Optical correction

It is important to recognize that refractive errors, whether corrected or uncorrected, have a major impact on strabismus and its management. Correcting a refractive error may result in better control of a misalignment solely by gaining optimal visual acuity. Conversely, previously uncorrected strabismus patients can be made symptomatic by correcting their refractive error by encouraging spontaneous alternation or switch of fixation to their non-dominant eye.¹

All children 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% to 2% cyclopentolate (Mydrilate, Cyclogyl), 1% tropicamide (Mydriacyl), or 1% atropine. Cyclopentolate is commonly preferred over atropine due to its shorter onset and duration of action. 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 or 1% tropicamide in children with dark irides.² The difference between atropine and cyclopentolate cycloplegia has consistently been measured to be between 0.4 and 0.7 D,^2–5 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.^2–3 Therefore, in accommodative esotropia with a residual deviation, an atropine refraction is still essential to uncover the maximal amount of hyperopia that needs to be corrected. Atropine eye drops do not sting: cyclopentolate drops do. The pupillary dilation achieved with any of these agents allows the funduscopic exam which is mandatory during the initial evaluation of a strabismus patient to exclude visual axis opacification or abnormalities of the posterior segments.

Considering the optimal optical correction for each patient is essential in the management of their strabismus. In acquired esotropia, particularly accommodative esotropia, all degrees of hypermetropia may be significant, and should be fully corrected with spectacles. The full hyperopic correction should be prescribed in order to remove all accommodative convergence. 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. In patients with surgical overcorrections of partially accommodative esotropia, reduction of the hypermetropic correction of more than +2.50 D may be associated with long-term instability of the alignment: such overcorrections should be avoided.^6,7

In intermittent exotropia (X(T)), it may seem logical not to correct moderate to high hypermetropia since the convergence induced by the hypermetropia has a beneficial effect on the control of the exodeviation. A small study of children with moderate to high hyperopia (3−7 D) and X(T) showed that full hyperopic prescription corrected the manifest deviation and improved the binocular status in all the patients.⁸ A larger study demonstrated that with partial spectacle correction the mean exodeviation increased on average 10 PD (prism diopters) in one-third of patients with X(T) and moderate hyperopia compared to patients with fully corrected hyperopia, emmetropia, or myopia.⁹ Nevertheless, high hyperopia, anisometropia, significant astigmatism, and myopia should always be corrected in patients with X(T). High hyperopic patients may not be accommodating fully and experiencing blur, resulting in poor control of the X(T). It is unclear if moderate hyperopia needs to be corrected unless the patient has decreased vision, or if surgical planning should be done wearing moderate hyperopic corrections to improve long-term surgical outcomes.⁹

For the treatment of X(T), over-minus lens therapy has been proposed (1.5−4 D additional minus) to stimulate accommodative convergence and improve control of the deviation. In three prospective studies of children treated with over-minus glasses, some form of improvement (depending on the outcome measure used) was seen in 45−70% of patients.^10–12 This improvement was seen irrespective of the initial angle of deviation and in some patients was maintained after discontinuation of therapy. Since the reports on the natural history and spontaneous improvement of X(T) are contradictory, and there is a lack of control groups for these interventions, it is difficult to ascertain their real efficacy. Over-minus therapy does not appear to increase myopia.^12,13

Other non-surgical interventions prescribed for the treatment of X(T) include part-time occlusion, fusional vergence exercises, and atropine. The rationale behind these interventions is that they may improve the control of the intermittent 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.^14,15 (see section on Occlusion therapy).

Bifocals are indicated in children with a high AC/A (accommodative convergence to accommodation ratio) accommodative esotropia (convergence excess esotropia), where the full hypermetropic spectacle correction aligns the eyes for distance fixation, but a residual esotropia is seen at near. 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 is often prescribed). Some physicians order an add of +3.00 D, on the grounds that the average fixation distance at near is 3 cm. Others prescribe the minimum near addition which controls the near esotropia. The recommended strategy is then to gradually and incrementally reduce the bifocal correction.

Occlusion therapy

Some authorities recommend treating amblyopia fully before strabismus surgery even though in some cases this means delaying surgery for many months. 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.^16,17

A recent study reported that, with partially accommodative esotropia, the mean angle of deviation decreased significantly after occlusion treatment for amblyopia and noticed that occlusion may resolve the non-accommodative component of the deviation obviating the need for surgery. Surgery would have been performed in 81% of patients if planned before the termination of amblyopia treatment, compared to 38% of patients that ultimately required surgery.¹⁸ Another study showed that 61% of 46 small-angle strabismic children 1.5 to 9 years of age, undergoing part-time treatment with Bangerter foils for amblyopia, developed motor fusion with no additional interventions.¹⁹

