The exoshift under anesthesia correlates with probable changes in medial rectus innervation after surgery for...

To study the outward deviation of the eyes (exoshift) under anesthesia, in a variety of clinical settings in order to improve our understanding of how medial rectus recessions change alignment and innervation.

Pre-operative and intraoperative eye deviations were measured before surgery and under Stage 3 of general anesthesia using a modified Krimsky test in 5 groups: 1) Unoperated infantile esotropia (N=60); 2) Undercorrected infantile esotropia (N=27); 3) Corrected infantile esotropia with subsequent vertical deviations (N=17); 4) Superior oblique palsies without horizontal deviations (N=21); and 5) Late consecutive exotropia (N=16).

Group 2 averaged half the esotropic deviation of Group 1 (19.8 +/-7.4 ET vs. 42.1 +/-18.3 ET), but had a nearly identical exoshift (41.9 +/-13.2 vs. 41.8 +/-13.6, p=0.96). Group 4 (orthotropic) and Group 5 (exotropic) demonstrated smaller and nearly identical exoshifts (26.0 +/-8.3 vs.24.0 +/-9.3, p=0.50). Group 3 had significantly less exotropia (30.1 +/-6.0) than Group 1 and 2 (p=0.002 for both), but significantly more exoshift than Group 4 (p=0.04) or Group 5 (p=0.067).

Contracture of the lateral rectus reduced the deviations after undercorrecting surgery, but the exoshift remained unchanged. Medial rectus recession by itself has no effect on normal level seen in consecutive exotropia. These data combined with a reasonable set of assumptions regarding the state of contracture (expansure) of the horizontal rectus in a variety of pre- and post-operative settings lead to the conclusion that setting the eyes straight with a successful bilateral medical rectus recession reduces the pre-operative hyperinnervation of medial rectus almost to normal, with a small amount of residual hyperinnervation remaining to overcome the increased contacture of the lateral rectus which occurs due to increased exotropia during sleep after a bilateral medial rectus recession. When this small residual hyperinnervation decreases to normal, consecutive exotropia develops, owing to continued increased contracture of the lateral recti.