The terminology surrounding very small angle strabismus has become very confused. The terms microstrabismus, microsquint, monofixation pattern (or syndrome), and subnormal binocular vision all refer to the same or similar conditions. Microtropia is my preferred term of choice.
Microtropia may be found as an apparently primary condition, or it may be present as a residual deviation after the treatment of a larger strabismus.
It is usually an eso-deviation with a form of binocular single vision, but there are exceptional cases of microhypertropia that usually result from surgical intervention of a large angle hypertropia and the occurrence of microexotropia has been reported.
The cause of primary microtropia is unknown, but there may be a genetic element and onset is from birth. As anisometropia is a common factor, the blurred image in the more ametropic eye might result in a foveal suppression scotoma with fixation on the edge of this scotoma. Secondary microtropia is often the result of vision therapy and/or surgery for a larger deviation, particularly in cases of early-onset such as infantile or primary comitant esotropia.
It has the following clinical characteristics:
The microtropia is between 1 and 10∆ in size.
There is often a difference between the refractive errors in the two eyes of more than 1.50D of hyperopia.
As the deviation may not be apparent on the cover test, amblyopia may be the first indication of the microtropia. Usually, the acuity is reduced one or two lines to 6/9 or 6/12.
Central fixation is always lost in microtropia.
Abnormal retinal correspondence
Harmonious anomalous retinal correspondence is present in microtropia.
The eyes in microtropia seem to be held in the nearly straight position of the small angle by the fusional impulses provided by peripheral vision.
In many cases of microtropia, the angle of the deviation may increase on the alternating cover test or even if one eye is covered for a slightly longer time than normal for the unilateral cover test. When the cover is removed from both the eyes, the eye that was last covered will be seen to return to the microtropia position. There is the appearance of a heterophoria in spite of the microtropia. It is as if a heterophoric movement is superimposed on the strabismus. The apparent heterophoria may be larger and more obvious than the microtropia, which as discussed above may not show at all on the cover test. This cover test recovery movement can be described as an anomalous fusional movement.
Low-grade stereopsis has been reported in microtropia although it is not always detected with standard clinical tests. It has been proposed that all cases of strabismus, including microtropia, perform sub-normally at random dot stereopsis tests.
In young children, optimum glasses should be recommended and the visual acuity monitored, with the possibility of occlusion therapy monitored. In older people, there isn’t much to do.