Four prism dioptre reflex test

Four-base-out is performed at distance only, because suppression scotomas in micro-strabismus are usually small and by using a near target, the examiner might cause the image of the target to land outside of the suppressed area and any signs of micro-strabismus or monofixation might be masked.

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.

[citation needed] 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 occurs with the interest of finding better visual acuity, however all patients found with eccentric fixation have amblyopia, suppression, anisometropia and poorer stereopsis.

Eccentric fixation utilises an abnormal retinal correspondence point and not the fovea, no movement under the prism is seen when placed over the deviated eye as the image falls onto the suppression scotoma.