4 prism diopter base-out test presentation

In depth of four prism diopter base-out test

1. 4 prism diopter base-out test Sudama kharbind BOVS,NAMS-4 th BATCH

2. 4 Prism diopter reflex text The  Four   Prism Dioptre Reflex Test   - is an objective, non-dissociative test used to assess the ocular alignment of both eyes by assessing motor fusion. Through the use of a 4 dioptre base out prism,  diplopia  is induced which is the driving force for the eyes to change fixation and therefore re-gain bifoveal fixation meaning, they overcome that amount of power.

3. Indications for use This test is performed on patients suspected to have small angle deviations of less than 10 prism dioptres, a  microtropia , that may or may not have been observed on cover test because of subtle eye movements. The test determines whether the patient has bifoveal fixation or monofixation despite their eyes seeming straight. [2]

4. Equipment Fixation target, does not have to be accommodative 4 dioptre prism, either loose or prism bar Bright lighting conditions

5. Method of assessment As it is an objective test, few instructions are required to be given to the patient. 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]

6. Interpretation of results/ recording Normal response If the patient's eyes are aligned and is bifoveal, the shifting of the image caused by the prism will produce a movement towards the apex of the prism (of the eye under the prism), and the fellow eye will have an outward movement in the same direction of the same magnitude due to  Hering's law of equal innervation . Simultaneously the fellow eye produces a fusional convergence movement as there is no central suppression of that fellow eye. This is a result of overcoming the diplopia experienced. [1]

7. Contd.. Abnormal response: central suppression In cases where the patient has central suppression the following will occur . When the prism is placed in front of the non-deviating eye, both eye will produce a conjugate movement in the direction of the prism apex. However unlike a normal response, the fellow deviated eye will not make a corrective movement because diplopia has not been appreciated since the image will fall into the suppression scotoma . Whereas when the prism is placed in front of the deviated eye, the image instantly falls into the suppression scotoma, diplopia is not detected. This causes the eye under the prism to remain stationary, therefore the fellow eye does not make a conjugate movement

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