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November 1998

Intermittent Exotropia: Far Distance Target for Surgical Dosage—Reply

Arch Ophthalmol. 1998;116(11):1550-1551. doi:

In reply

Dr Ing raises 2 important and thoughtful questions, 1 implied and 1 stated. I appreciate the opportunity to reply to both. Implied is the question: Why was my overcorrection rate, which was slightly more than 2%, so much smaller than that reported by others?

An inherent problem in the application of data from clinical trials is generalization. Most studies are conducted on a strictly defined patient population, yet there is a natural tendency to apply the conclusions to other populations. Herein lies the answer to Dr Ing's first question. Because of the degree of cooperation that was needed for the measurements performed, my study was limited to patients older than 3 years. As stated in my conclusion "The results cannot be extrapolated with confidence from this study to patients who have these criteria that were excluded."1(p193) All 3 of the studies cited by Dr Ing included patients as young as 1 year, and all 3 of them found that the incidence of overcorrection after surgery for intermittent exotropia is inveresly related to age.24 Pratt-Johnson et al2 found a 23% incidence of the monofixation syndrome after surgery for intermittent exotropia in children younger than 4 years, but only 6% in children older than 4 years. Richard and Parks3 reported a 12% overcorrection rate for children younger than 3 years, 5% between ages 3 and 6 years, and 0% for children older than 6 years. Presumably if a child's vision is initially overcorrected after surgery for intermittent exotropia (as is desired), the ability for bifoveal fusion may be rapidly lost if the child is very young. I know that if my study included patients younger than 3 years, the overcorrection rate would have been closer to that in other reports. I also tend to vigorously intervene with prisms and echothiophate iodide if an initial overcorrection persist too long after surgery for exotropia. Perhaps this further reduced my rate of persistent overcorrection. Finally, patients with the monofixation syndrome and exotropia before surgery almost always persist in having the monofixation syndrome after surgery.5 Some of these patients have a microtropia preoperatively on which a larger latent or intermittently manifest deviation is superimposed. Four such patients were excluded from my study because of the stated criteria that the deviation had to be intermittent preoperatively and the presence of a manifest microtropia placed them outside the inclusion criteria. These patients would have also raised my overcorrection rate, as most of them had small angle esotropia and monofixation syndrome after surgery. One might argue such patients should not have been excluded. They were excluded because I wished to study a homogeneous population, and these patients seem different to me than intermittent exotropes with bifoveal motor fusion. The exclusion criteria were applied equally to both treatment groups and inclusion of these patients should not have influenced the conclusions.

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