horror fusionis
Horror fusionis is an active avoidance of bifoveal stimulation in a patient with strabismus. The term comes from Bielschowsky, and it should not be applied loosely to other forms of absent or deficient sensory fusion.1
The sign appears as a haploscopic device or rotary prisms move the image in the deviating eye toward its fovea. Once the two images reach the foveas, they suddenly separate more widely, and no adjustment brings the foveal images together. Bielschowsky attributed the separation to a change in the angle of squint, and he believed defective development of cortical sensory connections might underlie the condition.1 Patients describe the images jumping over each other, and crossed diplopia changes abruptly to uncrossed diplopia as the prism is increased.2
Horror fusionis is rare except where it is elicited artificially, as in the chasing that orthoptists see on the major amblyoscope.1 Many affected patients are congenital esotropes with strabismus since early childhood. The condition should be distinguished from aniseikonia, small-angle harmonious anomalous retinal correspondence and deep foveal suppression. Surgery cannot eliminate the diplopia, so the condition must be identified beforehand. Treatment aims to relieve intractable diplopia with occlusion or hypnosis.2
More detail
Sensory fusion disruption syndrome is the closest confusable condition. Those patients can superimpose the diplopic images with prisms and report them on top of each other but not together, yet they cannot attain sensory fusion. The syndrome usually follows closed head trauma. Some patients with horror fusionis find widely separated images easier to ignore, and surgery that reduces the angle may worsen their symptoms.2 In the red-glass test, prisms may reduce a large deviation, but fully correcting the angle is inadvisable because it may provoke horror fusionis.3
Sources (8)
- von Noorden and Campos, Binocular Vision and Ocular Motility, ch. 9
- Pickwell's Binocular Vision Anomalies, ch. 14
- von Noorden and Campos, Binocular Vision and Ocular Motility, ch. 12
- Rutstein, R., Daum, K. (1997). Anomalies of Binocular Vision: Diagnosis & Management, Mosby.
- Caloroso, E.E., Rouse, M.W. (1993). Clinical Management of Strabismus. Butterworth-Heinemann.
- Scheiman, M., Wick, B. (2014). Clinical Management of Binocular Vision: Heterophoric, Accommodative, and Eye Movement Disorders. 4th ed., Lippincott Williams & Wilkins.
- Rowe, F.J. (2012). Clinical Orthoptics. 3rd ed., Wiley-Blackwell.
- Noorden, G.K.V., Campos, E.C. (2002). Binocular Vision and Ocular Motility: Theory and Management of Strabismus. 6th ed., Mosby.
Reviewed October 2026