[The 4-prism diopter base-in test in diagnosis of exophoria with asthenopia and compensation by accommodative convergence].

About 30% of our patients suffering from purely binocular asthenopia showed lower binocular than monocular visual acuity. Cover test examination revealed exophoria at near fixation, which could be regarded to be physiological. Furthermore, the Pola test as well as Graefe's prism diplopia test showed no pathological findings. When given 4 prism base-in, a better binocular acuity was achieved and exophoria at for distance fixation was revealed. Often the prisms base-in had to be increased slowly and an operative treatment was indicated in 80%. The test is described in detail and examples from typical case studies are demonstrated. The importance of the test for understanding asthenopia in cases with heterophoria is discussed.