Sustained improvement of reading symptoms following botulinum toxin A injection for convergence insufficiency

We evaluated the use of botulinum toxin A in adults with convergence insufficiency in whom prior treatment had failed.

We studied 8 patients (median age 36 years, range 17 to 77 years) with reading symptoms due to convergence insufficiency defined as an exodeviation greater at near, not exceeding 10 PD in the distance measured by prism and alternate cover test, and either convergence near point>6 cm or reduced fusional amplitudes. All patients were still symptomatic after prior treatment by convergence exercises (n=8), base-in prism glasses (n=5) or strabismus surgery (n=2). Five patients received injection of 5 IU botulinum toxin in 0.1 ml saline to one lateral rectus muscle, two received 2.5 IU, and one received 2.5 IU to both lateral rectus muscles.

At 1 month post injection, all patients had an initial reduction of exodeviation from baseline (median 9 PD, p=0.008) at near, although 2 patients had a temporary intermittent esotropia in the distance with diplopia associated with difficulty driving. At 6 months, when the pharmacological effect of botulinum toxin had completely worn off, patients still maintained a small reduction of exodeviation (median 4 PD, p=0.3) at near. Reading symptoms improved in 7 of 8 patients at 1-month post injection, and in all patients at 6 months. Two patients had health-related quality of life assessed with the Adult Strabismus 20 Questionnaire, showing improved Reading Function scores at 6 months. Interestingly, 3 patients reported improved reading despite returning to the baseline angle at 6 months, and 2 of 4 with 12-month follow-up still reported improvement.

In adult convergence insufficiency, botulinum toxin injection to a lateral rectus muscle improves reading symptoms beyond the duration of the pure pharmacological effect. Botulinum toxin injection may be useful in management of adult convergence insufficiency, although repeat injections may be needed.