Botulinum toxin injection for restrictive myopathy of thyroid-associated orbitopathy: success rate and predictive factors

Abstract

To evaluate the rate of and predictive factors for successful treatment of restrictive myopathy in thyroid-associated orbitopathy (TAO) using botulinum toxin injection. Twenty patients with restrictive myopathy of TAO were enrolled in the study. Abnormal thyroid function test results were not a prerequisite for inclusion. In each extraocular muscle 25 units of botulinum toxinA were injected. The success rate, calculated at 2years or last follow-up before surgery, was defined as proportion of the cases with esotropia of <10(Δ), vertical deviation of <5(Δ), and no diplopia in primary position and downgaze for at least 1year. Both univariate analysis and multivariate logistic regressions were performed to identify the factors associated with success. The procedure was successful in 11 cases (55%): in 8 patients with predominantly esotropia, 1 patient with predominantly hypotropia, and 2 patients of mixed type. Four factors were significantly associated with the success: type of deviation (P=0.007), lower amounts of hypotropia (P=0.001) and esotropia (P=0.05), and lower degree of extorsion (P=0.01). In the multivariate logistic regression, only lower amount of hypotropia was significantly associated with the success (P=0.09, OR=1.36). Botulinum toxin injection can be an effective alternative for the treatment of the restrictive myopathy in TAO. The best candidates for injection of the toxin are patients with esotropia, smaller angle of horizontal and vertical deviations, and lower degree of extorsion.