Approach
The treatment of strabismus is dependent on the type and aetiology of the abnormality and is directed by the evaluating ophthalmologist.
The presence of strabismus prevents the development of binocular visual functions such as depth perception and stereopsis in childhood. Binocular visual function can only be allowed to develop normally, or be restored if underdeveloped, if the strabismus is successfully treated at a young age. However, strabismus may resolve spontaneously in children aged <4 months, particularly if there is an intermittent, variable, or small (<40 prism dioptres) deviation.[2]
Initial non-surgical therapies
Correction of refractive errors
The visual acuity of all patients with strabismus should be assessed; if a significant refractive error is present, it should be corrected using spectacles or contact lenses.
This may lead to realignment of the eyes in accommodative esotropia (with correction of hyperopia) and some cases of intermittent exotropia. In children, the refractive error needs to be measured under cycloplegia (e.g., 30 minutes after application of cycloplegic eye drops such as cyclopentolate 1%).[21]
Reversal of monovision (if present), whereby the vision in the dominant eye is corrected for distance, and the non-dominant eye is corrected for close-up vision, may also be required to resolve symptoms for some adult patients.[28]
If esotropia does not respond to an initial prescription or if it recurs after surgery, repeat the cycloplegic refraction before diagnosing a non-accommodative component.[2]
Treatment of amblyopia
Although amblyopia can occur at any age prior to the development of visual maturity, it does not affect patients with adult-onset strabismus.
The treatment of amblyopia should be undertaken in children and considered in young adolescents. The evidence for its successful resolution following treatment in adults is poor.
If amblyopia is present, it should be treated before, or in parallel with, correction of the strabismus, as it has been shown that the surgical alignment of the eyes is more successful in patients with mild or no amblyopia than in those with moderate or severe amblyopia.[29]
Treatment of amblyopia involves occlusion of the sound eye with a patch, or penalisation of the sound eye either by optical means (i.e., spectacles), or pharmaceutical means (atropine 1% eye drops), or both.[30] Evidence from one systematic review has suggested that occlusion, while wearing necessary refractive correction, appears to be more effective than refractive correction alone in the treatment of strabismic amblyopia.[22] See Amblyopia (Management approach).
Treatment of diplopia
Diplopia may be absent in children with strabismus as a result of suppression (active process of the central nervous system by which the visual input of one eye is ignored) of the image created by the non-fixating eye.
Occlusion: although diplopia can be easily treated by covering one eye with a patch, this is rarely a long-term solution. In children with amblyopia the sound eye is covered (occluded), and in the absence of amblyopia the non-deviating eye is covered. If neither amblyopia nor an obviously deviating eye is present, alternate patching is used. To prevent the development of amblyopia, patching should be limited to a maximum of 6 hours a day, with a recommended duration of between 2 and 6 hours.[2] Occlusion should be used with caution in intermittent exotropia due to the risk of developing amblyopia.
Prisms: used in children with acute-onset strabismus to promote fusion (superimposition of images of both eyes) and avoid amblyopia, but rarely helpful in intermittent exotropia. In adults, they can be used to treat diplopia.[28] Prisms change the path of light, displacing the image of a given object. They are therefore used to superimpose the images of the two eyes when the visual axes are not aligned, thus allowing fusion. As prisms do not change the position of the eyes, they remain deviated under the prism. Prisms work best in comitant strabismus (when the angle of the misalignment is constant in different positions of gaze), and if the angle of the deviation is small.
Treatment of intermittent exotropia
In young children with intermittent exotropia and good fusional control, surgery is rarely needed because they infrequently deteriorate to constant exotropia or reduced stereopsis.[2]
Over-minus prescription: spectacles with a higher than necessary negative power may be prescribed to reduce angle of deviation and promote binocularity, although this is only a temporary measure and may cause asthenopia.[31][32]
Occlusion: covering one eye with a patch may result in improved control of the deviation. One Cochrane review found patching to confer a clinical benefit compared with observation in children aged 12 months to 10 years.[31] This is undertaken with caution, owing to the risk of developing amblyopia. In the presence of amblyopia, the sound eye is covered, and in its absence, alternate patching for 2-6 hours a day is used.[2]
Either of these techniques can be used and the other should be considered if the first fails.[32]
Orthoptic exercises
Both children and adults can present with convergence insufficiency and should be offered orthoptic exercises to improve symptoms associated with near viewing.[2]
The exercises can be administered in the office or at home. A review article found no scientific evidence to support the use of optometric vision therapy in the treatment of other forms of strabismus.[33]
Orthoptic exercises should not delay the application of standard therapy.
