Fixation disparity
Encyclopedia
Fixation disparity exists when there is a small misalignment of the eyes when viewing with binocular vision
. The misaligment may be vertical, horizontal or both. The misalignment (a few minutes of arc
) is much smaller than that of strabismus
, which prevents binocular vision, although it may reduce a patient's level of stereopsis
. A patient may or may not have fixation disparity and a patient may have a different fixation disparity at distance than near.
The Mallett Fixation Disparity Unit
Instrument used to measure the associated heterophoria (or compensating prism). It consists of a small central fixation letter X surrounded by two letters O, one on each side of X, the three letters being seen binocularly, and two coloured polarized vertical bars in line with the centre of the X which are seen by each eye separately. The instrument can be swung through 90º to measure any vertical fixation disparity. The associated phoria is indicated by the misalignment of the two polarized bars when the subject fixates the X through cross-polarized filters in front of the eyes. The amount of associated phoria is given by the value of the base-in or base-out prism power necessary to produce alignment. The unit can also be used to detect suppression. See Disparometer; associated heterophoria; uncompensated heterophoria.
Binocular vision
Binocular vision is vision in which both eyes are used together. The word binocular comes from two Latin roots, bini for double, and oculus for eye. Having two eyes confers at least four advantages over having one. First, it gives a creature a spare eye in case one is damaged. Second, it gives a...
. The misaligment may be vertical, horizontal or both. The misalignment (a few minutes of arc
Minute of arc
A minute of arc, arcminute, or minute of angle , is a unit of angular measurement equal to one sixtieth of one degree. In turn, a second of arc or arcsecond is one sixtieth of one minute of arc....
) is much smaller than that of strabismus
Strabismus
Strabismus is a condition in which the eyes are not properly aligned with each other. It typically involves a lack of coordination between the extraocular muscles, which prevents bringing the gaze of each eye to the same point in space and preventing proper binocular vision, which may adversely...
, which prevents binocular vision, although it may reduce a patient's level of stereopsis
Stereopsis
Stereopsis refers to impression of depth that is perceived when a scene is viewed with both eyes by someone with normal binocular vision. Binocular viewing of a scene creates two slightly different images of the scene in the two eyes due the the eyes' different positions on the head...
. A patient may or may not have fixation disparity and a patient may have a different fixation disparity at distance than near.
Measurement of fixation disparity
There are several methods to quantify fixation disparity. The Mallett card, the Bernell lantern slide, the Wessen Card and the Disparometer may be used. A patient's associated phoria is the amount of prism needed to reduce their fixation disparity to zero minutes of arc.The Mallett Fixation Disparity Unit
Instrument used to measure the associated heterophoria (or compensating prism). It consists of a small central fixation letter X surrounded by two letters O, one on each side of X, the three letters being seen binocularly, and two coloured polarized vertical bars in line with the centre of the X which are seen by each eye separately. The instrument can be swung through 90º to measure any vertical fixation disparity. The associated phoria is indicated by the misalignment of the two polarized bars when the subject fixates the X through cross-polarized filters in front of the eyes. The amount of associated phoria is given by the value of the base-in or base-out prism power necessary to produce alignment. The unit can also be used to detect suppression. See Disparometer; associated heterophoria; uncompensated heterophoria.
See also
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- Vision therapyVision therapyVision therapy, also known as visual training, vision training, or visual therapy, is a broad group of techniques aimed at correcting and improving binocular, oculomotor, visual processing, and perceptual disorders."-Historical development:...