In the absence of reduced visual acuity or amblyopia, there still remains considerable controversy of proper treatment course for X(T). Recent studies have shown that observation only is a valid course of action for these patients [24,25▪▪]. In a study of 327 children in the United Kingdom, 53% were observed without any treatment and the findings revealed that the majority of these children remained stable with no associated deterioration in their strabismus but also no clinically significant improvement .
The first large prospective randomized study attempting to determine the effectiveness of part-time patching for treatment of X(T) found that deterioration of the clinical picture over a 6-month period is uncommon with or without patching treatment [25▪▪]. The participants were randomized to patching were treated with patching one eye or patching the eyes alternately at the investigators’ discretion. No mention was made of the frequency of the deviation or whether one eye had more preference for fixation (amblyopia). Although there was a slightly lower deterioration rate with patching (0.6%) than with observation alone (6.1%), it was determined that both treatment options are reasonable for treating children 3–10 years of age with X(T). With the limited follow-up and the nondefinitive conclusion, the study offers some support for the efficacy of patching but the results are not significant enough for physicians to change their practice patterns. More long-term studies are needed to delineate if the study findings persist with time.
Overminus lens therapy induces accommodative convergence and is sometimes used as temporizing measure in X(T) to promote fusion and delay surgical correction. There have been few studies addressing the treatment of X(T) with overminus lenses. However, a more recent prospective study with 5-year follow-up showed 52% of 21 patients achieved a successful or good outcome with overminus lenses alone . The patients were initially treated with −1.00 to −3.00 lenses and after improvement in the angle of deviation and/or binocular function was noted the power of the lenses was reduced. The study found that in the short term (median 4 months), there was 100% improvement in the angle of deviation and binocular function. In the intermediate term (6–24 months), the lens power could not be reduced without recurrence of the X(T). Some cases required further treatment but others showed a smaller angle of deviation which was cosmetically acceptable with good binocular function. In the long-term (median 2.5 years), five patients achieved exophoria at near and distance fixation with good binocular function and this was maintained for at least 1 year after cessation of the treatment. The study showed no induced myopia from the minus lenses.
Overminus lens therapy can be considered as a primary treatment in X(T), especially in the short-term. They may be particularly useful in those patients with X(T) who have a high AC/A ratio or in patients who have a small-angle consecutive exotropia after strabismus surgery. Overminus lenses may cause asthenopia in older children.
The Convergence Insufficiency Treatment Trial produced sound scientific data to support orthoptic therapy for the treatment of convergence insufficiency . It has also has been shown to be effective in the treatment of convergence insufficiency-type X(T) with a maximum deviation of 25 prism diopters . In this prospective study of 30 patients, although the exodeviation remained essentially unchanged, orthoptic therapy achieved significant symptomatic relief and improved binocular function. Beyond 25 prism diopters, there was no significant improvement in symptoms. A more recent retrospective study of 74 patients utilized office and home-based therapy with prism exercises, pencil push-ups, 3D tests and dominant eye occlusion and showed improvement in 88% of patients with basic-type of X(T), 88.8% of patients with divergence excess-type X(T) and 100% of convergence insufficiency-type X(T) . Success was defined as relief of presenting symptoms associated with improved exodeviation or an exotropia less than 10 prism diopters.
Especially for convergence insufficiency-type and possibly other types of X(T), orthoptic therapy appears to be a viable treatment option. There are now computer-based orthoptic programs that are easily accessible and can be used at home by patients. However, it remains unclear as to which patients will respond to this therapy, what orthoptic exercises are most efficacious, how long and at what frequency does orthoptic therapy have to be performed for maximal efficacy and what is the long-term stability of these results. Therefore, most patients with X(T) are not offered this therapy at this time. Although orthoptic therapy has been used for the management of X(T) for over 50 years, there still remains a lack of large prospective, randomized scientific studies demonstrating its effectiveness and answering these questions.
Before Botox [Allergan, Inc. (acquired by Actavis March 2015, Parsippany, NJ, USA)] became a household word for cosmetic procedures, the initial research on botulinum toxin A was performed by Alan B. Scot MD for the treatment of strabismus . Many reasons why botulinum toxin has not commonly become the first-line therapy for the management of strabismus include the paucity of randomized controlled trials, the time required to realize the full effect of the injections, some uncertainty about dosing the medication, the expense of the toxin and practioners’ inexperience with using it. However, this treatment is rapid, less invasive and has the benefit of not altering the extraocular muscles which preserves future surgical options in the event of recurrence. Botulinum toxin has been and remains an effective therapy for the management of strabismus, including X(T), however, long-term studies are needed to assess the stability of its results with time.
