Four prism dioptre reflex test

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. 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.

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.
On the cover component of cover test a small manifest deviation may be documented, however the patient passes the Worth's Four Lights Test or Bagolini Striated Glasses Test, indicating binocular single vision, but fails a bifoveal test such as the Lang I/II.
The 4 PRT therefore confirms the presence of a microtropia whether it be with identity (i.e. with eccentric viewing) or without identity (i.e. extension of Panum’s fusion area).

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.
The target is a single isolated distance target of approximately 1-2 lines better than best corrected acuity at distance. It repeated for both eyes. Four-base-out is a distance method because suppression scotomas in micro-strabismus is usually small and by using a near target, the examiner might cause the image of the target to land outside of said suppression zone and any signs of micro-strabismus or monofixation might be masked.

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.

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