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Positive relative accommodation (PRA) in biology, is a measure of the maximum ability to stimulate eye accommodation while maintaining clear, single binocular vision. [1] This measurement is typically obtained by an orthoptist , ophthalmologist or optometrist during an eye examination using a phoropter .
However, errors in this relationship can cause problems, such as hyperopic individuals having a tendency for crossed eyes because of the over exertion of their accommodation system. Clinically, accommodative convergence is measured as a ratio of convergence, measured in prism diopters, to accommodation, measured in diopters of near demand.
Accommodation usually acts like a reflex, including part of the accommodation-convergence reflex, but it can also be consciously controlled. The main ways animals may change focus are: Changing the shape of the lens. Changing the position of the lens relative to the retina. Changing the axial length of the eyeball. Changing the shape of the cornea.
The symptoms and signs associated with convergence insufficiency are related to prolonged, visually demanding, near-centered tasks. They may include, but are not limited to, diplopia (double vision), asthenopia (eye strain), transient blurred vision, difficulty sustaining near-visual function, abnormal fatigue, headache, and abnormal postural adaptation, among others.
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Negative relative accommodation (NRA) was proposed by Joseph Kearney of Oxford University in 1967 as a measure of the maximum ability to relax accommodation while maintaining clear, single binocular vision. It is an indirect measurement of fusional vergence in binocular vision. [1]
Orthoptists are mainly involved with diagnosing and managing patients with binocular vision disorders which relate to amblyopia, eye movement disorders, extraocular muscle balance such as with version, refractive errors, vergence, accommodation imbalances, positive relative accommodation and negative relative accommodation. They work closely ...
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