Botulinum toxin for the treatment of strabismus

Background The use of botulinum toxin as an investigative and treatment modality for strabismus is well reported in the medical literature. However, it is unclear how effective it is in comparison with other treatment options for strabismus. Objectives The primary objective was to examine the efficacy of botulinum toxin therapy in the treatment of strabismus compared with alternative conservative or surgical treatment options. This review sought to ascertain those types of strabismus that particularly benefit from the use of botulinum toxin as a treatment option (such as small angle strabismus or strabismus with binocular potential, i.e. the potential to use both eyes together as a pair). The secondary objectives were to investigate the dose effect and complication rates associated with botulinum toxin. Search methods We searched CENTRAL, MEDLINE, Embase, LILACS and three trials registers on 6 July 2022, together with reference checking to identify additional studies. We did not use any date or language restrictions in the electronic searches for trials. Selection criteria We planned to include randomized controlled trials (RCTs) comparing botulinum toxin with strabismus surgery, botulinum toxin alternatives (i.e. bupivacaine) and conservative therapy such as orthoptic exercises, prisms, or lens therapy for people of any age with strabismus. All relevant RCTs identified in this update compared botulinum toxin with strabismus surgery. Data collection and analysis We used standard methods expected by Cochrane and assessed the certainty of the body of evidence using GRADE. Main results We included four RCTs with 242 participants that enrolled adults with esotropia or exotropia, children with acquired esotropia, and children with infantile esotropia. The follow-up period ranged from six to 36 months. Two studies were conducted in Spain, and one each in Canada and South Africa. We judged the included studies to have a mixture of low, unclear and high risk of bias. We did not consider any of the included studies to be at low risk of bias for all domains. All four studies reported the proportion of participants who improved or corrected strabismus, defined as <= 10 prism diopters (PD) at six months (two studies) or <= 8 PD at one year (two studies). Low-certainty evidence suggested that participants treated with the surgery may be more likely to improve or correct strabismus compared with those who treated with botulinum toxin (risk ratio (RR) 0.72, 95% confidence interval (CI) 0.53 to 0.99; I-2 = 50%; 4 studies, 242 participants; low-certainty evidence). One study, which enrolled 110 children with infantile esotropia, suggested that surgery may reduce the incidence of additional surgical intervention required, but the evidence was very uncertain (RR 3.05, 95% CI 1.34 to 6.91; 1 study, 101 participants; very low-certainty evidence). Two studies conducted in Spain compared botulinum toxin with surgery in children who required retreatment for acquired or infantile esotropia. These two studies provided low-certainty evidence that botulinum toxin may have little to no effect on achieving sensory fusion (RR 0.88, 95% CI 0.63 to 1.23; I-2 = 0%; 2 studies, 102 participants) and stereopsis (RR 0.86, 95% CI 0.59 to 1.25; I-2 = 0%; 2 studies, 102 participants) compared with surgery. Three studies reported non-serious adverse events. Partial transient ptosis (range 16.7% to 37.0%) and transient vertical deviation (range 5.6% to 18.5%) were observed among participants treated with botulinum toxin in three studies. In one study, 44.7% participants in the surgery group experienced discomfort. No studies reported serious adverse events or postintervention quality of life. Authors' conclusions It remains unclear whether botulinum toxin may be an alternative to strabismus surgery as an independent treatment modality among certain types of strabismus because we found only low and very low-certainty evidence in this review update. Low-certainty evidence suggests that strabismus surgery may be preferable to botulinum toxin injection to improve or correct strabismus when types of strabismus and different age groups are combined. We found low-certainty evidence suggesting botulinum toxin may have little to no effect on achievement of binocular single vision compared with surgery in children with acquired or infantile esotropia. We did not find sufficient evidence to draw any meaningful conclusions with respect to need for additional surgery, quality of life, and serious adverse events. We identified three ongoing trials comparing botulinum toxin with conventional surgeries in the varying types of strabismus, whose results will provide relevant evidence for our stated objectives. Future trials should be rigorously designed, and investigators should analyze outcome data appropriately and report adequate information to provide evidence of high certainty. Quality of life and cost-effectiveness should be examined in addition to clinical and safety outcomes.