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The many enigmas of intermittent exotropia
  1. Creig S Hoyt,
  2. Alexei Pesic
  1. University of California, San Francisco, California, USA
  1. Correspondence to Professor Creig S Hoyt, University of California, San Francisco, CA 94143, USA; creighoyt{at}gmail.com

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The Squint Club, a group of ophthalmologists with strabismus expertise, meets once a year to discuss difficult and unique clinical cases, new ideas about treatment and unique notions of pathophysiology regarding various disorders of ocular motor alignment. It was therefore surprising and noteworthy when several years ago at this meeting the late Arthur Rosenbaum presented a paper in which he stated that he thought the most perplexing and difficult form of strabismus in his practice was intermittent exotropia. Many in the audience were surprised by this statement. An unstated concern was the thought that intermittent exotropia might be too simple a condition for this august body to discuss. After all, intermittent exotropia appears to be an uncomplicated form of strabismus that ought to be easily treated with an expectation of consistent and excellent outcomes since it is characterised by:

  1. Occurrence in developmentally and neurologically normal children (unlike the case of infantile exotropia).1

  2. Initial normal binocular development at both distance and near fixation.

  3. A relatively low incidence of amblyopia which when it does occur is mild.2

  4. Occurrence of dissociated strabismus is less common than in infantile forms of strabismus.3

  5. Surgery appears to be the treatment of choice for all but the smallest deviations.4

Rosenbaum, however, convincingly made his point by emphasising that there was much that remained enigmatic about intermittent exotropia. These enigmas are not merely of academic interest, he argued, but central to the problems of managing children with intermittent exotropia. He highlighted a number of his concerns regarding intermittent exotropia:

  1. The natural history of untreated intermittent exotropia remains incompletely defined.5

  2. Even the widely accepted notion that in most cases it deteriorates over time has been challenged.6

  3. Precise clinical indications for when in the course of the disorder strabismus surgery should be performed have yet to be determined.7

  4. Long-term outcomes of surgically treated patients are disappointing with an unacceptably high rate of persistent or recurrent exodeviations and consecutive esodeviations.8

  5. The factors accounting for poor surgical outcomes are poorly understood.9

In the last several years, numerous investigators have attempted to address some of the concerns raised by Rosenbaum about the management of intermittent exotropia. In this issue of the journal, Buck and coworkers10 in an excellent study describe the clinical outcomes of an observational study. A total of 460 children under the age of 12 years with intermittent exotropia were recruited from 26 centres in the UK. These patients had been diagnosed within 1 year of the study period and were managed by their ophthalmologists in their preferred fashion. No standard treatment protocols were established by the authors of the study. The report details the clinical outcomes of 87 children who underwent surgery during the study period (approximately 18 months). It seems noteworthy that a little less than a fifth of the patients underwent surgery within 2.5 years of diagnosis. In eight centres none of their study patients underwent surgery. This would seem to highlight the lack of consensus among paediatric eye specialists as to the indications for surgery for intermittent exotropia. Another important and controversial issue in this study group is that mean age of the children undergoing surgery was 4.8 years. This is considerably younger than other recent studies.8 It is also surprising in light of the fact that even Parks and his coworkers have advocated avoiding early surgery in intermittent exotropia for fear of creating a consecutive esotropia with suppression and amblyopia.11

While the short-term outcomes in this study are reasonably good, as the authors clearly state, they remain less than ideal for a group of children who once had normal binocular reflexes and stereoacuity at distance and near fixation. On motor criteria alone, 36% had excellent alignment, 28% had fair but 36% had poor alignment (XT>15 or ET>4). Sensory results were similar with worsening or loss of near stereoacuity in 37%. These results are not surprising to experienced strabismus surgeons but re-emphasise our incomplete understanding of intermittent exotropia and how to treat it.

