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The patient is asked to fixate on a target while the examiner places a 4 prism dioptre base-out prism over the patient's eye, observing the response of the fellow eye. [ 1 ] The target is a single isolated distance target of approximately 1-2 lines better than best corrected acuity at distance.
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).
The Maddox rod test is a subjective method to measure horizontal and vertical deviations of the eyes, using a red line and a white light. The double Maddox rod test can also assess torsion and cyclotropias, by comparing the angles of two red lines.
The Hirschberg test, also known as the Hirschberg corneal reflex test, is a method to assess ocular misalignment by observing the light reflex on the cornea. It can detect exotropia, esotropia, hypertropia or hypotropia and is used in optometry and ophthalmology.
Dissociated vertical deviation (DVD) is an eye condition that causes one eye to drift upward spontaneously or after being covered. It is often associated with squints and can be diagnosed and treated by ophthalmologists.
Esotropia is a condition in which one or both eyes deviate inward, causing a cross-eyed appearance. It can be constant or intermittent, and has different types and causes, such as hyperopia, congenital, or accommodative factors.
Hypertropia is a condition of misalignment of the eyes, where one eye is higher than the other. It can be congenital or acquired, and may lead to amblyopia, diplopia or cosmetic defect. Learn about the possible causes, diagnosis and surgical options for hypertropia.
Prentice's rule is a formula to calculate the amount of prism correction in a lens based on decentration and lens power. It can be used for prescribing, tolerance control, or determining unprescribed prism in eyeglasses.