PubMed · 5816144
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H Yamamoto. 1969. [Study of accommodative convergence].. https://pubmed.ncbi.nlm.nih.gov/5816144/
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BACKGROUND: Patients with exotropia often have a slow-to-dissipate fusional mechanism at near, which masks the true near deviation. Consequently, determination of the accommodation convergence-accommodation (AC/A) ratio in patients with exotropia must be based on near measurements obtained after prolonged monocular occlusion (typically 1 hour). When determined in that manner, the presence of a high AC/A ratio before surgery in an exotropic patient has been reported to be predictive of an esotropia at near after surgery. OBJECTIVE: To investigate the diagnosis and management of exotropia with a high AC/A ratio. METHODS: Three hundred four consecutive patients with exotropia were studied. In addition to the usual measurements, measurements were obtained at near after 1 hour of monocular occlusion, with and without additional +3.00-diopter lenses. Also, a gradient AC/A ratio was obtained by using additional minus lenses at distance fixation. RESULTS: One hundred fifty-four (50.7%) of 304 patients would have been thought to have a high AC/A ratio if that diagnosis was based on measurements obtained before prolonged monocular occlusion. In fact, only 22 patients (7.2%) actually had a high AC/A ratio; 132 patients (43.4%) had a pseudo-high AC/A ratio. Six of 22 patients with a high AC/A ratio underwent surgery to correct the exotropia. The presence of a high AC/A ratio before surgery had sensitivity, specificity, and positive and negative predictive values of 100% for predicting a postoperative esotropia at near associated with a high AC/A ratio. The remaining 16 patients with high AC/A ratios were treated with overcorrecting minus lens therapy (including a bifocal). Ten of them have been followed up to at least 18 years of age, by which time 9 have shown normalization of the AC/A ratio. CONCLUSIONS: Near measurements used to calculate the AC/A ratio in exotropic patients must be made after prolonged monocular occlusion. Otherwise, many patients with a pseudo-high AC/A ratio will be thought to have a true high AC/A ratio. The presence of a high AC/A ratio is infrequent in patients with esotropia, but it is highly predictive of a postoperative esotropia at near fixation.
Little is known about the developmental plasticity of the vergence and accommodative systems, an important issue since abnormalities can lead to visual problems, e.g. strabismus. One way of artificially altering the links between accommodation and vergence is to vary the stimulus to vergence while fixing the accommodative stimulus, as is found in virtual reality displays. While it would be of interest to study developmental plasticity in this situation, since many children are exposed to games machines which use this arrangement, no studies to date have tackled this issue. There is, however, some indication that long-term VR viewing in adults can lead to visual problems. It seems important to determine the safety of these systems for the developing human visual system before they come into common use. In this paper, adaptation of the accommodation and vergence systems and the effect of VR viewing in adults is discussed. The sparse literature on adaptation in children is then reviewed, and suggestions made for approaches that would enhance our knowledge of plasticity of accommodation and vergence in children.