Botulinum toxin therapy of strabismus
Botulinum toxin therapy of strabismus is a medical technique in which botulinum toxin is injected into selected extraocular muscles in order to reduce the misalignment of the eyes. The injection of the toxin to treat strabismus, reported upon in 1981, is known as the first ever use of botulinum toxin for therapeutic purposes. Today, the injection of botulinum toxin into the muscles that surround the eyes is one of the available options in the management of strabismus; its use is limited to particular situations and its over-all efficacy is so far considered equivocal. Other, more widespread elements in strabismus management are vision therapy and occlusion therapy, corrective glasses (or contact lenses) and prism glasses, and strabismus surgery.
The effects that are due only to the toxin itself (including the side effects) generally wear off within 3 to 4 months. In contrast, improvements in alignment may potentially be long-lasting, provided binocular vision has been achieved and stabilized. Reliable indications for optimal dosages have not been established. There are indications that botulinum toxin therapy is similarly successful as strabismus surgery for patients with binocular vision and that it is less successful than surgery for those who have no binocular vision.
Botulinum toxin is the most acutely lethal toxin that is known. It is produced by the bacterium clostridium botulinum. It acts inside nerve terminals by decreasing the release of acetylcholine, blocking neuromuscular transmission and thereby causing flaccid muscular paralysis. As a result, the muscle is weakened for about 3 to 4 months.
For treating strabismus, the toxin is used in much diluted form, and the injection is targeted to reach specific muscles that move the eye, thereby temporarily weakening the selected muscles.
After local or general anaesthesia has been applied, the botulinum toxin is injected directly into the selected eye muscles using a specially designed needle electrode that is connected to an electromyography (EMG) apparatus as well as to a syringe containing the botulinum toxin solution.
When under local anaesthesia, the patient is asked to move the eyes just before the toxin is injected. This results in an EMG signal which provides instant feedback on the correct placement of the needle. If the patient is a young child, general anaesthesia is always used.
The duration of the intervention is one to two minutes if the person performing the procedure has sufficient experience.
The dosage to be used cannot be determined with precision, as no reliable relation between dose and effect could be established so far. The toxicity of botulinum toxin varies from one lot to the next; furthermore, the body may show an immunoreaction by which the efficacy of subsequent treatments is reduced.
Botulinum toxin is considered as an alternative to surgery in certain clinical situations. A study performed in the 1980s found outcomes of surgery to be "more predictable and longer lasting" than those of botulinum toxin therapy. As stated in a review article of 2007, its use for strabismus "varies enormously in different cities and countries for no apparent reason."
In a small-scale study, adults whose reading difficulties due to convergence insufficiency had been unsuccessfully addressed by convergence exercises, base-in prism glasses or strabismus surgery showed improved reading after botulinum toxin therapy, maintaining improved reading remaining also after six months.
Use as primary therapy
Botulinum toxin is considered a useful alternative to surgery in particular cases, for example for persons unfit for general anaesthesia, in evolving or unstable clinical conditions, after unsuccessful surgery, or to provide short-term relief from diplopia.
For patients who have had healthy vision heretofore until a small, horizontal deviation set in suddenly, the injection of botulinum toxin may allow them to maintain the binocular vision skills that had been acquired earlier.
Some consider botulinum injections to be a treatment option for children with small- to moderate-angle infantile esotropia. Studies have provided indications that performing injections into both medial rectus muscles may be more effective than an injection into one medial rectus muscle alone.
Botulinum toxin therapy has been reported to be similarly successful as strabismus surgery for patients with binocular vision and less successful than surgery for those who have no binocular vision.
Intra- and postoperational use
Botulinum toxin has also been used postoperatively for improving the alignment in patients with over- or undercorrection after strabismus surgery, leading to rapid elimination of postoperative diplopia but possibly requiring repeated injections or reoperation later on. It is considered particularly useful for patients who have the potential for binocular vision; success rates are higher for treating postoperative esotropia than for treating postoperative exotropia.
It has also been employed in combination with strabismus surgery in cases in which there is a large horizontal eye deviation and eye muscle surgery on both eyes (binocular surgery) is not an option for other reasons.
The most common side effects are droopy eyelids (ptosis) and over- or undercorrections; a further common side effects are diplopia and inadvertent vertical deviation (hypo- or hypertropia). The side effects typically resolve in 3–4 months.
Vision-threatening complications are rare, and the intervention is generally considered safe, also when performed repeatedly.
Alan B. Scott first injected botulinum toxin into extraocular muscles since the early 1970s and published his results 1981, giving rise to a wide scope of clinical research on the use of the toxin.
- Rowe FJ, Noonan CP (Feb 15, 2012). "Botulinum toxin for the treatment of strabismus". Cochrane Database of Systematic Reviews (2): CD006499 (3rd rev.). doi:10.1002/14651858.CD006499.pub3. PMID 22336817.
- "Chapter 25: Chemodenervation of Extraocular Muscles – Botulinum Toxin" (PDF)., pages 559–565. In: Gunter K. von Noorden, Emilio C. Campos: Binocular Vision and Ocular Motility: Theory and Management of Strabismus, Sixth Edition. Ophthalmology Books & Manuals (Cyber Sight), Orbis International
- Flanders M et al (June 1987). "Injection of type A botulinum toxin into extraocular muscles for correction of strabismus". Can. J. Ophthalmol. 22 (4). pp. 212–217. PMID 3607594.
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- Jon Peiter Saunte, Jonathan M. Holmes, Sustained improvement of reading symptoms following botulinum toxin A injection for convergence insufficiency, Informa Healthcare, September 2014, Vol. 22, No. 3 , Pages 95-99 (downloaded online 11 August 2014) doi:10.3109/09273972.2014.907815
- Ripley L, Rowe FJ (July–September 2007). "Use of botulinum toxin in small-angle heterotropia and decompensating heterophoria: a review of the literature". Strabismus 15 (3). pp. 165–171. PMID 17763254.
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- de Alba Campomanes AG, Binenbaum G, Campomanes Eguiarte G (April 2010). "Comparison of botulinum toxin with surgery as primary treatment for infantile esotropia". J AAPOS 14 (2). pp. 111–116. doi:10.1016/j.jaapos.2009.12.162. PMID 20451851.
- Wutthiphan S. (2008). "Botulinum toxin A in surgically overcorrected and undercorrected strabismus". J. Med. Assoc. Thai. 91 (Suppl. 1). pp. 86–91. PMID 18672599.
- Nilza Minguini et al (March 2012). "Surgery with intraoperative botulinum toxin-A injection for the treatment of large-angle horizontal strabismus: a pilot study". Clinics (Sao Paulo) 67 (3). pp. 279–282. doi:10.6061/clinics/2012(03)13.
- Scott AB (1981). "Botulinum toxin injection of eye muscles to correct strabismus". Trans Am Ophthalmol Soc. (79). pp. 734–770.