Twenty selected patients with amblyopia but with no apparent strabismus were found after careful examination to have an ultrasmall-angle strabismus with specific sensory adaptations. Detection and study of this microstrabismus required special sensory tests including, the bifoveal correspondence, Bagolini striated glasses, the 4-diopter prism, and monocular fixation behavior tests in addition to standard examination techniques. In each patient, a microtropia not detectable with the cover test was demonstrated by showing that these patients used an eccentric area in the deviating eye for both monocular and binocular fixation, thus masking the tropia. It was shown also that the eccentric area in the deviating eye possessed certain characteristics of a true fovea, including harmonious anomalous retinal correspondence with the fovea of the sound eye. In most of the 20 patients, varying degrees of anisometropia was found.
Helveston EM, von Noorden GK. MicrotropiaA Newly Defined Entity. Arch Ophthalmol. 1967;78(3):272-281. doi:10.1001/archopht.1967.00980030274003