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A fixation disparity is not constant within a certain observer, but can vary depending on the viewing conditions. If test prisms with increasing amount are placed in front of the observer’s eyes, the fixation disparity changes in the eso direction with base-in prisms and in the exo direction with base-out prisms (Fig. 3).
From left to right: an accommodative fixation stick, a vertical prism bar, and a horizontal prism bar. The PFR involves placing a prism bar in front of an eye. In a patient with BSV, a natural shift of the eye occurs. When measuring horizontal fusion ranges, base in prisms assess fusional divergence while base out prisms assess fusional ...
The strength of the prism is increased until the streak of the light passes through the centre of the prism, as the strength of the prism indicates the amount of deviation present. The Maddox rod is a handheld instrument composed of red parallel plano convex cylinder lens , which refracts light rays so that a point source of light is seen as a ...
The prism cover test (PCT) is an objective measurement and the gold standard in measuring strabismus, i.e. ocular misalignment, or a deviation of the eye. [1] It is used by ophthalmologists and orthoptists in order to measure the vertical and horizontal deviation and includes both manifest and latent components. [1]
[2] Accommodative vergence: blur-driven vergence. Fusional vergence (also: disparity vergence, disparity-driven vergence, or reflex vergence [2]): vergence induced by a stimulus to binocular fusion. Proximal vergence: vergence due to the awareness of a fixation object being near or far in the absence of disparity and of cues for accommodation ...
Numbers and arrows should be seen clearly. Relaxation of accommodation can result in an increase in exophoria and a decrease in esophoria, leading to an inaccurate result; The examiner should check the function of the Maddox Wing Instrument before use; the septa can be easily bent, leading to the septa not covering the intended view.
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Exophoria is a form of heterophoria in which there is a tendency of the eyes to deviate outward. [1] During examination, when the eyes are dissociated, the visual axes will appear to diverge away from one another. [2] The axis deviation in exophoria is usually mild compared with that of exotropia.