[Influence of prism overcorrection on residual postoperative strabismus]

Prism overcorrection is a complementary procedure to reduce the residual postoperative esotropia caused by anomalous retinal correspondence (ARC) after surgery for esotropia. We have investigated the results of this treatment.

For Group 1, the files of 63 patients treated with postoperative prism overcorrection in the Department of Ophthalmology, University of Giessen, were evaluated regarding the pre-and postoperative squint angles (SPCT, simultaneous and APCT, alternate prism and cover test) and the angle of ARC, determined by the red filter test and by the increase of esotropia after neutralisation of the squint angle (APCT). Postoperatively, squint angles had been measured immediately (APCT) after removal of the eye patch and 10 minutes to 2 hours later (SPCT and APCT). Subsequently, prism overcorrection was performed with a Fresnel prism foil (40 PD basis temporally) in front of the fellow eye for a maximum of 3 months. After 3 months, SPCT and APCT were performed. For Group 2, the files of 28 patients with a preoperative angle of ARC of 5 degrees or more, treated in the Department of Ophthalmology, University of Heidelberg, were evaluated. These patients had not been treated with prism overcorrection. The squint angles had been measured (SPCT and APCT) on the preoperative day, on the first postoperative day, a few hours after removing the eye patch, and after three months.

For Group 1, at surgery, the patients were 4 to 12 years old (median: 6.2 years). The preoperative squint angle ranged from + 5 degrees to + 27 degrees (median: + 12 degrees) in the SPCT and from + 7 degrees to + 27 degrees (median: + 14 degrees) in the APCT. The angle of ARC was between + 4 degrees and + 15 degrees (median: + 7 degrees). Both combined recess and resect surgery (with or without additional oblique muscle surgery) or bilateral retroequatorial medial rectus myopexy, in part depending on the squint angle pattern with medial rectus recession, were performed. Immediately after removing the patch, the squint angle (APCT) was between - 10 degrees and + 5 degrees (median: + 1 degrees). Ten minutes to 2 hours later, the manifest squint angle ranged from 0 to + 12 degrees (median: + 7 degrees). The angle had decreased significantly to - 6 to + 12 degrees (median: + 5 degrees) after 3 months. For Group 2, the patients' ages were between 5 and 12 years (median: 6.5 years). The squint angles ranged from + 11.5 degrees to + 35 degrees (median: + 20 degrees) in the SPCT and APCT. The angle of ARC was between + 5 degrees and + 17 degrees (median: + 8 degrees). Combined recess and resect surgery or bilateral recession of the medial rectus (with or without oblique muscle surgery both) were performed. The SPCT several hours after removal of the patch showed angles of - 4 degrees to + 14 degrees (median: + 4.25 degrees). Three months later the manifest squint angles ranged from - 5 degrees to + 14 degrees (median: + 3 degrees).

After prism overcorrection (Group 1), there was a significant reduction of the residual esotropia. Without this additional treatment (Group 2), there was no significant change in the postoperative squint angle. However, neither the positive outcome in Group 1 nor the difference to Group 2 do unequivocally prove that there is a beneficial effect of prism overcorrection, since preoperative conditions were different and the sample size in Group 2 was small, especially after matching for equal preoperative conditions. A spontaneous reduction of the postoperative esotropia cannot be excluded. Further studies are necessary in order to investigate the specific effect of prism overcorrection.