13th Bielschowsky Lecture
Accommodation and Convergence
-Ratios, Linkages, Styles, and Mental Somersaults
Anna Horwood, PhD, DBOT
Professor of Orthoptics and Visual Development
University of Reading, UK
Understanding the linkages between accommodation and convergence is fundamental to understanding concomitant strabismus, heterophoria and convergence and accommodation anomalies. The lecture will present an alternative conceptual framework around these two systems which fits clinical characteristics and responses to treatment as well, or better than, current models. The framework is based on the different weights the visual system places on the main cues to target position in depth across development and between patient groups.
Strabismologists, developmentalists and all healthcare professionals managing patients with concomitant strabismus
Accommodative convergence has traditionally been considered the major driver to the motor responses involved in near fixation; existing in a fixed, inflexible relationship expressed as the AC/A ratio. This viewpoint, however, only fits a small number of clinical diagnoses and fails to explain many others.
Our research suggests that the majority of non-strabismic people, and patients with intermittent strabismus, use binocular disparity as their primary visual cue, with blur and proximal/looming cues having less weight. The convergence to accommodation (CA/C) linkage is therefore more important than the AC/A relationship. Between-diagnosis differences in the relative balance between AC/A and CA/C relationships can explain many clinical findings.
Increased awareness of accommodation / convergence linkages, their strengths, their development and variability, which can be used to explain clinical findings and predict responses to common treatments.
Instead of thinking “accommodation drives convergence” , or even “convergence drives accommodation, we should instead think of the visual and non-visual cues which drive both.
- Horwood A, Riddell P. Can misalignments in typical infants be used as a model for infantile esotropia? Invest Ophthalmol Vis Sci. 2004;45(2):714-20.
- Horwood A, Riddell P. The use of cues to convergence and accommodation in naïve, uninstructed participants. Vision Research. 2008;48(15):1613-24.
- Horwood A, Riddell P. Hypo-accommodation responses in hypermetropic infants and children. BJOphth 2011;95( 2):231-7.
- Horwood A, Riddell P. Evidence that convergence rather than accommodation controls intermittent distance exotropia. Acta Ophthalmol Scand. 2012
- Horwood A, Riddell P. Developmental changes in the balance of disparity, blur and looming/proximity cues to drive ocular alignment and focus. Perception. 2013;42:693-715.