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Is botulinum toxin helpful in squint management?
  1. JOHN ELSTON
  1. Oxford Eye Hospital, Walton Street
  2. Oxford OX2 6HE

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    The first report of the experimental use of botulinum toxin relevant to ophthalmologists was published from Alan Scott’s laboratory in 1973, and concerned the creation of incomitant strabismus in rhesus monkeys by injection and paralysis of the medial rectus muscle.1 With the optimal dosage of botulinum toxin A, when the paralysis resolved a concomitant exodeviation persisted and it appeared that in these animals the agonist:antagonist muscle balance had been altered permanently. On this basis the technique appeared to be a possible alternative to strabismus surgery. The initial publications on the effect of botulinum toxin injection on concomitant strabismus in adults were open non-comparative studies, but appeared to substantiate the hypothesis that a permanent realignment of the eyes could be achieved.2 However, the strong tendency for recurrence of the deviation in the absence of sensory and motor binocularity before treatment was highlighted in later publications.3 In a comparative trial of botulinum toxin injection versus surgery for the treatment of strabismus in adults without fusion, surgery gave considerably better results.4For many strabismus surgeons interest in botulinum toxin treatment as a directly comparable alternative to surgery waned.

    There is less literature on the use of botulinum toxin in childhood strabismus; this is partly because the electromyography guided injection technique requires a local anaesthetic and is inapplicable under general anaesthesia.5 The technique can be used with local anaesthetic and intravenous ketamine in children, but this agent frequently causes nightmares and hallucinations. Also, side effects of the botulinum toxin owing to spread to other extraocular muscles and the levator are commoner in children because of the small orbit. In the UK, ethics committees have refused consent to carry out comparative studies in patients with congenital esotropia which, because of the possibility of establishing some degree of binocularity, is arguably the group with the best potential for long term alignment. Moreover, many ophthalmologists feel intuitively that strabismus surgery is a better option in children.

    Against this background the paper from Tejedor and Rodriguez (p 110) is an important contribution. In a group of children needing retreatment after previous surgery for acquired esotropia, there was no difference in the motor and sensory outcomes between those treated with botulinum toxin and those reoperated.

    The importance of this paper is threefold. Firstly, it addresses a specifically defined management issue by randomly allocating one form of treatment or another. This approach is widely recognised to be the appropriate test of a novel treatment, but one that is rarely practised particularly in the surgical specialties. The precise role of botulinum toxin in strabismus management remains unclear to many ophthalmologists, because of the lack of clear guidelines that can only be developed from controlled studies such as this. Secondly, the paper demonstrates the usefulness of botulinum toxin in the defined clinical circumstances. The treatment was safe, as effective as surgery, and did not produce side effects (for example, vertical deviations) that might prejudice the long term outcome. Injection treatment is also repeatable, does not induce scar tissue, and does not preclude the possibility of further surgery at a later stage. Thirdly, the outcomes described suggest that there may be a difference in the potential for botulinum toxin treatment to alter long term ocular alignment in children as opposed to adults. This may be partly because the children treated in this study had some fusional potential, whereas many of the adults treated in other studies do not. It may also be that the structural changes in the extraocular muscles described after botulinum toxin injections have more impact in the child than in the adult and permanently alter the agonist:antagonist balance.6

    As predicted by Scott in 1973 botulinum toxin treatment has proved to be highly effective for many forms of focal dystonia in which it is now the treatment of choice.7 Its role in strabismus management is clear in muscle paresis and other situations where binocular potential is good.8 9 More thoughtful well designed studies such as that of Tejedor and Rodriguez are still needed to clearly define its role in other situations.

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