February

A long-established success of β0ToxinA®: the management of strabismus

Strabismus, blepharospasm, and other ocular motility disorders were amongst the first conditions to be treated with the botulin toxin, with official approval by the U.S. Food and Drug Administration being granted in 1989, and are still considered some of the few "label" therapy indications.Extensive research in the past 20 years have analysed the dose-effect relationship, the success of repeat doses on, and all sorts of implications of a treatment used both as the primary method of treatment of ocular deviation to substitute traditional surgical correction, and as management of patients, especially adults, with recent surgical over- or under-corrections.

Nearly all kinds of strabismus benefit from botulinum toxin treatment, with best and longest results in horizontal strabismus, and good results in vertical strabismus and lid retraction. In concomitant strabismus it is above all valuable in small angle cases, and its success rate is very high in sensory strabismus as well.

Injections of botulinum A toxin are administered into the extraocular muscle to weaken it, at the initial dose of 1.25-5 U depending on the severity of the strabismus, with paralysis occurring four to five days after the injection. Treatment should be assessed every 7-14 days after an injection, and, if there is an adequate response, the same dose can be continued. If the response is not adequate, the dose can be increased two-fold. The muscle gradually recovers normal activity as new neuromuscular junctions are formed by sprouting from presynaptic axons. The duration of effect lasts approximately three months, and in some cases much longer (up to over 400 days has been reported in the literature). Long-term studies (in particular the 8-year 1998 London study published by Horgan SE, Lee JP, Bunce C.) show a trend toward fewer injections with time. Best results are obtained with repeated small doses. Generally, a 65% reduction of the strabismus angle after two to three injections can be expected. The maximum correction of strabismus has been 40 prism dioptres. If binocular functions are re-established when the eyes are realigned, as the treated muscle recovers function, the incomitant strabismus may not recur.

news02 eyeIn paralytic strabismus, which is due to weakness of extraocular muscles — for example, the lateral rectus in abducens nerve palsy — injections into the ipsilateral antagonist (medial rectus) can weaken contracture of this muscle. Thus the contracture of the homolateral antagonist can be overcome and not seldom singular binocular vision obtained again. In cases of moderate pareses which recover spontaneously, the muscle weakening effect of the toxin on the antagonist helps to restore binocular single vision.

In cases of chronic paralytic strabismus, instead, β0ToxinA® injected into the antagonist facilitates the surgical intervention on the paralyzed muscle.
In patients with concomitant strabismus, who have compromised or absent binocular fusion, treatment is cosmetic as permanent ocular realignment cannot be expected. In secondary strabismus resulting from transient monocular vision loss (such as posttraumatic cataract), toxin injections can help to establish whether binocular cooperation is still present. If so, the patient would be a candidate for surgery to restore ocular function.

In some cases, such as congenital esotropia, in commitant squints with large angles and in chronic or intermittent exotropia, surgery is still the preferred modality of treatment. Yet, treatment with β0ToxinA® can be performed to help realign the eye before more definitive surgery (for instance in dysthyroid eye disease).

It should be noted, though, that, in spite of excellent results in most strabismus cases, β0ToxinA® injections are ineffective in patients with restrictive strabismus.Literature is abundant in the paediatric field as well, with research performed on several hundred children, generally with excellent results. Yet the Biglan, Burnstine, Rogers, and Saunders study proves that chemodenervation of an extraocular muscle is not as successful as traditional surgery for treatment of infantile esotropia and other comitant deviations.

The advantages of β0ToxinA® injections over traditional surgery are its ease of administration (it can be performed under topical anaesthesia), absence of scar as there is no incision, and little or no postoperative discomfort. Indeed, with injections into the lids and the extraocular muscles, it is usually a safe, simple technique, with no general systemic effect or local complications. It goes without saying, though, that, as this condition presents very localised muscle overactivity in delicate places, the injections should not be attempted without electromyographic (EMG) guidance. Special care should be taken as retrobulbar hemorrhages sufficient to compromise retinal circulation have occurred from needle penetrations into the orbit. It is therefore recommended that appropriate instruments to decompress the orbit be available, together with an ophthalmoscope to diagnose any problems. Complications include transient ptosis or vertical deviation of the globe, especially with higher doses, caused by effects on extraocular muscles adjacent to the injection site (with a 16-17% incidence rate). Side effects of paralysis in one or more extraocular muscles may be spatial disorientation, or double vision, which are usually alleviated by covering the affected eye.

References:

  • Biglan AW, Burnstine RA, Rogers GL, Saunders RA. Management of strabismus with botulinum A toxin. Ophthalmology. 1989 Jul;96(7):935-43.
  • Elston JS, JP Lee, CM Powell, C Hogg and P Clark Treatment of strabismus in adults with botulinum toxin A. British Journal of Ophthalmology, 1985, Vol 69, 718-724
  • Horgan SE, Lee JP, Bunce C. The long-term use of botulinum toxin for adult strabismus. J Pediatr Ophthalmol Strabismus. 1998 Jan-Feb;35(1):9-16; quiz 44-5.
  • Huber A. [Use of botulinum toxin in ophthalmology] [Article in German] Ther Umsch. 1990 Apr;47(4):320-8.
  • Münchau A, Bhatia K P, Uses of botulinum toxin injection in medicine today BMJ. 2000 January 15; 320(7228): 161–165.
  • Scott AB. Botulinum toxin injection into extraocular muscles as an alternative to strabismus surgery. Ophthalmology. 1980 Oct;87(10):1044-9.
  • Scott AB. Botulinum toxin injection of eye muscles to correct strabismus. Trans Am Ophthalmol Soc. 1981;79:734-770.
  • Scott AB, Magoon EH, McNeer KW, Stager DR. Botulinum treatment of childhood strabismus. Ophthalmology. 1990 Nov;97(11):1434-8.

We hope you enjoyed this article.
Watch out for next month issue of our newsletter.

Contatti

SeaLab Europe LLP - Suite 36, 88-90 Hatton Garden, Holborn, EC1N 8PN, Londra (Regno Unito)

Telefono: +44 7077073252

Fax: +44 7077077606

Newsletter

Per ricevere la nostra newsletter registrati al nostro esclusivo "botoxina Foundation members Club" inserendo la tua email:

sottomarino newsletter

Potrai aggiudicarti un tour sul nostro battiscafo marino e nel nostro laboratorio.