Heterotropia is a generic term for manifest deviation of the eyes (manifest strabismus; cf. heterophoria), synonymous with squint. This may be obvious; an amblyopic eye, with poor visual acuity and fixation, may become deviated. Sometimes it may be more subtle, coming to attention only with the patient’s complaint of diplopia.
Using the alternate cover (cross cover) test, in which binocular fixation is not permitted, an imbalance in the visual axes may be demonstrated, but this will not distinguish between heterotropia and heterophoria. To make this distinction the cover test is required: if the uncovered eye moves to adopt fixation then heterotropia is confirmed.
Tropias may be in the horizontal (esotropia, exotropia) or vertical plane (hypertropia, hypotropia).
Shaunak S, O’Sullivan E, Kennard C. Eye movements. In: Hughes RAC (ed.). Neurological Investigations. London: BMJ Publishing, 1997: 253-282