There remains no clearly defined threshold for patients when considering surgical intervention in X(T). Commonly considered are increasing angle of exodeviation, worsening control or frequency of the exotropia, inability to fuse and maintain stereovision and the presence and severity of symptoms. Recent work has attempted to further elucidate the decision to perform strabismus surgery for children with X(T). After accounting for poor control of the exodeviation at distance, a retrospective study of 106 children reported that children with X(T) whose accompanying parent reported poorer health-related quality of life (HRQOL) were more likely to undergo surgery than those who reported better HRQOL [34▪]. Both the parents’ concern about the child's ability to function and their psychosocial concern about their child were associated with the decision to perform surgery. Of the clinical measures, the distance control, near control and near angle of deviation were found to be associated with the decision for surgery. Another study recognized that there is a high variability of control in X(T) throughout the day and even within minutes and that, if using control of the strabismus as the clinical decision to perform surgery, several assessments of are necessary to provide a representative measure .
There has been an on-going debate in the literature as to what surgical procedure constitutes the best treatment for X(T). Traditionally, bilateral lateral rectus recession (BLRecess), unilateral lateral rectus recession (ULRecess), medial rectus resection (MResect) and unilateral lateral rectus recession combined with medial rectus resection (R&R) have been used as the standards of surgical treatment. Even with comparisons of the larger cohort studies the conclusions of which procedure is most efficacious remain weak. Within the last several years, this debate has continued with many recent studies targeting outcomes of specific types of X(T). In 85 children with basic-type X(T), it was found that R&R was more effective than BLRecess resulting in better surgical alignment and less undercorrections . In a retrospective study of 128 patients, surgical outcomes after 2 years were assessed in basic-type X(T) and found to be similar with R&R and BLRecess . However, this study showed the final outcomes were better in the BLRecess group than the R&R group possibly due to the lower recurrence rate of exotropia in the BLRecess group.
In a retrospective study of 180 children with moderate angle X(T) of 20–25 prism diopters, ULRecess was compared with R&R and both procedures were found to be similar in the surgical outcomes [38▪]. After more than 2-years follow-up, surgical success was achieved in only 60.9% ULRecess patients and 56.1% R&R patients which the authors found disappointing. They reported that direct comparison of their success rates to other similar studies was not possible due to the lack of similarity between the studies including different lengths of follow-up and criteria for surgical success. This was echoed in a literature review of surgical outcomes over the last 10 years finding lack of harmony in outcome reporting was counterproductive and the authors suggested four core features for reporting for future studies: alignment, near stereoacuity, control score and quality-of-life measure [39▪].
Another study on surgical treatment showed that an improved R&R procedure resulted in better surgical success for convergence insufficiency-type X(T) [40▪]. This improved R&R procedure used an MResect based on the near deviation with the LRecess based on the distance exodeviation. This procedure was then compared with a unilateral MResect as well as a bilateral MResect and found that, although all surgical procedures reduced the near-distance differences, the improved R&R procedure had better ocular alignment at 6-months follow-up.
Surgical management of X(T) has been known to result in a high rate of undercorrections and overcorrections. With a reported incidence of 1.5–27%, consecutive esotropia may result in diplopia, reduction in stereovision and development of amblyopia [41▪▪]. Consecutive esotropia has recently been shown to occur in higher rates in patients with the following predisposing factors: divergence excess-type X(T), BLRecess, amblyopia, younger age at diagnosis and surgery, shorter duration from onset to surgery and overcorrection of at least 20 prism diopters at postoperative day 1 [42▪▪]. With patients with a long-term surgical follow-up of 10 years it was found that an increased risk of poor outcomes requiring reoperations occurred with patients with anisometropia, lateral incomitance and immediate postoperative undercorrections . However, in a study of 150 consecutive patients, it was found that surgery had the highest success rate for treatment of X(T) when it was combined with preoperative orthoptic therapy/occlusion therapy in comparison with other treatments for X(T) including observation, orthoptic/occlusion therapy alone and surgery alone . It may be the combination of orthoptic therapy with surgery that better addresses the neural-ocular communication needed to maintain ocular alignment than surgery alone.
Papers of particular interest, published within the annual period of review, have been highlighted as:
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This article factored in parental concerns, independent of patient's clinical severity, in the decision to undergoing strabismus surgery.
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