Botulinum Toxin Augmented Surgery vs Conventional Surgery in the Management of Large Angle Horizontal Deviations

Strabismus is defined as inability of the two eyes to align their visual axes to a common object of regard. Horizontal strabismus may be in the form of esotropia or exotropia depending on whether the visual axes and the eyes are converging or diverging respectively.

There are multiple surgical techniques used to treat horizontal strabismus with the main goal being to align the visual axes so that binocular vision may develop. The most common initial treatment is either bilateral rectus muscle recessions or unilateral recession resection surgery. For large angle deviations (>50 prism diopters [PD]), surgery on 2 muscles alone may not be adequate to correct the strabismus with high reoperation rates. Approaches to these patients have included large bilateral muscle recessions, supramaximal unilateral recession resection procedure, three or four horizontal muscle surgery, or botulinum toxin augmented surgery.

Botulinum toxin exerts its effect by interfering with release of acetylcholine at the neuromuscular junction, thus inhibiting muscle contraction. Although the pharmacologic effect of botulinum toxin is temporary, permanent results in patients with strabismus may occur. These effects include a change in the length-tension curves of the muscles and permanent changes in sarcomere density and myosin heavy chain composition.

Botulinum toxin alone has been advocated by McNeer and associates as a primary treatment for infantile esotropia in children with smaller angles of deviation. The advantage of this approach is that incisional surgery is not required. However, it was found that botulinum alone was less effective in establishing binocularity when compared to standard surgery. This is likely because multiple injections are often needed, which prolongs the duration of misalignment. On the other hand, the concomitant use of botulinum toxin and surgery allowed for rapid establishment of alignment. Other advantages of botulinum augmentation are that only 2 muscles are operated upon, thus decreasing anesthesia duration and the risk of surgical complications, and the lateral rectus muscles are preserved for additional surgery, if needed.

Botulinum toxin augmentation of strabismus surgery was reported in several previous studies. Owens et al. reported the successful intraoperative use of botulinum toxin to augment monocular recession-resection surgeries in three large-angle exotropia patients. Two of these patients had orthotropia ±10 PD at their final examinations, while the third patient demonstrated a stable 18 PD exotropia which was cosmetically satisfactory. Khan reported 8 patients with > 60 PD esotropia treated with botulinum-augmented horizontal muscle surgery. Six of 8 had deviations of 10 PD or less following surgery, with an average follow-up of 9 months. Özkan et al. used botulinum augmentation in a group of older patients (age range 5-50 years) with large-angle esotropia of different etiologies, with a success rate of 57% and an average follow-up of 14 months. Minguini et al. compared the results of surgery alone versus surgery plus treatment with botulinum toxin for treatment of 23 patient with large angle strabismus. Lueder et al. evaluated the long-term outcomes in patients with infantile esotropia who had been followed for more than 2 years after treatment. The results were good, with a 74% success rate. This study demonstrated that this procedure is an effective treatment for infants with large-angle infantile esotropia, with stable results over time2. Based on historical comparisons, it appears equally as effective as 3- or 4-muscle surgery and more effective than large bilateral medial rectus recessions alone.

Ideally, a prospective randomized study should be performed to more definitively determine the effectiveness of the addition of intraoperative botulinum toxin compared to surgery alone.