| Evidence that convergence rather than accommodation controls intermittent distance exotropia. | |
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MedLine Citation:
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PMID: 22280437 Owner: NLM Status: Publisher |
Abstract/OtherAbstract:
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Purpose: This study considered whether vergence drives accommodation or accommodation drives vergence during the control of distance exotropia for near fixation. High accommodative convergence to accommodation (AC/A) ratios are often used to explain this control, but the role of convergence to drive accommodation (the CA/C relationship) is rarely considered. Atypical CA/C characteristics could equally, or better, explain common clinical findings. Methods: Nineteen distance exotropes, aged 4-11 years, were compared while controlling their deviation with 27 non-exotropic controls aged 5-9 years. Simultaneous vergence and accommodation responses were measured to a range of targets incorporating different combinations of blur, disparity and looming cues at four fixation distances between 2 m and 33 cm. Stimulus and response AC/A and CA/C ratios were calculated. Results: Accommodation responses for near targets (p = 0.017) and response gains (p = 0.026) were greater in the exotropes than in the controls. Despite higher clinical stimulus AC/A ratios, the distance exotropes showed lower laboratory response AC/A ratios (p = 0.02), but significantly higher CA/C ratios (p = 0.02). All the exotropes, whether the angle changed most with lenses ('controlled by accommodation') or on occlusion ('controlled by fusion'), used binocular disparity not blur as their main cue to target distance. Conclusions: Increased vergence demand to control intermittent distance exotropia for near also drives significantly more accommodation. Minus lens therapy is more likely to act by correcting overaccommodation driven by controlling convergence, rather than by inducing blur-driven vergence. The use of convergence as a major drive to accommodation explains many clinical characteristics of distance exotropia, including apparently high near stimulus AC/A ratios. |
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Authors:
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Anna M Horwood; Patricia M Riddell |
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Publication Detail:
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Type: JOURNAL ARTICLE Date: 2012-1-26 |
Journal Detail:
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Title: Acta ophthalmologica Volume: - ISSN: 1755-3768 ISO Abbreviation: - Publication Date: 2012 Jan |
Date Detail:
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Created Date: 2012-1-27 Completed Date: - Revised Date: - |
Medline Journal Info:
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Nlm Unique ID: 101468102 Medline TA: Acta Ophthalmol Country: - |
Other Details:
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Languages: ENG Pagination: - Citation Subset: - |
Copyright Information:
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© 2012 The Authors. Acta Ophthalmologica © 2012 Acta Ophthalmologica Scandinavica Foundation. |
Affiliation:
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Infant Vision Laboratory, School of Psychology & Clinical Language Sciences, University of Reading, Reading, UK Orthoptic Department, Royal Berkshire Hospital, Reading, UK. |
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From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine
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