monofixation syndrome

A state of binocular vision in which one fovea is suppressed during binocular viewing while peripheral fusion, fusional vergence amplitudes and gross stereopsis are kept. It can exist with or without a measurable deviation of the eyes^1,2. A suppression scotoma about 2.5 to 3 degrees across removes the central image of one eye, and the periphery of the retina continues to fuse and holds the eyes nearly straight¹.

The syndrome is often treated as a synonym for microtropia, but strictly it also includes patients with straight eyes and a foveal scotoma, so microtropia is its commonest form rather than its definition^1,2,3. It should be suspected when acuity in one eye is 20/25 to 20/40, stereoacuity is 30 to 80 seconds of arc and the cover test shows little or nothing; the 4 prism diopter base-out test confirms the central scotoma⁴.