Date Published: January 18, 2019
Publisher: Public Library of Science
Author(s): Anna Przekoracka-Krawczyk, Krzysztof Piotr Michalak, Paulina Pyżalska, Markus Lappe.
Vergence prism adaptability was evaluated in subjects with high symptomatic and asymptomatic heterophoria and compared to individuals with a heterophoria within normal range (the control group). A computer haploscope was used to measure phoria values and changes in the eye position after introducing 6 prism diopters base out in front of the right eye. Phoria values were measured with a nonius paradigm every minute for a period of 10 minutes during adaptation. The results showed that subjects with symptomatic heterophoria are characterized by a lower rate of prism adaptation and adapted to a smaller extent with respect to the control group. The group with high but asymptomatic heterophoria showed prolonged adaptation time but after several minutes of binocular viewing the subjects were able to adapt to the prism to a level similar to the control group. These findings suggest that an impairment in the slow vergence control system may be responsible for the inability to fully reduce vergence effort, which results in poor vergence ranges and asthenopic symptoms during prolonged viewing.
During natural viewing the two eyes are directed very precisely to a target of regard and the image of the target object is projected close to the center of the retina in each eye. In a nonstrabismic individual, the two images can be fused in the brain if they are of adequate quality. However, under specific clinical and laboratory test conditions the target may be presented to only one eye to prevent fusion so that an angular deviation of the non-stimulated eye may occur. This deviation is referred to as heterophoria or phoria . Even small amounts of phoria may lead to visual symptoms (asthenopia) unless they are compensated by the vergence system. Inversely, even significant phoria may remain asymptomatic if the individual level of compensatory skills is sufficient [2–4]. Thus, it seems that compensatory and adaptation processes are needed to improve functional efficiency and accuracy of the visual system. One of the adaptation processes related to heterophoria is vergence prism adaptation (VPA) also known as phoria adaptation. As a result of VPA induced phoria returns to baseline (tonic) value [5, 6]. This can be the case after several minutes of sustained viewing of a near object or when looking through a prism, which increases vergence effort. This shift in the amount of phoria is probably a consequence of a changed tonic position of the eyes  and is a useful mechanism to reduce near visual stress during prolonged viewing . This adaptation mechanism may be activated in response to disparity evoked using a stereoscope or prism spectacles as well as in response to accommodation through lenses [8–11].
First of all, statistical analysis showed that absolute phoria at distance and near for the CPh and the dCPh groups was significantly higher than for the NPh group (distance phoria: Z = -4.78, p < 0.001 for CPh and NPh, respectively and Z = -4.77, p < 0.001 for dCPh and NPh, respectively; near phoria: Z = -4.72, p < 0.001 for CPh and NPh, respectively and Z = -4.64, p < 0.001 for dCPh and NPh, respectively). However there were no significant differences in phoria values at distance or at near between the CPh and the dCPh groups (distance phoria: Z = -1.05, p = 0.295; near phoria: Z = -0.79, p = 0.426) (see Table 1). The present study focused on the investigation of VPA skills in symptomatic and asymptomatic individuals with heterophoria. We tried to answer the question whether impaired VPA may be related to asthenopic symptoms independent of the amount of phoria. Source: http://doi.org/10.1371/journal.pone.0211039