second course of occlusion therapy because of recurrence; most of the patients were also treated with overminus spectacles and base-in prism. In 74% of patients, conservative treatment was able to defer surgery for at least one year. Twenty years after the initial occlusion therapy, 42 patients were still being followed, of whom 22 never had strabismus surgery.

PEDIG also conducted a multicenter RCT to determine the effectiveness of part-time patching (3 hours per day) compared with observation alone for reducing the risk of IXT deterioration over 6 months among 324 previously untreated children 3 to <11 years of age. The study found a small treatment effect of patching over observation but the proportion of patients who deteriorated in both groups was very small. At 6 months, deterioration occurred in only 6% of the patients in the observation group and 0.6% in the patching group. There were no statistically significant differences in rates of improvement in deviation magnitude, motor control, or stereoacuity in either group. Similar findings were reported for the 12–35 months of age cohort, where the rate of deterioration at 6 months was 2.2% for the patching group and between 2.3% and 4.6% for the observation group, making it unlikely that part-time patching provides a meaningful short-term benefit for young children with IXT. However, it also seems evident that immediate surgical intervention is not necessary to prevent IXT from deteriorating.

Some practitioners recommend treating amblyopia fully before strabismus surgery, even though in some cases this means delaying surgical treatment. Two prospective studies evaluated the need for completion of amblyopia therapy versus early operation with continuation of occlusion postoperatively. Neither study detected a significant difference in motor or sensory outcome whether amblyopia was fully or only partially treated before surgery, provided that amblyopia treatment continued after surgery. In one study, the mean angle of deviation decreased significantly after occlusion treatment for amblyopia in patients with partially accommodative esotropia, and noted that occlusion may resolve the non-accommodative component of the deviation, obviating the need for surgery. In that study, surgery would have been performed in 81% of patients if performed before the termination of amblyopia treatment, compared to 38% of patients that ultimately required surgery. Another study showed that 61% of 46 small-angle strabismic children undergoing part-time treatment with Bangerter foils for amblyopia, developed motor fusion with no additional interventions.

Prisms

In adult patients, prismatic correction can be used to restore binocularity, relieve diplopia and asthenopic symptoms, and alleviate abnormal head positions. Prismatic correction is best suited for smaller angle and comitant strabismus. Larger amounts of prism can cause image distortion and add weight and thickness to the lens. As a general guideline, 1 PD adds 1 mm of lens thickness. Fresnel prisms are available up to 40 PD but prisms larger than 12 PD induce degradation of optical quality, decreased visual acuity, and contrast sensitivity. These press-on flexible vinyl prisms can be applied to a portion of the lens to treat non-comitant strabismus. Prisms have limited value in the management of early childhood strabismus. If a child has a temporary deviation that is expected to improve, as in consecutive esotropia after surgery for exotropia, temporary prisms may be used to maintain binocularity while the deviation resolves or while awaiting surgical intervention. Prisms can be used for preoperative evaluation and calculation of surgical dosages (prism adaptation test) in patients with strabismus. Base-in prisms for reading can be used in patients with symptomatic convergence insufficiency.