Improvement in reading symptoms following botulinum toxin A injection for convergence insufficiency type intermittent exotropia (2014)

We evaluated injection of botulinum toxin A (BOTOX®, Allergan Inc., Irvine, CA) in patients with convergence insufficiency (CI) type intermittent exotropia (IXT) still symptomatic after convergence exercises. IXT can be divided into basic type, CI type and divergence excess type (Rosenbaum & Santiago 1999). CI type IXT is a common binocular vision disorder defined as an intermittent exodeviation at 10 PD or more at distance, and at least 10 PD more than distance deviation at near, measured by prism and alternate cover test (PACT), a remote near point of convergence, and often associated with reading problems (Hatt & Gnanaraj 2006). Management options for CI type IXT include base-in prisms, convergence exercises or surgery, but surgery may result in overcorrection at distance (Choi & Rosenbaum 2001), thus alternative treatments are desirable. Previous reports describe injections with botulinum toxin for exotropia can reduce the mean deviation 20 PD (Gardner et al. 2008), but little data exist for botulinum treatment of IXT. Recently, 10 patients with CI type IXT treated with botulinum toxin (Dysport) were presented by Dawson, and 80% had improvement 2–3 weeks later, but the poster abstract did not report specific data on reading symptoms (Dawson et al. 2012). To our knowledge, treatment for CI type IXT with Botox has not been reported in the peer-reviewed literature. We retrospectively identified five patients (median age 20 years, range 15 to 73 years) with CI type IXT who all had symptoms of reading difficulty. All patients demonstrated measurable stereoacuity (range 60–240 arc sec) measured by TNO (Laméris-Ootech, Netherlands). All patients had previously performed convergence exercises without sustaining improvement of symptoms. Near point of convergence was >6 cm in all patients at baseline (range 10-30 cm, median 12 cm). One patient had a history of head trauma with subsequent onset of reading problems. No patients had received previous strabismus surgery. All patients received injection of 2.5 IU Botox in 0.1 ml saline to one lateral rectus muscle assisted by electromyography, performed under topical anaesthesia. Evaluation of alignment and symptoms was recorded before Botox injection (baseline), after 1 month (24–41 days), after 6 months (171–208 days) and after 12 months (320–401 days). The patients were instructed to report reading symptoms as worse, unchanged, improved or recurred compared with baseline. Statistical analyses on changes from baseline were performed using signed rank tests (SAS, version-9.3, SAS Institute-Inc., Cary, NC). The Danish Data Protection Agency (Datatilsynet) approval was obtained for this study (J.Nr.2014-41-3038), which conformed to principles of the Declaration of Helsinki. In our study, at 1-month postinjection, all patients had an initial reduction of exodeviation from baseline (median reduction 30 PD, p = 0.25) at near (Table 1). At 6 months, patients still maintained a reduction of exodeviation (median reduction 23 PD, p = 0.06) at near. Reading symptoms improved in two of three patients at 1-month postinjection, in all patients at 6 months, and in three of four patients at 12 months. Interestingly, three patients reported improved reading symptoms despite almost returning to baseline angle at 6 months, and three of four with 12-month follow-up, still reported improvement. One patient experienced a right hypertropia lasting 4 weeks postinjection. No new persistent vertical deviation, pupillary change, ptosis or change in visual acuity was observed. This study has limitations, as it is a retrospective analysis, and the aetiology of the CI type IXT was not the same in all patients. In patients still symptomatic from CI type IXT after performing convergence exercises, despite uncertainty regarding mechanism, this study suggests Botox injection to a lateral rectus muscle may improve reading symptoms beyond the duration of the pure pharmacological effect, thus further studies are needed. Literature Search: (PUBMED 1966 to 17 July 2014, search term: botulinum toxin AND intermittent exotropia).