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The accommodative-convergence complex — A review
Published in Jan-Tjeerd de Faber, 28th European Strabismological Association Meeting, 2020
R.L. Brautaset, J.A.M. Jennings
Control system models have been used to aid the understanding and analysis of the human accommodation and vergence systems. The accommodation and vergence systems have been modelled as a dual interactive negative feedback model in which both accommodation and vergence are negative feedback systems (Fig. 1). The accommodation controller is triggered by blur and the level of accommodation altered in order to eliminate blur. The vergence system strives to eliminate disparity. In both systems, the forward controllers are leaky integrators. This means that the input and output will never be perfectly balanced and that a steady state error exists. Fixation disparity is the steady state error of the vergence system, whereas the lag/lead of accommodation is the steady state error of the accommodation system. Together, the steady state errors provide continuous stimulation to assist in maintaining a given accommodation and vergence level.
Predictor factors of prism effectiveness in young adults with convergence insufficiency
Published in Clinical and Experimental Optometry, 2021
Hassan Hashemi, Ebrahim jafarzadehpur, Payam Nabovati, Abbasali Yekta, Mohamadreza Aghamirsalim, Mohammad Kamali, Ali Mirzajani, Mehdi Khabazkhoob
In the present study, we used the Sheard’s criterion for prism prescription. As mentioned earlier, there are several methods for prescribing horizontal relieving prism in the literature, each with its own advantages and disadvantages. Concerning the Sheard’s criterion, its benefit has been reported in both esophoric and exophoric patients.5,17 The use of this method has been especially effective in cases of symptomatic presbyopic convergence insufficiency.16,17 The Percival’s criterion is less commonly used in clinical practice, although its effectiveness has been proven in some esophoric cases.17 A major limitation of these methods is that they are based on dissociated and less natural conditions. It is also possible to overestimate the amount of prism using these methods. Fixation disparity analysis is currently considered as the method of choice for prism prescribing in binocular disorders.20,21 One of the advantages of fixation disparity analysis is its associated nature.21 The amount of prism obtained from the fixation disparity analysis is usually less and more tolerable for the patient because it covers the non-compensatory part of the heterophoria.22 It is also possible to evaluate vergence adaptation by plotting the fixation disparity curve.23 One limitation of fixation disparity analysis is that drawing a curve is a time-consuming process and requires special equipment that may not be available in a primary care office.
Stereopsis and ocular alignment in Parkinson's disease patients with and without freezing of gait symptoms
Published in Clinical and Experimental Optometry, 2020
Mosaad Alhassan, Jeffery K Hovis, Quincy J Almeida
Vergence eye movements are affected in PD patients.2015 One way to evaluate the vergence system is to measure the fixation disparity curve. Fixation disparity is the small ocular misalignment of one eye, or both eyes when the two eyes are fixating on an object during normal binocular vision. The two images in the case offixation disparity do not stimulate corresponding retinal points, but they do fall within Panum's fusional area and so a single image is perceived.1949 The fixation disparity curve measures vergence adaptation. Moreover, it is important to look at how the fixation disparity data relates to stereopsis dysfunction in PD patients because a vergence eye movement deficit could contribute to reduced stereopsis.
Evaluation of the presence of a central fusion lock effect on fixation disparity curve parameters in symptomatic and asymptomatic subjects
Published in Clinical and Experimental Optometry, 2021
Maryam Mirzaeian, Hassan Hashemi, Foroozan Narooie-Noori, Payam Nabovati, Abbasali Yekta, Sara Sardari, Hadi Ostadimoghaddam, Asgar Doostdar, Mehdi Khabazkhoob
Fixation disparity (FD) is a condition in which the images of a binocularly fixated object are not imaged on exactly corresponding retinal points, but are still within Panum’s fusional areas. Therefore, there is central sensory fusion despite misalignment of the visual axes.1 Clinical tests of FD are important in detecting binocular vision problems,1,2 and planning treatment interventions using lenses, prisms, and vision therapy.3 Moreover, some studies have found that subjective measurements of FD are useful for diagnosis and treatment of decompensated heterophoria.4,5