Introduction: This retrospective study reports our experience on the use of botulinum toxin A (BTXA) in the treatment of sixth cranial nerve palsy at the Singapore National Eye Centre. BTXA is derived from clostridium botulinum; it causes temporary paralysis of the extraocular muscle (medial rectus) into which it is injected, thus preventing its contracture and allows the antagonist lateral rectus muscle to take up the slack and reduce or correct the ocular misalignment.
Methods: Nineteen patients had BTXA injection for estropia due to sixth cranial nerve palsy during the period September 1992 to August 1997. The sixth cranial nerve palsy was related to nasopharyngeal carcinoma in 76.7% of cases. Follow-up after the last injection ranged from zero (defaulted) to 21 months (mean 8, median 6 months).
Results: A total of 25 injections were given to 19 patients. Seven patients (36.8%) had final ocular alignment within 10 prism dioptres of orthotropia of which six achieved fusion at primary gaze position. There was no correlation between the number of injections per patient and the size of strabismus or grade of lateral rectus muscle function. The incidence of ptosis was 48%, subconjunctival haemorrhage 16% and hypertropia 16%.
Discussion: Our results suggest that those patients with smaller strabismus and a shorter time interval between onset of strabismus and botulinum injection tend to achieve better outcome in terms of fusion or ocular alignment within 10 prism dioptres of orthotropia. The treatment of strabismus with BTXA is an acceptable approach in selected patients. The procedure is simple, safe, cheap, effective, and avoids the risks of general anaesthesia. It can substitute for or eliminate the need for strabismus surgery in some cases of sixth nerve palsy.