Further implications of probable changes in medial rectus muscle innervation after surgery for infantile esotropia

The conclusion that setting the eyes of an infantile esotrope (ET) straight with a successful bilateral medial rectus muscle reseccion (MROU) reduces the preoperative hyperinnervation of these muscles almost to normal may have special implications for the best principles of management of infantile ET not detailed in a recent paper by Mimms, III, Miller and Schoolfield.

The same 113 infantile esotropes who provided data for the exoshift under anesthesia study had previously provided data for a dose-response curve. A simple geometric calculation was done to determine the amount of medial rectus (MR) recession necessary to compensate for contracture, and this was substracted from the dose-response value to reveal the additional mm of recession required to the MR sufficiently down the length-tension curve to compensate for the preoperative MR hyperinnervation.

The result of these calculations yielded a surprisingly narrow range of averaging 3.6 mm (range 3.2 to 3.8 mm) for a broad range of preoperative deviations (20 ET to 80 ET).

The conclusions which may be drawn from this analysis include an understanding of why infantiles (and other) esotropias tend to increase in size in a few weeks or months if unteated, verification of the wisdom of the common practice of measuring the deviation no more than a day or two prior to the surgery, and the common observation that unilateral medial rectus recessions of a given amount will generally produce significantly less than half of the effect of a similarly-sized bilateral medial rectus recession. Finally, this understanding explains the poor success rate of surgery in the presence of dense amblyopia with eccentric fixation.