What tonic force or forces continue to drive the eyes of many of these patients exotropic even after apparent successful surgery and in the presence of normal visual acuity and binocular reflexes? How can we better neutralise these forces? Why do some patients develop a persistent esotropia despite ‘appropriate’ surgical dosage? The authors are forthright, clear and disarming: they simply do not know. In comparing the group that was considered to have an excellent outcome to that with a poor outcome, there were no preoperative or operative characteristics that separated the two groups: gender, age at surgery, duration of exotropia, near angle of deviation, distance angle of deviation, near stereoacuity, prior non-surgical treatment, type of surgery or surgical dose. It should be noted that distance stereoacuity was not measured in this study as it was in the study by Pineles and colleagues.8 Can it really be true that careful calculation of surgical dosage is not a predictor of alignment outcome? Is there really no difference between bilateral and unilateral surgery in determining the postsurgical alignment of patients with intermittent exotropia? There are obviously strong advocates for one or the other of these surgical approaches.12 ,13 Will more long-term follow-up of this study group negate the conclusion that there are no clinical or surgical factors that are predictive of better alignment outcomes? For example, Choi and coworkers found no difference in the 2-year outcomes of patients with intermittent exotropia treated with bilateral rectus recession versus those treated with unilateral recession–resection procedures.14 With further follow-up, however, ocular motor outcomes were better in the group who underwent bilateral recessions. The authors attributed this to the higher exotropia recurrence rate in the recession–resection group.

Of special note is observation by Buck and coworkers that initial surgical overcorrection with a resulting esodeviation was not a predictor of a better alignment outcome. For many years, it was widely accepted that a small consecutive esotropia in the immediate postoperative period was the ideal outcome. The thinking was that the resulting diplopia would be the strongest possible stimulus for fusion. This notion has been called into question recently by other authors.9 ,15 Perhaps, the instability of the alignment in the immediate postoperative period (coupled with the difficulty in measuring it a child who is irritated as the result of recently undergoing eye surgery) accounts for the disagreement on this issue.16 Nevertheless, the question needs to be answered: what is the ideal target alignment for the immediate postoperative period?

The authors have left us with more questions than they have answered. The peculiar nature of intermittent exotropia continues to perplex us, but we must attempt to understand it better in order to provide better outcomes for our patients. The authors leave us with several challenging issues:

  1. Can we provide consistently better outcomes with surgical interventions than the spontaneous improvement rate in intermittent exotropia? This question cannot be properly answered without a large, long-term, prospective natural history study of intermittent exotropia. This will not be easy to complete and there will be issues with parental acceptance. Will a significant number of parents be willing to withhold surgical therapy even if the exodeviation increases and the binocular function decreases? Nevertheless, this type of study must be completed.

  2. Interventional studies need to be performed in a randomised controlled fashion. The Pediatric Eye Disease Investigator Group is currently recruiting patients for a study that will evaluate alignment outcomes in patients with intermittent exotropia treated by either bilateral lateral rectus recession versus unilateral recess/resect procedures. We look forward to seeing their results. Yet, other comparable studies will be required to further define the many questions surrounding intervention in this condition.

  3. Parental expectations frequently do not correlate with the outcome data produced by Buck and coworkers and other active investigators studying intermittent exotropia.17 Are we being completely honest in our preoperative counselling provided to families with a child with intermittent exotropia? While confessing the breadth of our ignorance may serve no one's best interest we should be quite clear about the reasonable expectations for surgery and the possibility of complications, especially overcorrection of the alignment.

Buck and coworkers are to be commended for completing such a large, careful, observational study. We hope that detailed follow-up of the study group will be provided in subsequent publications. We fully endorse their call for careful, randomised, prospective trials to evaluate the many questions surrounding the natural history and treatment of intermittent exotropia.

References

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Footnotes

  • Contributors CH is the main author of this editorial. AP researched the material and helped rewrite it.

  • Competing interests None.

  • Provenance and peer review Commissioned; internally peer reviewed.

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