10.1 Goals

Botulinum toxin can be used as a treatment for strabismus to improve eye alignment and promote binocularity.

10.2 Advantages

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  Less invasive procedure which preserves the extraocular muscles for later surgery if needed and for patients at risk of developing anterior segment ischemia.

  
  
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  Shorter procedure time, and as with incisional strabismus surgery, botulinum toxin injection in children requires general anesthesia but of significantly shorter duration. ¹

  
  
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  The weakened muscle is not disinserted, reducing the risk of anterior segment ischemia.

  
  
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  Can be used as a surgical treatment option for strabismus that is significantly variable or difficult to measure, with the understanding that incisional strabismus surgery may eventually be needed. The strabismus can thus be surgically managed earlier without the need for consistent measurements, as opposed to repeating the preoperative measurements multiple times and delaying surgery. Simultaneously, the potential for fusion can be assessed postoperatively.

  
  
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  For some patients, the need for incisional surgery may be eliminated if strabismus does not recur.

  
  
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  Lower risk of serious complications and possibly a lower risk of long-term overcorrections. ²

10.3 Expectations

In the early postoperative period after botulinum toxin injection, overcorrection of the deviation and ptosis often occur, ² which should be discussed in detail with the patient’s family preoperatively, so that the family may anticipate these findings and be less alarmed if they occur. The ptosis can be expected to resolve as the strabismus improves.

10.4 Key Principles

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  The mechanism of action of the toxin is at the neuromuscular junction where it blocks release of acetylcholine presynaptically. Paralysis of the muscle begins within 2 to 4 days and lasts for 8 to 12 weeks.

  
  
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  While the effect of the toxin at the neuromuscular junction is transient, the injected muscle and its antagonist appear to undergo remodeling. Weakening of the injected muscle allows mild contracture of the antagonist muscle and, in turn, lengthening of the injected muscle, which persist even after the toxin is no longer in effect.

  
  
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  In addition, taking advantage of the central drive for fusion, treatment with botulinum toxin to improve the eye alignment during the period of visual development in early childhood may promote the likelihood of a stable, long-term outcome.

  
  
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  The use of botulinum toxin is more dependent on the potential for fusion than incisional strabismus surgery to improve and stabilize the eye alignment.

  
  
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  The greater the volume of toxin that is injected, the higher the risk of adverse effects, specifically ptosis and spreading to other extraocular muscles, although they are transient. Larger volumes injected directly into the muscle may also result in more discomfort for patients.

10.5 Indications

Botulinum toxin has been used as a treatment for strabismus since the 1970s ³ and was approved by the Food and Drug Administration (FDA) in 1989 for the treatment of strabismus. ⁴ Botulinum toxin injection has been primarily reported as a treatment for various types of esotropia:

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  Moderate-angle esotropia of 20 to 35 prism diopters (PD) with potential for fusion, including infantile esotropia, ⁵ acute comitant esotropia, ¹ and cyclic esotropia. ⁶

  
  
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  Augmentation of medial rectus recessions in large angle esotropia greater than 50 to 60 PD for which recessions greater than 6.0 to 6.5 mm of the medial rectus muscles may increase the risk of overcorrection in the future.

  
  
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  In an acute abducens nerve palsy, weakening of the antagonist medial rectus muscle to reduce contracture of the medial rectus and to aid in the treatment of amblyopia in children while monitoring for recovery of the abducens nerve. ⁴

  
  
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  Weakening of the ipsilateral medial rectus muscle during a transposition procedure for an abducens nerve palsy or esotropic Duane syndrome to preserve the medial rectus muscle and decrease the risk of anterior segment ischemia. ⁷

Other reported types of strabismus and scenarios in which botulinum toxin injection may be beneficial include:

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  Residual strabismus or overcorrections, especially if intermittent, after prior surgery. ⁸

  
  
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  Partially accommodative esotropia ⁹ and esotropia in patients with cerebral palsy. ¹⁰

  
  
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  Patients with variable measurements or who are difficult to measure reliably may also be considered as candidates for treatment with botulinum toxin injection.

10.6 Contraindications

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  Botulinum toxin injection is less effective as a sole treatment in patients with large deviations greater than 40 PD and chronic paralytic strabismus.

  
  
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  Also appears to be less effective for exodeviations. ⁵

  
  
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  Known poor potential for fusion may preclude a satisfactory response to botulinum toxin treatment for strabismus.

10.7 Preoperative Preparation

A complete sensorimotor examination is performed as described in Chapter 1.7 Preoperative Preparation. Currently, there are no standardized guidelines for determining the number of units to inject based on the angle of the deviation or the type of strabismus. The author uses the following general guidelines using onabotulinum toxin A:

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  For each muscle, 5 units for 25 to 30 PD and 7.5 units for 35 to 40 PD.

  
  
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  Smaller doses of 2.5 units and up to 10 units may be used for smaller and larger deviations respectively.

  
  
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  For augmenting a medial rectus recession for patients with large angle esotropia (≥60 PD), 5 units are injected into each medial rectus muscle.

  
  
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  For a complete abducens nerve palsy, 5 to 7.5 units may be injected into the ipsilateral medial rectus muscle. For a partial abducens nerve palsy, 2.5 to 3 units may be injected.