Occlusion therapy, even in the absence of amblyopia, may be effective in improving the control of some forms of strabismus. Part-time (3−4 hours a day) unilateral or alternating occlusion is commonly prescribed for the treatment of X(T).^20,21 Occlusion may improve the control of the deviation by eliminating suppression. Commonly, the angle of the deviation decreases, and the relationship between distance and near deviation may change, altering the X(T)pattern.²¹ Unfortunately, most data supporting this and other non-surgical treatments for X(T) come from small retrospective and/or non-controlled studies. It remains unclear what is the most effective treatment for this condition.¹⁴

Prisms

Prisms have very limited value in the management of childhood strabismus. Where a child has a temporary deviation that might confidently be expected to improve, as in consecutive esotropia after surgery for exotropia, or in a post-viral sixth nerve palsy, temporary Fresnel membrane prisms may be used to maintain binocularity while the deviation resolves. Occasionally, prisms can be used to move unwanted images into an area of the retina that is suppressed in patients with diplopia. However, in anything other than the lowest powers, prisms significantly degrade the visual acuity, in addition to attracting dirt, fingerprints, etc.

Orthoptic exercises/vision therapy

Orthoptic exercises are sometimes used to teach patients how to use their fusion ability more efficiently. Orthoptic exercises may help in establishing control of strabismus in a very small group of patients with good fusion potential. The effectiveness of orthoptic exercises (and the broader practice of vision therapy) in treating most forms of strabismus is debated. A comprehensive review of the evidence supporting orthoptic exercises/vision therapy from the UK concluded that there was little evidence-based research to support these practices.²² A notable exception is the management of convergence insufficiency.²³

The Convergence Insufficiency Treatment Trial, a placebo-controlled randomized clinical trial, demonstrated that a 12-week office-based program of vergence and accommodative exercises (such as string-convergence and barrel-convergence) with home reinforcement was more effective than home-based “pencil push-ups,” home-based computer therapy plus “pencil push-ups,” or office-based placebo therapy in treating convergence insufficiency in children.²⁴ A similar conclusion was reached by the same researchers investigating convergence insufficiency treatment in young adults.²⁵

Drugs

Irreversible cholinesterase inhibitors, such as Ecothiopate (phospholine iodide 0.125%, 0.0625% and 0.0325%) drops, can be used to control accommodation and, thereby, accommodative convergence. Pilocarpine, a muscarinic receptor agonist, can be used to decrease the synkinetic convergence response. Contraction of the ciliary body produces increased convexity and forward-shifting of the lens, resulting in a myopic shift and reduction in the active accommodation effort made. The most common indications for these medications include persistent consecutive esotropia after surgical overcorrection of exotropia, small residual post-operative esotropia, and as a supplement to glasses in patients with high AC/A accommodative esotropia with residual esodeviation at near.

Phospholine iodide is rarely used now due to its limited commercial availability and side effects (supraorbital pain, poor vision especially at night, and iris cysts and lens opacities with long-term use). Systemic absorption of the drops causes a depletion of plasma cholinesterase making the patient susceptible to depolarizing muscle relaxants. If a child treated with cholinesterase inhibitors requires general anesthesia, a depolarizing relaxing agent such as succinylcholine should be avoided as prolonged respiratory paralysis may result. The depletion may last up to 6 weeks and parents should be advised appropriately. To reduce the risk of iris cyst formation, phenylephrine (5%) eye drops can be prescribed concurrently.

In hypermetropic esotropic children who do not tolerate their newly prescribed spectacles, atropine 1% once daily for 5 days can be used to blur the vision and give the child an incentive to wear glasses.

Botulinum toxin

The pharmacologic treatment of strabismus was pioneered by Scott, who experimented with the direct injection of various substances into the extraocular muscles. In 1990, botulinum toxin type A was granted FDA approval. Botulinum toxin type A (BTX-A) is a large protein molecule of 150 000 daltons. After intramuscular injection, the toxin enters and remains at the nerve terminal for several days to weeks inhibiting the release of acetylcholine by cleaving SNAP-25, a protein integral to the successful docking and release of the neurotransmitter in vesicles within the nerve endings resulting in muscle weakness or paralysis maximal within 3 to 5 days after the injection. Although an irreversible binding occurs, extrajunctional acetylcholine receptors may develop. The nerve reinnervates the muscle with a reversal of the paralysis and eventual recovery. Extraocular muscle paralysis usually lasts from 2 to 8 weeks.

The mechanism by which permanent alignment can occur after the paralyzing effect of botulinum toxin is gone is not completely understood. One theory is that if the antagonist muscle remains contracted for a certain time because of reduced force in treated muscle, it will develop structural alterations that shorten it^26,27 and decrease its elasticity (contracture). Conversely, the chemodenervated muscle will undergo lengthening as its antagonist contracts. This length adaptation is believed to occur through the addition and deletion of sarcomeres.²⁷ The alterations induced in extraocular muscles by botulinum treatment have been shown to be specific to the orbital singly innervated muscle fibers.^26,28

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