The main defect is a central suppression scotoma. The central retina has small receptive fields and is more sensitive to image blur or image disparity than the peripheral retina. Conditions that cause a suppression scotoma in the central retina but allow for peripheral fusion cause monofixation syndrome.
Some patients seem to have an inherited inability to bifixate.
Look for history of muscle surgery, amblyopia, or retinal disease or family history of strabismus. Using alternate cover testing may result in 2-3 times greater prism measurement due to fusional vergence mechanisms. Up to one third of the patients have no deviation with alternate cover. Stereoacuity Patients with monofixation syndrome have reduced stereopsis ranging from 300-67 arc seconds. Stereopsis tends to be better at near than at distance.