microtropia
A small-angle strabismus, usually an esotropia of less than about 6 prism diopters, that is fully adapted: the deviating eye has a central suppression scotoma, harmonious anomalous retinal correspondence, mild amblyopia and some eccentric fixation, while peripheral fusion and gross stereopsis are retained1,2.
In most cases three retinal points coincide in the deviating eye: the point used for fixation when the other eye is covered, the point that has acquired the fovea's visual direction, and the point on which the image falls during binocular viewing. This is called microtropia with identity. Because fixation already rests on that point, uncovering the eye calls for no refixation movement, and the cover test shows nothing1,2.
More detail
Microtropia is often treated as a synonym for monofixation syndrome, and some texts use the terms interchangeably3. Strictly, monofixation syndrome is the broader sensory state, which also includes patients with straight eyes and a foveal scotoma, so microtropia is its commonest form rather than its definition2,4. A primary microtropia has no history of a larger deviation; a secondary microtropia follows treatment of a larger strabismus1.
The condition should be suspected when acuity in one eye is one or two lines down, the refractive errors differ by more than 1.50 diopters, and the cover test is negative. The 4 prism diopter base-out test demonstrates the central scotoma. Fusional amplitudes are present, and a heterophoria can exist in the opposite direction, such as an exophoria over a micro-esotropia1,2.
Sources (9)
- Pickwell's Binocular Vision Anomalies, ch. 16, Microtropia
- von Noorden and Campos, Binocular Vision and Ocular Motility, ch. 16
- Kanski, Clinical Ophthalmology (2024), ch. 18
- Yanoff and Duker, Ophthalmology (2023), ch. 11
- Helveston, E.M. and von Noorden, G.K. (1967) 'Microtropia: A Newly Defined Entity', Archives of Ophthalmology, 78(3), pp. 272–281.
- Houston, C.A., Cleary, M., Dutton, G.N. and McFadzean, R.M. (1998) 'Clinical characteristics of microtropia--is microtropia a fixed phenomenon?', British Journal of Ophthalmology, 82(2), pp. 219–224.
- Lang, J. (1983) 'Microtropia', International Ophthalmology, 6(1), pp. 33–36.
- Parks, M.M. (1969) 'The monofixation syndrome', Transactions of the American Ophthalmological Society, 67, pp. 609–657.
- Rutstein, R.P., Corliss, D.A. and Fullard, R.J. (2006) 'Microtropia: clinical findings and management for the primary eye care practitioner', Optometry, 77(9), pp. 442–449.
Reviewed October 2026