Efficacy and complications of dose increments of botulinum toxin-A in the treatment of horizontal comitant strabismus

Purpose To investigate the efficacy and complications associated with dose increments of botulinum toxin-A (BTA) for comitant horizontal strabismus pa tients. Methods Twenty-five esotropic (ET) and 45 exotropic (XT) patients received 2.5-20 U of BTA injection. Parameters for achieving less than 10 prism diop tres (pd) of horizontal deviation and percentage correction of the pretreat ment deviation were assessed for injections of less than 10 U and more than 10 U of BTA. Induced ptosis and vertical deviation were examined within an d after 6 months of follow-up. Results The mean pretreatment deviations were 38.6 +/- 2.5 pd and 37.6 +/- 1.9 pd for the ET and XT groups, respectively. After receiving 1.6 and 1.5 injections on average, improvement to less than 10 pd at the primary positi on occurred in 32% of ET and 22% of XT patients; the difference was not sta tistically significant. The percentage corrections of the ET patients were 41.4 +/- 9.3% and 36.9 +/- 5.6% in those treated with less than 10 U and mo re than 10 U of BTA respectively; the difference between the two groups was insignificant. For the XT patients the values were 42.1 +/- 7.4% and 28.9 +/- 3.5% respectively, which also were not statistically significantly diff erent. Frequency of induced ptosis was more common in ET than XT patients ( p = 0.01) and this difference was more pronounced with increased doses of B TA (7.7% in ET and 5.3% in XT patients with less than 10 U of BTA, and 24.0 % in ET and 4.3% in XT patients with more than 10 U of BTA). Ptosis resolve d completely within 6 weeks in all cases. Induced vertical deviation with l ess than 10 U of BTA was encountered in one case of ET (11.1%, 9 pd) and in another case of XT (8.3%, 4 pd), increasing to 60.0% (2-20 pd) and 38.8% ( 4-16 pd) respectively with more than 10 U of BTA injection. In about a year , induced vertical deviation resolved in approximately 40%, and decreased i n 30% of the cases. Conclusion Increasing the dose of BTA is clinically effective in larger dev iations, although statistically indifferent, especially in ET compared with XT. However, an increased dose is accompanied by increased incidence of in duced ptosis and vertical deviation. Ptosis is temporary, but vertical devi ation may persist for a long time and may present a cosmetic problem for so me patients when more than 10 U of BTA is used.