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If the patient saw a red line to the right and white light to the left, they are said to have esotropia or esophoria (uncrossed diplopia) in which base out (BO) prisms of increasing strength are used until the lines are superimposed.
The Four Prism Dioptre Reflex Test (also known as the 4 PRT, or 4 Prism Dioptre Base-out Test) is an objective, non-dissociative test used to prove the alignment of both eyes (i.e. the presence of binocular single vision) by assessing motor fusion.
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).
Secondary MFS is a frequent outcome of surgical treatment of congenital esotropia. A study of 1981 showed MFS to result in the vast majority of cases if surgical alignment is reached before the age of 24 months and only in a minority of cases if it is reached later.
Either BASE IN for an exodeviation (eye turned out), BASE OUT for an esodeviation (eye turned in), BASE UP for a hypodeviation (eye turned down) or BASE DOWN for a hyperdeviation (eye turned up). [5] Steps: 1. The patient should be measured in primary position first and then in any other positions of gaze of concern.
Treatment options for esotropia include glasses to correct refractive errors (see accommodative esotropia below), the use of prisms, orthoptic exercises, or eye muscle surgery. The term is from Greek eso meaning "inward" and trope meaning "a turning".
For an abnormal result, based on where the light lands on the cornea, the examiner can detect if there is an exotropia (abnormal eye is turned out), esotropia (abnormal eye is turned in), hypertropia (abnormal eye higher than the normal one) or hypotropia (abnormal eye is lower than the normal one).
The test is indicated with the use of a presence of a prism in individuals with a strabismus and fusion is considered present if 4 lights are maintained, with or without the use of a prism. The W4LT can also be indicated when aiding a person to develop and strengthen their fusional capacities.
Prism correction is measured in prism dioptres. A prescription that specifies prism correction will also specify the "base". The base is the thickest part of the lens and is opposite from the apex. Light will be bent towards the base and the image will be shifted towards the apex.
If there is little or no vertical separation, vertical prism can be used to separate the lines. The patient is asked if one or both lines are tilted. The lines can be straightened subjectively by rotating the glasses in the trial frame and the degree of cyclotropia recorded.