Treatment of the underlying cause
As secondary strabismus may result from a number of underlying pathologies (such as cranial nerve and supranuclear palsies, Graves' disease, orbital fractures, and myasthenia gravis), it is essential that these conditions be identified and treated.
Surgery
The definitive treatment for most cases of primary and secondary strabismus is extraocular muscle surgery, particularly when the strabismus has been stable over several months.[34] Surgical intervention should be considered in infantile strabismus, comitant strabismus, sensory strabismus, and all stable misalignments (including paralytic strabismus). In acquired misalignments (e.g., paralytic), it is important to wait 6 to 12 months for potential spontaneous recovery and for stabilisation. Psychosocial benefits of surgery may result from improved facial appearances or function, even in individuals with non-diplopic strabismus.[28]
The basic principles of surgery are to increase (strengthen) or decrease (weaken) muscle function. This can be achieved by recessions, resections, and plications. Recessions involve moving the insertion of a muscle posteriorly on the eye to weaken the muscle. Resections involve removing a section of the muscle to strengthen it, which can also be done by plication. The use of adjustable sutures allows for refinement of the position of the extraocular muscle in the early post-operative period and may result in improved surgical outcomes in some patients.[35]
Decisions on whether and when to operate are made by the consulting ophthalmologist and may be complex in some patients. For infantile-onset exotropia, early surgery can improve sensory outcomes if the exotropia is constant, but it rarely achieves normal binocular function. Therefore, surgery is usually indicated for intermittent exotropia if there has been progression to constant or near-constant deviation, if there is reduced stereopsis, and/or if the patient or family report negative effects on social interactions. Effective initial surgical procedures in these cases include unilateral recess-resect and bilateral lateral rectus recession.[2] For infantile-onset esotropia, early surgical correction probably improves sensory outcomes by decreasing exposure, but it probably does not improve motor outcomes compared with later surgery. There is also insufficient evidence to favour any one surgical approach over another for esotropia.[2][36]
Surgery is generally required when correcting childhood strabismus in adults, but the procedures can be more challenging and should address the sequelae of previous surgery.[18] If the potential for binocular vision is poor, surgery at any age may still be an appropriate treatment to restore a normal appearance.[18][19]
Severe complications and poor or very poor outcomes are estimated to arise in 1 per 400 and 1 per 2400 surgeries, respectively.[37]
Chemodenervation
Botulinum toxin type A selectively blocks the release of acetylcholine at the neuromuscular junction and leads to chemodenervation. If injected intramuscularly, a temporary flaccid paralysis results, with muscle function usually recovering within 2 months. It is therefore considered most helpful in the treatment of strabismus that is expected to change, such as post-operative residual strabismus and acute paralytic strabismus (particularly that secondary to abducens nerve palsy).
In most patients with strabismus, surgical treatment is preferable to chemodenervation because the effect of the latter is temporary, as demonstrated by Cochrane reviews comparing the two approaches. One review concluded that botulinum toxin appears to be inferior for both primary treatment and re-treatment, that it may lead to more surgical re-treatments, and that it may not help to achieve binocularity when compared with surgery.[38] The other review indicated that surgery may achieve better eye alignment compared with botulinum toxin injections, and that it is associated with minimal risk.[36]
Botulinum toxin type A may help to alleviate strabismus secondary to acute paresis of the abducens nerve (cranial nerve VI) by injection into the medial rectus muscle to weaken the muscle while the acute insult heals. Repeated injections can also be used as a long-term alternative to surgery in patients who are not eligible for surgery due to co-morbidities, with one study describing good outcomes with up to 68 injections.[39] Botulinum toxin might also be useful in the treatment of consecutive exotropia, especially after multiple eye muscle surgeries.[40]
The main adverse effect is ptosis (in up to 40% of patients), which is usually reversible.[38]
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