Prism Adaptation Study

The Effect of Prism Adaptation on the Angle of Deviation in Convergence Excess Esotropia and Possible Consequences for Surgical Planning

Convergence excess esotropia describes a heterophoria with binocular single vision (BSV) on distance viewing that becomes esotropic on accommodation upon near fixation. Prism adaptation test (PAT) is a procedure routinely used to reveal the maximum angle of deviation preoperatively for many types of strabismus and has been shown to improve surgical outcomes; however, it is not conventionally used for convergence excess esotropia.

AIM

This study compares the angle of deviation at 1/3 m and 6 m before and after prism adaptation in subjects with convergence excess esotropia to determine if a masked distance angle of deviation can be demonstrated similar to the masked near angle of deviation in some intermittent exotropes. Surgical results are reported.

RESULTS

Fifty-eight children with convergence excess esotropia were prescribed prism adaptation prior to strabismus surgery and 49 met the inclusion criteria. A median increase in the angle of deviation of 20 prism dioptres (PD) was seen at both 1/3 m and 6 m following PAT. These changes were statistically significant (p < 0.001) at both distances. Following one surgical procedure, 83.6% were fully binocular postoperatively.

CONCLUSION

Prism adaptation frequently reveals an otherwise masked large distance angle of deviation in convergence excess esotropia. Convergence excess esotropia can be subdivided into two categories: true and simulated. Those with true convergence excess exhibit a manifest convergent strabismus when viewing a close object and a small, well-controlled latent strabismus upon fixation of a distant object. Those with simulated convergence excess have a comparable near deviation to those with true convergence excess but can be shown to have a distance deviation that approaches the size of the near strabismus once the normal fusional mechanisms are disrupted by a period of prism adaptation.

A retrospective evaluation of bilateral medial rectus recession for management of accommodative esotropia according to prism-adapted motor response preoperatively

The prism adaptation test (PAT) is a preoperative tool that may fine-tune surgical dosage and reduce under- and overcorrection in pediatric partially accommodative esotropia; however, it is resource intensive and the benefits are uncertain. PAT involves correction of esodeviation with prisms, with subsequent assessment for and quantification of change in angle of esodeviation, thereby augmenting the surgical target angle in a subset of patients. We evaluated PAT response and postoperative outcomes in a cohort of children who underwent bilateral medial rectus recession and found that 36% of patients showed a requirement for increase of prism dosage to retain orthotropia during PAT; these patients did better than those whose deviation was stable, with postoperative rate of motor success (defined as ≤10^Δ esotropia) of 100% versus 56%. PAT may be a useful positive prognostic test, and it also identifies a substantial patient population who may avoid undercorrection, the prism builders. However, this cohort may do better postoperatively regardless of the target angle for surgery. Additional randomized studies are required to demonstrate definitive benefit of PAT. Identification of the builder phenotype prior to commencing adaptation may reduce the workload involved in the PAT technique.

Preoperative prism adaptation for acquired esotropia: long-term results

PURPOSE

The efficacy of preoperative prism adaptation in subjects with acquired esotropia has been established at 6 and 12 months postoperatively. We evaluated the outcomes of subjects with acquired esotropia who had preoperative prism adaptation and were followed longer than 12 months.

METHODS

A retrospective analysis was undertaken of 2 groups of subjects with acquired esotropia who underwent bilateral medial rectus recessions based on the distance angle of deviation and were followed more than 12 months postoperatively. Group A subjects had preoperative prism adaptation to determine the target angle for corrective surgery. In Group B subjects, surgery was based on the maximum angle of strabismus at distance, determined by alternate prism cover test without preoperative prism adaptation.

RESULTS

We compared 17 subjects in Group A and 19 subjects in Group B. The postoperative follow-up period was 3 +/- 1.7 years in Group A and 4.8 +/- 1.8 years in Group B. The age at the last visit was 9.1 +/- 2 years in Group A and 10 +/- 2 years in Group B. Postoperative residual esotropia was 2.6 +/- 2.5 PD at distance in Group A patients and 6.6 +/- 5.9 PD in group B patients (P =.002). Residual esotropia at near was 3 +/- 3.8 PD in Group A and 11.5 +/- 8.12 PD in Group B (P <.01). More Group B subjects required bifocal spectacles to achieve optimal alignment at near (P =.001).

CONCLUSION

Acquired esotropia subjects operated on for their distance prism-adapted angle maintained better motor alignment over a long-term follow-up period when compared with nonprism-adapted subjects operated on for their distance angle.